1 //! Types that 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 //! A [`Pin<P>`] ensures that the pointee of any pointer type `P` 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<T>` and `&mut T` allow replacing and
15 //! moving the values they contain: you can move out of a `Box<T>`, or you can use [`mem::swap`].
16 //! [`Pin<P>`] wraps a pointer type `P`, 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<P>`] does not let clients
19 //! actually obtain a `Box<T>` or `&mut T` to pinned data, which implies that you cannot use
20 //! operations such as [`mem::swap`]:
22 //! use std::pin::Pin;
23 //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
24 //! // `mem::swap` needs `&mut T`, but we cannot get it.
25 //! // We are stuck, we cannot swap the contents of these references.
26 //! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
27 //! // we are not allowed to use it for moving things out of the `Pin`.
31 //! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
32 //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, `Pin<P>`
33 //! prevents certain *values* (pointed to by pointers wrapped in `Pin<P>`) from being
34 //! moved by making it impossible to call methods that require `&mut T` on them
35 //! (like [`mem::swap`]).
37 //! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
38 //! [`Deref`] and [`DerefMut`]. A `Pin<P>` where `P: Deref` should be considered
39 //! as a "`P`-style pointer" to a pinned `P::Target` -- so, a `Pin<Box<T>>` is
40 //! an owned pointer to a pinned `T`, and a `Pin<Rc<T>>` is a reference-counted
41 //! pointer to a pinned `T`.
42 //! For correctness, [`Pin<P>`] relies on the [`Deref`] and [`DerefMut`] implementations
43 //! to not move out of their `self` parameter, and to only ever return a pointer
44 //! to pinned data when they are called on a pinned pointer.
48 //! However, these restrictions are usually not necessary. Many types are always freely
49 //! movable, even when pinned, because they do not rely on having a stable address.
50 //! This includes all the basic types (like `bool`, `i32`, references)
51 //! as well as types consisting solely of these types.
52 //! Types that do not care about pinning implement the [`Unpin`] auto-trait, which
53 //! cancels the effect of [`Pin<P>`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function
54 //! identically, as do `Pin<&mut T>` and `&mut T`.
56 //! Note that pinning and `Unpin` only affect the pointed-to type `P::Target`, not the pointer
57 //! type `P` itself that got wrapped in `Pin<P>`. For example, whether or not `Box<T>` is
58 //! `Unpin` has no effect on the behavior of `Pin<Box<T>>` (here, `T` is the
61 //! # Example: self-referential struct
64 //! use std::pin::Pin;
65 //! use std::marker::PhantomPinned;
66 //! use std::ptr::NonNull;
68 //! // This is a self-referential struct since the slice field points to the data field.
69 //! // We cannot inform the compiler about that with a normal reference,
70 //! // since this pattern cannot be described with the usual borrowing rules.
71 //! // Instead we use a raw pointer, though one which is known to not be null,
72 //! // since we know it's pointing at the string.
73 //! struct Unmovable {
75 //! slice: NonNull<String>,
76 //! _pin: PhantomPinned,
80 //! // To ensure the data doesn't move when the function returns,
81 //! // we place it in the heap where it will stay for the lifetime of the object,
82 //! // and the only way to access it would be through a pointer to it.
83 //! fn new(data: String) -> Pin<Box<Self>> {
84 //! let res = Unmovable {
86 //! // we only create the pointer once the data is in place
87 //! // otherwise it will have already moved before we even started
88 //! slice: NonNull::dangling(),
89 //! _pin: PhantomPinned,
91 //! let mut boxed = Box::pin(res);
93 //! let slice = NonNull::from(&boxed.data);
94 //! // we know this is safe because modifying a field doesn't move the whole struct
96 //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
97 //! Pin::get_unchecked_mut(mut_ref).slice = slice;
103 //! let unmoved = Unmovable::new("hello".to_string());
104 //! // The pointer should point to the correct location,
105 //! // so long as the struct hasn't moved.
