# Rust Compiler Error Index

## E0001

#### Note: this error code is no longer emitted by the compiler.

This error suggests that the expression arm corresponding to the noted pattern will never be reached as for all possible values of the expression being matched, one of the preceding patterns will match.

This means that perhaps some of the preceding patterns are too general, this one is too specific or the ordering is incorrect.

For example, the following `match` block has too many arms:

    match Some(0) {
        Some(bar) => {/* ... */}
        x => {/* ... */} // This handles the `None` case
        _ => {/* ... */} // All possible cases have already been handled
    }Run

`match` blocks have their patterns matched in order, so, for example, putting a wildcard arm above a more specific arm will make the latter arm irrelevant.

Ensure the ordering of the match arm is correct and remove any superfluous arms.

## E0002

#### Note: this error code is no longer emitted by the compiler.

This error indicates that an empty match expression is invalid because the type it is matching on is non-empty (there exist values of this type). In safe code it is impossible to create an instance of an empty type, so empty match expressions are almost never desired. This error is typically fixed by adding one or more cases to the match expression.

An example of an empty type is `enum Empty { }`. So, the following will work:

    enum Empty {}

    fn foo(x: Empty) {
        match x {
            // empty
        }
    }Run

However, this won't:

ⓘThis example deliberately fails to compile

    fn foo(x: Option<String>) {
        match x {
            // empty
        }
    }Run

## E0004

This error indicates that the compiler cannot guarantee a matching pattern for one or more possible inputs to a match expression. Guaranteed matches are required in order to assign values to match expressions, or alternatively, determine the flow of execution. Erroneous code example:

ⓘThis example deliberately fails to compile

    enum Terminator {
        HastaLaVistaBaby,
        TalkToMyHand,
    }

    let x = Terminator::HastaLaVistaBaby;

    match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered
        Terminator::TalkToMyHand => {}
    }Run

If you encounter this error you must alter your patterns so that every possible value of the input type is matched. For types with a small number of variants (like enums) you should probably cover all cases explicitly. Alternatively, the underscore `_` wildcard pattern can be added after all other patterns to match "anything else". Example:

    enum Terminator {
        HastaLaVistaBaby,
        TalkToMyHand,
    }

    let x = Terminator::HastaLaVistaBaby;

    match x {
        Terminator::TalkToMyHand => {}
        Terminator::HastaLaVistaBaby => {}
    }

    // or:

    match x {
        Terminator::TalkToMyHand => {}
        _ => {}
    }Run

## E0005

Patterns used to bind names must be irrefutable, that is, they must guarantee that a name will be extracted in all cases. Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = Some(1);
    let Some(y) = x;
    // error: refutable pattern in local binding: `None` not coveredRun

If you encounter this error you probably need to use a `match` or `if let` to deal with the possibility of failure. Example:

    let x = Some(1);

    match x {
        Some(y) => {
            // do something
        },
        None => {}
    }

    // or:

    if let Some(y) = x {
        // do something
    }Run

## E0007

This error indicates that the bindings in a match arm would require a value to be moved into more than one location, thus violating unique ownership. Code like the following is invalid as it requires the entire `Option<String>` to be moved into a variable called `op_string` while simultaneously requiring the inner `String` to be moved into a variable called `s`.

ⓘThis example deliberately fails to compile

    let x = Some("s".to_string());

    match x {
        op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
        None => {},
    }Run

See also the error E0303.

## E0008

Names bound in match arms retain their type in pattern guards. As such, if a name is bound by move in a pattern, it should also be moved to wherever it is referenced in the pattern guard code. Doing so however would prevent the name from being available in the body of the match arm. Consider the following:

ⓘThis example deliberately fails to compile

    match Some("hi".to_string()) {
        Some(s) if s.len() == 0 => {}, // use s.
        _ => {},
    }Run

The variable `s` has type `String`, and its use in the guard is as a variable of type `String`. The guard code effectively executes in a separate scope to the body of the arm, so the value would be moved into this anonymous scope and therefore becomes unavailable in the body of the arm.

The problem above can be solved by using the `ref` keyword.

    match Some("hi".to_string()) {
        Some(ref s) if s.len() == 0 => {},
        _ => {},
    }Run

Though this example seems innocuous and easy to solve, the problem becomes clear when it encounters functions which consume the value:

ⓘThis example deliberately fails to compile

    struct A{}

    impl A {
        fn consume(self) -> usize {
            0
        }
    }

    fn main() {
        let a = Some(A{});
        match a {
            Some(y) if y.consume() > 0 => {}
            _ => {}
        }
    }Run

In this situation, even the `ref` keyword cannot solve it, since borrowed content cannot be moved. This problem cannot be solved generally. If the value can be cloned, here is a not-so-specific solution:

    #[derive(Clone)]
    struct A{}

    impl A {
        fn consume(self) -> usize {
            0
        }
    }

    fn main() {
        let a = Some(A{});
        match a{
            Some(ref y) if y.clone().consume() > 0 => {}
            _ => {}
        }
    }Run

If the value will be consumed in the pattern guard, using its clone will not move its ownership, so the code works.

## E0009

In a pattern, all values that don't implement the `Copy` trait have to be bound the same way. The goal here is to avoid binding simultaneously by-move and by-ref.

This limitation may be removed in a future version of Rust.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct X { x: (), }

    let x = Some((X { x: () }, X { x: () }));
    match x {
        Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the
                                //        same pattern
        None => panic!()
    }Run

You have two solutions:

Solution #1: Bind the pattern's values the same way.

    struct X { x: (), }

    let x = Some((X { x: () }, X { x: () }));
    match x {
        Some((ref y, ref z)) => {},
        // or Some((y, z)) => {}
        None => panic!()
    }Run

Solution #2: Implement the `Copy` trait for the `X` structure.

However, please keep in mind that the first solution should be preferred.

    #[derive(Clone, Copy)]
    struct X { x: (), }

    let x = Some((X { x: () }, X { x: () }));
    match x {
        Some((y, ref z)) => {},
        None => panic!()
    }Run

## E0010

The value of statics and constants must be known at compile time, and they live for the entire lifetime of a program. Creating a boxed value allocates memory on the heap at runtime, and therefore cannot be done at compile time. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(box_syntax)]

    const CON : Box<i32> = box 0;Run

## E0013

Static and const variables can refer to other const variables. But a const variable cannot refer to a static variable. For example, `Y` cannot refer to `X` here:

ⓘThis example deliberately fails to compile

    static X: i32 = 42;
    const Y: i32 = X;Run

To fix this, the value can be extracted as a const and then used:

    const A: i32 = 42;
    static X: i32 = A;
    const Y: i32 = A;Run

## E0015

The only functions that can be called in static or constant expressions are `const` functions, and struct/enum constructors. `const` functions are only available on a nightly compiler. Rust currently does not support more general compile-time function execution.

    const FOO: Option<u8> = Some(1); // enum constructor
    struct Bar {x: u8}
    const BAR: Bar = Bar {x: 1}; // struct constructorRun

See RFC 911 for more details on the design of `const fn`s.

## E0017

References in statics and constants may only refer to immutable values. Erroneous code example:

ⓘThis example deliberately fails to compile

    static X: i32 = 1;
    const C: i32 = 2;

    // these three are not allowed:
    const CR: &mut i32 = &mut C;
    static STATIC_REF: &'static mut i32 = &mut X;
    static CONST_REF: &'static mut i32 = &mut C;Run

Statics are shared everywhere, and if they refer to mutable data one might violate memory safety since holding multiple mutable references to shared data is not allowed.

If you really want global mutable state, try using `static mut` or a global `UnsafeCell`.

## E0019

A function call isn't allowed in the const's initialization expression because the expression's value must be known at compile-time. Erroneous code example:

ⓘThis example deliberately fails to compile

    enum Test {
        V1
    }

    impl Test {
        fn test(&self) -> i32 {
            12
        }
    }

    fn main() {
        const FOO: Test = Test::V1;

        const A: i32 = FOO.test(); // You can't call Test::func() here!
    }Run

Remember: you can't use a function call inside a const's initialization expression! However, you can totally use it anywhere else:

    enum Test {
        V1
    }

    impl Test {
        fn func(&self) -> i32 {
            12
        }
    }

    fn main() {
        const FOO: Test = Test::V1;

        FOO.func(); // here is good
        let x = FOO.func(); // or even here!
    }Run

## E0023

A pattern used to match against an enum variant must provide a sub-pattern for each field of the enum variant. This error indicates that a pattern attempted to extract an incorrect number of fields from a variant.

    enum Fruit {
        Apple(String, String),
        Pear(u32),
    }Run

Here the `Apple` variant has two fields, and should be matched against like so:

    enum Fruit {
        Apple(String, String),
        Pear(u32),
    }

    let x = Fruit::Apple(String::new(), String::new());

    // Correct.
    match x {
        Fruit::Apple(a, b) => {},
        _ => {}
    }Run

Matching with the wrong number of fields has no sensible interpretation:

ⓘThis example deliberately fails to compile

    enum Fruit {
        Apple(String, String),
        Pear(u32),
    }

    let x = Fruit::Apple(String::new(), String::new());

    // Incorrect.
    match x {
        Fruit::Apple(a) => {},
        Fruit::Apple(a, b, c) => {},
    }Run

Check how many fields the enum was declared with and ensure that your pattern uses the same number.

## E0025

Each field of a struct can only be bound once in a pattern. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        a: u8,
        b: u8,
    }

    fn main(){
        let x = Foo { a:1, b:2 };

        let Foo { a: x, a: y } = x;
        // error: field `a` bound multiple times in the pattern
    }Run

Each occurrence of a field name binds the value of that field, so to fix this error you will have to remove or alter the duplicate uses of the field name. Perhaps you misspelled another field name? Example:

    struct Foo {
        a: u8,
        b: u8,
    }

    fn main(){
        let x = Foo { a:1, b:2 };

        let Foo { a: x, b: y } = x; // ok!
    }Run

## E0026

This error indicates that a struct pattern attempted to extract a non-existent field from a struct. Struct fields are identified by the name used before the colon `:` so struct patterns should resemble the declaration of the struct type being matched.

    // Correct matching.
    struct Thing {
        x: u32,
        y: u32
    }

    let thing = Thing { x: 1, y: 2 };

    match thing {
        Thing { x: xfield, y: yfield } => {}
    }Run

If you are using shorthand field patterns but want to refer to the struct field by a different name, you should rename it explicitly.

Change this:

ⓘThis example deliberately fails to compile

    struct Thing {
        x: u32,
        y: u32
    }

    let thing = Thing { x: 0, y: 0 };

    match thing {
        Thing { x, z } => {}
    }Run

To this:

    struct Thing {
        x: u32,
        y: u32
    }

    let thing = Thing { x: 0, y: 0 };

    match thing {
        Thing { x, y: z } => {}
    }Run

## E0027

This error indicates that a pattern for a struct fails to specify a sub-pattern for every one of the struct's fields. Ensure that each field from the struct's definition is mentioned in the pattern, or use `..` to ignore unwanted fields.

For example:

ⓘThis example deliberately fails to compile

    struct Dog {
        name: String,
        age: u32,
    }

    let d = Dog { name: "Rusty".to_string(), age: 8 };

    // This is incorrect.
    match d {
        Dog { age: x } => {}
    }Run

This is correct (explicit):

    struct Dog {
        name: String,
        age: u32,
    }

    let d = Dog { name: "Rusty".to_string(), age: 8 };

    match d {
        Dog { name: ref n, age: x } => {}
    }

    // This is also correct (ignore unused fields).
    match d {
        Dog { age: x, .. } => {}
    }Run

## E0029

In a match expression, only numbers and characters can be matched against a range. This is because the compiler checks that the range is non-empty at compile-time, and is unable to evaluate arbitrary comparison functions. If you want to capture values of an orderable type between two end-points, you can use a guard.

ⓘThis example deliberately fails to compile

    let string = "salutations !";

    // The ordering relation for strings can't be evaluated at compile time,
    // so this doesn't work:
    match string {
        "hello" ..= "world" => {}
        _ => {}
    }

    // This is a more general version, using a guard:
    match string {
        s if s >= "hello" && s <= "world" => {}
        _ => {}
    }Run

## E0030

When matching against a range, the compiler verifies that the range is non-empty. Range patterns include both end-points, so this is equivalent to requiring the start of the range to be less than or equal to the end of the range.

For example:

ⓘThis example deliberately fails to compile

    match 5u32 {
        // This range is ok, albeit pointless.
        1 ..= 1 => {}
        // This range is empty, and the compiler can tell.
        1000 ..= 5 => {}
    }Run

## E0033

This error indicates that a pointer to a trait type cannot be implicitly dereferenced by a pattern. Every trait defines a type, but because the size of trait implementors isn't fixed, this type has no compile-time size. Therefore, all accesses to trait types must be through pointers. If you encounter this error you should try to avoid dereferencing the pointer.

ⓘThis example deliberately fails to compile

    let trait_obj: &SomeTrait = &"some_value";

    // This tries to implicitly dereference to create an unsized local variable.
    let &invalid = trait_obj;

    // You can call methods without binding to the value being pointed at.
    trait_obj.method_one();
    trait_obj.method_two();Run

You can read more about trait objects in the Trait Objects section of the Reference.

## E0034

The compiler doesn't know what method to call because more than one method has the same prototype. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Test;

    trait Trait1 {
        fn foo();
    }

    trait Trait2 {
        fn foo();
    }

    impl Trait1 for Test { fn foo() {} }
    impl Trait2 for Test { fn foo() {} }

    fn main() {
        Test::foo() // error, which foo() to call?
    }Run

To avoid this error, you have to keep only one of them and remove the others. So let's take our example and fix it:

    struct Test;

    trait Trait1 {
        fn foo();
    }

    impl Trait1 for Test { fn foo() {} }

    fn main() {
        Test::foo() // and now that's good!
    }Run

However, a better solution would be using fully explicit naming of type and trait:

    struct Test;

    trait Trait1 {
        fn foo();
    }

    trait Trait2 {
        fn foo();
    }

    impl Trait1 for Test { fn foo() {} }
    impl Trait2 for Test { fn foo() {} }

    fn main() {
        <Test as Trait1>::foo()
    }Run

One last example:

    trait F {
        fn m(&self);
    }

    trait G {
        fn m(&self);
    }

    struct X;

    impl F for X { fn m(&self) { println!("I am F"); } }
    impl G for X { fn m(&self) { println!("I am G"); } }

    fn main() {
        let f = X;

        F::m(&f); // it displays "I am F"
        G::m(&f); // it displays "I am G"
    }Run

## E0038

Trait objects like `Box<Trait>` can only be constructed when certain requirements are satisfied by the trait in question.

Trait objects are a form of dynamic dispatch and use a dynamically sized type for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed pointer is a 'fat pointer' that contains an extra pointer to a table of methods (among other things) for dynamic dispatch. This design mandates some restrictions on the types of traits that are allowed to be used in trait objects, which are collectively termed as 'object safety' rules.

Attempting to create a trait object for a non object-safe trait will trigger this error.

There are various rules:

### The trait cannot require `Self: Sized`

When `Trait` is treated as a type, the type does not implement the special `Sized` trait, because the type does not have a known size at compile time and can only be accessed behind a pointer. Thus, if we have a trait like the following:

    trait Foo where Self: Sized {

    }Run

We cannot create an object of type `Box<Foo>` or `&Foo` since in this case `Self` would not be `Sized`.

Generally, `Self: Sized` is used to indicate that the trait should not be used as a trait object. If the trait comes from your own crate, consider removing this restriction.

### Method references the `Self` type in its arguments or return type

This happens when a trait has a method like the following:

    trait Trait {
        fn foo(&self) -> Self;
    }

    impl Trait for String {
        fn foo(&self) -> Self {
            "hi".to_owned()
        }
    }

    impl Trait for u8 {
        fn foo(&self) -> Self {
            1
        }
    }Run

(Note that `&self` and `&mut self` are okay, it's additional `Self` types which cause this problem.)

In such a case, the compiler cannot predict the return type of `foo()` in a situation like the following:

ⓘThis example deliberately fails to compile

    trait Trait {
        fn foo(&self) -> Self;
    }

    fn call_foo(x: Box<Trait>) {
        let y = x.foo(); // What type is y?
        // ...
    }Run

If only some methods aren't object-safe, you can add a `where Self: Sized` bound on them to mark them as explicitly unavailable to trait objects. The functionality will still be available to all other implementers, including `Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).

    trait Trait {
        fn foo(&self) -> Self where Self: Sized;
        // more functions
    }Run

Now, `foo()` can no longer be called on a trait object, but you will now be allowed to make a trait object, and that will be able to call any object-safe methods. With such a bound, one can still call `foo()` on types implementing that trait that aren't behind trait objects.

### Method has generic type parameters

As mentioned before, trait objects contain pointers to method tables. So, if we have:

    trait Trait {
        fn foo(&self);
    }

    impl Trait for String {
        fn foo(&self) {
            // implementation 1
        }
    }

    impl Trait for u8 {
        fn foo(&self) {
            // implementation 2
        }
    }
    // ...Run

At compile time each implementation of `Trait` will produce a table containing the various methods (and other items) related to the implementation.

This works fine, but when the method gains generic parameters, we can have a problem.

Usually, generic parameters get monomorphized. For example, if I have

    fn foo<T>(x: T) {
        // ...
    }Run

The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any other type substitution is different. Hence the compiler generates the implementation on-demand. If you call `foo()` with a `bool` parameter, the compiler will only generate code for `foo::<bool>()`. When we have additional type parameters, the number of monomorphized implementations the compiler generates does not grow drastically, since the compiler will only generate an implementation if the function is called with unparametrized substitutions (i.e., substitutions where none of the substituted types are themselves parametrized).

However, with trait objects we have to make a table containing every object that implements the trait. Now, if it has type parameters, we need to add implementations for every type that implements the trait, and there could theoretically be an infinite number of types.

For example, with:

    trait Trait {
        fn foo<T>(&self, on: T);
        // more methods
    }

    impl Trait for String {
        fn foo<T>(&self, on: T) {
            // implementation 1
        }
    }

    impl Trait for u8 {
        fn foo<T>(&self, on: T) {
            // implementation 2
        }
    }

    // 8 more implementationsRun

Now, if we have the following code:

ⓘThis example deliberately fails to compile

    fn call_foo(thing: Box<Trait>) {
        thing.foo(true); // this could be any one of the 8 types above
        thing.foo(1);
        thing.foo("hello");
    }Run

We don't just need to create a table of all implementations of all methods of `Trait`, we need to create such a table, for each different type fed to `foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3 types being fed to `foo()`) = 30 implementations!

With real world traits these numbers can grow drastically.

To fix this, it is suggested to use a `where Self: Sized` bound similar to the fix for the sub-error above if you do not intend to call the method with type parameters:

    trait Trait {
        fn foo<T>(&self, on: T) where Self: Sized;
        // more methods
    }Run

If this is not an option, consider replacing the type parameter with another trait object (e.g., if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number of types you intend to feed to this method is limited, consider manually listing out the methods of different types.

### Method has no receiver

Methods that do not take a `self` parameter can't be called since there won't be a way to get a pointer to the method table for them.

    trait Foo {
        fn foo() -> u8;
    }Run

This could be called as `<Foo as Foo>::foo()`, which would not be able to pick an implementation.

Adding a `Self: Sized` bound to these methods will generally make this compile.

    trait Foo {
        fn foo() -> u8 where Self: Sized;
    }Run

### The trait cannot contain associated constants

Just like static functions, associated constants aren't stored on the method table. If the trait or any subtrait contain an associated constant, they cannot be made into an object.

ⓘThis example deliberately fails to compile

    trait Foo {
        const X: i32;
    }

    impl Foo {}Run

A simple workaround is to use a helper method instead:

    trait Foo {
        fn x(&self) -> i32;
    }Run

### The trait cannot use `Self` as a type parameter in the supertrait listing

This is similar to the second sub-error, but subtler. It happens in situations like the following:

ⓘThis example deliberately fails to compile

    trait Super<A> {}

    trait Trait: Super<Self> {
    }

    struct Foo;

    impl Super<Foo> for Foo{}

    impl Trait for Foo {}Run

Here, the supertrait might have methods as follows:

    trait Super<A> {
        fn get_a(&self) -> A; // note that this is object safe!
    }Run

If the trait `Foo` was deriving from something like `Super<String>` or `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type `get_a()` will definitely return an object of that type.

However, if it derives from `Super<Self>`, even though `Super` is object safe, the method `get_a()` would return an object of unknown type when called on the function. `Self` type parameters let us make object safe traits no longer safe, so they are forbidden when specifying supertraits.

There's no easy fix for this, generally code will need to be refactored so that you no longer need to derive from `Super<Self>`.

## E0040

It is not allowed to manually call destructors in Rust. It is also not necessary to do this since `drop` is called automatically whenever a value goes out of scope.

Here's an example of this error:

ⓘThis example deliberately fails to compile

    struct Foo {
        x: i32,
    }

    impl Drop for Foo {
        fn drop(&mut self) {
            println!("kaboom");
        }
    }

    fn main() {
        let mut x = Foo { x: -7 };
        x.drop(); // error: explicit use of destructor method
    }Run

## E0044

You can't use type or const parameters on foreign items. Example of erroneous code:

ⓘThis example deliberately fails to compile

    extern { fn some_func<T>(x: T); }Run

To fix this, replace the generic parameter with the specializations that you need:

    extern { fn some_func_i32(x: i32); }
    extern { fn some_func_i64(x: i64); }Run

## E0045

Rust only supports variadic parameters for interoperability with C code in its FFI. As such, variadic parameters can only be used with functions which are using the C ABI. Examples of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(unboxed_closures)]

    extern "rust-call" { fn foo(x: u8, ...); }

    // or

    fn foo(x: u8, ...) {}Run

To fix such code, put them in an extern "C" block:

    extern "C" {
        fn foo (x: u8, ...);
    }Run

## E0046

Items are missing in a trait implementation. Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo();
    }

    struct Bar;

    impl Foo for Bar {}
    // error: not all trait items implemented, missing: `foo`Run

When trying to make some type implement a trait `Foo`, you must, at minimum, provide implementations for all of `Foo`'s required methods (meaning the methods that do not have default implementations), as well as any required trait items like associated types or constants. Example:

    trait Foo {
        fn foo();
    }

    struct Bar;

    impl Foo for Bar {
        fn foo() {} // ok!
    }Run

## E0049

This error indicates that an attempted implementation of a trait method has the wrong number of type or const parameters.

For example, the trait below has a method `foo` with a type parameter `T`, but the implementation of `foo` for the type `Bar` is missing this parameter:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo<T: Default>(x: T) -> Self;
    }

    struct Bar;

    // error: method `foo` has 0 type parameters but its trait declaration has 1
    // type parameter
    impl Foo for Bar {
        fn foo(x: bool) -> Self { Bar }
    }Run

## E0050

This error indicates that an attempted implementation of a trait method has the wrong number of function parameters.

For example, the trait below has a method `foo` with two function parameters (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits the `u8` parameter:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo(&self, x: u8) -> bool;
    }

    struct Bar;

    // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
    // has 2
    impl Foo for Bar {
        fn foo(&self) -> bool { true }
    }Run

## E0053

The parameters of any trait method must match between a trait implementation and the trait definition.

Here are a couple examples of this error:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo(x: u16);
        fn bar(&self);
    }

    struct Bar;

    impl Foo for Bar {
        // error, expected u16, found i16
        fn foo(x: i16) { }

        // error, types differ in mutability
        fn bar(&mut self) { }
    }Run

## E0054

It is not allowed to cast to a bool. If you are trying to cast a numeric type to a bool, you can compare it with zero instead:

ⓘThis example deliberately fails to compile

    let x = 5;

    // Not allowed, won't compile
    let x_is_nonzero = x as bool;Run

    let x = 5;

    // Ok
    let x_is_nonzero = x != 0;Run

## E0055

During a method call, a value is automatically dereferenced as many times as needed to make the value's type match the method's receiver. The catch is that the compiler will only attempt to dereference a number of times up to the recursion limit (which can be set via the `recursion_limit` attribute).

For a somewhat artificial example:

ⓘThis example deliberately fails to compile

    #![recursion_limit="5"]

    struct Foo;

    impl Foo {
        fn foo(&self) {}
    }

    fn main() {
        let foo = Foo;
        let ref_foo = &&&&&Foo;

        // error, reached the recursion limit while auto-dereferencing `&&&&&Foo`
        ref_foo.foo();
    }Run

One fix may be to increase the recursion limit. Note that it is possible to create an infinite recursion of dereferencing, in which case the only fix is to somehow break the recursion.

## E0057

When invoking closures or other implementations of the function traits `Fn`, `FnMut` or `FnOnce` using call notation, the number of parameters passed to the function must match its definition.

An example using a closure:

ⓘThis example deliberately fails to compile

    let f = |x| x * 3;
    let a = f();        // invalid, too few parameters
    let b = f(4);       // this works!
    let c = f(2, 3);    // invalid, too many parametersRun

A generic function must be treated similarly:

    fn foo<F: Fn()>(f: F) {
        f(); // this is valid, but f(3) would not work
    }Run

## E0059

The built-in function traits are generic over a tuple of the function arguments. If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses (`Fn(T) -> U`) to denote the function trait, the type parameter should be a tuple. Otherwise function call notation cannot be used and the trait will not be implemented by closures.

The most likely source of this error is using angle-bracket notation without wrapping the function argument type into a tuple, for example:

ⓘThis example deliberately fails to compile

    #![feature(unboxed_closures)]

    fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }Run

It can be fixed by adjusting the trait bound like this:

    #![feature(unboxed_closures)]

    fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }Run

Note that `(T,)` always denotes the type of a 1-tuple containing an element of type `T`. The comma is necessary for syntactic disambiguation.

## E0060

External C functions are allowed to be variadic. However, a variadic function takes a minimum number of arguments. For example, consider C's variadic `printf` function:

    use std::os::raw::{c_char, c_int};

    extern "C" {
        fn printf(_: *const c_char, ...) -> c_int;
    }Run

Using this declaration, it must be called with at least one argument, so simply calling `printf()` is invalid. But the following uses are allowed:

    unsafe {
        use std::ffi::CString;

        let fmt = CString::new("test\n").unwrap();
        printf(fmt.as_ptr());

        let fmt = CString::new("number = %d\n").unwrap();
        printf(fmt.as_ptr(), 3);

        let fmt = CString::new("%d, %d\n").unwrap();
        printf(fmt.as_ptr(), 10, 5);
    }Run

## E0061

The number of arguments passed to a function must match the number of arguments specified in the function signature.

For example, a function like:

    fn f(a: u16, b: &str) {}Run

Must always be called with exactly two arguments, e.g., `f(2, "test")`.

Note that Rust does not have a notion of optional function arguments or variadic functions (except for its C-FFI).

## E0062

This error indicates that during an attempt to build a struct or struct-like enum variant, one of the fields was specified more than once. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        x: i32,
    }

    fn main() {
        let x = Foo {
                    x: 0,
                    x: 0, // error: field `x` specified more than once
                };
    }Run

Each field should be specified exactly one time. Example:

    struct Foo {
        x: i32,
    }

    fn main() {
        let x = Foo { x: 0 }; // ok!
    }Run

## E0063

This error indicates that during an attempt to build a struct or struct-like enum variant, one of the fields was not provided. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        x: i32,
        y: i32,
    }

    fn main() {
        let x = Foo { x: 0 }; // error: missing field: `y`
    }Run

Each field should be specified exactly once. Example:

    struct Foo {
        x: i32,
        y: i32,
    }

    fn main() {
        let x = Foo { x: 0, y: 0 }; // ok!
    }Run

## E0067

The left-hand side of a compound assignment expression must be a place expression. A place expression represents a memory location and includes item paths (ie, namespaced variables), dereferences, indexing expressions, and field references.

Let's start with some erroneous code examples:

ⓘThis example deliberately fails to compile

    use std::collections::LinkedList;

    // Bad: assignment to non-place expression
    LinkedList::new() += 1;

    // ...

    fn some_func(i: &mut i32) {
        i += 12; // Error : '+=' operation cannot be applied on a reference !
    }Run

And now some working examples:

    let mut i : i32 = 0;

    i += 12; // Good !

    // ...

    fn some_func(i: &mut i32) {
        *i += 12; // Good !
    }Run

## E0069

The compiler found a function whose body contains a `return;` statement but whose return type is not `()`. An example of this is:

ⓘThis example deliberately fails to compile

    // error
    fn foo() -> u8 {
        return;
    }Run

Since `return;` is just like `return ();`, there is a mismatch between the function's return type and the value being returned.

## E0070

The left-hand side of an assignment operator must be a place expression. A place expression represents a memory location and can be a variable (with optional namespacing), a dereference, an indexing expression or a field reference.

More details can be found in the Expressions section of the Reference.

