ee9cd7bb6a
Add a type annotation to improve error messages with type mismatches Add a link to the temporary-lifetime-extension section of the reference
1154 lines
51 KiB
Rust
1154 lines
51 KiB
Rust
//! Types that pin data to its location in memory.
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//!
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//! It is sometimes useful to have objects that are guaranteed not to move,
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//! in the sense that their placement in memory does not change, and can thus be relied upon.
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//! A prime example of such a scenario would be building self-referential structs,
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//! as moving an object with pointers to itself will invalidate them, which could cause undefined
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//! behavior.
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//!
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//! At a high level, a <code>[Pin]\<P></code> ensures that the pointee of any pointer type
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//! `P` has a stable location in memory, meaning it cannot be moved elsewhere
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//! and its memory cannot be deallocated until it gets dropped. We say that the
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//! pointee is "pinned". Things get more subtle when discussing types that
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//! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning)
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//! for more details.
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//!
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//! By default, all types in Rust are movable. Rust allows passing all types by-value,
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//! and common smart-pointer types such as <code>[Box]\<T></code> and <code>[&mut] T</code> allow
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//! replacing and moving the values they contain: you can move out of a <code>[Box]\<T></code>,
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//! or you can use [`mem::swap`]. <code>[Pin]\<P></code> wraps a pointer type `P`, so
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//! <code>[Pin]<[Box]\<T>></code> functions much like a regular <code>[Box]\<T></code>:
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//! when a <code>[Pin]<[Box]\<T>></code> gets dropped, so do its contents, and the memory gets
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//! deallocated. Similarly, <code>[Pin]<[&mut] T></code> is a lot like <code>[&mut] T</code>.
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//! However, <code>[Pin]\<P></code> does not let clients actually obtain a <code>[Box]\<T></code>
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//! or <code>[&mut] T</code> to pinned data, which implies that you cannot use operations such
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//! as [`mem::swap`]:
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//!
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//! ```
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//! use std::pin::Pin;
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//! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
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//! // `mem::swap` needs `&mut T`, but we cannot get it.
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//! // We are stuck, we cannot swap the contents of these references.
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//! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
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//! // we are not allowed to use it for moving things out of the `Pin`.
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//! }
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//! ```
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//!
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//! It is worth reiterating that <code>[Pin]\<P></code> does *not* change the fact that a Rust
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//! compiler considers all types movable. [`mem::swap`] remains callable for any `T`. Instead,
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//! <code>[Pin]\<P></code> prevents certain *values* (pointed to by pointers wrapped in
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//! <code>[Pin]\<P></code>) from being moved by making it impossible to call methods that require
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//! <code>[&mut] T</code> on them (like [`mem::swap`]).
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//!
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//! <code>[Pin]\<P></code> can be used to wrap any pointer type `P`, and as such it interacts with
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//! [`Deref`] and [`DerefMut`]. A <code>[Pin]\<P></code> where <code>P: [Deref]</code> should be
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//! considered as a "`P`-style pointer" to a pinned <code>P::[Target]</code> – so, a
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//! <code>[Pin]<[Box]\<T>></code> is an owned pointer to a pinned `T`, and a
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//! <code>[Pin]<[Rc]\<T>></code> is a reference-counted pointer to a pinned `T`.
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//! For correctness, <code>[Pin]\<P></code> relies on the implementations of [`Deref`] and
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//! [`DerefMut`] not to move out of their `self` parameter, and only ever to
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//! return a pointer to pinned data when they are called on a pinned pointer.
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//!
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//! # `Unpin`
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//!
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//! Many types are always freely movable, even when pinned, because they do not
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//! rely on having a stable address. This includes all the basic types (like
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//! [`bool`], [`i32`], and references) as well as types consisting solely of these
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//! types. Types that do not care about pinning implement the [`Unpin`]
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//! auto-trait, which cancels the effect of <code>[Pin]\<P></code>. For <code>T: [Unpin]</code>,
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//! <code>[Pin]<[Box]\<T>></code> and <code>[Box]\<T></code> function identically, as do
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//! <code>[Pin]<[&mut] T></code> and <code>[&mut] T</code>.
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//!
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//! Note that pinning and [`Unpin`] only affect the pointed-to type <code>P::[Target]</code>,
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//! not the pointer type `P` itself that got wrapped in <code>[Pin]\<P></code>. For example,
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//! whether or not <code>[Box]\<T></code> is [`Unpin`] has no effect on the behavior of
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//! <code>[Pin]<[Box]\<T>></code> (here, `T` is the pointed-to type).
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//!
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//! # Example: self-referential struct
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//!
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//! Before we go into more details to explain the guarantees and choices
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//! associated with <code>[Pin]\<P></code>, we discuss some examples for how it might be used.
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//! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee).
