1 //! Primitive traits and types representing basic properties of types.
3 //! Rust types can be classified in various useful ways according to
4 //! their intrinsic properties. These classifications are represented
7 #![stable(feature = "rust1", since = "1.0.0")]
9 use crate::cell::UnsafeCell;
11 use crate::fmt::Debug;
12 use crate::hash::Hash;
13 use crate::hash::Hasher;
15 /// Types that can be transferred across thread boundaries.
17 /// This trait is automatically implemented when the compiler determines it's
20 /// An example of a non-`Send` type is the reference-counting pointer
21 /// [`rc::Rc`][`Rc`]. If two threads attempt to clone [`Rc`]s that point to the same
22 /// reference-counted value, they might try to update the reference count at the
23 /// same time, which is [undefined behavior][ub] because [`Rc`] doesn't use atomic
24 /// operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring
25 /// some overhead) and thus is `Send`.
27 /// See [the Nomicon](../../nomicon/send-and-sync.html) for more details.
29 /// [`Rc`]: ../../std/rc/struct.Rc.html
30 /// [arc]: ../../std/sync/struct.Arc.html
31 /// [ub]: ../../reference/behavior-considered-undefined.html
32 #[stable(feature = "rust1", since = "1.0.0")]
33 #[cfg_attr(not(test), rustc_diagnostic_item = "send_trait")]
34 #[cfg_attr(not(bootstrap), lang = "send")]
35 #[rustc_on_unimplemented(
36 message = "`{Self}` cannot be sent between threads safely",
37 label = "`{Self}` cannot be sent between threads safely"
39 pub unsafe auto trait Send {
43 #[stable(feature = "rust1", since = "1.0.0")]
44 impl<T: ?Sized> !Send for *const T {}
45 #[stable(feature = "rust1", since = "1.0.0")]
46 impl<T: ?Sized> !Send for *mut T {}
48 /// Types with a constant size known at compile time.
50 /// All type parameters have an implicit bound of `Sized`. The special syntax
51 /// `?Sized` can be used to remove this bound if it's not appropriate.
54 /// # #![allow(dead_code)]
56 /// struct Bar<T: ?Sized>(T);
58 /// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
59 /// struct BarUse(Bar<[i32]>); // OK
62 /// The one exception is the implicit `Self` type of a trait. A trait does not
63 /// have an implicit `Sized` bound as this is incompatible with [trait object]s
64 /// where, by definition, the trait needs to work with all possible implementors,
65 /// and thus could be any size.
67 /// Although Rust will let you bind `Sized` to a trait, you won't
68 /// be able to use it to form a trait object later:
71 /// # #![allow(unused_variables)]
73 /// trait Bar: Sized { }
76 /// impl Foo for Impl { }
77 /// impl Bar for Impl { }
79 /// let x: &dyn Foo = &Impl; // OK
80 /// // let y: &dyn Bar = &Impl; // error: the trait `Bar` cannot
81 /// // be made into an object
84 /// [trait object]: ../../book/ch17-02-trait-objects.html
85 #[stable(feature = "rust1", since = "1.0.0")]
87 #[rustc_on_unimplemented(
88 message = "the size for values of type `{Self}` cannot be known at compilation time",
89 label = "doesn't have a size known at compile-time"
91 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
92 #[rustc_specialization_trait]
97 /// Types that can be "unsized" to a dynamically-sized type.
99 /// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and
100 /// `Unsize<dyn fmt::Debug>`.
102 /// All implementations of `Unsize` are provided automatically by the compiler.
104 /// `Unsize` is implemented for:
106 /// - `[T; N]` is `Unsize<[T]>`
107 /// - `T` is `Unsize<dyn Trait>` when `T: Trait`
108 /// - `Foo<..., T, ...>` is `Unsize<Foo<..., U, ...>>` if:
110 /// - Foo is a struct
111 /// - Only the last field of `Foo` has a type involving `T`
112 /// - `T` is not part of the type of any other fields
113 /// - `Bar<T>: Unsize<Bar<U>>`, if the last field of `Foo` has type `Bar<T>`
115 /// `Unsize` is used along with [`ops::CoerceUnsized`] to allow
116 /// "user-defined" containers such as [`Rc`] to contain dynamically-sized
117 /// types. See the [DST coercion RFC][RFC982] and [the nomicon entry on coercion][nomicon-coerce]
118 /// for more details.