Now, we can go further. Here are some erroneous code examples:

ⓘThis example deliberately fails to compile

    struct SomeStruct {
        x: i32,
        y: i32
    }

    const SOME_CONST : i32 = 12;

    fn some_other_func() {}

    fn some_function() {
        SOME_CONST = 14; // error : a constant value cannot be changed!
        1 = 3; // error : 1 isn't a valid place!
        some_other_func() = 4; // error : we can't assign value to a function!
        SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
                           // like a variable!
    }Run

And now let's give working examples:

    struct SomeStruct {
        x: i32,
        y: i32
    }
    let mut s = SomeStruct {x: 0, y: 0};

    s.x = 3; // that's good !

    // ...

    fn some_func(x: &mut i32) {
        *x = 12; // that's good !
    }Run

## E0071

You tried to use structure-literal syntax to create an item that is not a structure or enum variant.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    type U32 = u32;
    let t = U32 { value: 4 }; // error: expected struct, variant or union type,
                              // found builtin type `u32`Run

To fix this, ensure that the name was correctly spelled, and that the correct form of initializer was used.

For example, the code above can be fixed to:

    enum Foo {
        FirstValue(i32)
    }

    fn main() {
        let u = Foo::FirstValue(0i32);

        let t = 4;
    }Run

## E0072

When defining a recursive struct or enum, any use of the type being defined from inside the definition must occur behind a pointer (like `Box` or `&`). This is because structs and enums must have a well-defined size, and without the pointer, the size of the type would need to be unbounded.

Consider the following erroneous definition of a type for a list of bytes:

ⓘThis example deliberately fails to compile

    // error, invalid recursive struct type
    struct ListNode {
        head: u8,
        tail: Option<ListNode>,
    }Run

This type cannot have a well-defined size, because it needs to be arbitrarily large (since we would be able to nest `ListNode`s to any depth). Specifically,

    size of `ListNode` = 1 byte for `head`
                       + 1 byte for the discriminant of the `Option`
                       + size of `ListNode`

One way to fix this is by wrapping `ListNode` in a `Box`, like so:

    struct ListNode {
        head: u8,
        tail: Option<Box<ListNode>>,
    }Run

This works because `Box` is a pointer, so its size is well-known.

## E0073

#### Note: this error code is no longer emitted by the compiler.

You cannot define a struct (or enum) `Foo` that requires an instance of `Foo` in order to make a new `Foo` value. This is because there would be no way a first instance of `Foo` could be made to initialize another instance!

Here's an example of a struct that has this problem:

    struct Foo { x: Box<Foo> } // errorRun

One fix is to use `Option`, like so:

    struct Foo { x: Option<Box<Foo>> }Run

Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.

## E0074

#### Note: this error code is no longer emitted by the compiler.

When using the `#[simd]` attribute on a tuple struct, the components of the tuple struct must all be of a concrete, nongeneric type so the compiler can reason about how to use SIMD with them. This error will occur if the types are generic.

This will cause an error:

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Bad<T>(T, T, T);Run

This will not:

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Good(u32, u32, u32);Run

## E0075

The `#[simd]` attribute can only be applied to non empty tuple structs, because it doesn't make sense to try to use SIMD operations when there are no values to operate on.

This will cause an error:

ⓘThis example deliberately fails to compile

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Bad;Run

This will not:

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Good(u32);Run

## E0076

When using the `#[simd]` attribute to automatically use SIMD operations in tuple struct, the types in the struct must all be of the same type, or the compiler will trigger this error.

This will cause an error:

ⓘThis example deliberately fails to compile

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Bad(u16, u32, u32);Run

This will not:

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Good(u32, u32, u32);Run

## E0077

When using the `#[simd]` attribute on a tuple struct, the elements in the tuple must be machine types so SIMD operations can be applied to them.

This will cause an error:

ⓘThis example deliberately fails to compile

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Bad(String);Run

This will not:

    #![feature(repr_simd)]

    #[repr(simd)]
    struct Good(u32, u32, u32);Run

## E0080

This error indicates that the compiler was unable to sensibly evaluate an constant expression that had to be evaluated. Attempting to divide by 0 or causing integer overflow are two ways to induce this error. For example:

ⓘThis example deliberately fails to compile

    enum Enum {
        X = (1 << 500),
        Y = (1 / 0)
    }Run

Ensure that the expressions given can be evaluated as the desired integer type. See the FFI section of the Reference for more information about using a custom integer type:

https://doc.rust-lang.org/reference.html#ffi-attributes

## E0081

Enum discriminants are used to differentiate enum variants stored in memory. This error indicates that the same value was used for two or more variants, making them impossible to tell apart.

ⓘThis example deliberately fails to compile

    // Bad.
    enum Enum {
        P = 3,
        X = 3,
        Y = 5,
    }Run

    // Good.
    enum Enum {
        P,
        X = 3,
        Y = 5,
    }Run

Note that variants without a manually specified discriminant are numbered from top to bottom starting from 0, so clashes can occur with seemingly unrelated variants.

ⓘThis example deliberately fails to compile

    enum Bad {
        X,
        Y = 0
    }Run

Here `X` will have already been specified the discriminant 0 by the time `Y` is encountered, so a conflict occurs.

## E0084

An unsupported representation was attempted on a zero-variant enum.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(i32)]
    enum NightsWatch {} // error: unsupported representation for zero-variant enumRun

It is impossible to define an integer type to be used to represent zero-variant enum values because there are no zero-variant enum values. There is no way to construct an instance of the following type using only safe code. So you have two solutions. Either you add variants in your enum:

    #[repr(i32)]
    enum NightsWatch {
        JonSnow,
        Commander,
    }Run

or you remove the integer represention of your enum:

    enum NightsWatch {}Run

## E0087

#### Note: this error code is no longer emitted by the compiler.

Too many type arguments were supplied for a function. For example:

ⓘThis example deliberately fails to compile

    fn foo<T>() {}

    fn main() {
        foo::<f64, bool>(); // error: wrong number of type arguments:
                            //        expected 1, found 2
    }Run

The number of supplied arguments must exactly match the number of defined type parameters.

## E0088

#### Note: this error code is no longer emitted by the compiler.

You gave too many lifetime arguments. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn f() {}

    fn main() {
        f::<'static>() // error: wrong number of lifetime arguments:
                       //        expected 0, found 1
    }Run

Please check you give the right number of lifetime arguments. Example:

    fn f() {}

    fn main() {
        f() // ok!
    }Run

It's also important to note that the Rust compiler can generally determine the lifetime by itself. Example:

    struct Foo {
        value: String
    }

    impl Foo {
        // it can be written like this
        fn get_value<'a>(&'a self) -> &'a str { &self.value }
        // but the compiler works fine with this too:
        fn without_lifetime(&self) -> &str { &self.value }
    }

    fn main() {
        let f = Foo { value: "hello".to_owned() };

        println!("{}", f.get_value());
        println!("{}", f.without_lifetime());
    }Run

## E0089

#### Note: this error code is no longer emitted by the compiler.

Too few type arguments were supplied for a function. For example:

ⓘThis example deliberately fails to compile

    fn foo<T, U>() {}

    fn main() {
        foo::<f64>(); // error: wrong number of type arguments: expected 2, found 1
    }Run

Note that if a function takes multiple type arguments but you want the compiler to infer some of them, you can use type placeholders:

ⓘThis example deliberately fails to compile

    fn foo<T, U>(x: T) {}

    fn main() {
        let x: bool = true;
        foo::<f64>(x);    // error: wrong number of type arguments:
                          //        expected 2, found 1
        foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
    }Run

## E0090

#### Note: this error code is no longer emitted by the compiler.

You gave too few lifetime arguments. Example:

ⓘThis example deliberately fails to compile

    fn foo<'a: 'b, 'b: 'a>() {}

    fn main() {
        foo::<'static>(); // error: wrong number of lifetime arguments:
                          //        expected 2, found 1
    }Run

Please check you give the right number of lifetime arguments. Example:

    fn foo<'a: 'b, 'b: 'a>() {}

    fn main() {
        foo::<'static, 'static>();
    }Run

## E0091

You gave an unnecessary type or const parameter in a type alias. Erroneous code example:

ⓘThis example deliberately fails to compile

    type Foo<T> = u32; // error: type parameter `T` is unused
    // or:
    type Foo<A,B> = Box<A>; // error: type parameter `B` is unusedRun

Please check you didn't write too many parameters. Example:

    type Foo = u32; // ok!
    type Foo2<A> = Box<A>; // ok!Run

## E0092

You tried to declare an undefined atomic operation function. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn atomic_foo(); // error: unrecognized atomic operation
                         //        function
    }Run

Please check you didn't make a mistake in the function's name. All intrinsic functions are defined in librustc_codegen_llvm/intrinsic.rs and in libcore/intrinsics.rs in the Rust source code. Example:

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn atomic_fence(); // ok!
    }Run

## E0093

You declared an unknown intrinsic function. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn foo(); // error: unrecognized intrinsic function: `foo`
    }

    fn main() {
        unsafe {
            foo();
        }
    }Run

Please check you didn't make a mistake in the function's name. All intrinsic functions are defined in librustc_codegen_llvm/intrinsic.rs and in libcore/intrinsics.rs in the Rust source code. Example:

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn atomic_fence(); // ok!
    }

    fn main() {
        unsafe {
            atomic_fence();
        }
    }Run

## E0094

You gave an invalid number of type parameters to an intrinsic function. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
                                     //        of type parameters
    }Run

Please check that you provided the right number of type parameters and verify with the function declaration in the Rust source code. Example:

    #![feature(intrinsics)]

    extern "rust-intrinsic" {
        fn size_of<T>() -> usize; // ok!
    }Run

## E0106

This error indicates that a lifetime is missing from a type. If it is an error inside a function signature, the problem may be with failing to adhere to the lifetime elision rules (see below).

Here are some simple examples of where you'll run into this error:

ⓘThis example deliberately fails to compile

    struct Foo1 { x: &bool }
                  // ^ expected lifetime parameter
    struct Foo2<'a> { x: &'a bool } // correct

    struct Bar1 { x: Foo2 }
                  // ^^^^ expected lifetime parameter
    struct Bar2<'a> { x: Foo2<'a> } // correct

    enum Baz1 { A(u8), B(&bool), }
                      // ^ expected lifetime parameter
    enum Baz2<'a> { A(u8), B(&'a bool), } // correct

    type MyStr1 = &str;
               // ^ expected lifetime parameter
    type MyStr2<'a> = &'a str; // correctRun

Lifetime elision is a special, limited kind of inference for lifetimes in function signatures which allows you to leave out lifetimes in certain cases. For more background on lifetime elision see the book.

The lifetime elision rules require that any function signature with an elided output lifetime must either have

- exactly one input lifetime
- or, multiple input lifetimes, but the function must also be a method with a `&self` or `&mut self` receiver

In the first case, the output lifetime is inferred to be the same as the unique input lifetime. In the second case, the lifetime is instead inferred to be the same as the lifetime on `&self` or `&mut self`.

Here are some examples of elision errors:

ⓘThis example deliberately fails to compile

    // error, no input lifetimes
    fn foo() -> &str { }

    // error, `x` and `y` have distinct lifetimes inferred
    fn bar(x: &str, y: &str) -> &str { }

    // error, `y`'s lifetime is inferred to be distinct from `x`'s
    fn baz<'a>(x: &'a str, y: &str) -> &str { }Run

## E0107

This error means that an incorrect number of generic arguments were provided:

ⓘThis example deliberately fails to compile

    struct Foo<T> { x: T }

    struct Bar { x: Foo }             // error: wrong number of type arguments:
                                      //        expected 1, found 0
    struct Baz<S, T> { x: Foo<S, T> } // error: wrong number of type arguments:
                                      //        expected 1, found 2

    fn foo<T, U>(x: T, y: U) {}

    fn main() {
        let x: bool = true;
        foo::<bool>(x);                 // error: wrong number of type arguments:
                                        //        expected 2, found 1
        foo::<bool, i32, i32>(x, 2, 4); // error: wrong number of type arguments:
                                        //        expected 2, found 3
    }

    fn f() {}

    fn main() {
        f::<'static>(); // error: wrong number of lifetime arguments:
                        //        expected 0, found 1
    }Run

## E0109

You tried to provide a generic argument to a type which doesn't need it. Erroneous code example:

ⓘThis example deliberately fails to compile

    type X = u32<i32>; // error: type arguments are not allowed for this type
    type Y = bool<'static>; // error: lifetime parameters are not allowed on
                            //        this typeRun

Check that you used the correct argument and that the definition is correct.

Example:

    type X = u32; // ok!
    type Y = bool; // ok!Run

Note that generic arguments for enum variant constructors go after the variant, not after the enum. For example, you would write `Option::None::<u32>`, rather than `Option::<u32>::None`.

## E0110

#### Note: this error code is no longer emitted by the compiler.

You tried to provide a lifetime to a type which doesn't need it. See `E0109` for more details.

## E0116

You can only define an inherent implementation for a type in the same crate where the type was defined. For example, an `impl` block as below is not allowed since `Vec` is defined in the standard library:

ⓘThis example deliberately fails to compile

    impl Vec<u8> { } // errorRun

To fix this problem, you can do either of these things:

- define a trait that has the desired associated functions/types/constants and implement the trait for the type in question
- define a new type wrapping the type and define an implementation on the new type

Note that using the `type` keyword does not work here because `type` only introduces a type alias:

ⓘThis example deliberately fails to compile

    type Bytes = Vec<u8>;

    impl Bytes { } // error, same as aboveRun

## E0117

This error indicates a violation of one of Rust's orphan rules for trait implementations. The rule prohibits any implementation of a foreign trait (a trait defined in another crate) where

- the type that is implementing the trait is foreign
- all of the parameters being passed to the trait (if there are any) are also foreign.

Here's one example of this error:

ⓘThis example deliberately fails to compile

    impl Drop for u32 {}Run

To avoid this kind of error, ensure that at least one local type is referenced by the `impl`:

    pub struct Foo; // you define your type in your crate

    impl Drop for Foo { // and you can implement the trait on it!
        // code of trait implementation here
    }

    impl From<Foo> for i32 { // or you use a type from your crate as
                             // a type parameter
        fn from(i: Foo) -> i32 {
            0
        }
    }Run

Alternatively, define a trait locally and implement that instead:

    trait Bar {
        fn get(&self) -> usize;
    }

    impl Bar for u32 {
        fn get(&self) -> usize { 0 }
    }Run

For information on the design of the orphan rules, see RFC 1023.

## E0118

You're trying to write an inherent implementation for something which isn't a struct nor an enum. Erroneous code example:

ⓘThis example deliberately fails to compile

    impl (u8, u8) { // error: no base type found for inherent implementation
        fn get_state(&self) -> String {
            // ...
        }
    }Run

To fix this error, please implement a trait on the type or wrap it in a struct. Example:

    // we create a trait here
    trait LiveLongAndProsper {
        fn get_state(&self) -> String;
    }

    // and now you can implement it on (u8, u8)
    impl LiveLongAndProsper for (u8, u8) {
        fn get_state(&self) -> String {
            "He's dead, Jim!".to_owned()
        }
    }Run

Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct. For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`. Example:

    struct TypeWrapper((u8, u8));

    impl TypeWrapper {
        fn get_state(&self) -> String {
            "Fascinating!".to_owned()
        }
    }Run

## E0119

There are conflicting trait implementations for the same type. Example of erroneous code:

ⓘThis example deliberately fails to compile

    trait MyTrait {
        fn get(&self) -> usize;
    }

    impl<T> MyTrait for T {
        fn get(&self) -> usize { 0 }
    }

    struct Foo {
        value: usize
    }

    impl MyTrait for Foo { // error: conflicting implementations of trait
                           //        `MyTrait` for type `Foo`
        fn get(&self) -> usize { self.value }
    }Run

When looking for the implementation for the trait, the compiler finds both the `impl<T> MyTrait for T` where T is all types and the `impl MyTrait for Foo`. Since a trait cannot be implemented multiple times, this is an error. So, when you write:

    trait MyTrait {
        fn get(&self) -> usize;
    }

    impl<T> MyTrait for T {
        fn get(&self) -> usize { 0 }
    }Run

This makes the trait implemented on all types in the scope. So if you try to implement it on another one after that, the implementations will conflict. Example:

    trait MyTrait {
        fn get(&self) -> usize;
    }

    impl<T> MyTrait for T {
        fn get(&self) -> usize { 0 }
    }

    struct Foo;

    fn main() {
        let f = Foo;

        f.get(); // the trait is implemented so we can use it
    }Run

## E0120

An attempt was made to implement Drop on a trait, which is not allowed: only structs and enums can implement Drop. An example causing this error:

ⓘThis example deliberately fails to compile

    trait MyTrait {}

    impl Drop for MyTrait {
        fn drop(&mut self) {}
    }Run

A workaround for this problem is to wrap the trait up in a struct, and implement Drop on that. An example is shown below:

    trait MyTrait {}
    struct MyWrapper<T: MyTrait> { foo: T }

    impl <T: MyTrait> Drop for MyWrapper<T> {
        fn drop(&mut self) {}
    }
    Run

Alternatively, wrapping trait objects requires something like the following:

    trait MyTrait {}

    //or Box<MyTrait>, if you wanted an owned trait object
    struct MyWrapper<'a> { foo: &'a MyTrait }

    impl <'a> Drop for MyWrapper<'a> {
        fn drop(&mut self) {}
    }Run

## E0121

In order to be consistent with Rust's lack of global type inference, type and const placeholders are disallowed by design in item signatures.

Examples of this error include:

ⓘThis example deliberately fails to compile

    fn foo() -> _ { 5 } // error, explicitly write out the return type instead

    static BAR: _ = "test"; // error, explicitly write out the type insteadRun

## E0124

You declared two fields of a struct with the same name. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        field1: i32,
        field1: i32, // error: field is already declared
    }Run

Please verify that the field names have been correctly spelled. Example:

    struct Foo {
        field1: i32,
        field2: i32, // ok!
    }Run

## E0128

Type parameter defaults can only use parameters that occur before them. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo<T=U, U=()> {
        field1: T,
        filed2: U,
    }
    // error: type parameters with a default cannot use forward declared
    // identifiersRun

Since type parameters are evaluated in-order, you may be able to fix this issue by doing:

    struct Foo<U=(), T=U> {
        field1: T,
        filed2: U,
    }Run

Please also verify that this wasn't because of a name-clash and rename the type parameter if so.

## E0130

You declared a pattern as an argument in a foreign function declaration. Erroneous code example:

ⓘThis example deliberately fails to compile

    extern {
        fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
                                    //        function declarations
    }Run

Please replace the pattern argument with a regular one. Example:

    struct SomeStruct {
        a: u32,
        b: u32,
    }

    extern {
        fn foo(s: SomeStruct); // ok!
    }Run

Or:

    extern {
        fn foo(a: (u32, u32)); // ok!
    }Run

## E0131

It is not possible to define `main` with generic parameters. When `main` is present, it must take no arguments and return `()`. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main<T>() { // error: main function is not allowed to have generic parameters
    }Run

## E0132

A function with the `start` attribute was declared with type parameters.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(start)]

    #[start]
    fn f<T>() {}Run

It is not possible to declare type parameters on a function that has the `start` attribute. Such a function must have the following type signature (for more information, view the unstable book):

    fn(isize, *const *const u8) -> isize;Run

Example:

    #![feature(start)]

    #[start]
    fn my_start(argc: isize, argv: *const *const u8) -> isize {
        0
    }Run

## E0133

Unsafe code was used outside of an unsafe function or block.

Erroneous code example:

ⓘThis example deliberately fails to compile

    unsafe fn f() { return; } // This is the unsafe code

    fn main() {
        f(); // error: call to unsafe function requires unsafe function or block
    }Run

Using unsafe functionality is potentially dangerous and disallowed by safety checks. Examples:

- Dereferencing raw pointers
- Calling functions via FFI
- Calling functions marked unsafe

These safety checks can be relaxed for a section of the code by wrapping the unsafe instructions with an `unsafe` block. For instance:

    unsafe fn f() { return; }

    fn main() {
        unsafe { f(); } // ok!
    }Run

See also https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html

## E0136

A binary can only have one entry point, and by default that entry point is the function `main()`. If there are multiple such functions, please rename one.

## E0137

More than one function was declared with the `#[main]` attribute.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(main)]

    #[main]
    fn foo() {}

    #[main]
    fn f() {} // error: multiple functions with a #[main] attributeRun

This error indicates that the compiler found multiple functions with the `#[main]` attribute. This is an error because there must be a unique entry point into a Rust program. Example:

    #![feature(main)]

    #[main]
    fn f() {} // ok!Run

## E0138

More than one function was declared with the `#[start]` attribute.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(start)]

    #[start]
    fn foo(argc: isize, argv: *const *const u8) -> isize {}

    #[start]
    fn f(argc: isize, argv: *const *const u8) -> isize {}
    // error: multiple 'start' functionsRun

This error indicates that the compiler found multiple functions with the `#[start]` attribute. This is an error because there must be a unique entry point into a Rust program. Example:

    #![feature(start)]

    #[start]
    fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!Run

## E0139

#### Note: this error code is no longer emitted by the compiler.

There are various restrictions on transmuting between types in Rust; for example types being transmuted must have the same size. To apply all these restrictions, the compiler must know the exact types that may be transmuted. When type parameters are involved, this cannot always be done.

So, for example, the following is not allowed:

    use std::mem::transmute;

    struct Foo<T>(Vec<T>);

    fn foo<T>(x: Vec<T>) {
        // we are transmuting between Vec<T> and Foo<F> here
        let y: Foo<T> = unsafe { transmute(x) };
        // do something with y
    }Run

In this specific case there's a good chance that the transmute is harmless (but this is not guaranteed by Rust). However, when alignment and enum optimizations come into the picture, it's quite likely that the sizes may or may not match with different type parameter substitutions. It's not possible to check this for all possible types, so `transmute()` simply only accepts types without any unsubstituted type parameters.

If you need this, there's a good chance you're doing something wrong. Keep in mind that Rust doesn't guarantee much about the layout of different structs (even two structs with identical declarations may have different layouts). If there is a solution that avoids the transmute entirely, try it instead.

If it's possible, hand-monomorphize the code by writing the function for each possible type substitution. It's possible to use traits to do this cleanly, for example:

    use std::mem::transmute;

    struct Foo<T>(Vec<T>);

    trait MyTransmutableType: Sized {
        fn transmute(_: Vec<Self>) -> Foo<Self>;
    }

    impl MyTransmutableType for u8 {
        fn transmute(x: Vec<u8>) -> Foo<u8> {
            unsafe { transmute(x) }
        }
    }

    impl MyTransmutableType for String {
        fn transmute(x: Vec<String>) -> Foo<String> {
            unsafe { transmute(x) }
        }
    }

    // ... more impls for the types you intend to transmute

    fn foo<T: MyTransmutableType>(x: Vec<T>) {
        let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
        // do something with y
    }Run

Each impl will be checked for a size match in the transmute as usual, and since there are no unbound type parameters involved, this should compile unless there is a size mismatch in one of the impls.

It is also possible to manually transmute:

    unsafe {
        ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
    }Run

Note that this does not move `v` (unlike `transmute`), and may need a call to `mem::forget(v)` in case you want to avoid destructors being called.

## E0152

A lang item was redefined.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(lang_items)]

    #[lang = "arc"]
    struct Foo; // error: duplicate lang item found: `arc`Run

Lang items are already implemented in the standard library. Unless you are writing a free-standing application (e.g., a kernel), you do not need to provide them yourself.

You can build a free-standing crate by adding `#![no_std]` to the crate attributes:

ⓘThis example is not tested

    #![no_std]Run

See also the unstable book.

## E0154

#### Note: this error code is no longer emitted by the compiler.

Imports (`use` statements) are not allowed after non-item statements, such as variable declarations and expression statements.

Here is an example that demonstrates the error:

    fn f() {
        // Variable declaration before import
        let x = 0;
        use std::io::Read;
        // ...
    }Run

The solution is to declare the imports at the top of the block, function, or file.

Here is the previous example again, with the correct order:

    fn f() {
        use std::io::Read;
        let x = 0;
        // ...
    }Run

See the Declaration Statements section of the reference for more information about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

## E0158

`const` and `static` mean different things. A `const` is a compile-time constant, an alias for a literal value. This property means you can match it directly within a pattern.

The `static` keyword, on the other hand, guarantees a fixed location in memory. This does not always mean that the value is constant. For example, a global mutex can be declared `static` as well.

If you want to match against a `static`, consider using a guard instead:

    static FORTY_TWO: i32 = 42;

    match Some(42) {
        Some(x) if x == FORTY_TWO => {}
        _ => {}
    }Run

## E0161

A value was moved. However, its size was not known at compile time, and only values of a known size can be moved.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(box_syntax)]

    fn main() {
        let array: &[isize] = &[1, 2, 3];
        let _x: Box<[isize]> = box *array;
        // error: cannot move a value of type [isize]: the size of [isize] cannot
        //        be statically determined
    }Run

In Rust, you can only move a value when its size is known at compile time.

To work around this restriction, consider "hiding" the value behind a reference: either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move it around as usual. Example:

    #![feature(box_syntax)]

    fn main() {
        let array: &[isize] = &[1, 2, 3];
        let _x: Box<&[isize]> = box array; // ok!
    }Run

## E0162

#### Note: this error code is no longer emitted by the compiler.

An if-let pattern attempts to match the pattern, and enters the body if the match was successful. If the match is irrefutable (when it cannot fail to match), use a regular `let`-binding instead. For instance:

    struct Irrefutable(i32);
    let irr = Irrefutable(0);

    // This fails to compile because the match is irrefutable.
    if let Irrefutable(x) = irr {
        // This body will always be executed.
        // ...
    }

Try this instead:

    struct Irrefutable(i32);
    let irr = Irrefutable(0);

    let Irrefutable(x) = irr;
    println!("{}", x);Run

## E0164

This error means that an attempt was made to match a struct type enum variant as a non-struct type:

ⓘThis example deliberately fails to compile

    enum Foo { B { i: u32 } }

    fn bar(foo: Foo) -> u32 {
        match foo {
            Foo::B(i) => i, // error E0164
        }
    }Run

Try using `{}` instead:

    enum Foo { B { i: u32 } }

    fn bar(foo: Foo) -> u32 {
        match foo {
            Foo::B{i} => i,
        }
    }Run

## E0165

#### Note: this error code is no longer emitted by the compiler.

A while-let pattern attempts to match the pattern, and enters the body if the match was successful. If the match is irrefutable (when it cannot fail to match), use a regular `let`-binding inside a `loop` instead. For instance:

    struct Irrefutable(i32);
    let irr = Irrefutable(0);

    // This fails to compile because the match is irrefutable.
    while let Irrefutable(x) = irr {
        // ...
    }

Try this instead:

    struct Irrefutable(i32);
    let irr = Irrefutable(0);

    loop {
        let Irrefutable(x) = irr;
        // ...
    }Run

## E0170

Enum variants are qualified by default. For example, given this type:

    enum Method {
        GET,
        POST,
    }Run

You would match it using:

    enum Method {
        GET,
        POST,
    }

    let m = Method::GET;

    match m {
        Method::GET => {},
        Method::POST => {},
    }Run

If you don't qualify the names, the code will bind new variables named "GET" and "POST" instead. This behavior is likely not what you want, so `rustc` warns when that happens.

Qualified names are good practice, and most code works well with them. But if you prefer them unqualified, you can import the variants into scope:

    use Method::*;
    enum Method { GET, POST }Run

If you want others to be able to import variants from your module directly, use `pub use`:

    pub use Method::*;
    pub enum Method { GET, POST }Run

## E0178

In types, the `+` type operator has low precedence, so it is often necessary to use parentheses.

For example:

ⓘThis example deliberately fails to compile

    trait Foo {}

    struct Bar<'a> {
        w: &'a Foo + Copy,   // error, use &'a (Foo + Copy)
        x: &'a Foo + 'a,     // error, use &'a (Foo + 'a)
        y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
        z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
    }Run

More details can be found in RFC 438.

## E0183

No description.

## E0184

Explicitly implementing both Drop and Copy for a type is currently disallowed. This feature can make some sense in theory, but the current implementation is incorrect and can lead to memory unsafety (see issue #20126), so it has been disabled for now.

## E0185

An associated function for a trait was defined to be static, but an implementation of the trait declared the same function to be a method (i.e., to take a `self` parameter).

Here's an example of this error:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo();
    }

    struct Bar;

    impl Foo for Bar {
        // error, method `foo` has a `&self` declaration in the impl, but not in
        // the trait
        fn foo(&self) {}
    }Run

## E0186

An associated function for a trait was defined to be a method (i.e., to take a `self` parameter), but an implementation of the trait declared the same function to be static.

Here's an example of this error:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo(&self);
    }

    struct Bar;

    impl Foo for Bar {
        // error, method `foo` has a `&self` declaration in the trait, but not in
        // the impl
        fn foo() {}
    }Run

## E0191

Trait objects need to have all associated types specified. Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Trait {
        type Bar;
    }

    type Foo = Trait; // error: the value of the associated type `Bar` (from
                      //        the trait `Trait`) must be specifiedRun

Please verify you specified all associated types of the trait and that you used the right trait. Example:

    trait Trait {
        type Bar;
    }

    type Foo = Trait<Bar=i32>; // ok!Run

## E0192

Negative impls are only allowed for auto traits. For more information see the opt-in builtin traits RFC.

