1 //! Types that pin data to its location in memory.
3 //! It is sometimes useful to have objects that are guaranteed not to 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 //! as moving an object with pointers to itself will invalidate them, which could cause undefined
9 //! At a high level, a [`Pin<P>`] ensures that the pointee of any pointer type
10 //! `P` has a stable location in memory, meaning it cannot be moved elsewhere
11 //! and its memory cannot be deallocated until it gets dropped. We say that the
12 //! pointee is "pinned". Things get more subtle when discussing types that
13 //! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning)
16 //! By default, all types in Rust are movable. Rust allows passing all types by-value,
17 //! and common smart-pointer types such as [`Box<T>`] and `&mut T` allow replacing and
18 //! moving the values they contain: you can move out of a [`Box<T>`], or you can use [`mem::swap`].
19 //! [`Pin<P>`] wraps a pointer type `P`, so [`Pin`]`<`[`Box`]`<T>>` functions much like a regular
20 //! [`Box<T>`]: when a [`Pin`]`<`[`Box`]`<T>>` gets dropped, so do its contents, and the memory gets
21 //! deallocated. Similarly, [`Pin`]`<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does
22 //! not let clients actually obtain a [`Box<T>`] or `&mut T` to pinned data, which implies that you
23 //! cannot use operations such as [`mem::swap`]:
26 //! use std::pin::Pin;
27 //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
28 //! // `mem::swap` needs `&mut T`, but we cannot get it.
29 //! // We are stuck, we cannot swap the contents of these references.
30 //! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
31 //! // we are not allowed to use it for moving things out of the `Pin`.
35 //! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
36 //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, [`Pin<P>`]
37 //! prevents certain *values* (pointed to by pointers wrapped in [`Pin<P>`]) from being
38 //! moved by making it impossible to call methods that require `&mut T` on them
39 //! (like [`mem::swap`]).
41 //! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
42 //! [`Deref`] and [`DerefMut`]. A [`Pin<P>`] where `P: Deref` should be considered
43 //! as a "`P`-style pointer" to a pinned `P::Target` -- so, a [`Pin`]`<`[`Box`]`<T>>` is
44 //! an owned pointer to a pinned `T`, and a [`Pin`]`<`[`Rc`]`<T>>` is a reference-counted
45 //! pointer to a pinned `T`.
46 //! For correctness, [`Pin<P>`] relies on the implementations of [`Deref`] and
47 //! [`DerefMut`] not to move out of their `self` parameter, and only ever to
48 //! return a pointer to pinned data when they are called on a pinned pointer.
52 //! Many types are always freely movable, even when pinned, because they do not
53 //! rely on having a stable address. This includes all the basic types (like
54 //! [`bool`], [`i32`], and references) as well as types consisting solely of these
55 //! types. Types that do not care about pinning implement the [`Unpin`]
56 //! auto-trait, which cancels the effect of [`Pin<P>`]. For `T: Unpin`,
57 //! [`Pin`]`<`[`Box`]`<T>>` and [`Box<T>`] function identically, as do [`Pin`]`<&mut T>` and
60 //! Note that pinning and [`Unpin`] only affect the pointed-to type `P::Target`, not the pointer
61 //! type `P` itself that got wrapped in [`Pin<P>`]. For example, whether or not [`Box<T>`] is
62 //! [`Unpin`] has no effect on the behavior of [`Pin`]`<`[`Box`]`<T>>` (here, `T` is the
65 //! # Example: self-referential struct
67 //! Before we go into more details to explain the guarantees and choices
68 //! associated with `Pin<T>`, we discuss some examples for how it might be used.
69 //! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee).
72 //! use std::pin::Pin;
73 //! use std::marker::PhantomPinned;
74 //! use std::ptr::NonNull;
76 //! // This is a self-referential struct because the slice field points to the data field.
77 //! // We cannot inform the compiler about that with a normal reference,
78 //! // as this pattern cannot be described with the usual borrowing rules.
79 //! // Instead we use a raw pointer, though one which is known not to be null,
80 //! // as we know it's pointing at the string.
81 //! struct Unmovable {
83 //! slice: NonNull<String>,
84 //! _pin: PhantomPinned,
88 //! // To ensure the data doesn't move when the function returns,
89 //! // we place it in the heap where it will stay for the lifetime of the object,
90 //! // and the only way to access it would be through a pointer to it.