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//!
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//! ```rust
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//! use std::pin::Pin;
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//! use std::marker::PhantomPinned;
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//! use std::ptr::NonNull;
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//!
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//! // This is a self-referential struct because the slice field points to the data field.
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//! // We cannot inform the compiler about that with a normal reference,
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//! // as this pattern cannot be described with the usual borrowing rules.
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//! // Instead we use a raw pointer, though one which is known not to be null,
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//! // as we know it's pointing at the string.
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//! struct Unmovable {
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//! data: String,
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//! slice: NonNull<String>,
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//! _pin: PhantomPinned,
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//! }
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//!
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//! impl Unmovable {
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//! // To ensure the data doesn't move when the function returns,
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//! // we place it in the heap where it will stay for the lifetime of the object,
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//! // and the only way to access it would be through a pointer to it.
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//! fn new(data: String) -> Pin<Box<Self>> {
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//! let res = Unmovable {
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//! data,
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//! // we only create the pointer once the data is in place
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//! // otherwise it will have already moved before we even started
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//! slice: NonNull::dangling(),
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//! _pin: PhantomPinned,
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//! };
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//! let mut boxed = Box::pin(res);
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//!
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//! let slice = NonNull::from(&boxed.data);
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//! // we know this is safe because modifying a field doesn't move the whole struct
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//! unsafe {
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//! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
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//! Pin::get_unchecked_mut(mut_ref).slice = slice;
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//! }
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//! boxed
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//! }
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//! }
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//!
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//! let unmoved = Unmovable::new("hello".to_string());
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//! // The pointer should point to the correct location,
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//! // so long as the struct hasn't moved.
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//! // Meanwhile, we are free to move the pointer around.
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//! # #[allow(unused_mut)]
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//! let mut still_unmoved = unmoved;
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//! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
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//!
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//! // Since our type doesn't implement Unpin, this will fail to compile:
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//! // let mut 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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//! # Example: intrusive doubly-linked list
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//!
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//! In an intrusive doubly-linked list, the collection does not actually allocate
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//! the memory for the elements itself. Allocation is controlled by the clients,
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//! and elements can live on a stack frame that lives shorter than the collection does.
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//!
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//! To make this work, every element has pointers to its predecessor and successor in
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//! the list. Elements can only be added when they are pinned, because moving the elements
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//! around would invalidate the pointers. Moreover, the [`Drop`][Drop] implementation of a linked
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//! list element will patch the pointers of its predecessor and successor to remove itself
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//! from the list.
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//!
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//! Crucially, we have to be able to rely on [`drop`] being called. If an element
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//! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it
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//! from its neighboring elements would become invalid, which would break the data structure.
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//!
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//! Therefore, pinning also comes with a [`drop`]-related guarantee.
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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, repurposing, or
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//! otherwise invalidating the memory used to store the data is restricted, too.
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//! Concretely, for pinned data you have to maintain the invariant
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//! that *its memory will not get invalidated or repurposed from the moment it gets pinned until
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//! when [`drop`] is called*. Only once [`drop`] returns or panics, the memory may be reused.
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//!
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//! Memory can be "invalidated" by deallocation, but also by
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//! replacing a <code>[Some]\(v)</code> by [`None`], or calling [`Vec::set_len`] to "kill" some
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//! elements off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without
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//! calling the destructor first. None of this is allowed for pinned data without calling [`drop`].
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//!
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//! This is exactly the kind of guarantee that the intrusive linked list from the previous
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//! section needs to function correctly.
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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 to not ever call [`drop`] on a pinned element (e.g., you can still
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//! call [`mem::forget`] on a <code>[Pin]<[Box]\<T>></code>). In the example of the doubly-linked
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//! list, that element would just stay in the list. However you must not 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 uses pinning (such as the two examples above), you have to be careful
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//! when implementing [`Drop`][Drop]. The [`drop`] function takes <code>[&mut] self</code>, but this
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//! is called *even if your type was previously pinned*! It is as if the
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//! compiler automatically called [`Pin::get_unchecked_mut`].
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//!
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//! This can never cause a problem in safe code because implementing a type that
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//! relies on pinning requires unsafe code, but be aware that deciding to make
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//! use of pinning in your type (for example by implementing some operation on
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//! <code>[Pin]<[&]Self></code> or <code>[Pin]<[&mut] Self></code>) has consequences for your
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//! [`Drop`][Drop]implementation as well: if an element of your type could have been pinned,
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//! you must treat [`Drop`][Drop] as implicitly taking <code>[Pin]<[&mut] Self></code>.
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//!
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//! For example, you could implement [`Drop`][Drop] as follows:
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//!
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//! ```rust,no_run
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//! # use std::pin::Pin;
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//! # struct Type { }
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//! impl Drop for Type {
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//! fn drop(&mut self) {
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//! // `new_unchecked` is okay because we know this value is never used
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//! // again after being dropped.