## E0193

#### Note: this error code is no longer emitted by the compiler.

`where` clauses must use generic type parameters: it does not make sense to use them otherwise. An example causing this error:

    trait Foo {
        fn bar(&self);
    }

    #[derive(Copy,Clone)]
    struct Wrapper<T> {
        Wrapped: T
    }

    impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
        fn bar(&self) { }
    }Run

This use of a `where` clause is strange - a more common usage would look something like the following:

    trait Foo {
        fn bar(&self);
    }

    #[derive(Copy,Clone)]
    struct Wrapper<T> {
        Wrapped: T
    }
    impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
        fn bar(&self) { }
    }Run

Here, we're saying that the implementation exists on Wrapper only when the wrapped type `T` implements `Clone`. The `where` clause is important because some types will not implement `Clone`, and thus will not get this method.

In our erroneous example, however, we're referencing a single concrete type. Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no reason to also specify it in a `where` clause.

## E0194

A type parameter was declared which shadows an existing one. An example of this error:

ⓘThis example deliberately fails to compile

    trait Foo<T> {
        fn do_something(&self) -> T;
        fn do_something_else<T: Clone>(&self, bar: T);
    }Run

In this example, the trait `Foo` and the trait method `do_something_else` both define a type parameter `T`. This is not allowed: if the method wishes to define a type parameter, it must use a different name for it.

## E0195

Your method's lifetime parameters do not match the trait declaration. Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Trait {
        fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
    }

    struct Foo;

    impl Trait for Foo {
        fn bar<'a,'b>(x: &'a str, y: &'b str) {
        // error: lifetime parameters or bounds on method `bar`
        // do not match the trait declaration
        }
    }Run

The lifetime constraint `'b` for bar() implementation does not match the trait declaration. Ensure lifetime declarations match exactly in both trait declaration and implementation. Example:

    trait Trait {
        fn t<'a,'b:'a>(x: &'a str, y: &'b str);
    }

    struct Foo;

    impl Trait for Foo {
        fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
        }
    }Run

## E0197

Inherent implementations (one that do not implement a trait but provide methods associated with a type) are always safe because they are not implementing an unsafe trait. Removing the `unsafe` keyword from the inherent implementation will resolve this error.

ⓘThis example deliberately fails to compile

    struct Foo;

    // this will cause this error
    unsafe impl Foo { }
    // converting it to this will fix it
    impl Foo { }Run

## E0198

A negative implementation is one that excludes a type from implementing a particular trait. Not being able to use a trait is always a safe operation, so negative implementations are always safe and never need to be marked as unsafe.

ⓘThis example deliberately fails to compile

    #![feature(optin_builtin_traits)]

    struct Foo;

    // unsafe is unnecessary
    unsafe impl !Clone for Foo { }Run

This will compile:

ⓘThis example is not tested

    #![feature(optin_builtin_traits)]

    struct Foo;

    auto trait Enterprise {}

    impl !Enterprise for Foo { }Run

Please note that negative impls are only allowed for auto traits.

## E0199

Safe traits should not have unsafe implementations, therefore marking an implementation for a safe trait unsafe will cause a compiler error. Removing the unsafe marker on the trait noted in the error will resolve this problem.

ⓘThis example deliberately fails to compile

    struct Foo;

    trait Bar { }

    // this won't compile because Bar is safe
    unsafe impl Bar for Foo { }
    // this will compile
    impl Bar for Foo { }Run

## E0200

Unsafe traits must have unsafe implementations. This error occurs when an implementation for an unsafe trait isn't marked as unsafe. This may be resolved by marking the unsafe implementation as unsafe.

ⓘThis example deliberately fails to compile

    struct Foo;

    unsafe trait Bar { }

    // this won't compile because Bar is unsafe and impl isn't unsafe
    impl Bar for Foo { }
    // this will compile
    unsafe impl Bar for Foo { }Run

## E0201

It is an error to define two associated items (like methods, associated types, associated functions, etc.) with the same identifier.

For example:

ⓘThis example deliberately fails to compile

    struct Foo(u8);

    impl Foo {
        fn bar(&self) -> bool { self.0 > 5 }
        fn bar() {} // error: duplicate associated function
    }

    trait Baz {
        type Quux;
        fn baz(&self) -> bool;
    }

    impl Baz for Foo {
        type Quux = u32;

        fn baz(&self) -> bool { true }

        // error: duplicate method
        fn baz(&self) -> bool { self.0 > 5 }

        // error: duplicate associated type
        type Quux = u32;
    }Run

Note, however, that items with the same name are allowed for inherent `impl` blocks that don't overlap:

    struct Foo<T>(T);

    impl Foo<u8> {
        fn bar(&self) -> bool { self.0 > 5 }
    }

    impl Foo<bool> {
        fn bar(&self) -> bool { self.0 }
    }Run

## E0202

Inherent associated types were part of RFC 195 but are not yet implemented. See the tracking issue for the status of this implementation.

## E0203

No description.

## E0204

An attempt to implement the `Copy` trait for a struct failed because one of the fields does not implement `Copy`. To fix this, you must implement `Copy` for the mentioned field. Note that this may not be possible, as in the example of

ⓘThis example deliberately fails to compile

    struct Foo {
        foo : Vec<u32>,
    }

    impl Copy for Foo { }Run

This fails because `Vec<T>` does not implement `Copy` for any `T`.

Here's another example that will fail:

ⓘThis example deliberately fails to compile

    #[derive(Copy)]
    struct Foo<'a> {
        ty: &'a mut bool,
    }Run

This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this differs from the behavior for `&T`, which is always `Copy`).

## E0206

You can only implement `Copy` for a struct or enum. Both of the following examples will fail, because neither `[u8; 256]` nor `&'static mut Bar` (mutable reference to `Bar`) is a struct or enum:

ⓘThis example deliberately fails to compile

    type Foo = [u8; 256];
    impl Copy for Foo { } // error

    #[derive(Copy, Clone)]
    struct Bar;
    impl Copy for &'static mut Bar { } // errorRun

## E0207

Any type parameter or lifetime parameter of an `impl` must meet at least one of the following criteria:

- it appears in the implementing type of the impl, e.g. `impl<T> Foo<T>`
- for a trait impl, it appears in the implemented trait, e.g. `impl<T> SomeTrait<T> for Foo`
- it is bound as an associated type, e.g. `impl<T, U> SomeTrait for T where T: AnotherTrait<AssocType=U>`

### Error example 1

Suppose we have a struct `Foo` and we would like to define some methods for it. The following definition leads to a compiler error:

ⓘThis example deliberately fails to compile

    struct Foo;

    impl<T: Default> Foo {
    // error: the type parameter `T` is not constrained by the impl trait, self
    // type, or predicates [E0207]
        fn get(&self) -> T {
            <T as Default>::default()
        }
    }Run

The problem is that the parameter `T` does not appear in the implementing type (`Foo`) of the impl. In this case, we can fix the error by moving the type parameter from the `impl` to the method `get`:

    struct Foo;

    // Move the type parameter from the impl to the method
    impl Foo {
        fn get<T: Default>(&self) -> T {
            <T as Default>::default()
        }
    }Run

### Error example 2

As another example, suppose we have a `Maker` trait and want to establish a type `FooMaker` that makes `Foo`s:

ⓘThis example deliberately fails to compile

    trait Maker {
        type Item;
        fn make(&mut self) -> Self::Item;
    }

    struct Foo<T> {
        foo: T
    }

    struct FooMaker;

    impl<T: Default> Maker for FooMaker {
    // error: the type parameter `T` is not constrained by the impl trait, self
    // type, or predicates [E0207]
        type Item = Foo<T>;

        fn make(&mut self) -> Foo<T> {
            Foo { foo: <T as Default>::default() }
        }
    }Run

This fails to compile because `T` does not appear in the trait or in the implementing type.

One way to work around this is to introduce a phantom type parameter into `FooMaker`, like so:

    use std::marker::PhantomData;

    trait Maker {
        type Item;
        fn make(&mut self) -> Self::Item;
    }

    struct Foo<T> {
        foo: T
    }

    // Add a type parameter to `FooMaker`
    struct FooMaker<T> {
        phantom: PhantomData<T>,
    }

    impl<T: Default> Maker for FooMaker<T> {
        type Item = Foo<T>;

        fn make(&mut self) -> Foo<T> {
            Foo {
                foo: <T as Default>::default(),
            }
        }
    }Run

Another way is to do away with the associated type in `Maker` and use an input type parameter instead:

    // Use a type parameter instead of an associated type here
    trait Maker<Item> {
        fn make(&mut self) -> Item;
    }

    struct Foo<T> {
        foo: T
    }

    struct FooMaker;

    impl<T: Default> Maker<Foo<T>> for FooMaker {
        fn make(&mut self) -> Foo<T> {
            Foo { foo: <T as Default>::default() }
        }
    }Run

### Additional information

For more information, please see RFC 447.

## E0208

No description.

## E0210

This error indicates a violation of one of Rust's orphan rules for trait implementations. The rule concerns the use of type parameters in an implementation of a foreign trait (a trait defined in another crate), and states that type parameters must be "covered" by a local type. To understand what this means, it is perhaps easiest to consider a few examples.

If `ForeignTrait` is a trait defined in some external crate `foo`, then the following trait `impl` is an error:

ⓘThis example deliberately fails to compile

    extern crate foo;
    use foo::ForeignTrait;

    impl<T> ForeignTrait for T { } // errorRun

To work around this, it can be covered with a local type, `MyType`:

    struct MyType<T>(T);
    impl<T> ForeignTrait for MyType<T> { } // OkRun

Please note that a type alias is not sufficient.

For another example of an error, suppose there's another trait defined in `foo` named `ForeignTrait2` that takes two type parameters. Then this `impl` results in the same rule violation:

ⓘThis example is not tested

    struct MyType2;
    impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // errorRun

The reason for this is that there are two appearances of type parameter `T` in the `impl` header, both as parameters for `ForeignTrait2`. The first appearance is uncovered, and so runs afoul of the orphan rule.

Consider one more example:

ⓘThis example is not tested

    impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // OkRun

This only differs from the previous `impl` in that the parameters `T` and `MyType<T>` for `ForeignTrait2` have been swapped. This example does not violate the orphan rule; it is permitted.

To see why that last example was allowed, you need to understand the general rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:

ⓘThis example is not tested

    impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }Run

where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn` are types. One of the types `T0, ..., Tn` must be a local type (this is another orphan rule, see the explanation for E0117). Let `i` be the smallest integer such that `Ti` is a local type. Then no type parameter can appear in any of the `Tj` for `j < i`.

For information on the design of the orphan rules, see RFC 1023.

## E0212

No description.

## E0214

A generic type was described using parentheses rather than angle brackets. For example:

ⓘThis example deliberately fails to compile

    fn main() {
        let v: Vec(&str) = vec!["foo"];
    }Run

This is not currently supported: `v` should be defined as `Vec<&str>`. Parentheses are currently only used with generic types when defining parameters for `Fn`-family traits.

## E0220

You used an associated type which isn't defined in the trait. Erroneous code example:

ⓘThis example deliberately fails to compile

    trait T1 {
        type Bar;
    }

    type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`

    // or:

    trait T2 {
        type Bar;

        // error: Baz is used but not declared
        fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
    }Run

Make sure that you have defined the associated type in the trait body. Also, verify that you used the right trait or you didn't misspell the associated type name. Example:

    trait T1 {
        type Bar;
    }

    type Foo = T1<Bar=i32>; // ok!

    // or:

    trait T2 {
        type Bar;
        type Baz; // we declare `Baz` in our trait.

        // and now we can use it here:
        fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
    }Run

## E0221

An attempt was made to retrieve an associated type, but the type was ambiguous. For example:

ⓘThis example deliberately fails to compile

    trait T1 {}
    trait T2 {}

    trait Foo {
        type A: T1;
    }

    trait Bar : Foo {
        type A: T2;
        fn do_something() {
            let _: Self::A;
        }
    }Run

In this example, `Foo` defines an associated type `A`. `Bar` inherits that type from `Foo`, and defines another associated type of the same name. As a result, when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined by `Foo` or the one defined by `Bar`.

There are two options to work around this issue. The first is simply to rename one of the types. Alternatively, one can specify the intended type using the following syntax:

    trait T1 {}
    trait T2 {}

    trait Foo {
        type A: T1;
    }

    trait Bar : Foo {
        type A: T2;
        fn do_something() {
            let _: <Self as Bar>::A;
        }
    }Run

## E0223

An attempt was made to retrieve an associated type, but the type was ambiguous. For example:

ⓘThis example deliberately fails to compile

    trait MyTrait {type X; }

    fn main() {
        let foo: MyTrait::X;
    }Run

The problem here is that we're attempting to take the type of X from MyTrait. Unfortunately, the type of X is not defined, because it's only made concrete in implementations of the trait. A working version of this code might look like:

    trait MyTrait {type X; }
    struct MyStruct;

    impl MyTrait for MyStruct {
        type X = u32;
    }

    fn main() {
        let foo: <MyStruct as MyTrait>::X;
    }Run

This syntax specifies that we want the X type from MyTrait, as made concrete in MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct might implement two different traits with identically-named associated types. This syntax allows disambiguation between the two.

## E0224

No description.

## E0225

You attempted to use multiple types as bounds for a closure or trait object. Rust does not currently support this. A simple example that causes this error:

ⓘThis example deliberately fails to compile

    fn main() {
        let _: Box<dyn std::io::Read + std::io::Write>;
    }Run

Auto traits such as Send and Sync are an exception to this rule: It's possible to have bounds of one non-builtin trait, plus any number of auto traits. For example, the following compiles correctly:

    fn main() {
        let _: Box<dyn std::io::Read + Send + Sync>;
    }Run

## E0226

No description.

## E0227

No description.

## E0228

No description.

## E0229

An associated type binding was done outside of the type parameter declaration and `where` clause. Erroneous code example:

ⓘThis example deliberately fails to compile

    pub trait Foo {
        type A;
        fn boo(&self) -> <Self as Foo>::A;
    }

    struct Bar;

    impl Foo for isize {
        type A = usize;
        fn boo(&self) -> usize { 42 }
    }

    fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
    // error: associated type bindings are not allowed hereRun

To solve this error, please move the type bindings in the type parameter declaration:

    fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!Run

Or in the `where` clause:

    fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}Run

## E0230

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

ⓘThis example deliberately fails to compile

    #![feature(on_unimplemented)]

    fn foo<T: Index<u8>>(x: T){}

    #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
    trait Index<Idx> { /* ... */ }

    foo(true); // `bool` does not implement `Index<u8>`Run

There will be an error about `bool` not implementing `Index<u8>`, followed by a note saying "the type `bool` cannot be indexed by `u8`".

As you can see, you can specify type parameters in curly braces for substitution with the actual types (using the regular format string syntax) in a given situation. Furthermore, `{Self}` will substitute to the type (in this case, `bool`) that we tried to use.

This error appears when the curly braces contain an identifier which doesn't match with any of the type parameters or the string `Self`. This might happen if you misspelled a type parameter, or if you intended to use literal curly braces. If it is the latter, escape the curly braces with a second curly brace of the same type; e.g., a literal `{` is `{{`.

## E0231

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

ⓘThis example deliberately fails to compile

    #![feature(on_unimplemented)]

    fn foo<T: Index<u8>>(x: T){}

    #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
    trait Index<Idx> { /* ... */ }

    foo(true); // `bool` does not implement `Index<u8>`Run

there will be an error about `bool` not implementing `Index<u8>`, followed by a note saying "the type `bool` cannot be indexed by `u8`".

As you can see, you can specify type parameters in curly braces for substitution with the actual types (using the regular format string syntax) in a given situation. Furthermore, `{Self}` will substitute to the type (in this case, `bool`) that we tried to use.

This error appears when the curly braces do not contain an identifier. Please add one of the same name as a type parameter. If you intended to use literal braces, use `{{` and `}}` to escape them.

## E0232

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error message for when a particular trait isn't implemented on a type placed in a position that needs that trait. For example, when the following code is compiled:

ⓘThis example deliberately fails to compile

    #![feature(on_unimplemented)]

    fn foo<T: Index<u8>>(x: T){}

    #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
    trait Index<Idx> { /* ... */ }

    foo(true); // `bool` does not implement `Index<u8>`Run

there will be an error about `bool` not implementing `Index<u8>`, followed by a note saying "the type `bool` cannot be indexed by `u8`".

For this to work, some note must be specified. An empty attribute will not do anything, please remove the attribute or add some helpful note for users of the trait.

## E0243

#### Note: this error code is no longer emitted by the compiler.

This error indicates that not enough type parameters were found in a type or trait.

For example, the `Foo` struct below is defined to be generic in `T`, but the type parameter is missing in the definition of `Bar`:

ⓘThis example deliberately fails to compile

    struct Foo<T> { x: T }

    struct Bar { x: Foo }Run

## E0244

#### Note: this error code is no longer emitted by the compiler.

This error indicates that too many type parameters were found in a type or trait.

For example, the `Foo` struct below has no type parameters, but is supplied with two in the definition of `Bar`:

ⓘThis example deliberately fails to compile

    struct Foo { x: bool }

    struct Bar<S, T> { x: Foo<S, T> }Run

## E0251

#### Note: this error code is no longer emitted by the compiler.

Two items of the same name cannot be imported without rebinding one of the items under a new local name.

An example of this error:

    use foo::baz;
    use bar::*; // error, do `use foo::baz as quux` instead on the previous line

    fn main() {}

    mod foo {
        pub struct baz;
    }

    mod bar {
        pub mod baz {}
    }Run

## E0252

Two items of the same name cannot be imported without rebinding one of the items under a new local name.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use foo::baz;
    use bar::baz; // error, do `use bar::baz as quux` instead

    fn main() {}

    mod foo {
        pub struct baz;
    }

    mod bar {
        pub mod baz {}
    }Run

You can use aliases in order to fix this error. Example:

    use foo::baz as foo_baz;
    use bar::baz; // ok!

    fn main() {}

    mod foo {
        pub struct baz;
    }

    mod bar {
        pub mod baz {}
    }Run

Or you can reference the item with its parent:

    use bar::baz;

    fn main() {
        let x = foo::baz; // ok!
    }

    mod foo {
        pub struct baz;
    }

    mod bar {
        pub mod baz {}
    }Run

## E0253

Attempt was made to import an unimportable value. This can happen when trying to import a method from a trait.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod foo {
        pub trait MyTrait {
            fn do_something();
        }
    }

    use foo::MyTrait::do_something;
    // error: `do_something` is not directly importable

    fn main() {}Run

It's invalid to directly import methods belonging to a trait or concrete type.

## E0254

Attempt was made to import an item whereas an extern crate with this name has already been imported.

Erroneous code example:

ⓘThis example deliberately fails to compile

    extern crate core;

    mod foo {
        pub trait core {
            fn do_something();
        }
    }

    use foo::core;  // error: an extern crate named `core` has already
                    //        been imported in this module

    fn main() {}Run

To fix this issue, you have to rename at least one of the two imports. Example:

    extern crate core as libcore; // ok!

    mod foo {
        pub trait core {
            fn do_something();
        }
    }

    use foo::core;

    fn main() {}Run

## E0255

You can't import a value whose name is the same as another value defined in the module.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use bar::foo; // error: an item named `foo` is already in scope

    fn foo() {}

    mod bar {
         pub fn foo() {}
    }

    fn main() {}Run

You can use aliases in order to fix this error. Example:

    use bar::foo as bar_foo; // ok!

    fn foo() {}

    mod bar {
         pub fn foo() {}
    }

    fn main() {}Run

Or you can reference the item with its parent:

    fn foo() {}

    mod bar {
         pub fn foo() {}
    }

    fn main() {
        bar::foo(); // we get the item by referring to its parent
    }Run

## E0256

#### Note: this error code is no longer emitted by the compiler.

You can't import a type or module when the name of the item being imported is the same as another type or submodule defined in the module.

An example of this error:

ⓘThis example deliberately fails to compile

    use foo::Bar; // error

    type Bar = u32;

    mod foo {
        pub mod Bar { }
    }

    fn main() {}Run

## E0259

The name chosen for an external crate conflicts with another external crate that has been imported into the current module.

Erroneous code example:

ⓘThis example deliberately fails to compile

    extern crate core;
    extern crate std as core;

    fn main() {}Run

The solution is to choose a different name that doesn't conflict with any external crate imported into the current module.

Correct example:

    extern crate core;
    extern crate std as other_name;

    fn main() {}Run

## E0260

The name for an item declaration conflicts with an external crate's name.

Erroneous code example:

ⓘThis example deliberately fails to compile

    extern crate core;

    struct core;

    fn main() {}Run

There are two possible solutions:

Solution #1: Rename the item.

    extern crate core;

    struct xyz;Run

Solution #2: Import the crate with a different name.

    extern crate core as xyz;

    struct abc;Run

See the Declaration Statements section of the reference for more information about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

## E0261

When using a lifetime like `'a` in a type, it must be declared before being used.

These two examples illustrate the problem:

ⓘThis example deliberately fails to compile

    // error, use of undeclared lifetime name `'a`
    fn foo(x: &'a str) { }

    struct Foo {
        // error, use of undeclared lifetime name `'a`
        x: &'a str,
    }Run

These can be fixed by declaring lifetime parameters:

    struct Foo<'a> {
        x: &'a str,
    }

    fn foo<'a>(x: &'a str) {}Run

Impl blocks declare lifetime parameters separately. You need to add lifetime parameters to an impl block if you're implementing a type that has a lifetime parameter of its own. For example:

ⓘThis example deliberately fails to compile

    struct Foo<'a> {
        x: &'a str,
    }

    // error,  use of undeclared lifetime name `'a`
    impl Foo<'a> {
        fn foo<'a>(x: &'a str) {}
    }Run

This is fixed by declaring the impl block like this:

    struct Foo<'a> {
        x: &'a str,
    }

    // correct
    impl<'a> Foo<'a> {
        fn foo(x: &'a str) {}
    }Run

## E0262

Declaring certain lifetime names in parameters is disallowed. For example, because the `'static` lifetime is a special built-in lifetime name denoting the lifetime of the entire program, this is an error:

ⓘThis example deliberately fails to compile

    // error, invalid lifetime parameter name `'static`
    fn foo<'static>(x: &'static str) { }Run

## E0263

A lifetime name cannot be declared more than once in the same scope. For example:

ⓘThis example deliberately fails to compile

    // error, lifetime name `'a` declared twice in the same scope
    fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }Run

## E0264

An unknown external lang item was used. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(lang_items)]

    extern "C" {
        #[lang = "cake"] // error: unknown external lang item: `cake`
        fn cake();
    }Run

A list of available external lang items is available in `src/librustc/middle/weak_lang_items.rs`. Example:

    #![feature(lang_items)]

    extern "C" {
        #[lang = "panic_impl"] // ok!
        fn cake();
    }Run

## E0267

This error indicates the use of a loop keyword (`break` or `continue`) inside a closure but outside of any loop. Erroneous code example:

ⓘThis example deliberately fails to compile

    let w = || { break; }; // error: `break` inside of a closureRun

`break` and `continue` keywords can be used as normal inside closures as long as they are also contained within a loop. To halt the execution of a closure you should instead use a return statement. Example:

    let w = || {
        for _ in 0..10 {
            break;
        }
    };

    w();Run

## E0268

This error indicates the use of a loop keyword (`break` or `continue`) outside of a loop. Without a loop to break out of or continue in, no sensible action can be taken. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn some_func() {
        break; // error: `break` outside of loop
    }Run

Please verify that you are using `break` and `continue` only in loops. Example:

    fn some_func() {
        for _ in 0..10 {
            break; // ok!
        }
    }Run

## E0271

This is because of a type mismatch between the associated type of some trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`) and another type `U` that is required to be equal to `T::Bar`, but is not. Examples follow.

Here is a basic example:

ⓘThis example deliberately fails to compile

    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = &'static str; }

    foo(3_i8);Run

Here is that same example again, with some explanatory comments:

ⓘThis example deliberately fails to compile

    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
    //                    ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
    //                        |            |
    //         This says `foo` can         |
    //           only be used with         |
    //              some type that         |
    //         implements `Trait`.         |
    //                                     |
    //                             This says not only must
    //                             `T` be an impl of `Trait`
    //                             but also that the impl
    //                             must assign the type `u32`
    //                             to the associated type.
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = &'static str; }
    //~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    //      |                             |
    // `i8` does have                     |
    // implementation                     |
    // of `Trait`...                      |
    //                     ... but it is an implementation
    //                     that assigns `&'static str` to
    //                     the associated type.

    foo(3_i8);
    // Here, we invoke `foo` with an `i8`, which does not satisfy
    // the constraint `<i8 as Trait>::AssociatedType=u32`, and
    // therefore the type-checker complains with this error code.Run

To avoid those issues, you have to make the types match correctly. So we can fix the previous examples like this:

    // Basic Example:
    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = &'static str; }

    foo(3_i8);

    // For-Loop Example:
    let vs = vec![1, 2, 3, 4];
    for v in &vs {
        match v {
            &1 => {}
            _ => {}
        }
    }Run

## E0275

This error occurs when there was a recursive trait requirement that overflowed before it could be evaluated. Often this means that there is unbounded recursion in resolving some type bounds.

For example, in the following code:

ⓘThis example deliberately fails to compile

    trait Foo {}

    struct Bar<T>(T);

    impl<T> Foo for T where Bar<T>: Foo {}Run

To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However, to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is clearly a recursive requirement that can't be resolved directly.

Consider changing your trait bounds so that they're less self-referential.

## E0276

This error occurs when a bound in an implementation of a trait does not match the bounds specified in the original trait. For example:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo<T>(x: T);
    }

    impl Foo for bool {
        fn foo<T>(x: T) where T: Copy {}
    }Run

Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can take any type `T`. However, in the `impl` for `bool`, we have added an extra bound that `T` is `Copy`, which isn't compatible with the original trait.

Consider removing the bound from the method or adding the bound to the original method definition in the trait.

## E0277

You tried to use a type which doesn't implement some trait in a place which expected that trait. Erroneous code example:

ⓘThis example deliberately fails to compile

    // here we declare the Foo trait with a bar method
    trait Foo {
        fn bar(&self);
    }

    // we now declare a function which takes an object implementing the Foo trait
    fn some_func<T: Foo>(foo: T) {
        foo.bar();
    }

    fn main() {
        // we now call the method with the i32 type, which doesn't implement
        // the Foo trait
        some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
    }Run

In order to fix this error, verify that the type you're using does implement the trait. Example:

    trait Foo {
        fn bar(&self);
    }

    fn some_func<T: Foo>(foo: T) {
        foo.bar(); // we can now use this method since i32 implements the
                   // Foo trait
    }

    // we implement the trait on the i32 type
    impl Foo for i32 {
        fn bar(&self) {}
    }

    fn main() {
        some_func(5i32); // ok!
    }Run

Or in a generic context, an erroneous code example would look like:

ⓘThis example deliberately fails to compile

    fn some_func<T>(foo: T) {
        println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
                               //        implemented for the type `T`
    }

    fn main() {
        // We now call the method with the i32 type,
        // which *does* implement the Debug trait.
        some_func(5i32);
    }Run

Note that the error here is in the definition of the generic function: Although we only call it with a parameter that does implement `Debug`, the compiler still rejects the function: It must work with all possible input types. In order to make this example compile, we need to restrict the generic type we're accepting:

    use std::fmt;

    // Restrict the input type to types that implement Debug.
    fn some_func<T: fmt::Debug>(foo: T) {
        println!("{:?}", foo);
    }

    fn main() {
        // Calling the method is still fine, as i32 implements Debug.
        some_func(5i32);

        // This would fail to compile now:
        // struct WithoutDebug;
        // some_func(WithoutDebug);
    }Run

Rust only looks at the signature of the called function, as such it must already specify all requirements that will be used for every type parameter.

## E0278

No description.

## E0279

No description.

## E0280

No description.

## E0281

#### Note: this error code is no longer emitted by the compiler.

You tried to supply a type which doesn't implement some trait in a location which expected that trait. This error typically occurs when working with `Fn`-based types. Erroneous code example:

    fn foo<F: Fn(usize)>(x: F) { }

    fn main() {
        // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
        // but the trait `core::ops::Fn<(usize,)>` is required
        // [E0281]
        foo(|y: String| { });
    }

The issue in this case is that `foo` is defined as accepting a `Fn` with one argument of type `String`, but the closure we attempted to pass to it requires one arguments of type `usize`.

## E0282

This error indicates that type inference did not result in one unique possible type, and extra information is required. In most cases this can be provided by adding a type annotation. Sometimes you need to specify a generic type parameter manually.

