1 //! Types which pin data to its location in memory
3 //! It is sometimes useful to have objects that are guaranteed to not move,
4 //! in the sense that their placement in memory does not change, and can thus be relied upon.
5 //! A prime example of such a scenario would be building self-referential structs,
6 //! since moving an object with pointers to itself will invalidate them,
7 //! which could cause undefined behavior.
9 //! [`Pin`] ensures that the pointee of any pointer type has a stable location in memory,
10 //! meaning it cannot be moved elsewhere and its memory cannot be deallocated
11 //! until it gets dropped. We say that the pointee is "pinned".
13 //! By default, all types in Rust are movable. Rust allows passing all types by-value,
14 //! and common smart-pointer types such as `Box` and `&mut` allow replacing and
15 //! moving the values they contain: you can move out of a `Box`, or you can use [`mem::swap`].
16 //! [`Pin`] wraps a pointer type, so `Pin<Box<T>>` functions much like a regular `Box<T>`
17 //! (when a `Pin<Box<T>>` gets dropped, so do its contents, and the memory gets deallocated).
18 //! Similarily, `Pin<&mut T>` is a lot like `&mut T`. However, [`Pin`] does not let clients actually
19 //! obtain a `Box` or reference to pinned data, which implies that you cannot use
20 //! operations such as [`mem::swap`]:
22 //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
23 //! // `mem::swap` needs `&mut T`, but we cannot get it.
24 //! // We are stuck, we cannot swap the contents of these references.
25 //! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
26 //! // we are not allowed to use it for moving things out of the `Pin`.
30 //! It is worth reiterating that [`Pin`] does *not* change the fact that a Rust compiler
31 //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, `Pin`
32 //! prevents certain *values* (pointed to by pointers wrapped in `Pin`) from being
33 //! moved by making it impossible to call methods like [`mem::swap`] on them.
37 //! However, these restrictions are usually not necessary. Many types are always freely
38 //! movable, even when pinned, because they do not rely on having a stable address.
39 //! This includes all the basic types (`bool`, `i32` and friends, references)
40 //! as well as types consisting solely of these types.
41 //! Types that do not care about pinning implement the [`Unpin`] auto-trait, which
42 //! nullifies the effect of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function
43 //! identically, as do `Pin<&mut T>` and `&mut T`.
45 //! Note that pinning and `Unpin` only affect the pointed-to type, not the pointer
46 //! type itself that got wrapped in `Pin`. For example, whether or not `Box<T>` is
47 //! `Unpin` has no effect on the behavior of `Pin<Box<T>>` (here, `T` is the
50 //! # Example: self-referential struct
53 //! use std::pin::Pin;
54 //! use std::marker::PhantomPinned;
55 //! use std::ptr::NonNull;
57 //! // This is a self-referential struct since the slice field points to the data field.
58 //! // We cannot inform the compiler about that with a normal reference,
59 //! // since this pattern cannot be described with the usual borrowing rules.
60 //! // Instead we use a raw pointer, though one which is known to not be null,
61 //! // since we know it's pointing at the string.
62 //! struct Unmovable {
64 //! slice: NonNull<String>,
65 //! _pin: PhantomPinned,
69 //! // To ensure the data doesn't move when the function returns,
70 //! // we place it in the heap where it will stay for the lifetime of the object,
71 //! // and the only way to access it would be through a pointer to it.
72 //! fn new(data: String) -> Pin<Box<Self>> {
73 //! let res = Unmovable {
75 //! // we only create the pointer once the data is in place
76 //! // otherwise it will have already moved before we even started
77 //! slice: NonNull::dangling(),
78 //! _pin: PhantomPinned,
80 //! let mut boxed = Box::pin(res);
82 //! let slice = NonNull::from(&boxed.data);
83 //! // we know this is safe because modifying a field doesn't move the whole struct
85 //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
86 //! Pin::get_unchecked_mut(mut_ref).slice = slice;
92 //! let unmoved = Unmovable::new("hello".to_string());
93 //! // The pointer should point to the correct location,
94 //! // so long as the struct hasn't moved.
95 //! // Meanwhile, we are free to move the pointer around.
96 //! # #[allow(unused_mut)]
97 //! let mut still_unmoved = unmoved;
98 //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
100 //! // Since our type doesn't implement Unpin, this will fail to compile:
101 //! // let new_unmoved = Unmovable::new("world".to_string());
102 //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
105 //! # Example: intrusive doubly-linked list
107 //! In an intrusive doubly-linked list, the collection does not actually allocate
108 //! the memory for the elements itself. Allocation is controlled by the clients,
109 //! and elements can live on a stack frame that lives shorter than the collection does.
