1//! Primitive traits and types representing basic properties of types.
2//!
3//! Rust types can be classified in various useful ways according to
4//! their intrinsic properties. These classifications are represented
5//! as traits.
6
7#![stable(feature = "rust1", since = "1.0.0")]
8
9use crate::cell::UnsafeCell;
10use crate::cmp;
11use crate::fmt::Debug;
12use crate::hash::Hash;
13use crate::hash::Hasher;
14
15/// Implements a given marker trait for multiple types at the same time.
16///
17/// The basic syntax looks like this:
18/// ```ignore private macro
19/// marker_impls! { MarkerTrait for u8, i8 }
20/// ```
21/// You can also implement `unsafe` traits
22/// ```ignore private macro
23/// marker_impls! { unsafe MarkerTrait for u8, i8 }
24/// ```
25/// Add attributes to all impls:
26/// ```ignore private macro
27/// marker_impls! {
28/// #[allow(lint)]
29/// #[unstable(feature = "marker_trait", issue = "none")]
30/// MarkerTrait for u8, i8
31/// }
32/// ```
33/// And use generics:
34/// ```ignore private macro
35/// marker_impls! {
36/// MarkerTrait for
37/// u8, i8,
38/// {T: ?Sized} *const T,
39/// {T: ?Sized} *mut T,
40/// {T: MarkerTrait} PhantomData<T>,
41/// u32,
42/// }
43/// ```
44#[unstable(feature = "internal_impls_macro", issue = "none")]
45macro marker_impls {
46 ( $(#[$($meta:tt)*])* $Trait:ident for $({$($bounds:tt)*})? $T:ty $(, $($rest:tt)*)? ) => {
47 $(#[$($meta)*])* impl< $($($bounds)*)? > $Trait for $T {}
48 marker_impls! { $(#[$($meta)*])* $Trait for $($($rest)*)? }
49 },
50 ( $(#[$($meta:tt)*])* $Trait:ident for ) => {},
51
52 ( $(#[$($meta:tt)*])* unsafe $Trait:ident for $({$($bounds:tt)*})? $T:ty $(, $($rest:tt)*)? ) => {
53 $(#[$($meta)*])* unsafe impl< $($($bounds)*)? > $Trait for $T {}
54 marker_impls! { $(#[$($meta)*])* unsafe $Trait for $($($rest)*)? }
55 },
56 ( $(#[$($meta:tt)*])* unsafe $Trait:ident for ) => {},
57}
58
59/// Types that can be transferred across thread boundaries.
60///
61/// This trait is automatically implemented when the compiler determines it's
62/// appropriate.
63///
64/// An example of a non-`Send` type is the reference-counting pointer
65/// [`rc::Rc`][`Rc`]. If two threads attempt to clone [`Rc`]s that point to the same
66/// reference-counted value, they might try to update the reference count at the
67/// same time, which is [undefined behavior][ub] because [`Rc`] doesn't use atomic
68/// operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring
69/// some overhead) and thus is `Send`.
70///
71/// See [the Nomicon](../../nomicon/send-and-sync.html) and the [`Sync`] trait for more details.
72///
73/// [`Rc`]: ../../std/rc/struct.Rc.html
74/// [arc]: ../../std/sync/struct.Arc.html
75/// [ub]: ../../reference/behavior-considered-undefined.html
76#[stable(feature = "rust1", since = "1.0.0")]
77#[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
78#[diagnostic::on_unimplemented(
79 message = "`{Self}` cannot be sent between threads safely",
80 label = "`{Self}` cannot be sent between threads safely"
81)]
82pub unsafe auto trait Send {
83 // empty.
84}
85
86#[stable(feature = "rust1", since = "1.0.0")]
87impl<T: ?Sized> !Send for *const T {}
88#[stable(feature = "rust1", since = "1.0.0")]
89impl<T: ?Sized> !Send for *mut T {}
90
91// Most instances arise automatically, but this instance is needed to link up `T: Sync` with
92// `&T: Send` (and it also removes the unsound default instance `T Send` -> `&T: Send` that would
93// otherwise exist).
94#[stable(feature = "rust1", since = "1.0.0")]
95unsafe impl<T: Sync + ?Sized> Send for &T {}
96
97/// Types with a constant size known at compile time.
98///
99/// All type parameters have an implicit bound of `Sized`. The special syntax
100/// `?Sized` can be used to remove this bound if it's not appropriate.
101///
102/// ```
103/// # #![allow(dead_code)]
104/// struct Foo<T>(T);
105/// struct Bar<T: ?Sized>(T);
106///
107/// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
108/// struct BarUse(Bar<[i32]>); // OK
109/// ```
110///
111/// The one exception is the implicit `Self` type of a trait. A trait does not
112/// have an implicit `Sized` bound as this is incompatible with [trait object]s
113/// where, by definition, the trait needs to work with all possible implementors,
114/// and thus could be any size.