A common example is the `collect` method on `Iterator`. It has a generic type parameter with a `FromIterator` bound, which for a `char` iterator is implemented by `Vec` and `String` among others. Consider the following snippet that reverses the characters of a string:

ⓘThis example deliberately fails to compile

    let x = "hello".chars().rev().collect();Run

In this case, the compiler cannot infer what the type of `x` should be: `Vec<char>` and `String` are both suitable candidates. To specify which type to use, you can use a type annotation on `x`:

    let x: Vec<char> = "hello".chars().rev().collect();Run

It is not necessary to annotate the full type. Once the ambiguity is resolved, the compiler can infer the rest:

    let x: Vec<_> = "hello".chars().rev().collect();Run

Another way to provide the compiler with enough information, is to specify the generic type parameter:

    let x = "hello".chars().rev().collect::<Vec<char>>();Run

Again, you need not specify the full type if the compiler can infer it:

    let x = "hello".chars().rev().collect::<Vec<_>>();Run

Apart from a method or function with a generic type parameter, this error can occur when a type parameter of a struct or trait cannot be inferred. In that case it is not always possible to use a type annotation, because all candidates have the same return type. For instance:

ⓘThis example deliberately fails to compile

    struct Foo<T> {
        num: T,
    }

    impl<T> Foo<T> {
        fn bar() -> i32 {
            0
        }

        fn baz() {
            let number = Foo::bar();
        }
    }Run

This will fail because the compiler does not know which instance of `Foo` to call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.

## E0283

This error occurs when the compiler doesn't have enough information to unambiguously choose an implementation.

For example:

ⓘThis example deliberately fails to compile

    trait Generator {
        fn create() -> u32;
    }

    struct Impl;

    impl Generator for Impl {
        fn create() -> u32 { 1 }
    }

    struct AnotherImpl;

    impl Generator for AnotherImpl {
        fn create() -> u32 { 2 }
    }

    fn main() {
        let cont: u32 = Generator::create();
        // error, impossible to choose one of Generator trait implementation
        // Should it be Impl or AnotherImpl, maybe something else?
    }Run

To resolve this error use the concrete type:

    trait Generator {
        fn create() -> u32;
    }

    struct AnotherImpl;

    impl Generator for AnotherImpl {
        fn create() -> u32 { 2 }
    }

    fn main() {
        let gen1 = AnotherImpl::create();

        // if there are multiple methods with same name (different traits)
        let gen2 = <AnotherImpl as Generator>::create();
    }Run

## E0284

This error occurs when the compiler is unable to unambiguously infer the return type of a function or method which is generic on return type, such as the `collect` method for `Iterator`s.

For example:

ⓘThis example deliberately fails to compile

    fn foo() -> Result<bool, ()> {
        let results = [Ok(true), Ok(false), Err(())].iter().cloned();
        let v: Vec<bool> = results.collect()?;
        // Do things with v...
        Ok(true)
    }Run

Here we have an iterator `results` over `Result<bool, ()>`. Hence, `results.collect()` can return any type implementing `FromIterator<Result<bool, ()>>`. On the other hand, the `?` operator can accept any type implementing `Try`.

The author of this code probably wants `collect()` to return a `Result<Vec<bool>, ()>`, but the compiler can't be sure that there isn't another type `T` implementing both `Try` and `FromIterator<Result<bool, ()>>` in scope such that `T::Ok == Vec<bool>`. Hence, this code is ambiguous and an error is returned.

To resolve this error, use a concrete type for the intermediate expression:

    fn foo() -> Result<bool, ()> {
        let results = [Ok(true), Ok(false), Err(())].iter().cloned();
        let v = {
            let temp: Result<Vec<bool>, ()> = results.collect();
            temp?
        };
        // Do things with v...
        Ok(true)
    }Run

Note that the type of `v` can now be inferred from the type of `temp`.

## E0297

#### Note: this error code is no longer emitted by the compiler.

Patterns used to bind names must be irrefutable. That is, they must guarantee that a name will be extracted in all cases. Instead of pattern matching the loop variable, consider using a `match` or `if let` inside the loop body. For instance:

ⓘThis example deliberately fails to compile

    let xs : Vec<Option<i32>> = vec![Some(1), None];

    // This fails because `None` is not covered.
    for Some(x) in xs {
        // ...
    }Run

Match inside the loop instead:

    let xs : Vec<Option<i32>> = vec![Some(1), None];

    for item in xs {
        match item {
            Some(x) => {},
            None => {},
        }
    }Run

Or use `if let`:

    let xs : Vec<Option<i32>> = vec![Some(1), None];

    for item in xs {
        if let Some(x) = item {
            // ...
        }
    }Run

## E0301

Mutable borrows are not allowed in pattern guards, because matching cannot have side effects. Side effects could alter the matched object or the environment on which the match depends in such a way, that the match would not be exhaustive. For instance, the following would not match any arm if mutable borrows were allowed:

ⓘThis example deliberately fails to compile

    match Some(()) {
        None => { },
        option if option.take().is_none() => {
            /* impossible, option is `Some` */
        },
        Some(_) => { } // When the previous match failed, the option became `None`.
    }Run

## E0302

Assignments are not allowed in pattern guards, because matching cannot have side effects. Side effects could alter the matched object or the environment on which the match depends in such a way, that the match would not be exhaustive. For instance, the following would not match any arm if assignments were allowed:

ⓘThis example deliberately fails to compile

    match Some(()) {
        None => { },
        option if { option = None; false } => { },
        Some(_) => { } // When the previous match failed, the option became `None`.
    }Run

## E0303

In certain cases it is possible for sub-bindings to violate memory safety. Updates to the borrow checker in a future version of Rust may remove this restriction, but for now patterns must be rewritten without sub-bindings.

Before:

ⓘThis example deliberately fails to compile

    match Some("hi".to_string()) {
        ref op_string_ref @ Some(s) => {},
        None => {},
    }Run

After:

    match Some("hi".to_string()) {
        Some(ref s) => {
            let op_string_ref = &Some(s);
            // ...
        },
        None => {},
    }Run

The `op_string_ref` binding has type `&Option<&String>` in both cases.

See also https://github.com/rust-lang/rust/issues/14587

## E0307

No description.

## E0308

This error occurs when the compiler was unable to infer the concrete type of a variable. It can occur for several cases, the most common of which is a mismatch in the expected type that the compiler inferred for a variable's initializing expression, and the actual type explicitly assigned to the variable.

For example:

ⓘThis example deliberately fails to compile

    let x: i32 = "I am not a number!";
    //     ~~~   ~~~~~~~~~~~~~~~~~~~~
    //      |             |
    //      |    initializing expression;
    //      |    compiler infers type `&str`
    //      |
    //    type `i32` assigned to variable `x`Run

## E0309

The type definition contains some field whose type requires an outlives annotation. Outlives annotations (e.g., `T: 'a`) are used to guarantee that all the data in T is valid for at least the lifetime `'a`. This scenario most commonly arises when the type contains an associated type reference like `<T as SomeTrait<'a>>::Output`, as shown in this example:

ⓘThis example deliberately fails to compile

    // This won't compile because the applicable impl of
    // `SomeTrait` (below) requires that `T: 'a`, but the struct does
    // not have a matching where-clause.
    struct Foo<'a, T> {
        foo: <T as SomeTrait<'a>>::Output,
    }

    trait SomeTrait<'a> {
        type Output;
    }

    impl<'a, T> SomeTrait<'a> for T
    where
        T: 'a,
    {
        type Output = u32;
    }Run

Here, the where clause `T: 'a` that appears on the impl is not known to be satisfied on the struct. To make this example compile, you have to add a where-clause like `T: 'a` to the struct definition:

    struct Foo<'a, T>
    where
        T: 'a,
    {
        foo: <T as SomeTrait<'a>>::Output
    }

    trait SomeTrait<'a> {
        type Output;
    }

    impl<'a, T> SomeTrait<'a> for T
    where
        T: 'a,
    {
        type Output = u32;
    }Run

## E0310

Types in type definitions have lifetimes associated with them that represent how long the data stored within them is guaranteed to be live. This lifetime must be as long as the data needs to be alive, and missing the constraint that denotes this will cause this error.

ⓘThis example deliberately fails to compile

    // This won't compile because T is not constrained to the static lifetime
    // the reference needs
    struct Foo<T> {
        foo: &'static T
    }Run

This will compile, because it has the constraint on the type parameter:

    struct Foo<T: 'static> {
        foo: &'static T
    }Run

## E0311

No description.

## E0312

No description.

## E0313

No description.

## E0314

No description.

## E0315

No description.

## E0316

No description.

## E0317

This error occurs when an `if` expression without an `else` block is used in a context where a type other than `()` is expected, for example a `let` expression:

ⓘThis example deliberately fails to compile

    fn main() {
        let x = 5;
        let a = if x == 5 { 1 };
    }Run

An `if` expression without an `else` block has the type `()`, so this is a type error. To resolve it, add an `else` block having the same type as the `if` block.

## E0320

No description.

## E0321

A cross-crate opt-out trait was implemented on something which wasn't a struct or enum type. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(optin_builtin_traits)]

    struct Foo;

    impl !Sync for Foo {}

    unsafe impl Send for &'static Foo {}
    // error: cross-crate traits with a default impl, like `core::marker::Send`,
    //        can only be implemented for a struct/enum type, not
    //        `&'static Foo`Run

Only structs and enums are permitted to impl Send, Sync, and other opt-out trait, and the struct or enum must be local to the current crate. So, for example, `unsafe impl Send for Rc<Foo>` is not allowed.

## E0322

The `Sized` trait is a special trait built-in to the compiler for types with a constant size known at compile-time. This trait is automatically implemented for types as needed by the compiler, and it is currently disallowed to explicitly implement it for a type.

## E0323

An associated const was implemented when another trait item was expected. Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Foo {
        type N;
    }

    struct Bar;

    impl Foo for Bar {
        const N : u32 = 0;
        // error: item `N` is an associated const, which doesn't match its
        //        trait `<Bar as Foo>`
    }Run

Please verify that the associated const wasn't misspelled and the correct trait was implemented. Example:

    struct Bar;

    trait Foo {
        type N;
    }

    impl Foo for Bar {
        type N = u32; // ok!
    }Run

Or:

    struct Bar;

    trait Foo {
        const N : u32;
    }

    impl Foo for Bar {
        const N : u32 = 0; // ok!
    }Run

## E0324

A method was implemented when another trait item was expected. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Bar;

    trait Foo {
        const N : u32;

        fn M();
    }

    impl Foo for Bar {
        fn N() {}
        // error: item `N` is an associated method, which doesn't match its
        //        trait `<Bar as Foo>`
    }Run

To fix this error, please verify that the method name wasn't misspelled and verify that you are indeed implementing the correct trait items. Example:

    struct Bar;

    trait Foo {
        const N : u32;

        fn M();
    }

    impl Foo for Bar {
        const N : u32 = 0;

        fn M() {} // ok!
    }Run

## E0325

An associated type was implemented when another trait item was expected. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Bar;

    trait Foo {
        const N : u32;
    }

    impl Foo for Bar {
        type N = u32;
        // error: item `N` is an associated type, which doesn't match its
        //        trait `<Bar as Foo>`
    }Run

Please verify that the associated type name wasn't misspelled and your implementation corresponds to the trait definition. Example:

    struct Bar;

    trait Foo {
        type N;
    }

    impl Foo for Bar {
        type N = u32; // ok!
    }Run

Or:

    struct Bar;

    trait Foo {
        const N : u32;
    }

    impl Foo for Bar {
        const N : u32 = 0; // ok!
    }Run

## E0326

The types of any associated constants in a trait implementation must match the types in the trait definition. This error indicates that there was a mismatch.

Here's an example of this error:

ⓘThis example deliberately fails to compile

    trait Foo {
        const BAR: bool;
    }

    struct Bar;

    impl Foo for Bar {
        const BAR: u32 = 5; // error, expected bool, found u32
    }Run

## E0328

The Unsize trait should not be implemented directly. All implementations of Unsize are provided automatically by the compiler.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(unsize)]

    use std::marker::Unsize;

    pub struct MyType;

    impl<T> Unsize<T> for MyType {}Run

If you are defining your own smart pointer type and would like to enable conversion from a sized to an unsized type with the DST coercion system, use `CoerceUnsized` instead.

    #![feature(coerce_unsized)]

    use std::ops::CoerceUnsized;

    pub struct MyType<T: ?Sized> {
        field_with_unsized_type: T,
    }

    impl<T, U> CoerceUnsized<MyType<U>> for MyType<T>
        where T: CoerceUnsized<U> {}Run

## E0364

Private items cannot be publicly re-exported. This error indicates that you attempted to `pub use` a type or value that was not itself public.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod foo {
        const X: u32 = 1;
    }

    pub use foo::X;

    fn main() {}Run

The solution to this problem is to ensure that the items that you are re-exporting are themselves marked with `pub`:

    mod foo {
        pub const X: u32 = 1;
    }

    pub use foo::X;

    fn main() {}Run

See the 'Use Declarations' section of the reference for more information on this topic:

https://doc.rust-lang.org/reference.html#use-declarations

## E0365

Private modules cannot be publicly re-exported. This error indicates that you attempted to `pub use` a module that was not itself public.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod foo {
        pub const X: u32 = 1;
    }

    pub use foo as foo2;

    fn main() {}Run

The solution to this problem is to ensure that the module that you are re-exporting is itself marked with `pub`:

    pub mod foo {
        pub const X: u32 = 1;
    }

    pub use foo as foo2;

    fn main() {}Run

See the 'Use Declarations' section of the reference for more information on this topic:

https://doc.rust-lang.org/reference.html#use-declarations

## E0366

An attempt was made to implement `Drop` on a concrete specialization of a generic type. An example is shown below:

ⓘThis example deliberately fails to compile

    struct Foo<T> {
        t: T
    }

    impl Drop for Foo<u32> {
        fn drop(&mut self) {}
    }Run

This code is not legal: it is not possible to specialize `Drop` to a subset of implementations of a generic type. One workaround for this is to wrap the generic type, as shown below:

    struct Foo<T> {
        t: T
    }

    struct Bar {
        t: Foo<u32>
    }

    impl Drop for Bar {
        fn drop(&mut self) {}
    }Run

## E0367

An attempt was made to implement `Drop` on a specialization of a generic type. An example is shown below:

ⓘThis example deliberately fails to compile

    trait Foo{}

    struct MyStruct<T> {
        t: T
    }

    impl<T: Foo> Drop for MyStruct<T> {
        fn drop(&mut self) {}
    }Run

This code is not legal: it is not possible to specialize `Drop` to a subset of implementations of a generic type. In order for this code to work, `MyStruct` must also require that `T` implements `Foo`. Alternatively, another option is to wrap the generic type in another that specializes appropriately:

    trait Foo{}

    struct MyStruct<T> {
        t: T
    }

    struct MyStructWrapper<T: Foo> {
        t: MyStruct<T>
    }

    impl <T: Foo> Drop for MyStructWrapper<T> {
        fn drop(&mut self) {}
    }Run

## E0368

This error indicates that a binary assignment operator like `+=` or `^=` was applied to a type that doesn't support it. For example:

ⓘThis example deliberately fails to compile

    let mut x = 12f32; // error: binary operation `<<` cannot be applied to
                       //        type `f32`

    x <<= 2;Run

To fix this error, please check that this type implements this binary operation. Example:

    let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait

    x <<= 2; // ok!Run

It is also possible to overload most operators for your own type by implementing the `[OP]Assign` traits from `std::ops`.

Another problem you might be facing is this: suppose you've overloaded the `+` operator for some type `Foo` by implementing the `std::ops::Add` trait for `Foo`, but you find that using `+=` does not work, as in this example:

ⓘThis example deliberately fails to compile

    use std::ops::Add;

    struct Foo(u32);

    impl Add for Foo {
        type Output = Foo;

        fn add(self, rhs: Foo) -> Foo {
            Foo(self.0 + rhs.0)
        }
    }

    fn main() {
        let mut x: Foo = Foo(5);
        x += Foo(7); // error, `+= cannot be applied to the type `Foo`
    }Run

This is because `AddAssign` is not automatically implemented, so you need to manually implement it for your type.

## E0369

A binary operation was attempted on a type which doesn't support it. Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = 12f32; // error: binary operation `<<` cannot be applied to
                   //        type `f32`

    x << 2;Run

To fix this error, please check that this type implements this binary operation. Example:

    let x = 12u32; // the `u32` type does implement it:
                   // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html

    x << 2; // ok!Run

It is also possible to overload most operators for your own type by implementing traits from `std::ops`.

String concatenation appends the string on the right to the string on the left and may require reallocation. This requires ownership of the string on the left. If something should be added to a string literal, move the literal to the heap by allocating it with `to_owned()` like in `"Your text".to_owned()`.

## E0370

The maximum value of an enum was reached, so it cannot be automatically set in the next enum value. Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(i64)]
    enum Foo {
        X = 0x7fffffffffffffff,
        Y, // error: enum discriminant overflowed on value after
           //        9223372036854775807: i64; set explicitly via
           //        Y = -9223372036854775808 if that is desired outcome
    }Run

To fix this, please set manually the next enum value or put the enum variant with the maximum value at the end of the enum. Examples:

    #[repr(i64)]
    enum Foo {
        X = 0x7fffffffffffffff,
        Y = 0, // ok!
    }Run

Or:

    #[repr(i64)]
    enum Foo {
        Y = 0, // ok!
        X = 0x7fffffffffffffff,
    }Run

## E0371

When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by definition, so it is not useful to do this.

Example:

ⓘThis example deliberately fails to compile

    trait Foo { fn foo(&self) { } }
    trait Bar: Foo { }
    trait Baz: Bar { }

    impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
    impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
    impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
    impl Baz for Bar { } // Note: This is OKRun

## E0373

This error occurs when an attempt is made to use data captured by a closure, when that data may no longer exist. It's most commonly seen when attempting to return a closure:

ⓘThis example deliberately fails to compile

    fn foo() -> Box<Fn(u32) -> u32> {
        let x = 0u32;
        Box::new(|y| x + y)
    }Run

Notice that `x` is stack-allocated by `foo()`. By default, Rust captures closed-over data by reference. This means that once `foo()` returns, `x` no longer exists. An attempt to access `x` within the closure would thus be unsafe.

Another situation where this might be encountered is when spawning threads:

ⓘThis example deliberately fails to compile

    fn foo() {
        let x = 0u32;
        let y = 1u32;

        let thr = std::thread::spawn(|| {
            x + y
        });
    }Run

Since our new thread runs in parallel, the stack frame containing `x` and `y` may well have disappeared by the time we try to use them. Even if we call `thr.join()` within foo (which blocks until `thr` has completed, ensuring the stack frame won't disappear), we will not succeed: the compiler cannot prove that this behaviour is safe, and so won't let us do it.

The solution to this problem is usually to switch to using a `move` closure. This approach moves (or copies, where possible) data into the closure, rather than taking references to it. For example:

    fn foo() -> Box<Fn(u32) -> u32> {
        let x = 0u32;
        Box::new(move |y| x + y)
    }Run

Now that the closure has its own copy of the data, there's no need to worry about safety.

## E0374

A struct without a field containing an unsized type cannot implement `CoerceUnsized`. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    struct Foo<T: ?Sized> {
        a: i32,
    }

    // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
    impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
        where T: CoerceUnsized<U> {}Run

`CoerceUnsized` is used to coerce one struct containing an unsized type into another struct containing a different unsized type. If the struct doesn't have any fields of unsized types then you don't need explicit coercion to get the types you want. To fix this you can either not try to implement `CoerceUnsized` or you can add a field that is unsized to the struct.

Example:

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    // We don't need to impl `CoerceUnsized` here.
    struct Foo {
        a: i32,
    }

    // We add the unsized type field to the struct.
    struct Bar<T: ?Sized> {
        a: i32,
        b: T,
    }

    // The struct has an unsized field so we can implement
    // `CoerceUnsized` for it.
    impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
        where T: CoerceUnsized<U> {}Run

Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc` and `Arc` to be able to mark that they can coerce unsized types that they are pointing at.

## E0375

A struct with more than one field containing an unsized type cannot implement `CoerceUnsized`. This only occurs when you are trying to coerce one of the types in your struct to another type in the struct. In this case we try to impl `CoerceUnsized` from `T` to `U` which are both types that the struct takes. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    struct Foo<T: ?Sized, U: ?Sized> {
        a: i32,
        b: T,
        c: U,
    }

    // error: Struct `Foo` has more than one unsized field.
    impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}Run

`CoerceUnsized` only allows for coercion from a structure with a single unsized type field to another struct with a single unsized type field. In fact Rust only allows for a struct to have one unsized type in a struct and that unsized type must be the last field in the struct. So having two unsized types in a single struct is not allowed by the compiler. To fix this use only one field containing an unsized type in the struct and then use multiple structs to manage each unsized type field you need.

Example:

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    struct Foo<T: ?Sized> {
        a: i32,
        b: T,
    }

    impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
        where T: CoerceUnsized<U> {}

    fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
        Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
    }Run

## E0376

The type you are trying to impl `CoerceUnsized` for is not a struct. `CoerceUnsized` can only be implemented for a struct. Unsized types are already able to be coerced without an implementation of `CoerceUnsized` whereas a struct containing an unsized type needs to know the unsized type field it's containing is able to be coerced. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    struct Foo<T: ?Sized> {
        a: T,
    }

    // error: The type `U` is not a struct
    impl<T, U> CoerceUnsized<U> for Foo<T> {}Run

The `CoerceUnsized` trait takes a struct type. Make sure the type you are providing to `CoerceUnsized` is a struct with only the last field containing an unsized type.

Example:

    #![feature(coerce_unsized)]
    use std::ops::CoerceUnsized;

    struct Foo<T> {
        a: T,
    }

    // The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
    impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}Run

Note that in Rust, structs can only contain an unsized type if the field containing the unsized type is the last and only unsized type field in the struct.

## E0377

No description.

## E0378

The `DispatchFromDyn` trait currently can only be implemented for builtin pointer types and structs that are newtype wrappers around them — that is, the struct must have only one field (except for`PhantomData`), and that field must itself implement `DispatchFromDyn`.

Examples:

    #![feature(dispatch_from_dyn, unsize)]
    use std::{
        marker::Unsize,
        ops::DispatchFromDyn,
    };

    struct Ptr<T: ?Sized>(*const T);

    impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T>
    where
        T: Unsize<U>,
    {}Run

    #![feature(dispatch_from_dyn)]
    use std::{
        ops::DispatchFromDyn,
        marker::PhantomData,
    };

    struct Wrapper<T> {
        ptr: T,
        _phantom: PhantomData<()>,
    }

    impl<T, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T>
    where
        T: DispatchFromDyn<U>,
    {}Run

Example of illegal `DispatchFromDyn` implementation (illegal because of extra field)

    #![feature(dispatch_from_dyn)]
    use std::ops::DispatchFromDyn;

    struct WrapperExtraField<T> {
        ptr: T,
        extra_stuff: i32,
    }

    impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
    where
        T: DispatchFromDyn<U>,
    {}

## E0379

Trait methods cannot be declared `const` by design. For more information, see RFC 911.

## E0380

Auto traits cannot have methods or associated items. For more information see the opt-in builtin traits RFC.

## E0381

It is not allowed to use or capture an uninitialized variable. For example:

ⓘThis example deliberately fails to compile

    fn main() {
        let x: i32;
        let y = x; // error, use of possibly uninitialized variable
    }Run

To fix this, ensure that any declared variables are initialized before being used. Example:

    fn main() {
        let x: i32 = 0;
        let y = x; // ok!
    }Run

## E0382

This error occurs when an attempt is made to use a variable after its contents have been moved elsewhere. For example:

ⓘThis example deliberately fails to compile

    struct MyStruct { s: u32 }

    fn main() {
        let mut x = MyStruct{ s: 5u32 };
        let y = x;
        x.s = 6;
        println!("{}", x.s);
    }Run

Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out of `x` when we set `y`. This is fundamental to Rust's ownership system: outside of workarounds like `Rc`, a value cannot be owned by more than one variable.

Sometimes we don't need to move the value. Using a reference, we can let another function borrow the value without changing its ownership. In the example below, we don't actually have to move our string to `calculate_length`, we can give it a reference to it with `&` instead.

    fn main() {
        let s1 = String::from("hello");

        let len = calculate_length(&s1);

        println!("The length of '{}' is {}.", s1, len);
    }

    fn calculate_length(s: &String) -> usize {
        s.len()
    }Run

A mutable reference can be created with `&mut`.

Sometimes we don't want a reference, but a duplicate. All types marked `Clone` can be duplicated by calling `.clone()`. Subsequent changes to a clone do not affect the original variable.

Most types in the standard library are marked `Clone`. The example below demonstrates using `clone()` on a string. `s1` is first set to "many", and then copied to `s2`. Then the first character of `s1` is removed, without affecting `s2`. "any many" is printed to the console.

    fn main() {
        let mut s1 = String::from("many");
        let s2 = s1.clone();
        s1.remove(0);
        println!("{} {}", s1, s2);
    }Run

If we control the definition of a type, we can implement `Clone` on it ourselves with `#[derive(Clone)]`.

Some types have no ownership semantics at all and are trivial to duplicate. An example is `i32` and the other number types. We don't have to call `.clone()` to clone them, because they are marked `Copy` in addition to `Clone`. Implicit cloning is more convenient in this case. We can mark our own types `Copy` if all their members also are marked `Copy`.

In the example below, we implement a `Point` type. Because it only stores two integers, we opt-out of ownership semantics with `Copy`. Then we can `let p2 = p1` without `p1` being moved.

    #[derive(Copy, Clone)]
    struct Point { x: i32, y: i32 }

    fn main() {
        let mut p1 = Point{ x: -1, y: 2 };
        let p2 = p1;
        p1.x = 1;
        println!("p1: {}, {}", p1.x, p1.y);
        println!("p2: {}, {}", p2.x, p2.y);
    }Run

Alternatively, if we don't control the struct's definition, or mutable shared ownership is truly required, we can use `Rc` and `RefCell`:

    use std::cell::RefCell;
    use std::rc::Rc;

    struct MyStruct { s: u32 }

    fn main() {
        let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 }));
        let y = x.clone();
        x.borrow_mut().s = 6;
        println!("{}", x.borrow().s);
    }Run

With this approach, x and y share ownership of the data via the `Rc` (reference count type). `RefCell` essentially performs runtime borrow checking: ensuring that at most one writer or multiple readers can access the data at any one time.

If you wish to learn more about ownership in Rust, start with the chapter in the Book:

https://doc.rust-lang.org/book/ch04-00-understanding-ownership.html

## E0383

#### Note: this error code is no longer emitted by the compiler.

This error occurs when an attempt is made to partially reinitialize a structure that is currently uninitialized.

For example, this can happen when a drop has taken place:

ⓘThis example deliberately fails to compile

    struct Foo {
        a: u32,
    }
    impl Drop for Foo {
        fn drop(&mut self) { /* ... */ }
    }

    let mut x = Foo { a: 1 };
    drop(x); // `x` is now uninitialized
    x.a = 2; // error, partial reinitialization of uninitialized structure `t`Run

This error can be fixed by fully reinitializing the structure in question:

    struct Foo {
        a: u32,
    }
    impl Drop for Foo {
        fn drop(&mut self) { /* ... */ }
    }

    let mut x = Foo { a: 1 };
    drop(x);
    x = Foo { a: 2 };Run

## E0384

This error occurs when an attempt is made to reassign an immutable variable. For example:

ⓘThis example deliberately fails to compile

    fn main() {
        let x = 3;
        x = 5; // error, reassignment of immutable variable
    }Run

By default, variables in Rust are immutable. To fix this error, add the keyword `mut` after the keyword `let` when declaring the variable. For example:

    fn main() {
        let mut x = 3;
        x = 5;
    }Run

## E0387

#### Note: this error code is no longer emitted by the compiler.

This error occurs when an attempt is made to mutate or mutably reference data that a closure has captured immutably. Examples of this error are shown below:

ⓘThis example deliberately fails to compile

    // Accepts a function or a closure that captures its environment immutably.
    // Closures passed to foo will not be able to mutate their closed-over state.
    fn foo<F: Fn()>(f: F) { }

    // Attempts to mutate closed-over data. Error message reads:
    // `cannot assign to data in a captured outer variable...`
    fn mutable() {
        let mut x = 0u32;
        foo(|| x = 2);
    }

    // Attempts to take a mutable reference to closed-over data.  Error message
    // reads: `cannot borrow data mutably in a captured outer variable...`
    fn mut_addr() {
        let mut x = 0u32;
        foo(|| { let y = &mut x; });
    }Run

The problem here is that foo is defined as accepting a parameter of type `Fn`. Closures passed into foo will thus be inferred to be of type `Fn`, meaning that they capture their context immutably.