120 /// [`ops::CoerceUnsized`]: crate::ops::CoerceUnsized
121 /// [`Rc`]: ../../std/rc/struct.Rc.html
122 /// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
123 /// [nomicon-coerce]: ../../nomicon/coercions.html
124 #[unstable(feature = "unsize", issue = "27732")]
126 pub trait Unsize<T: ?Sized> {
130 /// Required trait for constants used in pattern matches.
132 /// Any type that derives `PartialEq` automatically implements this trait,
133 /// *regardless* of whether its type-parameters implement `Eq`.
135 /// If a `const` item contains some type that does not implement this trait,
136 /// then that type either (1.) does not implement `PartialEq` (which means the
137 /// constant will not provide that comparison method, which code generation
138 /// assumes is available), or (2.) it implements *its own* version of
139 /// `PartialEq` (which we assume does not conform to a structural-equality
142 /// In either of the two scenarios above, we reject usage of such a constant in
145 /// See also the [structural match RFC][RFC1445], and [issue 63438] which
146 /// motivated migrating from attribute-based design to this trait.
148 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
149 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
150 #[unstable(feature = "structural_match", issue = "31434")]
151 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
152 #[lang = "structural_peq"]
153 pub trait StructuralPartialEq {
157 /// Required trait for constants used in pattern matches.
159 /// Any type that derives `Eq` automatically implements this trait, *regardless*
160 /// of whether its type parameters implement `Eq`.
162 /// This is a hack to work around a limitation in our type system.
166 /// We want to require that types of consts used in pattern matches
167 /// have the attribute `#[derive(PartialEq, Eq)]`.
169 /// In a more ideal world, we could check that requirement by just checking that
170 /// the given type implements both the `StructuralPartialEq` trait *and*
171 /// the `Eq` trait. However, you can have ADTs that *do* `derive(PartialEq, Eq)`,
172 /// and be a case that we want the compiler to accept, and yet the constant's
173 /// type fails to implement `Eq`.
175 /// Namely, a case like this:
178 /// #[derive(PartialEq, Eq)]
179 /// struct Wrap<X>(X);
181 /// fn higher_order(_: &()) { }
183 /// const CFN: Wrap<fn(&())> = Wrap(higher_order);
193 /// (The problem in the above code is that `Wrap<fn(&())>` does not implement
194 /// `PartialEq`, nor `Eq`, because `for<'a> fn(&'a _)` does not implement those
197 /// Therefore, we cannot rely on naive check for `StructuralPartialEq` and
200 /// As a hack to work around this, we use two separate traits injected by each
201 /// of the two derives (`#[derive(PartialEq)]` and `#[derive(Eq)]`) and check
202 /// that both of them are present as part of structural-match checking.
203 #[unstable(feature = "structural_match", issue = "31434")]
204 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
205 #[lang = "structural_teq"]
206 pub trait StructuralEq {
210 /// Types whose values can be duplicated simply by copying bits.
212 /// By default, variable bindings have 'move semantics.' In other
223 /// // `x` has moved into `y`, and so cannot be used
225 /// // println!("{:?}", x); // error: use of moved value
228 /// However, if a type implements `Copy`, it instead has 'copy semantics':
231 /// // We can derive a `Copy` implementation. `Clone` is also required, as it's
232 /// // a supertrait of `Copy`.
233 /// #[derive(Debug, Copy, Clone)]
240 /// // `y` is a copy of `x`
242 /// println!("{:?}", x); // A-OK!
245 /// It's important to note that in these two examples, the only difference is whether you
246 /// are allowed to access `x` after the assignment. Under the hood, both a copy and a move
247 /// can result in bits being copied in memory, although this is sometimes optimized away.
249 /// ## How can I implement `Copy`?