115///
116/// Although Rust will let you bind `Sized` to a trait, you won't
117/// be able to use it to form a trait object later:
118///
119/// ```
120/// # #![allow(unused_variables)]
121/// trait Foo { }
122/// trait Bar: Sized { }
123///
124/// struct Impl;
125/// impl Foo for Impl { }
126/// impl Bar for Impl { }
127///
128/// let x: &dyn Foo = &Impl; // OK
129/// // let y: &dyn Bar = &Impl; // error: the trait `Bar` cannot
130/// // be made into an object
131/// ```
132///
133/// [trait object]: ../../book/ch17-02-trait-objects.html
134#[doc(alias = "?", alias = "?Sized")]
135#[stable(feature = "rust1", since = "1.0.0")]
136#[lang = "sized"]
137#[diagnostic::on_unimplemented(
138 message = "the size for values of type `{Self}` cannot be known at compilation time",
139 label = "doesn't have a size known at compile-time"
140)]
141#[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
142#[rustc_specialization_trait]
143#[rustc_deny_explicit_impl(implement_via_object = false)]
144#[rustc_coinductive]
145pub trait Sized {
146 // Empty.
147}
148
149/// Types that can be "unsized" to a dynamically-sized type.
150///
151/// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and
152/// `Unsize<dyn fmt::Debug>`.
153///
154/// All implementations of `Unsize` are provided automatically by the compiler.
155/// Those implementations are:
156///
157/// - Arrays `[T; N]` implement `Unsize<[T]>`.
158/// - A type implements `Unsize<dyn Trait + 'a>` if all of these conditions are met:
159/// - The type implements `Trait`.
160/// - `Trait` is object safe.
161/// - The type is sized.
162/// - The type outlives `'a`.
163/// - Structs `Foo<..., T1, ..., Tn, ...>` implement `Unsize<Foo<..., U1, ..., Un, ...>>`
164/// where any number of (type and const) parameters may be changed if all of these conditions
165/// are met:
166/// - Only the last field of `Foo` has a type involving the parameters `T1`, ..., `Tn`.
167/// - All other parameters of the struct are equal.
168/// - `Field<T1, ..., Tn>: Unsize<Field<U1, ..., Un>>`, where `Field<...>` stands for the actual
169/// type of the struct's last field.
170///
171/// `Unsize` is used along with [`ops::CoerceUnsized`] to allow
172/// "user-defined" containers such as [`Rc`] to contain dynamically-sized
173/// types. See the [DST coercion RFC][RFC982] and [the nomicon entry on coercion][nomicon-coerce]
174/// for more details.
175///
176/// [`ops::CoerceUnsized`]: crate::ops::CoerceUnsized
177/// [`Rc`]: ../../std/rc/struct.Rc.html
178/// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
179/// [nomicon-coerce]: ../../nomicon/coercions.html
180#[unstable(feature = "unsize", issue = "18598")]
181#[lang = "unsize"]
182#[rustc_deny_explicit_impl(implement_via_object = false)]
183pub trait Unsize<T: ?Sized> {
184 // Empty.
185}
186
187/// Required trait for constants used in pattern matches.
188///
189/// Any type that derives `PartialEq` automatically implements this trait,
190/// *regardless* of whether its type-parameters implement `PartialEq`.
191///
192/// If a `const` item contains some type that does not implement this trait,
193/// then that type either (1.) does not implement `PartialEq` (which means the
194/// constant will not provide that comparison method, which code generation
195/// assumes is available), or (2.) it implements *its own* version of
196/// `PartialEq` (which we assume does not conform to a structural-equality
197/// comparison).
198///
199/// In either of the two scenarios above, we reject usage of such a constant in
200/// a pattern match.
201///
202/// See also the [structural match RFC][RFC1445], and [issue 63438] which
203/// motivated migrating from an attribute-based design to this trait.
204///
205/// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
206/// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
207#[unstable(feature = "structural_match", issue = "31434")]
208#[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
209#[lang = "structural_peq"]
210pub trait StructuralPartialEq {
211 // Empty.
212}
213
214marker_impls! {
215 #[unstable(feature = "structural_match", issue = "31434")]
216 StructuralPartialEq for
217 usize, u8, u16, u32, u64, u128,
218 isize, i8, i16, i32, i64, i128,
219 bool,
220 char,
221 str /* Technically requires `[u8]: StructuralPartialEq` */,
222 (),
223 {T, const N: usize} [T; N],
224 {T} [T],
225 {T: ?Sized} &T,
226}
227
228/// Types whose values can be duplicated simply by copying bits.
229///
230/// By default, variable bindings have 'move semantics.' In other
231/// words:
232///
233/// ```
234/// #[derive(Debug)]
235/// struct Foo;
236///
237/// let x = Foo;
238///
239/// let y = x;
240///
241/// // `x` has moved into `y`, and so cannot be used
242///
243/// // println!("{x:?}"); // error: use of moved value
244/// ```
245///
246/// However, if a type implements `Copy`, it instead has 'copy semantics':
247///
248/// ```
249/// // We can derive a `Copy` implementation. `Clone` is also required, as it's
250/// // a supertrait of `Copy`.
251/// #[derive(Debug, Copy, Clone)]
252/// struct Foo;
253///
254/// let x = Foo;
255///
256/// let y = x;
257///
258/// // `y` is a copy of `x`
259///
260/// println!("{x:?}"); // A-OK!