If the definition of `foo` is under your control, the simplest solution is to capture the data mutably. This can be done by defining `foo` to take FnMut rather than Fn:

    fn foo<F: FnMut()>(f: F) { }Run

Alternatively, we can consider using the `Cell` and `RefCell` types to achieve interior mutability through a shared reference. Our example's `mutable` function could be redefined as below:

    use std::cell::Cell;

    fn foo<F: Fn()>(f: F) { }

    fn mutable() {
        let x = Cell::new(0u32);
        foo(|| x.set(2));
    }Run

You can read more about cell types in the API documentation:

https://doc.rust-lang.org/std/cell/

## E0388

E0388 was removed and is no longer issued.

## E0389

#### Note: this error code is no longer emitted by the compiler.

An attempt was made to mutate data using a non-mutable reference. This commonly occurs when attempting to assign to a non-mutable reference of a mutable reference (`&(&mut T)`).

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };
        let fancy_ref = &(&mut fancy);
        fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
        println!("{}", fancy_ref.num);
    }Run

Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an immutable reference to a value borrows it immutably. There can be multiple references of type `&(&mut T)` that point to the same value, so they must be immutable to prevent multiple mutable references to the same value.

To fix this, either remove the outer reference:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };

        let fancy_ref = &mut fancy;
        // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)

        fancy_ref.num = 6; // No error!

        println!("{}", fancy_ref.num);
    }Run

Or make the outer reference mutable:

    struct FancyNum {
        num: u8
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };

        let fancy_ref = &mut (&mut fancy);
        // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)

        fancy_ref.num = 6; // No error!

        println!("{}", fancy_ref.num);
    }Run

## E0390

You tried to implement methods for a primitive type. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        x: i32
    }

    impl *mut Foo {}
    // error: only a single inherent implementation marked with
    //        `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitiveRun

This isn't allowed, but using a trait to implement a method is a good solution. Example:

    struct Foo {
        x: i32
    }

    trait Bar {
        fn bar();
    }

    impl Bar for *mut Foo {
        fn bar() {} // ok!
    }Run

## E0391

This error indicates that some types or traits depend on each other and therefore cannot be constructed.

The following example contains a circular dependency between two traits:

ⓘThis example deliberately fails to compile

    trait FirstTrait : SecondTrait {

    }

    trait SecondTrait : FirstTrait {

    }Run

## E0392

This error indicates that a type or lifetime parameter has been declared but not actually used. Here is an example that demonstrates the error:

ⓘThis example deliberately fails to compile

    enum Foo<T> {
        Bar,
    }Run

If the type parameter was included by mistake, this error can be fixed by simply removing the type parameter, as shown below:

    enum Foo {
        Bar,
    }Run

Alternatively, if the type parameter was intentionally inserted, it must be used. A simple fix is shown below:

    enum Foo<T> {
        Bar(T),
    }Run

This error may also commonly be found when working with unsafe code. For example, when using raw pointers one may wish to specify the lifetime for which the pointed-at data is valid. An initial attempt (below) causes this error:

ⓘThis example deliberately fails to compile

    struct Foo<'a, T> {
        x: *const T,
    }Run

We want to express the constraint that Foo should not outlive `'a`, because the data pointed to by `T` is only valid for that lifetime. The problem is that there are no actual uses of `'a`. It's possible to work around this by adding a PhantomData type to the struct, using it to tell the compiler to act as if the struct contained a borrowed reference `&'a T`:

    use std::marker::PhantomData;

    struct Foo<'a, T: 'a> {
        x: *const T,
        phantom: PhantomData<&'a T>
    }Run

PhantomData can also be used to express information about unused type parameters.

## E0393

A type parameter which references `Self` in its default value was not specified. Example of erroneous code:

ⓘThis example deliberately fails to compile

    trait A<T=Self> {}

    fn together_we_will_rule_the_galaxy(son: &A) {}
    // error: the type parameter `T` must be explicitly specified in an
    //        object type because its default value `Self` references the
    //        type `Self`Run

A trait object is defined over a single, fully-defined trait. With a regular default parameter, this parameter can just be substituted in. However, if the default parameter is `Self`, the trait changes for each concrete type; i.e. `i32` will be expected to implement `A<i32>`, `bool` will be expected to implement `A<bool>`, etc... These types will not share an implementation of a fully-defined trait; instead they share implementations of a trait with different parameters substituted in for each implementation. This is irreconcilable with what we need to make a trait object work, and is thus disallowed. Making the trait concrete by explicitly specifying the value of the defaulted parameter will fix this issue. Fixed example:

    trait A<T=Self> {}

    fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!Run

## E0398

#### Note: this error code is no longer emitted by the compiler.

In Rust 1.3, the default object lifetime bounds are expected to change, as described in RFC 1156. You are getting a warning because the compiler thinks it is possible that this change will cause a compilation error in your code. It is possible, though unlikely, that this is a false alarm.

The heart of the change is that where `&'a Box<SomeTrait>` used to default to `&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here, `SomeTrait` is the name of some trait type). Note that the only types which are affected are references to boxes, like `&Box<SomeTrait>` or `&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>` are unaffected.

To silence this warning, edit your code to use an explicit bound. Most of the time, this means that you will want to change the signature of a function that you are calling. For example, if the error is reported on a call like `foo(x)`, and `foo` is defined as follows:

    fn foo(arg: &Box<SomeTrait>) { /* ... */ }Run

You might change it to:

    fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }Run

This explicitly states that you expect the trait object `SomeTrait` to contain references (with a maximum lifetime of `'a`).

## E0399

You implemented a trait, overriding one or more of its associated types but did not reimplement its default methods.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(associated_type_defaults)]

    pub trait Foo {
        type Assoc = u8;
        fn bar(&self) {}
    }

    impl Foo for i32 {
        // error - the following trait items need to be reimplemented as
        //         `Assoc` was overridden: `bar`
        type Assoc = i32;
    }Run

To fix this, add an implementation for each default method from the trait:

    #![feature(associated_type_defaults)]

    pub trait Foo {
        type Assoc = u8;
        fn bar(&self) {}
    }

    impl Foo for i32 {
        type Assoc = i32;
        fn bar(&self) {} // ok!
    }Run

## E0401

Inner items do not inherit type or const parameters from the functions they are embedded in.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo<T>(x: T) {
        fn bar(y: T) { // T is defined in the "outer" function
            // ..
        }
        bar(x);
    }Run

Nor will this:

ⓘThis example deliberately fails to compile

    fn foo<T>(x: T) {
        type MaybeT = Option<T>;
        // ...
    }Run

Or this:

ⓘThis example deliberately fails to compile

    fn foo<T>(x: T) {
        struct Foo {
            x: T,
        }
        // ...
    }Run

Items inside functions are basically just like top-level items, except that they can only be used from the function they are in.

There are a couple of solutions for this.

If the item is a function, you may use a closure:

    fn foo<T>(x: T) {
        let bar = |y: T| { // explicit type annotation may not be necessary
            // ..
        };
        bar(x);
    }Run

For a generic item, you can copy over the parameters:

    fn foo<T>(x: T) {
        fn bar<T>(y: T) {
            // ..
        }
        bar(x);
    }Run

    fn foo<T>(x: T) {
        type MaybeT<T> = Option<T>;
    }Run

Be sure to copy over any bounds as well:

    fn foo<T: Copy>(x: T) {
        fn bar<T: Copy>(y: T) {
            // ..
        }
        bar(x);
    }Run

    fn foo<T: Copy>(x: T) {
        struct Foo<T: Copy> {
            x: T,
        }
    }Run

This may require additional type hints in the function body.

In case the item is a function inside an `impl`, defining a private helper function might be easier:

    impl<T> Foo<T> {
        pub fn foo(&self, x: T) {
            self.bar(x);
        }

        fn bar(&self, y: T) {
            // ..
        }
    }Run

For default impls in traits, the private helper solution won't work, however closures or copying the parameters should still work.

## E0403

Some type parameters have the same name.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo<T, T>(s: T, u: T) {} // error: the name `T` is already used for a type
                                //        parameter in this type parameter listRun

Please verify that none of the type parameters are misspelled, and rename any clashing parameters. Example:

    fn foo<T, Y>(s: T, u: Y) {} // ok!Run

## E0404

You tried to use something which is not a trait in a trait position, such as a bound or `impl`.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo;
    struct Bar;

    impl Foo for Bar {} // error: `Foo` is not a traitRun

Another erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo;

    fn bar<T: Foo>(t: T) {} // error: `Foo` is not a traitRun

Please verify that you didn't misspell the trait's name or otherwise use the wrong identifier. Example:

    trait Foo {
        // some functions
    }
    struct Bar;

    impl Foo for Bar { // ok!
        // functions implementation
    }Run

or

    trait Foo {
        // some functions
    }

    fn bar<T: Foo>(t: T) {} // ok!Run

## E0405

The code refers to a trait that is not in scope.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo;

    impl SomeTrait for Foo {} // error: trait `SomeTrait` is not in scopeRun

Please verify that the name of the trait wasn't misspelled and ensure that it was imported. Example:

    // solution 1:
    use some_file::SomeTrait;

    // solution 2:
    trait SomeTrait {
        // some functions
    }

    struct Foo;

    impl SomeTrait for Foo { // ok!
        // implements functions
    }Run

## E0407

A definition of a method not in the implemented trait was given in a trait implementation.

Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn a();
    }

    struct Bar;

    impl Foo for Bar {
        fn a() {}
        fn b() {} // error: method `b` is not a member of trait `Foo`
    }Run

Please verify you didn't misspell the method name and you used the correct trait. First example:

    trait Foo {
        fn a();
        fn b();
    }

    struct Bar;

    impl Foo for Bar {
        fn a() {}
        fn b() {} // ok!
    }Run

Second example:

    trait Foo {
        fn a();
    }

    struct Bar;

    impl Foo for Bar {
        fn a() {}
    }

    impl Bar {
        fn b() {}
    }Run

## E0408

An "or" pattern was used where the variable bindings are not consistently bound across patterns.

Erroneous code example:

ⓘThis example deliberately fails to compile

    match x {
        Some(y) | None => { /* use y */ } // error: variable `y` from pattern #1 is
                                          //        not bound in pattern #2
        _ => ()
    }Run

Here, `y` is bound to the contents of the `Some` and can be used within the block corresponding to the match arm. However, in case `x` is `None`, we have not specified what `y` is, and the block will use a nonexistent variable.

To fix this error, either split into multiple match arms:

    let x = Some(1);
    match x {
        Some(y) => { /* use y */ }
        None => { /* ... */ }
    }Run

or, bind the variable to a field of the same type in all sub-patterns of the or pattern:

    let x = (0, 2);
    match x {
        (0, y) | (y, 0) => { /* use y */}
        _ => {}
    }Run

In this example, if `x` matches the pattern `(0, _)`, the second field is set to `y`. If it matches `(_, 0)`, the first field is set to `y`; so in all cases `y` is set to some value.

## E0409

An "or" pattern was used where the variable bindings are not consistently bound across patterns.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = (0, 2);
    match x {
        (0, ref y) | (y, 0) => { /* use y */} // error: variable `y` is bound with
                                              //        different mode in pattern #2
                                              //        than in pattern #1
        _ => ()
    }Run

Here, `y` is bound by-value in one case and by-reference in the other.

To fix this error, just use the same mode in both cases. Generally using `ref` or `ref mut` where not already used will fix this:

    let x = (0, 2);
    match x {
        (0, ref y) | (ref y, 0) => { /* use y */}
        _ => ()
    }Run

Alternatively, split the pattern:

    let x = (0, 2);
    match x {
        (y, 0) => { /* use y */ }
        (0, ref y) => { /* use y */}
        _ => ()
    }Run

## E0411

The `Self` keyword was used outside an impl, trait, or type definition.

Erroneous code example:

ⓘThis example deliberately fails to compile

    <Self>::foo; // error: use of `Self` outside of an impl, trait, or type
                 // definitionRun

The `Self` keyword represents the current type, which explains why it can only be used inside an impl, trait, or type definition. It gives access to the associated items of a type:

    trait Foo {
        type Bar;
    }

    trait Baz : Foo {
        fn bar() -> Self::Bar; // like this
    }Run

However, be careful when two types have a common associated type:

ⓘThis example deliberately fails to compile

    trait Foo {
        type Bar;
    }

    trait Foo2 {
        type Bar;
    }

    trait Baz : Foo + Foo2 {
        fn bar() -> Self::Bar;
        // error: ambiguous associated type `Bar` in bounds of `Self`
    }Run

This problem can be solved by specifying from which trait we want to use the `Bar` type:

    trait Foo {
        type Bar;
    }

    trait Foo2 {
        type Bar;
    }

    trait Baz : Foo + Foo2 {
        fn bar() -> <Self as Foo>::Bar; // ok!
    }Run

## E0412

The type name used is not in scope.

Erroneous code examples:

ⓘThis example deliberately fails to compile

    impl Something {} // error: type name `Something` is not in scope

    // or:

    trait Foo {
        fn bar(N); // error: type name `N` is not in scope
    }

    // or:

    fn foo(x: T) {} // type name `T` is not in scopeRun

To fix this error, please verify you didn't misspell the type name, you did declare it or imported it into the scope. Examples:

    struct Something;

    impl Something {} // ok!

    // or:

    trait Foo {
        type N;

        fn bar(_: Self::N); // ok!
    }

    // or:

    fn foo<T>(x: T) {} // ok!Run

Another case that causes this error is when a type is imported into a parent module. To fix this, you can follow the suggestion and use File directly or `use super::File;` which will import the types from the parent namespace. An example that causes this error is below:

ⓘThis example deliberately fails to compile

    use std::fs::File;

    mod foo {
        fn some_function(f: File) {}
    }Run

    use std::fs::File;

    mod foo {
        // either
        use super::File;
        // or
        // use std::fs::File;
        fn foo(f: File) {}
    }Run

## E0415

More than one function parameter have the same name.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo(f: i32, f: i32) {} // error: identifier `f` is bound more than
                              //        once in this parameter listRun

Please verify you didn't misspell parameters' name. Example:

    fn foo(f: i32, g: i32) {} // ok!Run

## E0416

An identifier is bound more than once in a pattern.

Erroneous code example:

ⓘThis example deliberately fails to compile

    match (1, 2) {
        (x, x) => {} // error: identifier `x` is bound more than once in the
                     //        same pattern
    }Run

Please verify you didn't misspell identifiers' name. Example:

    match (1, 2) {
        (x, y) => {} // ok!
    }Run

Or maybe did you mean to unify? Consider using a guard:

    match (A, B, C) {
        (x, x2, see) if x == x2 => { /* A and B are equal, do one thing */ }
        (y, z, see) => { /* A and B unequal; do another thing */ }
    }Run

## E0422

You are trying to use an identifier that is either undefined or not a struct. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main () {
        let x = Foo { x: 1, y: 2 };
    }Run

In this case, `Foo` is undefined, so it inherently isn't anything, and definitely not a struct.

ⓘThis example deliberately fails to compile

    fn main () {
        let foo = 1;
        let x = foo { x: 1, y: 2 };
    }Run

In this case, `foo` is defined, but is not a struct, so Rust can't use it as one.

## E0423

An identifier was used like a function name or a value was expected and the identifier exists but it belongs to a different namespace.

For (an erroneous) example, here a `struct` variant name were used as a function:

ⓘThis example deliberately fails to compile

    struct Foo { a: bool };

    let f = Foo();
    // error: expected function, found `Foo`
    // `Foo` is a struct name, but this expression uses it like a function nameRun

Please verify you didn't misspell the name of what you actually wanted to use here. Example:

    fn Foo() -> u32 { 0 }

    let f = Foo(); // ok!Run

It is common to forget the trailing `!` on macro invocations, which would also yield this error:

ⓘThis example deliberately fails to compile

    println("");
    // error: expected function, found macro `println`
    // did you mean `println!(...)`? (notice the trailing `!`)Run

Another case where this error is emitted is when a value is expected, but something else is found:

ⓘThis example deliberately fails to compile

    pub mod a {
        pub const I: i32 = 1;
    }

    fn h1() -> i32 {
        a.I
        //~^ ERROR expected value, found module `a`
        // did you mean `a::I`?
    }Run

## E0424

The `self` keyword was used in a static method.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo;

    impl Foo {
        fn bar(self) {}

        fn foo() {
            self.bar(); // error: `self` is not available in a static method.
        }
    }Run

Please check if the method's argument list should have contained `self`, `&self`, or `&mut self` (in case you didn't want to create a static method), and add it if so. Example:

    struct Foo;

    impl Foo {
        fn bar(self) {}

        fn foo(self) {
            self.bar(); // ok!
        }
    }Run

## E0425

An unresolved name was used.

Erroneous code examples:

ⓘThis example deliberately fails to compile

    something_that_doesnt_exist::foo;
    // error: unresolved name `something_that_doesnt_exist::foo`

    // or:

    trait Foo {
        fn bar() {
            Self; // error: unresolved name `Self`
        }
    }

    // or:

    let x = unknown_variable;  // error: unresolved name `unknown_variable`Run

Please verify that the name wasn't misspelled and ensure that the identifier being referred to is valid for the given situation. Example:

    enum something_that_does_exist {
        Foo,
    }Run

Or:

    mod something_that_does_exist {
        pub static foo : i32 = 0i32;
    }

    something_that_does_exist::foo; // ok!Run

Or:

    let unknown_variable = 12u32;
    let x = unknown_variable; // ok!Run

If the item is not defined in the current module, it must be imported using a `use` statement, like so:

    use foo::bar;
    bar();Run

If the item you are importing is not defined in some super-module of the current module, then it must also be declared as public (e.g., `pub fn`).

## E0426

An undeclared label was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    loop {
        break 'a; // error: use of undeclared label `'a`
    }Run

Please verify you spelt or declare the label correctly. Example:

    'a: loop {
        break 'a; // ok!
    }Run

## E0428

A type or module has been defined more than once.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Bar;
    struct Bar; // error: duplicate definition of value `Bar`Run

Please verify you didn't misspell the type/module's name or remove/rename the duplicated one. Example:

    struct Bar;
    struct Bar2; // ok!Run

## E0429

The `self` keyword cannot appear alone as the last segment in a `use` declaration.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use std::fmt::self; // error: `self` imports are only allowed within a { } listRun

To use a namespace itself in addition to some of its members, `self` may appear as part of a brace-enclosed list of imports:

    use std::fmt::{self, Debug};Run

If you only want to import the namespace, do so directly:

    use std::fmt;Run

## E0430

The `self` import appears more than once in the list.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use something::{self, self}; // error: `self` import can only appear once in
                                 //        the listRun

Please verify you didn't misspell the import name or remove the duplicated `self` import. Example:

    use something::{self}; // ok!Run

## E0431

An invalid `self` import was made.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use {self}; // error: `self` import can only appear in an import list with a
                //        non-empty prefixRun

You cannot import the current module into itself, please remove this import or verify you didn't misspell it.

## E0432

An import was unresolved.

Erroneous code example:

ⓘThis example deliberately fails to compile

    use something::Foo; // error: unresolved import `something::Foo`.Run

Paths in `use` statements are relative to the crate root. To import items relative to the current and parent modules, use the `self::` and `super::` prefixes, respectively. Also verify that you didn't misspell the import name and that the import exists in the module from where you tried to import it. Example:

    use self::something::Foo; // ok!

    mod something {
        pub struct Foo;
    }Run

Or, if you tried to use a module from an external crate, you may have missed the `extern crate` declaration (which is usually placed in the crate root):

    extern crate core; // Required to use the `core` crate

    use core::any;Run

## E0433

An undeclared type or module was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let map = HashMap::new();
    // error: failed to resolve: use of undeclared type or module `HashMap`Run

Please verify you didn't misspell the type/module's name or that you didn't forget to import it:

    use std::collections::HashMap; // HashMap has been imported.
    let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!Run

## E0434

This error indicates that a variable usage inside an inner function is invalid because the variable comes from a dynamic environment. Inner functions do not have access to their containing environment.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo() {
        let y = 5;
        fn bar() -> u32 {
            y // error: can't capture dynamic environment in a fn item; use the
              //        || { ... } closure form instead.
        }
    }Run

Functions do not capture local variables. To fix this error, you can replace the function with a closure:

    fn foo() {
        let y = 5;
        let bar = || {
            y
        };
    }Run

or replace the captured variable with a constant or a static item:

    fn foo() {
        static mut X: u32 = 4;
        const Y: u32 = 5;
        fn bar() -> u32 {
            unsafe {
                X = 3;
            }
            Y
        }
    }Run

## E0435

A non-constant value was used in a constant expression.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let foo = 42;
    let a: [u8; foo]; // error: attempt to use a non-constant value in a constantRun

To fix this error, please replace the value with a constant. Example:

    let a: [u8; 42]; // ok!Run

Or:

    const FOO: usize = 42;
    let a: [u8; FOO]; // ok!Run

## E0436

The functional record update syntax is only allowed for structs. (Struct-like enum variants don't qualify, for example.)

Erroneous code example:

ⓘThis example deliberately fails to compile

    enum PublicationFrequency {
        Weekly,
        SemiMonthly { days: (u8, u8), annual_special: bool },
    }

    fn one_up_competitor(competitor_frequency: PublicationFrequency)
                         -> PublicationFrequency {
        match competitor_frequency {
            PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
                days: (1, 15), annual_special: false
            },
            c @ PublicationFrequency::SemiMonthly{ .. } =>
                PublicationFrequency::SemiMonthly {
                    annual_special: true, ..c // error: functional record update
                                              //        syntax requires a struct
            }
        }
    }Run

Rewrite the expression without functional record update syntax:

    enum PublicationFrequency {
        Weekly,
        SemiMonthly { days: (u8, u8), annual_special: bool },
    }

    fn one_up_competitor(competitor_frequency: PublicationFrequency)
                         -> PublicationFrequency {
        match competitor_frequency {
            PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
                days: (1, 15), annual_special: false
            },
            PublicationFrequency::SemiMonthly{ days, .. } =>
                PublicationFrequency::SemiMonthly {
                    days, annual_special: true // ok!
            }
        }
    }Run

## E0437

Trait implementations can only implement associated types that are members of the trait in question. This error indicates that you attempted to implement an associated type whose name does not match the name of any associated type in the trait.

Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Foo {}

    impl Foo for i32 {
        type Bar = bool;
    }Run

The solution to this problem is to remove the extraneous associated type:

    trait Foo {}

    impl Foo for i32 {}Run

## E0438

Trait implementations can only implement associated constants that are members of the trait in question. This error indicates that you attempted to implement an associated constant whose name does not match the name of any associated constant in the trait.

Erroneous code example:

ⓘThis example deliberately fails to compile

    trait Foo {}

    impl Foo for i32 {
        const BAR: bool = true;
    }Run

The solution to this problem is to remove the extraneous associated constant:

    trait Foo {}

    impl Foo for i32 {}Run

## E0439

The length of the platform-intrinsic function `simd_shuffle` wasn't specified. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(platform_intrinsics)]

    extern "platform-intrinsic" {
        fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
        // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
    }Run

The `simd_shuffle` function needs the length of the array passed as last parameter in its name. Example:

    #![feature(platform_intrinsics)]

    extern "platform-intrinsic" {
        fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
    }Run

## E0445

A private trait was used on a public type parameter bound. Erroneous code examples:

ⓘThis example deliberately fails to compile

    #![deny(private_in_public)]

    trait Foo {
        fn dummy(&self) { }
    }

    pub trait Bar : Foo {} // error: private trait in public interface
    pub struct Bar2<T: Foo>(pub T); // same error
    pub fn foo<T: Foo> (t: T) {} // same errorRun

To solve this error, please ensure that the trait is also public. The trait can be made inaccessible if necessary by placing it into a private inner module, but it still has to be marked with `pub`. Example:

    pub trait Foo { // we set the Foo trait public
        fn dummy(&self) { }
    }

    pub trait Bar : Foo {} // ok!
    pub struct Bar2<T: Foo>(pub T); // ok!
    pub fn foo<T: Foo> (t: T) {} // ok!Run

## E0446

A private type was used in a public type signature. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![deny(private_in_public)]

    mod Foo {
        struct Bar(u32);

        pub fn bar() -> Bar { // error: private type in public interface
            Bar(0)
        }
    }Run

To solve this error, please ensure that the type is also public. The type can be made inaccessible if necessary by placing it into a private inner module, but it still has to be marked with `pub`. Example:

    mod Foo {
        pub struct Bar(u32); // we set the Bar type public

        pub fn bar() -> Bar { // ok!
            Bar(0)
        }
    }Run

## E0447

#### Note: this error code is no longer emitted by the compiler.

The `pub` keyword was used inside a function. Erroneous code example:

    fn foo() {
        pub struct Bar; // error: visibility has no effect inside functions
    }Run

Since we cannot access items defined inside a function, the visibility of its items does not impact outer code. So using the `pub` keyword in this context is invalid.

## E0448

The `pub` keyword was used inside a public enum. Erroneous code example:

ⓘThis example deliberately fails to compile

    pub enum Foo {
        pub Bar, // error: unnecessary `pub` visibility
    }Run

Since the enum is already public, adding `pub` on one its elements is unnecessary. Example:

ⓘThis example deliberately fails to compile

    enum Foo {
        pub Bar, // not ok!
    }Run

This is the correct syntax:

    pub enum Foo {
        Bar, // ok!
    }Run

## E0449

A visibility qualifier was used when it was unnecessary. Erroneous code examples:

ⓘThis example deliberately fails to compile

    struct Bar;

    trait Foo {
        fn foo();
    }

    pub impl Bar {} // error: unnecessary visibility qualifier

    pub impl Foo for Bar { // error: unnecessary visibility qualifier
        pub fn foo() {} // error: unnecessary visibility qualifier
    }Run

To fix this error, please remove the visibility qualifier when it is not required. Example:

    struct Bar;

    trait Foo {
        fn foo();
    }

    // Directly implemented methods share the visibility of the type itself,
    // so `pub` is unnecessary here
    impl Bar {}

    // Trait methods share the visibility of the trait, so `pub` is
    // unnecessary in either case
    impl Foo for Bar {
        fn foo() {}
    }Run

## E0451

A struct constructor with private fields was invoked. Erroneous code example:

ⓘThis example deliberately fails to compile

    mod Bar {
        pub struct Foo {
            pub a: isize,
            b: isize,
        }
    }

    let f = Bar::Foo{ a: 0, b: 0 }; // error: field `b` of struct `Bar::Foo`
                                    //        is privateRun

To fix this error, please ensure that all the fields of the struct are public, or implement a function for easy instantiation. Examples:

    mod Bar {
        pub struct Foo {
            pub a: isize,
            pub b: isize, // we set `b` field public
        }
    }

    let f = Bar::Foo{ a: 0, b: 0 }; // ok!Run

Or:

    mod Bar {
        pub struct Foo {
            pub a: isize,
            b: isize, // still private
        }

        impl Foo {
            pub fn new() -> Foo { // we create a method to instantiate `Foo`
                Foo { a: 0, b: 0 }
            }
        }
    }

    let f = Bar::Foo::new(); // ok!Run

## E0452

An invalid lint attribute has been given. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![allow(foo = "")] // error: malformed lint attributeRun

Lint attributes only accept a list of identifiers (where each identifier is a lint name). Ensure the attribute is of this form:

    #![allow(foo)] // ok!
    // or:
    #![allow(foo, foo2)] // ok!Run

## E0453

A lint check attribute was overruled by a `forbid` directive set as an attribute on an enclosing scope, or on the command line with the `-F` option.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![forbid(non_snake_case)]

    #[allow(non_snake_case)]
    fn main() {
        let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
                          //        forbid(non_snake_case)
    }Run

The `forbid` lint setting, like `deny`, turns the corresponding compiler warning into a hard error. Unlike `deny`, `forbid` prevents itself from being overridden by inner attributes.

If you're sure you want to override the lint check, you can change `forbid` to `deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a command-line option) to allow the inner lint check attribute:

    #![deny(non_snake_case)]

    #[allow(non_snake_case)]
    fn main() {
        let MyNumber = 2; // ok!
    }Run

Otherwise, edit the code to pass the lint check, and remove the overruled attribute:

    #![forbid(non_snake_case)]

    fn main() {
        let my_number = 2;
    }Run

## E0454

A link name was given with an empty name. Erroneous code example:

ⓘThis example is not tested

    #[link(name = "")] extern {} // error: #[link(name = "")] given with empty nameRun

The rust compiler cannot link to an external library if you don't give it its name. Example:

    #[link(name = "some_lib")] extern {} // ok!Run

## E0455

Linking with `kind=framework` is only supported when targeting macOS, as frameworks are specific to that operating system.

Erroneous code example:

ⓘThis example is not tested

    #[link(name = "FooCoreServices", kind = "framework")] extern {}
    // OS used to compile is Linux for exampleRun

To solve this error you can use conditional compilation:

    #[cfg_attr(target="macos", link(name = "FooCoreServices", kind = "framework"))]
    extern {}Run

See more: https://doc.rust-lang.org/reference/attributes.html#conditional-compilation

## E0456

No description.

## E0457

No description.

