//! 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. //! //! At a high level, a [Pin]\

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". Things get more subtle when discussing types that //! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning) //! for more details. //! //! By default, all types in Rust are movable. Rust allows passing all types by-value, //! and common smart-pointer types such as [Box]\ and [&mut] T allow replacing and //! moving the values they contain: you can move out of a [Box]\, or you can use [`mem::swap`]. //! [Pin]\

wraps a pointer type `P`, so [Pin]<[Box]\> functions much like a regular //! [Box]\: when a [Pin]<[Box]\> gets dropped, so do its contents, and the memory gets //! deallocated. Similarly, [Pin]<[&mut] T> is a lot like [&mut] T. However, [Pin]\

does //! not let clients actually obtain a [Box]\ or [&mut] T to pinned data, which implies that you //! cannot use operations such as [`mem::swap`]: //! //! ``` //! use std::pin::Pin; //! fn swap_pins(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]\

does *not* change the fact that a Rust compiler //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, [Pin]\

//! prevents certain *values* (pointed to by pointers wrapped in [Pin]\

) from being //! moved by making it impossible to call methods that require [&mut] T on them //! (like [`mem::swap`]). //! //! [Pin]\

can be used to wrap any pointer type `P`, and as such it interacts with //! [`Deref`] and [`DerefMut`]. A [Pin]\

where P: [Deref] should be considered //! as a "`P`-style pointer" to a pinned P::[Target] – so, a [Pin]<[Box]\> is //! an owned pointer to a pinned `T`, and a [Pin]<[Rc]\> is a reference-counted //! pointer to a pinned `T`. //! For correctness, [Pin]\

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]\

. For T: [Unpin], //! [Pin]<[Box]\> and [Box]\ 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]\

. For example, whether or not [Box]\ is //! [`Unpin`] has no effect on the behavior of [Pin]<[Box]\> (here, `T` is the //! pointed-to type). //! //! # Example: self-referential struct //! //! Before we go into more details to explain the guarantees and choices //! associated with [Pin]\

, we discuss some examples for how it might be used. //! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee). //! //! ```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, //! _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> { //! 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`][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 neighboring 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*. Only once [`drop`] returns or panics, the memory may be reused. //! //! 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. None of this is allowed for pinned data without calling [`drop`]. //! //! 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]\>). 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`][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`][Drop] //! implementation as well: if an element of your type could have been pinned, //! you must treat [`Drop`][Drop] as implicitly taking [Pin]<[&mut] Self>. //! //! For example, you could implement [`Drop`][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]\, [Box]\, or [RefCell]\. //! (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\ [Unpin] for Struct\. (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`][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]\: the destructor of [VecDeque]\ //! can fail to call [`drop`] on all elements if one of the destructors panics. This violates //! the [`Drop`][Drop] guarantee, because it can lead to elements being deallocated without //! their destructor being called. ([VecDeque]\ 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]\ and there is a [`take`][Option::take]-like operation with type //! fn([Pin]<[&mut] Struct\>) -> [Option]\, //! that operation can be used to move a `T` out of a pinned `Struct` – 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]\ //! 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(rc: Pin<&mut RefCell>) { //! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`. //! let rc_shr: &RefCell = 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]\ //! (using [RefCell]::get_pin_mut) and then move that content using the mutable //! reference we got later. //! //! ## Examples //! //! For a type like [Vec]\, both possibilities (structural pinning or not) make sense. //! A [Vec]\ 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]\ 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]\ without structural pinning could impl\ [Unpin] for [Vec]\, because the contents //! are never pinned and the [Vec]\ 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]\: [Unpin] holds for all `T`. //! It makes sense to do this for pointer types, because moving the [Box]\ //! does not actually move the `T`: the [Box]\ can be freely movable (aka [`Unpin`]) even if //! the `T` is not. In fact, even [Pin]<[Box]\> 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]\ and //! [Pin]<[Box]\>, 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). //! //! [Deref]: crate::ops::Deref //! [`Deref`]: crate::ops::Deref //! [Target]: crate::ops::Deref::Target //! [`DerefMut`]: crate::ops::DerefMut //! [`mem::swap`]: crate::mem::swap //! [`mem::forget`]: crate::mem::forget //! [Vec]: ../../std/vec/struct.Vec.html //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len //! [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]: crate::cell::RefCell //! [`drop`]: Drop::drop //! [VecDeque]: ../../std/collections/struct.VecDeque.html //! [`ptr::write`]: crate::ptr::write //! [`Future`]: crate::future::Future //! [drop-impl]: #drop-implementation //! [drop-guarantee]: #drop-guarantee //! [`poll`]: crate::future::Future::poll //! [&]: ../../std/primitive.reference.html //! [&mut]: ../../std/primitive.reference.html //! [`unsafe`]: ../../std/keyword.unsafe.html #![stable(feature = "pin", since = "1.33.0")] use crate::cmp::{self, PartialEq, PartialOrd}; use crate::fmt; use crate::hash::{Hash, Hasher}; use crate::marker::{Sized, Unpin}; use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver}; /// 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.* /// /// [`pin` module]: self // // Note: the `Clone` derive below causes unsoundness as it's possible to implement // `Clone` for mutable references. // See for more details. #[stable(feature = "pin", since = "1.33.0")] #[lang = "pin"] #[fundamental] #[repr(transparent)] #[derive(Copy, Clone)] pub struct Pin