251 /// There are two ways to implement `Copy` on your type. The simplest is to use `derive`:
254 /// #[derive(Copy, Clone)]
258 /// You can also implement `Copy` and `Clone` manually:
263 /// impl Copy for MyStruct { }
265 /// impl Clone for MyStruct {
266 /// fn clone(&self) -> MyStruct {
272 /// There is a small difference between the two: the `derive` strategy will also place a `Copy`
273 /// bound on type parameters, which isn't always desired.
275 /// ## What's the difference between `Copy` and `Clone`?
277 /// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of
278 /// `Copy` is not overloadable; it is always a simple bit-wise copy.
280 /// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`] can
281 /// provide any type-specific behavior necessary to duplicate values safely. For example,
282 /// the implementation of [`Clone`] for [`String`] needs to copy the pointed-to string
283 /// buffer in the heap. A simple bitwise copy of [`String`] values would merely copy the
284 /// pointer, leading to a double free down the line. For this reason, [`String`] is [`Clone`]
287 /// [`Clone`] is a supertrait of `Copy`, so everything which is `Copy` must also implement
288 /// [`Clone`]. If a type is `Copy` then its [`Clone`] implementation only needs to return `*self`
289 /// (see the example above).
291 /// ## When can my type be `Copy`?
293 /// A type can implement `Copy` if all of its components implement `Copy`. For example, this
294 /// struct can be `Copy`:
297 /// # #[allow(dead_code)]
298 /// #[derive(Copy, Clone)]
305 /// A struct can be `Copy`, and [`i32`] is `Copy`, therefore `Point` is eligible to be `Copy`.
306 /// By contrast, consider
309 /// # #![allow(dead_code)]
311 /// struct PointList {
312 /// points: Vec<Point>,
316 /// The struct `PointList` cannot implement `Copy`, because [`Vec<T>`] is not `Copy`. If we
317 /// attempt to derive a `Copy` implementation, we'll get an error:
320 /// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`
323 /// Shared references (`&T`) are also `Copy`, so a type can be `Copy`, even when it holds
324 /// shared references of types `T` that are *not* `Copy`. Consider the following struct,
325 /// which can implement `Copy`, because it only holds a *shared reference* to our non-`Copy`
326 /// type `PointList` from above:
329 /// # #![allow(dead_code)]
330 /// # struct PointList;
331 /// #[derive(Copy, Clone)]
332 /// struct PointListWrapper<'a> {
333 /// point_list_ref: &'a PointList,
337 /// ## When *can't* my type be `Copy`?
339 /// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
340 /// mutable reference. Copying [`String`] would duplicate responsibility for managing the
341 /// [`String`]'s buffer, leading to a double free.
343 /// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's
344 /// managing some resource besides its own [`size_of::<T>`] bytes.
346 /// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get
347 /// the error [E0204].
349 /// [E0204]: ../../error-index.html#E0204
351 /// ## When *should* my type be `Copy`?
353 /// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though,
354 /// that implementing `Copy` is part of the public API of your type. If the type might become
355 /// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to
356 /// avoid a breaking API change.
358 /// ## Additional implementors
360 /// In addition to the [implementors listed below][impls],
361 /// the following types also implement `Copy`:
363 /// * Function item types (i.e., the distinct types defined for each function)
364 /// * Function pointer types (e.g., `fn() -> i32`)
365 /// * Array types, for all sizes, if the item type also implements `Copy` (e.g., `[i32; 123456]`)
366 /// * Tuple types, if each component also implements `Copy` (e.g., `()`, `(i32, bool)`)
367 /// * Closure types, if they capture no value from the environment
368 /// or if all such captured values implement `Copy` themselves.
369 /// Note that variables captured by shared reference always implement `Copy`
370 /// (even if the referent doesn't),
371 /// while variables captured by mutable reference never implement `Copy`.
373 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
374 /// [`String`]: ../../std/string/struct.String.html
375 /// [`size_of::<T>`]: crate::mem::size_of
376 /// [impls]: #implementors
377 #[stable(feature = "rust1", since = "1.0.0")]
379 // FIXME(matthewjasper) This allows copying a type that doesn't implement
380 // `Copy` because of unsatisfied lifetime bounds (copying `A<'_>` when only
381 // `A<'static>: Copy` and `A<'_>: Clone`).