## E0458

An unknown "kind" was specified for a link attribute. Erroneous code example:

ⓘThis example is not tested

    #[link(kind = "wonderful_unicorn")] extern {}
    // error: unknown kind: `wonderful_unicorn`Run

Please specify a valid "kind" value, from one of the following:

- static
- dylib
- framework

## E0459

A link was used without a name parameter. Erroneous code example:

ⓘThis example is not tested

    #[link(kind = "dylib")] extern {}
    // error: #[link(...)] specified without `name = "foo"`Run

Please add the name parameter to allow the rust compiler to find the library you want. Example:

    #[link(kind = "dylib", name = "some_lib")] extern {} // ok!Run

## E0460

No description.

## E0461

No description.

## E0462

No description.

## E0463

A plugin/crate was declared but cannot be found. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(plugin)]
    #![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
    extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`Run

You need to link your code to the relevant crate in order to be able to use it (through Cargo or the `-L` option of rustc example). Plugins are crates as well, and you link to them the same way.

## E0464

No description.

## E0465

No description.

## E0466

Macro import declarations were malformed.

Erroneous code examples:

ⓘThis example deliberately fails to compile

    #[macro_use(a_macro(another_macro))] // error: invalid import declaration
    extern crate core as some_crate;

    #[macro_use(i_want = "some_macros")] // error: invalid import declaration
    extern crate core as another_crate;Run

This is a syntax error at the level of attribute declarations. The proper syntax for macro imports is the following:

ⓘThis example is not tested

    // In some_crate:
    #[macro_export]
    macro_rules! get_tacos {
        ...
    }

    #[macro_export]
    macro_rules! get_pimientos {
        ...
    }

    // In your crate:
    #[macro_use(get_tacos, get_pimientos)] // It imports `get_tacos` and
    extern crate some_crate;               // `get_pimientos` macros from some_crateRun

If you would like to import all exported macros, write `macro_use` with no arguments.

## E0468

A non-root module attempts to import macros from another crate.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    mod foo {
        #[macro_use(debug_assert)]  // error: must be at crate root to import
        extern crate core;          //        macros from another crate
        fn run_macro() { debug_assert!(true); }
    }Run

Only `extern crate` imports at the crate root level are allowed to import macros.

Either move the macro import to crate root or do without the foreign macros. This will work:

    #[macro_use(debug_assert)]
    extern crate core;

    mod foo {
        fn run_macro() { debug_assert!(true); }
    }Run

## E0469

A macro listed for import was not found.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[macro_use(drink, be_merry)] // error: imported macro not found
    extern crate alloc;

    fn main() {
        // ...
    }Run

Either the listed macro is not contained in the imported crate, or it is not exported from the given crate.

This could be caused by a typo. Did you misspell the macro's name?

Double-check the names of the macros listed for import, and that the crate in question exports them.

A working version would be:

ⓘThis example is not tested

    // In some_crate crate:
    #[macro_export]
    macro_rules! eat {
        ...
    }

    #[macro_export]
    macro_rules! drink {
        ...
    }

    // In your crate:
    #[macro_use(eat, drink)]
    extern crate some_crate; //ok!Run

## E0472

No description.

## E0473

No description.

## E0474

No description.

## E0475

No description.

## E0476

No description.

## E0477

No description.

## E0478

A lifetime bound was not satisfied.

Erroneous code example:

ⓘThis example deliberately fails to compile

    // Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
    // outlive all the superbounds from the trait (`'kiss`, in this example).

    trait Wedding<'t>: 't { }

    struct Prince<'kiss, 'SnowWhite> {
        child: Box<Wedding<'kiss> + 'SnowWhite>,
        // error: lifetime bound not satisfied
    }Run

In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss` lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix this issue, you need to specify it:

    trait Wedding<'t>: 't { }

    struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
                                              // longer than 'SnowWhite.
        child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
    }Run

## E0479

No description.

## E0480

No description.

## E0481

No description.

## E0482

No description.

## E0483

No description.

## E0484

No description.

## E0485

No description.

## E0486

No description.

## E0487

No description.

## E0488

No description.

## E0489

No description.

## E0490

No description.

## E0491

A reference has a longer lifetime than the data it references.

Erroneous code example:

ⓘThis example deliberately fails to compile

    trait SomeTrait<'a> {
        type Output;
    }

    impl<'a, T> SomeTrait<'a> for T {
        type Output = &'a T; // compile error E0491
    }Run

Here, the problem is that a reference type like `&'a T` is only valid if all the data in T outlives the lifetime `'a`. But this impl as written is applicable to any lifetime `'a` and any type `T` -- we have no guarantee that `T` outlives `'a`. To fix this, you can add a where clause like `where T: 'a`.

    trait SomeTrait<'a> {
        type Output;
    }

    impl<'a, T> SomeTrait<'a> for T
    where
        T: 'a,
    {
        type Output = &'a T; // compile error E0491
    }Run

## E0492

A borrow of a constant containing interior mutability was attempted. Erroneous code example:

ⓘThis example deliberately fails to compile

    use std::sync::atomic::AtomicUsize;

    const A: AtomicUsize = AtomicUsize::new(0);
    static B: &'static AtomicUsize = &A;
    // error: cannot borrow a constant which may contain interior mutability,
    //        create a static insteadRun

A `const` represents a constant value that should never change. If one takes a `&` reference to the constant, then one is taking a pointer to some memory location containing the value. Normally this is perfectly fine: most values can't be changed via a shared `&` pointer, but interior mutability would allow it. That is, a constant value could be mutated. On the other hand, a `static` is explicitly a single memory location, which can be mutated at will.

So, in order to solve this error, either use statics which are `Sync`:

    use std::sync::atomic::AtomicUsize;

    static A: AtomicUsize = AtomicUsize::new(0);
    static B: &'static AtomicUsize = &A; // ok!Run

You can also have this error while using a cell type:

ⓘThis example deliberately fails to compile

    use std::cell::Cell;

    const A: Cell<usize> = Cell::new(1);
    const B: &Cell<usize> = &A;
    // error: cannot borrow a constant which may contain interior mutability,
    //        create a static instead

    // or:
    struct C { a: Cell<usize> }

    const D: C = C { a: Cell::new(1) };
    const E: &Cell<usize> = &D.a; // error

    // or:
    const F: &C = &D; // errorRun

This is because cell types do operations that are not thread-safe. Due to this, they don't implement Sync and thus can't be placed in statics.

However, if you still wish to use these types, you can achieve this by an unsafe wrapper:

    use std::cell::Cell;
    use std::marker::Sync;

    struct NotThreadSafe<T> {
        value: Cell<T>,
    }

    unsafe impl<T> Sync for NotThreadSafe<T> {}

    static A: NotThreadSafe<usize> = NotThreadSafe { value : Cell::new(1) };
    static B: &'static NotThreadSafe<usize> = &A; // ok!Run

Remember this solution is unsafe! You will have to ensure that accesses to the cell are synchronized.

## E0493

No description.

## E0495

No description.

## E0496

A lifetime name is shadowing another lifetime name. Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo<'a> {
        a: &'a i32,
    }

    impl<'a> Foo<'a> {
        fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
                               //        name that is already in scope
        }
    }Run

Please change the name of one of the lifetimes to remove this error. Example:

    struct Foo<'a> {
        a: &'a i32,
    }

    impl<'a> Foo<'a> {
        fn f<'b>(x: &'b i32) { // ok!
        }
    }

    fn main() {
    }Run

## E0497

A stability attribute was used outside of the standard library. Erroneous code example:

ⓘThis example deliberately fails to compile

    #[stable] // error: stability attributes may not be used outside of the
              //        standard library
    fn foo() {}Run

It is not possible to use stability attributes outside of the standard library. Also, for now, it is not possible to write deprecation messages either.

## E0498

No description.

## E0499

A variable was borrowed as mutable more than once. Erroneous code example:

ⓘThis example deliberately fails to compile

    let mut i = 0;
    let mut x = &mut i;
    let mut a = &mut i;
    x;
    // error: cannot borrow `i` as mutable more than once at a timeRun

Please note that in rust, you can either have many immutable references, or one mutable reference. Take a look at https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html for more information. Example:

    let mut i = 0;
    let mut x = &mut i; // ok!

    // or:
    let mut i = 0;
    let a = &i; // ok!
    let b = &i; // still ok!
    let c = &i; // still ok!
    b;
    a;Run

## E0500

A borrowed variable was used by a closure. Example of erroneous code:

ⓘThis example deliberately fails to compile

    fn you_know_nothing(jon_snow: &mut i32) {
        let nights_watch = &jon_snow;
        let starks = || {
            *jon_snow = 3; // error: closure requires unique access to `jon_snow`
                           //        but it is already borrowed
        };
        println!("{}", nights_watch);
    }Run

In here, `jon_snow` is already borrowed by the `nights_watch` reference, so it cannot be borrowed by the `starks` closure at the same time. To fix this issue, you can create the closure after the borrow has ended:

    fn you_know_nothing(jon_snow: &mut i32) {
        let nights_watch = &jon_snow;
        println!("{}", nights_watch);
        let starks = || {
            *jon_snow = 3;
        };
    }Run

Or, if the type implements the `Clone` trait, you can clone it between closures:

    fn you_know_nothing(jon_snow: &mut i32) {
        let mut jon_copy = jon_snow.clone();
        let starks = || {
            *jon_snow = 3;
        };
        println!("{}", jon_copy);
    }Run

## E0501

This error indicates that a mutable variable is being used while it is still captured by a closure. Because the closure has borrowed the variable, it is not available for use until the closure goes out of scope.

Note that a capture will either move or borrow a variable, but in this situation, the closure is borrowing the variable. Take a look at http://rustbyexample.com/fn/closures/capture.html for more information about capturing.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    fn inside_closure(x: &mut i32) {
        // Actions which require unique access
    }

    fn outside_closure(x: &mut i32) {
        // Actions which require unique access
    }

    fn foo(a: &mut i32) {
        let mut bar = || {
            inside_closure(a)
        };
        outside_closure(a); // error: cannot borrow `*a` as mutable because previous
                            //        closure requires unique access.
        bar();
    }Run

To fix this error, you can finish using the closure before using the captured variable:

    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        let mut bar = || {
            inside_closure(a)
        };
        bar();
        // borrow on `a` ends.
        outside_closure(a); // ok!
    }Run

Or you can pass the variable as a parameter to the closure:

    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        let mut bar = |s: &mut i32| {
            inside_closure(s)
        };
        outside_closure(a);
        bar(a);
    }Run

It may be possible to define the closure later:

    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        outside_closure(a);
        let mut bar = || {
            inside_closure(a)
        };
        bar();
    }Run

## E0502

This error indicates that you are trying to borrow a variable as mutable when it has already been borrowed as immutable.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    fn bar(x: &mut i32) {}
    fn foo(a: &mut i32) {
        let ref y = a; // a is borrowed as immutable.
        bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
                //        as immutable
        println!("{}", y);
    }Run

To fix this error, ensure that you don't have any other references to the variable before trying to access it mutably:

    fn bar(x: &mut i32) {}
    fn foo(a: &mut i32) {
        bar(a);
        let ref y = a; // ok!
        println!("{}", y);
    }Run

For more information on the rust ownership system, take a look at https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html.

## E0503

A value was used after it was mutably borrowed.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    fn main() {
        let mut value = 3;
        // Create a mutable borrow of `value`.
        let borrow = &mut value;
        let _sum = value + 1; // error: cannot use `value` because
                              //        it was mutably borrowed
        println!("{}", borrow);
    }Run

In this example, `value` is mutably borrowed by `borrow` and cannot be used to calculate `sum`. This is not possible because this would violate Rust's mutability rules.

You can fix this error by finishing using the borrow before the next use of the value:

    fn main() {
        let mut value = 3;
        let borrow = &mut value;
        println!("{}", borrow);
        // The block has ended and with it the borrow.
        // You can now use `value` again.
        let _sum = value + 1;
    }Run

Or by cloning `value` before borrowing it:

    fn main() {
        let mut value = 3;
        // We clone `value`, creating a copy.
        let value_cloned = value.clone();
        // The mutable borrow is a reference to `value` and
        // not to `value_cloned`...
        let borrow = &mut value;
        // ... which means we can still use `value_cloned`,
        let _sum = value_cloned + 1;
        // even though the borrow only ends here.
        println!("{}", borrow);
    }Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

## E0504

#### Note: this error code is no longer emitted by the compiler.

This error occurs when an attempt is made to move a borrowed variable into a closure.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let fancy_num = FancyNum { num: 5 };
        let fancy_ref = &fancy_num;

        let x = move || {
            println!("child function: {}", fancy_num.num);
            // error: cannot move `fancy_num` into closure because it is borrowed
        };

        x();
        println!("main function: {}", fancy_ref.num);
    }Run

Here, `fancy_num` is borrowed by `fancy_ref` and so cannot be moved into the closure `x`. There is no way to move a value into a closure while it is borrowed, as that would invalidate the borrow.

If the closure can't outlive the value being moved, try using a reference rather than moving:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let fancy_num = FancyNum { num: 5 };
        let fancy_ref = &fancy_num;

        let x = move || {
            // fancy_ref is usable here because it doesn't move `fancy_num`
            println!("child function: {}", fancy_ref.num);
        };

        x();

        println!("main function: {}", fancy_num.num);
    }Run

If the value has to be borrowed and then moved, try limiting the lifetime of the borrow using a scoped block:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let fancy_num = FancyNum { num: 5 };

        {
            let fancy_ref = &fancy_num;
            println!("main function: {}", fancy_ref.num);
            // `fancy_ref` goes out of scope here
        }

        let x = move || {
            // `fancy_num` can be moved now (no more references exist)
            println!("child function: {}", fancy_num.num);
        };

        x();
    }Run

If the lifetime of a reference isn't enough, such as in the case of threading, consider using an `Arc` to create a reference-counted value:

    use std::sync::Arc;
    use std::thread;

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let fancy_ref1 = Arc::new(FancyNum { num: 5 });
        let fancy_ref2 = fancy_ref1.clone();

        let x = thread::spawn(move || {
            // `fancy_ref1` can be moved and has a `'static` lifetime
            println!("child thread: {}", fancy_ref1.num);
        });

        x.join().expect("child thread should finish");
        println!("main thread: {}", fancy_ref2.num);
    }Run

## E0505

A value was moved out while it was still borrowed.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Value {}

    fn borrow(val: &Value) {}

    fn eat(val: Value) {}

    fn main() {
        let x = Value{};
        let _ref_to_val: &Value = &x;
        eat(x);
        borrow(_ref_to_val);
    }Run

Here, the function `eat` takes ownership of `x`. However, `x` cannot be moved because the borrow to `_ref_to_val` needs to last till the function `borrow`. To fix that you can do a few different things:

- Try to avoid moving the variable.
- Release borrow before move.
- Implement the `Copy` trait on the type.

Examples:

    struct Value {}

    fn borrow(val: &Value) {}

    fn eat(val: &Value) {}

    fn main() {
        let x = Value{};

        let ref_to_val: &Value = &x;
        eat(&x); // pass by reference, if it's possible
        borrow(ref_to_val);
    }Run

Or:

    struct Value {}

    fn borrow(val: &Value) {}

    fn eat(val: Value) {}

    fn main() {
        let x = Value{};

        let ref_to_val: &Value = &x;
        borrow(ref_to_val);
        // ref_to_val is no longer used.
        eat(x);
    }Run

Or:

    #[derive(Clone, Copy)] // implement Copy trait
    struct Value {}

    fn borrow(val: &Value) {}

    fn eat(val: Value) {}

    fn main() {
        let x = Value{};
        let ref_to_val: &Value = &x;
        eat(x); // it will be copied here.
        borrow(ref_to_val);
    }Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

## E0506

This error occurs when an attempt is made to assign to a borrowed value.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy_num = FancyNum { num: 5 };
        let fancy_ref = &fancy_num;
        fancy_num = FancyNum { num: 6 };
        // error: cannot assign to `fancy_num` because it is borrowed

        println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
    }Run

Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't be assigned to a new value as it would invalidate the reference.

Alternatively, we can move out of `fancy_num` into a second `fancy_num`:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy_num = FancyNum { num: 5 };
        let moved_num = fancy_num;
        fancy_num = FancyNum { num: 6 };

        println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
    }Run

If the value has to be borrowed, try limiting the lifetime of the borrow using a scoped block:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy_num = FancyNum { num: 5 };

        {
            let fancy_ref = &fancy_num;
            println!("Ref: {}", fancy_ref.num);
        }

        // Works because `fancy_ref` is no longer in scope
        fancy_num = FancyNum { num: 6 };
        println!("Num: {}", fancy_num.num);
    }Run

Or by moving the reference into a function:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy_num = FancyNum { num: 5 };

        print_fancy_ref(&fancy_num);

        // Works because function borrow has ended
        fancy_num = FancyNum { num: 6 };
        println!("Num: {}", fancy_num.num);
    }

    fn print_fancy_ref(fancy_ref: &FancyNum){
        println!("Ref: {}", fancy_ref.num);
    }Run

## E0507

You tried to move out of a value which was borrowed. Erroneous code example:

ⓘThis example deliberately fails to compile

    use std::cell::RefCell;

    struct TheDarkKnight;

    impl TheDarkKnight {
        fn nothing_is_true(self) {}
    }

    fn main() {
        let x = RefCell::new(TheDarkKnight);

        x.borrow().nothing_is_true(); // error: cannot move out of borrowed content
    }Run

Here, the `nothing_is_true` method takes the ownership of `self`. However, `self` cannot be moved because `.borrow()` only provides an `&TheDarkKnight`, which is a borrow of the content owned by the `RefCell`. To fix this error, you have three choices:

- Try to avoid moving the variable.
- Somehow reclaim the ownership.
- Implement the `Copy` trait on the type.

Examples:

    use std::cell::RefCell;

    struct TheDarkKnight;

    impl TheDarkKnight {
        fn nothing_is_true(&self) {} // First case, we don't take ownership
    }

    fn main() {
        let x = RefCell::new(TheDarkKnight);

        x.borrow().nothing_is_true(); // ok!
    }Run

Or:

    use std::cell::RefCell;

    struct TheDarkKnight;

    impl TheDarkKnight {
        fn nothing_is_true(self) {}
    }

    fn main() {
        let x = RefCell::new(TheDarkKnight);
        let x = x.into_inner(); // we get back ownership

        x.nothing_is_true(); // ok!
    }Run

Or:

    use std::cell::RefCell;

    #[derive(Clone, Copy)] // we implement the Copy trait
    struct TheDarkKnight;

    impl TheDarkKnight {
        fn nothing_is_true(self) {}
    }

    fn main() {
        let x = RefCell::new(TheDarkKnight);

        x.borrow().nothing_is_true(); // ok!
    }Run

Moving a member out of a mutably borrowed struct will also cause E0507 error:

ⓘThis example deliberately fails to compile

    struct TheDarkKnight;

    impl TheDarkKnight {
        fn nothing_is_true(self) {}
    }

    struct Batcave {
        knight: TheDarkKnight
    }

    fn main() {
        let mut cave = Batcave {
            knight: TheDarkKnight
        };
        let borrowed = &mut cave;

        borrowed.knight.nothing_is_true(); // E0507
    }Run

It is fine only if you put something back. `mem::replace` can be used for that:

    use std::mem;

    let mut cave = Batcave {
        knight: TheDarkKnight
    };
    let borrowed = &mut cave;

    mem::replace(&mut borrowed.knight, TheDarkKnight).nothing_is_true(); // ok!Run

You can find more information about borrowing in the rust-book: http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html

## E0508

A value was moved out of a non-copy fixed-size array.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct NonCopy;

    fn main() {
        let array = [NonCopy; 1];
        let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`,
                               //        a non-copy fixed-size array
    }Run

The first element was moved out of the array, but this is not possible because `NonCopy` does not implement the `Copy` trait.

Consider borrowing the element instead of moving it:

    struct NonCopy;

    fn main() {
        let array = [NonCopy; 1];
        let _value = &array[0]; // Borrowing is allowed, unlike moving.
    }Run

Alternatively, if your type implements `Clone` and you need to own the value, consider borrowing and then cloning:

    #[derive(Clone)]
    struct NonCopy;

    fn main() {
        let array = [NonCopy; 1];
        // Now you can clone the array element.
        let _value = array[0].clone();
    }Run

## E0509

This error occurs when an attempt is made to move out of a value whose type implements the `Drop` trait.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct FancyNum {
        num: usize
    }

    struct DropStruct {
        fancy: FancyNum
    }

    impl Drop for DropStruct {
        fn drop(&mut self) {
            // Destruct DropStruct, possibly using FancyNum
        }
    }

    fn main() {
        let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
        let fancy_field = drop_struct.fancy; // Error E0509
        println!("Fancy: {}", fancy_field.num);
        // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
    }Run

Here, we tried to move a field out of a struct of type `DropStruct` which implements the `Drop` trait. However, a struct cannot be dropped if one or more of its fields have been moved.

Structs implementing the `Drop` trait have an implicit destructor that gets called when they go out of scope. This destructor may use the fields of the struct, so moving out of the struct could make it impossible to run the destructor. Therefore, we must think of all values whose type implements the `Drop` trait as single units whose fields cannot be moved.

This error can be fixed by creating a reference to the fields of a struct, enum, or tuple using the `ref` keyword:

    struct FancyNum {
        num: usize
    }

    struct DropStruct {
        fancy: FancyNum
    }

    impl Drop for DropStruct {
        fn drop(&mut self) {
            // Destruct DropStruct, possibly using FancyNum
        }
    }

    fn main() {
        let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
        let ref fancy_field = drop_struct.fancy; // No more errors!
        println!("Fancy: {}", fancy_field.num);
        // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
    }Run

Note that this technique can also be used in the arms of a match expression:

    struct FancyNum {
        num: usize
    }

    enum DropEnum {
        Fancy(FancyNum)
    }

    impl Drop for DropEnum {
        fn drop(&mut self) {
            // Destruct DropEnum, possibly using FancyNum
        }
    }

    fn main() {
        // Creates and enum of type `DropEnum`, which implements `Drop`
        let drop_enum = DropEnum::Fancy(FancyNum{num: 10});
        match drop_enum {
            // Creates a reference to the inside of `DropEnum::Fancy`
            DropEnum::Fancy(ref fancy_field) => // No error!
                println!("It was fancy-- {}!", fancy_field.num),
        }
        // implicit call to `drop_enum.drop()` as drop_enum goes out of scope
    }Run

## E0510

Cannot mutate place in this match guard.

When matching on a variable it cannot be mutated in the match guards, as this could cause the match to be non-exhaustive:

ⓘThis example deliberately fails to compile

    #![feature(nll, bind_by_move_pattern_guards)]
    let mut x = Some(0);
    match x {
        None => (),
        Some(_) if { x = None; false } => (),
        Some(v) => (), // No longer matches
    }Run

Here executing `x = None` would modify the value being matched and require us to go "back in time" to the `None` arm.

## E0511

Invalid monomorphization of an intrinsic function was used. Erroneous code example:

ⓘThis example is not tested

    #![feature(platform_intrinsics)]

    extern "platform-intrinsic" {
        fn simd_add<T>(a: T, b: T) -> T;
    }

    fn main() {
        unsafe { simd_add(0, 1); }
        // error: invalid monomorphization of `simd_add` intrinsic
    }Run

The generic type has to be a SIMD type. Example:

    #![feature(repr_simd)]
    #![feature(platform_intrinsics)]

    #[repr(simd)]
    #[derive(Copy, Clone)]
    struct i32x2(i32, i32);

    extern "platform-intrinsic" {
        fn simd_add<T>(a: T, b: T) -> T;
    }

    unsafe { simd_add(i32x2(0, 0), i32x2(1, 2)); } // ok!Run

## E0512

Transmute with two differently sized types was attempted. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn takes_u8(_: u8) {}

    fn main() {
        unsafe { takes_u8(::std::mem::transmute(0u16)); }
        // error: cannot transmute between types of different sizes,
        //        or dependently-sized types
    }Run

Please use types with same size or use the expected type directly. Example:

    fn takes_u8(_: u8) {}

    fn main() {
        unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
        // or:
        unsafe { takes_u8(0u8); } // ok!
    }Run

## E0514

No description.

## E0515

Cannot return value that references local variable

Local variables, function parameters and temporaries are all dropped before the end of the function body. So a reference to them cannot be returned.

ⓘThis example deliberately fails to compile

    #![feature(nll)]
    fn get_dangling_reference() -> &'static i32 {
        let x = 0;
        &x
    }Run

ⓘThis example deliberately fails to compile

    #![feature(nll)]
    use std::slice::Iter;
    fn get_dangling_iterator<'a>() -> Iter<'a, i32> {
        let v = vec![1, 2, 3];
        v.iter()
    }Run

Consider returning an owned value instead:

    use std::vec::IntoIter;

    fn get_integer() -> i32 {
        let x = 0;
        x
    }

    fn get_owned_iterator() -> IntoIter<i32> {
        let v = vec![1, 2, 3];
        v.into_iter()
    }Run

## E0516

The `typeof` keyword is currently reserved but unimplemented. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() {
        let x: typeof(92) = 92;
    }Run

Try using type inference instead. Example:

    fn main() {
        let x = 92;
    }Run

## E0517

This error indicates that a `#[repr(..)]` attribute was placed on an unsupported item.

Examples of erroneous code:

ⓘThis example deliberately fails to compile

    #[repr(C)]
    type Foo = u8;

    #[repr(packed)]
    enum Foo {Bar, Baz}

    #[repr(u8)]
    struct Foo {bar: bool, baz: bool}

    #[repr(C)]
    impl Foo {
        // ...
    }Run

- The `#[repr(C)]` attribute can only be placed on structs and enums.
- The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
- The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.

These attributes do not work on typedefs, since typedefs are just aliases.

Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the discriminant size for enums with no data fields on any of the variants, e.g. `enum Color {Red, Blue, Green}`, effectively setting the size of the enum to the size of the provided type. Such an enum can be cast to a value of the same type as well. In short, `#[repr(u8)]` makes the enum behave like an integer with a constrained set of allowed values.

Only field-less enums can be cast to numerical primitives, so this attribute will not apply to structs.

`#[repr(packed)]` reduces padding to make the struct size smaller. The representation of enums isn't strictly defined in Rust, and this attribute won't work on enums.

`#[repr(simd)]` will give a struct consisting of a homogeneous series of machine types (i.e., `u8`, `i32`, etc) a representation that permits vectorization via SIMD. This doesn't make much sense for enums since they don't consist of a single list of data.

## E0518

This error indicates that an `#[inline(..)]` attribute was incorrectly placed on something other than a function or method.

Examples of erroneous code:

ⓘThis example deliberately fails to compile

    #[inline(always)]
    struct Foo;

    #[inline(never)]
    impl Foo {
        // ...
    }Run

`#[inline]` hints the compiler whether or not to attempt to inline a method or function. By default, the compiler does a pretty good job of figuring this out itself, but if you feel the need for annotations, `#[inline(always)]` and `#[inline(never)]` can override or force the compiler's decision.

If you wish to apply this attribute to all methods in an impl, manually annotate each method; it is not possible to annotate the entire impl with an `#[inline]` attribute.

## E0519

No description.

## E0520

A non-default implementation was already made on this type so it cannot be specialized further. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(specialization)]

    trait SpaceLlama {
        fn fly(&self);
    }

    // applies to all T
    impl<T> SpaceLlama for T {
        default fn fly(&self) {}
    }

    // non-default impl
    // applies to all `Clone` T and overrides the previous impl
    impl<T: Clone> SpaceLlama for T {
        fn fly(&self) {}
    }

    // since `i32` is clone, this conflicts with the previous implementation
    impl SpaceLlama for i32 {
        default fn fly(&self) {}
        // error: item `fly` is provided by an `impl` that specializes
        //        another, but the item in the parent `impl` is not marked
        //        `default` and so it cannot be specialized.
    }Run

Specialization only allows you to override `default` functions in implementations.

To fix this error, you need to mark all the parent implementations as default. Example:

    #![feature(specialization)]

    trait SpaceLlama {
        fn fly(&self);
    }

    // applies to all T
    impl<T> SpaceLlama for T {
        default fn fly(&self) {} // This is a parent implementation.
    }

    // applies to all `Clone` T; overrides the previous impl
    impl<T: Clone> SpaceLlama for T {
        default fn fly(&self) {} // This is a parent implementation but was
                                 // previously not a default one, causing the error
    }

    // applies to i32, overrides the previous two impls
    impl SpaceLlama for i32 {
        fn fly(&self) {} // And now that's ok!
    }Run

## E0521

No description.

## E0522

The lang attribute is intended for marking special items that are built-in to Rust itself. This includes special traits (like `Copy` and `Sized`) that affect how the compiler behaves, as well as special functions that may be automatically invoked (such as the handler for out-of-bounds accesses when indexing a slice). Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(lang_items)]

    #[lang = "cookie"]
    fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
        loop {}
    }Run

## E0523

No description.

## E0524

No description.