{ pointer: P, } // The following implementations aren't derived in order to avoid soundness // issues. `&self.pointer` should not be accessible to untrusted trait // implementations. // // See for more details. #[stable(feature = "pin_trait_impls", since = "1.41.0")] impl PartialEq> for Pin

where P::Target: PartialEq, { fn eq(&self, other: &Pin) -> bool { P::Target::eq(self, other) } fn ne(&self, other: &Pin) -> bool { P::Target::ne(self, other) } } #[stable(feature = "pin_trait_impls", since = "1.41.0")] impl> Eq for Pin

{} #[stable(feature = "pin_trait_impls", since = "1.41.0")] impl PartialOrd> for Pin

where P::Target: PartialOrd, { fn partial_cmp(&self, other: &Pin) -> Option { P::Target::partial_cmp(self, other) } fn lt(&self, other: &Pin) -> bool { P::Target::lt(self, other) } fn le(&self, other: &Pin) -> bool { P::Target::le(self, other) } fn gt(&self, other: &Pin) -> bool { P::Target::gt(self, other) } fn ge(&self, other: &Pin) -> bool { P::Target::ge(self, other) } } #[stable(feature = "pin_trait_impls", since = "1.41.0")] impl> Ord for Pin

{ fn cmp(&self, other: &Self) -> cmp::Ordering { P::Target::cmp(self, other) } } #[stable(feature = "pin_trait_impls", since = "1.41.0")] impl> Hash for Pin

{ fn hash(&self, state: &mut H) { P::Target::hash(self, state); } } impl> Pin

{ /// Construct a new `Pin

` 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. #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin", since = "1.33.0")] pub const fn new(pointer: P) -> Pin

{ // SAFETY: the value pointed to is `Unpin`, and so has no requirements // around pinning. unsafe { Pin::new_unchecked(pointer) } } /// Unwraps this `Pin

` 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. #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin_into_inner", since = "1.39.0")] pub const fn into_inner(pin: Pin

) -> P { pin.pointer } } impl Pin

{ /// Construct a new `Pin

` 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

` 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(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`). /// /// Similarly, calling `Pin::new_unchecked` on an `Rc` 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(mut x: Rc) { /// let pinned = unsafe { Pin::new_unchecked(Rc::clone(&x)) }; /// { /// 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`]: crate::mem::swap #[lang = "new_unchecked"] #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin", since = "1.33.0")] pub const unsafe fn new_unchecked(pointer: P) -> Pin

{ Pin { pointer } } /// Gets a pinned shared reference from this pinned pointer. /// /// This is a generic method to go from `&Pin>` to `Pin<&T>`. /// It is safe because, as part of the contract of `Pin::new_unchecked`, /// the pointee cannot move after `Pin>` 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> { // SAFETY: see documentation on this function unsafe { Pin::new_unchecked(&*self.pointer) } } /// Unwraps this `Pin

