]>
git.proxmox.com Git - rustc.git/blob - library/core/src/marker.rs
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 #[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 #[stable(feature = "rust1", since = "1.0.0")]
86 #[rustc_on_unimplemented(
87 message
= "the size for values of type `{Self}` cannot be known at compilation time",
88 label
= "doesn't have a size known at compile-time"
90 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
91 #[rustc_specialization_trait]
96 /// Types that can be "unsized" to a dynamically-sized type.
98 /// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and
99 /// `Unsize<dyn fmt::Debug>`.
101 /// All implementations of `Unsize` are provided automatically by the compiler.
103 /// `Unsize` is implemented for:
105 /// - `[T; N]` is `Unsize<[T]>`
106 /// - `T` is `Unsize<dyn Trait>` when `T: Trait`
107 /// - `Foo<..., T, ...>` is `Unsize<Foo<..., U, ...>>` if:
109 /// - Foo is a struct
110 /// - Only the last field of `Foo` has a type involving `T`
111 /// - `T` is not part of the type of any other fields
112 /// - `Bar<T>: Unsize<Bar<U>>`, if the last field of `Foo` has type `Bar<T>`
114 /// `Unsize` is used along with [`ops::CoerceUnsized`] to allow
115 /// "user-defined" containers such as [`Rc`] to contain dynamically-sized
116 /// types. See the [DST coercion RFC][RFC982] and [the nomicon entry on coercion][nomicon-coerce]
117 /// for more details.
119 /// [`ops::CoerceUnsized`]: crate::ops::CoerceUnsized
120 /// [`Rc`]: ../../std/rc/struct.Rc.html
121 /// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
122 /// [nomicon-coerce]: ../../nomicon/coercions.html
123 #[unstable(feature = "unsize", issue = "27732")]
125 pub trait Unsize
<T
: ?Sized
> {
129 /// Required trait for constants used in pattern matches.
131 /// Any type that derives `PartialEq` automatically implements this trait,
132 /// *regardless* of whether its type-parameters implement `Eq`.
134 /// If a `const` item contains some type that does not implement this trait,
135 /// then that type either (1.) does not implement `PartialEq` (which means the
136 /// constant will not provide that comparison method, which code generation
137 /// assumes is available), or (2.) it implements *its own* version of
138 /// `PartialEq` (which we assume does not conform to a structural-equality
141 /// In either of the two scenarios above, we reject usage of such a constant in
144 /// See also the [structural match RFC][RFC1445], and [issue 63438] which
145 /// motivated migrating from attribute-based design to this trait.
147 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
148 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
149 #[unstable(feature = "structural_match", issue = "31434")]
150 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
151 #[lang = "structural_peq"]
152 pub trait StructuralPartialEq
{
156 /// Required trait for constants used in pattern matches.
158 /// Any type that derives `Eq` automatically implements this trait, *regardless*
159 /// of whether its type-parameters implement `Eq`.
161 /// This is a hack to workaround a limitation in our type-system.
165 /// We want to require that types of consts used in pattern matches
166 /// have the attribute `#[derive(PartialEq, Eq)]`.
168 /// In a more ideal world, we could check that requirement by just checking that
169 /// the given type implements both (1.) the `StructuralPartialEq` trait *and*
170 /// (2.) the `Eq` trait. However, you can have ADTs that *do* `derive(PartialEq, Eq)`,
171 /// and be a case that we want the compiler to accept, and yet the constant's
172 /// type fails to implement `Eq`.
174 /// Namely, a case like this:
177 /// #[derive(PartialEq, Eq)]
178 /// struct Wrap<X>(X);
179 /// fn higher_order(_: &()) { }
180 /// const CFN: Wrap<fn(&())> = Wrap(higher_order);
189 /// (The problem in the above code is that `Wrap<fn(&())>` does not implement
190 /// `PartialEq`, nor `Eq`, because `for<'a> fn(&'a _)` does not implement those
193 /// Therefore, we cannot rely on naive check for `StructuralPartialEq` and
196 /// As a hack to work around this, we use two separate traits injected by each
197 /// of the two derives (`#[derive(PartialEq)]` and `#[derive(Eq)]`) and check
198 /// that both of them are present as part of structural-match checking.
