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")]
34 #[rustc_on_unimplemented(
35 message = "`{Self}` cannot be sent between threads safely",
36 label = "`{Self}` cannot be sent between threads safely"
38 pub unsafe auto trait Send {
42 #[stable(feature = "rust1", since = "1.0.0")]
43 impl<T: ?Sized> !Send for *const T {}
44 #[stable(feature = "rust1", since = "1.0.0")]
45 impl<T: ?Sized> !Send for *mut T {}
47 /// Types with a constant size known at compile time.
49 /// All type parameters have an implicit bound of `Sized`. The special syntax
50 /// `?Sized` can be used to remove this bound if it's not appropriate.
53 /// # #![allow(dead_code)]
55 /// struct Bar<T: ?Sized>(T);
57 /// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
58 /// struct BarUse(Bar<[i32]>); // OK
61 /// The one exception is the implicit `Self` type of a trait. A trait does not
62 /// have an implicit `Sized` bound as this is incompatible with [trait object]s
63 /// where, by definition, the trait needs to work with all possible implementors,
64 /// and thus could be any size.
66 /// Although Rust will let you bind `Sized` to a trait, you won't
67 /// be able to use it to form a trait object later:
70 /// # #![allow(unused_variables)]
72 /// trait Bar: Sized { }
75 /// impl Foo for Impl { }
76 /// impl Bar for Impl { }
78 /// let x: &dyn Foo = &Impl; // OK
79 /// // let y: &dyn Bar = &Impl; // error: the trait `Bar` cannot
80 /// // be made into an object
83 /// [trait object]: ../../book/ch17-02-trait-objects.html
84 #[doc(alias = "?", alias = "?Sized")]
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.
103 /// Those implementations are:
105 /// - Arrays `[T; N]` implement `Unsize<[T]>`.
106 /// - Types implementing a trait `Trait` also implement `Unsize<dyn Trait>`.
107 /// - Structs `Foo<..., T, ...>` implement `Unsize<Foo<..., U, ...>>` if all of these conditions
109 /// - `T: Unsize<U>`.
110 /// - Only the last field of `Foo` has a type involving `T`.
111 /// - `Bar<T>: Unsize<Bar<U>>`, where `Bar<T>` stands for the actual type of that last field.
113 /// `Unsize` is used along with [`ops::CoerceUnsized`] to allow
114 /// "user-defined" containers such as [`Rc`] to contain dynamically-sized
115 /// types. See the [DST coercion RFC][RFC982] and [the nomicon entry on coercion][nomicon-coerce]
116 /// for more details.
118 /// [`ops::CoerceUnsized`]: crate::ops::CoerceUnsized
119 /// [`Rc`]: ../../std/rc/struct.Rc.html
120 /// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
121 /// [nomicon-coerce]: ../../nomicon/coercions.html
122 #[unstable(feature = "unsize", issue = "27732")]
124 pub trait Unsize<T: ?Sized> {
128 /// Required trait for constants used in pattern matches.
130 /// Any type that derives `PartialEq` automatically implements this trait,
131 /// *regardless* of whether its type-parameters implement `Eq`.
133 /// If a `const` item contains some type that does not implement this trait,
134 /// then that type either (1.) does not implement `PartialEq` (which means the
135 /// constant will not provide that comparison method, which code generation
136 /// assumes is available), or (2.) it implements *its own* version of
137 /// `PartialEq` (which we assume does not conform to a structural-equality
140 /// In either of the two scenarios above, we reject usage of such a constant in
143 /// See also the [structural match RFC][RFC1445], and [issue 63438] which
144 /// motivated migrating from attribute-based design to this trait.
146 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
147 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
148 #[unstable(feature = "structural_match", issue = "31434")]
149 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
150 #[lang = "structural_peq"]
151 pub trait StructuralPartialEq {
155 /// Required trait for constants used in pattern matches.
157 /// Any type that derives `Eq` automatically implements this trait, *regardless*
158 /// of whether its type parameters implement `Eq`.
