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

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    #[doc(primitive = "bool")]
    #[doc(alias = "true")]
    #[doc(alias = "false")]
    //
    /// The boolean type.
    ///
    /// The `bool` represents a value, which could only be either `true` or `false`. If you cast
    /// a `bool` into an integer, `true` will be 1 and `false` will be 0.
    ///
    /// # Basic usage
    ///
    /// `bool` implements various traits, such as [`BitAnd`], [`BitOr`], [`Not`], etc.,
    /// which allow us to perform boolean operations using `&`, `|` and `!`.
    ///
    /// `if` always demands a `bool` value. [`assert!`], being an important macro in testing,
    /// checks whether an expression returns `true`.
    ///
    /// ```
    /// let bool_val = true & false | false;
    /// assert!(!bool_val);
    /// ```
    ///
    /// [`assert!`]: macro.assert.html
    /// [`BitAnd`]: ops/trait.BitAnd.html
    /// [`BitOr`]: ops/trait.BitOr.html
    /// [`Not`]: ops/trait.Not.html
    ///
    /// # Examples
    ///
    /// A trivial example of the usage of `bool`,
    ///
    /// ```
    /// let praise_the_borrow_checker = true;
    ///
    /// // using the `if` conditional
    /// if praise_the_borrow_checker {
    ///     println!("oh, yeah!");
    /// } else {
    ///     println!("what?!!");
    /// }
    ///
    /// // ... or, a match pattern
    /// match praise_the_borrow_checker {
    ///     true => println!("keep praising!"),
    ///     false => println!("you should praise!"),
    /// }
    /// ```
    ///
    /// Also, since `bool` implements the [`Copy`](marker/trait.Copy.html) trait, we don't
    /// have to worry about the move semantics (just like the integer and float primitives).
    ///
    /// Now an example of `bool` cast to integer type:
    ///
    /// ```
    /// assert_eq!(true as i32, 1);
    /// assert_eq!(false as i32, 0);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_bool { }