106 //! // Meanwhile, we are free to move the pointer around.
107 //! # #[allow(unused_mut)]
108 //! let mut still_unmoved = unmoved;
109 //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
111 //! // Since our type doesn't implement Unpin, this will fail to compile:
112 //! // let mut new_unmoved = Unmovable::new("world".to_string());
113 //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
116 //! # Example: intrusive doubly-linked list
118 //! In an intrusive doubly-linked list, the collection does not actually allocate
119 //! the memory for the elements itself. Allocation is controlled by the clients,
120 //! and elements can live on a stack frame that lives shorter than the collection does.
122 //! To make this work, every element has pointers to its predecessor and successor in
123 //! the list. Elements can only be added when they are pinned, because moving the elements
124 //! around would invalidate the pointers. Moreover, the `Drop` implementation of a linked
125 //! list element will patch the pointers of its predecessor and successor to remove itself
128 //! Crucially, we have to be able to rely on `drop` being called. If an element
129 //! could be deallocated or otherwise invalidated without calling `drop`, the pointers into it
130 //! from its neighbouring elements would become invalid, which would break the data structure.
132 //! Therefore, pinning also comes with a `drop`-related guarantee.
134 //! # `Drop` guarantee
136 //! The purpose of pinning is to be able to rely on the placement of some data in memory.
137 //! To make this work, not just moving the data is restricted; deallocating, repurposing, or
138 //! otherwise invalidating the memory used to store the data is restricted, too.
139 //! Concretely, for pinned data you have to maintain the invariant
140 //! that *its memory will not get invalidated from the moment it gets pinned until
141 //! when `drop` is called*. Memory can be invalidated by deallocation, but also by
142 //! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
145 //! This is exactly the kind of guarantee that the intrusive linked list from the previous
146 //! section needs to function correctly.
148 //! Notice that this guarantee does *not* mean that memory does not leak! It is still
149 //! completely okay not to ever call `drop` on a pinned element (e.g., you can still
150 //! call [`mem::forget`] on a `Pin<Box<T>>`). In the example of the doubly-linked
151 //! list, that element would just stay in the list. However you may not free or reuse the storage
152 //! *without calling `drop`*.
154 //! # `Drop` implementation
156 //! If your type uses pinning (such as the two examples above), you have to be careful
157 //! when implementing `Drop`. The `drop` function takes `&mut self`, but this
158 //! is called *even if your type was previously pinned*! It is as if the
159 //! compiler automatically called `get_unchecked_mut`.
161 //! This can never cause a problem in safe code because implementing a type that
162 //! relies on pinning requires unsafe code, but be aware that deciding to make
163 //! use of pinning in your type (for example by implementing some operation on
164 //! `Pin<&Self>` or `Pin<&mut Self>`) has consequences for your `Drop`
165 //! implementation as well: if an element of your type could have been pinned,
166 //! you must treat Drop as implicitly taking `Pin<&mut Self>`.
168 //! In particular, if your type is `#[repr(packed)]`, the compiler will automatically
169 //! move fields around to be able to drop them. As a consequence, you cannot use
170 //! pinning with a `#[repr(packed)]` type.
172 //! # Projections and Structural Pinning
174 //! One interesting question arises when considering the interaction of pinning and
175 //! the fields of a struct. When can a struct have a "pinning projection", i.e.,
176 //! an operation with type `fn(Pin<&Struct>) -> Pin<&Field>`?
177 //! In a similar vein, when can a generic wrapper type (such as `Vec<T>`, `Box<T>`, or `RefCell<T>`)
178 //! have an operation with type `fn(Pin<&Wrapper<T>>) -> Pin<&T>`?
180 //! Having a pinning projection for some field means that pinning is "structural":
181 //! when the wrapper is pinned, the field must be considered pinned, too.
182 //! After all, the pinning projection lets us get a `Pin<&Field>`.