160 /// This is a hack to work around a limitation in our type system.
164 /// We want to require that types of consts used in pattern matches
165 /// have the attribute `#[derive(PartialEq, Eq)]`.
167 /// In a more ideal world, we could check that requirement by just checking that
168 /// the given type implements both the `StructuralPartialEq` trait *and*
169 /// the `Eq` trait. However, you can have ADTs that *do* `derive(PartialEq, Eq)`,
170 /// and be a case that we want the compiler to accept, and yet the constant's
171 /// type fails to implement `Eq`.
173 /// Namely, a case like this:
176 /// #[derive(PartialEq, Eq)]
177 /// struct Wrap<X>(X);
179 /// fn higher_order(_: &()) { }
181 /// const CFN: Wrap<fn(&())> = Wrap(higher_order);
191 /// (The problem in the above code is that `Wrap<fn(&())>` does not implement
192 /// `PartialEq`, nor `Eq`, because `for<'a> fn(&'a _)` does not implement those
195 /// Therefore, we cannot rely on naive check for `StructuralPartialEq` and
198 /// As a hack to work around this, we use two separate traits injected by each
199 /// of the two derives (`#[derive(PartialEq)]` and `#[derive(Eq)]`) and check
200 /// that both of them are present as part of structural-match checking.
201 #[unstable(feature = "structural_match", issue = "31434")]
202 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
203 #[lang = "structural_teq"]
204 pub trait StructuralEq {
208 /// Types whose values can be duplicated simply by copying bits.
210 /// By default, variable bindings have 'move semantics.' In other
221 /// // `x` has moved into `y`, and so cannot be used
223 /// // println!("{x:?}"); // error: use of moved value
226 /// However, if a type implements `Copy`, it instead has 'copy semantics':
229 /// // We can derive a `Copy` implementation. `Clone` is also required, as it's
230 /// // a supertrait of `Copy`.
231 /// #[derive(Debug, Copy, Clone)]
238 /// // `y` is a copy of `x`
240 /// println!("{x:?}"); // A-OK!
243 /// It's important to note that in these two examples, the only difference is whether you
244 /// are allowed to access `x` after the assignment. Under the hood, both a copy and a move
245 /// can result in bits being copied in memory, although this is sometimes optimized away.
247 /// ## How can I implement `Copy`?
249 /// There are two ways to implement `Copy` on your type. The simplest is to use `derive`:
252 /// #[derive(Copy, Clone)]
256 /// You can also implement `Copy` and `Clone` manually:
261 /// impl Copy for MyStruct { }
263 /// impl Clone for MyStruct {
264 /// fn clone(&self) -> MyStruct {
270 /// There is a small difference between the two: the `derive` strategy will also place a `Copy`
271 /// bound on type parameters, which isn't always desired.
273 /// ## What's the difference between `Copy` and `Clone`?
275 /// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of
276 /// `Copy` is not overloadable; it is always a simple bit-wise copy.
278 /// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`] can
279 /// provide any type-specific behavior necessary to duplicate values safely. For example,
280 /// the implementation of [`Clone`] for [`String`] needs to copy the pointed-to string
281 /// buffer in the heap. A simple bitwise copy of [`String`] values would merely copy the
282 /// pointer, leading to a double free down the line. For this reason, [`String`] is [`Clone`]
285 /// [`Clone`] is a supertrait of `Copy`, so everything which is `Copy` must also implement
286 /// [`Clone`]. If a type is `Copy` then its [`Clone`] implementation only needs to return `*self`
287 /// (see the example above).
289 /// ## When can my type be `Copy`?
291 /// A type can implement `Copy` if all of its components implement `Copy`. For example, this
292 /// struct can be `Copy`:
295 /// # #[allow(dead_code)]
296 /// #[derive(Copy, Clone)]
303 /// A struct can be `Copy`, and [`i32`] is `Copy`, therefore `Point` is eligible to be `Copy`.
