keyword_docs.rs.html -- source
Description: Source to the Rust file `src/libstd/keyword_docs.rs`.
Keywords: rust, rustlang, rust-lang

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    #[doc(keyword = "as")]
    //
    /// Cast between types, or rename an import.
    ///
    /// `as` is most commonly used to turn primitive types into other primitive types, but it has other
    /// uses that include turning pointers into addresses, addresses into pointers, and pointers into
    /// other pointers.
    ///
    /// ```rust
    /// let thing1: u8 = 89.0 as u8;
    /// assert_eq!('B' as u32, 66);
    /// assert_eq!(thing1 as char, 'Y');
    /// let thing2: f32 = thing1 as f32 + 10.5;
    /// assert_eq!(true as u8 + thing2 as u8, 100);
    /// ```
    ///
    /// In general, any cast that can be performed via ascribing the type can also be done using `as`,
    /// so instead of writing `let x: u32 = 123`, you can write `let x = 123 as u32` (Note: `let x: u32
    /// = 123` would be best in that situation). The same is not true in the other direction, however,
    /// explicitly using `as` allows a few more coercions that aren't allowed implicitly, such as
    /// changing the type of a raw pointer or turning closures into raw pointers.
    ///
    /// Other places `as` is used include as extra syntax for [`crate`] and `use`, to change the name
    /// something is imported as.
    ///
    /// For more information on what `as` is capable of, see the [Reference]
    ///
    /// [Reference]: ../reference/expressions/operator-expr.html#type-cast-expressions
    /// [`crate`]: keyword.crate.html
    mod as_keyword { }

    #[doc(keyword = "break")]
    //
    /// Exit early from a loop.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod break_keyword { }

    #[doc(keyword = "const")]
    //
    /// Compile-time constants and deterministic functions.
    ///
    /// Sometimes a certain value is used many times throughout a program, and it can become
    /// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
    /// make it a variable that gets carried around to each function that needs it. In these cases, the
    /// `const` keyword provides a convenient alternative to code duplication.
    ///
    /// ```rust
    /// const THING: u32 = 0xABAD1DEA;
    ///
    /// let foo = 123 + THING;
    /// ```
    ///
    /// Constants must be explicitly typed, unlike with `let` you can't ignore its type and let the
    /// compiler figure it out. Any constant value can be defined in a const, which in practice happens
    /// to be most things that would be reasonable to have a constant (barring `const fn`s). For
    /// example, you can't have a File as a `const`.
    ///
    /// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses
    /// all others in a Rust program. For example, if you wanted to define a constant string, it would
    /// look like this:
    ///
    /// ```rust
    /// const WORDS: &str = "hello rust!";
    /// ```
    ///
    /// Thanks to static lifetime elision, you usually don't have to explicitly use 'static:
    ///
    /// ```rust
    /// const WORDS: &str = "hello convenience!";
    /// ```
    ///
    /// `const` items looks remarkably similar to `static` items, which introduces some confusion as
    /// to which one should be used at which times. To put it simply, constants are inlined wherever
    /// they're used, making using them identical to simply replacing the name of the const with its
    /// value. Static variables on the other hand point to a single location in memory, which all
    /// accesses share. This means that, unlike with constants, they can't have destructors, and act as
    /// a single value across the entire codebase.
    ///
    /// Constants, as with statics, should always be in SCREAMING_SNAKE_CASE.
    ///
    /// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
    /// T` and `*mut T`. More about that can be read at the [pointer] primitive part of the Rust docs.
    ///
    /// For more detail on `const`, see the [Rust Book] or the [Reference]
    ///
    /// [pointer]: primitive.pointer.html
    /// [Rust Book]:
    /// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
    /// [Reference]: ../reference/items/constant-items.html
    mod const_keyword { }

    #[doc(keyword = "continue")]
    //
    /// Skip to the next iteration of a loop.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod continue_keyword { }