91 //! fn new(data: String) -> Pin<Box<Self>> {
92 //! let res = Unmovable {
94 //! // we only create the pointer once the data is in place
95 //! // otherwise it will have already moved before we even started
96 //! slice: NonNull::dangling(),
97 //! _pin: PhantomPinned,
99 //! let mut boxed = Box::pin(res);
101 //! let slice = NonNull::from(&boxed.data);
102 //! // we know this is safe because modifying a field doesn't move the whole struct
104 //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
105 //! Pin::get_unchecked_mut(mut_ref).slice = slice;
111 //! let unmoved = Unmovable::new("hello".to_string());
112 //! // The pointer should point to the correct location,
113 //! // so long as the struct hasn't moved.
114 //! // Meanwhile, we are free to move the pointer around.
115 //! # #[allow(unused_mut)]
116 //! let mut still_unmoved = unmoved;
117 //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
119 //! // Since our type doesn't implement Unpin, this will fail to compile:
120 //! // let mut new_unmoved = Unmovable::new("world".to_string());
121 //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
124 //! # Example: intrusive doubly-linked list
126 //! In an intrusive doubly-linked list, the collection does not actually allocate
127 //! the memory for the elements itself. Allocation is controlled by the clients,
128 //! and elements can live on a stack frame that lives shorter than the collection does.
130 //! To make this work, every element has pointers to its predecessor and successor in
131 //! the list. Elements can only be added when they are pinned, because moving the elements
132 //! around would invalidate the pointers. Moreover, the [`Drop`] implementation of a linked
133 //! list element will patch the pointers of its predecessor and successor to remove itself
136 //! Crucially, we have to be able to rely on [`drop`] being called. If an element
137 //! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it
138 //! from its neighboring elements would become invalid, which would break the data structure.
140 //! Therefore, pinning also comes with a [`drop`]-related guarantee.
142 //! # `Drop` guarantee
144 //! The purpose of pinning is to be able to rely on the placement of some data in memory.
145 //! To make this work, not just moving the data is restricted; deallocating, repurposing, or
146 //! otherwise invalidating the memory used to store the data is restricted, too.
147 //! Concretely, for pinned data you have to maintain the invariant
148 //! that *its memory will not get invalidated or repurposed from the moment it gets pinned until
149 //! when [`drop`] is called*. Only once [`drop`] returns or panics, the memory may be reused.
151 //! Memory can be "invalidated" by deallocation, but also by
152 //! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
153 //! off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without
154 //! calling the destructor first. None of this is allowed for pinned data without calling [`drop`].
156 //! This is exactly the kind of guarantee that the intrusive linked list from the previous
157 //! section needs to function correctly.
159 //! Notice that this guarantee does *not* mean that memory does not leak! It is still
160 //! completely okay not ever to call [`drop`] on a pinned element (e.g., you can still
161 //! call [`mem::forget`] on a [`Pin`]`<`[`Box`]`<T>>`). In the example of the doubly-linked
162 //! list, that element would just stay in the list. However you may not free or reuse the storage
163 //! *without calling [`drop`]*.
165 //! # `Drop` implementation
167 //! If your type uses pinning (such as the two examples above), you have to be careful
168 //! when implementing [`Drop`]. The [`drop`] function takes `&mut self`, but this
169 //! is called *even if your type was previously pinned*! It is as if the
170 //! compiler automatically called [`Pin::get_unchecked_mut`].
172 //! This can never cause a problem in safe code because implementing a type that
173 //! relies on pinning requires unsafe code, but be aware that deciding to make
174 //! use of pinning in your type (for example by implementing some operation on
175 //! [`Pin`]`<&Self>` or [`Pin`]`<&mut Self>`) has consequences for your [`Drop`]
176 //! implementation as well: if an element of your type could have been pinned,
177 //! you must treat [`Drop`] as implicitly taking [`Pin`]`<&mut Self>`.
179 //! For example, you could implement `Drop` as follows:
182 //! # use std::pin::Pin;
183 //! # struct Type { }
184 //! impl Drop for Type {
185 //! fn drop(&mut self) {
186 //! // `new_unchecked` is okay because we know this value is never used
187 //! // again after being dropped.
188 //! inner_drop(unsafe { Pin::new_unchecked(self)});
189 //! fn inner_drop(this: Pin<&mut Type>) {
190 //! // Actual drop code goes here.
196 //! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that
197 //! you do not accidentally use `self`/`this` in a way that is in conflict with pinning.