304 /// By contrast, consider
307 /// # #![allow(dead_code)]
309 /// struct PointList {
310 /// points: Vec<Point>,
314 /// The struct `PointList` cannot implement `Copy`, because [`Vec<T>`] is not `Copy`. If we
315 /// attempt to derive a `Copy` implementation, we'll get an error:
318 /// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`
321 /// Shared references (`&T`) are also `Copy`, so a type can be `Copy`, even when it holds
322 /// shared references of types `T` that are *not* `Copy`. Consider the following struct,
323 /// which can implement `Copy`, because it only holds a *shared reference* to our non-`Copy`
324 /// type `PointList` from above:
327 /// # #![allow(dead_code)]
328 /// # struct PointList;
329 /// #[derive(Copy, Clone)]
330 /// struct PointListWrapper<'a> {
331 /// point_list_ref: &'a PointList,
335 /// ## When *can't* my type be `Copy`?
337 /// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
338 /// mutable reference. Copying [`String`] would duplicate responsibility for managing the
339 /// [`String`]'s buffer, leading to a double free.
341 /// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's
342 /// managing some resource besides its own [`size_of::<T>`] bytes.
344 /// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get
345 /// the error [E0204].
347 /// [E0204]: ../../error_codes/E0204.html
349 /// ## When *should* my type be `Copy`?
351 /// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though,
352 /// that implementing `Copy` is part of the public API of your type. If the type might become
353 /// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to
354 /// avoid a breaking API change.
356 /// ## Additional implementors
358 /// In addition to the [implementors listed below][impls],
359 /// the following types also implement `Copy`:
361 /// * Function item types (i.e., the distinct types defined for each function)
362 /// * Function pointer types (e.g., `fn() -> i32`)
363 /// * Closure types, if they capture no value from the environment
364 /// or if all such captured values implement `Copy` themselves.
365 /// Note that variables captured by shared reference always implement `Copy`
366 /// (even if the referent doesn't),
367 /// while variables captured by mutable reference never implement `Copy`.
369 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
370 /// [`String`]: ../../std/string/struct.String.html
371 /// [`size_of::<T>`]: crate::mem::size_of
372 /// [impls]: #implementors
373 #[stable(feature = "rust1", since = "1.0.0")]
375 // FIXME(matthewjasper) This allows copying a type that doesn't implement
376 // `Copy` because of unsatisfied lifetime bounds (copying `A<'_>` when only
377 // `A<'static>: Copy` and `A<'_>: Clone`).
378 // We have this attribute here for now only because there are quite a few
379 // existing specializations on `Copy` that already exist in the standard
380 // library, and there's no way to safely have this behavior right now.
381 #[rustc_unsafe_specialization_marker]
382 #[rustc_diagnostic_item = "Copy"]
383 pub trait Copy: Clone {
387 /// Derive macro generating an impl of the trait `Copy`.
388 #[rustc_builtin_macro]
389 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
390 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
391 pub macro Copy($item:item) {
392 /* compiler built-in */
395 /// Types for which it is safe to share references between threads.
397 /// This trait is automatically implemented when the compiler determines
398 /// it's appropriate.
400 /// The precise definition is: a type `T` is [`Sync`] if and only if `&T` is
401 /// [`Send`]. In other words, if there is no possibility of
402 /// [undefined behavior][ub] (including data races) when passing
403 /// `&T` references between threads.
405 /// As one would expect, primitive types like [`u8`] and [`f64`]
406 /// are all [`Sync`], and so are simple aggregate types containing them,
407 /// like tuples, structs and enums. More examples of basic [`Sync`]
408 /// types include "immutable" types like `&T`, and those with simple
409 /// inherited mutability, such as [`Box<T>`][box], [`Vec<T>`][vec] and
410 /// most other collection types. (Generic parameters need to be [`Sync`]
411 /// for their container to be [`Sync`].)
