<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `libcore/marker.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>marker.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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</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. A trait does not</span>
<span class="doccomment">/// have an implicit `Sized` bound as this is incompatible with [trait object]s</span>
<span class="doccomment">/// where, by definition, the trait needs to work with all possible implementors,</span>
<span class="doccomment">/// and thus could be any size.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Although Rust will let you bind `Sized` to a trait, you won't</span>
<span class="doccomment">/// be able to use it to form a trait object later:</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">/// ## Additional implementors</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In addition to the [implementors listed below][impls],</span>
<span class="doccomment">/// the following types also implement `Copy`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// * Function item types (i.e. the distinct types defined for each function)</span>
<span class="doccomment">/// * Function pointer types (e.g. `fn() -> i32`)</span>
<span class="doccomment">/// * Array types, for all sizes, if the item type also implements `Copy` (e.g. `[i32; 123456]`)</span>
<span class="doccomment">/// * Tuple types, if each component also implements `Copy` (e.g. `()`, `(i32, bool)`)</span>
<span class="doccomment">/// * Closure types, if they capture no value from the environment</span>
<span class="doccomment">/// or if all such captured values implement `Copy` themselves.</span>
<span class="doccomment">/// Note that variables captured by shared reference always implement `Copy`</span>
<span class="doccomment">/// (even if the referent doesn't),</span>
<span class="doccomment">/// while variables captured by mutable reference never implement `Copy`.</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="doccomment">/// [impls]: #implementors</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="ident">message</span><span class="op">=</span><span class="string">"`{Self}` cannot be shared between threads safely"</span>,
<span class="ident">label</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">// FIXME(estebank): once support to add notes in `rustc_on_unimplemented`</span>
<span class="comment">// lands in beta, and it has been extended to check whether a closure is</span>
<span class="comment">// anywhere in the requirement chain, extend it as such (#48534):</span>
<span class="comment">// ```</span>
<span class="comment">// on(</span>
<span class="comment">// closure,</span>
<span class="comment">// note="`{Self}` cannot be shared safely, consider marking the closure `move`"</span>
<span class="comment">// ),</span>
<span class="comment">// ```</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">_</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> {}
<span class="doccomment">/// Types which can be moved out of a `Pin`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The `Unpin` trait is used to control the behavior of the [`Pin`] type. If a</span>
<span class="doccomment">/// type implements `Unpin`, it is safe to move a value of that type out of the</span>
<span class="doccomment">/// `Pin` pointer.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This trait is automatically implemented for almost every type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Pin`]: ../mem/struct.Pin.html</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"pin"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"49150"</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">Unpin</span> {}
<span class="doccomment">/// Implementations of `Copy` for primitive types.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Implementations that cannot be described in Rust</span>
<span class="doccomment">/// are implemented in `SelectionContext::copy_clone_conditions()` in librustc.</span>
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">not</span>(<span class="ident">stage0</span>))]</span>
<span class="kw">mod</span> <span class="ident">copy_impls</span> {
<span class="kw">use</span> <span class="kw">super</span>::<span class="ident">Copy</span>;
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">impl_copy</span> {
($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span>:<span class="ident">ty</span>)<span class="kw-2">*</span>) <span class="op">=></span> {
$(
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Copy</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> {}
)<span class="op">*</span>
}
}
<span class="macro">impl_copy</span><span class="macro">!</span> {
<span class="ident">usize</span> <span class="ident">u8</span> <span class="ident">u16</span> <span class="ident">u32</span> <span class="ident">u64</span> <span class="ident">u128</span>
<span class="ident">isize</span> <span class="ident">i8</span> <span class="ident">i16</span> <span class="ident">i32</span> <span class="ident">i64</span> <span class="ident">i128</span>
<span class="ident">f32</span> <span class="ident">f64</span>
<span class="ident">bool</span> <span class="ident">char</span>
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"never_type"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"35121"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Copy</span> <span class="kw">for</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">Copy</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="ident">Copy</span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// Shared references can be copied, but mutable references *cannot*!</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="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">Copy</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span> {}
}
</pre>
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