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 //! 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
17 //! [`Box<T>`]: when a [`Pin`]`<`[`Box`]`<T>>` gets dropped, so do its contents, and the memory gets
18 //! deallocated. Similarly, [`Pin`]`<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does
19 //! not let clients actually obtain a [`Box<T>`] or `&mut T` to pinned data, which implies that you
20 //! cannot use operations such as [`mem::swap`]:
23 //! use std::pin::Pin;
24 //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
25 //! // `mem::swap` needs `&mut T`, but we cannot get it.
26 //! // We are stuck, we cannot swap the contents of these references.
27 //! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
28 //! // we are not allowed to use it for moving things out of the `Pin`.
32 //! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
33 //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, [`Pin<P>`]
34 //! prevents certain *values* (pointed to by pointers wrapped in [`Pin<P>`]) from being
35 //! moved by making it impossible to call methods that require `&mut T` on them
36 //! (like [`mem::swap`]).
38 //! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
39 //! [`Deref`] and [`DerefMut`]. A [`Pin<P>`] where `P: Deref` should be considered
40 //! as a "`P`-style pointer" to a pinned `P::Target` -- so, a [`Pin`]`<`[`Box`]`<T>>` is
41 //! an owned pointer to a pinned `T`, and a [`Pin`]`<`[`Rc`]`<T>>` is a reference-counted
42 //! pointer to a pinned `T`.
43 //! For correctness, [`Pin<P>`] relies on the implementations of [`Deref`] and
44 //! [`DerefMut`] not to move out of their `self` parameter, and only ever to
45 //! return a pointer to pinned data when they are called on a pinned pointer.
49 //! Many types are always freely movable, even when pinned, because they do not
50 //! rely on having a stable address. This includes all the basic types (like
51 //! [`bool`], [`i32`], and references) as well as types consisting solely of these
52 //! types. Types that do not care about pinning implement the [`Unpin`]
53 //! auto-trait, which cancels the effect of [`Pin<P>`]. For `T: Unpin`,
54 //! [`Pin`]`<`[`Box`]`<T>>` and [`Box<T>`] function identically, as do [`Pin`]`<&mut T>` and
57 //! Note that pinning and [`Unpin`] only affect the pointed-to type `P::Target`, not the pointer
58 //! type `P` itself that got wrapped in [`Pin<P>`]. For example, whether or not [`Box<T>`] is
59 //! [`Unpin`] has no effect on the behavior of [`Pin`]`<`[`Box`]`<T>>` (here, `T` is the
62 //! # Example: self-referential struct
65 //! use std::pin::Pin;
66 //! use std::marker::PhantomPinned;
67 //! use std::ptr::NonNull;
69 //! // This is a self-referential struct because the slice field points to the data field.
70 //! // We cannot inform the compiler about that with a normal reference,
71 //! // as this pattern cannot be described with the usual borrowing rules.
72 //! // Instead we use a raw pointer, though one which is known not to be null,
73 //! // as we know it's pointing at the string.
74 //! struct Unmovable {
76 //! slice: NonNull<String>,
77 //! _pin: PhantomPinned,
81 //! // To ensure the data doesn't move when the function returns,
82 //! // we place it in the heap where it will stay for the lifetime of the object,
83 //! // and the only way to access it would be through a pointer to it.
84 //! fn new(data: String) -> Pin<Box<Self>> {
85 //! let res = Unmovable {
87 //! // we only create the pointer once the data is in place
88 //! // otherwise it will have already moved before we even started
89 //! slice: NonNull::dangling(),
90 //! _pin: PhantomPinned,
92 //! let mut boxed = Box::pin(res);
94 //! let slice = NonNull::from(&boxed.data);
95 //! // we know this is safe because modifying a field doesn't move the whole struct
97 //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
98 //! Pin::get_unchecked_mut(mut_ref).slice = slice;
104 //! let unmoved = Unmovable::new("hello".to_string());
105 //! // The pointer should point to the correct location,
106 //! // so long as the struct hasn't moved.