413 /// A somewhat surprising consequence of the definition is that `&mut T`
414 /// is `Sync` (if `T` is `Sync`) even though it seems like that might
415 /// provide unsynchronized mutation. The trick is that a mutable
416 /// reference behind a shared reference (that is, `& &mut T`)
417 /// becomes read-only, as if it were a `& &T`. Hence there is no risk
420 /// Types that are not `Sync` are those that have "interior
421 /// mutability" in a non-thread-safe form, such as [`Cell`][cell]
422 /// and [`RefCell`][refcell]. These types allow for mutation of
423 /// their contents even through an immutable, shared reference. For
424 /// example the `set` method on [`Cell<T>`][cell] takes `&self`, so it requires
425 /// only a shared reference [`&Cell<T>`][cell]. The method performs no
426 /// synchronization, thus [`Cell`][cell] cannot be `Sync`.
428 /// Another example of a non-`Sync` type is the reference-counting
429 /// pointer [`Rc`][rc]. Given any reference [`&Rc<T>`][rc], you can clone
430 /// a new [`Rc<T>`][rc], modifying the reference counts in a non-atomic way.
432 /// For cases when one does need thread-safe interior mutability,
433 /// Rust provides [atomic data types], as well as explicit locking via
434 /// [`sync::Mutex`][mutex] and [`sync::RwLock`][rwlock]. These types
435 /// ensure that any mutation cannot cause data races, hence the types
436 /// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe
437 /// analogue of [`Rc`][rc].
439 /// Any types with interior mutability must also use the
440 /// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which
441 /// can be mutated through a shared reference. Failing to doing this is
442 /// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing
443 /// from `&T` to `&mut T` is invalid.
445 /// See [the Nomicon][nomicon-send-and-sync] for more details about `Sync`.
447 /// [box]: ../../std/boxed/struct.Box.html
448 /// [vec]: ../../std/vec/struct.Vec.html
449 /// [cell]: crate::cell::Cell
450 /// [refcell]: crate::cell::RefCell
451 /// [rc]: ../../std/rc/struct.Rc.html
452 /// [arc]: ../../std/sync/struct.Arc.html
453 /// [atomic data types]: crate::sync::atomic
454 /// [mutex]: ../../std/sync/struct.Mutex.html
455 /// [rwlock]: ../../std/sync/struct.RwLock.html
456 /// [unsafecell]: crate::cell::UnsafeCell
457 /// [ub]: ../../reference/behavior-considered-undefined.html
458 /// [transmute]: crate::mem::transmute
459 /// [nomicon-send-and-sync]: ../../nomicon/send-and-sync.html
460 #[stable(feature = "rust1", since = "1.0.0")]
461 #[cfg_attr(not(test), rustc_diagnostic_item = "Sync")]
463 #[rustc_on_unimplemented(
464 message = "`{Self}` cannot be shared between threads safely",
465 label = "`{Self}` cannot be shared between threads safely"
467 pub unsafe auto trait Sync {
468 // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`
469 // lands in beta, and it has been extended to check whether a closure is
470 // anywhere in the requirement chain, extend it as such (#48534):
474 // note="`{Self}` cannot be shared safely, consider marking the closure `move`"
481 #[stable(feature = "rust1", since = "1.0.0")]
482 impl<T: ?Sized> !Sync for *const T {}
483 #[stable(feature = "rust1", since = "1.0.0")]
484 impl<T: ?Sized> !Sync for *mut T {}
488 #[stable(feature = "rust1", since = "1.0.0")]
489 impl<T: ?Sized> Hash for $t<T> {
491 fn hash<H: Hasher>(&self, _: &mut H) {}
494 #[stable(feature = "rust1", since = "1.0.0")]
495 impl<T: ?Sized> cmp::PartialEq for $t<T> {
496 fn eq(&self, _other: &$t<T>) -> bool {
501 #[stable(feature = "rust1", since = "1.0.0")]
502 impl<T: ?Sized> cmp::Eq for $t<T> {}
504 #[stable(feature = "rust1", since = "1.0.0")]
505 impl<T: ?Sized> cmp::PartialOrd for $t<T> {
