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Exclude single type parameters from links in core::pin for more vis…
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…ual consistency.
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steffahn committed Jan 5, 2021
1 parent d3915c5 commit ceaeb24
Showing 1 changed file with 33 additions and 35 deletions.
68 changes: 33 additions & 35 deletions library/core/src/pin.rs
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//! as moving an object with pointers to itself will invalidate them, which could cause undefined
//! behavior.
//!
//! At a high level, a [`Pin<P>`] ensures that the pointee of any pointer type
//! At a high level, a <code>[Pin]\<P></code> 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<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 <code>[Pin]<[Box]\<T>></code> functions much like a regular
//! [`Box<T>`]: when a <code>[Pin]<[Box]\<T>></code> gets dropped, so do its contents, and the memory gets
//! deallocated. Similarly, <code>[Pin]<&mut T></code> 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
//! and common smart-pointer types such as <code>[Box]\<T></code> and `&mut T` allow replacing and
//! moving the values they contain: you can move out of a <code>[Box]\<T></code>, or you can use [`mem::swap`].
//! <code>[Pin]\<P></code> wraps a pointer type `P`, so <code>[Pin]<[Box]\<T>></code> functions much like a regular
//! <code>[Box]\<T></code>: when a <code>[Pin]<[Box]\<T>></code> gets dropped, so do its contents, and the memory gets
//! deallocated. Similarly, <code>[Pin]<&mut T></code> is a lot like `&mut T`. However, <code>[Pin]\<P></code> does
//! not let clients actually obtain a <code>[Box]\<T></code> or `&mut T` to pinned data, which implies that you
//! cannot use operations such as [`mem::swap`]:
//!
//! ```
Expand All @@ -32,18 +32,18 @@
//! }
//! ```
//!
//! 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
//! It is worth reiterating that <code>[Pin]\<P></code> does *not* change the fact that a Rust compiler
//! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, <code>[Pin]\<P></code>
//! prevents certain *values* (pointed to by pointers wrapped in <code>[Pin]\<P></code>) 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
//! <code>[Pin]\<P></code> can be used to wrap any pointer type `P`, and as such it interacts with
//! [`Deref`] and [`DerefMut`]. A <code>[Pin]\<P></code> where `P: Deref` should be considered
//! as a "`P`-style pointer" to a pinned `P::Target` -- so, a <code>[Pin]<[Box]\<T>></code> is
//! an owned pointer to a pinned `T`, and a <code>[Pin]<[Rc]\<T>></code> is a reference-counted
//! pointer to a pinned `T`.
//! For correctness, [`Pin<P>`] relies on the implementations of [`Deref`] and
//! For correctness, <code>[Pin]\<P></code> 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.
//!
Expand All @@ -53,12 +53,12 @@
//! 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`,
//! <code>[Pin]<[Box]\<T>></code> and [`Box<T>`] function identically, as do <code>[Pin]<&mut T></code> and
//! auto-trait, which cancels the effect of <code>[Pin]\<P></code>. For `T: Unpin`,
//! <code>[Pin]<[Box]\<T>></code> and <code>[Box]\<T></code> function identically, as do <code>[Pin]<&mut T></code> 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
//! type `P` itself that got wrapped in <code>[Pin]\<P></code>. For example, whether or not <code>[Box]\<T></code> is
//! [`Unpin`] has no effect on the behavior of <code>[Pin]<[Box]\<T>></code> (here, `T` is the
//! pointed-to type).
//!
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//! 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
//! replacing a <code>[Some]\(v)</code> 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`].
//!
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//! that turn <code>[Pin]<&mut Struct></code> into a reference to the field, but what
//! type should that reference have? Is it <code>[Pin]<&mut Field></code> 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>`].
//! container/wrapper types such as <code>[Vec]\<T></code>, <code>[Box]\<T></code>, or <code>[RefCell]\<T></code>.
//! (This question applies to both mutable and shared references, we just
//! use the more common case of mutable references here for illustration.)
//!
Expand Down Expand Up @@ -292,19 +292,19 @@
//! 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>`]
//! This can be tricky, as witnessed by <code>[VecDeque]\<T></code>: the destructor of <code>[VecDeque]\<T></code>
//! 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
//! their destructor being called. (<code>[VecDeque]\<T></code> 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
//! <code>[Option]\<T></code> 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>`]
//! For a more complex example of moving data out of a pinned type, imagine if <code>[RefCell]\<T></code>
//! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
//! Then we could do the following:
//! ```compile_fail
Expand All @@ -315,30 +315,30 @@
//! 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 <code>[RefCell]\<T></code>
//! (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 possibilities (structural pinning or not) make sense.
//! A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut` methods to get
//! For a type like <code>[Vec]\<T></code>, both possibilities (structural pinning or not) make sense.
//! A <code>[Vec]\<T></code> 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)
//! [`pop`][Vec::pop] on a pinned <code>[Vec]\<T></code> 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.
//! A <code>[Vec]\<T></code> without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
//! are never pinned and the <code>[Vec]\<T></code> 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
//! does not actually move the `T`: the <code>[Box]\<T></code> can be freely movable (aka `Unpin`) even if
//! the `T` is not. In fact, even <code>[Pin]<[Box]\<T>></code> and <code>[Pin]<&mut T></code> 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
//! pointers themselves can be moved without moving the pinned data. For both <code>[Box]\<T></code> and
//! <code>[Pin]<[Box]\<T>></code>, whether the content is pinned is entirely independent of whether the
//! pointer is pinned, meaning pinning is *not* structural.
//!
Expand All @@ -353,17 +353,15 @@
//! [`DerefMut`]: crate::ops::DerefMut
//! [`mem::swap`]: crate::mem::swap
//! [`mem::forget`]: crate::mem::forget
//! [`Box<T>`]: ../../std/boxed/struct.Box.html
//! [`Vec<T>`]: ../../std/vec/struct.Vec.html
//! [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<T>`]: crate::cell::RefCell
//! [RefCell]: crate::cell::RefCell
//! [`drop`]: Drop::drop
//! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
//! [`Some(v)`]: Some
//! [VecDeque]: ../../std/collections/struct.VecDeque.html
//! [`ptr::write`]: crate::ptr::write
//! [`Future`]: crate::future::Future
//! [drop-impl]: #drop-implementation
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