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//! Types that pin data to its location in memory. //! //! It is sometimes useful to have objects that are guaranteed not to move, //! in the sense that their placement in memory does not change, and can thus be relied upon. //! A prime example of such a scenario would be building self-referential structs, //! as moving an object with pointers to itself will invalidate them, which could cause undefined //! behavior. //! //! A [`Pin<P>`] ensures that the pointee of any pointer type `P` has a stable location in memory, //! meaning it cannot be moved elsewhere and its memory cannot be deallocated //! until it gets dropped. We say that the pointee is "pinned". //! //! By default, all types in Rust are movable. Rust allows passing all types by-value, //! and common smart-pointer types such as [`Box<T>`] and `&mut T` allow replacing and //! moving the values they contain: you can move out of a [`Box<T>`], or you can use [`mem::swap`]. //! [`Pin<P>`] wraps a pointer type `P`, so [`Pin`]`<`[`Box`]`<T>>` functions much like a regular //! [`Box<T>`]: when a [`Pin`]`<`[`Box`]`<T>>` gets dropped, so do its contents, and the memory gets //! deallocated. Similarly, [`Pin`]`<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does //! not let clients actually obtain a [`Box<T>`] or `&mut T` to pinned data, which implies that you //! cannot use operations such as [`mem::swap`]: //! //! ``` //! use std::pin::Pin; //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) { //! // `mem::swap` needs `&mut T`, but we cannot get it. //! // We are stuck, we cannot swap the contents of these references. //! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason: //! // we are not allowed to use it for moving things out of the `Pin`. //! } //! ``` //! //! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, [`Pin<P>`] //! prevents certain *values* (pointed to by pointers wrapped in [`Pin<P>`]) from being //! moved by making it impossible to call methods that require `&mut T` on them //! (like [`mem::swap`]). //! //! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with //! [`Deref`] and [`DerefMut`]. A [`Pin<P>`] where `P: Deref` should be considered //! as a "`P`-style pointer" to a pinned `P::Target` -- so, a [`Pin`]`<`[`Box`]`<T>>` is //! an owned pointer to a pinned `T`, and a [`Pin`]`<`[`Rc`]`<T>>` is a reference-counted //! pointer to a pinned `T`. //! For correctness, [`Pin<P>`] relies on the implementations of [`Deref`] and //! [`DerefMut`] not to move out of their `self` parameter, and only ever to //! return a pointer to pinned data when they are called on a pinned pointer. //! //! # `Unpin` //! //! Many types are always freely movable, even when pinned, because they do not //! rely on having a stable address. This includes all the basic types (like //! [`bool`], [`i32`], and references) as well as types consisting solely of these //! types. Types that do not care about pinning implement the [`Unpin`] //! auto-trait, which cancels the effect of [`Pin<P>`]. For `T: Unpin`, //! [`Pin`]`<`[`Box`]`<T>>` and [`Box<T>`] function identically, as do [`Pin`]`<&mut T>` and //! `&mut T`. //! //! Note that pinning and [`Unpin`] only affect the pointed-to type `P::Target`, not the pointer //! type `P` itself that got wrapped in [`Pin<P>`]. For example, whether or not [`Box<T>`] is //! [`Unpin`] has no effect on the behavior of [`Pin`]`<`[`Box`]`<T>>` (here, `T` is the //! pointed-to type). //! //! # Example: self-referential struct //! //! ```rust //! use std::pin::Pin; //! use std::marker::PhantomPinned; //! use std::ptr::NonNull; //! //! // This is a self-referential struct because the slice field points to the data field. //! // We cannot inform the compiler about that with a normal reference, //! // as this pattern cannot be described with the usual borrowing rules. //! // Instead we use a raw pointer, though one which is known not to be null, //! // as we know it's pointing at the string. //! struct Unmovable { //! data: String, //! slice: NonNull<String>, //! _pin: PhantomPinned, //! } //! //! impl Unmovable { //! // To ensure the data doesn't move when the function returns, //! // we place it in the heap where it will stay for the lifetime of the object, //! // and the only way to access it would be through a pointer to it. //! fn new(data: String) -> Pin<Box<Self>> { //! let res = Unmovable { //! data, //! // we only create the pointer once the data is in place //! // otherwise it will have already moved before we even started //! slice: NonNull::dangling(), //! _pin: PhantomPinned, //! }; //! let mut boxed = Box::pin(res); //! //! let slice = NonNull::from(&boxed.data); //! // we