382 // We have this attribute here for now only because there are quite a few
383 // existing specializations on `Copy` that already exist in the standard
384 // library, and there's no way to safely have this behavior right now.
385 #[rustc_unsafe_specialization_marker]
386 pub trait Copy: Clone {
390 /// Derive macro generating an impl of the trait `Copy`.
391 #[rustc_builtin_macro]
392 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
393 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
394 pub macro Copy($item:item) {
395 /* compiler built-in */
398 /// Types for which it is safe to share references between threads.
400 /// This trait is automatically implemented when the compiler determines
401 /// it's appropriate.
403 /// The precise definition is: a type `T` is [`Sync`] if and only if `&T` is
404 /// [`Send`]. In other words, if there is no possibility of
405 /// [undefined behavior][ub] (including data races) when passing
406 /// `&T` references between threads.
408 /// As one would expect, primitive types like [`u8`] and [`f64`]
409 /// are all [`Sync`], and so are simple aggregate types containing them,
410 /// like tuples, structs and enums. More examples of basic [`Sync`]
411 /// types include "immutable" types like `&T`, and those with simple
412 /// inherited mutability, such as [`Box<T>`][box], [`Vec<T>`][vec] and
413 /// most other collection types. (Generic parameters need to be [`Sync`]
414 /// for their container to be [`Sync`].)
416 /// A somewhat surprising consequence of the definition is that `&mut T`
417 /// is `Sync` (if `T` is `Sync`) even though it seems like that might
418 /// provide unsynchronized mutation. The trick is that a mutable
419 /// reference behind a shared reference (that is, `& &mut T`)
420 /// becomes read-only, as if it were a `& &T`. Hence there is no risk
423 /// Types that are not `Sync` are those that have "interior
424 /// mutability" in a non-thread-safe form, such as [`Cell`][cell]
425 /// and [`RefCell`][refcell]. These types allow for mutation of
426 /// their contents even through an immutable, shared reference. For
427 /// example the `set` method on [`Cell<T>`][cell] takes `&self`, so it requires
428 /// only a shared reference [`&Cell<T>`][cell]. The method performs no
429 /// synchronization, thus [`Cell`][cell] cannot be `Sync`.
431 /// Another example of a non-`Sync` type is the reference-counting
432 /// pointer [`Rc`][rc]. Given any reference [`&Rc<T>`][rc], you can clone
433 /// a new [`Rc<T>`][rc], modifying the reference counts in a non-atomic way.
435 /// For cases when one does need thread-safe interior mutability,
436 /// Rust provides [atomic data types], as well as explicit locking via
437 /// [`sync::Mutex`][mutex] and [`sync::RwLock`][rwlock]. These types
438 /// ensure that any mutation cannot cause data races, hence the types
439 /// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe
440 /// analogue of [`Rc`][rc].
442 /// Any types with interior mutability must also use the
443 /// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which
444 /// can be mutated through a shared reference. Failing to doing this is
445 /// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing
446 /// from `&T` to `&mut T` is invalid.
448 /// See [the Nomicon][nomicon-send-and-sync] for more details about `Sync`.
