improve Pin documentation
This commit is contained in:
Ralf Jung
parent
32471f7ea4
commit
4059889539
2 changed files with 190 additions and 20 deletions
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@ -597,7 +597,8 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
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/// Types which can be safely moved after being pinned.
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///
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/// Since Rust itself has no notion of immovable types, and will consider moves to always be safe,
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/// Since Rust itself has no notion of immovable types, and will consider moves
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/// (e.g. through assignment or [`mem::replace`]) to always be safe,
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/// this trait cannot prevent types from moving by itself.
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///
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/// Instead it can be used to prevent moves through the type system,
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@ -606,7 +607,12 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
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/// See the [`pin module`] documentation for more information on pinning.
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///
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/// Implementing this trait lifts the restrictions of pinning off a type,
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/// which then allows it to move out with functions such as [`replace`].
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/// which then allows it to move out with functions such as [`mem::replace`].
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///
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/// `Unpin` has no consequence at all for non-pinned data. In particular,
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/// [`mem::replace`] will happily move `!Unpin` data. However, you cannot use
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/// [`mem::replace`] on data wrapped inside a [`Pin`], and *that* is what makes
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/// this system work.
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///
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/// So this, for example, can only be done on types implementing `Unpin`:
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///
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@ -623,7 +629,7 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
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///
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/// This trait is automatically implemented for almost every type.
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///
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/// [`replace`]: ../../std/mem/fn.replace.html
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/// [`mem::replace`]: ../../std/mem/fn.replace.html
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/// [`Pin`]: ../pin/struct.Pin.html
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/// [`pin module`]: ../../std/pin/index.html
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#[stable(feature = "pin", since = "1.33.0")]
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@ -16,7 +16,7 @@
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//! but doesn't allow moving `T`. The pointer value itself (the `Box`) can still be moved,
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//! but the value behind it cannot.
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//!
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//! Since data can be moved out of `&mut` and `Box` with functions such as [`swap`],
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//! Since data can be moved out of `&mut` and `Box` with functions such as [`mem::swap`],
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//! changing the location of the underlying data, [`Pin`] prohibits accessing the
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//! underlying pointer type (the `&mut` or `Box`) directly, and provides its own set of
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//! APIs for accessing and using the value. [`Pin`] also guarantees that no other
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@ -24,21 +24,22 @@
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//! self-references and other special behaviors that are only possible for unmovable
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//! values.
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//!
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//! It is worth reiterating that [`Pin`] does *not* change the fact that the Rust compiler
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//! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, `Pin`
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//! prevents certain *values* (pointed to by pointers wrapped in `Pin`) from being
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//! moved by making it impossible to call methods like [`mem::swap`] on them.
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//!
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//! # `Unpin`
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//!
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//! However, these restrictions are usually not necessary. Many types are always freely
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//! movable. These types implement the [`Unpin`] auto-trait, which nullifies the effect
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//! of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function identically, as do
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//! `Pin<&mut T>` and `&mut T`.
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//! movable, even when pinned. These types implement the [`Unpin`] auto-trait, which
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//! nullifies the effect of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function
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//! identically, as do `Pin<&mut T>` and `&mut T`.
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//!
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//! Note that pinning and `Unpin` only affect the pointed-to type. For example, whether
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//! or not `Box<T>` is `Unpin` has no affect on the behavior of `Pin<Box<T>>`. Similarly,
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//! `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves, even though the
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//! `T` underneath them isn't, because the pointers in `Pin<Box<_>>` and `Pin<&mut _>`
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//! are always freely movable, even if the data they point to isn't.
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//!
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//! [`Pin`]: struct.Pin.html
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//! [`Unpin`]: ../../std/marker/trait.Unpin.html
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//! [`swap`]: ../../std/mem/fn.swap.html
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//! [`Box`]: ../../std/boxed/struct.Box.html
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//! Note that pinning and `Unpin` only affect the pointed-to type, not the pointer
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//! type itself that got wrapped in `Pin`. For example, whether or not `Box<T>` is
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//! `Unpin` has no affect on the behavior of `Pin<Box<T>>` (here, `T` is the
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//! pointed-to type).