    #[doc(primitive = "never")]
    #[doc(alias = "!")]
    //
    /// The `!` type, also called "never".
    ///
    /// `!` represents the type of computations which never resolve to any value at all. For example,
    /// the [`exit`] function `fn exit(code: i32) -> !` exits the process without ever returning, and
    /// so returns `!`.
    ///
    /// `break`, `continue` and `return` expressions also have type `!`. For example we are allowed to
    /// write:
    ///
    /// ```
    /// #![feature(never_type)]
    /// # fn foo() -> u32 {
    /// let x: ! = {
    ///     return 123
    /// };
    /// # }
    /// ```
    ///
    /// Although the `let` is pointless here, it illustrates the meaning of `!`. Since `x` is never
    /// assigned a value (because `return` returns from the entire function), `x` can be given type
    /// `!`. We could also replace `return 123` with a `panic!` or a never-ending `loop` and this code
    /// would still be valid.
    ///
    /// A more realistic usage of `!` is in this code:
    ///
    /// ```
    /// # fn get_a_number() -> Option<u32> { None }
    /// # loop {
    /// let num: u32 = match get_a_number() {
    ///     Some(num) => num,
    ///     None => break,
    /// };
    /// # }
    /// ```
    ///
    /// Both match arms must produce values of type [`u32`], but since `break` never produces a value
    /// at all we know it can never produce a value which isn't a [`u32`]. This illustrates another
    /// behaviour of the `!` type - expressions with type `!` will coerce into any other type.
    ///
    /// [`u32`]: primitive.str.html
    /// [`exit`]: process/fn.exit.html
    ///
    /// # `!` and generics
    ///
    /// ## Infallible errors
    ///
    /// The main place you'll see `!` used explicitly is in generic code. Consider the [`FromStr`]
    /// trait:
    ///
    /// ```
    /// trait FromStr: Sized {
    ///     type Err;
    ///     fn from_str(s: &str) -> Result<Self, Self::Err>;
    /// }
    /// ```
    ///
    /// When implementing this trait for [`String`] we need to pick a type for [`Err`]. And since
    /// converting a string into a string will never result in an error, the appropriate type is `!`.
    /// (Currently the type actually used is an enum with no variants, though this is only because `!`
    /// was added to Rust at a later date and it may change in the future.) With an [`Err`] type of
    /// `!`, if we have to call [`String::from_str`] for some reason the result will be a
    /// [`Result<String, !>`] which we can unpack like this:
    ///
    /// ```ignore (string-from-str-error-type-is-not-never-yet)
    /// #[feature(exhaustive_patterns)]
    /// // NOTE: this does not work today!
    /// let Ok(s) = String::from_str("hello");
    /// ```
    ///
    /// Since the [`Err`] variant contains a `!`, it can never occur. If the `exhaustive_patterns`
    /// feature is present this means we can exhaustively match on [`Result<T, !>`] by just taking the
    /// [`Ok`] variant. This illustrates another behaviour of `!` - it can be used to "delete" certain
    /// enum variants from generic types like `Result`.
    ///
    /// ## Infinite loops
    ///
    /// While [`Result<T, !>`] is very useful for removing errors, `!` can also be used to remove
    /// successes as well. If we think of [`Result<T, !>`] as "if this function returns, it has not
    /// errored," we get a very intuitive idea of [`Result<!, E>`] as well: if the function returns, it
    /// *has* errored.
    ///
    /// For example, consider the case of a simple web server, which can be simplified to:
    ///
    /// ```ignore (hypothetical-example)
    /// loop {
    ///     let (client, request) = get_request().expect("disconnected");
    ///     let response = request.process();
    ///     response.send(client);
    /// }
    /// ```
    ///
    /// Currently, this isn't ideal, because we simply panic whenever we fail to get a new connection.
    /// Instead, we'd like to keep track of this error, like this:
    ///
    /// ```ignore (hypothetical-example)
    /// loop {
    ///     match get_request() {
    ///         Err(err) => break err,
    ///         Ok((client, request)) => {
    ///             let response = request.process();
    ///             response.send(client);
    ///         },
    ///     }
    /// }
    /// ```
    ///
    /// Now, when the server disconnects, we exit the loop with an error instead of panicking. While it
    /// might be intuitive to simply return the error, we might want to wrap it in a [`Result<!, E>`]
    /// instead:
    ///
    /// ```ignore (hypothetical-example)
    /// fn server_loop() -> Result<!, ConnectionError> {
    ///     loop {
    ///         let (client, request) = get_request()?;
    ///         let response = request.process();
    ///         response.send(client);
    ///     }
    /// }
    /// ```
    ///
    /// Now, we can use `?` instead of `match`, and the return type makes a lot more sense: if the loop
    /// ever stops, it means that an error occurred. We don't even have to wrap the loop in an `Ok`
    /// because `!` coerces to `Result<!, ConnectionError>` automatically.
    ///
    /// [`String::from_str`]: str/trait.FromStr.html#tymethod.from_str
    /// [`Result<String, !>`]: result/enum.Result.html
    /// [`Result<T, !>`]: result/enum.Result.html
    /// [`Result<!, E>`]: result/enum.Result.html
    /// [`Ok`]: result/enum.Result.html#variant.Ok
    /// [`String`]: string/struct.String.html
    /// [`Err`]: result/enum.Result.html#variant.Err
    /// [`FromStr`]: str/trait.FromStr.html
    ///
    /// # `!` and traits
    ///
    /// When writing your own traits, `!` should have an `impl` whenever there is an obvious `impl`
    /// which doesn't `panic!`. As it turns out, most traits can have an `impl` for `!`. Take [`Debug`]
    /// for example:
    ///
    /// ```
    /// #![feature(never_type)]
    /// # use std::fmt;
    /// # trait Debug {
    /// # fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
    /// # }
    /// impl Debug for ! {
    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         *self
    ///     }
    /// }
    /// ```
    ///
    /// Once again we're using `!`'s ability to coerce into any other type, in this case
    /// [`fmt::Result`]. Since this method takes a `&!` as an argument we know that it can never be
    /// called (because there is no value of type `!` for it to be called with). Writing `*self`
    /// essentially tells the compiler "We know that this code can never be run, so just treat the
    /// entire function body as having type [`fmt::Result`]". This pattern can be used a lot when
    /// implementing traits for `!`. Generally, any trait which only has methods which take a `self`
    /// parameter should have such an impl.
    ///
    /// On the other hand, one trait which would not be appropriate to implement is [`Default`]:
    ///
    /// ```
    /// trait Default {
    ///     fn default() -> Self;
    /// }
    /// ```
    ///
    /// Since `!` has no values, it has no default value either. It's true that we could write an
    /// `impl` for this which simply panics, but the same is true for any type (we could `impl
    /// Default` for (eg.) [`File`] by just making [`default()`] panic.)
    ///
    /// [`fmt::Result`]: fmt/type.Result.html
    /// [`File`]: fs/struct.File.html
    /// [`Debug`]: fmt/trait.Debug.html
    /// [`Default`]: default/trait.Default.html
    /// [`default()`]: default/trait.Default.html#tymethod.default
    ///
    #[unstable(feature = "never_type", issue = "35121")]
    mod prim_never { }