107 //! // Meanwhile, we are free to move the pointer around.
108 //! # #[allow(unused_mut)]
109 //! let mut still_unmoved = unmoved;
110 //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
112 //! // Since our type doesn't implement Unpin, this will fail to compile:
113 //! // let mut new_unmoved = Unmovable::new("world".to_string());
114 //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
117 //! # Example: intrusive doubly-linked list
119 //! In an intrusive doubly-linked list, the collection does not actually allocate
120 //! the memory for the elements itself. Allocation is controlled by the clients,
121 //! and elements can live on a stack frame that lives shorter than the collection does.
123 //! To make this work, every element has pointers to its predecessor and successor in
124 //! the list. Elements can only be added when they are pinned, because moving the elements
125 //! around would invalidate the pointers. Moreover, the [`Drop`] implementation of a linked
126 //! list element will patch the pointers of its predecessor and successor to remove itself
129 //! Crucially, we have to be able to rely on [`drop`] being called. If an element
130 //! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it
131 //! from its neighbouring elements would become invalid, which would break the data structure.
133 //! Therefore, pinning also comes with a [`drop`]-related guarantee.
135 //! # `Drop` guarantee
137 //! The purpose of pinning is to be able to rely on the placement of some data in memory.
138 //! To make this work, not just moving the data is restricted; deallocating, repurposing, or
139 //! otherwise invalidating the memory used to store the data is restricted, too.
140 //! Concretely, for pinned data you have to maintain the invariant
141 //! that *its memory will not get invalidated or repurposed from the moment it gets pinned until
142 //! when [`drop`] is called*. Memory can be invalidated by deallocation, but also by
143 //! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
144 //! off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without
145 //! calling the destructor first.
147 //! This is exactly the kind of guarantee that the intrusive linked list from the previous
148 //! section needs to function correctly.
150 //! Notice that this guarantee does *not* mean that memory does not leak! It is still
151 //! completely okay not ever to call [`drop`] on a pinned element (e.g., you can still
152 //! call [`mem::forget`] on a [`Pin`]`<`[`Box`]`<T>>`). In the example of the doubly-linked
153 //! list, that element would just stay in the list. However you may not free or reuse the storage
154 //! *without calling [`drop`]*.
156 //! # `Drop` implementation
158 //! If your type uses pinning (such as the two examples above), you have to be careful
159 //! when implementing [`Drop`]. The [`drop`] function takes `&mut self`, but this
160 //! is called *even if your type was previously pinned*! It is as if the
161 //! compiler automatically called [`Pin::get_unchecked_mut`].
163 //! This can never cause a problem in safe code because implementing a type that
164 //! relies on pinning requires unsafe code, but be aware that deciding to make
165 //! use of pinning in your type (for example by implementing some operation on
166 //! [`Pin`]`<&Self>` or [`Pin`]`<&mut Self>`) has consequences for your [`Drop`]
167 //! implementation as well: if an element of your type could have been pinned,
168 //! you must treat [`Drop`] as implicitly taking [`Pin`]`<&mut Self>`.
170 //! For example, you could implement `Drop` as follows:
173 //! # use std::pin::Pin;
174 //! # struct Type { }
175 //! impl Drop for Type {
176 //! fn drop(&mut self) {
177 //! // `new_unchecked` is okay because we know this value is never used
178 //! // again after being dropped.
179 //! inner_drop(unsafe { Pin::new_unchecked(self)});
180 //! fn inner_drop(this: Pin<&mut Type>) {
181 //! // Actual drop code goes here.
187 //! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that
188 //! you do not accidentally use `self`/`this` in a way that is in conflict with pinning.
190 //! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically
191 //! move fields around to be able to drop them. It might even do
192 //! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use
193 //! pinning with a `#[repr(packed)]` type.