    #[doc(keyword = "crate")]
    //
    /// A Rust binary or library.
    ///
    /// The primary use of the `crate` keyword is as a part of `extern crate` declarations, which are
    /// used to specify a dependency on a crate external to the one it's declared in. Crates are the
    /// fundamental compilation unit of Rust code, and can be seen as libraries or projects. More can
    /// be read about crates in the [Reference].
    ///
    /// ```rust ignore
    /// extern crate rand;
    /// extern crate my_crate as thing;
    /// extern crate std; // implicitly added to the root of every Rust project
    /// ```
    ///
    /// The `as` keyword can be used to change what the crate is referred to as in your project. If a
    /// crate name includes a dash, it is implicitly imported with the dashes replaced by underscores.
    ///
    /// `crate` can also be used as in conjunction with `pub` to signify that the item it's attached to
    /// is public only to other members of the same crate it's in.
    ///
    /// ```rust
    /// # #[allow(unused_imports)]
    /// pub(crate) use std::io::Error as IoError;
    /// pub(crate) enum CoolMarkerType { }
    /// pub struct PublicThing {
    ///     pub(crate) semi_secret_thing: bool,
    /// }
    /// ```
    ///
    /// `crate` is also used to represent the absolute path of a module, where `crate` refers to the
    /// root of the current crate. For instance, `crate::foo::bar` refers to the name `bar` inside the
    /// module `foo`, from anywhere else in the same crate.
    ///
    /// [Reference]: ../reference/items/extern-crates.html
    mod crate_keyword { }

    #[doc(keyword = "else")]
    //
    /// What to do when an [`if`] condition does not hold.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`if`]: keyword.if.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod else_keyword { }

    #[doc(keyword = "enum")]
    //
    /// A type that can be any one of several variants.
    ///
    /// Enums in Rust are similar to those of other compiled languages like C, but have important
    /// differences that make them considerably more powerful. What Rust calls enums are more commonly
    /// known as [Algebraic Data Types][ADT] if you're coming from a functional programming background.
    /// The important detail is that each enum variant can have data to go along with it.
    ///
    /// ```rust
    /// # struct Coord;
    /// enum SimpleEnum {
    ///     FirstVariant,
    ///     SecondVariant,
    ///     ThirdVariant,
    /// }
    ///
    /// enum Location {
    ///     Unknown,
    ///     Anonymous,
    ///     Known(Coord),
    /// }
    ///
    /// enum ComplexEnum {
    ///     Nothing,
    ///     Something(u32),
    ///     LotsOfThings {
    ///         usual_struct_stuff: bool,
    ///         blah: String,
    ///     }
    /// }
    ///
    /// enum EmptyEnum { }
    /// ```
    ///
    /// The first enum shown is the usual kind of enum you'd find in a C-style language. The second
    /// shows off a hypothetical example of something storing location data, with `Coord` being any
    /// other type that's needed, for example a struct. The third example demonstrates the kind of
    /// data a variant can store, ranging from nothing, to a tuple, to an anonymous struct.
    ///
    /// Instantiating enum variants involves explicitly using the enum's name as its namespace,
    /// followed by one of its variants. `SimpleEnum::SecondVariant` would be an example from above.
    /// When data follows along with a variant, such as with rust's built-in [`Option`] type, the data
    /// is added as the type describes, for example `Option::Some(123)`. The same follows with
    /// struct-like variants, with things looking like `ComplexEnum::LotsOfThings { usual_struct_stuff:
    /// true, blah: "hello!".to_string(), }`. Empty Enums are similar to () in that they cannot be
    /// instantiated at all, and are used mainly to mess with the type system in interesting ways.
    ///
    /// For more information, take a look at the [Rust Book] or the [Reference]
    ///
    /// [ADT]: https://en.wikipedia.org/wiki/Algebraic_data_type
    /// [`Option`]: option/enum.Option.html
    /// [Rust Book]: ../book/ch06-01-defining-an-enum.html
    /// [Reference]: ../reference/items/enumerations.html
    mod enum_keyword { }