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//!
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//! # Examples
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//!
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@ -94,6 +95,106 @@
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//! // let new_unmoved = Unmovable::new("world".to_string());
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//! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
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//! ```
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//!
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//! # `Drop` guarantee
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//!
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//! The purpose of pinning is to be able to rely on the placement of some data in memory.
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//! To make this work, not just moving the data is restricted; deallocating or overwriting
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//! it is restricted, too. Concretely, for pinned data you have to maintain the invariant
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//! that *it will not get overwritten or deallocated until `drop` was called*.
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//! ("Overwriting" here refers to other ways of invalidating storage, such as switching
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//! from one enum variant to another.)
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//!
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//! The purpose of this guarantee is to allow data structures that store pointers
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//! to pinned data. For example, in an intrusive doubly-linked list, every element
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//! will have pointers to its predecessor and successor in the list. Every element
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//! will be pinned, because moving the elements around would invalidate the pointers.
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//! Moreover, the `Drop` implemenetation of a linked list element will patch the pointers
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//! of its predecessor and successor to remove itself from the list. Clearly, if an element
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//! could be deallocated or overwritten without calling `drop`, the pointers into it
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//! from its neighbouring elements would become invalid, breaking the data structure.
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//!
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//! Notice that this guarantee does *not* mean that memory does not leak! It is still
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//! completely okay not to ever call `drop` on a pinned element (e.g., you can still
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//! call [`mem::forget`] on a `Pin<Box<T>>`). What you may not do is free or reuse the storage
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//! without calling `drop`.
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//!
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//! # `Drop` implementation
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//!
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//! If your type relies on pinning (for example, because it contains internal
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//! references, or because you are implementing something like the intrusive
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//! doubly-linked list mentioned in the previous section), you have to be careful
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//! when implementing `Drop`: notice that `drop` takes `&mut self`, but this
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//! will be called even if your type was previously pinned! It is as if the
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//! compiler automatically called `get_unchecked_mut`. This can never cause
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//! a problem in safe code because implementing a type that relies on pinning
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//! requires unsafe code, but be aware that deciding to make use of pinning
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//! in your type (for example by implementing some operation on `Pin<&[mut] Self>`)
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//! has consequences for your `Drop` implemenetation as well.
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//!
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//! # Projections and Structural Pinning
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//!
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//! One interesting question arises when considering pinning and "container types" --
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//! types such as `Vec` or `Box` but also `RefCell`; types that serve as wrappers
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//! around other types. When can such a type have a "projection" operation, an
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//! operation with type `fn(Pin<&[mut] Container<T>>) -> Pin<&[mut] T>`?
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//! This does not just apply to generic container types, even for normal structs
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//! the question arises whether `fn(Pin<&[mut] Struct>) -> Pin<&[mut] Field>`
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//! is an operation that can be soundly added to the API.
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//!
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//! This question is closely related to the question of whether pinning is "structural":
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//! when you have pinned a container, have you pinned its contents? Adding a
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//! projection to the API answers that question with a "yes" by offering pinned access
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//! to the contents.
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//!
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//! In general, as the author of a type you get to decide whether pinning is structural, and
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//! whether projections are provided. However, there are a couple requirements to be
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//! upheld when adding projection operations:
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//!
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//! 1. The container must only be [`Unpin`] if all its fields are `Unpin`. This is the default,
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//! but `Unpin` is a safe trait, so as the author of the container it is your responsibility
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//! *not* to add something like `impl<T> Unpin for Container<T>`. (Notice that adding a
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//! projection operation requires unsafe code, so the fact that `Unpin` is a safe trait
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//! does not break the principle that you only have to worry about any of this if
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//! you use `unsafe`.)
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//! 2. The destructor of the container must not move out of its argument. This is the exact
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//! point that was raised in the [previous section][drop-impl]: `drop` takes `&mut self`,
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//! but the container (and hence its fields) might have been pinned before.
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//! You have to guarantee that you do not move a field inside your `Drop` implementation.
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//! 3. Your container type must *not* be `#[repr(packed)]`. Packed structs have their fields
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//! moved around when they are dropped to properly align them, which is in conflict with
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//! claiming that the fields are pinned when your struct is.