184 //! However, structural pinning comes with a few extra requirements, so not all
185 //! wrappers can be structural and hence not all wrappers can offer pinning projections:
187 //! 1. The wrapper must only be [`Unpin`] if all the structural fields are
188 //! `Unpin`. This is the default, but `Unpin` is a safe trait, so as the author of
189 //! the wrapper it is your responsibility *not* to add something like
190 //! `impl<T> Unpin for Wrapper<T>`. (Notice that adding a projection operation
191 //! requires unsafe code, so the fact that `Unpin` is a safe trait does not break
192 //! the principle that you only have to worry about any of this if you use `unsafe`.)
193 //! 2. The destructor of the wrapper must not move structural fields out of its argument. This
194 //! is the exact point that was raised in the [previous section][drop-impl]: `drop` takes
195 //! `&mut self`, but the wrapper (and hence its fields) might have been pinned before.
196 //! You have to guarantee that you do not move a field inside your `Drop` implementation.
197 //! In particular, as explained previously, this means that your wrapper type must *not*
198 //! be `#[repr(packed)]`.
199 //! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
200 //! once your wrapper is pinned, the memory that contains the
201 //! content is not overwritten or deallocated without calling the content's destructors.
202 //! This can be tricky, as witnessed by `VecDeque<T>`: the destructor of `VecDeque<T>` can fail
203 //! to call `drop` on all elements if one of the destructors panics. This violates the
204 //! `Drop` guarantee, because it can lead to elements being deallocated without
205 //! their destructor being called. (`VecDeque` has no pinning projections, so this
206 //! does not cause unsoundness.)
207 //! 4. You must not offer any other operations that could lead to data being moved out of
208 //! the fields when your type is pinned. For example, if the wrapper contains an
209 //! `Option<T>` and there is a `take`-like operation with type
210 //! `fn(Pin<&mut Wrapper<T>>) -> Option<T>`,
211 //! that operation can be used to move a `T` out of a pinned `Wrapper<T>` -- which means
212 //! pinning cannot be structural.
214 //! For a more complex example of moving data out of a pinned type, imagine if `RefCell<T>`
215 //! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
216 //! Then we could do the following:
218 //! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
219 //! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
220 //! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
221 //! let b = rc_shr.borrow_mut();
222 //! let content = &mut *b; // And here we have `&mut T` to the same data.
225 //! This is catastrophic, it means we can first pin the content of the `RefCell<T>`
226 //! (using `RefCell::get_pin_mut`) and then move that content using the mutable
227 //! reference we got later.
229 //! For a type like `Vec<T>`, both possibilites (structural pinning or not) make sense,
230 //! and the choice is up to the author. A `Vec<T>` with structural pinning could
231 //! have `get_pin`/`get_pin_mut` projections. However, it could *not* allow calling
232 //! `pop` on a pinned `Vec<T>` because that would move the (structurally pinned) contents!
233 //! Nor could it allow `push`, which might reallocate and thus also move the contents.
234 //! A `Vec<T>` without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
235 //! are never pinned and the `Vec<T>` itself is fine with being moved as well.
237 //! In the standard library, pointer types generally do not have structural pinning,
238 //! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
239 //! It makes sense to do this for pointer types, because moving the `Box<T>`
240 //! does not actually move the `T`: the `Box<T>` can be freely movable (aka `Unpin`) even if the `T`
241 //! is not. In fact, even `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves,
242 //! for the same reason: their contents (the `T`) are pinned, but the pointers themselves
243 //! can be moved without moving the pinned data. For both `Box<T>` and `Pin<Box<T>>`,
244 //! whether the content is pinned is entirely independent of whether the pointer is
245 //! pinned, meaning pinning is *not* structural.