199 //! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically
200 //! move fields around to be able to drop them. It might even do
201 //! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use
202 //! pinning with a `#[repr(packed)]` type.
204 //! # Projections and Structural Pinning
206 //! When working with pinned structs, the question arises how one can access the
207 //! fields of that struct in a method that takes just [`Pin`]`<&mut Struct>`.
208 //! The usual approach is to write helper methods (so called *projections*)
209 //! that turn [`Pin`]`<&mut Struct>` into a reference to the field, but what
210 //! type should that reference have? Is it [`Pin`]`<&mut Field>` or `&mut Field`?
211 //! The same question arises with the fields of an `enum`, and also when considering
212 //! container/wrapper types such as [`Vec<T>`], [`Box<T>`], or [`RefCell<T>`].
213 //! (This question applies to both mutable and shared references, we just
214 //! use the more common case of mutable references here for illustration.)
216 //! It turns out that it is actually up to the author of the data structure
217 //! to decide whether the pinned projection for a particular field turns
218 //! [`Pin`]`<&mut Struct>` into [`Pin`]`<&mut Field>` or `&mut Field`. There are some
219 //! constraints though, and the most important constraint is *consistency*:
220 //! every field can be *either* projected to a pinned reference, *or* have
221 //! pinning removed as part of the projection. If both are done for the same field,
222 //! that will likely be unsound!
224 //! As the author of a data structure you get to decide for each field whether pinning
225 //! "propagates" to this field or not. Pinning that propagates is also called "structural",
226 //! because it follows the structure of the type.
227 //! In the following subsections, we describe the considerations that have to be made
228 //! for either choice.
230 //! ## Pinning *is not* structural for `field`
232 //! It may seem counter-intuitive that the field of a pinned struct might not be pinned,
233 //! but that is actually the easiest choice: if a [`Pin`]`<&mut Field>` is never created,
234 //! nothing can go wrong! So, if you decide that some field does not have structural pinning,
235 //! all you have to ensure is that you never create a pinned reference to that field.
237 //! Fields without structural pinning may have a projection method that turns
238 //! [`Pin`]`<&mut Struct>` into `&mut Field`:
241 //! # use std::pin::Pin;
242 //! # type Field = i32;
243 //! # struct Struct { field: Field }
245 //! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field {
246 //! // This is okay because `field` is never considered pinned.
247 //! unsafe { &mut self.get_unchecked_mut().field }
252 //! You may also `impl Unpin for Struct` *even if* the type of `field`
253 //! is not [`Unpin`]. What that type thinks about pinning is not relevant
254 //! when no [`Pin`]`<&mut Field>` is ever created.
256 //! ## Pinning *is* structural for `field`
258 //! The other option is to decide that pinning is "structural" for `field`,
259 //! meaning that if the struct is pinned then so is the field.
261 //! This allows writing a projection that creates a [`Pin`]`<&mut Field>`, thus
262 //! witnessing that the field is pinned:
265 //! # use std::pin::Pin;
266 //! # type Field = i32;
267 //! # struct Struct { field: Field }
269 //! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> {
270 //! // This is okay because `field` is pinned when `self` is.
271 //! unsafe { self.map_unchecked_mut(|s| &mut s.field) }
276 //! However, structural pinning comes with a few extra requirements:
278 //! 1. The struct must only be [`Unpin`] if all the structural fields are
279 //! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of
280 //! the struct it is your responsibility *not* to add something like
281 //! `impl<T> Unpin for Struct<T>`. (Notice that adding a projection operation
282 //! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break
283 //! the principle that you only have to worry about any of this if you use `unsafe`.)
284 //! 2. The destructor of the struct must not move structural fields out of its argument. This
285 //! is the exact point that was raised in the [previous section][drop-impl]: `drop` takes
286 //! `&mut self`, but the struct (and hence its fields) might have been pinned before.
287 //! You have to guarantee that you do not move a field inside your [`Drop`] implementation.
288 //! In particular, as explained previously, this means that your struct must *not*
289 //! be `#[repr(packed)]`.
290 //! See that section for how to write [`drop`] in a way that the compiler can help you
291 //! not accidentally break pinning.
292 //! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
293 //! once your struct is pinned, the memory that contains the
294 //! content is not overwritten or deallocated without calling the content's destructors.