450 /// [box]: ../../std/boxed/struct.Box.html
451 /// [vec]: ../../std/vec/struct.Vec.html
452 /// [cell]: crate::cell::Cell
453 /// [refcell]: crate::cell::RefCell
454 /// [rc]: ../../std/rc/struct.Rc.html
455 /// [arc]: ../../std/sync/struct.Arc.html
456 /// [atomic data types]: crate::sync::atomic
457 /// [mutex]: ../../std/sync/struct.Mutex.html
458 /// [rwlock]: ../../std/sync/struct.RwLock.html
459 /// [unsafecell]: crate::cell::UnsafeCell
460 /// [ub]: ../../reference/behavior-considered-undefined.html
461 /// [transmute]: crate::mem::transmute
462 /// [nomicon-send-and-sync]: ../../nomicon/send-and-sync.html
463 #[stable(feature = "rust1", since = "1.0.0")]
464 #[cfg_attr(not(test), rustc_diagnostic_item = "sync_trait")]
466 #[rustc_on_unimplemented(
467 message = "`{Self}` cannot be shared between threads safely",
468 label = "`{Self}` cannot be shared between threads safely"
470 pub unsafe auto trait Sync {
471 // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`
472 // lands in beta, and it has been extended to check whether a closure is
473 // anywhere in the requirement chain, extend it as such (#48534):
477 // note="`{Self}` cannot be shared safely, consider marking the closure `move`"
484 #[stable(feature = "rust1", since = "1.0.0")]
485 impl<T: ?Sized> !Sync for *const T {}
486 #[stable(feature = "rust1", since = "1.0.0")]
487 impl<T: ?Sized> !Sync for *mut T {}
491 #[stable(feature = "rust1", since = "1.0.0")]
492 impl<T: ?Sized> Hash for $t<T> {
494 fn hash<H: Hasher>(&self, _: &mut H) {}
497 #[stable(feature = "rust1", since = "1.0.0")]
498 impl<T: ?Sized> cmp::PartialEq for $t<T> {
499 fn eq(&self, _other: &$t<T>) -> bool {
504 #[stable(feature = "rust1", since = "1.0.0")]
505 impl<T: ?Sized> cmp::Eq for $t<T> {}
507 #[stable(feature = "rust1", since = "1.0.0")]
508 impl<T: ?Sized> cmp::PartialOrd for $t<T> {
509 fn partial_cmp(&self, _other: &$t<T>) -> Option<cmp::Ordering> {
510 Option::Some(cmp::Ordering::Equal)
514 #[stable(feature = "rust1", since = "1.0.0")]
515 impl<T: ?Sized> cmp::Ord for $t<T> {
516 fn cmp(&self, _other: &$t<T>) -> cmp::Ordering {
521 #[stable(feature = "rust1", since = "1.0.0")]
522 impl<T: ?Sized> Copy for $t<T> {}
524 #[stable(feature = "rust1", since = "1.0.0")]
525 impl<T: ?Sized> Clone for $t<T> {
526 fn clone(&self) -> Self {
531 #[stable(feature = "rust1", since = "1.0.0")]
532 impl<T: ?Sized> Default for $t<T> {
533 fn default() -> Self {
538 #[unstable(feature = "structural_match", issue = "31434")]
539 impl<T: ?Sized> StructuralPartialEq for $t<T> {}
541 #[unstable(feature = "structural_match", issue = "31434")]
542 impl<T: ?Sized> StructuralEq for $t<T> {}
546 /// Zero-sized type used to mark things that "act like" they own a `T`.
548 /// Adding a `PhantomData<T>` field to your type tells the compiler that your
549 /// type acts as though it stores a value of type `T`, even though it doesn't
550 /// really. This information is used when computing certain safety properties.
552 /// For a more in-depth explanation of how to use `PhantomData<T>`, please see
553 /// [the Nomicon](../../nomicon/phantom-data.html).
555 /// # A ghastly note 👻👻👻
557 /// Though they both have scary names, `PhantomData` and 'phantom types' are
558 /// related, but not identical. A phantom type parameter is simply a type
559 /// parameter which is never used. In Rust, this often causes the compiler to
560 /// complain, and the solution is to add a "dummy" use by way of `PhantomData`.
564 /// ## Unused lifetime parameters
566 /// Perhaps the most common use case for `PhantomData` is a struct that has an
567 /// unused lifetime parameter, typically as part of some unsafe code. For
568 /// example, here is a struct `Slice` that has two pointers of type `*const T`,
569 /// presumably pointing into an array somewhere:
571 /// ```compile_fail,E0392
572 /// struct Slice<'a, T> {
578 /// The intention is that the underlying data is only valid for the
579 /// lifetime `'a`, so `Slice` should not outlive `'a`. However, this
580 /// intent is not expressed in the code, since there are no uses of
581 /// the lifetime `'a` and hence it is not clear what data it applies
582 /// to. We can correct this by telling the compiler to act *as if* the
583 /// `Slice` struct contained a reference `&'a T`:
586 /// use std::marker::PhantomData;
588 /// # #[allow(dead_code)]
589 /// struct Slice<'a, T: 'a> {
592 /// phantom: PhantomData<&'a T>,
596 /// This also in turn requires the annotation `T: 'a`, indicating
597 /// that any references in `T` are valid over the lifetime `'a`.