## E0525

A closure was used but didn't implement the expected trait.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct X;

    fn foo<T>(_: T) {}
    fn bar<T: Fn(u32)>(_: T) {}

    fn main() {
        let x = X;
        let closure = |_| foo(x); // error: expected a closure that implements
                                  //        the `Fn` trait, but this closure only
                                  //        implements `FnOnce`
        bar(closure);
    }Run

In the example above, `closure` is an `FnOnce` closure whereas the `bar` function expected an `Fn` closure. In this case, it's simple to fix the issue, you just have to implement `Copy` and `Clone` traits on `struct X` and it'll be ok:

    #[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
    struct X;

    fn foo<T>(_: T) {}
    fn bar<T: Fn(u32)>(_: T) {}

    fn main() {
        let x = X;
        let closure = |_| foo(x);
        bar(closure); // ok!
    }Run

To understand better how closures work in Rust, read: https://doc.rust-lang.org/book/ch13-01-closures.html

## E0526

No description.

## E0527

The number of elements in an array or slice pattern differed from the number of elements in the array being matched.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    let r = &[1, 2, 3, 4];
    match r {
        &[a, b] => { // error: pattern requires 2 elements but array
                     //        has 4
            println!("a={}, b={}", a, b);
        }
    }Run

Ensure that the pattern is consistent with the size of the matched array. Additional elements can be matched with `..`:

    #![feature(slice_patterns)]

    let r = &[1, 2, 3, 4];
    match r {
        &[a, b, ..] => { // ok!
            println!("a={}, b={}", a, b);
        }
    }Run

## E0528

An array or slice pattern required more elements than were present in the matched array.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(slice_patterns)]

    let r = &[1, 2];
    match r {
        &[a, b, c, rest..] => { // error: pattern requires at least 3
                                //        elements but array has 2
            println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
        }
    }Run

Ensure that the matched array has at least as many elements as the pattern requires. You can match an arbitrary number of remaining elements with `..`:

    #![feature(slice_patterns)]

    let r = &[1, 2, 3, 4, 5];
    match r {
        &[a, b, c, rest..] => { // ok!
            // prints `a=1, b=2, c=3 rest=[4, 5]`
            println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
        }
    }Run

## E0529

An array or slice pattern was matched against some other type.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    let r: f32 = 1.0;
    match r {
        [a, b] => { // error: expected an array or slice, found `f32`
            println!("a={}, b={}", a, b);
        }
    }Run

Ensure that the pattern and the expression being matched on are of consistent types:

    let r = [1.0, 2.0];
    match r {
        [a, b] => { // ok!
            println!("a={}, b={}", a, b);
        }
    }Run

## E0530

A binding shadowed something it shouldn't.

Erroneous code example:

ⓘThis example deliberately fails to compile

    static TEST: i32 = 0;

    let r: (i32, i32) = (0, 0);
    match r {
        TEST => {} // error: match bindings cannot shadow statics
    }Run

To fix this error, just change the binding's name in order to avoid shadowing one of the following:

- struct name
- struct/enum variant
- static
- const
- associated const

Fixed example:

    static TEST: i32 = 0;

    let r: (i32, i32) = (0, 0);
    match r {
        something => {} // ok!
    }Run

## E0531

No description.

## E0532

Pattern arm did not match expected kind.

Erroneous code example:

ⓘThis example deliberately fails to compile

    enum State {
        Succeeded,
        Failed(String),
    }

    fn print_on_failure(state: &State) {
        match *state {
            // error: expected unit struct/variant or constant, found tuple
            //        variant `State::Failed`
            State::Failed => println!("Failed"),
            _ => ()
        }
    }Run

To fix this error, ensure the match arm kind is the same as the expression matched.

Fixed example:

    enum State {
        Succeeded,
        Failed(String),
    }

    fn print_on_failure(state: &State) {
        match *state {
            State::Failed(ref msg) => println!("Failed with {}", msg),
            _ => ()
        }
    }Run

## E0533

No description.

## E0534

The `inline` attribute was malformed.

Erroneous code example:

ⓘThis example is not tested

    #[inline()] // error: expected one argument
    pub fn something() {}

    fn main() {}Run

The parenthesized `inline` attribute requires the parameter to be specified:

    #[inline(always)]
    fn something() {}Run

or:

    #[inline(never)]
    fn something() {}Run

Alternatively, a paren-less version of the attribute may be used to hint the compiler about inlining opportunity:

    #[inline]
    fn something() {}Run

For more information about the inline attribute, read: https://doc.rust-lang.org/reference.html#inline-attributes

## E0535

An unknown argument was given to the `inline` attribute.

Erroneous code example:

ⓘThis example is not tested

    #[inline(unknown)] // error: invalid argument
    pub fn something() {}

    fn main() {}Run

The `inline` attribute only supports two arguments:

- always
- never

All other arguments given to the `inline` attribute will return this error. Example:

    #[inline(never)] // ok!
    pub fn something() {}

    fn main() {}Run

For more information about the inline attribute, https: read://doc.rust-lang.org/reference.html#inline-attributes

## E0536

The `not` cfg-predicate was malformed.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[cfg(not())] // error: expected 1 cfg-pattern
    pub fn something() {}

    pub fn main() {}Run

The `not` predicate expects one cfg-pattern. Example:

    #[cfg(not(target_os = "linux"))] // ok!
    pub fn something() {}

    pub fn main() {}Run

For more information about the cfg attribute, read: https://doc.rust-lang.org/reference.html#conditional-compilation

## E0537

An unknown predicate was used inside the `cfg` attribute.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[cfg(unknown())] // error: invalid predicate `unknown`
    pub fn something() {}

    pub fn main() {}Run

The `cfg` attribute supports only three kinds of predicates:

- any
- all
- not

Example:

    #[cfg(not(target_os = "linux"))] // ok!
    pub fn something() {}

    pub fn main() {}Run

For more information about the cfg attribute, read: https://doc.rust-lang.org/reference.html#conditional-compilation

## E0538

Attribute contains same meta item more than once.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[deprecated(
        since="1.0.0",
        note="First deprecation note.",
        note="Second deprecation note." // error: multiple same meta item
    )]
    fn deprecated_function() {}Run

Meta items are the key-value pairs inside of an attribute. Each key may only be used once in each attribute.

To fix the problem, remove all but one of the meta items with the same key.

Example:

    #[deprecated(
        since="1.0.0",
        note="First deprecation note."
    )]
    fn deprecated_function() {}Run

## E0539

No description.

## E0540

No description.

## E0541

An unknown meta item was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[deprecated(
        since="1.0.0",
        // error: unknown meta item
        reason="Example invalid meta item. Should be 'note'")
    ]
    fn deprecated_function() {}Run

Meta items are the key-value pairs inside of an attribute. The keys provided must be one of the valid keys for the specified attribute.

To fix the problem, either remove the unknown meta item, or rename it if you provided the wrong name.

In the erroneous code example above, the wrong name was provided, so changing to a correct one it will fix the error. Example:

    #[deprecated(
        since="1.0.0",
        note="This is a valid meta item for the deprecated attribute."
    )]
    fn deprecated_function() {}Run

## E0542

No description.

## E0543

No description.

## E0544

No description.

## E0545

No description.

## E0546

No description.

## E0547

No description.

## E0549

No description.

## E0550

No description.

## E0551

No description.

## E0552

A unrecognized representation attribute was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(D)] // error: unrecognized representation hint
    struct MyStruct {
        my_field: usize
    }Run

You can use a `repr` attribute to tell the compiler how you want a struct or enum to be laid out in memory.

Make sure you're using one of the supported options:

    #[repr(C)] // ok!
    struct MyStruct {
        my_field: usize
    }Run

For more information about specifying representations, see the "Alternative Representations" section of the Rustonomicon.

## E0553

No description.

## E0554

Feature attributes are only allowed on the nightly release channel. Stable or beta compilers will not comply.

Example of erroneous code (on a stable compiler):

ⓘThis example is not tested

    #![feature(non_ascii_idents)] // error: #![feature] may not be used on the
                                  //        stable release channelRun

If you need the feature, make sure to use a nightly release of the compiler (but be warned that the feature may be removed or altered in the future).

## E0556

No description.

## E0557

A feature attribute named a feature that has been removed.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(managed_boxes)] // error: feature has been removedRun

Delete the offending feature attribute.

## E0559

An unknown field was specified into an enum's structure variant.

Erroneous code example:

ⓘThis example deliberately fails to compile

    enum Field {
        Fool { x: u32 },
    }

    let s = Field::Fool { joke: 0 };
    // error: struct variant `Field::Fool` has no field named `joke`Run

Verify you didn't misspell the field's name or that the field exists. Example:

    enum Field {
        Fool { joke: u32 },
    }

    let s = Field::Fool { joke: 0 }; // ok!Run

## E0560

An unknown field was specified into a structure.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Simba {
        mother: u32,
    }

    let s = Simba { mother: 1, father: 0 };
    // error: structure `Simba` has no field named `father`Run

Verify you didn't misspell the field's name or that the field exists. Example:

    struct Simba {
        mother: u32,
        father: u32,
    }

    let s = Simba { mother: 1, father: 0 }; // ok!Run

## E0561

No description.

## E0562

Abstract return types (written `impl Trait` for some trait `Trait`) are only allowed as function and inherent impl return types.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() {
        let count_to_ten: impl Iterator<Item=usize> = 0..10;
        // error: `impl Trait` not allowed outside of function and inherent method
        //        return types
        for i in count_to_ten {
            println!("{}", i);
        }
    }Run

Make sure `impl Trait` only appears in return-type position.

    fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
        0..n
    }

    fn main() {
        for i in count_to_n(10) {  // ok!
            println!("{}", i);
        }
    }Run

See RFC 1522 for more details.

## E0564

No description.

## E0565

A literal was used in a built-in attribute that doesn't support literals.

Erroneous code example:

ⓘThis example is not tested

    #[inline("always")] // error: unsupported literal
    pub fn something() {}Run

Literals in attributes are new and largely unsupported in built-in attributes. Work to support literals where appropriate is ongoing. Try using an unquoted name instead:

    #[inline(always)]
    pub fn something() {}Run

## E0566

No description.

## E0567

No description.

## E0568

No description.

## E0569

If an impl has a generic parameter with the `#[may_dangle]` attribute, then that impl must be declared as an `unsafe impl.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(dropck_eyepatch)]

    struct Foo<X>(X);
    impl<#[may_dangle] X> Drop for Foo<X> {
        fn drop(&mut self) { }
    }Run

In this example, we are asserting that the destructor for `Foo` will not access any data of type `X`, and require this assertion to be true for overall safety in our program. The compiler does not currently attempt to verify this assertion; therefore we must tag this `impl` as unsafe.

## E0570

The requested ABI is unsupported by the current target.

The rust compiler maintains for each target a blacklist of ABIs unsupported on that target. If an ABI is present in such a list this usually means that the target / ABI combination is currently unsupported by llvm.

If necessary, you can circumvent this check using custom target specifications.

## E0571

A `break` statement with an argument appeared in a non-`loop` loop.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    let result = while true {
        if satisfied(i) {
            break 2*i; // error: `break` with value from a `while` loop
        }
        i += 1;
    };Run

The `break` statement can take an argument (which will be the value of the loop expression if the `break` statement is executed) in `loop` loops, but not `for`, `while`, or `while let` loops.

Make sure `break value;` statements only occur in `loop` loops:

    let result = loop { // ok!
        if satisfied(i) {
            break 2*i;
        }
        i += 1;
    };Run

## E0572

A return statement was found outside of a function body.

Erroneous code example:

ⓘThis example deliberately fails to compile

    const FOO: u32 = return 0; // error: return statement outside of function body

    fn main() {}Run

To fix this issue, just remove the return keyword or move the expression into a function. Example:

    const FOO: u32 = 0;

    fn some_fn() -> u32 {
        return FOO;
    }

    fn main() {
        some_fn();
    }Run

## E0573

No description.

## E0574

No description.

## E0575

No description.

## E0576

No description.

## E0577

No description.

## E0578

No description.

## E0579

When matching against an exclusive range, the compiler verifies that the range is non-empty. Exclusive range patterns include the start point but not the end point, so this is equivalent to requiring the start of the range to be less than the end of the range.

For example:

ⓘThis example deliberately fails to compile

    match 5u32 {
        // This range is ok, albeit pointless.
        1 .. 2 => {}
        // This range is empty, and the compiler can tell.
        5 .. 5 => {}
    }Run

## E0580

The `main` function was incorrectly declared.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main(x: i32) { // error: main function has wrong type
        println!("{}", x);
    }Run

The `main` function prototype should never take arguments. Example:

    fn main() {
        // your code
    }Run

If you want to get command-line arguments, use `std::env::args`. To exit with a specified exit code, use `std::process::exit`.

## E0581

In a `fn` type, a lifetime appears only in the return type, and not in the arguments types.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() {
        // Here, `'a` appears only in the return type:
        let x: for<'a> fn() -> &'a i32;
    }Run

To fix this issue, either use the lifetime in the arguments, or use `'static`. Example:

    fn main() {
        // Here, `'a` appears only in the return type:
        let x: for<'a> fn(&'a i32) -> &'a i32;
        let y: fn() -> &'static i32;
    }Run

Note: The examples above used to be (erroneously) accepted by the compiler, but this was since corrected. See issue #33685 for more details.

## E0582

A lifetime appears only in an associated-type binding, and not in the input types to the trait.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn bar<F>(t: F)
        // No type can satisfy this requirement, since `'a` does not
        // appear in any of the input types (here, `i32`):
        where F: for<'a> Fn(i32) -> Option<&'a i32>
    {
    }

    fn main() { }Run

To fix this issue, either use the lifetime in the inputs, or use `'static`. Example:

    fn bar<F, G>(t: F, u: G)
        where F: for<'a> Fn(&'a i32) -> Option<&'a i32>,
              G: Fn(i32) -> Option<&'static i32>,
    {
    }

    fn main() { }Run

Note: The examples above used to be (erroneously) accepted by the compiler, but this was since corrected. See issue #33685 for more details.

## E0583

A file wasn't found for an out-of-line module.

Erroneous code example:

ⓘThis example is not tested

    mod file_that_doesnt_exist; // error: file not found for module

    fn main() {}Run

Please be sure that a file corresponding to the module exists. If you want to use a module named `file_that_doesnt_exist`, you need to have a file named `file_that_doesnt_exist.rs` or `file_that_doesnt_exist/mod.rs` in the same directory.

## E0584

No description.

## E0585

A documentation comment that doesn't document anything was found.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() {
        // The following doc comment will fail:
        /// This is a useless doc comment!
    }Run

Documentation comments need to be followed by items, including functions, types, modules, etc. Examples:

    /// I'm documenting the following struct:
    struct Foo;

    /// I'm documenting the following function:
    fn foo() {}Run

## E0586

An inclusive range was used with no end.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() {
        let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
        let x = &tmp[1..=]; // error: inclusive range was used with no end
    }Run

An inclusive range needs an end in order to include it. If you just need a start and no end, use a non-inclusive range (with `..`):

    fn main() {
        let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
        let x = &tmp[1..]; // ok!
    }Run

Or put an end to your inclusive range:

    fn main() {
        let tmp = vec![0, 1, 2, 3, 4, 4, 3, 3, 2, 1];
        let x = &tmp[1..=3]; // ok!
    }Run

## E0587

No description.

## E0588

No description.

## E0589

The value of `N` that was specified for `repr(align(N))` was not a power of two, or was greater than 2^29.

ⓘThis example deliberately fails to compile

    #[repr(align(15))] // error: invalid `repr(align)` attribute: not a power of two
    enum Foo {
        Bar(u64),
    }Run

## E0590

`break` or `continue` must include a label when used in the condition of a `while` loop.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    while break {}Run

To fix this, add a label specifying which loop is being broken out of:

    'foo: while break 'foo {}Run

## E0591

Per RFC 401, if you have a function declaration `foo`:

    // For the purposes of this explanation, all of these
    // different kinds of `fn` declarations are equivalent:
    struct S;
    fn foo(x: S) { /* ... */ }
    extern "C" { fn foo(x: S); }
    impl S { fn foo(self) { /* ... */ } }Run

the type of `foo` is not `fn(S)`, as one might expect. Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`. However, `typeof(foo)` can be coerced to a function pointer `fn(S)`, so you rarely notice this:

    let x: fn(S) = foo; // OK, coercesRun

The reason that this matter is that the type `fn(S)` is not specific to any particular function: it's a function pointer. So calling `x()` results in a virtual call, whereas `foo()` is statically dispatched, because the type of `foo` tells us precisely what function is being called.

As noted above, coercions mean that most code doesn't have to be concerned with this distinction. However, you can tell the difference when using transmute to convert a fn item into a fn pointer.

This is sometimes done as part of an FFI:

ⓘThis example deliberately fails to compile

    extern "C" fn foo(userdata: Box<i32>) {
        /* ... */
    }

    let f: extern "C" fn(*mut i32) = transmute(foo);
    callback(f);Run

Here, transmute is being used to convert the types of the fn arguments. This pattern is incorrect because, because the type of `foo` is a function item (`typeof(foo)`), which is zero-sized, and the target type (`fn()`) is a function pointer, which is not zero-sized. This pattern should be rewritten. There are a few possible ways to do this:

- change the original fn declaration to match the expected signature, and do the cast in the fn body (the preferred option)

- cast the fn item fo a fn pointer before calling transmute, as shown here:

      let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
      let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works tooRun

The same applies to transmutes to `*mut fn()`, which were observed in practice. Note though that use of this type is generally incorrect. The intention is typically to describe a function pointer, but just `fn()` alone suffices for that. `*mut fn()` is a pointer to a fn pointer. (Since these values are typically just passed to C code, however, this rarely makes a difference in practice.)

## E0592

This error occurs when you defined methods or associated functions with same name.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo;

    impl Foo {
        fn bar() {} // previous definition here
    }

    impl Foo {
        fn bar() {} // duplicate definition here
    }Run

A similar error is E0201. The difference is whether there is one declaration block or not. To avoid this error, you must give each `fn` a unique name.

    struct Foo;

    impl Foo {
        fn bar() {}
    }

    impl Foo {
        fn baz() {} // define with different name
    }Run

## E0593

You tried to supply an `Fn`-based type with an incorrect number of arguments than what was expected.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo<F: Fn()>(x: F) { }

    fn main() {
        // [E0593] closure takes 1 argument but 0 arguments are required
        foo(|y| { });
    }Run

## E0594

No description.

## E0595

#### Note: this error code is no longer emitted by the compiler.

Closures cannot mutate immutable captured variables.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = 3; // error: closure cannot assign to immutable local variable `x`
    let mut c = || { x += 1 };Run

Make the variable binding mutable:

    let mut x = 3; // ok!
    let mut c = || { x += 1 };Run

## E0596

This error occurs because you tried to mutably borrow a non-mutable variable.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    let x = 1;
    let y = &mut x; // error: cannot borrow mutablyRun

In here, `x` isn't mutable, so when we try to mutably borrow it in `y`, it fails. To fix this error, you need to make `x` mutable:

    let mut x = 1;
    let y = &mut x; // ok!Run

## E0597

This error occurs because a value was dropped while it was still borrowed

Example of erroneous code:

ⓘThis example deliberately fails to compile

    struct Foo<'a> {
        x: Option<&'a u32>,
    }

    let mut x = Foo { x: None };
    {
        let y = 0;
        x.x = Some(&y); // error: `y` does not live long enough
    }
    println!("{:?}", x.x);Run

In here, `y` is dropped at the end of the inner scope, but it is borrowed by `x` until the `println`. To fix the previous example, just remove the scope so that `y` isn't dropped until after the println

    struct Foo<'a> {
        x: Option<&'a u32>,
    }

    let mut x = Foo { x: None };

    let y = 0;
    x.x = Some(&y);

    println!("{:?}", x.x);Run

## E0598

No description.

## E0599

This error occurs when a method is used on a type which doesn't implement it:

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Mouth;

    let x = Mouth;
    x.chocolate(); // error: no method named `chocolate` found for type `Mouth`
                   //        in the current scopeRun

## E0600

An unary operator was used on a type which doesn't implement it.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    enum Question {
        Yes,
        No,
    }

    !Question::Yes; // error: cannot apply unary operator `!` to type `Question`Run

In this case, `Question` would need to implement the `std::ops::Not` trait in order to be able to use `!` on it. Let's implement it:

    use std::ops::Not;

    enum Question {
        Yes,
        No,
    }

    // We implement the `Not` trait on the enum.
    impl Not for Question {
        type Output = bool;

        fn not(self) -> bool {
            match self {
                Question::Yes => false, // If the `Answer` is `Yes`, then it
                                        // returns false.
                Question::No => true, // And here we do the opposite.
            }
        }
    }

    assert_eq!(!Question::Yes, false);
    assert_eq!(!Question::No, true);Run

## E0601

No `main` function was found in a binary crate. To fix this error, add a `main` function. For example:

    fn main() {
        // Your program will start here.
        println!("Hello world!");
    }Run

If you don't know the basics of Rust, you can go look to the Rust Book to get started: https://doc.rust-lang.org/book/

## E0602

An unknown lint was used on the command line.

Erroneous example:

    rustc -D bogus omse_file.rs

Maybe you just misspelled the lint name or the lint doesn't exist anymore. Either way, try to update/remove it in order to fix the error.

## E0603

A private item was used outside its scope.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod SomeModule {
        const PRIVATE: u32 = 0x_a_bad_1dea_u32; // This const is private, so we
                                                // can't use it outside of the
                                                // `SomeModule` module.
    }

    println!("const value: {}", SomeModule::PRIVATE); // error: constant `PRIVATE`
                                                      //        is privateRun

In order to fix this error, you need to make the item public by using the `pub` keyword. Example:

    mod SomeModule {
        pub const PRIVATE: u32 = 0x_a_bad_1dea_u32; // We set it public by using the
                                                    // `pub` keyword.
    }

    println!("const value: {}", SomeModule::PRIVATE); // ok!Run

## E0604

A cast to `char` was attempted on a type other than `u8`.

Erroneous code example:

ⓘThis example deliberately fails to compile

    0u32 as char; // error: only `u8` can be cast as `char`, not `u32`Run

As the error message indicates, only `u8` can be cast into `char`. Example:

    let c = 86u8 as char; // ok!
    assert_eq!(c, 'V');Run

For more information about casts, take a look at the Type cast section in The Reference Book.

## E0605

An invalid cast was attempted.

Erroneous code examples:

ⓘThis example deliberately fails to compile

    let x = 0u8;
    x as Vec<u8>; // error: non-primitive cast: `u8` as `std::vec::Vec<u8>`

    // Another example

    let v = core::ptr::null::<u8>(); // So here, `v` is a `*const u8`.
    v as &u8; // error: non-primitive cast: `*const u8` as `&u8`Run

Only primitive types can be cast into each other. Examples:

    let x = 0u8;
    x as u32; // ok!

    let v = core::ptr::null::<u8>();
    v as *const i8; // ok!Run

For more information about casts, take a look at the Type cast section in The Reference Book.

## E0606

An incompatible cast was attempted.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = &0u8; // Here, `x` is a `&u8`.
    let y: u32 = x as u32; // error: casting `&u8` as `u32` is invalidRun

When casting, keep in mind that only primitive types can be cast into each other. Example:

    let x = &0u8;
    let y: u32 = *x as u32; // We dereference it first and then cast it.Run

For more information about casts, take a look at the Type cast section in The Reference Book.

## E0607

A cast between a thin and a fat pointer was attempted.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let v = core::ptr::null::<u8>();
    v as *const [u8];Run

First: what are thin and fat pointers?

Thin pointers are "simple" pointers: they are purely a reference to a memory address.

Fat pointers are pointers referencing Dynamically Sized Types (also called DST). DST don't have a statically known size, therefore they can only exist behind some kind of pointers that contain additional information. Slices and trait objects are DSTs. In the case of slices, the additional information the fat pointer holds is their size.

To fix this error, don't try to cast directly between thin and fat pointers.

For more information about casts, take a look at the Type cast section in The Reference Book.

## E0608

An attempt to index into a type which doesn't implement the `std::ops::Index` trait was performed.

Erroneous code example:

ⓘThis example deliberately fails to compile

    0u8[2]; // error: cannot index into a value of type `u8`Run

To be able to index into a type it needs to implement the `std::ops::Index` trait. Example:

    let v: Vec<u8> = vec![0, 1, 2, 3];

    // The `Vec` type implements the `Index` trait so you can do:
    println!("{}", v[2]);Run

## E0609

Attempted to access a non-existent field in a struct.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct StructWithFields {
        x: u32,
    }

    let s = StructWithFields { x: 0 };
    println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`Run

To fix this error, check that you didn't misspell the field's name or that the field actually exists. Example:

    struct StructWithFields {
        x: u32,
    }

    let s = StructWithFields { x: 0 };
    println!("{}", s.x); // ok!Run

## E0610

Attempted to access a field on a primitive type.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let x: u32 = 0;
    println!("{}", x.foo); // error: `{integer}` is a primitive type, therefore
                           //        doesn't have fieldsRun

Primitive types are the most basic types available in Rust and don't have fields. To access data via named fields, struct types are used. Example:

    // We declare struct called `Foo` containing two fields:
    struct Foo {
        x: u32,
        y: i64,
    }

    // We create an instance of this struct:
    let variable = Foo { x: 0, y: -12 };
    // And we can now access its fields:
    println!("x: {}, y: {}", variable.x, variable.y);Run

For more information about primitives and structs, take a look at The Book: https://doc.rust-lang.org/book/ch03-02-data-types.html https://doc.rust-lang.org/book/ch05-00-structs.html

## E0614

Attempted to dereference a variable which cannot be dereferenced.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let y = 0u32;
    *y; // error: type `u32` cannot be dereferencedRun

Only types implementing `std::ops::Deref` can be dereferenced (such as `&T`). Example:

    let y = 0u32;
    let x = &y;
    // So here, `x` is a `&u32`, so we can dereference it:
    *x; // ok!Run

## E0615

Attempted to access a method like a field.

Erroneous code example:

ⓘThis example deliberately fails to compile

    struct Foo {
        x: u32,
    }

    impl Foo {
        fn method(&self) {}
    }

    let f = Foo { x: 0 };
    f.method; // error: attempted to take value of method `method` on type `Foo`Run

If you want to use a method, add `()` after it:

    f.method();Run

However, if you wanted to access a field of a struct check that the field name is spelled correctly. Example:

    println!("{}", f.x);Run

## E0616

Attempted to access a private field on a struct.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod some_module {
        pub struct Foo {
            x: u32, // So `x` is private in here.
        }

        impl Foo {
            pub fn new() -> Foo { Foo { x: 0 } }
        }
    }

    let f = some_module::Foo::new();
    println!("{}", f.x); // error: field `x` of struct `some_module::Foo` is privateRun

If you want to access this field, you have two options:

1. Set the field public:

    mod some_module {
        pub struct Foo {
            pub x: u32, // `x` is now public.
        }

        impl Foo {
            pub fn new() -> Foo { Foo { x: 0 } }
        }
    }

    let f = some_module::Foo::new();
    println!("{}", f.x); // ok!Run

2. Add a getter function:

    mod some_module {
        pub struct Foo {
            x: u32, // So `x` is still private in here.
        }

        impl Foo {
            pub fn new() -> Foo { Foo { x: 0 } }

            // We create the getter function here:
            pub fn get_x(&self) -> &u32 { &self.x }
        }
    }

    let f = some_module::Foo::new();
    println!("{}", f.get_x()); // ok!Run

## E0617

Attempted to pass an invalid type of variable into a variadic function.

Erroneous code example:

ⓘThis example deliberately fails to compile

    extern {
        fn printf(c: *const i8, ...);
    }

    unsafe {
        printf(::std::ptr::null(), 0f32);
        // error: can't pass an `f32` to variadic function, cast to `c_double`
    }Run

Certain Rust types must be cast before passing them to a variadic function, because of arcane ABI rules dictated by the C standard. To fix the error, cast the value to the type specified by the error message (which you may need to import from `std::os::raw`).

## E0618

Attempted to call something which isn't a function nor a method.

Erroneous code examples:

ⓘThis example deliberately fails to compile

    enum X {
        Entry,
    }

    X::Entry(); // error: expected function, found `X::Entry`

    // Or even simpler:
    let x = 0i32;
    x(); // error: expected function, found `i32`Run

Only functions and methods can be called using `()`. Example:

    // We declare a function:
    fn i_am_a_function() {}

    // And we call it:
    i_am_a_function();Run

## E0619

#### Note: this error code is no longer emitted by the compiler.

The type-checker needed to know the type of an expression, but that type had not yet been inferred.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let mut x = vec![];
    match x.pop() {
        Some(v) => {
            // Here, the type of `v` is not (yet) known, so we
            // cannot resolve this method call:
            v.to_uppercase(); // error: the type of this value must be known in
                              //        this context
        }
        None => {}
    }Run

Type inference typically proceeds from the top of the function to the bottom, figuring out types as it goes. In some cases -- notably method calls and overloadable operators like `*` -- the type checker may not have enough information yet to make progress. This can be true even if the rest of the function provides enough context (because the type-checker hasn't looked that far ahead yet). In this case, type annotations can be used to help it along.