    #[doc(keyword = "extern")]
    //
    /// Link to or import external code.
    ///
    /// The `extern` keyword is used in two places in Rust. One is in conjunction with the [`crate`]
    /// keyword to make your Rust code aware of other Rust crates in your project, i.e., `extern crate
    /// lazy_static;`. The other use is in foreign function interfaces (FFI).
    ///
    /// `extern` is used in two different contexts within FFI. The first is in the form of external
    /// blocks, for declaring function interfaces that Rust code can call foreign code by.
    ///
    /// ```rust ignore
    /// #[link(name = "my_c_library")]
    /// extern "C" {
    ///     fn my_c_function(x: i32) -> bool;
    /// }
    /// ```
    ///
    /// This code would attempt to link with `libmy_c_library.so` on unix-like systems and
    /// `my_c_library.dll` on Windows at runtime, and panic if it can't find something to link to. Rust
    /// code could then use `my_c_function` as if it were any other unsafe Rust function. Working with
    /// non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.
    ///
    /// The mirror use case of FFI is also done via the `extern` keyword:
    ///
    /// ```rust
    /// #[no_mangle]
    /// pub extern fn callable_from_c(x: i32) -> bool {
    ///     x % 3 == 0
    /// }
    /// ```
    ///
    /// If compiled as a dylib, the resulting .so could then be linked to from a C library, and the
    /// function could be used as if it was from any other library.
    ///
    /// For more information on FFI, check the [Rust book] or the [Reference].
    ///
    /// [Rust book]:
    /// ../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
    /// [Reference]: ../reference/items/external-blocks.html
    mod extern_keyword { }

    #[doc(keyword = "false")]
    //
    /// A value of type [`bool`] representing logical **false**.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`bool`]: primitive.bool.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod false_keyword { }

    #[doc(keyword = "fn")]
    //
    /// A function or function pointer.
    ///
    /// Functions are the primary way code is executed within Rust. Function blocks, usually just
    /// called functions, can be defined in a variety of different places and be assigned many
    /// different attributes and modifiers.
    ///
    /// Standalone functions that just sit within a module not attached to anything else are common,
    /// but most functions will end up being inside [`impl`] blocks, either on another type itself, or
    /// as a trait impl for that type.
    ///
    /// ```rust
    /// fn standalone_function() {
    ///     // code
    /// }
    ///
    /// pub fn public_thing(argument: bool) -> String {
    ///     // code
    ///     # "".to_string()
    /// }
    ///
    /// struct Thing {
    ///     foo: i32,
    /// }
    ///
    /// impl Thing {
    ///     pub fn new() -> Self {
    ///         Self {
    ///             foo: 42,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// In addition to presenting fixed types in the form of `fn name(arg: type, ..) -> return_type`,
    /// functions can also declare a list of type parameters along with trait bounds that they fall
    /// into.
    ///
    /// ```rust
    /// fn generic_function<T: Clone>(x: T) -> (T, T, T) {
    ///     (x.clone(), x.clone(), x.clone())
    /// }
    ///
    /// fn generic_where<T>(x: T) -> T
    ///     where T: std::ops::Add<Output = T> + Copy
    /// {
    ///     x + x + x
    /// }
    /// ```
    ///
    /// Declaring trait bounds in the angle brackets is functionally identical to using a `where`
    /// clause. It's up to the programmer to decide which works better in each situation, but `where`
    /// tends to be better when things get longer than one line.
    ///
    /// Along with being made public via `pub`, `fn` can also have an [`extern`] added for use in
    /// FFI.
    ///
    /// For more information on the various types of functions and how they're used, consult the [Rust
    /// book] or the [Reference].
    ///
    /// [`impl`]: keyword.impl.html
    /// [`extern`]: keyword.extern.html
    /// [Rust book]: ../book/ch03-03-how-functions-work.html
    /// [Reference]: ../reference/items/functions.html
    mod fn_keyword { }