195 //! # Projections and Structural Pinning
197 //! When working with pinned structs, the question arises how one can access the
198 //! fields of that struct in a method that takes just [`Pin`]`<&mut Struct>`.
199 //! The usual approach is to write helper methods (so called *projections*)
200 //! that turn [`Pin`]`<&mut Struct>` into a reference to the field, but what
201 //! type should that reference have? Is it [`Pin`]`<&mut Field>` or `&mut Field`?
202 //! The same question arises with the fields of an `enum`, and also when considering
203 //! container/wrapper types such as [`Vec<T>`], [`Box<T>`], or [`RefCell<T>`].
204 //! (This question applies to both mutable and shared references, we just
205 //! use the more common case of mutable references here for illustration.)
207 //! It turns out that it is actually up to the author of the data structure
208 //! to decide whether the pinned projection for a particular field turns
209 //! [`Pin`]`<&mut Struct>` into [`Pin`]`<&mut Field>` or `&mut Field`. There are some
210 //! constraints though, and the most important constraint is *consistency*:
211 //! every field can be *either* projected to a pinned reference, *or* have
212 //! pinning removed as part of the projection. If both are done for the same field,
213 //! that will likely be unsound!
215 //! As the author of a data structure you get to decide for each field whether pinning
216 //! "propagates" to this field or not. Pinning that propagates is also called "structural",
217 //! because it follows the structure of the type.
218 //! In the following subsections, we describe the considerations that have to be made
219 //! for either choice.
221 //! ## Pinning *is not* structural for `field`
223 //! It may seem counter-intuitive that the field of a pinned struct might not be pinned,
224 //! but that is actually the easiest choice: if a [`Pin`]`<&mut Field>` is never created,
225 //! nothing can go wrong! So, if you decide that some field does not have structural pinning,
226 //! all you have to ensure is that you never create a pinned reference to that field.
228 //! Fields without structural pinning may have a projection method that turns
229 //! [`Pin`]`<&mut Struct>` into `&mut Field`:
232 //! # use std::pin::Pin;
233 //! # type Field = i32;
234 //! # struct Struct { field: Field }
236 //! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field {
237 //! // This is okay because `field` is never considered pinned.
238 //! unsafe { &mut self.get_unchecked_mut().field }
243 //! You may also `impl Unpin for Struct` *even if* the type of `field`
244 //! is not [`Unpin`]. What that type thinks about pinning is not relevant
245 //! when no [`Pin`]`<&mut Field>` is ever created.
247 //! ## Pinning *is* structural for `field`
249 //! The other option is to decide that pinning is "structural" for `field`,
250 //! meaning that if the struct is pinned then so is the field.
252 //! This allows writing a projection that creates a [`Pin`]`<&mut Field>`, thus
253 //! witnessing that the field is pinned:
256 //! # use std::pin::Pin;
257 //! # type Field = i32;
258 //! # struct Struct { field: Field }
260 //! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> {
261 //! // This is okay because `field` is pinned when `self` is.
262 //! unsafe { self.map_unchecked_mut(|s| &mut s.field) }
267 //! However, structural pinning comes with a few extra requirements:
269 //! 1. The struct must only be [`Unpin`] if all the structural fields are
270 //! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of
271 //! the struct it is your responsibility *not* to add something like
272 //! `impl<T> Unpin for Struct<T>`. (Notice that adding a projection operation
273 //! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break
274 //! the principle that you only have to worry about any of this if you use `unsafe`.)
275 //! 2. The destructor of the struct must not move structural fields out of its argument. This
276 //! is the exact point that was raised in the [previous section][drop-impl]: `drop` takes
277 //! `&mut self`, but the struct (and hence its fields) might have been pinned before.
278 //! You have to guarantee that you do not move a field inside your [`Drop`] implementation.
279 //! In particular, as explained previously, this means that your struct must *not*
280 //! be `#[repr(packed)]`.