    #[doc(primitive = "char")]
    //
    /// A character type.
    ///
    /// The `char` type represents a single character. More specifically, since
    /// 'character' isn't a well-defined concept in Unicode, `char` is a '[Unicode
    /// scalar value]', which is similar to, but not the same as, a '[Unicode code
    /// point]'.
    ///
    /// [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
    /// [Unicode code point]: http://www.unicode.org/glossary/#code_point
    ///
    /// This documentation describes a number of methods and trait implementations on the
    /// `char` type. For technical reasons, there is additional, separate
    /// documentation in [the `std::char` module](char/index.html) as well.
    ///
    /// # Representation
    ///
    /// `char` is always four bytes in size. This is a different representation than
    /// a given character would have as part of a [`String`]. For example:
    ///
    /// ```
    /// let v = vec!['h', 'e', 'l', 'l', 'o'];
    ///
    /// // five elements times four bytes for each element
    /// assert_eq!(20, v.len() * std::mem::size_of::<char>());
    ///
    /// let s = String::from("hello");
    ///
    /// // five elements times one byte per element
    /// assert_eq!(5, s.len() * std::mem::size_of::<u8>());
    /// ```
    ///
    /// [`String`]: string/struct.String.html
    ///
    /// As always, remember that a human intuition for 'character' may not map to
    /// Unicode's definitions. For example, despite looking similar, the 'é'
    /// character is one Unicode code point while 'é' is two Unicode code points:
    ///
    /// ```
    /// let mut chars = "é".chars();
    /// // U+00e9: 'latin small letter e with acute'
    /// assert_eq!(Some('\u{00e9}'), chars.next());
    /// assert_eq!(None, chars.next());
    ///
    /// let mut chars = "é".chars();
    /// // U+0065: 'latin small letter e'
    /// assert_eq!(Some('\u{0065}'), chars.next());
    /// // U+0301: 'combining acute accent'
    /// assert_eq!(Some('\u{0301}'), chars.next());
    /// assert_eq!(None, chars.next());
    /// ```
    ///
    /// This means that the contents of the first string above _will_ fit into a
    /// `char` while the contents of the second string _will not_. Trying to create
    /// a `char` literal with the contents of the second string gives an error:
    ///
    /// ```text
    /// error: character literal may only contain one codepoint: 'é'
    /// let c = 'é';
    ///         ^^^
    /// ```
    ///
    /// Another implication of the 4-byte fixed size of a `char` is that
    /// per-`char` processing can end up using a lot more memory:
    ///
    /// ```
    /// let s = String::from("love: ❤️");
    /// let v: Vec<char> = s.chars().collect();
    ///
    /// assert_eq!(12, std::mem::size_of_val(&s[..]));
    /// assert_eq!(32, std::mem::size_of_val(&v[..]));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_char { }

    #[doc(primitive = "unit")]
    //
    /// The `()` type, sometimes called "unit" or "nil".
    ///
    /// The `()` type has exactly one value `()`, and is used when there
    /// is no other meaningful value that could be returned. `()` is most
    /// commonly seen implicitly: functions without a `-> ...` implicitly
    /// have return type `()`, that is, these are equivalent:
    ///
    /// ```rust
    /// fn long() -> () {}
    ///
    /// fn short() {}
    /// ```
    ///
    /// The semicolon `;` can be used to discard the result of an
    /// expression at the end of a block, making the expression (and thus
    /// the block) evaluate to `()`. For example,
    ///
    /// ```rust
    /// fn returns_i64() -> i64 {
    ///     1i64
    /// }
    /// fn returns_unit() {
    ///     1i64;
    /// }
    ///
    /// let is_i64 = {
    ///     returns_i64()
    /// };
    /// let is_unit = {
    ///     returns_i64();
    /// };
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_unit { }

    #[doc(primitive = "pointer")]
    //
    /// Raw, unsafe pointers, `*const T`, and `*mut T`.
    ///
    /// *[See also the `std::ptr` module](ptr/index.html).*
    ///
    /// Working with raw pointers in Rust is uncommon, typically limited to a few patterns.
    /// Raw pointers can be unaligned or [`null`]. However, when a raw pointer is
    /// dereferenced (using the `*` operator), it must be non-null and aligned.
    ///
    /// Storing through a raw pointer using `*ptr = data` calls `drop` on the old value, so
    /// [`write`] must be used if the type has drop glue and memory is not already
    /// initialized - otherwise `drop` would be called on the uninitialized memory.
    ///
    /// Use the [`null`] and [`null_mut`] functions to create null pointers, and the
    /// [`is_null`] method of the `*const T` and `*mut T` types to check for null.
    /// The `*const T` and `*mut T` types also define the [`offset`] method, for
    /// pointer math.
    ///
    /// # Common ways to create raw pointers
    ///
    /// ## 1. Coerce a reference (`&T`) or mutable reference (`&mut T`).
    ///
    /// ```
    /// let my_num: i32 = 10;
    /// let my_num_ptr: *const i32 = &my_num;
    /// let mut my_speed: i32 = 88;
    /// let my_speed_ptr: *mut i32 = &mut my_speed;
    /// ```
    ///
    /// To get a pointer to a boxed value, dereference the box:
    ///
    /// ```
    /// let my_num: Box<i32> = Box::new(10);
    /// let my_num_ptr: *const i32 = &*my_num;
    /// let mut my_speed: Box<i32> = Box::new(88);
    /// let my_speed_ptr: *mut i32 = &mut *my_speed;
    /// ```
    ///
    /// This does not take ownership of the original allocation
    /// and requires no resource management later,
    /// but you must not use the pointer after its lifetime.
    ///
    /// ## 2. Consume a box (`Box<T>`).
    ///
    /// The [`into_raw`] function consumes a box and returns
    /// the raw pointer. It doesn't destroy `T` or deallocate any memory.
    ///
    /// ```
    /// let my_speed: Box<i32> = Box::new(88);
    /// let my_speed: *mut i32 = Box::into_raw(my_speed);
    ///
    /// // By taking ownership of the original `Box<T>` though
    /// // we are obligated to put it together later to be destroyed.
    /// unsafe {
    ///     drop(Box::from_raw(my_speed));
    /// }
    /// ```
    ///
    /// Note that here the call to [`drop`] is for clarity - it indicates
    /// that we are done with the given value and it should be destroyed.
    ///
    /// ## 3. Get it from C.
    ///
    /// ```
    /// # #![feature(rustc_private)]
    /// extern crate libc;
    ///
    /// use std::mem;
    ///
    /// fn main() {
    ///     unsafe {
    ///         let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>()) as *mut i32;
    ///         if my_num.is_null() {
    ///             panic!("failed to allocate memory");
    ///         }
    ///         libc::free(my_num as *mut libc::c_void);
    ///     }
    /// }
    /// ```
    ///
    /// Usually you wouldn't literally use `malloc` and `free` from Rust,
    /// but C APIs hand out a lot of pointers generally, so are a common source
    /// of raw pointers in Rust.
    ///
    /// [`null`]: ../std/ptr/fn.null.html
    /// [`null_mut`]: ../std/ptr/fn.null_mut.html
    /// [`is_null`]: ../std/primitive.pointer.html#method.is_null
    /// [`offset`]: ../std/primitive.pointer.html#method.offset
    /// [`into_raw`]: ../std/boxed/struct.Box.html#method.into_raw
    /// [`drop`]: ../std/mem/fn.drop.html
    /// [`write`]: ../std/ptr/fn.write.html
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_pointer { }