    #[doc(keyword = "for")]
    //
    /// Iteration with [`in`], trait implementation with [`impl`], or [higher-ranked trait bounds]
    /// (`for<'a>`).
    ///
    /// The `for` keyword is used in many syntactic locations:
    ///
    /// * `for` is used in for-in-loops (see below).
    /// * `for` is used when implementing traits as in `impl Trait for Type` (see [`impl`] for more info
    ///   on that).
    /// * `for` is also used for [higher-ranked trait bounds] as in `for<'a> &'a T: PartialEq<i32>`.
    ///
    /// for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common
    /// practice within Rust, which is to loop over an iterator until that iterator returns `None` (or
    /// `break` is called).
    ///
    /// ```rust
    /// for i in 0..5 {
    ///     println!("{}", i * 2);
    /// }
    ///
    /// for i in std::iter::repeat(5) {
    ///     println!("turns out {} never stops being 5", i);
    ///     break; // would loop forever otherwise
    /// }
    ///
    /// 'outer: for x in 5..50 {
    ///     for y in 0..10 {
    ///         if x == y {
    ///             break 'outer;
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// As shown in the example above, `for` loops (along with all other loops) can be tagged, using
    /// similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the
    /// same tag to `break` breaks the tagged loop, which is useful for inner loops. It is definitely
    /// not a goto.
    ///
    /// A `for` loop expands as shown:
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// for loop_variable in iterator {
    ///     code()
    /// }
    /// ```
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// {
    ///     let mut _iter = std::iter::IntoIterator::into_iter(iterator);
    ///     loop {
    ///         match _iter.next() {
    ///             Some(loop_variable) => {
    ///                 code()
    ///             },
    ///             None => break,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// More details on the functionality shown can be seen at the [`IntoIterator`] docs.
    ///
    /// For more information on for-loops, see the [Rust book] or the [Reference].
    ///
    /// [`in`]: keyword.in.html
    /// [`impl`]: keyword.impl.html
    /// [higher-ranked trait bounds]: ../reference/trait-bounds.html#higher-ranked-trait-bounds
    /// [`IntoIterator`]: iter/trait.IntoIterator.html
    /// [Rust book]:
    /// ../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    /// [Reference]: ../reference/expressions/loop-expr.html#iterator-loops
    mod for_keyword { }

    #[doc(keyword = "if")]
    //
    /// Evaluate a block if a condition holds.
    ///
    /// `if` is a familiar construct to most programmers, and is the main way you'll often do logic in
    /// your code. However, unlike in most languages, `if` blocks can also act as expressions.
    ///
    /// ```rust
    /// # let rude = true;
    /// if 1 == 2 {
    ///     println!("whoops, mathematics broke");
    /// } else {
    ///     println!("everything's fine!");
    /// }
    ///
    /// let greeting = if rude {
    ///     "sup nerd."
    /// } else {
    ///     "hello, friend!"
    /// };
    ///
    /// if let Ok(x) = "123".parse::<i32>() {
    ///     println!("{} double that and you get {}!", greeting, x * 2);
    /// }
    /// ```
    ///
    /// Shown above are the three typical forms an `if` block comes in. First is the usual kind of
    /// thing you'd see in many languages, with an optional `else` block. Second uses `if` as an
    /// expression, which is only possible if all branches return the same type. An `if` expression can
    /// be used everywhere you'd expect. The third kind of `if` block is an `if let` block, which
    /// behaves similarly to using a `match` expression:
    ///
    /// ```rust
    /// if let Some(x) = Some(123) {
    ///     // code
    ///     # let _ = x;
    /// } else {
    ///     // something else
    /// }
    ///
    /// match Some(123) {
    ///     Some(x) => {
    ///         // code
    ///         # let _ = x;
    ///     },
    ///     _ => {
    ///         // something else
    ///     },
    /// }
    /// ```
    ///
    /// Each kind of `if` expression can be mixed and matched as needed.
    ///
    /// ```rust
    /// if true == false {
    ///     println!("oh no");
    /// } else if "something" == "other thing" {
    ///     println!("oh dear");
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     println!("uh oh");
    /// } else {
    ///     println!("phew, nothing's broken");
    /// }
    /// ```
    ///
    /// The `if` keyword is used in one other place in Rust, namely as a part of pattern matching
    /// itself, allowing patterns such as `Some(x) if x > 200` to be used.
    ///
    /// For more information on `if` expressions, see the [Rust book] or the [Reference].
    ///
    /// [Rust book]: ../book/ch03-05-control-flow.html#if-expressions
    /// [Reference]: ../reference/expressions/if-expr.html
    mod if_keyword { }