know this is safe because modifying a field doesn't move the whole struct //! unsafe { //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed); //! Pin::get_unchecked_mut(mut_ref).slice = slice; //! } //! boxed //! } //! } //! //! let unmoved = Unmovable::new("hello".to_string()); //! // The pointer should point to the correct location, //! // so long as the struct hasn't moved. //! // Meanwhile, we are free to move the pointer around. //! # #[allow(unused_mut)] //! let mut still_unmoved = unmoved; //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data)); //! //! // Since our type doesn't implement Unpin, this will fail to compile: //! // let mut new_unmoved = Unmovable::new("world".to_string()); //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved); //! ``` //! //! # Example: intrusive doubly-linked list //! //! In an intrusive doubly-linked list, the collection does not actually allocate //! the memory for the elements itself. Allocation is controlled by the clients, //! and elements can live on a stack frame that lives shorter than the collection does. //! //! To make this work, every element has pointers to its predecessor and successor in //! the list. Elements can only be added when they are pinned, because moving the elements //! around would invalidate the pointers. Moreover, the [`Drop`] implementation of a linked //! list element will patch the pointers of its predecessor and successor to remove itself //! from the list. //! //! Crucially, we have to be able to rely on [`drop`] being called. If an element //! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it //! from its neighbouring elements would become invalid, which would break the data structure. //! //! Therefore, pinning also comes with a [`drop`]-related guarantee. //! //! # `Drop` guarantee //! //! The purpose of pinning is to be able to rely on the placement of some data in memory. //! To make this work, not just moving the data is restricted; deallocating, repurposing, or //! otherwise invalidating the memory used to store the data is restricted, too. //! Concretely, for pinned data you have to maintain the invariant //! that *its memory will not get invalidated or repurposed from the moment it gets pinned until //! when [`drop`] is called*. Memory can be invalidated by deallocation, but also by //! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements //! off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without //! calling the destructor first. //! //! This is exactly the kind of guarantee that the intrusive linked list from the previous //! section needs to function correctly. //! //! Notice that this guarantee does *not* mean that memory does not leak! It is still //! completely okay not ever to call [`drop`] on a pinned element (e.g., you can still //! call [`mem::forget`] on a [`Pin`]`<`[`Box`]`<T>>`). In the example of the doubly-linked //! list, that element would just stay in the list. However you may not free or reuse the storage //! *without calling [`drop`]*. //! //! # `Drop` implementation //! //! If your type uses pinning (such as the two examples above), you have to be careful //! when implementing [`Drop`]. The [`drop`] function takes `&mut self`, but this //! is called *even if your type was previously pinned*! It is as if the //! compiler automatically called [`Pin::get_unchecked_mut`]. //! //! This can never cause a problem in safe code because implementing a type that //! relies on pinning requires unsafe code, but be aware that deciding to make //! use of pinning in your type (for example by implementing some operation on //! [`Pin`]`<&Self>` or [`Pin`]`<&mut Self>`) has consequences for your [`Drop`] //! implementation as well: if an element of your type could have been pinned, //! you must treat [`Drop`] as implicitly taking [`Pin`]`<&mut Self>`. //! //! For example, you could implement `Drop` as follows: //! //! ```rust,no_run //! # use std::pin::Pin; //! # struct Type { } //! impl Drop for Type { //! fn drop(&mut self) { //! // `new_unchecked` is okay because we know this value is never used //! // again after being dropped. //! inner_drop(unsafe { Pin::new_unchecked(self)}); //! fn inner_drop(this: Pin<&mut Type>) { //! // Actual drop code goes here. //! } //! } //! } //! ``` //! //! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that //! you do not accidentally use `self`/`this` in a way that is in conflict with pinning. //! //! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically //! move fields around to be able to drop them. It might even do //! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use //! pinning with a `#[repr(packed)]` type. //! //! # Projections and Structural Pinning //! //! When working with pinned structs, the question arises how one can access the //! fields of that struct in a method that takes just [`Pin`]`<&mut Struct>`. //! The usual approach is to write helper methods (so called *projections*) //! that turn [`Pin`]`<&mut Struct>` into a reference to the field, but what //! type should that reference have? Is it [`Pin`]`<&mut Field>` or `&mut Field`? //! The same question arises with the fields of an `enum`, and also when considering //! container/wrapper types such as [`Vec<T>`], [`Box<T>`], or [`RefCell<T>`]. //! (This question applies to both mutable and shared references, we just //! use the more common case of mutable references here for illustration.) //! //! It turns out that it is actually up to the author of the data structure //! to decide whether the pinned projection for a particular field turns //! [`Pin`]`<&mut Struct>` into [`Pin`]`<&mut Field>` or `&mut Field`. There are some //! constraints though, and the most important constraint is *consistency*: //! every field can be *either* projected to a pinned reference, *or* have //! pinning removed as part of the projection. If both are done for the same field, //! that will likely be unsound! //! //! As the author of a data structure you get to decide for each field whether pinning //! "propagates" to this field or not. Pinning that propagates is also called "structural", //! because it follows the structure of the type. //! In the following subsections, we describe the considerations that have to be made //! for either choice. //! //! ## Pinning *is not* structural for `field` //! //! It may seem counter-intuitive that the field of a pinned struct might not be pinned, //! but that is actually the easiest choice: if a [`Pin`]`<&mut Field>` is never created, //! nothing can go wrong! So, if you decide that some field does not have structural pinning, //! all you have to ensure is that you never create a pinned reference to that field. //! //! Fields without structural pinning may have a projection method that turns //! [`Pin`]`<&mut Struct>` into `&mut Field`: //! //! ```rust,no_run //! # use std::pin::Pin; //! # type Field = i32; //! # struct Struct { field: Field } //! impl Struct { //! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field { //! // This is okay because `field` is never considered pinned. //! unsafe { &mut self.get_unchecked_mut().field } //! } //! } //! ``` //! //! You may also `impl Unpin for Struct` *even if* the type of `field` //! is not [`Unpin`]. What that type thinks about pinning is not relevant //! when no [`Pin`]`<&mut Field>` is ever created. //! //! ## Pinning *is* structural for `field` //! //! The other option is to decide that pinning is "structural" for `field`, //! meaning that if the struct is pinned then so is the field. //! //! This allows writing a projection that creates a [`Pin`]`<&mut Field>`, thus //! witnessing that the field is pinned: //! //! ```rust,no_run //! # use std::pin::Pin; //! # type Field = i32; //! # struct Struct { field: Field } //! impl Struct { //! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> { //! // This is okay because `field` is pinned when `self` is. //! unsafe { self.map_unchecked_mut(|s| &mut s.field) } //! } //! } //! ``` //! //! However, structural pinning comes with a few extra requirements: //! //! 1. The struct must only be [`Unpin`] if all the structural fields are //! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of //! the struct it is your responsibility *not* to add something like //! `impl<T> Unpin for Struct<T>`. (Notice that adding a projection operation //! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break //! the principle that you only have to worry about any of this if you use `unsafe`.) //! 2. The destructor of the struct must not move structural fields out of its argument. This //! is the exact point that was raised in the [previous section][drop-impl]: `drop` takes //! `&mut self`, but the struct (and hence its fields) might have been pinned before. //! You have to guarantee that you do not move a field inside your [`Drop`] implementation. //! In particular, as explained previously, this means that your struct must *not* //! be `#[repr(packed)]`. //! See that section for how to write [`drop`] in a way that the compiler can help you //! not accidentally break pinning. //! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]: //! once your struct is pinned, the memory that contains the //! content is not overwritten or deallocated without calling the content's destructors. //! This can be tricky, as witnessed by [`VecDeque<T>`]: the destructor of [`VecDeque<T>`] //! can fail to call [`drop`] on all elements if one of the destructors panics. This violates //! the [`Drop`] guarantee, because it can lead to elements being deallocated without //! their destructor being called. ([`VecDeque<T>`] has no pinning projections, so this //! does not cause unsoundness.) //! 4. You must not offer any other operations that could lead to data being moved out of //! the structural fields when your type is pinned. For example, if the struct contains an //! [`Option<T>`] and there is a `take`-like operation with type //! `fn(Pin<&mut Struct<T>>) -> Option<T>`, //! that operation can be used to move a `T` out of a pinned `Struct<T>` -- which means //! pinning cannot be structural for the field holding this data. //! //! For a more complex example of moving data out of a pinned type, imagine if [`RefCell<T>`] //! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`. //! Then we could do the following: //! ```compile_fail //! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) { //! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`. //! let rc_shr: &RefCell<T> = rc.into_ref().get_ref(); //! let b = rc_shr.borrow_mut(); //! let content = &mut *b; // And here we have `&mut T` to the same data. //! } //! ``` //! This is catastrophic, it means we can first pin the content of the [`RefCell<T>`] //! (using `RefCell::get_pin_mut`) and then move that content using the mutable //! reference we got later. //! //! ## Examples //! //! For a type like [`Vec<T>`], both possibilites (structural pinning or not) make sense. //! A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut` methods to get //! pinned references to elements. However, it could *not* allow calling //! [`pop`][Vec::pop] on a pinned [`Vec<T>`] because that would move the (structurally pinned) //! contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also move the //! contents. //! //! A [`Vec<T>`] without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents //! are never pinned and the [`Vec<T>`] itself is fine with being moved as well. //! At that point pinning just has no effect on the vector at all. //! //! In the standard library, pointer types generally do not have structural pinning, //! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`. //! It makes sense to do this for pointer types, because moving the `Box<T>` //! does not actually move the `T`: the [`Box<T>`] can be freely movable (aka `Unpin`) even if //! the `T` is not. In fact, even [`Pin`]`<`[`Box`]`<T>>` and [`Pin`]`<&mut T>` are always //! [`Unpin`] themselves, for the same reason: their contents (the `T`) are pinned, but the //! pointers themselves can be moved without moving the pinned data. For both [`Box<T>`] and //! [`Pin`]`<`[`Box`]`<T>>`, whether the content is pinned is entirely independent of whether the //! pointer is pinned, meaning pinning is *not* structural. //! //! When implementing a [`Future`] combinator, you will usually need structural pinning //! for the nested futures, as you need to get pinned references to them to call [`poll`]. //! But if your combinator contains any other data that does not need to be pinned, //! you can make those fields not structural and hence freely access them with a //! mutable reference even when you just have [`Pin`]`<&mut Self>` (such as in your own //! [`poll`] implementation). //! //! [`Pin<P>`]: struct.Pin.html //! [`Unpin`]: ../marker/trait.Unpin.html //! [`Deref`]: ../ops/trait.Deref.html //! [`DerefMut`]: ../ops/trait.DerefMut.html //! [`mem::swap`]: ../mem/fn.swap.html //! [`mem::forget`]: ../mem/fn.forget.html //! [`Box<T>`]: ../../std/boxed/struct.Box.html //! [`Vec<T>`]: ../../std/vec/struct.Vec.html //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len //! [`Pin`]: struct.Pin.html //! [`Box`]: ../../std/boxed/struct.Box.html //! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop //! [Vec::push]: ../../std/vec/struct.Vec.html#method.push //! [`Rc`]: ../../std/rc/struct.Rc.html //! [`RefCell<T>`]: ../../std/cell/struct.RefCell.html //! [`Drop`]: ../../std/ops/trait.Drop.html //! [`drop`]: ../../std/ops/trait.Drop.html#tymethod.drop //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html //! [`Option<T>`]: ../../std/option/enum.Option.html //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html //! [`RefCell<T>`]: ../cell/struct.RefCell.html //! [`None`]: ../option/enum.Option.html#variant.None //! [`Some(v)`]: ../option/enum.Option.html#variant.Some //! [`ptr::write`]: ../ptr/fn.write.html //! [`Future`]: ../future/trait.Future.html //! [drop-impl]: #drop-implementation //! [drop-guarantee]: #drop-guarantee //! [`poll`]: ../../std/future/trait.Future.html#tymethod.poll //! [`Pin::get_unchecked_mut`]: struct.Pin.html#method.get_unchecked_mut #![stable(feature = "pin", since = "1.33.0")] use crate::fmt; use crate::marker::{Sized, Unpin}; use crate::cmp::{self, PartialEq, PartialOrd}; use crate::ops::{Deref, DerefMut, Receiver, CoerceUnsized, DispatchFromDyn}; /// A pinned pointer. /// /// This is a wrapper around a kind of pointer which makes that pointer "pin" its /// value in place, preventing the value referenced by that pointer from being moved /// unless it implements [`Unpin`]. /// /// *See the [`pin` module] documentation for an explanation of pinning.