    #[doc(primitive = "array")]
    //
    /// A fixed-size array, denoted `[T; N]`, for the element type, `T`, and the
    /// non-negative compile-time constant size, `N`.
    ///
    /// There are two syntactic forms for creating an array:
    ///
    /// * A list with each element, i.e., `[x, y, z]`.
    /// * A repeat expression `[x; N]`, which produces an array with `N` copies of `x`.
    ///   The type of `x` must be [`Copy`][copy].
    ///
    /// Arrays of sizes from 0 to 32 (inclusive) implement the following traits if
    /// the element type allows it:
    ///
    /// - [`Debug`][debug]
    /// - [`IntoIterator`][intoiterator] (implemented for `&[T; N]` and `&mut [T; N]`)
    /// - [`PartialEq`][partialeq], [`PartialOrd`][partialord], [`Eq`][eq], [`Ord`][ord]
    /// - [`Hash`][hash]
    /// - [`AsRef`][asref], [`AsMut`][asmut]
    /// - [`Borrow`][borrow], [`BorrowMut`][borrowmut]
    /// - [`Default`][default]
    ///
    /// This limitation on the size `N` exists because Rust does not yet support
    /// code that is generic over the size of an array type. `[Foo; 3]` and `[Bar; 3]`
    /// are instances of same generic type `[T; 3]`, but `[Foo; 3]` and `[Foo; 5]` are
    /// entirely different types. As a stopgap, trait implementations are
    /// statically generated up to size 32.
    ///
    /// Arrays of *any* size are [`Copy`][copy] if the element type is [`Copy`][copy]
    /// and [`Clone`][clone] if the element type is [`Clone`][clone]. This works
    /// because [`Copy`][copy] and [`Clone`][clone] traits are specially known
    /// to the compiler.
    ///
    /// Arrays coerce to [slices (`[T]`)][slice], so a slice method may be called on
    /// an array. Indeed, this provides most of the API for working with arrays.
    /// Slices have a dynamic size and do not coerce to arrays.
    ///
    /// You can move elements out of an array with a slice pattern. If you want
    /// one element, see [`mem::replace`][replace].
    ///
    /// # Examples
    ///
    /// ```
    /// let mut array: [i32; 3] = [0; 3];
    ///
    /// array[1] = 1;
    /// array[2] = 2;
    ///
    /// assert_eq!([1, 2], &array[1..]);
    ///
    /// // This loop prints: 0 1 2
    /// for x in &array {
    ///     print!("{} ", x);
    /// }
    /// ```
    ///
    /// An array itself is not iterable:
    ///
    /// ```compile_fail,E0277
    /// let array: [i32; 3] = [0; 3];
    ///
    /// for x in array { }
    /// // error: the trait bound `[i32; 3]: std::iter::Iterator` is not satisfied
    /// ```
    ///
    /// The solution is to coerce the array to a slice by calling a slice method:
    ///
    /// ```
    /// # let array: [i32; 3] = [0; 3];
    /// for x in array.iter() { }
    /// ```
    ///
    /// If the array has 32 or fewer elements (see above), you can also use the
    /// array reference's [`IntoIterator`] implementation:
    ///
    /// ```
    /// # let array: [i32; 3] = [0; 3];
    /// for x in &array { }
    /// ```
    ///
    /// You can use a slice pattern to move elements out of an array:
    ///
    /// ```
    /// fn move_away(_: String) { /* Do interesting things. */ }
    ///
    /// let [john, roa] = ["John".to_string(), "Roa".to_string()];
    /// move_away(john);
    /// move_away(roa);
    /// ```
    ///
    /// [slice]: primitive.slice.html
    /// [copy]: marker/trait.Copy.html
    /// [clone]: clone/trait.Clone.html
    /// [debug]: fmt/trait.Debug.html
    /// [intoiterator]: iter/trait.IntoIterator.html
    /// [partialeq]: cmp/trait.PartialEq.html
    /// [partialord]: cmp/trait.PartialOrd.html
    /// [eq]: cmp/trait.Eq.html
    /// [ord]: cmp/trait.Ord.html
    /// [hash]: hash/trait.Hash.html
    /// [asref]: convert/trait.AsRef.html
    /// [asmut]: convert/trait.AsMut.html
    /// [borrow]: borrow/trait.Borrow.html
    /// [borrowmut]: borrow/trait.BorrowMut.html
    /// [default]: default/trait.Default.html
    /// [replace]: mem/fn.replace.html
    /// [`IntoIterator`]: iter/trait.IntoIterator.html
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_array { }