    #[doc(keyword = "impl")]
    //
    /// Implement some functionality for a type.
    ///
    /// The `impl` keyword is primarily used to define implementations on types. Inherent
    /// implementations are standalone, while trait implementations are used to implement traits for
    /// types, or other traits.
    ///
    /// Functions and consts can both be defined in an implementation. A function defined in an
    /// `impl` block can be standalone, meaning it would be called like `Foo::bar()`. If the function
    /// takes `self`, `&self`, or `&mut self` as its first argument, it can also be called using
    /// method-call syntax, a familiar feature to any object oriented programmer, like `foo.bar()`.
    ///
    /// ```rust
    /// struct Example {
    ///     number: i32,
    /// }
    ///
    /// impl Example {
    ///     fn boo() {
    ///         println!("boo! Example::boo() was called!");
    ///     }
    ///
    ///     fn answer(&mut self) {
    ///         self.number += 42;
    ///     }
    ///
    ///     fn get_number(&self) -> i32 {
    ///         self.number
    ///     }
    /// }
    ///
    /// trait Thingy {
    ///     fn do_thingy(&self);
    /// }
    ///
    /// impl Thingy for Example {
    ///     fn do_thingy(&self) {
    ///         println!("doing a thing! also, number is {}!", self.number);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on implementations, see the [Rust book][book1] or the [Reference].
    ///
    /// The other use of the `impl` keyword is in `impl Trait` syntax, which can be seen as a shorthand
    /// for "a concrete type that implements this trait". Its primary use is working with closures,
    /// which have type definitions generated at compile time that can't be simply typed out.
    ///
    /// ```rust
    /// fn thing_returning_closure() -> impl Fn(i32) -> bool {
    ///     println!("here's a closure for you!");
    ///     |x: i32| x % 3 == 0
    /// }
    /// ```
    ///
    /// For more information on `impl Trait` syntax, see the [Rust book][book2].
    ///
    /// [book1]: ../book/ch05-03-method-syntax.html
    /// [Reference]: ../reference/items/implementations.html
    /// [book2]: ../book/ch10-02-traits.html#returning-types-that-implement-traits
    mod impl_keyword { }

    #[doc(keyword = "in")]
    //
    /// Iterate over a series of values with [`for`].
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`for`]: keyword.for.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod in_keyword { }