295 //! This can be tricky, as witnessed by [`VecDeque<T>`]: the destructor of [`VecDeque<T>`]
296 //! can fail to call [`drop`] on all elements if one of the destructors panics. This violates
297 //! the [`Drop`] guarantee, because it can lead to elements being deallocated without
298 //! their destructor being called. ([`VecDeque<T>`] has no pinning projections, so this
299 //! does not cause unsoundness.)
300 //! 4. You must not offer any other operations that could lead to data being moved out of
301 //! the structural fields when your type is pinned. For example, if the struct contains an
302 //! [`Option<T>`] and there is a `take`-like operation with type
303 //! `fn(Pin<&mut Struct<T>>) -> Option<T>`,
304 //! that operation can be used to move a `T` out of a pinned `Struct<T>` -- which means
305 //! pinning cannot be structural for the field holding this data.
307 //! For a more complex example of moving data out of a pinned type, imagine if [`RefCell<T>`]
308 //! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
309 //! Then we could do the following:
311 //! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
312 //! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
313 //! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
314 //! let b = rc_shr.borrow_mut();
315 //! let content = &mut *b; // And here we have `&mut T` to the same data.
318 //! This is catastrophic, it means we can first pin the content of the [`RefCell<T>`]
319 //! (using `RefCell::get_pin_mut`) and then move that content using the mutable
320 //! reference we got later.
324 //! For a type like [`Vec<T>`], both possibilities (structural pinning or not) make sense.
325 //! A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut` methods to get
326 //! pinned references to elements. However, it could *not* allow calling
327 //! [`pop`][Vec::pop] on a pinned [`Vec<T>`] because that would move the (structurally pinned)
328 //! contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also move the
331 //! A [`Vec<T>`] without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
332 //! are never pinned and the [`Vec<T>`] itself is fine with being moved as well.
333 //! At that point pinning just has no effect on the vector at all.
335 //! In the standard library, pointer types generally do not have structural pinning,
336 //! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
337 //! It makes sense to do this for pointer types, because moving the `Box<T>`
338 //! does not actually move the `T`: the [`Box<T>`] can be freely movable (aka `Unpin`) even if
339 //! the `T` is not. In fact, even [`Pin`]`<`[`Box`]`<T>>` and [`Pin`]`<&mut T>` are always
340 //! [`Unpin`] themselves, for the same reason: their contents (the `T`) are pinned, but the
341 //! pointers themselves can be moved without moving the pinned data. For both [`Box<T>`] and
342 //! [`Pin`]`<`[`Box`]`<T>>`, whether the content is pinned is entirely independent of whether the
343 //! pointer is pinned, meaning pinning is *not* structural.
345 //! When implementing a [`Future`] combinator, you will usually need structural pinning
346 //! for the nested futures, as you need to get pinned references to them to call [`poll`].
347 //! But if your combinator contains any other data that does not need to be pinned,
348 //! you can make those fields not structural and hence freely access them with a
349 //! mutable reference even when you just have [`Pin`]`<&mut Self>` (such as in your own
350 //! [`poll`] implementation).
353 //! [`Deref`]: crate::ops::Deref
354 //! [`DerefMut`]: crate::ops::DerefMut
355 //! [`mem::swap`]: crate::mem::swap
356 //! [`mem::forget`]: crate::mem::forget
357 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
358 //! [`Vec<T>`]: ../../std/vec/struct.Vec.html
359 //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
360 //! [`Box`]: ../../std/boxed/struct.Box.html
361 //! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop
362 //! [Vec::push]: ../../std/vec/struct.Vec.html#method.push
363 //! [`Rc`]: ../../std/rc/struct.Rc.html
364 //! [`RefCell<T>`]: crate::cell::RefCell
365 //! [`drop`]: Drop::drop
366 //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
367 //! [`Option<T>`]: Option
368 //! [`Some(v)`]: Some
369 //! [`ptr::write`]: crate::ptr::write
370 //! [`Future`]: crate::future::Future
371 //! [drop-impl]: #drop-implementation
372 //! [drop-guarantee]: #drop-guarantee
373 //! [`poll`]: crate::future::Future::poll
375 #![stable(feature = "pin", since = "1.33.0")]
377 use crate::cmp::{self, PartialEq, PartialOrd};
379 use crate::hash::{Hash, Hasher};
380 use crate::marker::{Sized, Unpin};
381 use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
383 /// A pinned pointer.