    #[doc(primitive = "slice")]
    #[doc(alias = "[")]
    #[doc(alias = "]")]
    #[doc(alias = "[]")]
    /// A dynamically-sized view into a contiguous sequence, `[T]`.
    ///
    /// *[See also the `std::slice` module](slice/index.html).*
    ///
    /// Slices are a view into a block of memory represented as a pointer and a
    /// length.
    ///
    /// ```
    /// // slicing a Vec
    /// let vec = vec![1, 2, 3];
    /// let int_slice = &vec[..];
    /// // coercing an array to a slice
    /// let str_slice: &[&str] = &["one", "two", "three"];
    /// ```
    ///
    /// Slices are either mutable or shared. The shared slice type is `&[T]`,
    /// while the mutable slice type is `&mut [T]`, where `T` represents the element
    /// type. For example, you can mutate the block of memory that a mutable slice
    /// points to:
    ///
    /// ```
    /// let mut x = [1, 2, 3];
    /// let x = &mut x[..]; // Take a full slice of `x`.
    /// x[1] = 7;
    /// assert_eq!(x, &[1, 7, 3]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_slice { }

    #[doc(primitive = "str")]
    //
    /// String slices.
    ///
    /// *[See also the `std::str` module](str/index.html).*
    ///
    /// The `str` type, also called a 'string slice', is the most primitive string
    /// type. It is usually seen in its borrowed form, `&str`. It is also the type
    /// of string literals, `&'static str`.
    ///
    /// String slices are always valid UTF-8.
    ///
    /// # Examples
    ///
    /// String literals are string slices:
    ///
    /// ```
    /// let hello = "Hello, world!";
    ///
    /// // with an explicit type annotation
    /// let hello: &'static str = "Hello, world!";
    /// ```
    ///
    /// They are `'static` because they're stored directly in the final binary, and
    /// so will be valid for the `'static` duration.
    ///
    /// # Representation
    ///
    /// A `&str` is made up of two components: a pointer to some bytes, and a
    /// length. You can look at these with the [`as_ptr`] and [`len`] methods:
    ///
    /// ```
    /// use std::slice;
    /// use std::str;
    ///
    /// let story = "Once upon a time...";
    ///
    /// let ptr = story.as_ptr();
    /// let len = story.len();
    ///
    /// // story has nineteen bytes
    /// assert_eq!(19, len);
    ///
    /// // We can re-build a str out of ptr and len. This is all unsafe because
    /// // we are responsible for making sure the two components are valid:
    /// let s = unsafe {
    ///     // First, we build a &[u8]...
    ///     let slice = slice::from_raw_parts(ptr, len);
    ///
    ///     // ... and then convert that slice into a string slice
    ///     str::from_utf8(slice)
    /// };
    ///
    /// assert_eq!(s, Ok(story));
    /// ```
    ///
    /// [`as_ptr`]: #method.as_ptr
    /// [`len`]: #method.len
    ///
    /// Note: This example shows the internals of `&str`. `unsafe` should not be
    /// used to get a string slice under normal circumstances. Use `as_slice`
    /// instead.
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_str { }

    #[doc(primitive = "tuple")]
    #[doc(alias = "(")]
    #[doc(alias = ")")]
    #[doc(alias = "()")]
    //
    /// A finite heterogeneous sequence, `(T, U, ..)`.
    ///
    /// Let's cover each of those in turn:
    ///
    /// Tuples are *finite*. In other words, a tuple has a length. Here's a tuple
    /// of length `3`:
    ///
    /// ```
    /// ("hello", 5, 'c');
    /// ```
    ///
    /// 'Length' is also sometimes called 'arity' here; each tuple of a different
    /// length is a different, distinct type.
    ///
    /// Tuples are *heterogeneous*. This means that each element of the tuple can
    /// have a different type. In that tuple above, it has the type:
    ///
    /// ```
    /// # let _:
    /// (&'static str, i32, char)
    /// # = ("hello", 5, 'c');
    /// ```
    ///
    /// Tuples are a *sequence*. This means that they can be accessed by position;
    /// this is called 'tuple indexing', and it looks like this:
    ///
    /// ```rust
    /// let tuple = ("hello", 5, 'c');
    ///
    /// assert_eq!(tuple.0, "hello");
    /// assert_eq!(tuple.1, 5);
    /// assert_eq!(tuple.2, 'c');
    /// ```
    ///
    /// The sequential nature of the tuple applies to its implementations of various
    /// traits.  For example, in `PartialOrd` and `Ord`, the elements are compared
    /// sequentially until the first non-equal set is found.
    ///
    /// For more about tuples, see [the book](../book/ch03-02-data-types.html#the-tuple-type).
    ///
    /// # Trait implementations
    ///
    /// If every type inside a tuple implements one of the following traits, then a
    /// tuple itself also implements it.
    ///
    /// * [`Clone`]
    /// * [`Copy`]
    /// * [`PartialEq`]
    /// * [`Eq`]
    /// * [`PartialOrd`]
    /// * [`Ord`]
    /// * [`Debug`]
    /// * [`Default`]
    /// * [`Hash`]
    ///
    /// [`Clone`]: clone/trait.Clone.html
    /// [`Copy`]: marker/trait.Copy.html
    /// [`PartialEq`]: cmp/trait.PartialEq.html
    /// [`Eq`]: cmp/trait.Eq.html
    /// [`PartialOrd`]: cmp/trait.PartialOrd.html
    /// [`Ord`]: cmp/trait.Ord.html
    /// [`Debug`]: fmt/trait.Debug.html
    /// [`Default`]: default/trait.Default.html
    /// [`Hash`]: hash/trait.Hash.html
    ///
    /// Due to a temporary restriction in Rust's type system, these traits are only
    /// implemented on tuples of arity 12 or less. In the future, this may change.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let tuple = ("hello", 5, 'c');
    ///
    /// assert_eq!(tuple.0, "hello");
    /// ```
    ///
    /// Tuples are often used as a return type when you want to return more than
    /// one value:
    ///
    /// ```
    /// fn calculate_point() -> (i32, i32) {
    ///     // Don't do a calculation, that's not the point of the example
    ///     (4, 5)
    /// }
    ///
    /// let point = calculate_point();
    ///
    /// assert_eq!(point.0, 4);
    /// assert_eq!(point.1, 5);
    ///
    /// // Combining this with patterns can be nicer.
    ///
    /// let (x, y) = calculate_point();
    ///
    /// assert_eq!(x, 4);
    /// assert_eq!(y, 5);
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_tuple { }