261/// ```
262///
263/// It's important to note that in these two examples, the only difference is whether you
264/// are allowed to access `x` after the assignment. Under the hood, both a copy and a move
265/// can result in bits being copied in memory, although this is sometimes optimized away.
266///
267/// ## How can I implement `Copy`?
268///
269/// There are two ways to implement `Copy` on your type. The simplest is to use `derive`:
270///
271/// ```
272/// #[derive(Copy, Clone)]
273/// struct MyStruct;
274/// ```
275///
276/// You can also implement `Copy` and `Clone` manually:
277///
278/// ```
279/// struct MyStruct;
280///
281/// impl Copy for MyStruct { }
282///
283/// impl Clone for MyStruct {
284/// fn clone(&self) -> MyStruct {
285/// *self
286/// }
287/// }
288/// ```
289///
290/// There is a small difference between the two: the `derive` strategy will also place a `Copy`
291/// bound on type parameters, which isn't always desired.
292///
293/// ## What's the difference between `Copy` and `Clone`?
294///
295/// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of
296/// `Copy` is not overloadable; it is always a simple bit-wise copy.
297///
298/// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`] can
299/// provide any type-specific behavior necessary to duplicate values safely. For example,
300/// the implementation of [`Clone`] for [`String`] needs to copy the pointed-to string
301/// buffer in the heap. A simple bitwise copy of [`String`] values would merely copy the
302/// pointer, leading to a double free down the line. For this reason, [`String`] is [`Clone`]
303/// but not `Copy`.
304///
305/// [`Clone`] is a supertrait of `Copy`, so everything which is `Copy` must also implement
306/// [`Clone`]. If a type is `Copy` then its [`Clone`] implementation only needs to return `*self`
307/// (see the example above).
308///
309/// ## When can my type be `Copy`?
310///
311/// A type can implement `Copy` if all of its components implement `Copy`. For example, this
312/// struct can be `Copy`:
313///
314/// ```
315/// # #[allow(dead_code)]
316/// #[derive(Copy, Clone)]
317/// struct Point {
318/// x: i32,
319/// y: i32,
320/// }
321/// ```
322///
323/// A struct can be `Copy`, and [`i32`] is `Copy`, therefore `Point` is eligible to be `Copy`.
324/// By contrast, consider
325///
326/// ```
327/// # #![allow(dead_code)]
328/// # struct Point;
329/// struct PointList {
330/// points: Vec<Point>,
331/// }
332/// ```
333///
334/// The struct `PointList` cannot implement `Copy`, because [`Vec<T>`] is not `Copy`. If we
335/// attempt to derive a `Copy` implementation, we'll get an error:
336///
337/// ```text
338/// the trait `Copy` cannot be implemented for this type; field `points` does not implement `Copy`
339/// ```
340///
341/// Shared references (`&T`) are also `Copy`, so a type can be `Copy`, even when it holds
342/// shared references of types `T` that are *not* `Copy`. Consider the following struct,
343/// which can implement `Copy`, because it only holds a *shared reference* to our non-`Copy`
344/// type `PointList` from above:
345///
346/// ```
347/// # #![allow(dead_code)]
348/// # struct PointList;
349/// #[derive(Copy, Clone)]
350/// struct PointListWrapper<'a> {
351/// point_list_ref: &'a PointList,
352/// }
353/// ```
354///
355/// ## When *can't* my type be `Copy`?
356///
357/// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
358/// mutable reference. Copying [`String`] would duplicate responsibility for managing the
359/// [`String`]'s buffer, leading to a double free.
360///
361/// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's
362/// managing some resource besides its own [`size_of::<T>`] bytes.
363///
364/// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get
365/// the error [E0204].
366///
367/// [E0204]: ../../error_codes/E0204.html
368///
369/// ## When *should* my type be `Copy`?
370///
371/// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though,
372/// that implementing `Copy` is part of the public API of your type. If the type might become
373/// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to
374/// avoid a breaking API change.
375///
376/// ## Additional implementors
377///
378/// In addition to the [implementors listed below][impls],
379/// the following types also implement `Copy`:
380///
381/// * Function item types (i.e., the distinct types defined for each function)
382/// * Function pointer types (e.g., `fn() -> i32`)
383/// * Closure types, if they capture no value from the environment
384/// or if all such captured values implement `Copy` themselves.
385/// Note that variables captured by shared reference always implement `Copy`
386/// (even if the referent doesn't),
387/// while variables captured by mutable reference never implement `Copy`.
388///
389/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
390/// [`String`]: ../../std/string/struct.String.html
391/// [`size_of::<T>`]: crate::mem::size_of
392/// [impls]: #implementors
393#[stable(feature = "rust1", since = "1.0.0")]
394#[lang = "copy"]
395// FIXME(matthewjasper) This allows copying a type that doesn't implement
396// `Copy` because of unsatisfied lifetime bounds (copying `A<'_>` when only
397// `A<'static>: Copy` and `A<'_>: Clone`).
398// We have this attribute here for now only because there are quite a few
399// existing specializations on `Copy` that already exist in the standard
400// library, and there's no way to safely have this behavior right now.