281 //! See that section for how to write [`drop`] in a way that the compiler can help you
282 //! not accidentally break pinning.
283 //! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
284 //! once your struct is pinned, the memory that contains the
285 //! content is not overwritten or deallocated without calling the content's destructors.
286 //! This can be tricky, as witnessed by [`VecDeque<T>`]: the destructor of [`VecDeque<T>`]
287 //! can fail to call [`drop`] on all elements if one of the destructors panics. This violates
288 //! the [`Drop`] guarantee, because it can lead to elements being deallocated without
289 //! their destructor being called. ([`VecDeque<T>`] has no pinning projections, so this
290 //! does not cause unsoundness.)
291 //! 4. You must not offer any other operations that could lead to data being moved out of
292 //! the structural fields when your type is pinned. For example, if the struct contains an
293 //! [`Option<T>`] and there is a `take`-like operation with type
294 //! `fn(Pin<&mut Struct<T>>) -> Option<T>`,
295 //! that operation can be used to move a `T` out of a pinned `Struct<T>` -- which means
296 //! pinning cannot be structural for the field holding this data.
298 //! For a more complex example of moving data out of a pinned type, imagine if [`RefCell<T>`]
299 //! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
300 //! Then we could do the following:
302 //! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
303 //! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
304 //! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
305 //! let b = rc_shr.borrow_mut();
306 //! let content = &mut *b; // And here we have `&mut T` to the same data.
309 //! This is catastrophic, it means we can first pin the content of the [`RefCell<T>`]
310 //! (using `RefCell::get_pin_mut`) and then move that content using the mutable
311 //! reference we got later.
315 //! For a type like [`Vec<T>`], both possibilites (structural pinning or not) make sense.
316 //! A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut` methods to get
317 //! pinned references to elements. However, it could *not* allow calling
318 //! [`pop`][Vec::pop] on a pinned [`Vec<T>`] because that would move the (structurally pinned)
319 //! contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also move the
322 //! A [`Vec<T>`] without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
323 //! are never pinned and the [`Vec<T>`] itself is fine with being moved as well.
324 //! At that point pinning just has no effect on the vector at all.
326 //! In the standard library, pointer types generally do not have structural pinning,
327 //! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
328 //! It makes sense to do this for pointer types, because moving the `Box<T>`
329 //! does not actually move the `T`: the [`Box<T>`] can be freely movable (aka `Unpin`) even if
330 //! the `T` is not. In fact, even [`Pin`]`<`[`Box`]`<T>>` and [`Pin`]`<&mut T>` are always
331 //! [`Unpin`] themselves, for the same reason: their contents (the `T`) are pinned, but the
332 //! pointers themselves can be moved without moving the pinned data. For both [`Box<T>`] and
333 //! [`Pin`]`<`[`Box`]`<T>>`, whether the content is pinned is entirely independent of whether the
334 //! pointer is pinned, meaning pinning is *not* structural.
336 //! When implementing a [`Future`] combinator, you will usually need structural pinning
337 //! for the nested futures, as you need to get pinned references to them to call [`poll`].
338 //! But if your combinator contains any other data that does not need to be pinned,
339 //! you can make those fields not structural and hence freely access them with a
340 //! mutable reference even when you just have [`Pin`]`<&mut Self>` (such as in your own
341 //! [`poll`] implementation).