    #[doc(primitive = "f32")]
    /// The 32-bit floating point type.
    ///
    /// *[See also the `std::f32` module](f32/index.html).*
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_f32 { }

    #[doc(primitive = "f64")]
    //
    /// The 64-bit floating point type.
    ///
    /// *[See also the `std::f64` module](f64/index.html).*
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_f64 { }

    #[doc(primitive = "i8")]
    //
    /// The 8-bit signed integer type.
    ///
    /// *[See also the `std::i8` module](i8/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_i8 { }

    #[doc(primitive = "i16")]
    //
    /// The 16-bit signed integer type.
    ///
    /// *[See also the `std::i16` module](i16/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_i16 { }

    #[doc(primitive = "i32")]
    //
    /// The 32-bit signed integer type.
    ///
    /// *[See also the `std::i32` module](i32/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_i32 { }

    #[doc(primitive = "i64")]
    //
    /// The 64-bit signed integer type.
    ///
    /// *[See also the `std::i64` module](i64/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_i64 { }

    #[doc(primitive = "i128")]
    //
    /// The 128-bit signed integer type.
    ///
    /// *[See also the `std::i128` module](i128/index.html).*
    #[stable(feature = "i128", since="1.26.0")]
    mod prim_i128 { }

    #[doc(primitive = "u8")]
    //
    /// The 8-bit unsigned integer type.
    ///
    /// *[See also the `std::u8` module](u8/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_u8 { }

    #[doc(primitive = "u16")]
    //
    /// The 16-bit unsigned integer type.
    ///
    /// *[See also the `std::u16` module](u16/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_u16 { }

    #[doc(primitive = "u32")]
    //
    /// The 32-bit unsigned integer type.
    ///
    /// *[See also the `std::u32` module](u32/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_u32 { }

    #[doc(primitive = "u64")]
    //
    /// The 64-bit unsigned integer type.
    ///
    /// *[See also the `std::u64` module](u64/index.html).*
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_u64 { }

    #[doc(primitive = "u128")]
    //
    /// The 128-bit unsigned integer type.
    ///
    /// *[See also the `std::u128` module](u128/index.html).*
    #[stable(feature = "i128", since="1.26.0")]
    mod prim_u128 { }

    #[doc(primitive = "isize")]
    //
    /// The pointer-sized signed integer type.
    ///
    /// *[See also the `std::isize` module](isize/index.html).*
    ///
    /// The size of this primitive is how many bytes it takes to reference any
    /// location in memory. For example, on a 32 bit target, this is 4 bytes
    /// and on a 64 bit target, this is 8 bytes.
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_isize { }

    #[doc(primitive = "usize")]
    //
    /// The pointer-sized unsigned integer type.
    ///
    /// *[See also the `std::usize` module](usize/index.html).*
    ///
    /// The size of this primitive is how many bytes it takes to reference any
    /// location in memory. For example, on a 32 bit target, this is 4 bytes
    /// and on a 64 bit target, this is 8 bytes.
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_usize { }