* /// /// [`Unpin`]: ../../std/marker/trait.Unpin.html /// [`pin` module]: ../../std/pin/index.html // // Note: the derives below, and the explicit `PartialEq` and `PartialOrd` // implementations, are allowed because they all only use `&P`, so they cannot move // the value behind `pointer`. #[stable(feature = "pin", since = "1.33.0")] #[lang = "pin"] #[fundamental] #[repr(transparent)] #[derive(Copy, Clone, Hash, Eq, Ord)] pub struct Pin<P> { pointer: P, } #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")] impl<P, Q> PartialEq<Pin<Q>> for Pin<P> where P: PartialEq<Q>, { fn eq(&self, other: &Pin<Q>) -> bool { self.pointer == other.pointer } fn ne(&self, other: &Pin<Q>) -> bool { self.pointer != other.pointer } } #[stable(feature = "pin_partialeq_partialord_impl_applicability", since = "1.34.0")] impl<P, Q> PartialOrd<Pin<Q>> for Pin<P> where P: PartialOrd<Q>, { fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> { self.pointer.partial_cmp(&other.pointer) } fn lt(&self, other: &Pin<Q>) -> bool { self.pointer < other.pointer } fn le(&self, other: &Pin<Q>) -> bool { self.pointer <= other.pointer } fn gt(&self, other: &Pin<Q>) -> bool { self.pointer > other.pointer } fn ge(&self, other: &Pin<Q>) -> bool { self.pointer >= other.pointer } } impl<P: Deref<Target: Unpin>> Pin<P> { /// Construct a new `Pin<P>` around a pointer to some data of a type that /// implements [`Unpin`]. /// /// Unlike `Pin::new_unchecked`, this method is safe because the pointer /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees. /// /// [`Unpin`]: ../../std/marker/trait.Unpin.html #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn new(pointer: P) -> Pin<P> { // Safety: the value pointed to is `Unpin`, and so has no requirements // around pinning. unsafe { Pin::new_unchecked(pointer) } } /// Unwraps this `Pin<P>` returning the underlying pointer. /// /// This requires that the data inside this `Pin` is [`Unpin`] so that we /// can ignore the pinning invariants when unwrapping it. /// /// [`Unpin`]: ../../std/marker/trait.Unpin.html #[stable(feature = "pin_into_inner", since = "1.39.0")] #[inline(always)] pub fn into_inner(pin: Pin<P>) -> P { pin.pointer } } impl<P: Deref> Pin<P> { /// Construct a new `Pin<P>` around a reference to some data of a type that /// may or may not implement `Unpin`. /// /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used /// instead. /// /// # Safety /// /// This constructor is unsafe because we cannot guarantee that the data /// pointed to by `pointer` is pinned, meaning that the data will not be moved or /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does /// not guarantee that the data `P` points to is pinned, that is a violation of /// the API contract and may lead to undefined behavior in later (safe) operations. /// /// By using this method, you are making a promise about the `P::Deref` and /// `P::DerefMut` implementations, if they exist. Most importantly, they /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref` /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer* /// and expect these methods to uphold the pinning invariants. /// Moreover, by calling this method you promise that the reference `P` /// dereferences to will not be moved out of again; in particular, it /// must not be possible to obtain a `&mut P::Target` and then /// move out of that reference (using, for example [`mem::swap`]). /// /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because /// while you are able to pin it for the given lifetime `'a`, you have no control /// over whether it is kept pinned once `'a` ends: /// ``` /// use std::mem; /// use std::pin::Pin; /// /// fn move_pinned_ref<T>(mut a: T, mut b: T) { /// unsafe { /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a); /// // This should mean the pointee `a` can never move again. /// } /// mem::swap(&mut a, &mut b); /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even /// // though we have previously pinned it! We have violated the pinning API