    #[doc(keyword = "let")]
    //
    /// Bind a value to a variable.
    ///
    /// The primary use for the `let` keyword is in `let` statements, which are used to introduce a new
    /// set of variables into the current scope, as given by a pattern.
    ///
    /// ```rust
    /// # #![allow(unused_assignments)]
    /// let thing1: i32 = 100;
    /// let thing2 = 200 + thing1;
    ///
    /// let mut changing_thing = true;
    /// changing_thing = false;
    ///
    /// let (part1, part2) = ("first", "second");
    ///
    /// struct Example {
    ///     a: bool,
    ///     b: u64,
    /// }
    ///
    /// let Example { a, b: _ } = Example {
    ///     a: true,
    ///     b: 10004,
    /// };
    /// assert!(a);
    /// ```
    ///
    /// The pattern is most commonly a single variable, which means no pattern matching is done and
    /// the expression given is bound to the variable. Apart from that, patterns used in `let` bindings
    /// can be as complicated as needed, given that the pattern is exhaustive. See the [Rust
    /// book][book1] for more information on pattern matching. The type of the pattern is optionally
    /// given afterwards, but if left blank is automatically inferred by the compiler if possible.
    ///
    /// Variables in Rust are immutable by default, and require the `mut` keyword to be made mutable.
    ///
    /// Multiple variables can be defined with the same name, known as shadowing. This doesn't affect
    /// the original variable in any way beyond being unable to directly access it beyond the point of
    /// shadowing. It continues to remain in scope, getting dropped only when it falls out of scope.
    /// Shadowed variables don't need to have the same type as the variables shadowing them.
    ///
    /// ```rust
    /// let shadowing_example = true;
    /// let shadowing_example = 123.4;
    /// let shadowing_example = shadowing_example as u32;
    /// let mut shadowing_example = format!("cool! {}", shadowing_example);
    /// shadowing_example += " something else!"; // not shadowing
    /// ```
    ///
    /// Other places the `let` keyword is used include along with [`if`], in the form of `if let`
    /// expressions. They're useful if the pattern being matched isn't exhaustive, such as with
    /// enumerations. `while let` also exists, which runs a loop with a pattern matched value until
    /// that pattern can't be matched.
    ///
    /// For more information on the `let` keyword, see the [Rust book][book2] or the [Reference]
    ///
    /// [book1]: ../book/ch06-02-match.html
    /// [`if`]: keyword.if.html
    /// [book2]: ../book/ch18-01-all-the-places-for-patterns.html#let-statements
    /// [Reference]: ../reference/statements.html#let-statements
    mod let_keyword { }

    #[doc(keyword = "while")]
    //
    /// Loop while a condition is upheld.
    ///
    /// A `while` expression is used for predicate loops. The `while` expression runs the conditional
    /// expression before running the loop body, then runs the loop body if the conditional
    /// expression evaluates to `true`, or exits the loop otherwise.
    ///
    /// ```rust
    /// let mut counter = 0;
    ///
    /// while counter < 10 {
    ///     println!("{}", counter);
    ///     counter += 1;
    /// }
    /// ```
    ///
    /// Like the [`for`] expression, we can use `break` and `continue`. A `while` expression
    /// cannot break with a value and always evaluates to `()` unlike [`loop`].
    ///
    /// ```rust
    /// let mut i = 1;
    ///
    /// while i < 100 {
    ///     i *= 2;
    ///     if i == 64 {
    ///         break; // Exit when `i` is 64.
    ///     }
    /// }
    /// ```
    ///
    /// As `if` expressions have their pattern matching variant in `if let`, so too do `while`
    /// expressions with `while let`. The `while let` expression matches the pattern against the
    /// expression, then runs the loop body if pattern matching succeeds, or exits the loop otherwise.
    /// We can use `break` and `continue` in `while let` expressions just like in `while`.
    ///
    /// ```rust
    /// let mut counter = Some(0);
    ///
    /// while let Some(i) = counter {
    ///     if i == 10 {
    ///         counter = None;
    ///     } else {
    ///         println!("{}", i);
    ///         counter = Some (i + 1);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on `while` and loops in general, see the [reference].
    ///
    /// [`for`]: keyword.for.html
    /// [`loop`]: keyword.loop.html
    /// [reference]: ../reference/expressions/loop-expr.html#predicate-loops
    mod while_keyword { }