599 /// When initializing a `Slice` you simply provide the value
600 /// `PhantomData` for the field `phantom`:
603 /// # #![allow(dead_code)]
604 /// # use std::marker::PhantomData;
605 /// # struct Slice<'a, T: 'a> {
606 /// # start: *const T,
608 /// # phantom: PhantomData<&'a T>,
610 /// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
611 /// let ptr = vec.as_ptr();
614 /// end: unsafe { ptr.add(vec.len()) },
615 /// phantom: PhantomData,
620 /// ## Unused type parameters
622 /// It sometimes happens that you have unused type parameters which
623 /// indicate what type of data a struct is "tied" to, even though that
624 /// data is not actually found in the struct itself. Here is an
625 /// example where this arises with [FFI]. The foreign interface uses
626 /// handles of type `*mut ()` to refer to Rust values of different
627 /// types. We track the Rust type using a phantom type parameter on
628 /// the struct `ExternalResource` which wraps a handle.
630 /// [FFI]: ../../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
633 /// # #![allow(dead_code)]
634 /// # trait ResType { }
635 /// # struct ParamType;
636 /// # mod foreign_lib {
637 /// # pub fn new(_: usize) -> *mut () { 42 as *mut () }
638 /// # pub fn do_stuff(_: *mut (), _: usize) {}
640 /// # fn convert_params(_: ParamType) -> usize { 42 }
641 /// use std::marker::PhantomData;
644 /// struct ExternalResource<R> {
645 /// resource_handle: *mut (),
646 /// resource_type: PhantomData<R>,
649 /// impl<R: ResType> ExternalResource<R> {
650 /// fn new() -> Self {
651 /// let size_of_res = mem::size_of::<R>();
653 /// resource_handle: foreign_lib::new(size_of_res),
654 /// resource_type: PhantomData,
658 /// fn do_stuff(&self, param: ParamType) {
659 /// let foreign_params = convert_params(param);
660 /// foreign_lib::do_stuff(self.resource_handle, foreign_params);
665 /// ## Ownership and the drop check
667 /// Adding a field of type `PhantomData<T>` indicates that your
668 /// type owns data of type `T`. This in turn implies that when your
669 /// type is dropped, it may drop one or more instances of the type
670 /// `T`. This has bearing on the Rust compiler's [drop check]
673 /// If your struct does not in fact *own* the data of type `T`, it is
674 /// better to use a reference type, like `PhantomData<&'a T>`
675 /// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so
676 /// as not to indicate ownership.
678 /// [drop check]: ../../nomicon/dropck.html
679 #[lang = "phantom_data"]
680 #[stable(feature = "rust1", since = "1.0.0")]
681 pub struct PhantomData<T: ?Sized>;
683 impls! { PhantomData }
686 #[stable(feature = "rust1", since = "1.0.0")]
687 unsafe impl<T: Sync + ?Sized> Send for &T {}
688 #[stable(feature = "rust1", since = "1.0.0")]
689 unsafe impl<T: Send + ?Sized> Send for &mut T {}
692 /// Compiler-internal trait used to indicate the type of enum discriminants.
694 /// This trait is automatically implemented for every type and does not add any
695 /// guarantees to [`mem::Discriminant`]. It is **undefined behavior** to transmute
696 /// between `DiscriminantKind::Discriminant` and `mem::Discriminant`.
698 /// [`mem::Discriminant`]: crate::mem::Discriminant
700 feature = "discriminant_kind",
702 reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
704 #[lang = "discriminant_kind"]
705 pub trait DiscriminantKind {
706 /// The type of the discriminant, which must satisfy the trait
707 /// bounds required by `mem::Discriminant`.
708 #[lang = "discriminant_type"]
709 type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
712 /// Compiler-internal trait used to determine whether a type contains
713 /// any `UnsafeCell` internally, but not through an indirection.
714 /// This affects, for example, whether a `static` of that type is
715 /// placed in read-only static memory or writable static memory.
