a19472a93e
Many tiny changes to stdlib doc comments to make them consistent (for example "Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph breaks, backticks for monospace style, and other minor nits. https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text
2459 lines
78 KiB
Rust
2459 lines
78 KiB
Rust
#[doc(keyword = "as")]
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//
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/// Cast between types, or rename an import.
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///
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/// `as` is most commonly used to turn primitive types into other primitive types, but it has other
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/// uses that include turning pointers into addresses, addresses into pointers, and pointers into
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/// other pointers.
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///
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/// ```rust
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/// let thing1: u8 = 89.0 as u8;
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/// assert_eq!('B' as u32, 66);
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/// assert_eq!(thing1 as char, 'Y');
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/// let thing2: f32 = thing1 as f32 + 10.5;
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/// assert_eq!(true as u8 + thing2 as u8, 100);
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/// ```
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///
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/// In general, any cast that can be performed via ascribing the type can also be done using `as`,
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/// so instead of writing `let x: u32 = 123`, you can write `let x = 123 as u32` (note: `let x: u32
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/// = 123` would be best in that situation). The same is not true in the other direction, however;
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/// explicitly using `as` allows a few more coercions that aren't allowed implicitly, such as
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/// changing the type of a raw pointer or turning closures into raw pointers.
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///
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/// `as` can be seen as the primitive for `From` and `Into`: `as` only works with primitives
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/// (`u8`, `bool`, `str`, pointers, ...) whereas `From` and `Into` also works with types like
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/// `String` or `Vec`.
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///
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/// `as` can also be used with the `_` placeholder when the destination type can be inferred. Note
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/// that this can cause inference breakage and usually such code should use an explicit type for
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/// both clarity and stability. This is most useful when converting pointers using `as *const _` or
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/// `as *mut _` though the [`cast`][const-cast] method is recommended over `as *const _` and it is
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/// [the same][mut-cast] for `as *mut _`: those methods make the intent clearer.
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///
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/// `as` is also used to rename imports in [`use`] and [`extern crate`][`crate`] statements:
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///
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/// ```
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/// # #[allow(unused_imports)]
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/// use std::{mem as memory, net as network};
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/// // Now you can use the names `memory` and `network` to refer to `std::mem` and `std::net`.
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/// ```
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/// For more information on what `as` is capable of, see the [Reference].
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///
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/// [Reference]: ../reference/expressions/operator-expr.html#type-cast-expressions
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/// [`crate`]: keyword.crate.html
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/// [`use`]: keyword.use.html
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/// [const-cast]: pointer::cast
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/// [mut-cast]: primitive.pointer.html#method.cast-1
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mod as_keyword {}
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#[doc(keyword = "break")]
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//
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/// Exit early from a loop or labelled block.
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///
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/// When `break` is encountered, execution of the associated loop body is
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/// immediately terminated.
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///
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/// ```rust
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/// let mut last = 0;
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///
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/// for x in 1..100 {
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/// if x > 12 {
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/// break;
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/// }
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/// last = x;
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/// }
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///
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/// assert_eq!(last, 12);
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/// println!("{last}");
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/// ```
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///
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/// A break expression is normally associated with the innermost loop enclosing the
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/// `break` but a label can be used to specify which enclosing loop is affected.
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///
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/// ```rust
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/// 'outer: for i in 1..=5 {
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/// println!("outer iteration (i): {i}");
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///
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/// '_inner: for j in 1..=200 {
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/// println!(" inner iteration (j): {j}");
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/// if j >= 3 {
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/// // breaks from inner loop, lets outer loop continue.
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/// break;
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/// }
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/// if i >= 2 {
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/// // breaks from outer loop, and directly to "Bye".
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/// break 'outer;
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/// }
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/// }
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/// }
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/// println!("Bye.");
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/// ```
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///
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/// When associated with `loop`, a break expression may be used to return a value from that loop.
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/// This is only valid with `loop` and not with any other type of loop.
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/// If no value is specified, `break;` returns `()`.
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/// Every `break` within a loop must return the same type.
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///
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/// ```rust
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/// let (mut a, mut b) = (1, 1);
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/// let result = loop {
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/// if b > 10 {
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/// break b;
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/// }
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/// let c = a + b;
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/// a = b;
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/// b = c;
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/// };
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/// // first number in Fibonacci sequence over 10:
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/// assert_eq!(result, 13);
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/// println!("{result}");
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/// ```
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///
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/// For more details consult the [Reference on "break expression"] and the [Reference on "break and
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/// loop values"].
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///
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/// [Reference on "break expression"]: ../reference/expressions/loop-expr.html#break-expressions
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/// [Reference on "break and loop values"]:
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/// ../reference/expressions/loop-expr.html#break-and-loop-values
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mod break_keyword {}
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#[doc(keyword = "const")]
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//
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/// Compile-time constants, compile-time evaluable functions, and raw pointers.
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///
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/// ## Compile-time constants
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///
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/// Sometimes a certain value is used many times throughout a program, and it can become
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/// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
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/// make it a variable that gets carried around to each function that needs it. In these cases, the
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/// `const` keyword provides a convenient alternative to code duplication:
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///
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/// ```rust
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/// const THING: u32 = 0xABAD1DEA;
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///
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/// let foo = 123 + THING;
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/// ```
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///
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/// Constants must be explicitly typed; unlike with `let`, you can't ignore their type and let the
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/// compiler figure it out. Any constant value can be defined in a `const`, which in practice happens
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/// to be most things that would be reasonable to have in a constant (barring `const fn`s). For
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/// example, you can't have a [`File`] as a `const`.
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///
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/// [`File`]: crate::fs::File
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///
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/// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses
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/// all others in a Rust program. For example, if you wanted to define a constant string, it would
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/// look like this:
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///
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/// ```rust
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/// const WORDS: &'static str = "hello rust!";
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/// ```
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///
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/// Thanks to static lifetime elision, you usually don't have to explicitly use `'static`:
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///
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/// ```rust
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/// const WORDS: &str = "hello convenience!";
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/// ```
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///
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/// `const` items looks remarkably similar to `static` items, which introduces some confusion as
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/// to which one should be used at which times. To put it simply, constants are inlined wherever
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/// they're used, making using them identical to simply replacing the name of the `const` with its
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/// value. Static variables, on the other hand, point to a single location in memory, which all
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/// accesses share. This means that, unlike with constants, they can't have destructors, and act as
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/// a single value across the entire codebase.
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///
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/// Constants, like statics, should always be in `SCREAMING_SNAKE_CASE`.
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///
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/// For more detail on `const`, see the [Rust Book] or the [Reference].
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///
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/// ## Compile-time evaluable functions
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///
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/// The other main use of the `const` keyword is in `const fn`. This marks a function as being
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/// callable in the body of a `const` or `static` item and in array initializers (commonly called
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/// "const contexts"). `const fn` are restricted in the set of operations they can perform, to
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/// ensure that they can be evaluated at compile-time. See the [Reference][const-eval] for more
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/// detail.
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///
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/// Turning a `fn` into a `const fn` has no effect on run-time uses of that function.
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///
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/// ## Other uses of `const`
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///
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/// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
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/// T` and `*mut T`. More about `const` as used in raw pointers can be read at the Rust docs for the [pointer primitive].
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///
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/// [pointer primitive]: pointer
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/// [Rust Book]: ../book/ch03-01-variables-and-mutability.html#constants
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/// [Reference]: ../reference/items/constant-items.html
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/// [const-eval]: ../reference/const_eval.html
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mod const_keyword {}
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#[doc(keyword = "continue")]
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//
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/// Skip to the next iteration of a loop.
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///
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/// When `continue` is encountered, the current iteration is terminated, returning control to the
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/// loop head, typically continuing with the next iteration.
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///
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/// ```rust
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/// // Printing odd numbers by skipping even ones
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/// for number in 1..=10 {
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/// if number % 2 == 0 {
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/// continue;
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/// }
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/// println!("{number}");
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/// }
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/// ```
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///
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/// Like `break`, `continue` is normally associated with the innermost enclosing loop, but labels
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/// may be used to specify the affected loop.
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///
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/// ```rust
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/// // Print Odd numbers under 30 with unit <= 5
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/// 'tens: for ten in 0..3 {
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/// '_units: for unit in 0..=9 {
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/// if unit % 2 == 0 {
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/// continue;
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/// }
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/// if unit > 5 {
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/// continue 'tens;
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/// }
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/// println!("{}", ten * 10 + unit);
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/// }
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/// }
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/// ```
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///
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/// See [continue expressions] from the reference for more details.
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///
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/// [continue expressions]: ../reference/expressions/loop-expr.html#continue-expressions
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mod continue_keyword {}
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#[doc(keyword = "crate")]
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//
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/// A Rust binary or library.
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///
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/// The primary use of the `crate` keyword is as a part of `extern crate` declarations, which are
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/// used to specify a dependency on a crate external to the one it's declared in. Crates are the
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/// fundamental compilation unit of Rust code, and can be seen as libraries or projects. More can
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/// be read about crates in the [Reference].
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///
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/// ```rust ignore
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/// extern crate rand;
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/// extern crate my_crate as thing;
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/// extern crate std; // implicitly added to the root of every Rust project
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/// ```
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///
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/// The `as` keyword can be used to change what the crate is referred to as in your project. If a
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/// crate name includes a dash, it is implicitly imported with the dashes replaced by underscores.
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///
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/// `crate` can also be used as in conjunction with `pub` to signify that the item it's attached to
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/// is public only to other members of the same crate it's in.