To fix this error, just specify the type of the variable. Example:

    let mut x: Vec<String> = vec![]; // We precise the type of the vec elements.
    match x.pop() {
        Some(v) => {
            v.to_uppercase(); // Since rustc now knows the type of the vec elements,
                              // we can use `v`'s methods.
        }
        None => {}
    }Run

## E0620

A cast to an unsized type was attempted.

Erroneous code example:

ⓘThis example deliberately fails to compile

    let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
                                      //        as `[usize]`Run

In Rust, some types don't have a known size at compile-time. For example, in a slice type like `[u32]`, the number of elements is not known at compile-time and hence the overall size cannot be computed. As a result, such types can only be manipulated through a reference (e.g., `&T` or `&mut T`) or other pointer-type (e.g., `Box` or `Rc`). Try casting to a reference instead:

    let x = &[1_usize, 2] as &[usize]; // ok!Run

## E0621

This error code indicates a mismatch between the lifetimes appearing in the function signature (i.e., the parameter types and the return type) and the data-flow found in the function body.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
                                                 //        required in the type of
                                                 //        `y`
        if x > y { x } else { y }
    }Run

In the code above, the function is returning data borrowed from either `x` or `y`, but the `'a` annotation indicates that it is returning data only from `x`. To fix the error, the signature and the body must be made to match. Typically, this is done by updating the function signature. So, in this case, we change the type of `y` to `&'a i32`, like so:

    fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
        if x > y { x } else { y }
    }Run

Now the signature indicates that the function data borrowed from either `x` or `y`. Alternatively, you could change the body to not return data from `y`:

    fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
        x
    }Run

## E0622

An intrinsic was declared without being a function.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(intrinsics)]
    extern "rust-intrinsic" {
        pub static breakpoint : unsafe extern "rust-intrinsic" fn();
        // error: intrinsic must be a function
    }

    fn main() { unsafe { breakpoint(); } }Run

An intrinsic is a function available for use in a given programming language whose implementation is handled specially by the compiler. In order to fix this error, just declare a function.

## E0623

No description.

## E0624

A private item was used outside of its scope.

Erroneous code example:

ⓘThis example deliberately fails to compile

    mod inner {
        pub struct Foo;

        impl Foo {
            fn method(&self) {}
        }
    }

    let foo = inner::Foo;
    foo.method(); // error: method `method` is privateRun

Two possibilities are available to solve this issue:

1. Only use the item in the scope it has been defined:

    mod inner {
        pub struct Foo;

        impl Foo {
            fn method(&self) {}
        }

        pub fn call_method(foo: &Foo) { // We create a public function.
            foo.method(); // Which calls the item.
        }
    }

    let foo = inner::Foo;
    inner::call_method(&foo); // And since the function is public, we can call the
                              // method through it.Run

2. Make the item public:

    mod inner {
        pub struct Foo;

        impl Foo {
            pub fn method(&self) {} // It's now public.
        }
    }

    let foo = inner::Foo;
    foo.method(); // Ok!Run

## E0625

No description.

## E0626

This error occurs because a borrow in a generator persists across a yield point.

ⓘThis example deliberately fails to compile

    let mut b = || {
        let a = &String::new(); // <-- This borrow...
        yield (); // ...is still in scope here, when the yield occurs.
        println!("{}", a);
    };
    Pin::new(&mut b).resume();Run

At present, it is not permitted to have a yield that occurs while a borrow is still in scope. To resolve this error, the borrow must either be "contained" to a smaller scope that does not overlap the yield or else eliminated in another way. So, for example, we might resolve the previous example by removing the borrow and just storing the integer by value:

    let mut b = || {
        let a = 3;
        yield ();
        println!("{}", a);
    };
    Pin::new(&mut b).resume();Run

This is a very simple case, of course. In more complex cases, we may wish to have more than one reference to the value that was borrowed -- in those cases, something like the `Rc` or `Arc` types may be useful.

This error also frequently arises with iteration:

ⓘThis example deliberately fails to compile

    let mut b = || {
      let v = vec![1,2,3];
      for &x in &v { // <-- borrow of `v` is still in scope...
        yield x; // ...when this yield occurs.
      }
    };
    Pin::new(&mut b).resume();Run

Such cases can sometimes be resolved by iterating "by value" (or using `into_iter()`) to avoid borrowing:

    let mut b = || {
      let v = vec![1,2,3];
      for x in v { // <-- Take ownership of the values instead!
        yield x; // <-- Now yield is OK.
      }
    };
    Pin::new(&mut b).resume();Run

If taking ownership is not an option, using indices can work too:

    let mut b = || {
      let v = vec![1,2,3];
      let len = v.len(); // (*)
      for i in 0..len {
        let x = v[i]; // (*)
        yield x; // <-- Now yield is OK.
      }
    };
    Pin::new(&mut b).resume();

    // (*) -- Unfortunately, these temporaries are currently required.
    // See <https://github.com/rust-lang/rust/issues/43122>.Run

## E0627

No description.

## E0628

No description.

## E0629

No description.

## E0630

No description.

## E0631

No description.

## E0632

No description.

## E0633

The `unwind` attribute was malformed.

Erroneous code example:

ⓘThis example is not tested

    #[unwind()] // error: expected one argument
    pub extern fn something() {}

    fn main() {}Run

The `#[unwind]` attribute should be used as follows:

- `#[unwind(aborts)]` -- specifies that if a non-Rust ABI function should abort the process if it attempts to unwind. This is the safer and preferred option.

- `#[unwind(allowed)]` -- specifies that a non-Rust ABI function should be allowed to unwind. This can easily result in Undefined Behavior (UB), so be careful.

NB. The default behavior here is "allowed", but this is unspecified and likely to change in the future.

## E0634

No description.

## E0635

The `#![feature]` attribute specified an unknown feature.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(nonexistent_rust_feature)] // error: unknown featureRun

## E0636

A `#![feature]` attribute was declared multiple times.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![allow(stable_features)]
    #![feature(rust1)]
    #![feature(rust1)] // error: the feature `rust1` has already been declaredRun

## E0637

No description.

## E0638

This error indicates that the struct, enum or enum variant must be matched non-exhaustively as it has been marked as `non_exhaustive`.

When applied within a crate, downstream users of the crate will need to use the `_` pattern when matching enums and use the `..` pattern when matching structs. Downstream crates cannot match against non-exhaustive enum variants.

For example, in the below example, since the enum is marked as `non_exhaustive`, it is required that downstream crates match non-exhaustively on it.

ⓘThis example is not tested

    use std::error::Error as StdError;

    #[non_exhaustive] pub enum Error {
       Message(String),
       Other,
    }

    impl StdError for Error {
       fn description(&self) -> &str {
            // This will not error, despite being marked as non_exhaustive, as this
            // enum is defined within the current crate, it can be matched
            // exhaustively.
            match *self {
               Message(ref s) => s,
               Other => "other or unknown error",
            }
       }
    }Run

An example of matching non-exhaustively on the above enum is provided below:

ⓘThis example is not tested

    use mycrate::Error;

    // This will not error as the non_exhaustive Error enum has been matched with a
    // wildcard.
    match error {
       Message(ref s) => ...,
       Other => ...,
       _ => ...,
    }Run

Similarly, for structs, match with `..` to avoid this error.

## E0639

This error indicates that the struct, enum or enum variant cannot be instantiated from outside of the defining crate as it has been marked as `non_exhaustive` and as such more fields/variants may be added in future that could cause adverse side effects for this code.

It is recommended that you look for a `new` function or equivalent in the crate's documentation.

## E0640

No description.

## E0641

No description.

## E0642

Trait methods currently cannot take patterns as arguments.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo((x, y): (i32, i32)); // error: patterns aren't allowed
                                    //        in trait methods
    }Run

You can instead use a single name for the argument:

    trait Foo {
        fn foo(x_and_y: (i32, i32)); // ok!
    }Run

## E0643

This error indicates that there is a mismatch between generic parameters and impl Trait parameters in a trait declaration versus its impl.

ⓘThis example deliberately fails to compile

    trait Foo {
        fn foo(&self, _: &impl Iterator);
    }
    impl Foo for () {
        fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible
                                              // signature for trait
    }Run

## E0644

A closure or generator was constructed that references its own type.

Erroneous example:

    fn fix<F>(f: &F)
      where F: Fn(&F)
    {
      f(&f);
    }

    fn main() {
      fix(&|y| {
        // Here, when `x` is called, the parameter `y` is equal to `x`.
      });
    }

Rust does not permit a closure to directly reference its own type, either through an argument (as in the example above) or by capturing itself through its environment. This restriction helps keep closure inference tractable.

The easiest fix is to rewrite your closure into a top-level function, or into a method. In some cases, you may also be able to have your closure call itself by capturing a `&Fn()` object or `fn()` pointer that refers to itself. That is permitting, since the closure would be invoking itself via a virtual call, and hence does not directly reference its own type.

## E0645

No description.

## E0646

It is not possible to define `main` with a where clause. Erroneous code example:

ⓘThis example deliberately fails to compile

    fn main() where i32: Copy { // error: main function is not allowed to have
                                // a where clause
    }Run

## E0647

It is not possible to define `start` with a where clause. Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(start)]

    #[start]
    fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
        //^ error: start function is not allowed to have a where clause
        0
    }Run

## E0648

`export_name` attributes may not contain null characters (`\0`).

ⓘThis example deliberately fails to compile

    #[export_name="\0foo"] // error: `export_name` may not contain null characters
    pub fn bar() {}Run

## E0657

No description.

## E0658

An unstable feature was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(u128)] // error: use of unstable library feature 'repr128'
    enum Foo {
        Bar(u64),
    }Run

If you're using a stable or a beta version of rustc, you won't be able to use any unstable features. In order to do so, please switch to a nightly version of rustc (by using rustup).

If you're using a nightly version of rustc, just add the corresponding feature to be able to use it:

    #![feature(repr128)]

    #[repr(u128)] // ok!
    enum Foo {
        Bar(u64),
    }Run

## E0659

An item usage is ambiguous.

Erroneous code example:

ⓘThis example deliberately fails to compile

    pub mod moon {
        pub fn foo() {}
    }

    pub mod earth {
        pub fn foo() {}
    }

    mod collider {
        pub use moon::*;
        pub use earth::*;
    }

    fn main() {
        collider::foo(); // ERROR: `foo` is ambiguous
    }Run

This error generally appears when two items with the same name are imported into a module. Here, the `foo` functions are imported and reexported from the `collider` module and therefore, when we're using `collider::foo()`, both functions collide.

To solve this error, the best solution is generally to keep the path before the item when using it. Example:

    pub mod moon {
        pub fn foo() {}
    }

    pub mod earth {
        pub fn foo() {}
    }

    mod collider {
        pub use moon;
        pub use earth;
    }

    fn main() {
        collider::moon::foo(); // ok!
        collider::earth::foo(); // ok!
    }Run

## E0666

No description.

## E0667

No description.

## E0668

Malformed inline assembly rejected by LLVM.

LLVM checks the validity of the constraints and the assembly string passed to it. This error implies that LLVM seems something wrong with the inline assembly call.

In particular, it can happen if you forgot the closing bracket of a register constraint (see issue #51430):

ⓘThis example is not tested

    #![feature(asm)]

    fn main() {
        let rax: u64;
        unsafe {
            asm!("" :"={rax"(rax));
            println!("Accumulator is: {}", rax);
        }
    }Run

## E0669

Cannot convert inline assembly operand to a single LLVM value.

This error usually happens when trying to pass in a value to an input inline assembly operand that is actually a pair of values. In particular, this can happen when trying to pass in a slice, for instance a `&str`. In Rust, these values are represented internally as a pair of values, the pointer and its length. When passed as an input operand, this pair of values can not be coerced into a register and thus we must fail with an error.

## E0670

Rust 2015 does not permit the use of `async fn`.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    async fn foo() {}Run

Switch to the Rust 2018 edition to use `async fn`.

## E0671

Const parameters cannot depend on type parameters. The following is therefore invalid:

ⓘThis example deliberately fails to compile

    #![feature(const_generics)]

    fn const_id<T, const N: T>() -> T { // error: const parameter
                                        // depends on type parameter
        N
    }Run

## E0687

No description.

## E0688

No description.

## E0689

This error indicates that the numeric value for the method being passed exists but the type of the numeric value or binding could not be identified.

The error happens on numeric literals:

ⓘThis example deliberately fails to compile

    2.0.neg();Run

and on numeric bindings without an identified concrete type:

ⓘThis example deliberately fails to compile

    let x = 2.0;
    x.neg();  // same error as aboveRun

Because of this, you must give the numeric literal or binding a type:

    use std::ops::Neg;

    let _ = 2.0_f32.neg();
    let x: f32 = 2.0;
    let _ = x.neg();
    let _ = (2.0 as f32).neg();Run

## E0690

A struct with the representation hint `repr(transparent)` had zero or more than one fields that were not guaranteed to be zero-sized.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(transparent)]
    struct LengthWithUnit<U> { // error: transparent struct needs exactly one
        value: f32,            //        non-zero-sized field, but has 2
        unit: U,
    }Run

Because transparent structs are represented exactly like one of their fields at run time, said field must be uniquely determined. If there is no field, or if there are multiple fields, it is not clear how the struct should be represented. Note that fields of zero-typed types (e.g., `PhantomData`) can also exist alongside the field that contains the actual data, they do not count for this error. When generic types are involved (as in the above example), an error is reported because the type parameter could be non-zero-sized.

To combine `repr(transparent)` with type parameters, `PhantomData` may be useful:

    use std::marker::PhantomData;

    #[repr(transparent)]
    struct LengthWithUnit<U> {
        value: f32,
        unit: PhantomData<U>,
    }Run

## E0691

A struct, enum, or union with the `repr(transparent)` representation hint contains a zero-sized field that requires non-trivial alignment.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #![feature(repr_align)]

    #[repr(align(32))]
    struct ForceAlign32;

    #[repr(transparent)]
    struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
                                       //        struct has alignment larger than 1Run

A transparent struct, enum, or union is supposed to be represented exactly like the piece of data it contains. Zero-sized fields with different alignment requirements potentially conflict with this property. In the example above, `Wrapper` would have to be aligned to 32 bytes even though `f32` has a smaller alignment requirement.

Consider removing the over-aligned zero-sized field:

    #[repr(transparent)]
    struct Wrapper(f32);Run

Alternatively, `PhantomData<T>` has alignment 1 for all `T`, so you can use it if you need to keep the field for some reason:

    #![feature(repr_align)]

    use std::marker::PhantomData;

    #[repr(align(32))]
    struct ForceAlign32;

    #[repr(transparent)]
    struct Wrapper(f32, PhantomData<ForceAlign32>);Run

Note that empty arrays `[T; 0]` have the same alignment requirement as the element type `T`. Also note that the error is conservatively reported even when the alignment of the zero-sized type is less than or equal to the data field's alignment.

## E0692

A `repr(transparent)` type was also annotated with other, incompatible representation hints.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(transparent, C)] // error: incompatible representation hints
    struct Grams(f32);Run

A type annotated as `repr(transparent)` delegates all representation concerns to another type, so adding more representation hints is contradictory. Remove either the `transparent` hint or the other hints, like this:

    #[repr(transparent)]
    struct Grams(f32);Run

Alternatively, move the other attributes to the contained type:

    #[repr(C)]
    struct Foo {
        x: i32,
        // ...
    }

    #[repr(transparent)]
    struct FooWrapper(Foo);Run

Note that introducing another `struct` just to have a place for the other attributes may have unintended side effects on the representation:

    #[repr(transparent)]
    struct Grams(f32);

    #[repr(C)]
    struct Float(f32);

    #[repr(transparent)]
    struct Grams2(Float); // this is not equivalent to `Grams` aboveRun

Here, `Grams2` is a not equivalent to `Grams` -- the former transparently wraps a (non-transparent) struct containing a single float, while `Grams` is a transparent wrapper around a float. This can make a difference for the ABI.

## E0693

No description.

## E0694

No description.

## E0695

A `break` statement without a label appeared inside a labeled block.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    loop {
        'a: {
            break;
        }
    }Run

Make sure to always label the `break`:

    'l: loop {
        'a: {
            break 'l;
        }
    }Run

Or if you want to `break` the labeled block:

    loop {
        'a: {
            break 'a;
        }
        break;
    }Run

## E0696

No description.

## E0697

No description.

## E0698

When using generators (or async) all type variables must be bound so a generator can be constructed.

Erroneous code example:

    #![feature(futures_api, async_await, await_macro)]
    async fn bar<T>() -> () {}

    async fn foo() {
      await!(bar());  // error: cannot infer type for `T`
    }

In the above example `T` is unknowable by the compiler. To fix this you must bind `T` to a concrete type such as `String` so that a generator can then be constructed:

ⓘThis code runs with edition 2018

    #![feature(futures_api, async_await, await_macro)]
    async fn bar<T>() -> () {}

    async fn foo() {
      await!(bar::<String>());
      //          ^^^^^^^^ specify type explicitly
    }Run

## E0699

A method was called on a raw pointer whose inner type wasn't completely known.

For example, you may have done something like:

ⓘThis example deliberately fails to compile

    let foo = &1;
    let bar = foo as *const _;
    if bar.is_null() {
        // ...
    }Run

Here, the type of `bar` isn't known; it could be a pointer to anything. Instead, specify a type for the pointer (preferably something that makes sense for the thing you're pointing to):

    let foo = &1;
    let bar = foo as *const i32;
    if bar.is_null() {
        // ...
    }Run

Even though `is_null()` exists as a method on any raw pointer, Rust shows this error because Rust allows for `self` to have arbitrary types (behind the arbitrary_self_types feature flag).

This means that someone can specify such a function:

ⓘThis example is not tested

    impl Foo {
        fn is_null(self: *const Self) -> bool {
            // do something else
        }
    }Run

and now when you call `.is_null()` on a raw pointer to `Foo`, there's ambiguity.

Given that we don't know what type the pointer is, and there's potential ambiguity for some types, we disallow calling methods on raw pointers when the type is unknown.

## E0700

The `impl Trait` return type captures lifetime parameters that do not appear within the `impl Trait` itself.

Erroneous code example:

    use std::cell::Cell;

    trait Trait<'a> { }

    impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }

    fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
    where 'x: 'y
    {
        x
    }

Here, the function `foo` returns a value of type `Cell<&'x u32>`, which references the lifetime `'x`. However, the return type is declared as `impl Trait<'y>` -- this indicates that `foo` returns "some type that implements `Trait<'y>`", but it also indicates that the return type only captures data referencing the lifetime `'y`. In this case, though, we are referencing data with lifetime `'x`, so this function is in error.

To fix this, you must reference the lifetime `'x` from the return type. For example, changing the return type to `impl Trait<'y> + 'x` would work:

    use std::cell::Cell;

    trait Trait<'a> { }

    impl<'a,'b> Trait<'b> for Cell<&'a u32> { }

    fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
    where 'x: 'y
    {
        x
    }Run

## E0701

This error indicates that a `#[non_exhaustive]` attribute was incorrectly placed on something other than a struct or enum.

Examples of erroneous code:

ⓘThis example deliberately fails to compile

    #[non_exhaustive]
    trait Foo { }Run

## E0703

No description.

## E0704

This error indicates that a incorrect visibility restriction was specified.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    mod foo {
        pub(foo) struct Bar {
            x: i32
        }
    }Run

To make struct `Bar` only visible in module `foo` the `in` keyword should be used:

    mod foo {
        pub(in crate::foo) struct Bar {
            x: i32
        }
    }Run

For more information see the Rust Reference on Visibility.

## E0705

A `#![feature]` attribute was declared for a feature that is stable in the current edition, but not in all editions.

Erroneous code example:

ⓘThis example is not tested

    #![feature(rust_2018_preview)]
    #![feature(test_2018_feature)] // error: the feature
                                   // `test_2018_feature` is
                                   // included in the Rust 2018 editionRun

## E0706

No description.

## E0707

No description.

## E0708

No description.

## E0709

No description.

## E0710

No description.

## E0711

No description.

## E0712

This error occurs because a borrow of a thread-local variable was made inside a function which outlived the lifetime of the function.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(nll)]
    #![feature(thread_local)]

    #[thread_local]
    static FOO: u8 = 3;

    fn main() {
        let a = &FOO; // error: thread-local variable borrowed past end of function

        std::thread::spawn(move || {
            println!("{}", a);
        });
    }Run

## E0713

This error occurs when an attempt is made to borrow state past the end of the lifetime of a type that implements the `Drop` trait.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(nll)]

    pub struct S<'a> { data: &'a mut String }

    impl<'a> Drop for S<'a> {
        fn drop(&mut self) { self.data.push_str("being dropped"); }
    }

    fn demo<'a>(s: S<'a>) -> &'a mut String { let p = &mut *s.data; p }Run

Here, `demo` tries to borrow the string data held within its argument `s` and then return that borrow. However, `S` is declared as implementing `Drop`.

Structs implementing the `Drop` trait have an implicit destructor that gets called when they go out of scope. This destructor gets exclusive access to the fields of the struct when it runs.

This means that when `s` reaches the end of `demo`, its destructor gets exclusive access to its `&mut`-borrowed string data. allowing another borrow of that string data (`p`), to exist across the drop of `s` would be a violation of the principle that `&mut`-borrows have exclusive, unaliased access to their referenced data.

This error can be fixed by changing `demo` so that the destructor does not run while the string-data is borrowed; for example by taking `S` by reference:

    #![feature(nll)]

    pub struct S<'a> { data: &'a mut String }

    impl<'a> Drop for S<'a> {
        fn drop(&mut self) { self.data.push_str("being dropped"); }
    }

    fn demo<'a>(s: &'a mut S<'a>) -> &'a mut String { let p = &mut *(*s).data; p }Run

Note that this approach needs a reference to S with lifetime `'a`. Nothing shorter than `'a` will suffice: a shorter lifetime would imply that after `demo` finishes executing, something else (such as the destructor!) could access `s.data` after the end of that shorter lifetime, which would again violate the `&mut`-borrow's exclusive access.

## E0714

A `#[marker]` trait contained an associated item.

The items of marker traits cannot be overridden, so there's no need to have them when they cannot be changed per-type anyway. If you wanted them for ergonomic reasons, consider making an extension trait instead.

## E0715

An `impl` for a `#[marker]` trait tried to override an associated item.

Because marker traits are allowed to have multiple implementations for the same type, it's not allowed to override anything in those implementations, as it would be ambiguous which override should actually be used.

## E0716

This error indicates that a temporary value is being dropped while a borrow is still in active use.

Erroneous code example:

ⓘThis example deliberately fails to compile

    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let p = bar(&foo());
             // ------ creates a temporary
    let q = *p;Run

Here, the expression `&foo()` is borrowing the expression `foo()`. As `foo()` is a call to a function, and not the name of a variable, this creates a temporary -- that temporary stores the return value from `foo()` so that it can be borrowed. You could imagine that `let p = bar(&foo());` is equivalent to this:

ⓘThis example deliberately fails to compile

    let p = {
      let tmp = foo(); // the temporary
      bar(&tmp)
    }; // <-- tmp is freed as we exit this block
    let q = p;Run

Whenever a temporary is created, it is automatically dropped (freed) according to fixed rules. Ordinarily, the temporary is dropped at the end of the enclosing statement -- in this case, after the `let`. This is illustrated in the example above by showing that `tmp` would be freed as we exit the block.

To fix this problem, you need to create a local variable to store the value in rather than relying on a temporary. For example, you might change the original program to the following:

    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let value = foo(); // dropped at the end of the enclosing block
    let p = bar(&value);
    let q = *p;Run

By introducing the explicit `let value`, we allocate storage that will last until the end of the enclosing block (when `value` goes out of scope). When we borrow `&value`, we are borrowing a local variable that already exists, and hence no temporary is created.

Temporaries are not always dropped at the end of the enclosing statement. In simple cases where the `&` expression is immediately stored into a variable, the compiler will automatically extend the lifetime of the temporary until the end of the enclosing block. Therefore, an alternative way to fix the original program is to write `let tmp = &foo()` and not `let tmp = foo()`:

    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let value = &foo();
    let p = bar(value);
    let q = *p;Run

Here, we are still borrowing `foo()`, but as the borrow is assigned directly into a variable, the temporary will not be dropped until the end of the enclosing block. Similar rules apply when temporaries are stored into aggregate structures like a tuple or struct:

    // Here, two temporaries are created, but
    // as they are stored directly into `value`,
    // they are not dropped until the end of the
    // enclosing block.
    fn foo() -> i32 { 22 }
    let value = (&foo(), &foo());Run

## E0717

No description.

## E0718

This error indicates that a `#[lang = ".."]` attribute was placed on the wrong type of item.

Examples of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(lang_items)]

    #[lang = "arc"]
    static X: u32 = 42;Run

## E0719

No description.

## E0720

An `impl Trait` type expands to a recursive type.

An `impl Trait` type must be expandable to a concrete type that contains no `impl Trait` types. For example the following example tries to create an `impl Trait` type `T` that is equal to `[T, T]`:

ⓘThis example deliberately fails to compile

    fn make_recursive_type() -> impl Sized {
        [make_recursive_type(), make_recursive_type()]
    }Run

## E0722

No description.

## E0723

An feature unstable in `const` contexts was used.

Erroneous code example:

ⓘThis example deliberately fails to compile

    trait T {}

    impl T for () {}

    const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
        ()
    }Run

To enable this feature on a nightly version of rustc, add the `const_fn` feature flag:

    #![feature(const_fn)]

    trait T {}

    impl T for () {}

    const fn foo() -> impl T {
        ()
    }Run

## E0724

No description.

## E0725

A feature attribute named a feature that was disallowed in the compiler command line flags.

Erroneous code example:

ⓘThis example is not tested

    #![feature(never_type)] // error: the feature `never_type` is not in
                            // the list of allowed featuresRun

Delete the offending feature attribute, or add it to the list of allowed features in the `-Z allow_features` flag.

## E0726

No description.

## E0727

No description.

## E0728

No description.

## E0729

Support for Non-Lexical Lifetimes (NLL) has been included in the Rust compiler since 1.31, and has been enabled on the 2015 edition since 1.36. The new borrow checker for NLL uncovered some bugs in the old borrow checker, which in some cases allowed unsound code to compile, resulting in memory safety issues.

### What do I do?

Change your code so the warning does no longer trigger. For backwards compatibility, this unsound code may still compile (with a warning) right now. However, at some point in the future, the compiler will no longer accept this code and will throw a hard error.

### Shouldn't you fix the old borrow checker?

The old borrow checker has known soundness issues that are basically impossible to fix. The new NLL-based borrow checker is the fix.

### Can I turn these warnings into errors by denying a lint?

No.

### When are these warnings going to turn into errors?

No formal timeline for turning the warnings into errors has been set. See GitHub issue 58781 for more information.

### Why do I get this message with code that doesn't involve borrowing?

There are some known bugs that trigger this message.

## E0730

An array without a fixed length was pattern-matched.

Example of erroneous code:

ⓘThis example deliberately fails to compile

    #![feature(const_generics)]

    fn is_123<const N: usize>(x: [u32; N]) -> bool {
        match x {
            [1, 2, 3] => true, // error: cannot pattern-match on an
                               //        array without a fixed length
            _ => false
        }
    }Run

Ensure that the pattern is consistent with the size of the matched array. Additional elements can be matched with `..`:

    #![feature(slice_patterns)]

    let r = &[1, 2, 3, 4];
    match r {
        &[a, b, ..] => { // ok!
            println!("a={}, b={}", a, b);
        }
    }Run

## E0731

An enum with the representation hint `repr(transparent)` had zero or more than one variants.

Erroneous code example:

ⓘThis example deliberately fails to compile

    #[repr(transparent)]
    enum Status { // error: transparent enum needs exactly one variant, but has 2
        Errno(u32),
        Ok,
    }Run

Because transparent enums are represented exactly like one of their variants at run time, said variant must be uniquely determined. If there is no variant, or if there are multiple variants, it is not clear how the enum should be represented.

## E0732

An `enum` with a discriminant must specify a `#[repr(inttype)]`.

A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit variant with a discriminant, or where there are both unit variants with discriminants and non-unit variants. This restriction ensures that there is a well-defined way to extract a variant's discriminant from a value; for instance:

    #![feature(arbitrary_enum_discriminant)]

    #[repr(u8)]
    enum Enum {
        Unit = 3,
        Tuple(u16) = 2,
        Struct {
            a: u8,
            b: u16,
        } = 1,
    }

    fn discriminant(v : &Enum) -> u8 {
        unsafe { *(v as *const Enum as *const u8) }
    }

    assert_eq!(3, discriminant(&Enum::Unit));
    assert_eq!(2, discriminant(&Enum::Tuple(5)));
    assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));Run