    #[doc(primitive = "reference")]
    #[doc(alias = "&")]
    //
    /// References, both shared and mutable.
    ///
    /// A reference represents a borrow of some owned value. You can get one by using the `&` or `&mut`
    /// operators on a value, or by using a `ref` or `ref mut` pattern.
    ///
    /// For those familiar with pointers, a reference is just a pointer that is assumed to be
    /// aligned, not null, and pointing to memory containing a valid value of `T` - for example,
    /// `&bool` can only point to an allocation containing the integer values `1` (`true`) or `0`
    /// (`false`), but creating a `&bool` that points to an allocation containing
    /// the value `3` causes undefined behaviour.
    /// In fact, `Option<&T>` has the same memory representation as a
    /// nullable but aligned pointer, and can be passed across FFI boundaries as such.
    ///
    /// In most cases, references can be used much like the original value. Field access, method
    /// calling, and indexing work the same (save for mutability rules, of course). In addition, the
    /// comparison operators transparently defer to the referent's implementation, allowing references
    /// to be compared the same as owned values.
    ///
    /// References have a lifetime attached to them, which represents the scope for which the borrow is
    /// valid. A lifetime is said to "outlive" another one if its representative scope is as long or
    /// longer than the other. The `'static` lifetime is the longest lifetime, which represents the
    /// total life of the program. For example, string literals have a `'static` lifetime because the
    /// text data is embedded into the binary of the program, rather than in an allocation that needs
    /// to be dynamically managed.
    ///
    /// `&mut T` references can be freely coerced into `&T` references with the same referent type, and
    /// references with longer lifetimes can be freely coerced into references with shorter ones.
    ///
    /// Reference equality by address, instead of comparing the values pointed to, is accomplished via
    /// implicit reference-pointer coercion and raw pointer equality via [`ptr::eq`], while
    /// [`PartialEq`] compares values.
    ///
    /// [`ptr::eq`]: ptr/fn.eq.html
    /// [`PartialEq`]: cmp/trait.PartialEq.html
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let five = 5;
    /// let other_five = 5;
    /// let five_ref = &five;
    /// let same_five_ref = &five;
    /// let other_five_ref = &other_five;
    ///
    /// assert!(five_ref == same_five_ref);
    /// assert!(five_ref == other_five_ref);
    ///
    /// assert!(ptr::eq(five_ref, same_five_ref));
    /// assert!(!ptr::eq(five_ref, other_five_ref));
    /// ```
    ///
    /// For more information on how to use references, see [the book's section on "References and
    /// Borrowing"][book-refs].
    ///
    /// [book-refs]: ../book/ch04-02-references-and-borrowing.html
    ///
    /// # Trait implementations
    ///
    /// The following traits are implemented for all `&T`, regardless of the type of its referent:
    ///
    /// * [`Copy`]
    /// * [`Clone`] \(Note that this will not defer to `T`'s `Clone` implementation if it exists!)
    /// * [`Deref`]
    /// * [`Borrow`]
    /// * [`Pointer`]
    ///
    /// [`Copy`]: marker/trait.Copy.html
    /// [`Clone`]: clone/trait.Clone.html
    /// [`Deref`]: ops/trait.Deref.html
    /// [`Borrow`]: borrow/trait.Borrow.html
    /// [`Pointer`]: fmt/trait.Pointer.html
    ///
    /// `&mut T` references get all of the above except `Copy` and `Clone` (to prevent creating
    /// multiple simultaneous mutable borrows), plus the following, regardless of the type of its
    /// referent:
    ///
    /// * [`DerefMut`]
    /// * [`BorrowMut`]
    ///
    /// [`DerefMut`]: ops/trait.DerefMut.html
    /// [`BorrowMut`]: borrow/trait.BorrowMut.html
    ///
    /// The following traits are implemented on `&T` references if the underlying `T` also implements
    /// that trait:
    ///
    /// * All the traits in [`std::fmt`] except [`Pointer`] and [`fmt::Write`]
    /// * [`PartialOrd`]
    /// * [`Ord`]
    /// * [`PartialEq`]
    /// * [`Eq`]
    /// * [`AsRef`]
    /// * [`Fn`] \(in addition, `&T` references get [`FnMut`] and [`FnOnce`] if `T: Fn`)
    /// * [`Hash`]
    /// * [`ToSocketAddrs`]
    ///
    /// [`std::fmt`]: fmt/index.html
    /// [`fmt::Write`]: fmt/trait.Write.html
    /// [`PartialOrd`]: cmp/trait.PartialOrd.html
    /// [`Ord`]: cmp/trait.Ord.html
    /// [`PartialEq`]: cmp/trait.PartialEq.html
    /// [`Eq`]: cmp/trait.Eq.html
    /// [`AsRef`]: convert/trait.AsRef.html
    /// [`Fn`]: ops/trait.Fn.html
    /// [`FnMut`]: ops/trait.FnMut.html
    /// [`FnOnce`]: ops/trait.FnOnce.html
    /// [`Hash`]: hash/trait.Hash.html
    /// [`ToSocketAddrs`]: net/trait.ToSocketAddrs.html
    ///
    /// `&mut T` references get all of the above except `ToSocketAddrs`, plus the following, if `T`
    /// implements that trait:
    ///
    /// * [`AsMut`]
    /// * [`FnMut`] \(in addition, `&mut T` references get [`FnOnce`] if `T: FnMut`)
    /// * [`fmt::Write`]
    /// * [`Iterator`]
    /// * [`DoubleEndedIterator`]
    /// * [`ExactSizeIterator`]
    /// * [`FusedIterator`]
    /// * [`TrustedLen`]
    /// * [`Send`] \(note that `&T` references only get `Send` if `T: Sync`)
    /// * [`io::Write`]
    /// * [`Read`]
    /// * [`Seek`]
    /// * [`BufRead`]
    ///
    /// [`AsMut`]: convert/trait.AsMut.html
    /// [`Iterator`]: iter/trait.Iterator.html
    /// [`DoubleEndedIterator`]: iter/trait.DoubleEndedIterator.html
    /// [`ExactSizeIterator`]: iter/trait.ExactSizeIterator.html
    /// [`FusedIterator`]: iter/trait.FusedIterator.html
    /// [`TrustedLen`]: iter/trait.TrustedLen.html
    /// [`Send`]: marker/trait.Send.html
    /// [`io::Write`]: io/trait.Write.html
    /// [`Read`]: io/trait.Read.html
    /// [`Seek`]: io/trait.Seek.html
    /// [`BufRead`]: io/trait.BufRead.html
    ///
    /// Note that due to method call deref coercion, simply calling a trait method will act like they
    /// work on references as well as they do on owned values! The implementations described here are
    /// meant for generic contexts, where the final type `T` is a type parameter or otherwise not
    /// locally known.
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_ref { }