199 #[unstable(feature = "structural_match", issue = "31434")]
200 #[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
201 #[lang = "structural_teq"]
202 pub trait StructuralEq
{
206 /// Types whose values can be duplicated simply by copying bits.
208 /// By default, variable bindings have 'move semantics.' In other
219 /// // `x` has moved into `y`, and so cannot be used
221 /// // println!("{:?}", x); // error: use of moved value
224 /// However, if a type implements `Copy`, it instead has 'copy semantics':
227 /// // We can derive a `Copy` implementation. `Clone` is also required, as it's
228 /// // a supertrait of `Copy`.
229 /// #[derive(Debug, Copy, Clone)]
236 /// // `y` is a copy of `x`
238 /// println!("{:?}", x); // A-OK!
241 /// It's important to note that in these two examples, the only difference is whether you
242 /// are allowed to access `x` after the assignment. Under the hood, both a copy and a move
243 /// can result in bits being copied in memory, although this is sometimes optimized away.
245 /// ## How can I implement `Copy`?
247 /// There are two ways to implement `Copy` on your type. The simplest is to use `derive`:
250 /// #[derive(Copy, Clone)]
254 /// You can also implement `Copy` and `Clone` manually:
259 /// impl Copy for MyStruct { }
261 /// impl Clone for MyStruct {
262 /// fn clone(&self) -> MyStruct {
268 /// There is a small difference between the two: the `derive` strategy will also place a `Copy`
269 /// bound on type parameters, which isn't always desired.
271 /// ## What's the difference between `Copy` and `Clone`?
273 /// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of
274 /// `Copy` is not overloadable; it is always a simple bit-wise copy.
276 /// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`] can
277 /// provide any type-specific behavior necessary to duplicate values safely. For example,
278 /// the implementation of [`Clone`] for [`String`] needs to copy the pointed-to string
279 /// buffer in the heap. A simple bitwise copy of [`String`] values would merely copy the
280 /// pointer, leading to a double free down the line. For this reason, [`String`] is [`Clone`]
283 /// [`Clone`] is a supertrait of `Copy`, so everything which is `Copy` must also implement
284 /// [`Clone`]. If a type is `Copy` then its [`Clone`] implementation only needs to return `*self`
285 /// (see the example above).
287 /// ## When can my type be `Copy`?
289 /// A type can implement `Copy` if all of its components implement `Copy`. For example, this
290 /// struct can be `Copy`:
293 /// # #[allow(dead_code)]
294 /// #[derive(Copy, Clone)]
301 /// A struct can be `Copy`, and [`i32`] is `Copy`, therefore `Point` is eligible to be `Copy`.
302 /// By contrast, consider
305 /// # #![allow(dead_code)]
307 /// struct PointList {
308 /// points: Vec<Point>,
312 /// The struct `PointList` cannot implement `Copy`, because [`Vec<T>`] is not `Copy`. If we
313 /// attempt to derive a `Copy` implementation, we'll get an error:
316 /// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`
319 /// Shared references (`&T`) are also `Copy`, so a type can be `Copy`, even when it holds
320 /// shared references of types `T` that are *not* `Copy`. Consider the following struct,
321 /// which can implement `Copy`, because it only holds a *shared reference* to our non-`Copy`
322 /// type `PointList` from above:
325 /// # #![allow(dead_code)]
326 /// # struct PointList;
327 /// #[derive(Copy, Clone)]
328 /// struct PointListWrapper<'a> {
329 /// point_list_ref: &'a PointList,
333 /// ## When *can't* my type be `Copy`?
335 /// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
336 /// mutable reference. Copying [`String`] would duplicate responsibility for managing the
337 /// [`String`]'s buffer, leading to a double free.
339 /// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's
340 /// managing some resource besides its own [`size_of::<T>`] bytes.
342 /// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get
343 /// the error [E0204].
345 /// [E0204]: ../../error-index.html#E0204
347 /// ## When *should* my type be `Copy`?
349 /// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though,
350 /// that implementing `Copy` is part of the public API of your type. If the type might become
351 /// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to
352 /// avoid a breaking API change.
354 /// ## Additional implementors
356 /// In addition to the [implementors listed below][impls],
357 /// the following types also implement `Copy`:
359 /// * Function item types (i.e., the distinct types defined for each function)
360 /// * Function pointer types (e.g., `fn() -> i32`)
361 /// * Array types, for all sizes, if the item type also implements `Copy` (e.g., `[i32; 123456]`)
362 /// * Tuple types, if each component also implements `Copy` (e.g., `()`, `(i32, bool)`)
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 pub trait Copy
: Clone
{
386 /// Derive macro generating an impl of the trait `Copy`.