401#[rustc_unsafe_specialization_marker]
402#[rustc_diagnostic_item = "Copy"]
403pub trait Copy: Clone {
404 // Empty.
405}
406
407/// Derive macro generating an impl of the trait `Copy`.
408#[rustc_builtin_macro]
409#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
410#[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
411pub macro Copy($item:item) {
412 /* compiler built-in */
413}
414
415// Implementations of `Copy` for primitive types.
416//
417// Implementations that cannot be described in Rust
418// are implemented in `traits::SelectionContext::copy_clone_conditions()`
419// in `rustc_trait_selection`.
420marker_impls! {
421 #[stable(feature = "rust1", since = "1.0.0")]
422 Copy for
423 usize, u8, u16, u32, u64, u128,
424 isize, i8, i16, i32, i64, i128,
425 f16, f32, f64, f128,
426 bool, char,
427 {T: ?Sized} *const T,
428 {T: ?Sized} *mut T,
429
430}
431
432#[unstable(feature = "never_type", issue = "35121")]
433impl Copy for ! {}
434
435/// Shared references can be copied, but mutable references *cannot*!
436#[stable(feature = "rust1", since = "1.0.0")]
437impl<T: ?Sized> Copy for &T {}
438
439/// Types for which it is safe to share references between threads.
440///
441/// This trait is automatically implemented when the compiler determines
442/// it's appropriate.
443///
444/// The precise definition is: a type `T` is [`Sync`] if and only if `&T` is
445/// [`Send`]. In other words, if there is no possibility of
446/// [undefined behavior][ub] (including data races) when passing
447/// `&T` references between threads.
448///
449/// As one would expect, primitive types like [`u8`] and [`f64`]
450/// are all [`Sync`], and so are simple aggregate types containing them,
451/// like tuples, structs and enums. More examples of basic [`Sync`]
452/// types include "immutable" types like `&T`, and those with simple
453/// inherited mutability, such as [`Box<T>`][box], [`Vec<T>`][vec] and
454/// most other collection types. (Generic parameters need to be [`Sync`]
455/// for their container to be [`Sync`].)
456///
457/// A somewhat surprising consequence of the definition is that `&mut T`
458/// is `Sync` (if `T` is `Sync`) even though it seems like that might
459/// provide unsynchronized mutation. The trick is that a mutable
460/// reference behind a shared reference (that is, `& &mut T`)
461/// becomes read-only, as if it were a `& &T`. Hence there is no risk
462/// of a data race.
463///
464/// A shorter overview of how [`Sync`] and [`Send`] relate to referencing:
465/// * `&T` is [`Send`] if and only if `T` is [`Sync`]
466/// * `&mut T` is [`Send`] if and only if `T` is [`Send`]
467/// * `&T` and `&mut T` are [`Sync`] if and only if `T` is [`Sync`]
468///
469/// Types that are not `Sync` are those that have "interior
470/// mutability" in a non-thread-safe form, such as [`Cell`][cell]
471/// and [`RefCell`][refcell]. These types allow for mutation of
472/// their contents even through an immutable, shared reference. For
473/// example the `set` method on [`Cell<T>`][cell] takes `&self`, so it requires
474/// only a shared reference [`&Cell<T>`][cell]. The method performs no
475/// synchronization, thus [`Cell`][cell] cannot be `Sync`.
476///
477/// Another example of a non-`Sync` type is the reference-counting
478/// pointer [`Rc`][rc]. Given any reference [`&Rc<T>`][rc], you can clone
479/// a new [`Rc<T>`][rc], modifying the reference counts in a non-atomic way.
480///
481/// For cases when one does need thread-safe interior mutability,
482/// Rust provides [atomic data types], as well as explicit locking via
483/// [`sync::Mutex`][mutex] and [`sync::RwLock`][rwlock]. These types
484/// ensure that any mutation cannot cause data races, hence the types
485/// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe
486/// analogue of [`Rc`][rc].
487///
488/// Any types with interior mutability must also use the
489/// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which
490/// can be mutated through a shared reference. Failing to doing this is
491/// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing
492/// from `&T` to `&mut T` is invalid.
493///
494/// See [the Nomicon][nomicon-send-and-sync] for more details about `Sync`.