247 //! [`Pin<P>`]: struct.Pin.html
248 //! [`Unpin`]: ../../std/marker/trait.Unpin.html
249 //! [`Deref`]: ../../std/ops/trait.Deref.html
250 //! [`DerefMut`]: ../../std/ops/trait.DerefMut.html
251 //! [`mem::swap`]: ../../std/mem/fn.swap.html
252 //! [`mem::forget`]: ../../std/mem/fn.forget.html
253 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
254 //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
255 //! [`None`]: ../../std/option/enum.Option.html#variant.None
256 //! [`Some(v)`]: ../../std/option/enum.Option.html#variant.Some
257 //! [drop-impl]: #drop-implementation
258 //! [drop-guarantee]: #drop-guarantee
260 #![stable(feature = "pin", since = "1.33.0")]
263 use marker::{Sized, Unpin};
264 use cmp::{self, PartialEq, PartialOrd};
265 use ops::{Deref, DerefMut, Receiver, CoerceUnsized, DispatchFromDyn};
267 /// A pinned pointer.
269 /// This is a wrapper around a kind of pointer which makes that pointer "pin" its
270 /// value in place, preventing the value referenced by that pointer from being moved
271 /// unless it implements [`Unpin`].
273 /// See the [`pin` module] documentation for further explanation on pinning.
275 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
276 /// [`pin` module]: ../../std/pin/index.html
278 // Note: the derives below, and the explicit `PartialEq` and `PartialOrd`
279 // implementations, are allowed because they all only use `&P`, so they cannot move
280 // the value behind `pointer`.
281 #[stable(feature = "pin", since = "1.33.0")]
285 #[derive(Copy, Clone, Hash, Eq, Ord)]
290 #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")]
291 impl<P, Q> PartialEq<Pin<Q>> for Pin<P>
295 fn eq(&self, other: &Pin<Q>) -> bool {
296 self.pointer == other.pointer
299 fn ne(&self, other: &Pin<Q>) -> bool {
300 self.pointer != other.pointer
304 #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")]
305 impl<P, Q> PartialOrd<Pin<Q>> for Pin<P>
309 fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
310 self.pointer.partial_cmp(&other.pointer)
313 fn lt(&self, other: &Pin<Q>) -> bool {
314 self.pointer < other.pointer
317 fn le(&self, other: &Pin<Q>) -> bool {
318 self.pointer <= other.pointer
321 fn gt(&self, other: &Pin<Q>) -> bool {
322 self.pointer > other.pointer
325 fn ge(&self, other: &Pin<Q>) -> bool {
326 self.pointer >= other.pointer
330 impl<P: Deref> Pin<P>
334 /// Construct a new `Pin<P>` around a pointer to some data of a type that
335 /// implements [`Unpin`].
337 /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
338 /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
340 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
341 #[stable(feature = "pin", since = "1.33.0")]
343 pub fn new(pointer: P) -> Pin<P> {
344 // Safety: the value pointed to is `Unpin`, and so has no requirements
346 unsafe { Pin::new_unchecked(pointer) }
350 impl<P: Deref> Pin<P> {
351 /// Construct a new `Pin<P>` around a reference to some data of a type that
352 /// may or may not implement `Unpin`.
354 /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
359 /// This constructor is unsafe because we cannot guarantee that the data
360 /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
361 /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
362 /// not guarantee that the data `P` points to is pinned, that is a violation of
363 /// the API contract and may lead to undefined behavior in later (safe) operations.
365 /// By using this method, you are making a promise about the `P::Deref` and
366 /// `P::DerefMut` implementations, if they exist. Most importantly, they
367 /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
368 /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
369 /// and expect these methods to uphold the pinning invariants.
370 /// Moreover, by calling this method you promise that the reference `P`
371 /// dereferences to will not be moved out of again; in particular, it
372 /// must not be possible to obtain a `&mut P::Target` and then
373 /// move out of that reference (using, for example [`mem::swap`]).
375 /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
376 /// while you are able to pin it for the given lifetime `'a`, you have no control
377 /// over whether it is kept pinned once `'a` ends:
380 /// use std::pin::Pin;
382 /// fn move_pinned_ref<T>(mut a: T, mut b: T) {
384 /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
385 /// // This should mean the pointee `a` can never move again.