343 //! [`Pin<P>`]: struct.Pin.html
344 //! [`Unpin`]: ../marker/trait.Unpin.html
345 //! [`Deref`]: ../ops/trait.Deref.html
346 //! [`DerefMut`]: ../ops/trait.DerefMut.html
347 //! [`mem::swap`]: ../mem/fn.swap.html
348 //! [`mem::forget`]: ../mem/fn.forget.html
349 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
350 //! [`Vec<T>`]: ../../std/vec/struct.Vec.html
351 //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
352 //! [`Pin`]: struct.Pin.html
353 //! [`Box`]: ../../std/boxed/struct.Box.html
354 //! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop
355 //! [Vec::push]: ../../std/vec/struct.Vec.html#method.push
356 //! [`Rc`]: ../../std/rc/struct.Rc.html
357 //! [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
358 //! [`Drop`]: ../../std/ops/trait.Drop.html
359 //! [`drop`]: ../../std/ops/trait.Drop.html#tymethod.drop
360 //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
361 //! [`Option<T>`]: ../../std/option/enum.Option.html
362 //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
363 //! [`RefCell<T>`]: ../cell/struct.RefCell.html
364 //! [`None`]: ../option/enum.Option.html#variant.None
365 //! [`Some(v)`]: ../option/enum.Option.html#variant.Some
366 //! [`ptr::write`]: ../ptr/fn.write.html
367 //! [`Future`]: ../future/trait.Future.html
368 //! [drop-impl]: #drop-implementation
369 //! [drop-guarantee]: #drop-guarantee
370 //! [`poll`]: ../../std/future/trait.Future.html#tymethod.poll
371 //! [`Pin::get_unchecked_mut`]: struct.Pin.html#method.get_unchecked_mut
372 //! [`bool`]: ../../std/primitive.bool.html
373 //! [`i32`]: ../../std/primitive.i32.html
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 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
392 /// [`pin` module]: ../../std/pin/index.html
394 // Note: the `Clone` derive below causes unsoundness as it's possible to implement
395 // `Clone` for mutable references.
396 // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311> for more details.
397 #[stable(feature = "pin", since = "1.33.0")]
401 #[derive(Copy, Clone)]
406 // The following implementations aren't derived in order to avoid soundness
407 // issues. `&self.pointer` should not be accessible to untrusted trait
410 // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73> for more details.
412 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
413 impl<P
: Deref
, Q
: Deref
> PartialEq
<Pin
<Q
>> for Pin
<P
>
415 P
::Target
: PartialEq
<Q
::Target
>,
417 fn eq(&self, other
: &Pin
<Q
>) -> bool
{
418 P
::Target
::eq(self, other
)
421 fn ne(&self, other
: &Pin
<Q
>) -> bool
{
422 P
::Target
::ne(self, other
)
426 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
427 impl<P
: Deref
<Target
: Eq
>> Eq
for Pin
<P
> {}
429 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
430 impl<P
: Deref
, Q
: Deref
> PartialOrd
<Pin
<Q
>> for Pin
<P
>
432 P
::Target
: PartialOrd
<Q
::Target
>,
434 fn partial_cmp(&self, other
: &Pin
<Q
>) -> Option
<cmp
::Ordering
> {
435 P
::Target
::partial_cmp(self, other
)
438 fn lt(&self, other
: &Pin
<Q
>) -> bool
{
439 P
::Target
::lt(self, other
)
442 fn le(&self, other
: &Pin
<Q
>) -> bool
{
443 P
::Target
::le(self, other
)
446 fn gt(&self, other
: &Pin
<Q
>) -> bool
{
447 P
::Target
::gt(self, other
)
450 fn ge(&self, other
: &Pin
<Q
>) -> bool
{
451 P
::Target
::ge(self, other
)
455 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
456 impl<P
: Deref
<Target
: Ord
>> Ord
for Pin
<P
> {
457 fn cmp(&self, other
: &Self) -> cmp
::Ordering
{
458 P
::Target
::cmp(self, other
)
462 #[stable(feature = "pin_trait_impls", since = "1.41.0")]
463 impl<P
: Deref
<Target
: Hash
>> Hash
for Pin
<P
> {
464 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
465 P
::Target
::hash(self, state
);
469 impl<P
: Deref
<Target
: Unpin
>> Pin
<P
> {
470 /// Construct a new `Pin<P>` around a pointer to some data of a type that
471 /// implements [`Unpin`].
