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Rollup merge of #62120 - GuillaumeGomez:add-missing-type-links, r=Centril

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Add missing type links in documentation

r? @rust-lang/docs
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Mazdak Farrokhzad 2019-07-26 18:56:34 +02:00 committed by GitHub
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src/libcore/pin.rs
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@ -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<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`]:
//! 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;
@ -30,15 +30,15 @@
//! ```
//!
//! 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
//! 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
//! [`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
@ -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<P>`]. For `T: Unpin`,
//! `Pin<Box<T>>` and `Box<T>` function identically, as do `Pin<&mut T>` and
//! [`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
//! 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
@ -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<Box<T>>`). 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`]`<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`*.
//! *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<T>`], [`Box<T>`], or [`RefCell<T>`].
//! (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<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
//! 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<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` has no pinning projections, so this
//! 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
//! [`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.
@ -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<T>`
//! 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
//! 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` on a pinned `Vec<T>` because that would move the (structurally pinned) contents!
//! Nor could it allow `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.
//! [`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.
//! 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`.
//! 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<P>`]: struct.Pin.html
//! [`Unpin`]: ../marker/trait.Unpin.html
@ -342,6 +348,16 @@
//! [`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
@ -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")]