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//! 4. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
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//! you must make sure that, once your container is pinned, the memory containing the
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//! content is not overwritten or deallocated without calling the content's destructors.
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//! This can be tricky, as witnessed by `VecDeque`: the destructor of `VecDeque` can fail
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//! to call `drop` on all elements if one of the destructors panics. This violates the
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//! `Drop` guarantee, because it can lead to elements being deallocated without
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//! their destructor being called.
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//! 5. You must not offer any other operations that could lead to data being moved out of
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//! the fields when your type is pinned. This is usually not a concern, but can become
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//! tricky when interior mutability is involved. For example, imagine `RefCell`
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//! would have a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
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//! This would be catastrophic, because it is possible to move out of a pinned
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//! `RefCell`: from `x: Pin<&mut RefCell<T>>`, use `let y = x.into_ref().get_ref()` to obtain
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//! `y: &RefCell<T>`, and from there use `y.borrow_mut().deref_mut()` to obtain `&mut T`
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//! which can be used with [`mem::swap`].
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//!
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//! On the other hand, if you decide *not* to offer any pinning projections, you
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//! are free to do `impl<T> Unpin for Container<T>`. In the standard library,
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//! we do this for all pointer types: `Box<T>: Unpin` holds for all `T`.
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//! It makes a lot of sense to do this for pointer types, because moving the `Box<T>`
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//! does not actually move the `T`: the `Box<T>` can be freely movable even if the `T`
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//! is not. In fact, even `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves,
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//! for the same reason.
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//!
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//! [`Pin`]: struct.Pin.html
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//! [`Unpin`]: ../../std/marker/trait.Unpin.html
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//! [`mem::swap`]: ../../std/mem/fn.swap.html
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//! [`mem::forget`]: ../../std/mem/fn.forget.html
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//! [`Box`]: ../../std/boxed/struct.Box.html
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//! [drop-impl]: #drop-implementation
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//! [drop-guarantee]: #drop-guarantee
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#![stable(feature = "pin", since = "1.33.0")]
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@ -170,7 +271,12 @@ where
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P::Target: Unpin,
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{
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/// Construct a new `Pin` around a pointer to some data of a type that
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/// implements `Unpin`.
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/// implements [`Unpin`].
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///
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/// Unlike `Pin::new_unchecked`, this method is safe because the pointer
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/// `P` dereferences to an [`Unpin`] type, which nullifies the pinning guarantees.
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///
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/// [`Unpin`]: ../../std/marker/trait.Unpin.html
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub fn new(pointer: P) -> Pin<P> {
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@ -191,8 +297,33 @@ impl<P: Deref> Pin<P> {
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/// not guarantee that the data `P` points to is pinned, constructing a
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/// `Pin<P>` is undefined behavior.
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///
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/// By using this method, you are making a promise about the `P::Deref` and
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/// `P::DerefMut` implementations, if they exist. Most importantly, they
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/// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
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/// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
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/// and expect these methods to uphold the pinning invariants.
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/// Moreover, by calling this method you promise that the reference `P`
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/// dereferences to will not be moved out of again; in particular, it
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/// must not be possible to obtain a `&mut P::Target` and then
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/// move out of that reference (using, for example [`replace`]).
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///
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/// For example, the following is a *violation* of `Pin`'s safety:
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/// ```
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/// use std::mem;
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/// use std::pin::Pin;
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///
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/// fn foo<T>(mut a: T, b: T) {
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/// unsafe { let p = Pin::new_unchecked(&mut a); } // should mean `a` can never move again
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/// let a2 = mem::replace(&mut a, b);
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/// // the address of `a` changed to `a2`'s stack slot, so `a` got moved even
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/// // though we have previously pinned it!
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/// }
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/// ```
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///
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/// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
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/// instead.
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///
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/// [`replace`]: ../../std/mem/fn.replace.html
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
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@ -200,6 +331,12 @@ impl<P: Deref> Pin<P> {
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}
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/// Gets a pinned shared reference from this pinned pointer.
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///
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/// This is a generic method to go from `&Pin<SmartPointer<T>>` to `Pin<&T>`.
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/// It is safe because, as part of the contract of `Pin::new_unchecked`,
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/// the pointee cannot move after `Pin<SmartPointer<T>>` got created.
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/// "Malicious" implementations of `SmartPointer::Deref` are likewise
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/// ruled out by the contract of `Pin::new_unchecked`.
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
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@ -209,13 +346,22 @@ impl<P: Deref> Pin<P> {
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impl<P: DerefMut> Pin<P> {
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/// Gets a pinned mutable reference from this pinned pointer.
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///
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/// This is a generic method to go from `&mut Pin<SmartPointer<T>>` to `Pin<&mut T>`.
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/// It is safe because, as part of the contract of `Pin::new_unchecked`,
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/// the pointee cannot move after `Pin<SmartPointer<T>>` got created.
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/// "Malicious" implementations of `SmartPointer::DerefMut` are likewise
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/// ruled out by the contract of `Pin::new_unchecked`.
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
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unsafe { Pin::new_unchecked(&mut *self.pointer) }
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}
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/// Assign a new value to the memory behind the pinned reference.
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/// Assigns a new value to the memory behind the pinned reference.
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///
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/// This overwrites pinned data, but that is okay: its destructor gets
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/// run before being overwritten, so no pinning guarantee is violated.
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub fn set(self: &mut Pin<P>, value: P::Target)
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@ -227,10 +373,12 @@ impl<P: DerefMut> Pin<P> {
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}
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impl<'a, T: ?Sized> Pin<&'a T> {
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/// Construct a new pin by mapping the interior value.
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/// Constructs a new pin by mapping the interior value.
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///
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/// For example, if you wanted to get a `Pin` of a field of something,
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/// you could use this to get access to that field in one line of code.
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/// However, there are several gotchas with these "pinning projections";
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/// see the [`pin` module] documentation for further details on that topic.
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///
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/// # Safety
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///
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/// will not move so long as the argument value does not move (for example,
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/// because it is one of the fields of that value), and also that you do
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/// not move out of the argument you receive to the interior function.
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///
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/// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
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#[stable(feature = "pin", since = "1.33.0")]
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pub unsafe fn map_unchecked<U, F>(self: Pin<&'a T>, func: F) -> Pin<&'a U> where
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F: FnOnce(&T) -> &U,
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@ -249,11 +399,21 @@ impl<'a, T: ?Sized> Pin<&'a T> {
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/// Gets a shared reference out of a pin.
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///
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/// This is safe because it is not possible to move out of a shared reference.
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/// It may seem like there is an issue here with interior mutability: in fact,
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/// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
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/// not a problem as long as there does not also exist a `Pin<&T>` pointing
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/// to the same data, and `RefCell` does not let you create a pinned reference
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/// to its contents. See the discussion on ["pinning projections"] for further
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/// details.
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///
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/// Note: `Pin` also implements `Deref` to the target, which can be used
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/// to access the inner value. However, `Deref` only provides a reference
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/// that lives for as long as the borrow of the `Pin`, not the lifetime of
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/// the `Pin` itself. This method allows turning the `Pin` into a reference
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/// with the same lifetime as the original `Pin`.
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///
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/// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
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#[stable(feature = "pin", since = "1.33.0")]
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#[inline(always)]
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pub fn get_ref(self: Pin<&'a T>) -> &'a T {
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@ -306,6 +466,8 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
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///
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/// For example, if you wanted to get a `Pin` of a field of something,
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/// you could use this to get access to that field in one line of code.
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/// However, there are several gotchas with these "pinning projections";
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/// see the [`pin` module] documentation for further details on that topic.
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///
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/// # Safety
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///
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@ -313,6 +475,8 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
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/// will not move so long as the argument value does not move (for example,
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/// because it is one of the fields of that value), and also that you do
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/// not move out of the argument you receive to the interior function.
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///
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/// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
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#[stable(feature = "pin", since = "1.33.0")]
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pub unsafe fn map_unchecked_mut<U, F>(self: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
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F: FnOnce(&mut T) -> &mut U,
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