    #[doc(keyword = "loop")]
    //
    /// Loop indefinitely.
    ///
    /// `loop` is used to define the simplest kind of loop supported in Rust. It runs the code inside
    /// it until the code uses `break` or the program exits.
    ///
    /// ```rust
    /// loop {
    ///     println!("hello world forever!");
    ///     # break;
    /// }
    ///
    /// let mut i = 0;
    /// loop {
    ///     println!("i is {}", i);
    ///     if i > 10 {
    ///         break;
    ///     }
    ///     i += 1;
    /// }
    /// ```
    ///
    /// Unlike the other kinds of loops in Rust (`while`, `while let`, and `for`), loops can be used as
    /// expressions that return values via `break`.
    ///
    /// ```rust
    /// let mut i = 1;
    /// let something = loop {
    ///     i *= 2;
    ///     if i > 100 {
    ///         break i;
    ///     }
    /// };
    /// assert_eq!(something, 128);
    /// ```
    ///
    /// Every `break` in a loop has to have the same type. When it's not explicitly giving something,
    /// `break;` returns `()`.
    ///
    /// For more information on `loop` and loops in general, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/loop-expr.html
    mod loop_keyword { }

    #[doc(keyword = "match")]
    //
    /// Control flow based on pattern matching.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod match_keyword { }

    #[doc(keyword = "mod")]
    //
    /// Organize code into [modules].
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [modules]: ../reference/items/modules.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod mod_keyword { }

    #[doc(keyword = "move")]
    //
    /// Capture a [closure]'s environment by value.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [closure]: ../book/second-edition/ch13-01-closures.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod move_keyword { }

    #[doc(keyword = "mut")]
    //
    /// A mutable binding, reference, or pointer.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod mut_keyword { }

    #[doc(keyword = "pub")]
    //
    /// Make an item visible to others.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod pub_keyword { }

    #[doc(keyword = "ref")]
    //
    /// Bind by reference during pattern matching.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod ref_keyword { }

    #[doc(keyword = "return")]
    //
    /// Return a value from a function.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod return_keyword { }

    #[doc(keyword = "self")]
    //
    /// The receiver of a method, or the current module.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod self_keyword { }

    #[doc(keyword = "Self")]
    //
    /// The implementing type within a [`trait`] or [`impl`] block, or the current type within a type
    /// definition.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`impl`]: keyword.impl.html
    /// [`trait`]: keyword.trait.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod self_upper_keyword { }

    #[doc(keyword = "static")]
    //
    /// A place that is valid for the duration of a program.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod static_keyword { }