111 //! To make this work, every element has pointers to its predecessor and successor in
112 //! the list. Element can only be added when they are pinned, because moving the elements
113 //! around would invalidate the pointers. Moreover, the `Drop` implementation of a linked
114 //! list element will patch the pointers of its predecessor and successor to remove itself
117 //! To make this work, it is crucial taht we can actually rely on `drop` being called.
118 //! And, in fact, this is a guarantee that `Pin` provides.
120 //! # `Drop` guarantee
122 //! The purpose of pinning is to be able to rely on the placement of some data in memory.
123 //! To make this work, not just moving the data is restricted; deallocating, repurposing or
124 //! otherwise invalidating the memory used to store the data is restricted, too.
125 //! Concretely, for pinned data you have to maintain the invariant
126 //! that *its memory will not get invalidated from the moment it gets pinned until
127 //! when `drop` is called*. Memory can be invalidated by deallocation, but also by
128 //! replacing a `Some(v)` by `None`, or calling `Vec::set_len` to "kill" some elements
131 //! This is exactly the kind of guarantee that the intrusive linked list from the previous
132 //! section needs to function correctly. Clearly, if an element
133 //! could be deallocated or otherwise invalidated without calling `drop`, the pointers into it
134 //! from its neighbouring elements would become invalid, which would break the data structure.
136 //! Notice that this guarantee does *not* mean that memory does not leak! It is still
137 //! completely okay not to ever call `drop` on a pinned element (e.g., you can still
138 //! call [`mem::forget`] on a `Pin<Box<T>>`). In the example of the doubly-linked
139 //! list, that element would just stay in the list. However you may not free or reuse the storage
140 //! *without calling `drop`*.
142 //! # `Drop` implementation
144 //! If your type uses pinning (such as the two examples above), you have to be careful
145 //! when implementing `Drop`. The `drop` function takes `&mut self`, but this
146 //! is called *even if your type was previously pinned*! It is as if the
147 //! compiler automatically called `get_unchecked_mut`.
149 //! This can never cause a problem in safe code because implementing a type that relies on pinning
150 //! requires unsafe code, but be aware that deciding to make use of pinning
151 //! in your type (for example by implementing some operation on `Pin<&[mut] Self>`)
152 //! has consequences for your `Drop` implementation as well: if an element
153 //! of your type could have been pinned, you must treat Drop as implicitly taking
154 //! `Pin<&mut Self>`.
156 //! In particular, if your type is `#[repr(packed)]`, the compiler will automatically
157 //! move fields around to be able to drop them. As a consequence, you cannot use
158 //! pinning with a `#[repr(packed)]` type.
160 //! # Projections and Structural Pinning
162 //! One interesting question arises when considering the interaction of pinning and
163 //! the fields of a struct. When can a struct have a "pinning projection", i.e.,
164 //! an operation with type `fn(Pin<&[mut] Struct>) -> Pin<&[mut] Field>`?
165 //! In a similar vein, when can a container type (such as `Vec`, `Box`, or `RefCell`)
166 //! have an operation with type `fn(Pin<&[mut] Container<T>>) -> Pin<&[mut] T>`?
168 //! This question is closely related to the question of whether pinning is "structural":
169 //! when you have pinned a wrapper type, have you pinned its contents? Deciding this
170 //! is entirely up to the author of any given type. However, adding a
171 //! projection to the API answers that question with a "yes" by offering pinned access
172 //! to the contents. In that case, there are a couple requirements to be upheld:
174 //! 1. The wrapper must only be [`Unpin`] if all the fields one can project to are
175 //! `Unpin`. This is the default, but `Unpin` is a safe trait, so as the author of
176 //! the wrapper it is your responsibility *not* to add something like
177 //! `impl<T> Unpin for Container<T>`. (Notice that adding a projection operation
178 //! requires unsafe code, so the fact that `Unpin` is a safe trait does not break
179 //! the principle that you only have to worry about any of this if you use `unsafe`.)
180 //! 2. The destructor of the wrapper must not move out of its argument. This is the exact
181 //! point that was raised in the [previous section][drop-impl]: `drop` takes `&mut self`,
182 //! but the wrapper (and hence its fields) might have been pinned before.
183 //! You have to guarantee that you do not move a field inside your `Drop` implementation.
184 //! 3. Your wrapper type must *not* be `#[repr(packed)]`. Packed structs have their fields
185 //! moved around when they are dropped to properly align them, which is in conflict with
186 //! claiming that the fields are pinned when your struct is.