506 fn partial_cmp(&self, _other: &$t<T>) -> Option<cmp::Ordering> {
507 Option::Some(cmp::Ordering::Equal)
511 #[stable(feature = "rust1", since = "1.0.0")]
512 impl<T: ?Sized> cmp::Ord for $t<T> {
513 fn cmp(&self, _other: &$t<T>) -> cmp::Ordering {
518 #[stable(feature = "rust1", since = "1.0.0")]
519 impl<T: ?Sized> Copy for $t<T> {}
521 #[stable(feature = "rust1", since = "1.0.0")]
522 impl<T: ?Sized> Clone for $t<T> {
523 fn clone(&self) -> Self {
528 #[stable(feature = "rust1", since = "1.0.0")]
529 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
530 impl<T: ?Sized> const Default for $t<T> {
531 fn default() -> Self {
536 #[unstable(feature = "structural_match", issue = "31434")]
537 impl<T: ?Sized> StructuralPartialEq for $t<T> {}
539 #[unstable(feature = "structural_match", issue = "31434")]
540 impl<T: ?Sized> StructuralEq for $t<T> {}
544 /// Zero-sized type used to mark things that "act like" they own a `T`.
546 /// Adding a `PhantomData<T>` field to your type tells the compiler that your
547 /// type acts as though it stores a value of type `T`, even though it doesn't
548 /// really. This information is used when computing certain safety properties.
550 /// For a more in-depth explanation of how to use `PhantomData<T>`, please see
551 /// [the Nomicon](../../nomicon/phantom-data.html).
553 /// # A ghastly note 👻👻👻
555 /// Though they both have scary names, `PhantomData` and 'phantom types' are
556 /// related, but not identical. A phantom type parameter is simply a type
557 /// parameter which is never used. In Rust, this often causes the compiler to
558 /// complain, and the solution is to add a "dummy" use by way of `PhantomData`.
562 /// ## Unused lifetime parameters
564 /// Perhaps the most common use case for `PhantomData` is a struct that has an
565 /// unused lifetime parameter, typically as part of some unsafe code. For
566 /// example, here is a struct `Slice` that has two pointers of type `*const T`,
567 /// presumably pointing into an array somewhere:
569 /// ```compile_fail,E0392
570 /// struct Slice<'a, T> {
576 /// The intention is that the underlying data is only valid for the
577 /// lifetime `'a`, so `Slice` should not outlive `'a`. However, this
578 /// intent is not expressed in the code, since there are no uses of
579 /// the lifetime `'a` and hence it is not clear what data it applies
580 /// to. We can correct this by telling the compiler to act *as if* the
581 /// `Slice` struct contained a reference `&'a T`:
584 /// use std::marker::PhantomData;
586 /// # #[allow(dead_code)]
587 /// struct Slice<'a, T: 'a> {
590 /// phantom: PhantomData<&'a T>,
594 /// This also in turn requires the annotation `T: 'a`, indicating
595 /// that any references in `T` are valid over the lifetime `'a`.
597 /// When initializing a `Slice` you simply provide the value
598 /// `PhantomData` for the field `phantom`:
601 /// # #![allow(dead_code)]
602 /// # use std::marker::PhantomData;
603 /// # struct Slice<'a, T: 'a> {
604 /// # start: *const T,
606 /// # phantom: PhantomData<&'a T>,
608 /// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
609 /// let ptr = vec.as_ptr();
612 /// end: unsafe { ptr.add(vec.len()) },
613 /// phantom: PhantomData,
618 /// ## Unused type parameters
620 /// It sometimes happens that you have unused type parameters which
621 /// indicate what type of data a struct is "tied" to, even though that
622 /// data is not actually found in the struct itself. Here is an
623 /// example where this arises with [FFI]. The foreign interface uses
624 /// handles of type `*mut ()` to refer to Rust values of different
625 /// types. We track the Rust type using a phantom type parameter on
626 /// the struct `ExternalResource` which wraps a handle.