387 /// mem::swap(&mut a, &mut b);
388 /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even
389 /// // though we have previously pinned it! We have violated the pinning API contract.
392 /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
394 /// Similarily, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
395 /// aliases to the same data that are not subject to the pinning restrictions:
398 /// use std::pin::Pin;
400 /// fn move_pinned_rc<T>(mut x: Rc<T>) {
401 /// let pinned = unsafe { Pin::new_unchecked(x.clone()) };
403 /// let p: Pin<&T> = pinned.as_ref();
404 /// // This should mean the pointee can never move again.
407 /// let content = Rc::get_mut(&mut x).unwrap();
408 /// // Now, if `x` was the only reference, we have a mutable reference to
409 /// // data that we pinned above, which we could use to move it as we have
410 /// // seen in the previous example. We have violated the pinning API contract.
414 /// [`mem::swap`]: ../../std/mem/fn.swap.html
415 #[stable(feature = "pin", since = "1.33.0")]
417 pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
421 /// Gets a pinned shared reference from this pinned pointer.
423 /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
424 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
425 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
426 /// "Malicious" implementations of `Pointer::Deref` are likewise
427 /// ruled out by the contract of `Pin::new_unchecked`.
428 #[stable(feature = "pin", since = "1.33.0")]
430 pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
431 unsafe { Pin::new_unchecked(&*self.pointer) }
435 impl<P: DerefMut> Pin<P> {
436 /// Gets a pinned mutable reference from this pinned pointer.
438 /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
439 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
440 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
441 /// "Malicious" implementations of `Pointer::DerefMut` are likewise
442 /// ruled out by the contract of `Pin::new_unchecked`.
443 #[stable(feature = "pin", since = "1.33.0")]
445 pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
446 unsafe { Pin::new_unchecked(&mut *self.pointer) }
449 /// Assigns a new value to the memory behind the pinned reference.
451 /// This overwrites pinned data, but that is okay: its destructor gets
452 /// run before being overwritten, so no pinning guarantee is violated.
453 #[stable(feature = "pin", since = "1.33.0")]
455 pub fn set(self: &mut Pin<P>, value: P::Target)
459 *(self.pointer) = value;
463 impl<'a, T: ?Sized> Pin<&'a T> {
464 /// Constructs a new pin by mapping the interior value.
466 /// For example, if you wanted to get a `Pin` of a field of something,
467 /// you could use this to get access to that field in one line of code.
468 /// However, there are several gotchas with these "pinning projections";
469 /// see the [`pin` module] documentation for further details on that topic.
473 /// This function is unsafe. You must guarantee that the data you return
474 /// will not move so long as the argument value does not move (for example,
475 /// because it is one of the fields of that value), and also that you do
476 /// not move out of the argument you receive to the interior function.
478 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
479 #[stable(feature = "pin", since = "1.33.0")]
480 pub unsafe fn map_unchecked<U, F>(self: Pin<&'a T>, func: F) -> Pin<&'a U> where
483 let pointer = &*self.pointer;
484 let new_pointer = func(pointer);
485 Pin::new_unchecked(new_pointer)
488 /// Gets a shared reference out of a pin.
490 /// This is safe because it is not possible to move out of a shared reference.
491 /// It may seem like there is an issue here with interior mutability: in fact,
492 /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
493 /// not a problem as long as there does not also exist a `Pin<&T>` pointing
494 /// to the same data, and `RefCell<T>` does not let you create a pinned reference
495 /// to its contents. See the discussion on ["pinning projections"] for further
498 /// Note: `Pin` also implements `Deref` to the target, which can be used
499 /// to access the inner value. However, `Deref` only provides a reference
500 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
501 /// the `Pin` itself. This method allows turning the `Pin` into a reference
502 /// with the same lifetime as the original `Pin`.