187 //! 4. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
188 //! once your wrapper is pinned, the memory that contains the
189 //! content is not overwritten or deallocated without calling the content's destructors.
190 //! This can be tricky, as witnessed by `VecDeque`: the destructor of `VecDeque` can fail
191 //! to call `drop` on all elements if one of the destructors panics. This violates the
192 //! `Drop` guarantee, because it can lead to elements being deallocated without
193 //! their destructor being called. (`VecDeque` has no pinning projections, so this
194 //! does not cause unsoundness.)
195 //! 5. You must not offer any other operations that could lead to data being moved out of
196 //! the fields when your type is pinned. This is usually not a concern, but can become
197 //! tricky when interior mutability is involved. For example, imagine `RefCell`
198 //! would have a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
199 //! Then we could do the following:
201 //! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>) {
202 //! { let p = rc.as_mut().get_pin_mut(); } // here we get pinned access to the `T`
203 //! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
204 //! let b = rc_shr.borrow_mut();
205 //! let content = &mut *b; // and here we have `&mut T` to the same data
208 //! This is catastrophic, it means we can first pin the content of the `RefCell`
209 //! (using `RefCell::get_pin_mut`) and then move that content using the mutable
210 //! reference we got later.
212 //! On the other hand, if you decide *not* to offer any pinning projections, you
213 //! are free to `impl<T> Unpin for Container<T>`. In the standard library,
214 //! this is done for all pointer types: `Box<T>: Unpin` holds for all `T`.
215 //! It makes sense to do this for pointer types, because moving the `Box<T>`
216 //! does not actually move the `T`: the `Box<T>` can be freely movable even if the `T`
217 //! is not. In fact, even `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves,
218 //! for the same reason.
220 //! [`Pin`]: struct.Pin.html
221 //! [`Unpin`]: ../../std/marker/trait.Unpin.html
222 //! [`mem::swap`]: ../../std/mem/fn.swap.html
223 //! [`mem::forget`]: ../../std/mem/fn.forget.html
224 //! [`Box`]: ../../std/boxed/struct.Box.html
225 //! [drop-impl]: #drop-implementation
226 //! [drop-guarantee]: #drop-guarantee
228 #![stable(feature = "pin", since = "1.33.0")]
231 use marker::{Sized, Unpin};
232 use cmp::{self, PartialEq, PartialOrd};
233 use ops::{Deref, DerefMut, Receiver, CoerceUnsized, DispatchFromDyn};
235 /// A pinned pointer.
237 /// This is a wrapper around a kind of pointer which makes that pointer "pin" its
238 /// value in place, preventing the value referenced by that pointer from being moved
239 /// unless it implements [`Unpin`].
241 /// See the [`pin` module] documentation for further explanation on pinning.
243 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
244 /// [`pin` module]: ../../std/pin/index.html
246 // Note: the derives below, and the explicit `PartialEq` and `PartialOrd`
247 // implementations, are allowed because they all only use `&P`, so they cannot move
248 // the value behind `pointer`.
249 #[stable(feature = "pin", since = "1.33.0")]
250 #[cfg_attr(not(stage0), lang = "pin")]
253 #[derive(Copy, Clone, Hash, Eq, Ord)]
258 #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")]
259 impl<P, Q> PartialEq<Pin<Q>> for Pin<P>
263 fn eq(&self, other: &Pin<Q>) -> bool {
264 self.pointer == other.pointer
267 fn ne(&self, other: &Pin<Q>) -> bool {
268 self.pointer != other.pointer
272 #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")]
273 impl<P, Q> PartialOrd<Pin<Q>> for Pin<P>
277 fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
278 self.pointer.partial_cmp(&other.pointer)
281 fn lt(&self, other: &Pin<Q>) -> bool {
282 self.pointer < other.pointer
285 fn le(&self, other: &Pin<Q>) -> bool {
286 self.pointer <= other.pointer
289 fn gt(&self, other: &Pin<Q>) -> bool {
290 self.pointer > other.pointer
293 fn ge(&self, other: &Pin<Q>) -> bool {
294 self.pointer >= other.pointer
298 impl<P: Deref> Pin<P>
302 /// Construct a new `Pin` around a pointer to some data of a type that
303 /// implements [`Unpin`].
305 /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
306 /// `P` dereferences to an [`Unpin`] type, which nullifies the pinning guarantees.