628 /// [FFI]: ../../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
631 /// # #![allow(dead_code)]
632 /// # trait ResType { }
633 /// # struct ParamType;
634 /// # mod foreign_lib {
635 /// # pub fn new(_: usize) -> *mut () { 42 as *mut () }
636 /// # pub fn do_stuff(_: *mut (), _: usize) {}
638 /// # fn convert_params(_: ParamType) -> usize { 42 }
639 /// use std::marker::PhantomData;
642 /// struct ExternalResource<R> {
643 /// resource_handle: *mut (),
644 /// resource_type: PhantomData<R>,
647 /// impl<R: ResType> ExternalResource<R> {
648 /// fn new() -> Self {
649 /// let size_of_res = mem::size_of::<R>();
651 /// resource_handle: foreign_lib::new(size_of_res),
652 /// resource_type: PhantomData,
656 /// fn do_stuff(&self, param: ParamType) {
657 /// let foreign_params = convert_params(param);
658 /// foreign_lib::do_stuff(self.resource_handle, foreign_params);
663 /// ## Ownership and the drop check
665 /// Adding a field of type `PhantomData<T>` indicates that your
666 /// type owns data of type `T`. This in turn implies that when your
667 /// type is dropped, it may drop one or more instances of the type
668 /// `T`. This has bearing on the Rust compiler's [drop check]
671 /// If your struct does not in fact *own* the data of type `T`, it is
672 /// better to use a reference type, like `PhantomData<&'a T>`
673 /// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so
674 /// as not to indicate ownership.
676 /// [drop check]: ../../nomicon/dropck.html
677 #[lang = "phantom_data"]
678 #[stable(feature = "rust1", since = "1.0.0")]
679 pub struct PhantomData<T: ?Sized>;
681 impls! { PhantomData }
684 #[stable(feature = "rust1", since = "1.0.0")]
685 unsafe impl<T: Sync + ?Sized> Send for &T {}
686 #[stable(feature = "rust1", since = "1.0.0")]
687 unsafe impl<T: Send + ?Sized> Send for &mut T {}
690 /// Compiler-internal trait used to indicate the type of enum discriminants.
692 /// This trait is automatically implemented for every type and does not add any
693 /// guarantees to [`mem::Discriminant`]. It is **undefined behavior** to transmute
694 /// between `DiscriminantKind::Discriminant` and `mem::Discriminant`.
696 /// [`mem::Discriminant`]: crate::mem::Discriminant
698 feature = "discriminant_kind",
700 reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
702 #[lang = "discriminant_kind"]
703 pub trait DiscriminantKind {
704 /// The type of the discriminant, which must satisfy the trait
705 /// bounds required by `mem::Discriminant`.
706 #[lang = "discriminant_type"]
707 type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
710 /// Compiler-internal trait used to determine whether a type contains
711 /// any `UnsafeCell` internally, but not through an indirection.
712 /// This affects, for example, whether a `static` of that type is
713 /// placed in read-only static memory or writable static memory.
715 pub(crate) unsafe auto trait Freeze {}
717 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
718 unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
719 unsafe impl<T: ?Sized> Freeze for *const T {}
720 unsafe impl<T: ?Sized> Freeze for *mut T {}
721 unsafe impl<T: ?Sized> Freeze for &T {}
722 unsafe impl<T: ?Sized> Freeze for &mut T {}
724 /// Types that can be safely moved after being pinned.
726 /// Rust itself has no notion of immovable types, and considers moves (e.g.,
727 /// through assignment or [`mem::replace`]) to always be safe.