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//! inner_drop(unsafe { Pin::new_unchecked(self)});
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//! fn inner_drop(this: Pin<&mut Type>) {
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//! // Actual drop code goes here.
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//! }
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//! }
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//! }
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//! ```
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//!
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//! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that
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//! you do not accidentally use `self`/`this` in a way that is in conflict with pinning.
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//!
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//! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically
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//! move fields around to be able to drop them. It might even do
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//! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use
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//! pinning with a `#[repr(packed)]` type.
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//!
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//! # Projections and Structural Pinning
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//!
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//! When working with pinned structs, the question arises how one can access the
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//! fields of that struct in a method that takes just <code>[Pin]<[&mut] Struct></code>.
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//! The usual approach is to write helper methods (so called *projections*)
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//! that turn <code>[Pin]<[&mut] Struct></code> into a reference to the field, but what type should
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//! that reference have? Is it <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>?
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//! The same question arises with the fields of an `enum`, and also when considering
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//! container/wrapper types such as <code>[Vec]\<T></code>, <code>[Box]\<T></code>,
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//! or <code>[RefCell]\<T></code>. (This question applies to both mutable and shared references,
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//! we just use the more common case of mutable references here for illustration.)
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//!
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//! It turns out that it is actually up to the author of the data structure to decide whether
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//! the pinned projection for a particular field turns <code>[Pin]<[&mut] Struct></code>
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//! into <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>. There are some
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//! constraints though, and the most important constraint is *consistency*:
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//! every field can be *either* projected to a pinned reference, *or* have
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//! pinning removed as part of the projection. If both are done for the same field,
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//! that will likely be unsound!
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//!
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//! As the author of a data structure you get to decide for each field whether pinning
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//! "propagates" to this field or not. Pinning that propagates is also called "structural",
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//! because it follows the structure of the type.
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//! In the following subsections, we describe the considerations that have to be made
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//! for either choice.
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//!
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//! ## Pinning *is not* structural for `field`
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//!
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//! It may seem counter-intuitive that the field of a pinned struct might not be pinned,
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//! but that is actually the easiest choice: if a <code>[Pin]<[&mut] Field></code> is never created,
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//! nothing can go wrong! So, if you decide that some field does not have structural pinning,
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//! all you have to ensure is that you never create a pinned reference to that field.
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//!
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//! Fields without structural pinning may have a projection method that turns
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//! <code>[Pin]<[&mut] Struct></code> into <code>[&mut] Field</code>:
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//!
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//! ```rust,no_run
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//! # use std::pin::Pin;
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//! # type Field = i32;
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//! # struct Struct { field: Field }
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//! impl Struct {
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//! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field {
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//! // This is okay because `field` is never considered pinned.
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//! unsafe { &mut self.get_unchecked_mut().field }
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//! }
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//! }
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//! ```
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//!
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//! You may also <code>impl [Unpin] for Struct</code> *even if* the type of `field`
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//! is not [`Unpin`]. What that type thinks about pinning is not relevant
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//! when no <code>[Pin]<[&mut] Field></code> is ever created.
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//!
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//! ## Pinning *is* structural for `field`
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//!
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//! The other option is to decide that pinning is "structural" for `field`,
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//! meaning that if the struct is pinned then so is the field.
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//!
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//! This allows writing a projection that creates a <code>[Pin]<[&mut] Field></code>, thus
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//! witnessing that the field is pinned:
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//!
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//! ```rust,no_run
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//! # use std::pin::Pin;
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//! # type Field = i32;
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//! # struct Struct { field: Field }
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//! impl Struct {
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//! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> {
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//! // This is okay because `field` is pinned when `self` is.
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//! unsafe { self.map_unchecked_mut(|s| &mut s.field) }
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//! }
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//! }
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//! ```
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//!
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//! However, structural pinning comes with a few extra requirements:
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//!
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//! 1. The struct must only be [`Unpin`] if all the structural fields are
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//! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of
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//! the struct it is your responsibility *not* to add something like
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//! <code>impl\<T> [Unpin] for Struct\<T></code>. (Notice that adding a projection operation
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//! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break
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//! the principle that you only have to worry about any of this if you use [`unsafe`].)
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//! 2. The destructor of the struct must not move structural fields out of its argument. This
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//! is the exact point that was raised in the [previous section][drop-impl]: [`drop`] takes
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//! <code>[&mut] self</code>, but the struct (and hence its fields) might have been pinned
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//! before. You have to guarantee that you do not move a field inside your [`Drop`][Drop]
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//! implementation. In particular, as explained previously, this means that your struct
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//! must *not* be `#[repr(packed)]`.