495///
496/// [box]: ../../std/boxed/struct.Box.html
497/// [vec]: ../../std/vec/struct.Vec.html
498/// [cell]: crate::cell::Cell
499/// [refcell]: crate::cell::RefCell
500/// [rc]: ../../std/rc/struct.Rc.html
501/// [arc]: ../../std/sync/struct.Arc.html
502/// [atomic data types]: crate::sync::atomic
503/// [mutex]: ../../std/sync/struct.Mutex.html
504/// [rwlock]: ../../std/sync/struct.RwLock.html
505/// [unsafecell]: crate::cell::UnsafeCell
506/// [ub]: ../../reference/behavior-considered-undefined.html
507/// [transmute]: crate::mem::transmute
508/// [nomicon-send-and-sync]: ../../nomicon/send-and-sync.html
509#[stable(feature = "rust1", since = "1.0.0")]
510#[cfg_attr(not(test), rustc_diagnostic_item = "Sync")]
511#[lang = "sync"]
512#[rustc_on_unimplemented(
513 on(
514 _Self = "core::cell::once::OnceCell<T>",
515 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::OnceLock` instead"
516 ),
517 on(
518 _Self = "core::cell::Cell<u8>",
519 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead",
520 ),
521 on(
522 _Self = "core::cell::Cell<u16>",
523 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU16` instead",
524 ),
525 on(
526 _Self = "core::cell::Cell<u32>",
527 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU32` instead",
528 ),
529 on(
530 _Self = "core::cell::Cell<u64>",
531 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU64` instead",
532 ),
533 on(
534 _Self = "core::cell::Cell<usize>",
535 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicUsize` instead",
536 ),
537 on(
538 _Self = "core::cell::Cell<i8>",
539 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI8` instead",
540 ),
541 on(
542 _Self = "core::cell::Cell<i16>",
543 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI16` instead",
544 ),
545 on(
546 _Self = "core::cell::Cell<i32>",
547 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead",
548 ),
549 on(
550 _Self = "core::cell::Cell<i64>",
551 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI64` instead",
552 ),
553 on(
554 _Self = "core::cell::Cell<isize>",
555 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicIsize` instead",
556 ),
557 on(
558 _Self = "core::cell::Cell<bool>",
559 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead",
560 ),
561 on(
562 all(
563 _Self = "core::cell::Cell<T>",
564 not(_Self = "core::cell::Cell<u8>"),
565 not(_Self = "core::cell::Cell<u16>"),
566 not(_Self = "core::cell::Cell<u32>"),
567 not(_Self = "core::cell::Cell<u64>"),
568 not(_Self = "core::cell::Cell<usize>"),
569 not(_Self = "core::cell::Cell<i8>"),
570 not(_Self = "core::cell::Cell<i16>"),
571 not(_Self = "core::cell::Cell<i32>"),
572 not(_Self = "core::cell::Cell<i64>"),
573 not(_Self = "core::cell::Cell<isize>"),
574 not(_Self = "core::cell::Cell<bool>")
575 ),
576 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`",
577 ),
578 on(
579 _Self = "core::cell::RefCell<T>",
580 note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
581 ),
582 message = "`{Self}` cannot be shared between threads safely",
583 label = "`{Self}` cannot be shared between threads safely"
584)]
585pub unsafe auto trait Sync {
586 // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`
587 // lands in beta, and it has been extended to check whether a closure is
588 // anywhere in the requirement chain, extend it as such (#48534):
589 // ```
590 // on(
591 // closure,
592 // note="`{Self}` cannot be shared safely, consider marking the closure `move`"
593 // ),
594 // ```
595
596 // Empty
597}
598
599#[stable(feature = "rust1", since = "1.0.0")]
600impl<T: ?Sized> !Sync for *const T {}
601#[stable(feature = "rust1", since = "1.0.0")]
602impl<T: ?Sized> !Sync for *mut T {}
603
604/// Zero-sized type used to mark things that "act like" they own a `T`.
605///
606/// Adding a `PhantomData<T>` field to your type tells the compiler that your
607/// type acts as though it stores a value of type `T`, even though it doesn't
608/// really. This information is used when computing certain safety properties.
609///
610/// For a more in-depth explanation of how to use `PhantomData<T>`, please see
611/// [the Nomicon](../../nomicon/phantom-data.html).
612///
613/// # A ghastly note 👻👻👻
614///
615/// Though they both have scary names, `PhantomData` and 'phantom types' are
616/// related, but not identical. A phantom type parameter is simply a type
617/// parameter which is never used. In Rust, this often causes the compiler to
618/// complain, and the solution is to add a "dummy" use by way of `PhantomData`.
619///
620/// # Examples
621///
622/// ## Unused lifetime parameters
623///
624/// Perhaps the most common use case for `PhantomData` is a struct that has an
625/// unused lifetime parameter, typically as part of some unsafe code. For
626/// example, here is a struct `Slice` that has two pointers of type `*const T`,
627/// presumably pointing into an array somewhere:
628///
629/// ```compile_fail,E0392
630/// struct Slice<'a, T> {
631/// start: *const T,
632/// end: *const T,
633/// }
634/// ```
635///
636/// The intention is that the underlying data is only valid for the
637/// lifetime `'a`, so `Slice` should not outlive `'a`. However, this
638/// intent is not expressed in the code, since there are no uses of
639/// the lifetime `'a` and hence it is not clear what data it applies
640/// to. We can correct this by telling the compiler to act *as if* the
641/// `Slice` struct contained a reference `&'a T`:
642///
643/// ```
644/// use std::marker::PhantomData;
645///
646/// # #[allow(dead_code)]
647/// struct Slice<'a, T> {
648/// start: *const T,
649/// end: *const T,
650/// phantom: PhantomData<&'a T>,
651/// }
652/// ```
653///
654/// This also in turn infers the lifetime bound `T: 'a`, indicating
655/// that any references in `T` are valid over the lifetime `'a`.