473 /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
474 /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
476 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
477 #[stable(feature = "pin", since = "1.33.0")]
479 pub fn new(pointer
: P
) -> Pin
<P
> {
480 // Safety: the value pointed to is `Unpin`, and so has no requirements
482 unsafe { Pin::new_unchecked(pointer) }
485 /// Unwraps this `Pin<P>` returning the underlying pointer.
487 /// This requires that the data inside this `Pin` is [`Unpin`] so that we
488 /// can ignore the pinning invariants when unwrapping it.
490 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
491 #[stable(feature = "pin_into_inner", since = "1.39.0")]
493 pub fn into_inner(pin
: Pin
<P
>) -> P
{
498 impl<P
: Deref
> Pin
<P
> {
499 /// Construct a new `Pin<P>` around a reference to some data of a type that
500 /// may or may not implement `Unpin`.
502 /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
507 /// This constructor is unsafe because we cannot guarantee that the data
508 /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
509 /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
510 /// not guarantee that the data `P` points to is pinned, that is a violation of
511 /// the API contract and may lead to undefined behavior in later (safe) operations.
513 /// By using this method, you are making a promise about the `P::Deref` and
514 /// `P::DerefMut` implementations, if they exist. Most importantly, they
515 /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
516 /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
517 /// and expect these methods to uphold the pinning invariants.
518 /// Moreover, by calling this method you promise that the reference `P`
519 /// dereferences to will not be moved out of again; in particular, it
520 /// must not be possible to obtain a `&mut P::Target` and then
521 /// move out of that reference (using, for example [`mem::swap`]).
523 /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
524 /// while you are able to pin it for the given lifetime `'a`, you have no control
525 /// over whether it is kept pinned once `'a` ends:
528 /// use std::pin::Pin;
530 /// fn move_pinned_ref<T>(mut a: T, mut b: T) {
532 /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
533 /// // This should mean the pointee `a` can never move again.
535 /// mem::swap(&mut a, &mut b);
536 /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even
537 /// // though we have previously pinned it! We have violated the pinning API contract.
540 /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
542 /// Similarily, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
543 /// aliases to the same data that are not subject to the pinning restrictions:
546 /// use std::pin::Pin;
548 /// fn move_pinned_rc<T>(mut x: Rc<T>) {
549 /// let pinned = unsafe { Pin::new_unchecked(x.clone()) };
551 /// let p: Pin<&T> = pinned.as_ref();
552 /// // This should mean the pointee can never move again.
555 /// let content = Rc::get_mut(&mut x).unwrap();
556 /// // Now, if `x` was the only reference, we have a mutable reference to
557 /// // data that we pinned above, which we could use to move it as we have
558 /// // seen in the previous example. We have violated the pinning API contract.
562 /// [`mem::swap`]: ../../std/mem/fn.swap.html
563 #[stable(feature = "pin", since = "1.33.0")]
565 pub unsafe fn new_unchecked(pointer
: P
) -> Pin
<P
> {
569 /// Gets a pinned shared reference from this pinned pointer.
571 /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
572 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
573 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
574 /// "Malicious" implementations of `Pointer::Deref` are likewise
575 /// ruled out by the contract of `Pin::new_unchecked`.
576 #[stable(feature = "pin", since = "1.33.0")]
578 pub fn as_ref(&self) -> Pin
<&P
::Target
> {
579 // SAFETY: see documentation on this function
580 unsafe { Pin::new_unchecked(&*self.pointer) }
583 /// Unwraps this `Pin<P>` returning the underlying pointer.
587 /// This function is unsafe. You must guarantee that you will continue to
588 /// treat the pointer `P` as pinned after you call this function, so that
589 /// the invariants on the `Pin` type can be upheld. If the code using the
590 /// resulting `P` does not continue to maintain the pinning invariants that
591 /// is a violation of the API contract and may lead to undefined behavior in
592 /// later (safe) operations.