385 /// This is a wrapper around a kind of pointer which makes that pointer "pin" its
386 /// value in place, preventing the value referenced by that pointer from being moved
387 /// unless it implements [`Unpin`].
389 /// *See the [`pin` module] documentation for an explanation of pinning.*
391 /// [`pin` module]: self
393 // Note: the `Clone` derive below causes unsoundness as it's possible to implement
394 // `Clone` for mutable references.
395 // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311> for more details.
396 #[stable(feature = "pin", since = "1.33.0")]
400 #[derive(Copy, Clone)]
405 // The following implementations aren't derived in order to avoid soundness
406 // issues. `&self.pointer` should not be accessible to untrusted trait
409 // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73> for more details.
411 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
412 impl<P: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<P>
414 P::Target: PartialEq<Q::Target>,
416 fn eq(&self, other: &Pin<Q>) -> bool {
417 P::Target::eq(self, other)
420 fn ne(&self, other: &Pin<Q>) -> bool {
421 P::Target::ne(self, other)
425 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
426 impl<P: Deref<Target: Eq>> Eq for Pin<P> {}
428 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
429 impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P>
431 P::Target: PartialOrd<Q::Target>,
433 fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
434 P::Target::partial_cmp(self, other)
437 fn lt(&self, other: &Pin<Q>) -> bool {
438 P::Target::lt(self, other)
441 fn le(&self, other: &Pin<Q>) -> bool {
442 P::Target::le(self, other)
445 fn gt(&self, other: &Pin<Q>) -> bool {
446 P::Target::gt(self, other)
449 fn ge(&self, other: &Pin<Q>) -> bool {
450 P::Target::ge(self, other)
454 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
455 impl<P: Deref<Target: Ord>> Ord for Pin<P> {
456 fn cmp(&self, other: &Self) -> cmp::Ordering {
457 P::Target::cmp(self, other)
461 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
462 impl<P: Deref<Target: Hash>> Hash for Pin<P> {
463 fn hash<H: Hasher>(&self, state: &mut H) {
464 P::Target::hash(self, state);
468 impl<P: Deref<Target: Unpin>> Pin<P> {
469 /// Construct a new `Pin<P>` around a pointer to some data of a type that
470 /// implements [`Unpin`].
472 /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
473 /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
474 #[stable(feature = "pin", since = "1.33.0")]
476 pub fn new(pointer: P) -> Pin<P> {
477 // Safety: the value pointed to is `Unpin`, and so has no requirements
479 unsafe { Pin::new_unchecked(pointer) }
482 /// Unwraps this `Pin<P>` returning the underlying pointer.
484 /// This requires that the data inside this `Pin` is [`Unpin`] so that we
485 /// can ignore the pinning invariants when unwrapping it.
486 #[stable(feature = "pin_into_inner", since = "1.39.0")]
488 pub fn into_inner(pin: Pin<P>) -> P {
493 impl<P: Deref> Pin<P> {
494 /// Construct a new `Pin<P>` around a reference to some data of a type that
495 /// may or may not implement `Unpin`.
497 /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
502 /// This constructor is unsafe because we cannot guarantee that the data
503 /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
504 /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
505 /// not guarantee that the data `P` points to is pinned, that is a violation of
506 /// the API contract and may lead to undefined behavior in later (safe) operations.
508 /// By using this method, you are making a promise about the `P::Deref` and
509 /// `P::DerefMut` implementations, if they exist. Most importantly, they
510 /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
511 /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
512 /// and expect these methods to uphold the pinning invariants.
513 /// Moreover, by calling this method you promise that the reference `P`
514 /// dereferences to will not be moved out of again; in particular, it
515 /// must not be possible to obtain a `&mut P::Target` and then
516 /// move out of that reference (using, for example [`mem::swap`]).
518 /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
519 /// while you are able to pin it for the given lifetime `'a`, you have no control
520 /// over whether it is kept pinned once `'a` ends:
523 /// use std::pin::Pin;
525 /// fn move_pinned_ref<T>(mut a: T, mut b: T) {
527 /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
528 /// // This should mean the pointee `a` can never move again.
530 /// mem::swap(&mut a, &mut b);
531 /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even
532 /// // though we have previously pinned it! We have violated the pinning API contract.
535 /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
537 /// Similarly, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
538 /// aliases to the same data that are not subject to the pinning restrictions:
541 /// use std::pin::Pin;
543 /// fn move_pinned_rc<T>(mut x: Rc<T>) {
544 /// let pinned = unsafe { Pin::new_unchecked(x.clone()) };
546 /// let p: Pin<&T> = pinned.as_ref();
547 /// // This should mean the pointee can never move again.