656///
657/// When initializing a `Slice` you simply provide the value
658/// `PhantomData` for the field `phantom`:
659///
660/// ```
661/// # #![allow(dead_code)]
662/// # use std::marker::PhantomData;
663/// # struct Slice<'a, T> {
664/// # start: *const T,
665/// # end: *const T,
666/// # phantom: PhantomData<&'a T>,
667/// # }
668/// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
669/// let ptr = vec.as_ptr();
670/// Slice {
671/// start: ptr,
672/// end: unsafe { ptr.add(vec.len()) },
673/// phantom: PhantomData,
674/// }
675/// }
676/// ```
677///
678/// ## Unused type parameters
679///
680/// It sometimes happens that you have unused type parameters which
681/// indicate what type of data a struct is "tied" to, even though that
682/// data is not actually found in the struct itself. Here is an
683/// example where this arises with [FFI]. The foreign interface uses
684/// handles of type `*mut ()` to refer to Rust values of different
685/// types. We track the Rust type using a phantom type parameter on
686/// the struct `ExternalResource` which wraps a handle.
687///
688/// [FFI]: ../../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
689///
690/// ```
691/// # #![allow(dead_code)]
692/// # trait ResType { }
693/// # struct ParamType;
694/// # mod foreign_lib {
695/// # pub fn new(_: usize) -> *mut () { 42 as *mut () }
696/// # pub fn do_stuff(_: *mut (), _: usize) {}
697/// # }
698/// # fn convert_params(_: ParamType) -> usize { 42 }
699/// use std::marker::PhantomData;
700/// use std::mem;
701///
702/// struct ExternalResource<R> {
703/// resource_handle: *mut (),
704/// resource_type: PhantomData<R>,
705/// }
706///
707/// impl<R: ResType> ExternalResource<R> {
708/// fn new() -> Self {
709/// let size_of_res = mem::size_of::<R>();
710/// Self {
711/// resource_handle: foreign_lib::new(size_of_res),
712/// resource_type: PhantomData,
713/// }
714/// }
715///
716/// fn do_stuff(&self, param: ParamType) {
717/// let foreign_params = convert_params(param);
718/// foreign_lib::do_stuff(self.resource_handle, foreign_params);
719/// }
720/// }
721/// ```
722///
723/// ## Ownership and the drop check
724///
725/// The exact interaction of `PhantomData` with drop check **may change in the future**.
726///
727/// Currently, adding a field of type `PhantomData<T>` indicates that your type *owns* data of type
728/// `T` in very rare circumstances. This in turn has effects on the Rust compiler's [drop check]
729/// analysis. For the exact rules, see the [drop check] documentation.
730///
731/// ## Layout
732///
733/// For all `T`, the following are guaranteed:
734/// * `size_of::<PhantomData<T>>() == 0`
735/// * `align_of::<PhantomData<T>>() == 1`
736///
737/// [drop check]: Drop#drop-check
738#[lang = "phantom_data"]
739#[stable(feature = "rust1", since = "1.0.0")]
740pub struct PhantomData<T: ?Sized>;
741
742#[stable(feature = "rust1", since = "1.0.0")]
743impl<T: ?Sized> Hash for PhantomData<T> {
744 #[inline]
745 fn hash<H: Hasher>(&self, _: &mut H) {}
746}
747
748#[stable(feature = "rust1", since = "1.0.0")]
749impl<T: ?Sized> cmp::PartialEq for PhantomData<T> {
750 fn eq(&self, _other: &PhantomData<T>) -> bool {
751 true
752 }
753}
754
755#[stable(feature = "rust1", since = "1.0.0")]
756impl<T: ?Sized> cmp::Eq for PhantomData<T> {}
757
758#[stable(feature = "rust1", since = "1.0.0")]
759impl<T: ?Sized> cmp::PartialOrd for PhantomData<T> {
760 fn partial_cmp(&self, _other: &PhantomData<T>) -> Option<cmp::Ordering> {
761 Option::Some(cmp::Ordering::Equal)
762 }
763}
764
765#[stable(feature = "rust1", since = "1.0.0")]
766impl<T: ?Sized> cmp::Ord for PhantomData<T> {
767 fn cmp(&self, _other: &PhantomData<T>) -> cmp::Ordering {
768 cmp::Ordering::Equal
769 }
770}
771
772#[stable(feature = "rust1", since = "1.0.0")]
773impl<T: ?Sized> Copy for PhantomData<T> {}
774
775#[stable(feature = "rust1", since = "1.0.0")]
776impl<T: ?Sized> Clone for PhantomData<T> {
777 fn clone(&self) -> Self {
778 Self
779 }
780}
781
782#[stable(feature = "rust1", since = "1.0.0")]
783impl<T: ?Sized> Default for PhantomData<T> {
784 fn default() -> Self {
785 Self
786 }
787}
788
789#[unstable(feature = "structural_match", issue = "31434")]
790impl<T: ?Sized> StructuralPartialEq for PhantomData<T> {}
791
792/// Compiler-internal trait used to indicate the type of enum discriminants.