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//! See that section for how to write [`drop`] in a way that the compiler can help you
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//! not accidentally break pinning.
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//! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
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//! once your struct is pinned, the memory that contains 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 <code>[VecDeque]\<T></code>: the destructor of
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//! <code>[VecDeque]\<T></code> can fail to call [`drop`] on all elements if one of the
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//! destructors panics. This violates the [`Drop`][Drop] guarantee, because it can lead to
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//! elements being deallocated without their destructor being called.
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//! (<code>[VecDeque]\<T></code> has no pinning projections, so this
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//! does not cause unsoundness.)
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//! 4. You must not offer any other operations that could lead to data being moved out of
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//! the structural fields when your type is pinned. For example, if the struct contains an
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//! <code>[Option]\<T></code> and there is a [`take`][Option::take]-like operation with type
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//! <code>fn([Pin]<[&mut] Struct\<T>>) -> [Option]\<T></code>,
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//! that operation can be used to move a `T` out of a pinned `Struct<T>` – which means
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//! pinning cannot be structural for the field holding this data.
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//!
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//! For a more complex example of moving data out of a pinned type,
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//! imagine if <code>[RefCell]\<T></code> had a method
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//! <code>fn get_pin_mut(self: [Pin]<[&mut] Self>) -> [Pin]<[&mut] T></code>.
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//! Then we could do the following:
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//! ```compile_fail
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//! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
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//! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
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//! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
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//! let b = rc_shr.borrow_mut();
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//! let content = &mut *b; // And here we have `&mut T` to the same data.
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//! }
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//! ```
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//! This is catastrophic, it means we can first pin the content of the
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//! <code>[RefCell]\<T></code> (using <code>[RefCell]::get_pin_mut</code>) and then move that
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//! content using the mutable reference we got later.
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//!
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//! ## Examples
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//!
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//! For a type like <code>[Vec]\<T></code>, both possibilities (structural pinning or not) make
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//! sense. A <code>[Vec]\<T></code> with structural pinning could have `get_pin`/`get_pin_mut`
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//! methods to get pinned references to elements. However, it could *not* allow calling
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//! [`pop`][Vec::pop] on a pinned <code>[Vec]\<T></code> because that would move the (structurally
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//! pinned) contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also
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//! move the contents.
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//!
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//! A <code>[Vec]\<T></code> without structural pinning could
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//! <code>impl\<T> [Unpin] for [Vec]\<T></code>, because the contents are never pinned
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//! and the <code>[Vec]\<T></code> itself is fine with being moved as well.
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//! At that point pinning just has no effect on the vector at all.
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//!
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//! In the standard library, pointer types generally do not have structural pinning,
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//! and thus they do not offer pinning projections. This is why <code>[Box]\<T>: [Unpin]</code>
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//! holds for all `T`. It makes sense to do this for pointer types, because moving the
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//! <code>[Box]\<T></code> does not actually move the `T`: the <code>[Box]\<T></code> can be freely
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//! movable (aka [`Unpin`]) even if the `T` is not. In fact, even <code>[Pin]<[Box]\<T>></code> and
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//! <code>[Pin]<[&mut] T></code> are always [`Unpin`] themselves, for the same reason:
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//! their contents (the `T`) are pinned, but the pointers themselves can be moved without moving
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//! the pinned data. For both <code>[Box]\<T></code> and <code>[Pin]<[Box]\<T>></code>,
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//! whether the content is pinned is entirely independent of whether the
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//! pointer is pinned, meaning pinning is *not* structural.
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//!
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//! When implementing a [`Future`] combinator, you will usually need structural pinning
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//! for the nested futures, as you need to get pinned references to them to call [`poll`].
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//! But if your combinator contains any other data that does not need to be pinned,
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//! you can make those fields not structural and hence freely access them with a
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//! mutable reference even when you just have <code>[Pin]<[&mut] Self></code> (such as in your own
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//! [`poll`] implementation).
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//!
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//! [Deref]: crate::ops::Deref "ops::Deref"
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//! [`Deref`]: crate::ops::Deref "ops::Deref"
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//! [Target]: crate::ops::Deref::Target "ops::Deref::Target"
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//! [`DerefMut`]: crate::ops::DerefMut "ops::DerefMut"
|
||
//! [`mem::swap`]: crate::mem::swap "mem::swap"
|
||
//! [`mem::forget`]: crate::mem::forget "mem::forget"
|
||
//! [Vec]: ../../std/vec/struct.Vec.html "Vec"
|
||
//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len "Vec::set_len"
|
||
//! [Box]: ../../std/boxed/struct.Box.html "Box"
|
||
//! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop "Vec::pop"
|
||
//! [Vec::push]: ../../std/vec/struct.Vec.html#method.push "Vec::push"
|
||
//! [Rc]: ../../std/rc/struct.Rc.html "rc::Rc"
|
||
//! [RefCell]: crate::cell::RefCell "cell::RefCell"
|
||
//! [`drop`]: Drop::drop
|
||
//! [VecDeque]: ../../std/collections/struct.VecDeque.html "collections::VecDeque"
|
||
//! [`ptr::write`]: crate::ptr::write "ptr::write"
|
||
//! [`Future`]: crate::future::Future "future::Future"
|
||
//! [drop-impl]: #drop-implementation
|
||
//! [drop-guarantee]: #drop-guarantee
|
||
//! [`poll`]: crate::future::Future::poll "future::Future::poll"
|
||
//! [&]: reference "shared reference"
|
||
//! [&mut]: reference "mutable reference"
|
||
//! [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
|
||
|
||
#![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 <https://internals.rust-lang.org/t/unsoundness-in-pin/11311> for more details.