717 pub(crate) unsafe auto trait Freeze {}
719 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
720 unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
721 unsafe impl<T: ?Sized> Freeze for *const T {}
722 unsafe impl<T: ?Sized> Freeze for *mut T {}
723 unsafe impl<T: ?Sized> Freeze for &T {}
724 unsafe impl<T: ?Sized> Freeze for &mut T {}
726 /// Types that can be safely moved after being pinned.
728 /// Rust itself has no notion of immovable types, and considers moves (e.g.,
729 /// through assignment or [`mem::replace`]) to always be safe.
731 /// The [`Pin`][Pin] type is used instead to prevent moves through the type
732 /// system. Pointers `P<T>` wrapped in the [`Pin<P<T>>`][Pin] wrapper can't be
733 /// moved out of. See the [`pin` module] documentation for more information on
736 /// Implementing the `Unpin` trait for `T` lifts the restrictions of pinning off
737 /// the type, which then allows moving `T` out of [`Pin<P<T>>`][Pin] with
738 /// functions such as [`mem::replace`].
740 /// `Unpin` has no consequence at all for non-pinned data. In particular,
741 /// [`mem::replace`] happily moves `!Unpin` data (it works for any `&mut T`, not
742 /// just when `T: Unpin`). However, you cannot use [`mem::replace`] on data
743 /// wrapped inside a [`Pin<P<T>>`][Pin] because you cannot get the `&mut T` you
744 /// need for that, and *that* is what makes this system work.
746 /// So this, for example, can only be done on types implementing `Unpin`:
749 /// # #![allow(unused_must_use)]
751 /// use std::pin::Pin;
753 /// let mut string = "this".to_string();
754 /// let mut pinned_string = Pin::new(&mut string);
756 /// // We need a mutable reference to call `mem::replace`.
757 /// // We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
758 /// // but that is only possible because `String` implements `Unpin`.
759 /// mem::replace(&mut *pinned_string, "other".to_string());
762 /// This trait is automatically implemented for almost every type.
764 /// [`mem::replace`]: crate::mem::replace
765 /// [Pin]: crate::pin::Pin
766 /// [`pin` module]: crate::pin
767 #[stable(feature = "pin", since = "1.33.0")]
768 #[rustc_on_unimplemented(
769 note = "consider using `Box::pin`",
770 message = "`{Self}` cannot be unpinned"
773 pub auto trait Unpin {}
775 /// A marker type which does not implement `Unpin`.
777 /// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
778 #[stable(feature = "pin", since = "1.33.0")]
779 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
780 pub struct PhantomPinned;
782 #[stable(feature = "pin", since = "1.33.0")]
783 impl !Unpin for PhantomPinned {}
785 #[stable(feature = "pin", since = "1.33.0")]
786 impl<'a, T: ?Sized + 'a> Unpin for &'a T {}
788 #[stable(feature = "pin", since = "1.33.0")]
789 impl<'a, T: ?Sized + 'a> Unpin for &'a mut T {}
791 #[stable(feature = "pin_raw", since = "1.38.0")]
792 impl<T: ?Sized> Unpin for *const T {}
794 #[stable(feature = "pin_raw", since = "1.38.0")]
795 impl<T: ?Sized> Unpin for *mut T {}
797 /// Implementations of `Copy` for primitive types.
799 /// Implementations that cannot be described in Rust
800 /// are implemented in `traits::SelectionContext::copy_clone_conditions()`
801 /// in `rustc_trait_selection`.
806 macro_rules! impl_copy {
809 #[stable(feature = "rust1", since = "1.0.0")]
816 usize u8 u16 u32 u64 u128
817 isize i8 i16 i32 i64 i128
822 #[unstable(feature = "never_type", issue = "35121")]
825 #[stable(feature = "rust1", since = "1.0.0")]
826 impl<T: ?Sized> Copy for *const T {}
828 #[stable(feature = "rust1", since = "1.0.0")]
829 impl<T: ?Sized> Copy for *mut T {}
831 /// Shared references can be copied, but mutable references *cannot*!
832 #[stable(feature = "rust1", since = "1.0.0")]
833 impl<T: ?Sized> Copy for &T {}