550 /// let content = Rc::get_mut(&mut x).unwrap();
551 /// // Now, if `x` was the only reference, we have a mutable reference to
552 /// // data that we pinned above, which we could use to move it as we have
553 /// // seen in the previous example. We have violated the pinning API contract.
557 /// [`mem::swap`]: crate::mem::swap
558 #[lang = "new_unchecked"]
559 #[stable(feature = "pin", since = "1.33.0")]
561 pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
565 /// Gets a pinned shared reference from this pinned pointer.
567 /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
568 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
569 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
570 /// "Malicious" implementations of `Pointer::Deref` are likewise
571 /// ruled out by the contract of `Pin::new_unchecked`.
572 #[stable(feature = "pin", since = "1.33.0")]
574 pub fn as_ref(&self) -> Pin<&P::Target> {
575 // SAFETY: see documentation on this function
576 unsafe { Pin::new_unchecked(&*self.pointer) }
579 /// Unwraps this `Pin<P>` returning the underlying pointer.
583 /// This function is unsafe. You must guarantee that you will continue to
584 /// treat the pointer `P` as pinned after you call this function, so that
585 /// the invariants on the `Pin` type can be upheld. If the code using the
586 /// resulting `P` does not continue to maintain the pinning invariants that
587 /// is a violation of the API contract and may lead to undefined behavior in
588 /// later (safe) operations.
590 /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used
592 #[stable(feature = "pin_into_inner", since = "1.39.0")]
594 pub unsafe fn into_inner_unchecked(pin: Pin<P>) -> P {
599 impl<P: DerefMut> Pin<P> {
600 /// Gets a pinned mutable reference from this pinned pointer.
602 /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
603 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
604 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
605 /// "Malicious" implementations of `Pointer::DerefMut` are likewise
606 /// ruled out by the contract of `Pin::new_unchecked`.
608 /// This method is useful when doing multiple calls to functions that consume the pinned type.
613 /// use std::pin::Pin;
617 /// fn method(self: Pin<&mut Self>) {
621 /// fn call_method_twice(mut self: Pin<&mut Self>) {
622 /// // `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
623 /// self.as_mut().method();
624 /// self.as_mut().method();
628 #[stable(feature = "pin", since = "1.33.0")]
630 pub fn as_mut(&mut self) -> Pin<&mut P::Target> {
631 // SAFETY: see documentation on this function
632 unsafe { Pin::new_unchecked(&mut *self.pointer) }
635 /// Assigns a new value to the memory behind the pinned reference.
637 /// This overwrites pinned data, but that is okay: its destructor gets
638 /// run before being overwritten, so no pinning guarantee is violated.
639 #[stable(feature = "pin", since = "1.33.0")]
641 pub fn set(&mut self, value: P::Target)
645 *(self.pointer) = value;
649 impl<'a, T: ?Sized> Pin<&'a T> {
650 /// Constructs a new pin by mapping the interior value.
652 /// For example, if you wanted to get a `Pin` of a field of something,
653 /// you could use this to get access to that field in one line of code.
654 /// However, there are several gotchas with these "pinning projections";
655 /// see the [`pin` module] documentation for further details on that topic.
659 /// This function is unsafe. You must guarantee that the data you return
660 /// will not move so long as the argument value does not move (for example,
661 /// because it is one of the fields of that value), and also that you do
662 /// not move out of the argument you receive to the interior function.
664 /// [`pin` module]: self#projections-and-structural-pinning
665 #[stable(feature = "pin", since = "1.33.0")]
666 pub unsafe fn map_unchecked<U, F>(self, func: F) -> Pin<&'a U>
671 let pointer = &*self.pointer;
672 let new_pointer = func(pointer);
674 // SAFETY: the safety contract for `new_unchecked` must be
675 // upheld by the caller.
676 unsafe { Pin::new_unchecked(new_pointer) }
679 /// Gets a shared reference out of a pin.
681 /// This is safe because it is not possible to move out of a shared reference.