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
#[lang = "pin"]
|
||
#[fundamental]
|
||
#[repr(transparent)]
|
||
#[derive(Copy, Clone)]
|
||
pub struct Pin<P> {
|
||
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 <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73> for more details.
|
||
|
||
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
|
||
impl<P: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<P>
|
||
where
|
||
P::Target: PartialEq<Q::Target>,
|
||
{
|
||
fn eq(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::eq(self, other)
|
||
}
|
||
|
||
fn ne(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::ne(self, other)
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
|
||
impl<P: Deref<Target: Eq>> Eq for Pin<P> {}
|
||
|
||
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
|
||
impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P>
|
||
where
|
||
P::Target: PartialOrd<Q::Target>,
|
||
{
|
||
fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
|
||
P::Target::partial_cmp(self, other)
|
||
}
|
||
|
||
fn lt(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::lt(self, other)
|
||
}
|
||
|
||
fn le(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::le(self, other)
|
||
}
|
||
|
||
fn gt(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::gt(self, other)
|
||
}
|
||
|
||
fn ge(&self, other: &Pin<Q>) -> bool {
|
||
P::Target::ge(self, other)
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
|
||
impl<P: Deref<Target: Ord>> Ord for Pin<P> {
|
||
fn cmp(&self, other: &Self) -> cmp::Ordering {
|
||
P::Target::cmp(self, other)
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
|
||
impl<P: Deref<Target: Hash>> Hash for Pin<P> {
|
||
fn hash<H: Hasher>(&self, state: &mut H) {
|
||
P::Target::hash(self, state);
|
||
}
|
||
}
|
||
|
||
impl<P: Deref<Target: Unpin>> Pin<P> {
|
||
/// Construct a new `Pin<P>` 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<P> {
|
||
// SAFETY: the value pointed to is `Unpin`, and so has no requirements
|
||
// around pinning.
|
||
unsafe { Pin::new_unchecked(pointer) }
|
||
}
|
||
|
||
/// Unwraps this `Pin<P>` 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>) -> P {
|
||
pin.pointer
|
||
}
|
||
}
|
||
|
||
impl<P: Deref> Pin<P> {
|
||
/// Construct a new `Pin<P>` 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<P>` 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<T>(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<T>` 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<T>(mut x: Rc<T>) {
|
||
/// 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<P> {
|
||
Pin { pointer }
|
||
}
|
||
|
||
/// Gets a pinned shared reference from this pinned pointer.
|
||
///
|
||
/// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
|
||
/// It is safe because, as part of the contract of `Pin::new_unchecked`,
|
||
/// the pointee cannot move after `Pin<Pointer<T>>` 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<P>` 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>) -> P {
|
||
pin.pointer
|
||
}
|
||
}
|
||
|
||
impl<P: DerefMut> Pin<P> {
|
||
/// Gets a pinned mutable reference from this pinned pointer.
|
||
///
|
||
/// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
|
||
/// It is safe because, as part of the contract of `Pin::new_unchecked`,
|
||
/// the pointee cannot move after `Pin<Pointer<T>>` 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<U, F>(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<T>`. However, this is
|
||
/// not a problem as long as there does not also exist a `Pin<&T>` pointing
|
||
/// to the same data, and `RefCell<T>` 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)]
|
||
#[must_use]
|
||
#[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)]
|
||
#[must_use = "`self` will be dropped if the result is not used"]
|
||
#[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)]
|
||
#[must_use = "`self` will be dropped if the result is not used"]
|
||
#[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)]
|
||
#[must_use = "`self` will be dropped if the result is not used"]
|
||
#[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
|
||
#[must_use = "`self` will be dropped if the result is not used"]
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
pub unsafe fn map_unchecked_mut<U, F>(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<T: ?Sized> 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<'a, P: DerefMut> Pin<&'a mut Pin<P>> {
|
||
/// Gets a pinned mutable reference from this nested pinned pointer.