594 /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used
597 /// [`Unpin`]: ../../std/marker/trait.Unpin.html
598 /// [`Pin::into_inner`]: #method.into_inner
599 #[stable(feature = "pin_into_inner", since = "1.39.0")]
601 pub unsafe fn into_inner_unchecked(pin
: Pin
<P
>) -> P
{
606 impl<P
: DerefMut
> Pin
<P
> {
607 /// Gets a pinned mutable reference from this pinned pointer.
609 /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
610 /// It is safe because, as part of the contract of `Pin::new_unchecked`,
611 /// the pointee cannot move after `Pin<Pointer<T>>` got created.
612 /// "Malicious" implementations of `Pointer::DerefMut` are likewise
613 /// ruled out by the contract of `Pin::new_unchecked`.
615 /// This method is useful when doing multiple calls to functions that consume the pinned type.
620 /// use std::pin::Pin;
624 /// fn method(self: Pin<&mut Self>) {
628 /// fn call_method_twice(mut self: Pin<&mut Self>) {
629 /// // `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
630 /// self.as_mut().method();
631 /// self.as_mut().method();
635 #[stable(feature = "pin", since = "1.33.0")]
637 pub fn as_mut(&mut self) -> Pin
<&mut P
::Target
> {
638 // SAFETY: see documentation on this function
639 unsafe { Pin::new_unchecked(&mut *self.pointer) }
642 /// Assigns a new value to the memory behind the pinned reference.
644 /// This overwrites pinned data, but that is okay: its destructor gets
645 /// run before being overwritten, so no pinning guarantee is violated.
646 #[stable(feature = "pin", since = "1.33.0")]
648 pub fn set(&mut self, value
: P
::Target
)
652 *(self.pointer
) = value
;
656 impl<'a
, T
: ?Sized
> Pin
<&'a T
> {
657 /// Constructs a new pin by mapping the interior value.
659 /// For example, if you wanted to get a `Pin` of a field of something,
660 /// you could use this to get access to that field in one line of code.
661 /// However, there are several gotchas with these "pinning projections";
662 /// see the [`pin` module] documentation for further details on that topic.
666 /// This function is unsafe. You must guarantee that the data you return
667 /// will not move so long as the argument value does not move (for example,
668 /// because it is one of the fields of that value), and also that you do
669 /// not move out of the argument you receive to the interior function.
671 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
672 #[stable(feature = "pin", since = "1.33.0")]
673 pub unsafe fn map_unchecked
<U
, F
>(self, func
: F
) -> Pin
<&'a U
>
677 let pointer
= &*self.pointer
;
678 let new_pointer
= func(pointer
);
679 Pin
::new_unchecked(new_pointer
)
682 /// Gets a shared reference out of a pin.
684 /// This is safe because it is not possible to move out of a shared reference.
685 /// It may seem like there is an issue here with interior mutability: in fact,
686 /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
687 /// not a problem as long as there does not also exist a `Pin<&T>` pointing
688 /// to the same data, and `RefCell<T>` does not let you create a pinned reference
689 /// to its contents. See the discussion on ["pinning projections"] for further
692 /// Note: `Pin` also implements `Deref` to the target, which can be used
693 /// to access the inner value. However, `Deref` only provides a reference
694 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
695 /// the `Pin` itself. This method allows turning the `Pin` into a reference
696 /// with the same lifetime as the original `Pin`.
698 /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
699 #[stable(feature = "pin", since = "1.33.0")]
701 pub fn get_ref(self) -> &'a T
{
706 impl<'a
, T
: ?Sized
> Pin
<&'a
mut T
> {
707 /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
708 #[stable(feature = "pin", since = "1.33.0")]
710 pub fn into_ref(self) -> Pin
<&'a T
> {
711 Pin { pointer: self.pointer }
714 /// Gets a mutable reference to the data inside of this `Pin`.
716 /// This requires that the data inside this `Pin` is `Unpin`.