504 /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
505 #[stable(feature = "pin", since = "1.33.0")]
507 pub fn get_ref(self: Pin<&'a T>) -> &'a T {
512 impl<'a, T: ?Sized> Pin<&'a mut T> {
513 /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
514 #[stable(feature = "pin", since = "1.33.0")]
516 pub fn into_ref(self: Pin<&'a mut T>) -> Pin<&'a T> {
517 Pin { pointer: self.pointer }
520 /// Gets a mutable reference to the data inside of this `Pin`.
522 /// This requires that the data inside this `Pin` is `Unpin`.
524 /// Note: `Pin` also implements `DerefMut` to the data, which can be used
525 /// to access the inner value. However, `DerefMut` only provides a reference
526 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
527 /// the `Pin` itself. This method allows turning the `Pin` into a reference
528 /// with the same lifetime as the original `Pin`.
529 #[stable(feature = "pin", since = "1.33.0")]
531 pub fn get_mut(self: Pin<&'a mut T>) -> &'a mut T
537 /// Gets a mutable reference to the data inside of this `Pin`.
541 /// This function is unsafe. You must guarantee that you will never move
542 /// the data out of the mutable reference you receive when you call this
543 /// function, so that the invariants on the `Pin` type can be upheld.
545 /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
547 #[stable(feature = "pin", since = "1.33.0")]
549 pub unsafe fn get_unchecked_mut(self: Pin<&'a mut T>) -> &'a mut T {
553 /// Construct a new pin by mapping the interior value.
555 /// For example, if you wanted to get a `Pin` of a field of something,
556 /// you could use this to get access to that field in one line of code.
557 /// However, there are several gotchas with these "pinning projections";
558 /// see the [`pin` module] documentation for further details on that topic.
562 /// This function is unsafe. You must guarantee that the data you return
563 /// will not move so long as the argument value does not move (for example,
564 /// because it is one of the fields of that value), and also that you do
565 /// not move out of the argument you receive to the interior function.
567 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
568 #[stable(feature = "pin", since = "1.33.0")]
569 pub unsafe fn map_unchecked_mut<U, F>(self: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
570 F: FnOnce(&mut T) -> &mut U,
572 let pointer = Pin::get_unchecked_mut(self);
573 let new_pointer = func(pointer);
574 Pin::new_unchecked(new_pointer)
578 #[stable(feature = "pin", since = "1.33.0")]
579 impl<P: Deref> Deref for Pin<P> {
580 type Target = P::Target;
581 fn deref(&self) -> &P::Target {
582 Pin::get_ref(Pin::as_ref(self))
586 #[stable(feature = "pin", since = "1.33.0")]
587 impl<P: DerefMut> DerefMut for Pin<P>
591 fn deref_mut(&mut self) -> &mut P::Target {
592 Pin::get_mut(Pin::as_mut(self))
596 #[unstable(feature = "receiver_trait", issue = "0")]
597 impl<P: Receiver> Receiver for Pin<P> {}
599 #[stable(feature = "pin", since = "1.33.0")]
600 impl<P: fmt::Debug> fmt::Debug for Pin<P> {
601 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
602 fmt::Debug::fmt(&self.pointer, f)
606 #[stable(feature = "pin", since = "1.33.0")]
607 impl<P: fmt::Display> fmt::Display for Pin<P> {
608 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
609 fmt::Display::fmt(&self.pointer, f)
613 #[stable(feature = "pin", since = "1.33.0")]
614 impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
615 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
616 fmt::Pointer::fmt(&self.pointer, f)
620 // Note: this means that any impl of `CoerceUnsized` that allows coercing from
621 // a type that impls `Deref<Target=impl !Unpin>` to a type that impls
622 // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound
623 // for other reasons, though, so we just need to take care not to allow such
624 // impls to land in std.
625 #[stable(feature = "pin", since = "1.33.0")]
626 impl<P, U> CoerceUnsized<Pin<U>> for Pin<P>
631 #[stable(feature = "pin", since = "1.33.0")]
632 impl<'a, P, U> DispatchFromDyn<Pin<U>> for Pin<P>
634 P: DispatchFromDyn<U>,