387 #[rustc_builtin_macro]
388 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
389 #[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
390 pub macro Copy($item
:item
) {
391 /* compiler built-in */
394 /// Types for which it is safe to share references between threads.
396 /// This trait is automatically implemented when the compiler determines
397 /// it's appropriate.
399 /// The precise definition is: a type `T` is [`Sync`] if and only if `&T` is
400 /// [`Send`]. In other words, if there is no possibility of
401 /// [undefined behavior][ub] (including data races) when passing
402 /// `&T` references between threads.
404 /// As one would expect, primitive types like [`u8`] and [`f64`]
405 /// are all [`Sync`], and so are simple aggregate types containing them,
406 /// like tuples, structs and enums. More examples of basic [`Sync`]
407 /// types include "immutable" types like `&T`, and those with simple
408 /// inherited mutability, such as [`Box<T>`][box], [`Vec<T>`][vec] and
409 /// most other collection types. (Generic parameters need to be [`Sync`]
410 /// for their container to be [`Sync`].)
412 /// A somewhat surprising consequence of the definition is that `&mut T`
413 /// is `Sync` (if `T` is `Sync`) even though it seems like that might
414 /// provide unsynchronized mutation. The trick is that a mutable
415 /// reference behind a shared reference (that is, `& &mut T`)
416 /// becomes read-only, as if it were a `& &T`. Hence there is no risk
419 /// Types that are not `Sync` are those that have "interior
420 /// mutability" in a non-thread-safe form, such as [`Cell`][cell]
421 /// and [`RefCell`][refcell]. These types allow for mutation of
422 /// their contents even through an immutable, shared reference. For
423 /// example the `set` method on [`Cell<T>`][cell] takes `&self`, so it requires
424 /// only a shared reference [`&Cell<T>`][cell]. The method performs no
425 /// synchronization, thus [`Cell`][cell] cannot be `Sync`.
427 /// Another example of a non-`Sync` type is the reference-counting
428 /// pointer [`Rc`][rc]. Given any reference [`&Rc<T>`][rc], you can clone
429 /// a new [`Rc<T>`][rc], modifying the reference counts in a non-atomic way.
431 /// For cases when one does need thread-safe interior mutability,
432 /// Rust provides [atomic data types], as well as explicit locking via
433 /// [`sync::Mutex`][mutex] and [`sync::RwLock`][rwlock]. These types
434 /// ensure that any mutation cannot cause data races, hence the types
435 /// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe
436 /// analogue of [`Rc`][rc].
438 /// Any types with interior mutability must also use the
439 /// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which
440 /// can be mutated through a shared reference. Failing to doing this is
441 /// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing
442 /// from `&T` to `&mut T` is invalid.
444 /// See [the Nomicon][nomicon-send-and-sync] for more details about `Sync`.