|
||
///
|
||
/// This is a generic method to go from `Pin<&mut Pin<Pointer<T>>>` to `Pin<&mut T>`. It is
|
||
/// safe because the existence of a `Pin<Pointer<T>>` ensures that the pointee, `T`, cannot
|
||
/// move in the future, and this method does not enable the pointee to move. "Malicious"
|
||
/// implementations of `P::DerefMut` are likewise ruled out by the contract of
|
||
/// `Pin::new_unchecked`.
|
||
#[unstable(feature = "pin_deref_mut", issue = "86918")]
|
||
#[must_use = "`self` will be dropped if the result is not used"]
|
||
#[inline(always)]
|
||
pub fn as_deref_mut(self) -> Pin<&'a mut P::Target> {
|
||
// SAFETY: What we're asserting here is that going from
|
||
//
|
||
// Pin<&mut Pin<P>>
|
||
//
|
||
// to
|
||
//
|
||
// Pin<&mut P::Target>
|
||
//
|
||
// is safe.
|
||
//
|
||
// We need to ensure that two things hold for that to be the case:
|
||
//
|
||
// 1) Once we give out a `Pin<&mut P::Target>`, an `&mut P::Target` will not be given out.
|
||
// 2) By giving out a `Pin<&mut P::Target>`, we do not risk of violating `Pin<&mut Pin<P>>`
|
||
//
|
||
// The existence of `Pin<P>` is sufficient to guarantee #1: since we already have a
|
||
// `Pin<P>`, it must already uphold the pinning guarantees, which must mean that
|
||
// `Pin<&mut P::Target>` does as well, since `Pin::as_mut` is safe. We do not have to rely
|
||
// on the fact that P is _also_ pinned.
|
||
//
|
||
// For #2, we need to ensure that code given a `Pin<&mut P::Target>` cannot cause the
|
||
// `Pin<P>` to move? That is not possible, since `Pin<&mut P::Target>` no longer retains
|
||
// any access to the `P` itself, much less the `Pin<P>`.
|
||
unsafe { self.get_unchecked_mut() }.as_mut()
|
||
}
|
||
}
|
||
|
||
impl<T: ?Sized> 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<P: Deref> Deref for Pin<P> {
|
||
type Target = P::Target;
|
||
fn deref(&self) -> &P::Target {
|
||
Pin::get_ref(Pin::as_ref(self))
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> {
|
||
fn deref_mut(&mut self) -> &mut P::Target {
|
||
Pin::get_mut(Pin::as_mut(self))
|
||
}
|
||
}
|
||
|
||
#[unstable(feature = "receiver_trait", issue = "none")]
|
||
impl<P: Receiver> Receiver for Pin<P> {}
|
||
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
impl<P: fmt::Debug> fmt::Debug for Pin<P> {
|
||
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
||
fmt::Debug::fmt(&self.pointer, f)
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
impl<P: fmt::Display> fmt::Display for Pin<P> {
|
||
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
||
fmt::Display::fmt(&self.pointer, f)
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
|
||
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<Target=impl !Unpin>` to a type that impls
|
||
// `Deref<Target=Unpin>` 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<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}
|
||
|
||
#[stable(feature = "pin", since = "1.33.0")]
|
||
impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
|
||
|
||
/// Constructs a <code>[Pin]<[&mut] T></code>, by pinning[^1] a `value: T` _locally_[^2]
|
||
/// (≠ [in the heap][`Box::pin`]).
|
||
///
|
||
/// [^1]: If the (type `T` of the) given value does not implement [`Unpin`], then this
|
||
/// effectively pins the `value` in memory, where it will be unable to be moved.
|
||
/// Otherwise, <code>[Pin]<[&mut] T></code> behaves like <code>[&mut] T</code>, and operations such
|
||
/// as [`mem::replace()`][crate::mem::replace] will allow extracting that value, and therefore,
|
||
/// moving it.
|
||
/// See [the `Unpin` section of the `pin` module][self#unpin] for more info.
|
||
///
|
||
/// [^2]: This is usually dubbed "stack"-pinning. And whilst local values are almost always located
|
||
/// in the stack (_e.g._, when within the body of a non-`async` function), the truth is that inside
|
||
/// the body of an `async fn` or block —more generally, the body of a generator— any locals crossing
|
||
/// an `.await` point —a `yield` point— end up being part of the state captured by the `Future` —by
|
||
/// the `Generator`—, and thus will be stored wherever that one is.