` 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. #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin_into_inner", since = "1.39.0")] pub const unsafe fn into_inner_unchecked(pin: Pin

) -> P { pin.pointer } } impl Pin

{ /// Gets a pinned mutable reference from this pinned pointer. /// /// This is a generic method to go from `&mut Pin>` to `Pin<&mut T>`. /// It is safe because, as part of the contract of `Pin::new_unchecked`, /// the pointee cannot move after `Pin>` 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> { // SAFETY: see documentation on this function 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]: self#projections-and-structural-pinning #[stable(feature = "pin", since = "1.33.0")] pub unsafe fn map_unchecked(self, func: F) -> Pin<&'a U> where U: ?Sized, F: FnOnce(&T) -> &U, { let pointer = &*self.pointer; let new_pointer = func(pointer); // SAFETY: the safety contract for `new_unchecked` must be // upheld by the caller. unsafe { 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`. However, this is /// not a problem as long as there does not also exist a `Pin<&T>` pointing /// to the same data, and `RefCell` 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"]: self#projections-and-structural-pinning #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin", since = "1.33.0")] pub const 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. #[inline(always)] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] #[stable(feature = "pin", since = "1.33.0")] pub const 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`. #[inline(always)] #[stable(feature = "pin", since = "1.33.0")] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] pub const 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. #[inline(always)] #[stable(feature = "pin", since = "1.33.0")] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] pub const 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]: self#projections-and-structural-pinning #[stable(feature = "pin", since = "1.33.0")] pub unsafe fn map_unchecked_mut(self, func: F) -> Pin<&'a mut U> where U: ?Sized, F: FnOnce(&mut T) -> &mut U, { // SAFETY: the caller is responsible for not moving the // value out of this reference. let pointer = unsafe { Pin::get_unchecked_mut(self) }; let new_pointer = func(pointer); // SAFETY: as the value of `this` is guaranteed to not have // been moved out, this call to `new_unchecked` is safe. unsafe { Pin::new_unchecked(new_pointer) } } } impl Pin<&'static T> { /// Get a pinned reference from a static reference. /// /// This is safe, because `T` is borrowed for the `'static` lifetime, which /// never ends. #[unstable(feature = "pin_static_ref", issue = "78186")] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] pub const fn static_ref(r: &'static T) -> Pin<&'static T> { // SAFETY: The 'static borrow guarantees the data will not be // moved/invalidated until it gets dropped (which is never). unsafe { Pin::new_unchecked(r) } } } impl Pin<&'static mut T> { /// Get a pinned mutable reference from a static mutable reference. /// /// This is safe, because `T` is borrowed for the `'static` lifetime, which /// never ends. #[unstable(feature = "pin_static_ref", issue = "78186")] #[rustc_const_unstable(feature = "const_pin", issue = "76654")] pub const fn static_mut(r: &'static mut T) -> Pin<&'static mut T> { // SAFETY: The 'static borrow guarantees the data will not be // moved/invalidated until it gets dropped (which is never). unsafe { Pin::new_unchecked(r) } } } #[stable(feature = "pin", since = "1.33.0")] impl Deref for Pin

{ type Target = P::Target; fn deref(&self) -> &P::Target { Pin::get_ref(Pin::as_ref(self)) } } #[stable(feature = "pin", since = "1.33.0")] impl> DerefMut for Pin

{ fn deref_mut(&mut self) -> &mut P::Target { Pin::get_mut(Pin::as_mut(self)) } } #[unstable(feature = "receiver_trait", issue = "none")] impl Receiver for Pin

{} #[stable(feature = "pin", since = "1.33.0")] impl fmt::Debug for Pin

{ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Debug::fmt(&self.pointer, f) } } #[stable(feature = "pin", since = "1.33.0")] impl fmt::Display for Pin

{ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Display::fmt(&self.pointer, f) } } #[stable(feature = "pin", since = "1.33.0")] impl fmt::Pointer for Pin

{ 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` to a type that impls // `Deref` 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 CoerceUnsized> for Pin

where P: CoerceUnsized {} #[stable(feature = "pin", since = "1.33.0")] impl DispatchFromDyn> for Pin

where P: DispatchFromDyn {}