446 /// [box]: ../../std/boxed/struct.Box.html
447 /// [vec]: ../../std/vec/struct.Vec.html
448 /// [cell]: crate::cell::Cell
449 /// [refcell]: crate::cell::RefCell
450 /// [rc]: ../../std/rc/struct.Rc.html
451 /// [arc]: ../../std/sync/struct.Arc.html
452 /// [atomic data types]: crate::sync::atomic
453 /// [mutex]: ../../std/sync/struct.Mutex.html
454 /// [rwlock]: ../../std/sync/struct.RwLock.html
455 /// [unsafecell]: crate::cell::UnsafeCell
456 /// [ub]: ../../reference/behavior-considered-undefined.html
457 /// [transmute]: crate::mem::transmute
458 /// [nomicon-send-and-sync]: ../../nomicon/send-and-sync.html
459 #[stable(feature = "rust1", since = "1.0.0")]
460 #[cfg_attr(not(test), rustc_diagnostic_item = "sync_trait")]
462 #[rustc_on_unimplemented(
463 message
= "`{Self}` cannot be shared between threads safely",
464 label
= "`{Self}` cannot be shared between threads safely"
466 pub unsafe auto trait Sync
{
467 // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`
468 // lands in beta, and it has been extended to check whether a closure is
469 // anywhere in the requirement chain, extend it as such (#48534):
473 // note="`{Self}` cannot be shared safely, consider marking the closure `move`"
480 #[stable(feature = "rust1", since = "1.0.0")]
481 impl<T
: ?Sized
> !Sync
for *const T {}
482 #[stable(feature = "rust1", since = "1.0.0")]
483 impl<T
: ?Sized
> !Sync
for *mut T {}
487 #[stable(feature = "rust1", since = "1.0.0")]
488 impl<T
: ?Sized
> Hash
for $t
<T
> {
490 fn hash
<H
: Hasher
>(&self, _
: &mut H
) {}
493 #[stable(feature = "rust1", since = "1.0.0")]
494 impl<T
: ?Sized
> cmp
::PartialEq
for $t
<T
> {
495 fn eq(&self, _other
: &$t
<T
>) -> bool
{
500 #[stable(feature = "rust1", since = "1.0.0")]
501 impl<T
: ?Sized
> cmp
::Eq
for $t
<T
> {}
503 #[stable(feature = "rust1", since = "1.0.0")]
504 impl<T
: ?Sized
> cmp
::PartialOrd
for $t
<T
> {
505 fn partial_cmp(&self, _other
: &$t
<T
>) -> Option
<cmp
::Ordering
> {
506 Option
::Some(cmp
::Ordering
::Equal
)
510 #[stable(feature = "rust1", since = "1.0.0")]
511 impl<T
: ?Sized
> cmp
::Ord
for $t
<T
> {
512 fn cmp(&self, _other
: &$t
<T
>) -> cmp
::Ordering
{
517 #[stable(feature = "rust1", since = "1.0.0")]
518 impl<T
: ?Sized
> Copy
for $t
<T
> {}
520 #[stable(feature = "rust1", since = "1.0.0")]
521 impl<T
: ?Sized
> Clone
for $t
<T
> {
522 fn clone(&self) -> Self {
527 #[stable(feature = "rust1", since = "1.0.0")]
528 impl<T
: ?Sized
> Default
for $t
<T
> {
529 fn default() -> Self {
534 #[unstable(feature = "structural_match", issue = "31434")]
535 impl<T
: ?Sized
> StructuralPartialEq
for $t
<T
> {}
537 #[unstable(feature = "structural_match", issue = "31434")]
538 impl<T
: ?Sized
> StructuralEq
for $t
<T
> {}
542 /// Zero-sized type used to mark things that "act like" they own a `T`.
544 /// Adding a `PhantomData<T>` field to your type tells the compiler that your
545 /// type acts as though it stores a value of type `T`, even though it doesn't
546 /// really. This information is used when computing certain safety properties.
548 /// For a more in-depth explanation of how to use `PhantomData<T>`, please see
549 /// [the Nomicon](../../nomicon/phantom-data.html).
551 /// # A ghastly note 👻👻👻
553 /// Though they both have scary names, `PhantomData` and 'phantom types' are
554 /// related, but not identical. A phantom type parameter is simply a type
555 /// parameter which is never used. In Rust, this often causes the compiler to
556 /// complain, and the solution is to add a "dummy" use by way of `PhantomData`.
560 /// ## Unused lifetime parameters
562 /// Perhaps the most common use case for `PhantomData` is a struct that has an
563 /// unused lifetime parameter, typically as part of some unsafe code. For
564 /// example, here is a struct `Slice` that has two pointers of type `*const T`,
565 /// presumably pointing into an array somewhere:
567 /// ```compile_fail,E0392
568 /// struct Slice<'a, T> {
574 /// The intention is that the underlying data is only valid for the
575 /// lifetime `'a`, so `Slice` should not outlive `'a`. However, this
576 /// intent is not expressed in the code, since there are no uses of
577 /// the lifetime `'a` and hence it is not clear what data it applies
578 /// to. We can correct this by telling the compiler to act *as if* the
579 /// `Slice` struct contained a reference `&'a T`:
582 /// use std::marker::PhantomData;
584 /// # #[allow(dead_code)]
585 /// struct Slice<'a, T: 'a> {
588 /// phantom: PhantomData<&'a T>,
592 /// This also in turn requires the annotation `T: 'a`, indicating
593 /// that any references in `T` are valid over the lifetime `'a`.