|
||
///
|
||
/// ## Examples
|
||
///
|
||
/// ### Basic usage
|
||
///
|
||
/// ```rust
|
||
/// #![feature(pin_macro)]
|
||
/// # use core::marker::PhantomPinned as Foo;
|
||
/// use core::pin::{pin, Pin};
|
||
///
|
||
/// fn stuff(foo: Pin<&mut Foo>) {
|
||
/// // …
|
||
/// # let _ = foo;
|
||
/// }
|
||
///
|
||
/// let pinned_foo = pin!(Foo { /* … */ });
|
||
/// stuff(pinned_foo);
|
||
/// // or, directly:
|
||
/// stuff(pin!(Foo { /* … */ }));
|
||
/// ```
|
||
///
|
||
/// ### Manually polling a `Future` (wihout `Unpin` bounds)
|
||
///
|
||
/// ```rust
|
||
/// #![feature(pin_macro)]
|
||
/// use std::{
|
||
/// future::Future,
|
||
/// pin::pin,
|
||
/// task::{Context, Poll},
|
||
/// thread,
|
||
/// };
|
||
/// # use std::{sync::Arc, task::Wake, thread::Thread};
|
||
///
|
||
/// # /// A waker that wakes up the current thread when called.
|
||
/// # struct ThreadWaker(Thread);
|
||
/// #
|
||
/// # impl Wake for ThreadWaker {
|
||
/// # fn wake(self: Arc<Self>) {
|
||
/// # self.0.unpark();
|
||
/// # }
|
||
/// # }
|
||
/// #
|
||
/// /// Runs a future to completion.
|
||
/// fn block_on<Fut: Future>(fut: Fut) -> Fut::Output {
|
||
/// let waker_that_unparks_thread = // …
|
||
/// # Arc::new(ThreadWaker(thread::current())).into();
|
||
/// let mut cx = Context::from_waker(&waker_that_unparks_thread);
|
||
/// // Pin the future so it can be polled.
|
||
/// let mut pinned_fut = pin!(fut);
|
||
/// loop {
|
||
/// match pinned_fut.as_mut().poll(&mut cx) {
|
||
/// Poll::Pending => thread::park(),
|
||
/// Poll::Ready(res) => return res,
|
||
/// }
|
||
/// }
|
||
/// }
|
||
/// #
|
||
/// # assert_eq!(42, block_on(async { 42 }));
|
||
/// ```
|
||
///
|
||
/// ### With `Generator`s
|
||
///
|
||
/// ```rust
|
||
/// #![feature(generators, generator_trait, pin_macro)]
|
||
/// use core::{
|
||
/// ops::{Generator, GeneratorState},
|
||
/// pin::pin,
|
||
/// };
|
||
///
|
||
/// fn generator_fn() -> impl Generator<Yield = usize, Return = ()> /* not Unpin */ {
|
||
/// // Allow generator to be self-referential (not `Unpin`)
|
||
/// // vvvvvv so that locals can cross yield points.
|
||
/// static || {
|
||
/// let foo = String::from("foo"); // --+
|
||
/// yield 0; // | <- crosses yield point!
|
||
/// println!("{}", &foo); // <----------+
|
||
/// yield foo.len();
|
||
/// }
|
||
/// }
|
||
///
|
||
/// fn main() {
|
||
/// let mut generator = pin!(generator_fn());
|
||
/// match generator.as_mut().resume(()) {
|
||
/// GeneratorState::Yielded(0) => {},
|
||
/// _ => unreachable!(),
|
||
/// }
|
||
/// match generator.as_mut().resume(()) {
|
||
/// GeneratorState::Yielded(3) => {},
|
||
/// _ => unreachable!(),
|
||
/// }
|
||
/// match generator.resume(()) {
|
||
/// GeneratorState::Yielded(_) => unreachable!(),
|
||
/// GeneratorState::Complete(()) => {},
|
||
/// }
|
||
/// }
|
||
/// ```
|
||
///
|
||
/// ## Remarks
|
||
///
|
||
/// Precisely because a value is pinned to local storage, the resulting <code>[Pin]<[&mut] T></code>
|
||
/// reference ends up borrowing a local tied to that block: it can't escape it.
|
||
///
|
||
/// The following, for instance, fails to compile:
|
||
///
|
||
/// ```rust,compile_fail
|
||
/// #![feature(pin_macro)]
|
||
/// use core::pin::{pin, Pin};
|
||
/// # use core::{marker::PhantomPinned as Foo, mem::drop as stuff};
|
||
///
|
||
/// let x: Pin<&mut Foo> = {
|
||
/// let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
|
||
/// x
|
||
/// }; // <- Foo is dropped
|
||
/// stuff(x); // Error: use of dropped value
|
||
/// ```
|
||
///
|
||
/// <details><summary>Error message</summary>
|
||
///
|
||
/// ```rust
|
||
/// # const _IGNORE: &str = stringify! {
|
||
/// error[E0716]: temporary value dropped while borrowed
|
||
/// --> src/main.rs:9:28
|
||
/// |
|
||
/// 8 | let x: Pin<&mut Foo> = {
|
||
/// | - borrow later stored here
|
||
/// 9 | let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
|
||
/// | ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
|
||
/// 10 | x
|
||
/// 11 | }; // <- Foo is dropped
|
||
/// | - temporary value is freed at the end of this statement
|
||
/// |
|
||
/// = note: consider using a let binding to create a longer lived value
|
||
/// # };
|
||
/// ```
|
||
///
|
||
/// </details>
|
||
///
|
||
/// This makes [`pin!`] **unsuitable to pin values when intending to _return_ them**. Instead, the
|
||
/// value is expected to be passed around _unpinned_ until the point where it is to be consumed,
|
||
/// where it is then useful and even sensible to pin the value locally using [`pin!`].