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///
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/// ```rust
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/// # #[allow(unused_imports)]
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/// pub(crate) use std::io::Error as IoError;
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/// pub(crate) enum CoolMarkerType { }
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/// pub struct PublicThing {
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/// pub(crate) semi_secret_thing: bool,
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/// }
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/// ```
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///
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/// `crate` is also used to represent the absolute path of a module, where `crate` refers to the
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/// root of the current crate. For instance, `crate::foo::bar` refers to the name `bar` inside the
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/// module `foo`, from anywhere else in the same crate.
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///
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/// [Reference]: ../reference/items/extern-crates.html
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mod crate_keyword {}
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#[doc(keyword = "else")]
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//
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/// What expression to evaluate when an [`if`] condition evaluates to [`false`].
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///
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/// `else` expressions are optional. When no else expressions are supplied it is assumed to evaluate
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/// to the unit type `()`.
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///
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/// The type that the `else` blocks evaluate to must be compatible with the type that the `if` block
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/// evaluates to.
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///
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/// As can be seen below, `else` must be followed by either: `if`, `if let`, or a block `{}` and it
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/// will return the value of that expression.
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///
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/// ```rust
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/// let result = if true == false {
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/// "oh no"
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/// } else if "something" == "other thing" {
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/// "oh dear"
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/// } else if let Some(200) = "blarg".parse::<i32>().ok() {
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/// "uh oh"
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/// } else {
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/// println!("Sneaky side effect.");
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/// "phew, nothing's broken"
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/// };
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/// ```
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///
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/// Here's another example but here we do not try and return an expression:
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///
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/// ```rust
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/// if true == false {
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/// println!("oh no");
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/// } else if "something" == "other thing" {
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/// println!("oh dear");
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/// } else if let Some(200) = "blarg".parse::<i32>().ok() {
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/// println!("uh oh");
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/// } else {
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/// println!("phew, nothing's broken");
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/// }
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/// ```
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///
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/// The above is _still_ an expression but it will always evaluate to `()`.
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///
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/// There is possibly no limit to the number of `else` blocks that could follow an `if` expression
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/// however if you have several then a [`match`] expression might be preferable.
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///
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/// Read more about control flow in the [Rust Book].
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///
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/// [Rust Book]: ../book/ch03-05-control-flow.html#handling-multiple-conditions-with-else-if
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/// [`match`]: keyword.match.html
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/// [`false`]: keyword.false.html
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/// [`if`]: keyword.if.html
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mod else_keyword {}
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#[doc(keyword = "enum")]
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//
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/// A type that can be any one of several variants.
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///
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/// Enums in Rust are similar to those of other compiled languages like C, but have important
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/// differences that make them considerably more powerful. What Rust calls enums are more commonly
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/// known as [Algebraic Data Types][ADT] if you're coming from a functional programming background.
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/// The important detail is that each enum variant can have data to go along with it.
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///
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/// ```rust
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/// # struct Coord;
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/// enum SimpleEnum {
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/// FirstVariant,
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/// SecondVariant,
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/// ThirdVariant,
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/// }
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///
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/// enum Location {
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/// Unknown,
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/// Anonymous,
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/// Known(Coord),
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/// }
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///
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/// enum ComplexEnum {
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/// Nothing,
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/// Something(u32),
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/// LotsOfThings {
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/// usual_struct_stuff: bool,
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/// blah: String,
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/// }
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/// }
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///
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/// enum EmptyEnum { }
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/// ```
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///
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/// The first enum shown is the usual kind of enum you'd find in a C-style language. The second
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/// shows off a hypothetical example of something storing location data, with `Coord` being any
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/// other type that's needed, for example a struct. The third example demonstrates the kind of
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/// data a variant can store, ranging from nothing, to a tuple, to an anonymous struct.
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///
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/// Instantiating enum variants involves explicitly using the enum's name as its namespace,
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/// followed by one of its variants. `SimpleEnum::SecondVariant` would be an example from above.
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/// When data follows along with a variant, such as with rust's built-in [`Option`] type, the data
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/// is added as the type describes, for example `Option::Some(123)`. The same follows with
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/// struct-like variants, with things looking like `ComplexEnum::LotsOfThings { usual_struct_stuff:
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/// true, blah: "hello!".to_string(), }`. Empty Enums are similar to [`!`] in that they cannot be
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/// instantiated at all, and are used mainly to mess with the type system in interesting ways.
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///
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/// For more information, take a look at the [Rust Book] or the [Reference]
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///
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/// [ADT]: https://en.wikipedia.org/wiki/Algebraic_data_type
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/// [Rust Book]: ../book/ch06-01-defining-an-enum.html
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/// [Reference]: ../reference/items/enumerations.html
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mod enum_keyword {}
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#[doc(keyword = "extern")]
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//
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/// Link to or import external code.
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///
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/// The `extern` keyword is used in two places in Rust. One is in conjunction with the [`crate`]
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/// keyword to make your Rust code aware of other Rust crates in your project, i.e., `extern crate
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/// lazy_static;`. The other use is in foreign function interfaces (FFI).
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///
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/// `extern` is used in two different contexts within FFI. The first is in the form of external
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/// blocks, for declaring function interfaces that Rust code can call foreign code by.
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///
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/// ```rust ignore
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/// #[link(name = "my_c_library")]
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/// extern "C" {
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/// fn my_c_function(x: i32) -> bool;
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/// }
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/// ```
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///
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/// This code would attempt to link with `libmy_c_library.so` on unix-like systems and
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/// `my_c_library.dll` on Windows at runtime, and panic if it can't find something to link to. Rust
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/// code could then use `my_c_function` as if it were any other unsafe Rust function. Working with
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/// non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.
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///
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/// The mirror use case of FFI is also done via the `extern` keyword:
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///
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/// ```rust
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/// #[no_mangle]
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/// pub extern "C" fn callable_from_c(x: i32) -> bool {
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/// x % 3 == 0
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/// }
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/// ```
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///
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/// If compiled as a dylib, the resulting .so could then be linked to from a C library, and the
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/// function could be used as if it was from any other library.
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///
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/// For more information on FFI, check the [Rust book] or the [Reference].
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///
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/// [Rust book]:
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/// ../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
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/// [Reference]: ../reference/items/external-blocks.html
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/// [`crate`]: keyword.crate.html
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mod extern_keyword {}
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#[doc(keyword = "false")]
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//
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|
/// A value of type [`bool`] representing logical **false**.
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|
///
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/// `false` is the logical opposite of [`true`].
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///
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|
/// See the documentation for [`true`] for more information.
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///
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|
/// [`true`]: keyword.true.html
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mod false_keyword {}
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#[doc(keyword = "fn")]
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//
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|
/// A function or function pointer.
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|
///
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|
/// Functions are the primary way code is executed within Rust. Function blocks, usually just
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/// called functions, can be defined in a variety of different places and be assigned many
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/// different attributes and modifiers.
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///
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|
/// Standalone functions that just sit within a module not attached to anything else are common,
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/// but most functions will end up being inside [`impl`] blocks, either on another type itself, or
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/// as a trait impl for that type.
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///
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/// ```rust
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|
/// fn standalone_function() {
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/// // code
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/// }
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|
///
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|
/// pub fn public_thing(argument: bool) -> String {
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/// // code
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|
/// # "".to_string()
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|
/// }
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///
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/// struct Thing {
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/// foo: i32,
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/// }
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///
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|
/// impl Thing {
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|
/// pub fn new() -> Self {
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/// Self {
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/// foo: 42,
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/// }
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/// }
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/// }
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/// ```
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///
|
|
/// In addition to presenting fixed types in the form of `fn name(arg: type, ..) -> return_type`,
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/// functions can also declare a list of type parameters along with trait bounds that they fall
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/// into.
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///
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/// ```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 anything that implements [`IntoIterator`] until the
|
|
/// iterator returned by `.into_iter()` returns `None` (or the loop body uses `break`).
|
|
///
|
|
/// ```rust
|
|
/// for i in 0..5 {
|
|
/// println!("{}", i * 2);
|
|
/// }
|
|
///
|
|
/// for i in std::iter::repeat(5) {
|
|
/// println!("turns out {i} never stops being 5");
|
|
/// 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 result = match IntoIterator::into_iter(iterator) {
|
|
/// mut iter => loop {
|
|
/// match iter.next() {
|
|
/// None => break,
|
|
/// Some(loop_variable) => { code(); },
|
|
/// };
|
|
/// },
|
|
/// };
|
|
/// result
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// 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].
|
|
///
|
|
/// See also, [`loop`], [`while`].
|
|
///
|
|
/// [`in`]: keyword.in.html
|
|
/// [`impl`]: keyword.impl.html
|
|
/// [`loop`]: keyword.loop.html
|
|
/// [`while`]: keyword.while.html
|
|
/// [higher-ranked trait bounds]: ../reference/trait-bounds.html#higher-ranked-trait-bounds
|
|
/// [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 expression immediately following `in` must implement the [`IntoIterator`] trait.
|
|
///
|
|
/// ## Literal Examples:
|
|
///
|
|
/// * `for _ in 1..3 {}` - Iterate over an exclusive range up to but excluding 3.
|
|
/// * `for _ in 1..=3 {}` - Iterate over an inclusive range up to and including 3.
|
|
///
|
|
/// (Read more about [range patterns])
|
|
///
|
|
/// [`IntoIterator`]: ../book/ch13-04-performance.html
|
|
/// [range patterns]: ../reference/patterns.html?highlight=range#range-patterns
|
|
/// [`for`]: keyword.for.html
|
|
///
|
|
/// The other use of `in` is with the keyword `pub`. It allows users to declare an item as visible
|
|
/// only within a given scope.
|
|
///
|
|
/// ## Literal Example:
|
|
///
|
|
/// * `pub(in crate::outer_mod) fn outer_mod_visible_fn() {}` - fn is visible in `outer_mod`
|
|
///
|
|
/// Starting with the 2018 edition, paths for `pub(in path)` must start with `crate`, `self` or
|
|
/// `super`. The 2015 edition may also use paths starting with `::` or modules from the crate root.
|
|
///
|
|
/// For more information, see the [Reference].
|
|
///
|
|
/// [Reference]: ../reference/visibility-and-privacy.html#pubin-path-pubcrate-pubsuper-and-pubself
|
|
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].
|
|
///
|
|
/// See also, [`for`], [`loop`].
|
|
///
|
|
/// [`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 = 1;
|
|
/// loop {
|
|
/// println!("i is {i}");
|
|
/// if i > 100 {
|
|
/// break;
|
|
/// }
|
|
/// i *= 2;
|
|
/// }
|
|
/// assert_eq!(i, 128);
|
|
/// ```
|
|
///
|
|
/// 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].
|
|
///
|
|
/// See also, [`for`], [`while`].
|
|
///
|
|
/// [`for`]: keyword.for.html
|
|
/// [`while`]: keyword.while.html
|
|
/// [Reference]: ../reference/expressions/loop-expr.html
|
|
mod loop_keyword {}
|
|
|
|
#[doc(keyword = "match")]
|
|
//
|
|
/// Control flow based on pattern matching.
|
|
///
|
|
/// `match` can be used to run code conditionally. Every pattern must
|
|
/// be handled exhaustively either explicitly or by using wildcards like
|
|
/// `_` in the `match`. Since `match` is an expression, values can also be
|
|
/// returned.
|
|
///
|
|
/// ```rust
|
|
/// let opt = Option::None::<usize>;
|
|
/// let x = match opt {
|
|
/// Some(int) => int,
|
|
/// None => 10,
|
|
/// };
|
|
/// assert_eq!(x, 10);
|
|
///
|
|
/// let a_number = Option::Some(10);
|
|
/// match a_number {
|
|
/// Some(x) if x <= 5 => println!("0 to 5 num = {x}"),
|
|
/// Some(x @ 6..=10) => println!("6 to 10 num = {x}"),
|
|
/// None => panic!(),
|
|
/// // all other numbers
|
|
/// _ => panic!(),
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// `match` can be used to gain access to the inner members of an enum
|
|
/// and use them directly.
|
|
///
|
|
/// ```rust
|
|
/// enum Outer {
|
|
/// Double(Option<u8>, Option<String>),
|
|
/// Single(Option<u8>),
|
|
/// Empty
|
|
/// }
|
|
///
|
|
/// let get_inner = Outer::Double(None, Some(String::new()));
|
|
/// match get_inner {
|
|
/// Outer::Double(None, Some(st)) => println!("{st}"),
|
|
/// Outer::Single(opt) => println!("{opt:?}"),
|
|
/// _ => panic!(),
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// For more information on `match` and matching in general, see the [Reference].
|
|
///
|
|
/// [Reference]: ../reference/expressions/match-expr.html
|
|
mod match_keyword {}
|
|
|
|
#[doc(keyword = "mod")]
|
|
//
|
|
/// Organize code into [modules].
|
|
///
|
|
/// Use `mod` to create new [modules] to encapsulate code, including other
|
|
/// modules:
|
|
///
|
|
/// ```
|
|
/// mod foo {
|
|
/// mod bar {
|
|
/// type MyType = (u8, u8);
|
|
/// fn baz() {}
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Like [`struct`]s and [`enum`]s, a module and its content are private by
|
|
/// default, inaccessible to code outside of the module.
|
|
///
|
|
/// To learn more about allowing access, see the documentation for the [`pub`]
|
|
/// keyword.
|
|
///
|
|
/// [`enum`]: keyword.enum.html
|
|
/// [`pub`]: keyword.pub.html
|
|
/// [`struct`]: keyword.struct.html
|
|
/// [modules]: ../reference/items/modules.html
|
|
mod mod_keyword {}
|
|
|
|
#[doc(keyword = "move")]
|
|
//
|
|
/// Capture a [closure]'s environment by value.
|
|
///
|
|
/// `move` converts any variables captured by reference or mutable reference
|
|
/// to variables captured by value.
|
|
///
|
|
/// ```rust
|
|
/// let data = vec![1, 2, 3];
|
|
/// let closure = move || println!("captured {data:?} by value");
|
|
///
|
|
/// // data is no longer available, it is owned by the closure
|
|
/// ```
|
|
///
|
|
/// Note: `move` closures may still implement [`Fn`] or [`FnMut`], even though
|
|
/// they capture variables by `move`. This is because the traits implemented by
|
|
/// a closure type are determined by *what* the closure does with captured
|
|
/// values, not *how* it captures them:
|
|
///
|
|
/// ```rust
|
|
/// fn create_fn() -> impl Fn() {
|
|
/// let text = "Fn".to_owned();
|
|
/// move || println!("This is a: {text}")
|
|
/// }
|
|
///
|
|
/// let fn_plain = create_fn();
|
|
/// fn_plain();
|
|
/// ```
|
|
///
|
|
/// `move` is often used when [threads] are involved.
|
|
///
|
|
/// ```rust
|
|
/// let data = vec![1, 2, 3];
|
|
///
|
|
/// std::thread::spawn(move || {
|
|
/// println!("captured {data:?} by value")
|
|
/// }).join().unwrap();
|
|
///
|
|
/// // data was moved to the spawned thread, so we cannot use it here
|
|
/// ```
|
|
///
|
|
/// `move` is also valid before an async block.
|
|
///
|
|
/// ```rust
|
|
/// let capture = "hello".to_owned();
|
|
/// let block = async move {
|
|
/// println!("rust says {capture} from async block");
|
|
/// };
|
|
/// ```
|
|
///
|
|
/// For more information on the `move` keyword, see the [closures][closure] section
|
|
/// of the Rust book or the [threads] section.
|
|
///
|
|
/// [closure]: ../book/ch13-01-closures.html
|
|
/// [threads]: ../book/ch16-01-threads.html#using-move-closures-with-threads
|
|
mod move_keyword {}
|
|
|
|
#[doc(keyword = "mut")]
|
|
//
|
|
/// A mutable variable, reference, or pointer.
|
|
///
|
|
/// `mut` can be used in several situations. The first is mutable variables,
|
|
/// which can be used anywhere you can bind a value to a variable name. Some
|
|
/// examples:
|
|
///
|
|
/// ```rust
|
|
/// // A mutable variable in the parameter list of a function.
|
|
/// fn foo(mut x: u8, y: u8) -> u8 {
|
|
/// x += y;
|
|
/// x
|
|
/// }
|
|
///
|
|
/// // Modifying a mutable variable.
|
|
/// # #[allow(unused_assignments)]
|
|
/// let mut a = 5;
|
|
/// a = 6;
|
|
///
|
|
/// assert_eq!(foo(3, 4), 7);
|
|
/// assert_eq!(a, 6);
|
|
/// ```
|
|
///
|
|
/// The second is mutable references. They can be created from `mut` variables
|
|
/// and must be unique: no other variables can have a mutable reference, nor a
|
|
/// shared reference.
|
|
///
|
|
/// ```rust
|
|
/// // Taking a mutable reference.
|
|
/// fn push_two(v: &mut Vec<u8>) {
|
|
/// v.push(2);
|
|
/// }
|
|
///
|
|
/// // A mutable reference cannot be taken to a non-mutable variable.
|
|
/// let mut v = vec![0, 1];
|
|
/// // Passing a mutable reference.
|
|
/// push_two(&mut v);
|
|
///
|
|
/// assert_eq!(v, vec![0, 1, 2]);
|
|
/// ```
|
|
///
|
|
/// ```rust,compile_fail,E0502
|
|
/// let mut v = vec![0, 1];
|
|
/// let mut_ref_v = &mut v;
|
|
/// ##[allow(unused)]
|
|
/// let ref_v = &v;
|
|
/// mut_ref_v.push(2);
|
|
/// ```
|
|
///
|
|
/// Mutable raw pointers work much like mutable references, with the added
|
|
/// possibility of not pointing to a valid object. The syntax is `*mut Type`.
|
|
///
|
|
/// More information on mutable references and pointers can be found in the [Reference].
|
|
///
|
|
/// [Reference]: ../reference/types/pointer.html#mutable-references-mut
|
|
mod mut_keyword {}
|
|
|
|
#[doc(keyword = "pub")]
|
|
//
|
|
/// Make an item visible to others.
|
|
///
|
|
/// The keyword `pub` makes any module, function, or data structure accessible from inside
|
|
/// of external modules. The `pub` keyword may also be used in a `use` declaration to re-export
|
|
/// an identifier from a namespace.
|
|
///
|
|
/// For more information on the `pub` keyword, please see the visibility section
|
|
/// of the [reference] and for some examples, see [Rust by Example].
|
|
///
|
|
/// [reference]:../reference/visibility-and-privacy.html?highlight=pub#visibility-and-privacy
|
|
/// [Rust by Example]:../rust-by-example/mod/visibility.html
|
|
mod pub_keyword {}
|
|
|
|
#[doc(keyword = "ref")]
|
|
//
|
|
/// Bind by reference during pattern matching.
|
|
///
|
|
/// `ref` annotates pattern bindings to make them borrow rather than move.
|
|
/// It is **not** a part of the pattern as far as matching is concerned: it does
|
|
/// not affect *whether* a value is matched, only *how* it is matched.
|
|
///
|
|
/// By default, [`match`] statements consume all they can, which can sometimes
|
|
/// be a problem, when you don't really need the value to be moved and owned:
|
|
///
|
|
/// ```compile_fail,E0382
|
|
/// let maybe_name = Some(String::from("Alice"));
|
|
/// // The variable 'maybe_name' is consumed here ...
|
|
/// match maybe_name {
|
|
/// Some(n) => println!("Hello, {n}"),
|
|
/// _ => println!("Hello, world"),
|
|
/// }
|
|
/// // ... and is now unavailable.
|
|
/// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
|
|
/// ```
|
|
///
|
|
/// Using the `ref` keyword, the value is only borrowed, not moved, making it
|
|
/// available for use after the [`match`] statement:
|
|
///
|
|
/// ```
|
|
/// let maybe_name = Some(String::from("Alice"));
|
|
/// // Using `ref`, the value is borrowed, not moved ...
|
|
/// match maybe_name {
|
|
/// Some(ref n) => println!("Hello, {n}"),
|
|
/// _ => println!("Hello, world"),
|
|
/// }
|
|
/// // ... so it's available here!
|
|
/// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
|
|
/// ```
|
|
///
|
|
/// # `&` vs `ref`
|
|
///
|
|
/// - `&` denotes that your pattern expects a reference to an object. Hence `&`
|
|
/// is a part of said pattern: `&Foo` matches different objects than `Foo` does.
|
|
///
|
|
/// - `ref` indicates that you want a reference to an unpacked value. It is not
|
|
/// matched against: `Foo(ref foo)` matches the same objects as `Foo(foo)`.
|
|
///
|
|
/// See also the [Reference] for more information.
|
|
///
|
|
/// [`match`]: keyword.match.html
|
|
/// [Reference]: ../reference/patterns.html#identifier-patterns
|
|
mod ref_keyword {}
|
|
|
|
#[doc(keyword = "return")]
|
|
//
|
|
/// Returns a value from a function.
|
|
///
|
|
/// A `return` marks the end of an execution path in a function:
|
|
///
|
|
/// ```
|
|
/// fn foo() -> i32 {
|
|
/// return 3;
|
|
/// }
|
|
/// assert_eq!(foo(), 3);
|
|
/// ```
|
|
///
|
|
/// `return` is not needed when the returned value is the last expression in the
|
|
/// function. In this case the `;` is omitted:
|
|
///
|
|
/// ```
|
|
/// fn foo() -> i32 {
|
|
/// 3
|
|
/// }
|
|
/// assert_eq!(foo(), 3);
|
|
/// ```
|
|
///
|
|
/// `return` returns from the function immediately (an "early return"):
|
|
///
|
|
/// ```no_run
|
|
/// use std::fs::File;
|
|
/// use std::io::{Error, ErrorKind, Read, Result};
|
|
///
|
|
/// fn main() -> Result<()> {
|
|
/// let mut file = match File::open("foo.txt") {
|
|
/// Ok(f) => f,
|
|
/// Err(e) => return Err(e),
|
|
/// };
|
|
///
|
|
/// let mut contents = String::new();
|
|
/// let size = match file.read_to_string(&mut contents) {
|
|
/// Ok(s) => s,
|
|
/// Err(e) => return Err(e),
|
|
/// };
|
|
///
|
|
/// if contents.contains("impossible!") {
|
|
/// return Err(Error::new(ErrorKind::Other, "oh no!"));
|
|
/// }
|
|
///
|
|
/// if size > 9000 {
|
|
/// return Err(Error::new(ErrorKind::Other, "over 9000!"));
|
|
/// }
|
|
///
|
|
/// assert_eq!(contents, "Hello, world!");
|
|
/// Ok(())
|
|
/// }
|
|
/// ```
|
|
mod return_keyword {}
|
|
|
|
#[doc(keyword = "self")]
|
|
//
|
|
/// The receiver of a method, or the current module.
|
|
///
|
|
/// `self` is used in two situations: referencing the current module and marking
|
|
/// the receiver of a method.
|
|
///
|
|
/// In paths, `self` can be used to refer to the current module, either in a
|
|
/// [`use`] statement or in a path to access an element:
|
|
///
|
|
/// ```
|
|
/// # #![allow(unused_imports)]
|
|
/// use std::io::{self, Read};
|
|
/// ```
|
|
///
|
|
/// Is functionally the same as:
|
|
///
|
|
/// ```
|
|
/// # #![allow(unused_imports)]
|
|
/// use std::io;
|
|
/// use std::io::Read;
|
|
/// ```
|
|
///
|
|
/// Using `self` to access an element in the current module:
|
|
///
|
|
/// ```
|
|
/// # #![allow(dead_code)]
|
|
/// # fn main() {}
|
|
/// fn foo() {}
|
|
/// fn bar() {
|
|
/// self::foo()
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// `self` as the current receiver for a method allows to omit the parameter
|
|
/// type most of the time. With the exception of this particularity, `self` is
|
|
/// used much like any other parameter:
|
|
///
|
|
/// ```
|
|
/// struct Foo(i32);
|
|
///
|
|
/// impl Foo {
|
|
/// // No `self`.
|
|
/// fn new() -> Self {
|
|
/// Self(0)
|
|
/// }
|
|
///
|
|
/// // Consuming `self`.
|
|
/// fn consume(self) -> Self {
|
|
/// Self(self.0 + 1)
|
|
/// }
|
|
///
|
|
/// // Borrowing `self`.
|
|
/// fn borrow(&self) -> &i32 {
|
|
/// &self.0
|
|
/// }
|
|
///
|
|
/// // Borrowing `self` mutably.
|
|
/// fn borrow_mut(&mut self) -> &mut i32 {
|
|
/// &mut self.0
|
|
/// }
|
|
/// }
|
|
///
|
|
/// // This method must be called with a `Type::` prefix.
|
|
/// let foo = Foo::new();
|
|
/// assert_eq!(foo.0, 0);
|
|
///
|
|
/// // Those two calls produces the same result.
|
|
/// let foo = Foo::consume(foo);
|
|
/// assert_eq!(foo.0, 1);
|
|
/// let foo = foo.consume();
|
|
/// assert_eq!(foo.0, 2);
|
|
///
|
|
/// // Borrowing is handled automatically with the second syntax.
|
|
/// let borrow_1 = Foo::borrow(&foo);
|
|
/// let borrow_2 = foo.borrow();
|
|
/// assert_eq!(borrow_1, borrow_2);
|
|
///
|
|
/// // Borrowing mutably is handled automatically too with the second syntax.
|
|
/// let mut foo = Foo::new();
|
|
/// *Foo::borrow_mut(&mut foo) += 1;
|
|
/// assert_eq!(foo.0, 1);
|
|
/// *foo.borrow_mut() += 1;
|
|
/// assert_eq!(foo.0, 2);
|
|
/// ```
|
|
///
|
|
/// Note that this automatic conversion when calling `foo.method()` is not
|
|
/// limited to the examples above. See the [Reference] for more information.
|
|
///
|
|
/// [`use`]: keyword.use.html
|
|
/// [Reference]: ../reference/items/associated-items.html#methods
|
|
mod self_keyword {}
|
|
|
|
// FIXME: Once rustdoc can handle URL conflicts on case insensitive file systems, we can remove the
|
|
// three next lines and put back: `#[doc(keyword = "Self")]`.
|
|
#[doc(alias = "Self")]
|
|
#[allow(rustc::existing_doc_keyword)]
|
|
#[doc(keyword = "SelfTy")]
|
|
//
|
|
/// The implementing type within a [`trait`] or [`impl`] block, or the current type within a type
|
|
/// definition.
|
|
///
|
|
/// Within a type definition:
|
|
///
|
|
/// ```
|
|
/// # #![allow(dead_code)]
|
|
/// struct Node {
|
|
/// elem: i32,
|
|
/// // `Self` is a `Node` here.
|
|
/// next: Option<Box<Self>>,
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// In an [`impl`] block:
|
|
///
|
|
/// ```
|
|
/// struct Foo(i32);
|
|
///
|
|
/// impl Foo {
|
|
/// fn new() -> Self {
|
|
/// Self(0)
|
|
/// }
|
|
/// }
|
|
///
|
|
/// assert_eq!(Foo::new().0, Foo(0).0);
|
|
/// ```
|
|
///
|
|
/// Generic parameters are implicit with `Self`:
|
|
///
|
|
/// ```
|
|
/// # #![allow(dead_code)]
|
|
/// struct Wrap<T> {
|
|
/// elem: T,
|
|
/// }
|
|
///
|
|
/// impl<T> Wrap<T> {
|
|
/// fn new(elem: T) -> Self {
|
|
/// Self { elem }
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// In a [`trait`] definition and related [`impl`] block:
|
|
///
|
|
/// ```
|
|
/// trait Example {
|
|
/// fn example() -> Self;
|
|
/// }
|
|
///
|
|
/// struct Foo(i32);
|
|
///
|
|
/// impl Example for Foo {
|
|
/// fn example() -> Self {
|
|
/// Self(42)
|
|
/// }
|
|
/// }
|
|
///
|
|
/// assert_eq!(Foo::example().0, Foo(42).0);
|
|
/// ```
|
|
///
|
|
/// [`impl`]: keyword.impl.html
|
|
/// [`trait`]: keyword.trait.html
|
|
mod self_upper_keyword {}
|
|
|
|
#[doc(keyword = "static")]
|
|
//
|
|
/// A static item is a value which is valid for the entire duration of your
|
|
/// program (a `'static` lifetime).
|
|
///
|
|
/// On the surface, `static` items seem very similar to [`const`]s: both contain
|
|
/// a value, both require type annotations and both can only be initialized with
|
|
/// constant functions and values. However, `static`s are notably different in
|
|
/// that they represent a location in memory. That means that you can have
|
|
/// references to `static` items and potentially even modify them, making them
|
|
/// essentially global variables.
|
|
///
|
|
/// Static items do not call [`drop`] at the end of the program.
|
|
///
|
|
/// There are two types of `static` items: those declared in association with
|
|
/// the [`mut`] keyword and those without.
|
|
///
|
|
/// Static items cannot be moved:
|
|
///
|
|
/// ```rust,compile_fail,E0507
|
|
/// static VEC: Vec<u32> = vec![];
|
|
///
|
|
/// fn move_vec(v: Vec<u32>) -> Vec<u32> {
|
|
/// v
|
|
/// }
|
|
///
|
|
/// // This line causes an error
|
|
/// move_vec(VEC);
|
|
/// ```
|
|
///
|
|
/// # Simple `static`s
|
|
///
|
|
/// Accessing non-[`mut`] `static` items is considered safe, but some
|
|
/// restrictions apply. Most notably, the type of a `static` value needs to
|
|
/// implement the [`Sync`] trait, ruling out interior mutability containers
|
|
/// like [`RefCell`]. See the [Reference] for more information.
|
|
///
|
|
/// ```rust
|
|
/// static FOO: [i32; 5] = [1, 2, 3, 4, 5];
|
|
///
|
|
/// let r1 = &FOO as *const _;
|
|
/// let r2 = &FOO as *const _;
|
|
/// // With a strictly read-only static, references will have the same address
|
|
/// assert_eq!(r1, r2);
|
|
/// // A static item can be used just like a variable in many cases
|
|
/// println!("{FOO:?}");
|
|
/// ```
|
|
///
|
|
/// # Mutable `static`s
|
|
///
|
|
/// If a `static` item is declared with the [`mut`] keyword, then it is allowed
|
|
/// to be modified by the program. However, accessing mutable `static`s can
|
|
/// cause undefined behavior in a number of ways, for example due to data races
|
|
/// in a multithreaded context. As such, all accesses to mutable `static`s
|
|
/// require an [`unsafe`] block.
|
|
///
|
|
/// Despite their unsafety, mutable `static`s are necessary in many contexts:
|
|
/// they can be used to represent global state shared by the whole program or in
|
|
/// [`extern`] blocks to bind to variables from C libraries.
|
|
///
|
|
/// In an [`extern`] block:
|
|
///
|
|
/// ```rust,no_run
|
|
/// # #![allow(dead_code)]
|
|
/// extern "C" {
|
|
/// static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Mutable `static`s, just like simple `static`s, have some restrictions that
|
|
/// apply to them. See the [Reference] for more information.
|
|
///
|
|
/// [`const`]: keyword.const.html
|
|
/// [`extern`]: keyword.extern.html
|
|
/// [`mut`]: keyword.mut.html
|
|
/// [`unsafe`]: keyword.unsafe.html
|
|
/// [`RefCell`]: cell::RefCell
|
|
/// [Reference]: ../reference/items/static-items.html
|
|
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 UpperCamelCase, 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::PhantomData
|
|
/// [book]: ../book/ch05-01-defining-structs.html
|
|
/// [reference]: ../reference/items/structs.html
|
|
mod struct_keyword {}
|
|
|
|
#[doc(keyword = "super")]
|
|
//
|
|
/// The parent of the current [module].
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// # fn main() {}
|
|
/// mod a {
|
|
/// pub fn foo() {}
|
|
/// }
|
|
/// mod b {
|
|
/// pub fn foo() {
|
|
/// super::a::foo(); // call a's foo function
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// It is also possible to use `super` multiple times: `super::super::foo`,
|
|
/// going up the ancestor chain.
|
|
///
|
|
/// See the [Reference] for more information.
|
|
///
|
|
/// [module]: ../reference/items/modules.html
|
|
/// [Reference]: ../reference/paths.html#super
|
|
mod super_keyword {}
|
|
|
|
#[doc(keyword = "trait")]
|
|
//
|
|
/// A common interface for a group of types.
|
|
///
|
|
/// A `trait` is like an interface that data types can implement. When a type
|
|
/// implements a trait it can be treated abstractly as that trait using generics
|
|
/// or trait objects.
|
|
///
|
|
/// Traits can be made up of three varieties of associated items:
|
|
///
|
|
/// - functions and methods
|
|
/// - types
|
|
/// - constants
|
|
///
|
|
/// Traits may also contain additional type parameters. Those type parameters
|
|
/// or the trait itself can be constrained by other traits.
|
|
///
|
|
/// Traits can serve as markers or carry other logical semantics that
|
|
/// aren't expressed through their items. When a type implements that
|
|
/// trait it is promising to uphold its contract. [`Send`] and [`Sync`] are two
|
|
/// such marker traits present in the standard library.
|
|
///
|
|
/// See the [Reference][Ref-Traits] for a lot more information on traits.
|
|
///
|
|
/// # Examples
|
|
///
|
|
/// Traits are declared using the `trait` keyword. Types can implement them
|
|
/// using [`impl`] `Trait` [`for`] `Type`:
|
|
///
|
|
/// ```rust
|
|
/// trait Zero {
|
|
/// const ZERO: Self;
|
|
/// fn is_zero(&self) -> bool;
|
|
/// }
|
|
///
|
|
/// impl Zero for i32 {
|
|
/// const ZERO: Self = 0;
|
|
///
|
|
/// fn is_zero(&self) -> bool {
|
|
/// *self == Self::ZERO
|
|
/// }
|
|
/// }
|
|
///
|
|
/// assert_eq!(i32::ZERO, 0);
|
|
/// assert!(i32::ZERO.is_zero());
|
|
/// assert!(!4.is_zero());
|
|
/// ```
|
|
///
|
|
/// With an associated type:
|
|
///
|
|
/// ```rust
|
|
/// trait Builder {
|
|
/// type Built;
|
|
///
|
|
/// fn build(&self) -> Self::Built;
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Traits can be generic, with constraints or without:
|
|
///
|
|
/// ```rust
|
|
/// trait MaybeFrom<T> {
|
|
/// fn maybe_from(value: T) -> Option<Self>
|
|
/// where
|
|
/// Self: Sized;
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Traits can build upon the requirements of other traits. In the example
|
|
/// below `Iterator` is a **supertrait** and `ThreeIterator` is a **subtrait**:
|
|
///
|
|
/// ```rust
|
|
/// trait ThreeIterator: Iterator {
|
|
/// fn next_three(&mut self) -> Option<[Self::Item; 3]>;
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Traits can be used in functions, as parameters:
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// fn debug_iter<I: Iterator>(it: I) where I::Item: std::fmt::Debug {
|
|
/// for elem in it {
|
|
/// println!("{elem:#?}");
|
|
/// }
|
|
/// }
|
|
///
|
|
/// // u8_len_1, u8_len_2 and u8_len_3 are equivalent
|
|
///
|
|
/// fn u8_len_1(val: impl Into<Vec<u8>>) -> usize {
|
|
/// val.into().len()
|
|
/// }
|
|
///
|
|
/// fn u8_len_2<T: Into<Vec<u8>>>(val: T) -> usize {
|
|
/// val.into().len()
|
|
/// }
|
|
///
|
|
/// fn u8_len_3<T>(val: T) -> usize
|
|
/// where
|
|
/// T: Into<Vec<u8>>,
|
|
/// {
|
|
/// val.into().len()
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Or as return types:
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// fn from_zero_to(v: u8) -> impl Iterator<Item = u8> {
|
|
/// (0..v).into_iter()
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// The use of the [`impl`] keyword in this position allows the function writer
|
|
/// to hide the concrete type as an implementation detail which can change
|
|
/// without breaking user's code.
|
|
///
|
|
/// # Trait objects
|
|
///
|
|
/// A *trait object* is an opaque value of another type that implements a set of
|
|
/// traits. A trait object implements all specified traits as well as their
|
|
/// supertraits (if any).
|
|
///
|
|
/// The syntax is the following: `dyn BaseTrait + AutoTrait1 + ... AutoTraitN`.
|
|
/// Only one `BaseTrait` can be used so this will not compile:
|
|
///
|
|
/// ```rust,compile_fail,E0225
|
|
/// trait A {}
|
|
/// trait B {}
|
|
///
|
|
/// let _: Box<dyn A + B>;
|
|
/// ```
|
|
///
|
|
/// Neither will this, which is a syntax error:
|
|
///
|
|
/// ```rust,compile_fail
|
|
/// trait A {}
|
|
/// trait B {}
|
|
///
|
|
/// let _: Box<dyn A + dyn B>;
|
|
/// ```
|
|
///
|
|
/// On the other hand, this is correct:
|
|
///
|
|
/// ```rust
|
|
/// trait A {}
|
|
///
|
|
/// let _: Box<dyn A + Send + Sync>;
|
|
/// ```
|
|
///
|
|
/// The [Reference][Ref-Trait-Objects] has more information about trait objects,
|
|
/// their limitations and the differences between editions.
|
|
///
|
|
/// # Unsafe traits
|
|
///
|
|
/// Some traits may be unsafe to implement. Using the [`unsafe`] keyword in
|
|
/// front of the trait's declaration is used to mark this:
|
|
///
|
|
/// ```rust
|
|
/// unsafe trait UnsafeTrait {}
|
|
///
|
|
/// unsafe impl UnsafeTrait for i32 {}
|
|
/// ```
|
|
///
|
|
/// # Differences between the 2015 and 2018 editions
|
|
///
|
|
/// In the 2015 edition the parameters pattern was not needed for traits:
|
|
///
|
|
/// ```rust,edition2015
|
|
/// # #![allow(anonymous_parameters)]
|
|
/// trait Tr {
|
|
/// fn f(i32);
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// This behavior is no longer valid in edition 2018.
|
|
///
|
|
/// [`for`]: keyword.for.html
|
|
/// [`impl`]: keyword.impl.html
|
|
/// [`unsafe`]: keyword.unsafe.html
|
|
/// [Ref-Traits]: ../reference/items/traits.html
|
|
/// [Ref-Trait-Objects]: ../reference/types/trait-object.html
|
|
mod trait_keyword {}
|
|
|
|
#[doc(keyword = "true")]
|
|
//
|
|
/// A value of type [`bool`] representing logical **true**.
|
|
///
|
|
/// Logically `true` is not equal to [`false`].
|
|
///
|
|
/// ## Control structures that check for **true**
|
|
///
|
|
/// Several of Rust's control structures will check for a `bool` condition evaluating to **true**.
|
|
///
|
|
/// * The condition in an [`if`] expression must be of type `bool`.
|
|
/// Whenever that condition evaluates to **true**, the `if` expression takes
|
|
/// on the value of the first block. If however, the condition evaluates
|
|
/// to `false`, the expression takes on value of the `else` block if there is one.
|
|
///
|
|
/// * [`while`] is another control flow construct expecting a `bool`-typed condition.
|
|
/// As long as the condition evaluates to **true**, the `while` loop will continually
|
|
/// evaluate its associated block.
|
|
///
|
|
/// * [`match`] arms can have guard clauses on them.
|
|
///
|
|
/// [`if`]: keyword.if.html
|
|
/// [`while`]: keyword.while.html
|
|
/// [`match`]: ../reference/expressions/match-expr.html#match-guards
|
|
/// [`false`]: keyword.false.html
|
|
mod true_keyword {}
|
|
|
|
#[doc(keyword = "type")]
|
|
//
|
|
/// Define an [alias] for an existing type.
|
|
///
|
|
/// The syntax is `type Name = ExistingType;`.
|
|
///
|
|
/// # Examples
|
|
///
|
|
/// `type` does **not** create a new type:
|
|
///
|
|
/// ```rust
|
|
/// type Meters = u32;
|
|
/// type Kilograms = u32;
|
|
///
|
|
/// let m: Meters = 3;
|
|
/// let k: Kilograms = 3;
|
|
///
|
|
/// assert_eq!(m, k);
|
|
/// ```
|
|
///
|
|
/// A type can be generic:
|
|
///
|
|
/// ```rust
|
|
/// # use std::sync::{Arc, Mutex};
|
|
/// type ArcMutex<T> = Arc<Mutex<T>>;
|
|
/// ```
|
|
///
|
|
/// In traits, `type` is used to declare an [associated type]:
|
|
///
|
|
/// ```rust
|
|
/// trait Iterator {
|
|
/// // associated type declaration
|
|
/// type Item;
|
|
/// fn next(&mut self) -> Option<Self::Item>;
|
|
/// }
|
|
///
|
|
/// struct Once<T>(Option<T>);
|
|
///
|
|
/// impl<T> Iterator for Once<T> {
|
|
/// // associated type definition
|
|
/// type Item = T;
|
|
/// fn next(&mut self) -> Option<Self::Item> {
|
|
/// self.0.take()
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// [`trait`]: keyword.trait.html
|
|
/// [associated type]: ../reference/items/associated-items.html#associated-types
|
|
/// [alias]: ../reference/items/type-aliases.html
|
|
mod type_keyword {}
|
|
|
|
#[doc(keyword = "unsafe")]
|
|
//
|
|
/// Code or interfaces whose [memory safety] cannot be verified by the type
|
|
/// system.
|
|
///
|
|
/// The `unsafe` keyword has two uses:
|
|
/// - to declare the existence of contracts the compiler can't check (`unsafe fn` and `unsafe
|
|
/// trait`),
|
|
/// - and to declare that a programmer has checked that these contracts have been upheld (`unsafe
|
|
/// {}` and `unsafe impl`, but also `unsafe fn` -- see below).
|
|
///
|
|
/// They are not mutually exclusive, as can be seen in `unsafe fn`: the body of an `unsafe fn` is,
|
|
/// by default, treated like an unsafe block. The `unsafe_op_in_unsafe_fn` lint can be enabled to
|
|
/// change that.
|
|
///
|
|
/// # Unsafe abilities
|
|
///
|
|
/// **No matter what, Safe Rust can't cause Undefined Behavior**. This is
|
|
/// referred to as [soundness]: a well-typed program actually has the desired
|
|
/// properties. The [Nomicon][nomicon-soundness] has a more detailed explanation
|
|
/// on the subject.
|
|
///
|
|
/// To ensure soundness, Safe Rust is restricted enough that it can be
|
|
/// automatically checked. Sometimes, however, it is necessary to write code
|
|
/// that is correct for reasons which are too clever for the compiler to
|
|
/// understand. In those cases, you need to use Unsafe Rust.
|
|
///
|
|
/// Here are the abilities Unsafe Rust has in addition to Safe Rust:
|
|
///
|
|
/// - Dereference [raw pointers]
|
|
/// - Implement `unsafe` [`trait`]s
|
|
/// - Call `unsafe` functions
|
|
/// - Mutate [`static`]s (including [`extern`]al ones)
|
|
/// - Access fields of [`union`]s
|
|
///
|
|
/// However, this extra power comes with extra responsibilities: it is now up to
|
|
/// you to ensure soundness. The `unsafe` keyword helps by clearly marking the
|
|
/// pieces of code that need to worry about this.
|
|
///
|
|
/// ## The different meanings of `unsafe`
|
|
///
|
|
/// Not all uses of `unsafe` are equivalent: some are here to mark the existence
|
|
/// of a contract the programmer must check, others are to say "I have checked
|
|
/// the contract, go ahead and do this". The following
|
|
/// [discussion on Rust Internals] has more in-depth explanations about this but
|
|
/// here is a summary of the main points:
|
|
///
|
|
/// - `unsafe fn`: calling this function means abiding by a contract the
|
|
/// compiler cannot enforce.
|
|
/// - `unsafe trait`: implementing the [`trait`] means abiding by a
|
|
/// contract the compiler cannot enforce.
|
|
/// - `unsafe {}`: the contract necessary to call the operations inside the
|
|
/// block has been checked by the programmer and is guaranteed to be respected.
|
|
/// - `unsafe impl`: the contract necessary to implement the trait has been
|
|
/// checked by the programmer and is guaranteed to be respected.
|
|
///
|
|
/// By default, `unsafe fn` also acts like an `unsafe {}` block
|
|
/// around the code inside the function. This means it is not just a signal to
|
|
/// the caller, but also promises that the preconditions for the operations
|
|
/// inside the function are upheld. Mixing these two meanings can be confusing, so the
|
|
/// `unsafe_op_in_unsafe_fn` lint can be enabled to warn against that and require explicit unsafe
|
|
/// blocks even inside `unsafe fn`.
|
|
///
|
|
/// See the [Rustonomicon] and the [Reference] for more information.
|
|
///
|
|
/// # Examples
|
|
///
|
|
/// ## Marking elements as `unsafe`
|
|
///
|
|
/// `unsafe` can be used on functions. Note that functions and statics declared
|
|
/// in [`extern`] blocks are implicitly marked as `unsafe` (but not functions
|
|
/// declared as `extern "something" fn ...`). Mutable statics are always unsafe,
|
|
/// wherever they are declared. Methods can also be declared as `unsafe`:
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// static mut FOO: &str = "hello";
|
|
///
|
|
/// unsafe fn unsafe_fn() {}
|
|
///
|
|
/// extern "C" {
|
|
/// fn unsafe_extern_fn();
|
|
/// static BAR: *mut u32;
|
|
/// }
|
|
///
|
|
/// trait SafeTraitWithUnsafeMethod {
|
|
/// unsafe fn unsafe_method(&self);
|
|
/// }
|
|
///
|
|
/// struct S;
|
|
///
|
|
/// impl S {
|
|
/// unsafe fn unsafe_method_on_struct() {}
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Traits can also be declared as `unsafe`:
|
|
///
|
|
/// ```rust
|
|
/// unsafe trait UnsafeTrait {}
|
|
/// ```
|
|
///
|
|
/// Since `unsafe fn` and `unsafe trait` indicate that there is a safety
|
|
/// contract that the compiler cannot enforce, documenting it is important. The
|
|
/// standard library has many examples of this, like the following which is an
|
|
/// extract from [`Vec::set_len`]. The `# Safety` section explains the contract
|
|
/// that must be fulfilled to safely call the function.
|
|
///
|
|
/// ```rust,ignore (stub-to-show-doc-example)
|
|
/// /// Forces the length of the vector to `new_len`.
|
|
/// ///
|
|
/// /// This is a low-level operation that maintains none of the normal
|
|
/// /// invariants of the type. Normally changing the length of a vector
|
|
/// /// is done using one of the safe operations instead, such as
|
|
/// /// `truncate`, `resize`, `extend`, or `clear`.
|
|
/// ///
|
|
/// /// # Safety
|
|
/// ///
|
|
/// /// - `new_len` must be less than or equal to `capacity()`.
|
|
/// /// - The elements at `old_len..new_len` must be initialized.
|
|
/// pub unsafe fn set_len(&mut self, new_len: usize)
|
|
/// ```
|
|
///
|
|
/// ## Using `unsafe {}` blocks and `impl`s
|
|
///
|
|
/// Performing `unsafe` operations requires an `unsafe {}` block:
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// #![deny(unsafe_op_in_unsafe_fn)]
|
|
///
|
|
/// /// Dereference the given pointer.
|
|
/// ///
|
|
/// /// # Safety
|
|
/// ///
|
|
/// /// `ptr` must be aligned and must not be dangling.
|
|
/// unsafe fn deref_unchecked(ptr: *const i32) -> i32 {
|
|
/// // SAFETY: the caller is required to ensure that `ptr` is aligned and dereferenceable.
|
|
/// unsafe { *ptr }
|
|
/// }
|
|
///
|
|
/// let a = 3;
|
|
/// let b = &a as *const _;
|
|
/// // SAFETY: `a` has not been dropped and references are always aligned,
|
|
/// // so `b` is a valid address.
|
|
/// unsafe { assert_eq!(*b, deref_unchecked(b)); };
|
|
/// ```
|
|
///
|
|
/// ## `unsafe` and traits
|
|
///
|
|
/// The interactions of `unsafe` and traits can be surprising, so let us contrast the
|
|
/// two combinations of safe `fn` in `unsafe trait` and `unsafe fn` in safe trait using two
|
|
/// examples:
|
|
///
|
|
/// ```rust
|
|
/// /// # Safety
|
|
/// ///
|
|
/// /// `make_even` must return an even number.
|
|
/// unsafe trait MakeEven {
|
|
/// fn make_even(&self) -> i32;
|
|
/// }
|
|
///
|
|
/// // SAFETY: Our `make_even` always returns something even.
|
|
/// unsafe impl MakeEven for i32 {
|
|
/// fn make_even(&self) -> i32 {
|
|
/// self << 1
|
|
/// }
|
|
/// }
|
|
///
|
|
/// fn use_make_even(x: impl MakeEven) {
|
|
/// if x.make_even() % 2 == 1 {
|
|
/// // SAFETY: this can never happen, because all `MakeEven` implementations
|
|
/// // ensure that `make_even` returns something even.
|
|
/// unsafe { std::hint::unreachable_unchecked() };
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// Note how the safety contract of the trait is upheld by the implementation, and is itself used to
|
|
/// uphold the safety contract of the unsafe function `unreachable_unchecked` called by
|
|
/// `use_make_even`. `make_even` itself is a safe function because its *callers* do not have to
|
|
/// worry about any contract, only the *implementation* of `MakeEven` is required to uphold a
|
|
/// certain contract. `use_make_even` is safe because it can use the promise made by `MakeEven`
|
|
/// implementations to uphold the safety contract of the `unsafe fn unreachable_unchecked` it calls.
|
|
///
|
|
/// It is also possible to have `unsafe fn` in a regular safe `trait`:
|
|
///
|
|
/// ```rust
|
|
/// # #![feature(never_type)]
|
|
/// #![deny(unsafe_op_in_unsafe_fn)]
|
|
///
|
|
/// trait Indexable {
|
|
/// const LEN: usize;
|
|
///
|
|
/// /// # Safety
|
|
/// ///
|
|
/// /// The caller must ensure that `idx < LEN`.
|
|
/// unsafe fn idx_unchecked(&self, idx: usize) -> i32;
|
|
/// }
|
|
///
|
|
/// // The implementation for `i32` doesn't need to do any contract reasoning.
|
|
/// impl Indexable for i32 {
|
|
/// const LEN: usize = 1;
|
|
///
|
|
/// unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
|
|
/// debug_assert_eq!(idx, 0);
|
|
/// *self
|
|
/// }
|
|
/// }
|
|
///
|
|
/// // The implementation for arrays exploits the function contract to
|
|
/// // make use of `get_unchecked` on slices and avoid a run-time check.
|
|
/// impl Indexable for [i32; 42] {
|
|
/// const LEN: usize = 42;
|
|
///
|
|
/// unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
|
|
/// // SAFETY: As per this trait's documentation, the caller ensures
|
|
/// // that `idx < 42`.
|
|
/// unsafe { *self.get_unchecked(idx) }
|
|
/// }
|
|
/// }
|
|
///
|
|
/// // The implementation for the never type declares a length of 0,
|
|
/// // which means `idx_unchecked` can never be called.
|
|
/// impl Indexable for ! {
|
|
/// const LEN: usize = 0;
|
|
///
|
|
/// unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
|
|
/// // SAFETY: As per this trait's documentation, the caller ensures
|
|
/// // that `idx < 0`, which is impossible, so this is dead code.
|
|
/// unsafe { std::hint::unreachable_unchecked() }
|
|
/// }
|
|
/// }
|
|
///
|
|
/// fn use_indexable<I: Indexable>(x: I, idx: usize) -> i32 {
|
|
/// if idx < I::LEN {
|
|
/// // SAFETY: We have checked that `idx < I::LEN`.
|
|
/// unsafe { x.idx_unchecked(idx) }
|
|
/// } else {
|
|
/// panic!("index out-of-bounds")
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// This time, `use_indexable` is safe because it uses a run-time check to discharge the safety
|
|
/// contract of `idx_unchecked`. Implementing `Indexable` is safe because when writing
|
|
/// `idx_unchecked`, we don't have to worry: our *callers* need to discharge a proof obligation
|
|
/// (like `use_indexable` does), but the *implementation* of `get_unchecked` has no proof obligation
|
|
/// to contend with. Of course, the implementation of `Indexable` may choose to call other unsafe
|
|
/// operations, and then it needs an `unsafe` *block* to indicate it discharged the proof
|
|
/// obligations of its callees. (We enabled `unsafe_op_in_unsafe_fn`, so the body of `idx_unchecked`
|
|
/// is not implicitly an unsafe block.) For that purpose it can make use of the contract that all
|
|
/// its callers must uphold -- the fact that `idx < LEN`.
|
|
///
|
|
/// Formally speaking, an `unsafe fn` in a trait is a function with *preconditions* that go beyond
|
|
/// those encoded by the argument types (such as `idx < LEN`), whereas an `unsafe trait` can declare
|
|
/// that some of its functions have *postconditions* that go beyond those encoded in the return type
|
|
/// (such as returning an even integer). If a trait needs a function with both extra precondition
|
|
/// and extra postcondition, then it needs an `unsafe fn` in an `unsafe trait`.
|
|
///
|
|
/// [`extern`]: keyword.extern.html
|
|
/// [`trait`]: keyword.trait.html
|
|
/// [`static`]: keyword.static.html
|
|
/// [`union`]: keyword.union.html
|
|
/// [`impl`]: keyword.impl.html
|
|
/// [raw pointers]: ../reference/types/pointer.html
|
|
/// [memory safety]: ../book/ch19-01-unsafe-rust.html
|
|
/// [Rustonomicon]: ../nomicon/index.html
|
|
/// [nomicon-soundness]: ../nomicon/safe-unsafe-meaning.html
|
|
/// [soundness]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library
|
|
/// [Reference]: ../reference/unsafety.html
|
|
/// [discussion on Rust Internals]: https://internals.rust-lang.org/t/what-does-unsafe-mean/6696
|
|
mod unsafe_keyword {}
|
|
|
|
#[doc(keyword = "use")]
|
|
//
|
|
/// Import or rename items from other crates or modules.
|
|
///
|
|
/// Usually a `use` keyword is used to shorten the path required to refer to a module item.
|
|
/// The keyword may appear in modules, blocks and even functions, usually at the top.
|
|
///
|
|
/// The most basic usage of the keyword is `use path::to::item;`,
|
|
/// though a number of convenient shortcuts are supported:
|
|
///
|
|
/// * Simultaneously binding a list of paths with a common prefix,
|
|
/// using the glob-like brace syntax `use a::b::{c, d, e::f, g::h::i};`
|
|
/// * Simultaneously binding a list of paths with a common prefix and their common parent module,
|
|
/// using the [`self`] keyword, such as `use a::b::{self, c, d::e};`
|
|
/// * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`.
|
|
/// This can also be used with the last two features: `use a::b::{self as ab, c as abc}`.
|
|
/// * Binding all paths matching a given prefix,
|
|
/// using the asterisk wildcard syntax `use a::b::*;`.
|
|
/// * Nesting groups of the previous features multiple times,
|
|
/// such as `use a::b::{self as ab, c, d::{*, e::f}};`
|
|
/// * Reexporting with visibility modifiers such as `pub use a::b;`
|
|
/// * Importing with `_` to only import the methods of a trait without binding it to a name
|
|
/// (to avoid conflict for example): `use ::std::io::Read as _;`.
|
|
///
|
|
/// Using path qualifiers like [`crate`], [`super`] or [`self`] is supported: `use crate::a::b;`.
|
|
///
|
|
/// Note that when the wildcard `*` is used on a type, it does not import its methods (though
|
|
/// for `enum`s it imports the variants, as shown in the example below).
|
|
///
|
|
/// ```compile_fail,edition2018
|
|
/// enum ExampleEnum {
|
|
/// VariantA,
|
|
/// VariantB,
|
|
/// }
|
|
///
|
|
/// impl ExampleEnum {
|
|
/// fn new() -> Self {
|
|
/// Self::VariantA
|
|
/// }
|
|
/// }
|
|
///
|
|
/// use ExampleEnum::*;
|
|
///
|
|
/// // Compiles.
|
|
/// let _ = VariantA;
|
|
///
|
|
/// // Does not compile !
|
|
/// let n = new();
|
|
/// ```
|
|
///
|
|
/// For more information on `use` and paths in general, see the [Reference].
|
|
///
|
|
/// The differences about paths and the `use` keyword between the 2015 and 2018 editions
|
|
/// can also be found in the [Reference].
|
|
///
|
|
/// [`crate`]: keyword.crate.html
|
|
/// [`self`]: keyword.self.html
|
|
/// [`super`]: keyword.super.html
|
|
/// [Reference]: ../reference/items/use-declarations.html
|
|
mod use_keyword {}
|
|
|
|
#[doc(keyword = "where")]
|
|
//
|
|
/// Add constraints that must be upheld to use an item.
|
|
///
|
|
/// `where` allows specifying constraints on lifetime and generic parameters.
|
|
/// The [RFC] introducing `where` contains detailed information about the
|
|
/// keyword.
|
|
///
|
|
/// # Examples
|
|
///
|
|
/// `where` can be used for constraints with traits:
|
|
///
|
|
/// ```rust
|
|
/// fn new<T: Default>() -> T {
|
|
/// T::default()
|
|
/// }
|
|
///
|
|
/// fn new_where<T>() -> T
|
|
/// where
|
|
/// T: Default,
|
|
/// {
|
|
/// T::default()
|
|
/// }
|
|
///
|
|
/// assert_eq!(0.0, new());
|
|
/// assert_eq!(0.0, new_where());
|
|
///
|
|
/// assert_eq!(0, new());
|
|
/// assert_eq!(0, new_where());
|
|
/// ```
|
|
///
|
|
/// `where` can also be used for lifetimes.
|
|
///
|
|
/// This compiles because `longer` outlives `shorter`, thus the constraint is
|
|
/// respected:
|
|
///
|
|
/// ```rust
|
|
/// fn select<'short, 'long>(s1: &'short str, s2: &'long str, second: bool) -> &'short str
|
|
/// where
|
|
/// 'long: 'short,
|
|
/// {
|
|
/// if second { s2 } else { s1 }
|
|
/// }
|
|
///
|
|
/// let outer = String::from("Long living ref");
|
|
/// let longer = &outer;
|
|
/// {
|
|
/// let inner = String::from("Short living ref");
|
|
/// let shorter = &inner;
|
|
///
|
|
/// assert_eq!(select(shorter, longer, false), shorter);
|
|
/// assert_eq!(select(shorter, longer, true), longer);
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// On the other hand, this will not compile because the `where 'b: 'a` clause
|
|
/// is missing: the `'b` lifetime is not known to live at least as long as `'a`
|
|
/// which means this function cannot ensure it always returns a valid reference:
|
|
///
|
|
/// ```rust,compile_fail
|
|
/// fn select<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
|
|
/// {
|
|
/// if second { s2 } else { s1 }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// `where` can also be used to express more complicated constraints that cannot
|
|
/// be written with the `<T: Trait>` syntax:
|
|
///
|
|
/// ```rust
|
|
/// fn first_or_default<I>(mut i: I) -> I::Item
|
|
/// where
|
|
/// I: Iterator,
|
|
/// I::Item: Default,
|
|
/// {
|
|
/// i.next().unwrap_or_else(I::Item::default)
|
|
/// }
|
|
///
|
|
/// assert_eq!(first_or_default([1, 2, 3].into_iter()), 1);
|
|
/// assert_eq!(first_or_default(Vec::<i32>::new().into_iter()), 0);
|
|
/// ```
|
|
///
|
|
/// `where` is available anywhere generic and lifetime parameters are available,
|
|
/// as can be seen with the [`Cow`](crate::borrow::Cow) type from the standard
|
|
/// library:
|
|
///
|
|
/// ```rust
|
|
/// # #![allow(dead_code)]
|
|
/// pub enum Cow<'a, B>
|
|
/// where
|
|
/// B: ToOwned + ?Sized,
|
|
/// {
|
|
/// Borrowed(&'a B),
|
|
/// Owned(<B as ToOwned>::Owned),
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/0135-where.md
|
|
mod where_keyword {}
|
|
|
|
// 2018 Edition keywords
|
|
|
|
#[doc(alias = "promise")]
|
|
#[doc(keyword = "async")]
|
|
//
|
|
/// Returns a [`Future`] instead of blocking the current thread.
|
|
///
|
|
/// Use `async` in front of `fn`, `closure`, or a `block` to turn the marked code into a `Future`.
|
|
/// As such the code will not be run immediately, but will only be evaluated when the returned
|
|
/// future is [`.await`]ed.
|
|
///
|
|
/// We have written an [async book] detailing `async`/`await` and trade-offs compared to using threads.
|
|
///
|
|
/// ## Editions
|
|
///
|
|
/// `async` is a keyword from the 2018 edition onwards.
|
|
///
|
|
/// It is available for use in stable Rust from version 1.39 onwards.
|
|
///
|
|
/// [`Future`]: future::Future
|
|
/// [`.await`]: ../std/keyword.await.html
|
|
/// [async book]: https://rust-lang.github.io/async-book/
|
|
mod async_keyword {}
|
|
|
|
#[doc(keyword = "await")]
|
|
//
|
|
/// Suspend execution until the result of a [`Future`] is ready.
|
|
///
|
|
/// `.await`ing a future will suspend the current function's execution until the executor
|
|
/// has run the future to completion.
|
|
///
|
|
/// Read the [async book] for details on how [`async`]/`await` and executors work.
|
|
///
|
|
/// ## Editions
|
|
///
|
|
/// `await` is a keyword from the 2018 edition onwards.
|
|
///
|
|
/// It is available for use in stable Rust from version 1.39 onwards.
|
|
///
|
|
/// [`Future`]: future::Future
|
|
/// [async book]: https://rust-lang.github.io/async-book/
|
|
/// [`async`]: ../std/keyword.async.html
|
|
mod await_keyword {}
|
|
|
|
#[doc(keyword = "dyn")]
|
|
//
|
|
/// `dyn` is a prefix of a [trait object]'s type.
|
|
///
|
|
/// The `dyn` keyword is used to highlight that calls to methods on the associated `Trait`
|
|
/// are [dynamically dispatched]. To use the trait this way, it must be 'object safe'.
|
|
///
|
|
/// Unlike generic parameters or `impl Trait`, the compiler does not know the concrete type that
|
|
/// is being passed. That is, the type has been [erased].
|
|
/// As such, a `dyn Trait` reference contains _two_ pointers.
|
|
/// One pointer goes to the data (e.g., an instance of a struct).
|
|
/// Another pointer goes to a map of method call names to function pointers
|
|
/// (known as a virtual method table or vtable).
|
|
///
|
|
/// At run-time, when a method needs to be called on the `dyn Trait`, the vtable is consulted to get
|
|
/// the function pointer and then that function pointer is called.
|
|
///
|
|
/// See the Reference for more information on [trait objects][ref-trait-obj]
|
|
/// and [object safety][ref-obj-safety].
|
|
///
|
|
/// ## Trade-offs
|
|
///
|
|
/// The above indirection is the additional runtime cost of calling a function on a `dyn Trait`.
|
|
/// Methods called by dynamic dispatch generally cannot be inlined by the compiler.
|
|
///
|
|
/// However, `dyn Trait` is likely to produce smaller code than `impl Trait` / generic parameters as
|
|
/// the method won't be duplicated for each concrete type.
|
|
///
|
|
/// [trait object]: ../book/ch17-02-trait-objects.html
|
|
/// [dynamically dispatched]: https://en.wikipedia.org/wiki/Dynamic_dispatch
|
|
/// [ref-trait-obj]: ../reference/types/trait-object.html
|
|
/// [ref-obj-safety]: ../reference/items/traits.html#object-safety
|
|
/// [erased]: https://en.wikipedia.org/wiki/Type_erasure
|
|
mod dyn_keyword {}
|
|
|
|
#[doc(keyword = "union")]
|
|
//
|
|
/// The [Rust equivalent of a C-style union][union].
|
|
///
|
|
/// A `union` looks like a [`struct`] in terms of declaration, but all of its
|
|
/// fields exist in the same memory, superimposed over one another. For instance,
|
|
/// if we wanted some bits in memory that we sometimes interpret as a `u32` and
|
|
/// sometimes as an `f32`, we could write:
|
|
///
|
|
/// ```rust
|
|
/// union IntOrFloat {
|
|
/// i: u32,
|
|
/// f: f32,
|
|
/// }
|
|
///
|
|
/// let mut u = IntOrFloat { f: 1.0 };
|
|
/// // Reading the fields of a union is always unsafe
|
|
/// assert_eq!(unsafe { u.i }, 1065353216);
|
|
/// // Updating through any of the field will modify all of them
|
|
/// u.i = 1073741824;
|
|
/// assert_eq!(unsafe { u.f }, 2.0);
|
|
/// ```
|
|
///
|
|
/// # Matching on unions
|
|
///
|
|
/// It is possible to use pattern matching on `union`s. A single field name must
|
|
/// be used and it must match the name of one of the `union`'s field.
|
|
/// Like reading from a `union`, pattern matching on a `union` requires `unsafe`.
|
|
///
|
|
/// ```rust
|
|
/// union IntOrFloat {
|
|
/// i: u32,
|
|
/// f: f32,
|
|
/// }
|
|
///
|
|
/// let u = IntOrFloat { f: 1.0 };
|
|
///
|
|
/// unsafe {
|
|
/// match u {
|
|
/// IntOrFloat { i: 10 } => println!("Found exactly ten!"),
|
|
/// // Matching the field `f` provides an `f32`.
|
|
/// IntOrFloat { f } => println!("Found f = {f} !"),
|
|
/// }
|
|
/// }
|
|
/// ```
|
|
///
|
|
/// # References to union fields
|
|
///
|
|
/// All fields in a `union` are all at the same place in memory which means
|
|
/// borrowing one borrows the entire `union`, for the same lifetime:
|
|
///
|
|
/// ```rust,compile_fail,E0502
|
|
/// union IntOrFloat {
|
|
/// i: u32,
|
|
/// f: f32,
|
|
/// }
|
|
///
|
|
/// let mut u = IntOrFloat { f: 1.0 };
|
|
///
|
|
/// let f = unsafe { &u.f };
|
|
/// // This will not compile because the field has already been borrowed, even
|
|
/// // if only immutably
|
|
/// let i = unsafe { &mut u.i };
|
|
///
|
|
/// *i = 10;
|
|
/// println!("f = {f} and i = {i}");
|
|
/// ```
|
|
///
|
|
/// See the [Reference][union] for more information on `union`s.
|
|
///
|
|
/// [`struct`]: keyword.struct.html
|
|
/// [union]: ../reference/items/unions.html
|
|
mod union_keyword {}
|