595 /// When initializing a `Slice` you simply provide the value
596 /// `PhantomData` for the field `phantom`:
599 /// # #![allow(dead_code)]
600 /// # use std::marker::PhantomData;
601 /// # struct Slice<'a, T: 'a> {
602 /// # start: *const T,
604 /// # phantom: PhantomData<&'a T>,
606 /// fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
607 /// let ptr = vec.as_ptr();
610 /// end: unsafe { ptr.add(vec.len()) },
611 /// phantom: PhantomData,
616 /// ## Unused type parameters
618 /// It sometimes happens that you have unused type parameters which
619 /// indicate what type of data a struct is "tied" to, even though that
620 /// data is not actually found in the struct itself. Here is an
621 /// example where this arises with [FFI]. The foreign interface uses
622 /// handles of type `*mut ()` to refer to Rust values of different
623 /// types. We track the Rust type using a phantom type parameter on
624 /// the struct `ExternalResource` which wraps a handle.
626 /// [FFI]: ../../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
629 /// # #![allow(dead_code)]
630 /// # trait ResType { }
631 /// # struct ParamType;
632 /// # mod foreign_lib {
633 /// # pub fn new(_: usize) -> *mut () { 42 as *mut () }
634 /// # pub fn do_stuff(_: *mut (), _: usize) {}
636 /// # fn convert_params(_: ParamType) -> usize { 42 }
637 /// use std::marker::PhantomData;
640 /// struct ExternalResource<R> {
641 /// resource_handle: *mut (),
642 /// resource_type: PhantomData<R>,
645 /// impl<R: ResType> ExternalResource<R> {
646 /// fn new() -> Self {
647 /// let size_of_res = mem::size_of::<R>();
649 /// resource_handle: foreign_lib::new(size_of_res),
650 /// resource_type: PhantomData,
654 /// fn do_stuff(&self, param: ParamType) {
655 /// let foreign_params = convert_params(param);
656 /// foreign_lib::do_stuff(self.resource_handle, foreign_params);
661 /// ## Ownership and the drop check
663 /// Adding a field of type `PhantomData<T>` indicates that your
664 /// type owns data of type `T`. This in turn implies that when your
665 /// type is dropped, it may drop one or more instances of the type
666 /// `T`. This has bearing on the Rust compiler's [drop check]
669 /// If your struct does not in fact *own* the data of type `T`, it is
670 /// better to use a reference type, like `PhantomData<&'a T>`
671 /// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so
672 /// as not to indicate ownership.
674 /// [drop check]: ../../nomicon/dropck.html
675 #[lang = "phantom_data"]
676 #[stable(feature = "rust1", since = "1.0.0")]
677 pub struct PhantomData
<T
: ?Sized
>;
679 impls
! { PhantomData }
682 #[stable(feature = "rust1", since = "1.0.0")]
683 unsafe impl<T
: Sync
+ ?Sized
> Send
for &T {}
684 #[stable(feature = "rust1", since = "1.0.0")]
685 unsafe impl<T
: Send
+ ?Sized
> Send
for &mut T {}
688 /// Compiler-internal trait used to indicate the type of enum discriminants.
690 /// This trait is automatically implemented for every type and does not add any
691 /// guarantees to [`mem::Discriminant`]. It is **undefined behavior** to transmute
692 /// between `DiscriminantKind::Discriminant` and `mem::Discriminant`.
694 /// [`mem::Discriminant`]: crate::mem::Discriminant
696 feature
= "discriminant_kind",
698 reason
= "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
700 #[lang = "discriminant_kind"]
701 pub trait DiscriminantKind
{
702 /// The type of the discriminant, which must satisfy the trait
703 /// bounds required by `mem::Discriminant`.
704 #[lang = "discriminant_type"]
705 type Discriminant
: Clone
+ Copy
+ Debug
+ Eq
+ PartialEq
+ Hash
+ Send
+ Sync
+ Unpin
;
708 /// Compiler-internal trait used to determine whether a type contains
709 /// any `UnsafeCell` internally, but not through an indirection.
710 /// This affects, for example, whether a `static` of that type is
711 /// placed in read-only static memory or writable static memory.