    #[doc(primitive = "fn")]
    //
    /// Function pointers, like `fn(usize) -> bool`.
    ///
    /// *See also the traits [`Fn`], [`FnMut`], and [`FnOnce`].*
    ///
    /// [`Fn`]: ops/trait.Fn.html
    /// [`FnMut`]: ops/trait.FnMut.html
    /// [`FnOnce`]: ops/trait.FnOnce.html
    ///
    /// Function pointers are pointers that point to *code*, not data. They can be called
    /// just like functions. Like references, function pointers are, among other things, assumed to
    /// not be null, so if you want to pass a function pointer over FFI and be able to accommodate null
    /// pointers, make your type `Option<fn()>` with your required signature.
    ///
    /// Plain function pointers are obtained by casting either plain functions, or closures that don't
    /// capture an environment:
    ///
    /// ```
    /// fn add_one(x: usize) -> usize {
    ///     x + 1
    /// }
    ///
    /// let ptr: fn(usize) -> usize = add_one;
    /// assert_eq!(ptr(5), 6);
    ///
    /// let clos: fn(usize) -> usize = |x| x + 5;
    /// assert_eq!(clos(5), 10);
    /// ```
    ///
    /// In addition to varying based on their signature, function pointers come in two flavors: safe
    /// and unsafe. Plain `fn()` function pointers can only point to safe functions,
    /// while `unsafe fn()` function pointers can point to safe or unsafe functions.
    ///
    /// ```
    /// fn add_one(x: usize) -> usize {
    ///     x + 1
    /// }
    ///
    /// unsafe fn add_one_unsafely(x: usize) -> usize {
    ///     x + 1
    /// }
    ///
    /// let safe_ptr: fn(usize) -> usize = add_one;
    ///
    /// //ERROR: mismatched types: expected normal fn, found unsafe fn
    /// //let bad_ptr: fn(usize) -> usize = add_one_unsafely;
    ///
    /// let unsafe_ptr: unsafe fn(usize) -> usize = add_one_unsafely;
    /// let really_safe_ptr: unsafe fn(usize) -> usize = add_one;
    /// ```
    ///
    /// On top of that, function pointers can vary based on what ABI they use. This is achieved by
    /// adding the `extern` keyword to the type name, followed by the ABI in question. For example,
    /// `fn()` is different from `extern "C" fn()`, which itself is different from `extern "stdcall"
    /// fn()`, and so on for the various ABIs that Rust supports. Non-`extern` functions have an ABI
    /// of `"Rust"`, and `extern` functions without an explicit ABI have an ABI of `"C"`. For more
    /// information, see [the nomicon's section on foreign calling conventions][nomicon-abi].
    ///
    /// [nomicon-abi]: ../nomicon/ffi.html#foreign-calling-conventions
    ///
    /// Extern function declarations with the "C" or "cdecl" ABIs can also be *variadic*, allowing them
    /// to be called with a variable number of arguments. Normal rust functions, even those with an
    /// `extern "ABI"`, cannot be variadic. For more information, see [the nomicon's section on
    /// variadic functions][nomicon-variadic].
    ///
    /// [nomicon-variadic]: ../nomicon/ffi.html#variadic-functions
    ///
    /// These markers can be combined, so `unsafe extern "stdcall" fn()` is a valid type.
    ///
    /// Function pointers implement the following traits:
    ///
    /// * [`Clone`]
    /// * [`PartialEq`]
    /// * [`Eq`]
    /// * [`PartialOrd`]
    /// * [`Ord`]
    /// * [`Hash`]
    /// * [`Pointer`]
    /// * [`Debug`]
    ///
    /// [`Clone`]: clone/trait.Clone.html
    /// [`PartialEq`]: cmp/trait.PartialEq.html
    /// [`Eq`]: cmp/trait.Eq.html
    /// [`PartialOrd`]: cmp/trait.PartialOrd.html
    /// [`Ord`]: cmp/trait.Ord.html
    /// [`Hash`]: hash/trait.Hash.html
    /// [`Pointer`]: fmt/trait.Pointer.html
    /// [`Debug`]: fmt/trait.Debug.html
    ///
    /// Due to a temporary restriction in Rust's type system, these traits are only implemented on
    /// functions that take 12 arguments or less, with the `"Rust"` and `"C"` ABIs. In the future, this
    /// may change.
    ///
    /// In addition, function pointers of *any* signature, ABI, or safety are [`Copy`], and all *safe*
    /// function pointers implement [`Fn`], [`FnMut`], and [`FnOnce`]. This works because these traits
    /// are specially known to the compiler.
    ///
    /// [`Copy`]: marker/trait.Copy.html
    #[stable(feature = "rust1", since = "1.0.0")]
    mod prim_fn { }

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