contract. /// } /// ``` /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`). /// /// Similarily, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be /// aliases to the same data that are not subject to the pinning restrictions: /// ``` /// use std::rc::Rc; /// use std::pin::Pin; /// /// fn move_pinned_rc<T>(mut x: Rc<T>) { /// let pinned = unsafe { Pin::new_unchecked(x.clone()) }; /// { /// let p: Pin<&T> = pinned.as_ref(); /// // This should mean the pointee can never move again. /// } /// drop(pinned); /// let content = Rc::get_mut(&mut x).unwrap(); /// // Now, if `x` was the only reference, we have a mutable reference to /// // data that we pinned above, which we could use to move it as we have /// // seen in the previous example. We have violated the pinning API contract. /// } /// ``` /// /// [`mem::swap`]: ../../std/mem/fn.swap.html #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub unsafe fn new_unchecked(pointer: P) -> Pin<P> { Pin { pointer } } /// Gets a pinned shared reference from this pinned pointer. /// /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`. /// It is safe because, as part of the contract of `Pin::new_unchecked`, /// the pointee cannot move after `Pin<Pointer<T>>` got created. /// "Malicious" implementations of `Pointer::Deref` are likewise /// ruled out by the contract of `Pin::new_unchecked`. #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn as_ref(&self) -> Pin<&P::Target> { unsafe { Pin::new_unchecked(&*self.pointer) } } /// Unwraps this `Pin<P>` returning the underlying pointer. /// /// # Safety /// /// This function is unsafe. You must guarantee that you will continue to /// treat the pointer `P` as pinned after you call this function, so that /// the invariants on the `Pin` type can be upheld. If the code using the /// resulting `P` does not continue to maintain the pinning invariants that /// is a violation of the API contract and may lead to undefined behavior in /// later (safe) operations. /// /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used /// instead. /// /// [`Unpin`]: ../../std/marker/trait.Unpin.html /// [`Pin::into_inner`]: #method.into_inner #[stable(feature = "pin_into_inner", since = "1.39.0")] #[inline(always)] pub unsafe fn into_inner_unchecked(pin: Pin<P>) -> P { pin.pointer } } impl<P: DerefMut> Pin<P> { /// Gets a pinned mutable reference from this pinned pointer. /// /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`. /// It is safe because, as part of the contract of `Pin::new_unchecked`, /// the pointee cannot move after `Pin<Pointer<T>>` got created. /// "Malicious" implementations of `Pointer::DerefMut` are likewise /// ruled out by the contract of `Pin::new_unchecked`. /// /// This method is useful when doing multiple calls to functions that consume the pinned type. /// /// # Example /// /// ``` /// use std::pin::Pin; /// /// # struct Type {} /// impl Type { /// fn method(self: Pin<&mut Self>) { /// // do something /// } /// /// fn call_method_twice(mut self: Pin<&mut Self>) { /// // `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`. /// self.as_mut().method(); /// self.as_mut().method(); /// } /// } /// ``` #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn as_mut(&mut self) -> Pin<&mut P::Target> { unsafe { Pin::new_unchecked(&mut *self.pointer) } } /// Assigns a new value to the memory behind the pinned reference. /// /// This overwrites pinned data, but that is okay: its destructor gets /// run before being overwritten, so no pinning guarantee is violated. #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn set(&mut self, value: P::Target) where P::Target: Sized, { *(self.pointer) = value; } } impl<'a, T: ?Sized> Pin<&'a T> { /// Constructs a new pin by mapping the interior value. /// /// For example, if you wanted to get a `Pin` of a field of something, /// you could use this to get access to that field in one line of code. /// However, there are several gotchas with these "pinning projections"; /// see the [`pin` module] documentation for further details on that topic. /// /// # Safety /// /// This function is unsafe. You must guarantee that the data you return /// will not move so long as the argument value does not move (for example, /// because it is one of the fields of that value), and also that you do /// not move out of the argument you receive to the interior function. /// /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning #[stable(feature = "pin", since = "1.33.0")] pub unsafe fn map_unchecked<U, F>(self, func: F) -> Pin<&'a U> where F: FnOnce(&T) -> &U, { let pointer = &*self.pointer; let new_pointer = func(pointer); Pin::new_unchecked(new_pointer) } /// Gets a shared reference out of a pin. /// /// This is safe because it is