718 /// Note: `Pin` also implements `DerefMut` to the data, which can be used
719 /// to access the inner value. However, `DerefMut` only provides a reference
720 /// that lives for as long as the borrow of the `Pin`, not the lifetime of
721 /// the `Pin` itself. This method allows turning the `Pin` into a reference
722 /// with the same lifetime as the original `Pin`.
723 #[stable(feature = "pin", since = "1.33.0")]
725 pub fn get_mut(self) -> &'a
mut T
732 /// Gets a mutable reference to the data inside of this `Pin`.
736 /// This function is unsafe. You must guarantee that you will never move
737 /// the data out of the mutable reference you receive when you call this
738 /// function, so that the invariants on the `Pin` type can be upheld.
740 /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
742 #[stable(feature = "pin", since = "1.33.0")]
744 pub unsafe fn get_unchecked_mut(self) -> &'a
mut T
{
748 /// Construct a new pin by mapping the interior value.
750 /// For example, if you wanted to get a `Pin` of a field of something,
751 /// you could use this to get access to that field in one line of code.
752 /// However, there are several gotchas with these "pinning projections";
753 /// see the [`pin` module] documentation for further details on that topic.
757 /// This function is unsafe. You must guarantee that the data you return
758 /// will not move so long as the argument value does not move (for example,
759 /// because it is one of the fields of that value), and also that you do
760 /// not move out of the argument you receive to the interior function.
762 /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
763 #[stable(feature = "pin", since = "1.33.0")]
764 pub unsafe fn map_unchecked_mut
<U
, F
>(self, func
: F
) -> Pin
<&'a
mut U
>
766 F
: FnOnce(&mut T
) -> &mut U
,
768 let pointer
= Pin
::get_unchecked_mut(self);
769 let new_pointer
= func(pointer
);
770 Pin
::new_unchecked(new_pointer
)
774 #[stable(feature = "pin", since = "1.33.0")]
775 impl<P
: Deref
> Deref
for Pin
<P
> {
776 type Target
= P
::Target
;
777 fn deref(&self) -> &P
::Target
{
778 Pin
::get_ref(Pin
::as_ref(self))
782 #[stable(feature = "pin", since = "1.33.0")]
783 impl<P
: DerefMut
<Target
: Unpin
>> DerefMut
for Pin
<P
> {
784 fn deref_mut(&mut self) -> &mut P
::Target
{
785 Pin
::get_mut(Pin
::as_mut(self))
789 #[unstable(feature = "receiver_trait", issue = "0")]
790 impl<P
: Receiver
> Receiver
for Pin
<P
> {}
792 #[stable(feature = "pin", since = "1.33.0")]
793 impl<P
: fmt
::Debug
> fmt
::Debug
for Pin
<P
> {
794 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
795 fmt
::Debug
::fmt(&self.pointer
, f
)
799 #[stable(feature = "pin", since = "1.33.0")]
800 impl<P
: fmt
::Display
> fmt
::Display
for Pin
<P
> {
801 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
802 fmt
::Display
::fmt(&self.pointer
, f
)
806 #[stable(feature = "pin", since = "1.33.0")]
807 impl<P
: fmt
::Pointer
> fmt
::Pointer
for Pin
<P
> {
808 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
809 fmt
::Pointer
::fmt(&self.pointer
, f
)
813 // Note: this means that any impl of `CoerceUnsized` that allows coercing from
814 // a type that impls `Deref<Target=impl !Unpin>` to a type that impls
815 // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound
816 // for other reasons, though, so we just need to take care not to allow such
817 // impls to land in std.
818 #[stable(feature = "pin", since = "1.33.0")]
819 impl<P
, U
> CoerceUnsized
<Pin
<U
>> for Pin
<P
> where P
: CoerceUnsized
<U
> {}
821 #[stable(feature = "pin", since = "1.33.0")]
822 impl<P
, U
> DispatchFromDyn
<Pin
<U
>> for Pin
<P
> where P
: DispatchFromDyn
<U
> {}