diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 69d54f06407..561f8ef36ff 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -899,25 +899,37 @@ marker_impls! {
{T: ?Sized} &mut T,
}
-/// Types that can be safely moved after being pinned.
+/// Types that do not require any pinning guarantees.
///
-/// Rust itself has no notion of immovable types, and considers moves (e.g.,
-/// through assignment or [`mem::replace`]) to always be safe.
+/// For information on what "pinning" is, see the [`pin` module] documentation.
///
-/// The [`Pin`][Pin] type is used instead to prevent moves through the type
-/// system. Pointers `P<T>` wrapped in the [`Pin<P<T>>`][Pin] wrapper can't be
-/// moved out of. See the [`pin` module] documentation for more information on
-/// pinning.
+/// Implementing the `Unpin` trait for `T` expresses the fact that `T` is pinning-agnostic:
+/// it shall not expose nor rely on any pinning guarantees. This, in turn, means that a
+/// `Pin`-wrapped pointer to such a type can feature a *fully unrestricted* API.
+/// In other words, if `T: Unpin`, a value of type `T` will *not* be bound by the invariants
+/// which pinning otherwise offers, even when "pinned" by a [`Pin<Ptr>`] pointing at it.
+/// When a value of type `T` is pointed at by a [`Pin<Ptr>`], [`Pin`] will not restrict access
+/// to the pointee value like it normally would, thus allowing the user to do anything that they
+/// normally could with a non-[`Pin`]-wrapped `Ptr` to that value.
///
-/// Implementing the `Unpin` trait for `T` lifts the restrictions of pinning off
-/// the type, which then allows moving `T` out of [`Pin<P<T>>`][Pin] with
-/// functions such as [`mem::replace`].
+/// The idea of this trait is to alleviate the reduced ergonomics of APIs that require the use
+/// of [`Pin`] for soundness for some types, but which also want to be used by other types that
+/// don't care about pinning. The prime example of such an API is [`Future::poll`]. There are many
+/// [`Future`] types that don't care about pinning. These futures can implement `Unpin` and
+/// therefore get around the pinning related restrictions in the API, while still allowing the
+/// subset of [`Future`]s which *do* require pinning to be implemented soundly.
///
-/// `Unpin` has no consequence at all for non-pinned data. In particular,
-/// [`mem::replace`] happily moves `!Unpin` data (it works for any `&mut T`, not
-/// just when `T: Unpin`). However, you cannot use [`mem::replace`] on data
-/// wrapped inside a [`Pin<P<T>>`][Pin] because you cannot get the `&mut T` you
-/// need for that, and *that* is what makes this system work.
+/// For more discussion on the consequences of [`Unpin`] within the wider scope of the pinning
+/// system, see the [section about `Unpin`] in the [`pin` module].
+///
+/// `Unpin` has no consequence at all for non-pinned data. In particular, [`mem::replace`] happily
+/// moves `!Unpin` data, which would be immovable when pinned ([`mem::replace`] works for any
+/// `&mut T`, not just when `T: Unpin`).
+///
+/// *However*, you cannot use [`mem::replace`] on `!Unpin` data which is *pinned* by being wrapped
+/// inside a [`Pin<Ptr>`] pointing at it. This is because you cannot (safely) use a
+/// [`Pin<Ptr>`] to get an `&mut T` to its pointee value, which you would need to call
+/// [`mem::replace`], and *that* is what makes this system work.
///
/// So this, for example, can only be done on types implementing `Unpin`:
///
@@ -935,11 +947,22 @@ marker_impls! {
/// mem::replace(&mut *pinned_string, "other".to_string());
/// ```
///
-/// This trait is automatically implemented for almost every type.
+/// This trait is automatically implemented for almost every type. The compiler is free
+/// to take the conservative stance of marking types as [`Unpin`] so long as all of the types that
+/// compose its fields are also [`Unpin`]. This is because if a type implements [`Unpin`], then it
+/// is unsound for that type's implementation to rely on pinning-related guarantees for soundness,
+/// *even* when viewed through a "pinning" pointer! It is the responsibility of the implementor of
+/// a type that relies upon pinning for soundness to ensure that type is *not* marked as [`Unpin`]
+/// by adding [`PhantomPinned`] field. For more details, see the [`pin` module] docs.
///
-/// [`mem::replace`]: crate::mem::replace
-/// [Pin]: crate::pin::Pin
-/// [`pin` module]: crate::pin
+/// [`mem::replace`]: crate::mem::replace "mem replace"
+/// [`Future`]: crate::future::Future "Future"
+/// [`Future::poll`]: crate::future::Future::poll "Future poll"
+/// [`Pin`]: crate::pin::Pin "Pin"
+/// [`Pin<Ptr>`]: crate::pin::Pin "Pin"
+/// [`pin` module]: crate::pin "pin module"
+/// [section about `Unpin`]: crate::pin#unpin "pin module docs about unpin"
+/// [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
#[stable(feature = "pin", since = "1.33.0")]
#[diagnostic::on_unimplemented(
note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",
diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs
index 7d8c881eab8..bb6c81a486a 100644
--- a/library/core/src/pin.rs
+++ b/library/core/src/pin.rs
@@ -1,188 +1,616 @@
-//! Types that pin data to its location in memory.
+//! Types that pin data to a location in memory.
//!
-//! It is sometimes useful to have objects that are guaranteed not to move,
-//! in the sense that their placement in memory does not change, and can thus be relied upon.
-//! A prime example of such a scenario would be building self-referential structs,
-//! as moving an object with pointers to itself will invalidate them, which could cause undefined
-//! behavior.
+//! It is sometimes useful to be able to rely upon a certain value not being able to *move*,
+//! in the sense that its address in memory cannot change. This is useful especially when there
+//! are one or more [*pointers*][pointer] pointing at that value. The ability to rely on this
+//! guarantee that the value a [pointer] is pointing at (its **pointee**) will
//!
-//! At a high level, a <code>[Pin]\<P></code> ensures that the pointee of any pointer type
-//! `P` has a stable location in memory, meaning it cannot be moved elsewhere
-//! and its memory cannot be deallocated until it gets dropped. We say that the
-//! pointee is "pinned". Things get more subtle when discussing types that
-//! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning)
-//! for more details.
+//! 1. Not be *moved* out of its memory location
+//! 2. More generally, remain *valid* at that same memory location
//!
-//! By default, all types in Rust are movable. