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id="550">550</span> <span id="551">551</span> <span id="552">552</span> <span id="553">553</span> <span id="554">554</span> <span id="555">555</span> <span id="556">556</span> <span id="557">557</span> <span id="558">558</span> <span id="559">559</span> <span id="560">560</span> <span id="561">561</span> <span id="562">562</span> <span id="563">563</span> </pre><pre class="rust "> <span class="comment">// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT</span> <span class="comment">// file at the top-level directory of this distribution and at</span> <span class="comment">// http://rust-lang.org/COPYRIGHT.</span> <span class="comment">//</span> <span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span> <span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span> <span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span> <span class="comment">// option. This file may not be copied, modified, or distributed</span> <span class="comment">// except according to those terms.</span> <span class="doccomment">//! Primitive traits and types representing basic properties of types.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Rust types can be classified in various useful ways according to</span> <span class="doccomment">//! their intrinsic properties. These classifications are represented</span> <span class="doccomment">//! as traits.</span> <span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">use</span> <span class="ident">cell</span>::<span class="ident">UnsafeCell</span>; <span class="kw">use</span> <span class="ident">cmp</span>; <span class="kw">use</span> <span class="ident">hash</span>::<span class="ident">Hash</span>; <span class="kw">use</span> <span class="ident">hash</span>::<span class="ident">Hasher</span>; <span class="doccomment">/// Types that can be transferred across thread boundaries.</span> <span class="doccomment">///</span> <span class="doccomment">/// This trait is automatically implemented when the compiler determines it's</span> <span class="doccomment">/// appropriate.</span> <span class="doccomment">///</span> <span class="doccomment">/// An example of a non-`Send` type is the reference-counting pointer</span> <span class="doccomment">/// [`rc::Rc`][`Rc`]. If two threads attempt to clone [`Rc`]s that point to the same</span> <span class="doccomment">/// reference-counted value, they might try to update the reference count at the</span> <span class="doccomment">/// same time, which is [undefined behavior][ub] because [`Rc`] doesn't use atomic</span> <span class="doccomment">/// operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring</span> <span class="doccomment">/// some overhead) and thus is `Send`.</span> <span class="doccomment">///</span> <span class="doccomment">/// See [the Nomicon](../../nomicon/send-and-sync.html) for more details.</span> <span class="doccomment">///</span> <span class="doccomment">/// [`Rc`]: ../../std/rc/struct.Rc.html</span> <span class="doccomment">/// [arc]: ../../std/sync/struct.Arc.html</span> <span class="doccomment">/// [ub]: ../../reference/behavior-considered-undefined.html</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="attribute">#[<span class="ident">rustc_on_unimplemented</span> <span class="op">=</span> <span class="string">"`{Self}` cannot be sent between threads safely"</span>]</span> <span class="kw">pub</span> <span class="kw">unsafe</span> <span class="ident">auto</span> <span class="kw">trait</span> <span class="ident">Send</span> { <span class="comment">// empty.</span> } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">!</span><span class="ident">Send</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">T</span> { } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">!</span><span class="ident">Send</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> { } <span class="doccomment">/// Types with a constant size known at compile time.</span> <span class="doccomment">///</span> <span class="doccomment">/// All type parameters have an implicit bound of `Sized`. The special syntax</span> <span class="doccomment">/// `?Sized` can be used to remove this bound if it's not appropriate.</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #![allow(dead_code)]</span> <span class="doccomment">/// struct Foo<T>(T);</span> <span class="doccomment">/// struct Bar<T: ?Sized>(T);</span> <span class="doccomment">///</span> <span class="doccomment">/// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]</span> <span class="doccomment">/// struct BarUse(Bar<[i32]>); // OK</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// The one exception is the implicit `Self` type of a trait, which does not</span> <span class="doccomment">/// get an implicit `Sized` bound. This is because a `Sized` bound prevents</span> <span class="doccomment">/// the trait from being used to form a [trait object]:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #![allow(unused_variables)]</span> <span class="doccomment">/// trait Foo { }</span> <span class="doccomment">/// trait Bar: Sized { }</span> <span class="doccomment">///</span> <span class="doccomment">/// struct Impl;</span> <span class="doccomment">/// impl Foo for Impl { }</span> <span class="doccomment">/// impl Bar for Impl { }</span> <span class="doccomment">///</span> <span class="doccomment">/// let x: &Foo = &Impl; // OK</span> <span class="doccomment">/// // let y: &Bar = &Impl; // error: the trait `Bar` cannot</span> <span class="doccomment">/// // be made into an object</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// [trait object]: ../../book/first-edition/trait-objects.html</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"sized"</span>]</span> <span class="attribute">#[<span class="ident">rustc_on_unimplemented</span> <span class="op">=</span> <span class="string">"`{Self}` does not have a constant size known at compile-time"</span>]</span> <span class="attribute">#[<span class="ident">fundamental</span>]</span> <span class="comment">// for Default, for example, which requires that `[T]: !Default` be evaluatable</span> <span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Sized</span> { <span class="comment">// Empty.</span> } <span class="doccomment">/// Types that can be "unsized" to a dynamically-sized type.</span> <span class="doccomment">///</span> <span class="doccomment">/// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and</span> <span class="doccomment">/// `Unsize<fmt::Debug>`.</span> <span class="doccomment">///</span> <span class="doccomment">/// All implementations of `Unsize` are provided automatically by the compiler.</span> <span class="doccomment">///</span> <span class="doccomment">/// `Unsize` is implemented for:</span> <span class="doccomment">///</span> <span class="doccomment">/// - `[T; N]` is `Unsize<[T]>`</span> <span class="doccomment">/// - `T` is `Unsize<Trait>` when `T: Trait`</span> <span class="doccomment">/// - `Foo<..., T, ...>` is `Unsize<Foo<..., U, ...>>` if:</span> <span class="doccomment">/// - `T: Unsize<U>`</span> <span class="doccomment">/// - Foo is a struct</span> <span class="doccomment">/// - Only the last field of `Foo` has a type involving `T`</span> <span class="doccomment">/// - `T` is not part of the type of any other fields</span> <span class="doccomment">/// - `Bar<T>: Unsize<Bar<U>>`, if the last field of `Foo` has type `Bar<T>`</span> <span class="doccomment">///</span> <span class="doccomment">/// `Unsize` is used along with [`ops::CoerceUnsized`][coerceunsized] to allow</span> <span class="doccomment">/// "user-defined" containers such as [`rc::Rc`][rc] to contain dynamically-sized</span> <span class="doccomment">/// types. See the [DST coercion RFC][RFC982] and [the nomicon entry on coercion][nomicon-coerce]</span> <span class="doccomment">/// for more details.</span> <span class="doccomment">///</span> <span class="doccomment">/// [coerceunsized]: ../ops/trait.CoerceUnsized.html</span> <span class="doccomment">/// [rc]: ../../std/rc/struct.Rc.html</span> <span class="doccomment">/// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md</span> <span class="doccomment">/// [nomicon-coerce]: ../../nomicon/coercions.html</span> <span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"unsize"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"unsize"</span>]</span> <span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Unsize</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> { <span class="comment">// Empty.</span> } <span class="doccomment">/// Types whose values can be duplicated simply by copying bits.</span> <span class="doccomment">///</span> <span class="doccomment">/// By default, variable bindings have 'move semantics.' In other</span> <span class="doccomment">/// words:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// #[derive(Debug)]</span> <span class="doccomment">/// struct Foo;</span> <span class="doccomment">///</span> <span class="doccomment">/// let x = Foo;</span> <span class="doccomment">///</span> <span class="doccomment">/// let y = x;</span> <span class="doccomment">///</span> <span class="doccomment">/// // `x` has moved into `y`, and so cannot be used</span> <span class="doccomment">///</span> <span class="doccomment">/// // println!("{:?}", x); // error: use of moved value</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// However, if a type implements `Copy`, it instead has 'copy semantics':</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// // We can derive a `Copy` implementation. `Clone` is also required, as it's</span> <span class="doccomment">/// // a supertrait of `Copy`.</span> <span class="doccomment">/// #[derive(Debug, Copy, Clone)]</span> <span class="doccomment">/// struct Foo;</span> <span class="doccomment">///</span> <span class="doccomment">/// let x = Foo;</span> <span class="doccomment">///</span> <span class="doccomment">/// let y = x;</span> <span class="doccomment">///</span> <span class="doccomment">/// // `y` is a copy of `x`</span> <span class="doccomment">///</span> <span class="doccomment">/// println!("{:?}", x); // A-OK!</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// It's important to note that in these two examples, the only difference is whether you</span> <span class="doccomment">/// are allowed to access `x` after the assignment. Under the hood, both a copy and a move</span> <span class="doccomment">/// can result in bits being copied in memory, although this is sometimes optimized away.</span> <span class="doccomment">///</span> <span class="doccomment">/// ## How can I implement `Copy`?</span> <span class="doccomment">///</span> <span class="doccomment">/// There are two ways to implement `Copy` on your type. The simplest is to use `derive`:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// #[derive(Copy, Clone)]</span> <span class="doccomment">/// struct MyStruct;</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// You can also implement `Copy` and `Clone` manually:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// struct MyStruct;</span> <span class="doccomment">///</span> <span class="doccomment">/// impl Copy for MyStruct { }</span> <span class="doccomment">///</span> <span class="doccomment">/// impl Clone for MyStruct {</span> <span class="doccomment">/// fn clone(&self) -> MyStruct {</span> <span class="doccomment">/// *self</span> <span class="doccomment">/// }</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// There is a small difference between the two: the `derive` strategy will also place a `Copy`</span> <span class="doccomment">/// bound on type parameters, which isn't always desired.</span> <span class="doccomment">///</span> <span class="doccomment">/// ## What's the difference between `Copy` and `Clone`?</span> <span class="doccomment">///</span> <span class="doccomment">/// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of</span> <span class="doccomment">/// `Copy` is not overloadable; it is always a simple bit-wise copy.</span> <span class="doccomment">///</span> <span class="doccomment">/// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`] can</span> <span class="doccomment">/// provide any type-specific behavior necessary to duplicate values safely. For example,</span> <span class="doccomment">/// the implementation of [`Clone`] for [`String`] needs to copy the pointed-to string</span> <span class="doccomment">/// buffer in the heap. A simple bitwise copy of [`String`] values would merely copy the</span> <span class="doccomment">/// pointer, leading to a double free down the line. For this reason, [`String`] is [`Clone`]</span> <span class="doccomment">/// but not `Copy`.</span> <span class="doccomment">///</span> <span class="doccomment">/// [`Clone`] is a supertrait of `Copy`, so everything which is `Copy` must also implement</span> <span class="doccomment">/// [`Clone`]. If a type is `Copy` then its [`Clone`] implementation only needs to return `*self`</span> <span class="doccomment">/// (see the example above).</span> <span class="doccomment">///</span> <span class="doccomment">/// ## When can my type be `Copy`?</span> <span class="doccomment">///</span> <span class="doccomment">/// A type can implement `Copy` if all of its components implement `Copy`. For example, this</span> <span class="doccomment">/// struct can be `Copy`:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #[allow(dead_code)]</span> <span class="doccomment">/// struct Point {</span> <span class="doccomment">/// x: i32,</span> <span class="doccomment">/// y: i32,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// A struct can be `Copy`, and [`i32`] is `Copy`, therefore `Point` is eligible to be `Copy`.</span> <span class="doccomment">/// By contrast, consider</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #![allow(dead_code)]</span> <span class="doccomment">/// # struct Point;</span> <span class="doccomment">/// struct PointList {</span> <span class="doccomment">/// points: Vec<Point>,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// The struct `PointList` cannot implement `Copy`, because [`Vec<T>`] is not `Copy`. If we</span> <span class="doccomment">/// attempt to derive a `Copy` implementation, we'll get an error:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```text</span> <span class="doccomment">/// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// ## When *can't* my type be `Copy`?</span> <span class="doccomment">///</span> <span class="doccomment">/// Some types can't be copied safely. For example, copying `&mut T` would create an aliased</span> <span class="doccomment">/// mutable reference. Copying [`String`] would duplicate responsibility for managing the</span> <span class="doccomment">/// [`String`]'s buffer, leading to a double free.</span> <span class="doccomment">///</span> <span class="doccomment">/// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's</span> <span class="doccomment">/// managing some resource besides its own [`size_of::<T>`] bytes.</span> <span class="doccomment">///</span> <span class="doccomment">/// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get</span> <span class="doccomment">/// the error [E0204].</span> <span class="doccomment">///</span> <span class="doccomment">/// [E0204]: ../../error-index.html#E0204</span> <span class="doccomment">///</span> <span class="doccomment">/// ## When *should* my type be `Copy`?</span> <span class="doccomment">///</span> <span class="doccomment">/// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though,</span> <span class="doccomment">/// that implementing `Copy` is part of the public API of your type. If the type might become</span> <span class="doccomment">/// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to</span> <span class="doccomment">/// avoid a breaking API change.</span> <span class="doccomment">///</span> <span class="doccomment">/// [`Vec<T>`]: ../../std/vec/struct.Vec.html</span> <span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span> <span class="doccomment">/// [`Drop`]: ../../std/ops/trait.Drop.html</span> <span class="doccomment">/// [`size_of::<T>`]: ../../std/mem/fn.size_of.html</span> <span class="doccomment">/// [`Clone`]: ../clone/trait.Clone.html</span> <span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span> <span class="doccomment">/// [`i32`]: ../../std/primitive.i32.html</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"copy"</span>]</span> <span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Copy</span> : <span class="ident">Clone</span> { <span class="comment">// Empty.</span> } <span class="doccomment">/// Types for which it is safe to share references between threads.</span> <span class="doccomment">///</span> <span class="doccomment">/// This trait is automatically implemented when the compiler determines</span> <span class="doccomment">/// it's appropriate.</span> <span class="doccomment">///</span> <span class="doccomment">/// The precise definition is: a type `T` is `Sync` if `&T` is</span> <span class="doccomment">/// [`Send`][send]. In other words, if there is no possibility of</span> <span class="doccomment">/// [undefined behavior][ub] (including data races) when passing</span> <span class="doccomment">/// `&T` references between threads.</span> <span class="doccomment">///</span> <span class="doccomment">/// As one would expect, primitive types like [`u8`][u8] and [`f64`][f64]</span> <span class="doccomment">/// are all `Sync`, and so are simple aggregate types containing them,</span> <span class="doccomment">/// like tuples, structs and enums. More examples of basic `Sync`</span> <span class="doccomment">/// types include "immutable" types like `&T`, and those with simple</span> <span class="doccomment">/// inherited mutability, such as [`Box<T>`][box], [`Vec<T>`][vec] and</span> <span class="doccomment">/// most other collection types. (Generic parameters need to be `Sync`</span> <span class="doccomment">/// for their container to be `Sync`.)</span> <span class="doccomment">///</span> <span class="doccomment">/// A somewhat surprising consequence of the definition is that `&mut T`</span> <span class="doccomment">/// is `Sync` (if `T` is `Sync`) even though it seems like that might</span> <span class="doccomment">/// provide unsynchronized mutation. The trick is that a mutable</span> <span class="doccomment">/// reference behind a shared reference (that is, `& &mut T`)</span> <span class="doccomment">/// becomes read-only, as if it were a `& &T`. Hence there is no risk</span> <span class="doccomment">/// of a data race.</span> <span class="doccomment">///</span> <span class="doccomment">/// Types that are not `Sync` are those that have "interior</span> <span class="doccomment">/// mutability" in a non-thread-safe form, such as [`cell::Cell`][cell]</span> <span class="doccomment">/// and [`cell::RefCell`][refcell]. These types allow for mutation of</span> <span class="doccomment">/// their contents even through an immutable, shared reference. For</span> <span class="doccomment">/// example the `set` method on [`Cell<T>`][cell] takes `&self`, so it requires</span> <span class="doccomment">/// only a shared reference [`&Cell<T>`][cell]. The method performs no</span> <span class="doccomment">/// synchronization, thus [`Cell`][cell] cannot be `Sync`.</span> <span class="doccomment">///</span> <span class="doccomment">/// Another example of a non-`Sync` type is the reference-counting</span> <span class="doccomment">/// pointer [`rc::Rc`][rc]. Given any reference [`&Rc<T>`][rc], you can clone</span> <span class="doccomment">/// a new [`Rc<T>`][rc], modifying the reference counts in a non-atomic way.</span> <span class="doccomment">///</span> <span class="doccomment">/// For cases when one does need thread-safe interior mutability,</span> <span class="doccomment">/// Rust provides [atomic data types], as well as explicit locking via</span> <span class="doccomment">/// [`sync::Mutex`][mutex] and [`sync::RwLock`][rwlock]. These types</span> <span class="doccomment">/// ensure that any mutation cannot cause data races, hence the types</span> <span class="doccomment">/// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe</span> <span class="doccomment">/// analogue of [`Rc`][rc].</span> <span class="doccomment">///</span> <span class="doccomment">/// Any types with interior mutability must also use the</span> <span class="doccomment">/// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which</span> <span class="doccomment">/// can be mutated through a shared reference. Failing to doing this is</span> <span class="doccomment">/// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing</span> <span class="doccomment">/// from `&T` to `&mut T` is invalid.</span> <span class="doccomment">///</span> <span class="doccomment">/// See [the Nomicon](../../nomicon/send-and-sync.html) for more</span> <span class="doccomment">/// details about `Sync`.</span> <span class="doccomment">///</span> <span class="doccomment">/// [send]: trait.Send.html</span> <span class="doccomment">/// [u8]: ../../std/primitive.u8.html</span> <span class="doccomment">/// [f64]: ../../std/primitive.f64.html</span> <span class="doccomment">/// [box]: ../../std/boxed/struct.Box.html</span> <span class="doccomment">/// [vec]: ../../std/vec/struct.Vec.html</span> <span class="doccomment">/// [cell]: ../cell/struct.Cell.html</span> <span class="doccomment">/// [refcell]: ../cell/struct.RefCell.html</span> <span class="doccomment">/// [rc]: ../../std/rc/struct.Rc.html</span> <span class="doccomment">/// [arc]: ../../std/sync/struct.Arc.html</span> <span class="doccomment">/// [atomic data types]: ../sync/atomic/index.html</span> <span class="doccomment">/// [mutex]: ../../std/sync/struct.Mutex.html</span> <span class="doccomment">/// [rwlock]: ../../std/sync/struct.RwLock.html</span> <span class="doccomment">/// [unsafecell]: ../cell/struct.UnsafeCell.html</span> <span class="doccomment">/// [ub]: ../../reference/behavior-considered-undefined.html</span> <span class="doccomment">/// [transmute]: ../../std/mem/fn.transmute.html</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"sync"</span>]</span> <span class="attribute">#[<span class="ident">rustc_on_unimplemented</span> <span class="op">=</span> <span class="string">"`{Self}` cannot be shared between threads safely"</span>]</span> <span class="kw">pub</span> <span class="kw">unsafe</span> <span class="ident">auto</span> <span class="kw">trait</span> <span class="ident">Sync</span> { <span class="comment">// Empty</span> } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">!</span><span class="ident">Sync</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">T</span> { } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">!</span><span class="ident">Sync</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> { } <span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">impls</span>{ (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span>: <span class="ident">ident</span>) <span class="op">=></span> ( <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Hash</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="attribute">#[<span class="ident">inline</span>]</span> <span class="kw">fn</span> <span class="ident">hash</span><span class="op"><</span><span class="ident">H</span>: <span class="ident">Hasher</span><span class="op">></span>(<span class="kw-2">&</span><span class="self">self</span>, _: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">H</span>) { } } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">cmp</span>::<span class="ident">PartialEq</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="kw">fn</span> <span class="ident">eq</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">_other</span>: <span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) <span class="op">-></span> <span class="ident">bool</span> { <span class="bool-val">true</span> } } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">cmp</span>::<span class="ident">Eq</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">cmp</span>::<span class="ident">PartialOrd</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="kw">fn</span> <span class="ident">partial_cmp</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">_other</span>: <span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">cmp</span>::<span class="ident">Ordering</span><span class="op">></span> { <span class="prelude-ty">Option</span>::<span class="prelude-val">Some</span>(<span class="ident">cmp</span>::<span class="ident">Ordering</span>::<span class="ident">Equal</span>) } } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">cmp</span>::<span class="ident">Ord</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="kw">fn</span> <span class="ident">cmp</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">_other</span>: <span class="kw-2">&</span><span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) <span class="op">-></span> <span class="ident">cmp</span>::<span class="ident">Ordering</span> { <span class="ident">cmp</span>::<span class="ident">Ordering</span>::<span class="ident">Equal</span> } } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Copy</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Clone</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="kw">fn</span> <span class="ident">clone</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> } } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Default</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="kw">fn</span> <span class="ident">default</span>() <span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> } } ) } <span class="doccomment">/// Zero-sized type used to mark things that "act like" they own a `T`.</span> <span class="doccomment">///</span> <span class="doccomment">/// Adding a `PhantomData<T>` field to your type tells the compiler that your</span> <span class="doccomment">/// type acts as though it stores a value of type `T`, even though it doesn't</span> <span class="doccomment">/// really. This information is used when computing certain safety properties.</span> <span class="doccomment">///</span> <span class="doccomment">/// For a more in-depth explanation of how to use `PhantomData<T>`, please see</span> <span class="doccomment">/// [the Nomicon](../../nomicon/phantom-data.html).</span> <span class="doccomment">///</span> <span class="doccomment">/// # A ghastly note 👻👻👻</span> <span class="doccomment">///</span> <span class="doccomment">/// Though they both have scary names, `PhantomData` and 'phantom types' are</span> <span class="doccomment">/// related, but not identical. A phantom type parameter is simply a type</span> <span class="doccomment">/// parameter which is never used. In Rust, this often causes the compiler to</span> <span class="doccomment">/// complain, and the solution is to add a "dummy" use by way of `PhantomData`.</span> <span class="doccomment">///</span> <span class="doccomment">/// # Examples</span> <span class="doccomment">///</span> <span class="doccomment">/// ## Unused lifetime parameters</span> <span class="doccomment">///</span> <span class="doccomment">/// Perhaps the most common use case for `PhantomData` is a struct that has an</span> <span class="doccomment">/// unused lifetime parameter, typically as part of some unsafe code. For</span> <span class="doccomment">/// example, here is a struct `Slice` that has two pointers of type `*const T`,</span> <span class="doccomment">/// presumably pointing into an array somewhere:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```compile_fail,E0392</span> <span class="doccomment">/// struct Slice<'a, T> {</span> <span class="doccomment">/// start: *const T,</span> <span class="doccomment">/// end: *const T,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// The intention is that the underlying data is only valid for the</span> <span class="doccomment">/// lifetime `'a`, so `Slice` should not outlive `'a`. However, this</span> <span class="doccomment">/// intent is not expressed in the code, since there are no uses of</span> <span class="doccomment">/// the lifetime `'a` and hence it is not clear what data it applies</span> <span class="doccomment">/// to. We can correct this by telling the compiler to act *as if* the</span> <span class="doccomment">/// `Slice` struct contained a reference `&'a T`:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// use std::marker::PhantomData;</span> <span class="doccomment">///</span> <span class="doccomment">/// # #[allow(dead_code)]</span> <span class="doccomment">/// struct Slice<'a, T: 'a> {</span> <span class="doccomment">/// start: *const T,</span> <span class="doccomment">/// end: *const T,</span> <span class="doccomment">/// phantom: PhantomData<&'a T>,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// This also in turn requires the annotation `T: 'a`, indicating</span> <span class="doccomment">/// that any references in `T` are valid over the lifetime `'a`.</span> <span class="doccomment">///</span> <span class="doccomment">/// When initializing a `Slice` you simply provide the value</span> <span class="doccomment">/// `PhantomData` for the field `phantom`:</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #![allow(dead_code)]</span> <span class="doccomment">/// # use std::marker::PhantomData;</span> <span class="doccomment">/// # struct Slice<'a, T: 'a> {</span> <span class="doccomment">/// # start: *const T,</span> <span class="doccomment">/// # end: *const T,</span> <span class="doccomment">/// # phantom: PhantomData<&'a T>,</span> <span class="doccomment">/// # }</span> <span class="doccomment">/// fn borrow_vec<'a, T>(vec: &'a Vec<T>) -> Slice<'a, T> {</span> <span class="doccomment">/// let ptr = vec.as_ptr();</span> <span class="doccomment">/// Slice {</span> <span class="doccomment">/// start: ptr,</span> <span class="doccomment">/// end: unsafe { ptr.offset(vec.len() as isize) },</span> <span class="doccomment">/// phantom: PhantomData,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// ## Unused type parameters</span> <span class="doccomment">///</span> <span class="doccomment">/// It sometimes happens that you have unused type parameters which</span> <span class="doccomment">/// indicate what type of data a struct is "tied" to, even though that</span> <span class="doccomment">/// data is not actually found in the struct itself. Here is an</span> <span class="doccomment">/// example where this arises with [FFI]. The foreign interface uses</span> <span class="doccomment">/// handles of type `*mut ()` to refer to Rust values of different</span> <span class="doccomment">/// types. We track the Rust type using a phantom type parameter on</span> <span class="doccomment">/// the struct `ExternalResource` which wraps a handle.</span> <span class="doccomment">///</span> <span class="doccomment">/// [FFI]: ../../book/first-edition/ffi.html</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// # #![allow(dead_code)]</span> <span class="doccomment">/// # trait ResType { }</span> <span class="doccomment">/// # struct ParamType;</span> <span class="doccomment">/// # mod foreign_lib {</span> <span class="doccomment">/// # pub fn new(_: usize) -> *mut () { 42 as *mut () }</span> <span class="doccomment">/// # pub fn do_stuff(_: *mut (), _: usize) {}</span> <span class="doccomment">/// # }</span> <span class="doccomment">/// # fn convert_params(_: ParamType) -> usize { 42 }</span> <span class="doccomment">/// use std::marker::PhantomData;</span> <span class="doccomment">/// use std::mem;</span> <span class="doccomment">///</span> <span class="doccomment">/// struct ExternalResource<R> {</span> <span class="doccomment">/// resource_handle: *mut (),</span> <span class="doccomment">/// resource_type: PhantomData<R>,</span> <span class="doccomment">/// }</span> <span class="doccomment">///</span> <span class="doccomment">/// impl<R: ResType> ExternalResource<R> {</span> <span class="doccomment">/// fn new() -> ExternalResource<R> {</span> <span class="doccomment">/// let size_of_res = mem::size_of::<R>();</span> <span class="doccomment">/// ExternalResource {</span> <span class="doccomment">/// resource_handle: foreign_lib::new(size_of_res),</span> <span class="doccomment">/// resource_type: PhantomData,</span> <span class="doccomment">/// }</span> <span class="doccomment">/// }</span> <span class="doccomment">///</span> <span class="doccomment">/// fn do_stuff(&self, param: ParamType) {</span> <span class="doccomment">/// let foreign_params = convert_params(param);</span> <span class="doccomment">/// foreign_lib::do_stuff(self.resource_handle, foreign_params);</span> <span class="doccomment">/// }</span> <span class="doccomment">/// }</span> <span class="doccomment">/// ```</span> <span class="doccomment">///</span> <span class="doccomment">/// ## Ownership and the drop check</span> <span class="doccomment">///</span> <span class="doccomment">/// Adding a field of type `PhantomData<T>` indicates that your</span> <span class="doccomment">/// type owns data of type `T`. This in turn implies that when your</span> <span class="doccomment">/// type is dropped, it may drop one or more instances of the type</span> <span class="doccomment">/// `T`. This has bearing on the Rust compiler's [drop check]</span> <span class="doccomment">/// analysis.</span> <span class="doccomment">///</span> <span class="doccomment">/// If your struct does not in fact *own* the data of type `T`, it is</span> <span class="doccomment">/// better to use a reference type, like `PhantomData<&'a T>`</span> <span class="doccomment">/// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so</span> <span class="doccomment">/// as not to indicate ownership.</span> <span class="doccomment">///</span> <span class="doccomment">/// [drop check]: ../../nomicon/dropck.html</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"phantom_data"</span>]</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>; <span class="macro">impls</span><span class="macro">!</span> { <span class="ident">PhantomData</span> } <span class="kw">mod</span> <span class="ident">impls</span> { <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">Sync</span> <span class="op">+</span> <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Send</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span> {} <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">Send</span> <span class="op">+</span> <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Send</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {} } <span class="doccomment">/// Compiler-internal trait used to determine whether a type contains</span> <span class="doccomment">/// any `UnsafeCell` internally, but not through an indirection.</span> <span class="doccomment">/// This affects, for example, whether a `static` of that type is</span> <span class="doccomment">/// placed in read-only static memory or writable static memory.</span> <span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"freeze"</span>]</span> <span class="kw">unsafe</span> <span class="ident">auto</span> <span class="kw">trait</span> <span class="ident">Freeze</span> {} <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="op">!</span><span class="ident">Freeze</span> <span class="kw">for</span> <span class="ident">UnsafeCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {} <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Freeze</span> <span class="kw">for</span> <span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {} <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Freeze</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">T</span> {} <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Freeze</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {} <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Freeze</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span> {} <span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Freeze</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {} </pre> </section> <section id='search' class="content hidden"></section> <section class="footer"></section> <aside id="help" class="hidden"> <div> <h1 class="hidden">Help</h1> <div class="shortcuts"> <h2>Keyboard Shortcuts</h2> <dl> <dt><kbd>?</kbd></dt> <dd>Show this help dialog</dd> <dt><kbd>S</kbd></dt> <dd>Focus the search field</dd> <dt><kbd>↑</kbd></dt> <dd>Move up in search results</dd> <dt><kbd>↓</kbd></dt> <dd>Move down in search results</dd> <dt><kbd>↹</kbd></dt> <dd>Switch tab</dd> <dt><kbd>⏎</kbd></dt> <dd>Go to active search result</dd> <dt><kbd>+</kbd></dt> <dd>Expand all sections</dd> <dt><kbd>-</kbd></dt> <dd>Collapse all sections</dd> </dl> </div> <div class="infos"> <h2>Search Tricks</h2> <p> Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type. </p> <p> Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>. </p> <p> Search functions by type signature (e.g. <code>vec -> 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