682 /// It may seem like there is an issue here with interior mutability: in fact,
683 /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
684 /// not a problem as long as there does not also exist a `Pin<&T>` pointing
685 /// to the same data, and `RefCell<T>` does not let you create a pinned reference
686 /// to its contents. See the discussion on ["pinning projections"] for further
689 /// Note: `Pin` also implements `Deref` to the target, which can be used
690 /// to access the inner value. However, `Deref` only provides a reference
691 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
692 /// the `Pin` itself. This method allows turning the `Pin` into a reference
693 /// with the same lifetime as the original `Pin`.
695 /// ["pinning projections"]: self#projections-and-structural-pinning
696 #[stable(feature = "pin", since = "1.33.0")]
698 pub fn get_ref(self) -> &'a T {
703 impl<'a, T: ?Sized> Pin<&'a mut T> {
704 /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
705 #[stable(feature = "pin", since = "1.33.0")]
707 pub fn into_ref(self) -> Pin<&'a T> {
708 Pin { pointer: self.pointer }
711 /// Gets a mutable reference to the data inside of this `Pin`.
713 /// This requires that the data inside this `Pin` is `Unpin`.
715 /// Note: `Pin` also implements `DerefMut` to the data, which can be used
716 /// to access the inner value. However, `DerefMut` only provides a reference
717 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
718 /// the `Pin` itself. This method allows turning the `Pin` into a reference
719 /// with the same lifetime as the original `Pin`.
720 #[stable(feature = "pin", since = "1.33.0")]
722 pub fn get_mut(self) -> &'a mut T
729 /// Gets a mutable reference to the data inside of this `Pin`.
733 /// This function is unsafe. You must guarantee that you will never move
734 /// the data out of the mutable reference you receive when you call this
735 /// function, so that the invariants on the `Pin` type can be upheld.
737 /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
739 #[stable(feature = "pin", since = "1.33.0")]
741 pub unsafe fn get_unchecked_mut(self) -> &'a mut T {
745 /// Construct a new pin by mapping the interior value.
747 /// For example, if you wanted to get a `Pin` of a field of something,
748 /// you could use this to get access to that field in one line of code.
749 /// However, there are several gotchas with these "pinning projections";
750 /// see the [`pin` module] documentation for further details on that topic.
754 /// This function is unsafe. You must guarantee that the data you return
755 /// will not move so long as the argument value does not move (for example,
756 /// because it is one of the fields of that value), and also that you do
757 /// not move out of the argument you receive to the interior function.
759 /// [`pin` module]: self#projections-and-structural-pinning
760 #[stable(feature = "pin", since = "1.33.0")]
761 pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U>
764 F: FnOnce(&mut T) -> &mut U,
766 // SAFETY: the caller is responsible for not moving the
767 // value out of this reference.
768 let pointer = unsafe { Pin::get_unchecked_mut(self) };
769 let new_pointer = func(pointer);
770 // SAFETY: as the value of `this` is guaranteed to not have
771 // been moved out, this call to `new_unchecked` is safe.
772 unsafe { Pin::new_unchecked(new_pointer) }
776 #[stable(feature = "pin", since = "1.33.0")]
777 impl<P: Deref> Deref for Pin<P> {
778 type Target = P::Target;
779 fn deref(&self) -> &P::Target {
780 Pin::get_ref(Pin::as_ref(self))
784 #[stable(feature = "pin", since = "1.33.0")]
785 impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> {
786 fn deref_mut(&mut self) -> &mut P::Target {
787 Pin::get_mut(Pin::as_mut(self))
791 #[unstable(feature = "receiver_trait", issue = "none")]
792 impl<P: Receiver> Receiver for Pin<P> {}
794 #[stable(feature = "pin", since = "1.33.0")]
795 impl<P: fmt::Debug> fmt::Debug for Pin<P> {
796 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
797 fmt::Debug::fmt(&self.pointer, f)
801 #[stable(feature = "pin", since = "1.33.0")]
802 impl<P: fmt::Display> fmt::Display for Pin<P> {
803 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
804 fmt::Display::fmt(&self.pointer, f)
808 #[stable(feature = "pin", since = "1.33.0")]
809 impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
810 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
811 fmt::Pointer::fmt(&self.pointer, f)
815 // Note: this means that any impl of `CoerceUnsized` that allows coercing from
816 // a type that impls `Deref<Target=impl !Unpin>` to a type that impls
817 // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound
818 // for other reasons, though, so we just need to take care not to allow such
819 // impls to land in std.
820 #[stable(feature = "pin", since = "1.33.0")]
821 impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}
823 #[stable(feature = "pin", since = "1.33.0")]
824 impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}