713 pub(crate) unsafe auto trait Freeze {}
715 impl<T
: ?Sized
> !Freeze
for UnsafeCell
<T
> {}
716 unsafe impl<T
: ?Sized
> Freeze
for PhantomData
<T
> {}
717 unsafe impl<T
: ?Sized
> Freeze
for *const T {}
718 unsafe impl<T
: ?Sized
> Freeze
for *mut T {}
719 unsafe impl<T
: ?Sized
> Freeze
for &T {}
720 unsafe impl<T
: ?Sized
> Freeze
for &mut T {}
722 /// Types that can be safely moved after being pinned.
724 /// Rust itself has no notion of immovable types, and considers moves (e.g.,
725 /// through assignment or [`mem::replace`]) to always be safe.
727 /// The [`Pin`][Pin] type is used instead to prevent moves through the type
728 /// system. Pointers `P<T>` wrapped in the [`Pin<P<T>>`][Pin] wrapper can't be
729 /// moved out of. See the [`pin` module] documentation for more information on
732 /// Implementing the `Unpin` trait for `T` lifts the restrictions of pinning off
733 /// the type, which then allows moving `T` out of [`Pin<P<T>>`][Pin] with
734 /// functions such as [`mem::replace`].
736 /// `Unpin` has no consequence at all for non-pinned data. In particular,
737 /// [`mem::replace`] happily moves `!Unpin` data (it works for any `&mut T`, not
738 /// just when `T: Unpin`). However, you cannot use [`mem::replace`] on data
739 /// wrapped inside a [`Pin<P<T>>`][Pin] because you cannot get the `&mut T` you
740 /// need for that, and *that* is what makes this system work.
742 /// So this, for example, can only be done on types implementing `Unpin`:
745 /// # #![allow(unused_must_use)]
747 /// use std::pin::Pin;
749 /// let mut string = "this".to_string();
750 /// let mut pinned_string = Pin::new(&mut string);
752 /// // We need a mutable reference to call `mem::replace`.
753 /// // We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
754 /// // but that is only possible because `String` implements `Unpin`.
755 /// mem::replace(&mut *pinned_string, "other".to_string());
758 /// This trait is automatically implemented for almost every type.
760 /// [`mem::replace`]: crate::mem::replace
761 /// [Pin]: crate::pin::Pin
762 /// [`pin` module]: crate::pin
763 #[stable(feature = "pin", since = "1.33.0")]
764 #[rustc_on_unimplemented(
765 on(_Self
= "std::future::Future", note
= "consider using `Box::pin`",),
766 message
= "`{Self}` cannot be unpinned"
769 pub auto trait Unpin {}
771 /// A marker type which does not implement `Unpin`.
773 /// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
774 #[stable(feature = "pin", since = "1.33.0")]
775 #[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
776 pub struct PhantomPinned
;
778 #[stable(feature = "pin", since = "1.33.0")]
779 impl !Unpin
for PhantomPinned {}
781 #[stable(feature = "pin", since = "1.33.0")]
782 impl<'a
, T
: ?Sized
+ 'a
> Unpin
for &'a T {}
784 #[stable(feature = "pin", since = "1.33.0")]
785 impl<'a
, T
: ?Sized
+ 'a
> Unpin
for &'a
mut T {}
787 #[stable(feature = "pin_raw", since = "1.38.0")]
788 impl<T
: ?Sized
> Unpin
for *const T {}
790 #[stable(feature = "pin_raw", since = "1.38.0")]
791 impl<T
: ?Sized
> Unpin
for *mut T {}
793 /// Implementations of `Copy` for primitive types.
795 /// Implementations that cannot be described in Rust
796 /// are implemented in `traits::SelectionContext::copy_clone_conditions()`
797 /// in `rustc_trait_selection`.
802 macro_rules
! impl_copy
{
805 #[stable(feature = "rust1", since = "1.0.0")]
812 usize u8 u16 u32 u64 u128
813 isize i8 i16 i32 i64 i128
818 #[unstable(feature = "never_type", issue = "35121")]
821 #[stable(feature = "rust1", since = "1.0.0")]
822 impl<T
: ?Sized
> Copy
for *const T {}
824 #[stable(feature = "rust1", since = "1.0.0")]
825 impl<T
: ?Sized
> Copy
for *mut T {}
827 /// Shared references can be copied, but mutable references *cannot*!
828 #[stable(feature = "rust1", since = "1.0.0")]
829 impl<T
: ?Sized
> Copy
for &T {}