308 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
309 #[stable(feature = "pin", since = "1.33.0")]
311 pub fn new(pointer: P) -> Pin<P> {
312 // Safety: the value pointed to is `Unpin`, and so has no requirements
314 unsafe { Pin::new_unchecked(pointer) }
318 impl<P: Deref> Pin<P> {
319 /// Construct a new `Pin` around a reference to some data of a type that
320 /// may or may not implement `Unpin`.
324 /// This constructor is unsafe because we cannot guarantee that the data
325 /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
326 /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
327 /// not guarantee that the data `P` points to is pinned, constructing a
328 /// `Pin<P>` is unsafe. In particular, calling `Pin::new_unchecked`
329 /// on an `&'a mut T` is unsafe because while you are able to pin it for the given
330 /// lifetime `'a`, you have no control over whether it is kept pinned once `'a`
331 /// ends. A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
333 /// By using this method, you are making a promise about the `P::Deref` and
334 /// `P::DerefMut` implementations, if they exist. Most importantly, they
335 /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
336 /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
337 /// and expect these methods to uphold the pinning invariants.
338 /// Moreover, by calling this method you promise that the reference `P`
339 /// dereferences to will not be moved out of again; in particular, it
340 /// must not be possible to obtain a `&mut P::Target` and then
341 /// move out of that reference (using, for example [`mem::swap`]).
343 /// For example, the following is a *violation* of `Pin`'s safety:
346 /// use std::pin::Pin;
348 /// fn foo<T>(mut a: T, mut b: T) {
349 /// unsafe { let p = Pin::new_unchecked(&mut a); } // should mean `a` can never move again
350 /// mem::swap(&mut a, &mut b);
351 /// // the address of `a` changed to `b`'s stack slot, so `a` got moved even
352 /// // though we have previously pinned it!
356 /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
359 /// [`mem::swap`]: ../../std/mem/fn.swap.html
360 #[stable(feature = "pin", since = "1.33.0")]
362 pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
366 /// Gets a pinned shared reference from this pinned pointer.
368 /// This is a generic method to go from `&Pin<SmartPointer<T>>` to `Pin<&T>`.
369 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
370 /// the pointee cannot move after `Pin<SmartPointer<T>>` got created.
371 /// "Malicious" implementations of `SmartPointer::Deref` are likewise
372 /// ruled out by the contract of `Pin::new_unchecked`.
373 #[stable(feature = "pin", since = "1.33.0")]
375 pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
376 unsafe { Pin::new_unchecked(&*self.pointer) }
380 impl<P: DerefMut> Pin<P> {
381 /// Gets a pinned mutable reference from this pinned pointer.
383 /// This is a generic method to go from `&mut Pin<SmartPointer<T>>` to `Pin<&mut T>`.
384 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
385 /// the pointee cannot move after `Pin<SmartPointer<T>>` got created.
386 /// "Malicious" implementations of `SmartPointer::DerefMut` are likewise
387 /// ruled out by the contract of `Pin::new_unchecked`.
388 #[stable(feature = "pin", since = "1.33.0")]
390 pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
391 unsafe { Pin::new_unchecked(&mut *self.pointer) }
394 /// Assigns a new value to the memory behind the pinned reference.
396 /// This overwrites pinned data, but that is okay: its destructor gets
397 /// run before being overwritten, so no pinning guarantee is violated.
398 #[stable(feature = "pin", since = "1.33.0")]
400 pub fn set(self: &mut Pin<P>, value: P::Target)
404 *(self.pointer) = value;
408 impl<'a, T: ?Sized> Pin<&'a T> {
409 /// Constructs a new pin by mapping the interior value.
411 /// For example, if you wanted to get a `Pin` of a field of something,
412 /// you could use this to get access to that field in one line of code.
413 /// However, there are several gotchas with these "pinning projections";
414 /// see the [`pin` module] documentation for further details on that topic.
418 /// This function is unsafe. You must guarantee that the data you return
419 /// will not move so long as the argument value does not move (for example,
420 /// because it is one of the fields of that value), and also that you do
421 /// not move out of the argument you receive to the interior function.
423 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
424 #[stable(feature = "pin", since = "1.33.0")]
425 pub unsafe fn map_unchecked<U, F>(self: Pin<&'a T>, func: F) -> Pin<&'a U> where
428 let pointer = &*self.pointer;
429 let new_pointer = func(pointer);
430 Pin::new_unchecked(new_pointer)
433 /// Gets a shared reference out of a pin.
435 /// This is safe because it is not possible to move out of a shared reference.