793///
794/// This trait is automatically implemented for every type and does not add any
795/// guarantees to [`mem::Discriminant`]. It is **undefined behavior** to transmute
796/// between `DiscriminantKind::Discriminant` and `mem::Discriminant`.
797///
798/// [`mem::Discriminant`]: crate::mem::Discriminant
799#[unstable(
800 feature = "discriminant_kind",
801 issue = "none",
802 reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
803)]
804#[lang = "discriminant_kind"]
805#[rustc_deny_explicit_impl(implement_via_object = false)]
806pub trait DiscriminantKind {
807 /// The type of the discriminant, which must satisfy the trait
808 /// bounds required by `mem::Discriminant`.
809 #[lang = "discriminant_type"]
810 type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
811}
812
813/// Used to determine whether a type contains
814/// any `UnsafeCell` internally, but not through an indirection.
815/// This affects, for example, whether a `static` of that type is
816/// placed in read-only static memory or writable static memory.
817/// This can be used to declare that a constant with a generic type
818/// will not contain interior mutability, and subsequently allow
819/// placing the constant behind references.
820///
821/// # Safety
822///
823/// This trait is a core part of the language, it is just expressed as a trait in libcore for
824/// convenience. Do *not* implement it for other types.
825// FIXME: Eventually this trait should become `#[rustc_deny_explicit_impl]`.
826// That requires porting the impls below to native internal impls.
827#[lang = "freeze"]
828#[unstable(feature = "freeze", issue = "121675")]
829pub unsafe auto trait Freeze {}
830
831#[unstable(feature = "freeze", issue = "121675")]
832impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
833marker_impls! {
834 #[unstable(feature = "freeze", issue = "121675")]
835 unsafe Freeze for
836 {T: ?Sized} PhantomData<T>,
837 {T: ?Sized} *const T,
838 {T: ?Sized} *mut T,
839 {T: ?Sized} &T,
840 {T: ?Sized} &mut T,
841}
842
843/// Types that do not require any pinning guarantees.
844///
845/// For information on what "pinning" is, see the [`pin` module] documentation.
846///
847/// Implementing the `Unpin` trait for `T` expresses the fact that `T` is pinning-agnostic:
848/// it shall not expose nor rely on any pinning guarantees. This, in turn, means that a
849/// `Pin`-wrapped pointer to such a type can feature a *fully unrestricted* API.
850/// In other words, if `T: Unpin`, a value of type `T` will *not* be bound by the invariants
851/// which pinning otherwise offers, even when "pinned" by a [`Pin<Ptr>`] pointing at it.
852/// When a value of type `T` is pointed at by a [`Pin<Ptr>`], [`Pin`] will not restrict access
853/// to the pointee value like it normally would, thus allowing the user to do anything that they
854/// normally could with a non-[`Pin`]-wrapped `Ptr` to that value.
855///
856/// The idea of this trait is to alleviate the reduced ergonomics of APIs that require the use
857/// of [`Pin`] for soundness for some types, but which also want to be used by other types that
858/// don't care about pinning. The prime example of such an API is [`Future::poll`]. There are many
859/// [`Future`] types that don't care about pinning. These futures can implement `Unpin` and
860/// therefore get around the pinning related restrictions in the API, while still allowing the
861/// subset of [`Future`]s which *do* require pinning to be implemented soundly.
862///
863/// For more discussion on the consequences of [`Unpin`] within the wider scope of the pinning
864/// system, see the [section about `Unpin`] in the [`pin` module].
865///
866/// `Unpin` has no consequence at all for non-pinned data. In particular, [`mem::replace`] happily
867/// moves `!Unpin` data, which would be immovable when pinned ([`mem::replace`] works for any
868/// `&mut T`, not just when `T: Unpin`).
869///
870/// *However*, you cannot use [`mem::replace`] on `!Unpin` data which is *pinned* by being wrapped
871/// inside a [`Pin<Ptr>`] pointing at it. This is because you cannot (safely) use a
872/// [`Pin<Ptr>`] to get an `&mut T` to its pointee value, which you would need to call
873/// [`mem::replace`], and *that* is what makes this system work.
874///
875/// So this, for example, can only be done on types implementing `Unpin`:
876///
877/// ```rust
878/// # #![allow(unused_must_use)]
879/// use std::mem;
880/// use std::pin::Pin;
881///
882/// let mut string = "this".to_string();
883/// let mut pinned_string = Pin::new(&mut string);
884///
885/// // We need a mutable reference to call `mem::replace`.
886/// // We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
887/// // but that is only possible because `String` implements `Unpin`.
888/// mem::replace(&mut *pinned_string, "other".to_string());
889/// ```
890///
891/// This trait is automatically implemented for almost every type. The compiler is free
892/// to take the conservative stance of marking types as [`Unpin`] so long as all of the types that
893/// compose its fields are also [`Unpin`]. This is because if a type implements [`Unpin`], then it
894/// is unsound for that type's implementation to rely on pinning-related guarantees for soundness,
895/// *even* when viewed through a "pinning" pointer! It is the responsibility of the implementor of
896/// a type that relies upon pinning for soundness to ensure that type is *not* marked as [`Unpin`]
897/// by adding [`PhantomPinned`] field. For more details, see the [`pin` module] docs.