    #[doc(keyword = "struct")]
    //
    /// A type that is composed of other types.
    ///
    /// Structs in Rust come in three flavors: Structs with named fields, tuple structs, and unit
    /// structs.
    ///
    /// ```rust
    /// struct Regular {
    ///     field1: f32,
    ///     field2: String,
    ///     pub field3: bool
    /// }
    ///
    /// struct Tuple(u32, String);
    ///
    /// struct Unit;
    /// ```
    ///
    /// Regular structs are the most commonly used. Each field defined within them has a name and a
    /// type, and once defined can be accessed using `example_struct.field` syntax. The fields of a
    /// struct share its mutability, so `foo.bar = 2;` would only be valid if `foo` was mutable. Adding
    /// `pub` to a field makes it visible to code in other modules, as well as allowing it to be
    /// directly accessed and modified.
    ///
    /// Tuple structs are similar to regular structs, but its fields have no names. They are used like
    /// tuples, with deconstruction possible via `let TupleStruct(x, y) = foo;` syntax. For accessing
    /// individual variables, the same syntax is used as with regular tuples, namely `foo.0`, `foo.1`,
    /// etc, starting at zero.
    ///
    /// Unit structs are most commonly used as marker. They have a size of zero bytes, but unlike empty
    /// enums they can be instantiated, making them isomorphic to the unit type `()`. Unit structs are
    /// useful when you need to implement a trait on something, but don't need to store any data inside
    /// it.
    ///
    /// # Instantiation
    ///
    /// Structs can be instantiated in different ways, all of which can be mixed and
    /// matched as needed. The most common way to make a new struct is via a constructor method such as
    /// `new()`, but when that isn't available (or you're writing the constructor itself), struct
    /// literal syntax is used:
    ///
    /// ```rust
    /// # struct Foo { field1: f32, field2: String, etc: bool }
    /// let example = Foo {
    ///     field1: 42.0,
    ///     field2: "blah".to_string(),
    ///     etc: true,
    /// };
    /// ```
    ///
    /// It's only possible to directly instantiate a struct using struct literal syntax when all of its
    /// fields are visible to you.
    ///
    /// There are a handful of shortcuts provided to make writing constructors more convenient, most
    /// common of which is the Field Init shorthand. When there is a variable and a field of the same
    /// name, the assignment can be simplified from `field: field` into simply `field`. The following
    /// example of a hypothetical constructor demonstrates this:
    ///
    /// ```rust
    /// struct User {
    ///     name: String,
    ///     admin: bool,
    /// }
    ///
    /// impl User {
    ///     pub fn new(name: String) -> Self {
    ///         Self {
    ///             name,
    ///             admin: false,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// Another shortcut for struct instantiation is available, used when you need to make a new
    /// struct that has the same values as most of a previous struct of the same type, called struct
    /// update syntax:
    ///
    /// ```rust
    /// # struct Foo { field1: String, field2: () }
    /// # let thing = Foo { field1: "".to_string(), field2: () };
    /// let updated_thing = Foo {
    ///     field1: "a new value".to_string(),
    ///     ..thing
    /// };
    /// ```
    ///
    /// Tuple structs are instantiated in the same way as tuples themselves, except with the struct's
    /// name as a prefix: `Foo(123, false, 0.1)`.
    ///
    /// Empty structs are instantiated with just their name, and don't need anything else. `let thing =
    /// EmptyStruct;`
    ///
    /// # Style conventions
    ///
    /// Structs are always written in CamelCase, with few exceptions. While the trailing comma on a
    /// struct's list of fields can be omitted, it's usually kept for convenience in adding and
    /// removing fields down the line.
    ///
    /// For more information on structs, take a look at the [Rust Book][book] or the
    /// [Reference][reference].
    ///
    /// [`PhantomData`]: marker/struct.PhantomData.html
    /// [book]: ../book/ch05-01-defining-structs.html
    /// [reference]: ../reference/items/structs.html
    mod struct_keyword { }

    #[doc(keyword = "super")]
    //
    /// The parent of the current [module].
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [module]: ../reference/items/modules.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod super_keyword { }

    #[doc(keyword = "trait")]
    //
    /// A common interface for a class of types.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod trait_keyword { }

    #[doc(keyword = "true")]
    //
    /// A value of type [`bool`] representing logical **true**.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`bool`]: primitive.bool.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod true_keyword { }

    #[doc(keyword = "type")]
    //
    /// Define an alias for an existing type.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod type_keyword { }

    #[doc(keyword = "unsafe")]
    //
    /// Code or interfaces whose [memory safety] cannot be verified by the type system.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [memory safety]: ../book/ch19-01-unsafe-rust.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod unsafe_keyword { }

    #[doc(keyword = "use")]
    //
    /// Import or rename items from other crates or modules.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod use_keyword { }

    #[doc(keyword = "where")]
    //
    /// Add constraints that must be upheld to use an item.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod where_keyword { }

    // 2018 Edition keywords

    #[unstable(feature = "async_await", issue = "50547")]
    #[doc(keyword = "async")]
    //
    /// Return a [`Future`] instead of blocking the current thread.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`Future`]: ./future/trait.Future.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod async_keyword { }

    #[unstable(feature = "async_await", issue = "50547")]
    #[doc(keyword = "await")]
    //
    /// Suspend execution until the result of a [`Future`] is ready.
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [`Future`]: ./future/trait.Future.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod await_keyword { }

    #[doc(keyword = "dyn")]
    //
    /// Name the type of a [trait object].
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [trait object]: ../book/ch17-02-trait-objects.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod dyn_keyword { }

    #[doc(keyword = "union")]
    //
    /// The [Rust equivalent of a C-style union][union].
    ///
    /// The documentation for this keyword is [not yet complete]. Pull requests welcome!
    ///
    /// [union]: ../reference/items/unions.html
    /// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
    mod union_keyword { }

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## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)