729 /// The [`Pin`][Pin] type is used instead to prevent moves through the type
730 /// system. Pointers `P<T>` wrapped in the [`Pin<P<T>>`][Pin] wrapper can't be
731 /// moved out of. See the [`pin` module] documentation for more information on
734 /// Implementing the `Unpin` trait for `T` lifts the restrictions of pinning off
735 /// the type, which then allows moving `T` out of [`Pin<P<T>>`][Pin] with
736 /// functions such as [`mem::replace`].
738 /// `Unpin` has no consequence at all for non-pinned data. In particular,
739 /// [`mem::replace`] happily moves `!Unpin` data (it works for any `&mut T`, not
740 /// just when `T: Unpin`). However, you cannot use [`mem::replace`] on data
741 /// wrapped inside a [`Pin<P<T>>`][Pin] because you cannot get the `&mut T` you
742 /// need for that, and *that* is what makes this system work.
744 /// So this, for example, can only be done on types implementing `Unpin`:
747 /// # #![allow(unused_must_use)]
749 /// use std::pin::Pin;
751 /// let mut string = "this".to_string();
752 /// let mut pinned_string = Pin::new(&mut string);
754 /// // We need a mutable reference to call `mem::replace`.
755 /// // We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
756 /// // but that is only possible because `String` implements `Unpin`.
757 /// mem::replace(&mut *pinned_string, "other".to_string());
760 /// This trait is automatically implemented for almost every type.
762 /// [`mem::replace`]: crate::mem::replace
763 /// [Pin]: crate::pin::Pin
764 /// [`pin` module]: crate::pin
765 #[stable(feature = "pin", since = "1.33.0")]
766 #[rustc_on_unimplemented(
767 note = "consider using `Box::pin`",
768 message = "`{Self}` cannot be unpinned"
771 pub auto trait Unpin {}
773 /// A marker type which does not implement `Unpin`.
775 /// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
776 #[stable(feature = "pin", since = "1.33.0")]
777 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
778 pub struct PhantomPinned;
780 #[stable(feature = "pin", since = "1.33.0")]
781 impl !Unpin for PhantomPinned {}
783 #[stable(feature = "pin", since = "1.33.0")]
784 impl<'a, T: ?Sized + 'a> Unpin for &'a T {}
786 #[stable(feature = "pin", since = "1.33.0")]
787 impl<'a, T: ?Sized + 'a> Unpin for &'a mut T {}
789 #[stable(feature = "pin_raw", since = "1.38.0")]
790 impl<T: ?Sized> Unpin for *const T {}
792 #[stable(feature = "pin_raw", since = "1.38.0")]
793 impl<T: ?Sized> Unpin for *mut T {}
795 /// A marker for types that can be dropped.
797 /// This should be used for `~const` bounds,
798 /// as non-const bounds will always hold for every type.
799 #[unstable(feature = "const_trait_impl", issue = "67792")]
801 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
802 pub trait Destruct {}
804 /// A marker for tuple types.
806 /// The implementation of this trait is built-in and cannot be implemented
807 /// for any user type.
808 #[unstable(feature = "tuple_trait", issue = "none")]
809 #[lang = "tuple_trait"]
810 #[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
813 /// Implementations of `Copy` for primitive types.
815 /// Implementations that cannot be described in Rust
816 /// are implemented in `traits::SelectionContext::copy_clone_conditions()`
817 /// in `rustc_trait_selection`.
822 macro_rules! impl_copy {
825 #[stable(feature = "rust1", since = "1.0.0")]
832 usize u8 u16 u32 u64 u128
833 isize i8 i16 i32 i64 i128
838 #[unstable(feature = "never_type", issue = "35121")]
841 #[stable(feature = "rust1", since = "1.0.0")]
842 impl<T: ?Sized> Copy for *const T {}
844 #[stable(feature = "rust1", since = "1.0.0")]
845 impl<T: ?Sized> Copy for *mut T {}
847 /// Shared references can be copied, but mutable references *cannot*!
848 #[stable(feature = "rust1", since = "1.0.0")]
849 impl<T: ?Sized> Copy for &T {}