|
||
///
|
||
/// If you really need to return a pinned value, consider using [`Box::pin`] instead.
|
||
///
|
||
/// On the other hand, pinning to the stack[<sup>2</sup>](#fn2) using [`pin!`] is likely to be
|
||
/// cheaper than pinning into a fresh heap allocation using [`Box::pin`]. Moreover, by virtue of not
|
||
/// even needing an allocator, [`pin!`] is the main non-`unsafe` `#![no_std]`-compatible [`Pin`]
|
||
/// constructor.
|
||
///
|
||
/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
|
||
#[unstable(feature = "pin_macro", issue = "93178")]
|
||
pub macro pin($value:expr $(,)?) {
|
||
// This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
|
||
// review such a hypothetical macro (that any user-code could define):
|
||
//
|
||
// ```rust
|
||
// macro_rules! pin {( $value:expr ) => (
|
||
// match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block.
|
||
// $crate::pin::Pin::<&mut _>::new_unchecked(at_value)
|
||
// }}
|
||
// )}
|
||
// ```
|
||
//
|
||
// Safety:
|
||
// - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls
|
||
// that would break `Pin`'s invariants.
|
||
// - `{ $value }` is braced, making it a _block expression_, thus **moving**
|
||
// the given `$value`, and making it _become an **anonymous** temporary_.
|
||
// By virtue of being anonynomous, it can no longer be accessed, thus
|
||
// preventing any attemps to `mem::replace` it or `mem::forget` it, _etc._
|
||
//
|
||
// This gives us a `pin!` definition that is sound, and which works, but only
|
||
// in certain scenarios:
|
||
// - If the `pin!(value)` expression is _directly_ fed to a function call:
|
||
// `let poll = pin!(fut).poll(cx);`
|
||
// - If the `pin!(value)` expression is part of a scrutinee:
|
||
// ```rust
|
||
// match pin!(fut) { pinned_fut => {
|
||
// pinned_fut.as_mut().poll(...);
|
||
// pinned_fut.as_mut().poll(...);
|
||
// }} // <- `fut` is dropped here.
|
||
// ```
|
||
// Alas, it doesn't work for the more straight-forward use-case: `let` bindings.
|
||
// ```rust
|
||
// let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement
|
||
// pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed
|
||
// // note: consider using a `let` binding to create a longer lived value
|
||
// ```
|
||
// - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66
|
||
//
|
||
// This makes such a macro incredibly unergonomic in practice, and the reason most macros
|
||
// out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`)
|
||
// instead of featuring the more intuitive ergonomics of an expression macro.
|
||
//
|
||
// Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a
|
||
// temporary is dropped at the end of its enclosing statement when it is part of the parameters
|
||
// given to function call, which has precisely been the case with our `Pin::new_unchecked()`!
|
||
// For instance,
|
||
// ```rust
|
||
// let p = Pin::new_unchecked(&mut <temporary>);
|
||
// ```
|
||
// becomes:
|
||
// ```rust
|
||
// let p = { let mut anon = <temporary>; &mut anon };
|
||
// ```
|
||
//
|
||
// However, when using a literal braced struct to construct the value, references to temporaries
|
||
// can then be taken. This makes Rust change the lifespan of such temporaries so that they are,
|
||
// instead, dropped _at the end of the enscoping block_.
|
||
// For instance,
|
||
// ```rust
|
||
// let p = Pin { pointer: &mut <temporary> };
|
||
// ```
|
||
// becomes:
|
||
// ```rust
|
||
// let mut anon = <temporary>;
|
||
// let p = Pin { pointer: &mut anon };
|
||
// ```
|
||
// which is *exactly* what we want.
|
||
//
|
||
// See https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension
|
||
// for more info.
|
||
//
|
||
// Finally, we don't hit problems _w.r.t._ the privacy of the `pointer` field, or the
|
||
// unqualified `Pin` name, thanks to `decl_macro`s being _fully_ hygienic (`def_site` hygiene).
|
||
Pin::<&mut _> { pointer: &mut { $value } }
|
||
}
|