diff --git a/src/libcore/pin.rs b/src/libcore/pin.rs index c063cee52270..2feaab7a09c9 100644 --- a/src/libcore/pin.rs +++ b/src/libcore/pin.rs @@ -11,13 +11,13 @@ //! 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` 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>` functions much like a regular `Box`: -//! when a `Pin>` 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`]: +//! 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; @@ -30,15 +30,15 @@ //! ``` //! //! 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 +//! 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>` is -//! an owned pointer to a pinned `T`, and a `Pin>` is a reference-counted +//! [`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 @@ -48,15 +48,15 @@ //! //! 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 +//! [`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>` and `Box` function identically, as do `Pin<&mut T>` and +//! [`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>` (here, `T` is the +//! 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 @@ -122,15 +122,15 @@ //! //! 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 +//! 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 +//! 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. +//! Therefore, pinning also comes with a [`drop`]-related guarantee. //! //! # `Drop` guarantee //! @@ -139,7 +139,7 @@ //! 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 +//! 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. @@ -148,26 +148,27 @@ //! 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>`). In the example of the doubly-linked +//! 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`*. +//! *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 +//! 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 `get_unchecked_mut`. +//! 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` +//! [`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>`. +//! 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 { } @@ -182,7 +183,8 @@ //! } //! } //! ``` -//! The function `inner_drop` has the type that `drop` *should* have, so this makes sure that +//! +//! 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 @@ -192,10 +194,10 @@ //! # 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>`. +//! 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`? +//! 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 @@ -203,7 +205,7 @@ //! //! 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 +//! [`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, @@ -218,12 +220,13 @@ //! ## 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, +//! 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`: +//! [`Pin`]`<&mut Struct>` into `&mut Field`: +//! //! ```rust,no_run //! # use std::pin::Pin; //! # type Field = i32; @@ -237,16 +240,17 @@ //! ``` //! //! 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. +//! 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 +//! 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; @@ -262,30 +266,30 @@ //! 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 +//! [`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 +//! 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. +//! 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 +//! 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` guarantee, because it can lead to elements being deallocated without -//! their destructor being called. (`VecDeque` has no pinning projections, so this +//! 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`] 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`-like operation with type +//! [`Option`] and there is a `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. @@ -301,37 +305,39 @@ //! 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` +//! 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 possibilites (structural pinning or not) make sense. -//! A `Vec` with structural pinning could have `get_pin`/`get_pin_mut` methods to get +//! 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` on a pinned `Vec` because that would move the (structurally pinned) contents! -//! Nor could it allow `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. +//! [`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>` 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>`, -//! whether the content is pinned is entirely independent of whether the pointer is -//! pinned, meaning pinning is *not* structural. +//! 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`. +//! 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). +//! mutable reference even when you just have [`Pin`]`<&mut Self>` (such as in your own +//! [`poll`] implementation). //! //! [`Pin

`]: struct.Pin.html //! [`Unpin`]: ../marker/trait.Unpin.html @@ -342,6 +348,16 @@ //! [`Box`]: ../../std/boxed/struct.Box.html //! [`Vec`]: ../../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`]: ../../std/cell/struct.RefCell.html +//! [`Drop`]: ../../std/ops/trait.Drop.html +//! [`drop`]: ../../std/ops/trait.Drop.html#tymethod.drop +//! [`VecDeque`]: ../../std/collections/struct.VecDeque.html +//! [`Option`]: ../../std/option/enum.Option.html //! [`VecDeque`]: ../../std/collections/struct.VecDeque.html //! [`RefCell`]: ../cell/struct.RefCell.html //! [`None`]: ../option/enum.Option.html#variant.None @@ -350,6 +366,8 @@ //! [`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")]