not possible to move out of a shared reference. /// It may seem like there is an issue here with interior mutability: in fact, /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is /// not a problem as long as there does not also exist a `Pin<&T>` pointing /// to the same data, and `RefCell<T>` does not let you create a pinned reference /// to its contents. See the discussion on ["pinning projections"] for further /// details. /// /// Note: `Pin` also implements `Deref` to the target, which can be used /// to access the inner value. However, `Deref` only provides a reference /// that lives for as long as the borrow of the `Pin`, not the lifetime of /// the `Pin` itself. This method allows turning the `Pin` into a reference /// with the same lifetime as the original `Pin`. /// /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn get_ref(self) -> &'a T { self.pointer } } impl<'a, T: ?Sized> Pin<&'a mut T> { /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime. #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn into_ref(self) -> Pin<&'a T> { Pin { pointer: self.pointer } } /// Gets a mutable reference to the data inside of this `Pin`. /// /// This requires that the data inside this `Pin` is `Unpin`. /// /// Note: `Pin` also implements `DerefMut` to the data, which can be used /// to access the inner value. However, `DerefMut` only provides a reference /// that lives for as long as the borrow of the `Pin`, not the lifetime of /// the `Pin` itself. This method allows turning the `Pin` into a reference /// with the same lifetime as the original `Pin`. #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub fn get_mut(self) -> &'a mut T where T: Unpin, { self.pointer } /// Gets a mutable reference to the data inside of this `Pin`. /// /// # Safety /// /// This function is unsafe. You must guarantee that you will never move /// the data out of the mutable reference you receive when you call this /// function, so that the invariants on the `Pin` type can be upheld. /// /// If the underlying data is `Unpin`, `Pin::get_mut` should be used /// instead. #[stable(feature = "pin", since = "1.33.0")] #[inline(always)] pub unsafe fn get_unchecked_mut(self) -> &'a mut T { self.pointer } /// Construct a new pin by mapping the interior value. /// /// For example, if you wanted to get a `Pin` of a field of something, /// you could use this to get access to that field in one line of code. /// However, there are several gotchas with these "pinning projections"; /// see the [`pin` module] documentation for further details on that topic. /// /// # Safety /// /// This function is unsafe. You must guarantee that the data you return /// will not move so long as the argument value does not move (for example, /// because it is one of the fields of that value), and also that you do /// not move out of the argument you receive to the interior function. /// /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning #[stable(feature = "pin", since = "1.33.0")] pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U> where F: FnOnce(&mut T) -> &mut U, { let pointer = Pin::get_unchecked_mut(self); let new_pointer = func(pointer); Pin::new_unchecked(new_pointer) } } #[stable(feature = "pin", since = "1.33.0")] impl<P: Deref> Deref for Pin<P> { type Target = P::Target; fn deref(&self) -> &P::Target { Pin::get_ref(Pin::as_ref(self)) } } #[stable(feature = "pin", since = "1.33.0")] impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> { fn deref_mut(&mut self) -> &mut P::Target { Pin::get_mut(Pin::as_mut(self)) } } #[unstable(feature = "receiver_trait", issue = "0")] impl<P: Receiver> Receiver for Pin<P> {} #[stable(feature = "pin", since = "1.33.0")] impl<P: fmt::Debug> fmt::Debug for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Debug::fmt(&self.pointer, f) } } #[stable(feature = "pin", since = "1.33.0")] impl<P: fmt::Display> fmt::Display for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Display::fmt(&self.pointer, f) } } #[stable(feature = "pin", since = "1.33.0")] impl<P: fmt::Pointer> fmt::Pointer for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Pointer::fmt(&self.pointer, f) } } // Note: this means that any impl of `CoerceUnsized` that allows coercing from // a type that impls `Deref<Target=impl !Unpin>` to a type that impls // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound // for other reasons, though, so we just need to take care not to allow such // impls to land in std. #[stable(feature = "pin", since = "1.33.0")] impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U>, {} #[stable(feature = "pin", since = "1.33.0")] impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U>, {}