898///
899/// [`mem::replace`]: crate::mem::replace "mem replace"
900/// [`Future`]: crate::future::Future "Future"
901/// [`Future::poll`]: crate::future::Future::poll "Future poll"
902/// [`Pin`]: crate::pin::Pin "Pin"
903/// [`Pin<Ptr>`]: crate::pin::Pin "Pin"
904/// [`pin` module]: crate::pin "pin module"
905/// [section about `Unpin`]: crate::pin#unpin "pin module docs about unpin"
906/// [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
907#[stable(feature = "pin", since = "1.33.0")]
908#[diagnostic::on_unimplemented(
909 note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",
910 message = "`{Self}` cannot be unpinned"
911)]
912#[lang = "unpin"]
913pub auto trait Unpin {}
914
915/// A marker type which does not implement `Unpin`.
916///
917/// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
918#[stable(feature = "pin", since = "1.33.0")]
919#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
920pub struct PhantomPinned;
921
922#[stable(feature = "pin", since = "1.33.0")]
923impl !Unpin for PhantomPinned {}
924
925marker_impls! {
926 #[stable(feature = "pin", since = "1.33.0")]
927 Unpin for
928 {T: ?Sized} &T,
929 {T: ?Sized} &mut T,
930}
931
932marker_impls! {
933 #[stable(feature = "pin_raw", since = "1.38.0")]
934 Unpin for
935 {T: ?Sized} *const T,
936 {T: ?Sized} *mut T,
937}
938
939/// A marker for types that can be dropped.
940///
941/// This should be used for `~const` bounds,
942/// as non-const bounds will always hold for every type.
943#[unstable(feature = "const_trait_impl", issue = "67792")]
944#[lang = "destruct"]
945#[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
946#[rustc_deny_explicit_impl(implement_via_object = false)]
947#[const_trait]
948pub trait Destruct {}
949
950/// A marker for tuple types.
951///
952/// The implementation of this trait is built-in and cannot be implemented
953/// for any user type.
954#[unstable(feature = "tuple_trait", issue = "none")]
955#[lang = "tuple_trait"]
956#[diagnostic::on_unimplemented(message = "`{Self}` is not a tuple")]
957#[rustc_deny_explicit_impl(implement_via_object = false)]
958pub trait Tuple {}
959
960/// A marker for pointer-like types.
961///
962/// All types that have the same size and alignment as a `usize` or
963/// `*const ()` automatically implement this trait.
964#[unstable(feature = "pointer_like_trait", issue = "none")]
965#[lang = "pointer_like"]
966#[diagnostic::on_unimplemented(
967 message = "`{Self}` needs to have the same ABI as a pointer",
968 label = "`{Self}` needs to be a pointer-like type"
969)]
970pub trait PointerLike {}
971
972/// A marker for types which can be used as types of `const` generic parameters.
973///
974/// These types must have a proper equivalence relation (`Eq`) and it must be automatically
975/// derived (`StructuralPartialEq`). There's a hard-coded check in the compiler ensuring
976/// that all fields are also `ConstParamTy`, which implies that recursively, all fields
977/// are `StructuralPartialEq`.
978#[lang = "const_param_ty"]
979#[unstable(feature = "adt_const_params", issue = "95174")]
980#[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
981#[allow(multiple_supertrait_upcastable)]
982pub trait ConstParamTy: StructuralPartialEq + Eq {}
983
984/// Derive macro generating an impl of the trait `ConstParamTy`.
985#[rustc_builtin_macro]
986#[unstable(feature = "adt_const_params", issue = "95174")]
987pub macro ConstParamTy($item:item) {
988 /* compiler built-in */
989}
990
991// FIXME(adt_const_params): handle `ty::FnDef`/`ty::Closure`
992marker_impls! {
993 #[unstable(feature = "adt_const_params", issue = "95174")]
994 ConstParamTy for
995 usize, u8, u16, u32, u64, u128,
996 isize, i8, i16, i32, i64, i128,
997 bool,
998 char,
999 str /* Technically requires `[u8]: ConstParamTy` */,
1000 {T: ConstParamTy, const N: usize} [T; N],
1001 {T: ConstParamTy} [T],
1002 {T: ?Sized + ConstParamTy} &T,
1003}
1004
1005// FIXME(adt_const_params): Add to marker_impls call above once not in bootstrap
1006#[unstable(feature = "adt_const_params", issue = "95174")]
1007impl ConstParamTy for () {}
1008
1009/// A common trait implemented by all function pointers.
1010#[unstable(
1011 feature = "fn_ptr_trait",
1012 issue = "none",
1013 reason = "internal trait for implementing various traits for all function pointers"
1014)]
1015#[lang = "fn_ptr_trait"]
1016#[rustc_deny_explicit_impl(implement_via_object = false)]
1017pub trait FnPtr: Copy + Clone {
1018 /// Returns the address of the function pointer.
1019 #[lang = "fn_ptr_addr"]
1020 fn addr(self) -> *const ();
1021}
1022