436 /// It may seem like there is an issue here with interior mutability: in fact,
437 /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
438 /// not a problem as long as there does not also exist a `Pin<&T>` pointing
439 /// to the same data, and `RefCell` does not let you create a pinned reference
440 /// to its contents. See the discussion on ["pinning projections"] for further
443 /// Note: `Pin` also implements `Deref` to the target, which can be used
444 /// to access the inner value. However, `Deref` only provides a reference
445 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
446 /// the `Pin` itself. This method allows turning the `Pin` into a reference
447 /// with the same lifetime as the original `Pin`.
449 /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
450 #[stable(feature = "pin", since = "1.33.0")]
452 pub fn get_ref(self: Pin<&'a T>) -> &'a T {
457 impl<'a, T: ?Sized> Pin<&'a mut T> {
458 /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
459 #[stable(feature = "pin", since = "1.33.0")]
461 pub fn into_ref(self: Pin<&'a mut T>) -> Pin<&'a T> {
462 Pin { pointer: self.pointer }
465 /// Gets a mutable reference to the data inside of this `Pin`.
467 /// This requires that the data inside this `Pin` is `Unpin`.
469 /// Note: `Pin` also implements `DerefMut` to the data, which can be used
470 /// to access the inner value. However, `DerefMut` only provides a reference
471 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
472 /// the `Pin` itself. This method allows turning the `Pin` into a reference
473 /// with the same lifetime as the original `Pin`.
474 #[stable(feature = "pin", since = "1.33.0")]
476 pub fn get_mut(self: Pin<&'a mut T>) -> &'a mut T
482 /// Gets a mutable reference to the data inside of this `Pin`.
486 /// This function is unsafe. You must guarantee that you will never move
487 /// the data out of the mutable reference you receive when you call this
488 /// function, so that the invariants on the `Pin` type can be upheld.
490 /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
492 #[stable(feature = "pin", since = "1.33.0")]
494 pub unsafe fn get_unchecked_mut(self: Pin<&'a mut T>) -> &'a mut T {
498 /// Construct a new pin by mapping the interior value.
500 /// For example, if you wanted to get a `Pin` of a field of something,
501 /// you could use this to get access to that field in one line of code.
502 /// However, there are several gotchas with these "pinning projections";
503 /// see the [`pin` module] documentation for further details on that topic.
507 /// This function is unsafe. You must guarantee that the data you return
508 /// will not move so long as the argument value does not move (for example,
509 /// because it is one of the fields of that value), and also that you do
510 /// not move out of the argument you receive to the interior function.
512 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
513 #[stable(feature = "pin", since = "1.33.0")]
514 pub unsafe fn map_unchecked_mut<U, F>(self: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
515 F: FnOnce(&mut T) -> &mut U,
517 let pointer = Pin::get_unchecked_mut(self);
518 let new_pointer = func(pointer);
519 Pin::new_unchecked(new_pointer)
523 #[stable(feature = "pin", since = "1.33.0")]
524 impl<P: Deref> Deref for Pin<P> {
525 type Target = P::Target;
526 fn deref(&self) -> &P::Target {
527 Pin::get_ref(Pin::as_ref(self))
531 #[stable(feature = "pin", since = "1.33.0")]
532 impl<P: DerefMut> DerefMut for Pin<P>
536 fn deref_mut(&mut self) -> &mut P::Target {
537 Pin::get_mut(Pin::as_mut(self))
541 #[unstable(feature = "receiver_trait", issue = "0")]
542 impl<P: Receiver> Receiver for Pin<P> {}
544 #[stable(feature = "pin", since = "1.33.0")]
545 impl<P: fmt::Debug> fmt::Debug for Pin<P> {
546 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
547 fmt::Debug::fmt(&self.pointer, f)
551 #[stable(feature = "pin", since = "1.33.0")]
552 impl<P: fmt::Display> fmt::Display for Pin<P> {
553 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
554 fmt::Display::fmt(&self.pointer, f)
558 #[stable(feature = "pin", since = "1.33.0")]
559 impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
560 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
561 fmt::Pointer::fmt(&self.pointer, f)
565 // Note: this means that any impl of `CoerceUnsized` that allows coercing from
566 // a type that impls `Deref<Target=impl !Unpin>` to a type that impls
567 // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound
568 // for other reasons, though, so we just need to take care not to allow such
569 // impls to land in std.
570 #[stable(feature = "pin", since = "1.33.0")]
571 impl<P, U> CoerceUnsized<Pin<U>> for Pin<P>
576 #[stable(feature = "pin", since = "1.33.0")]
577 impl<'a, P, U> DispatchFromDyn<Pin<U>> for Pin<P>
579 P: DispatchFromDyn<U>,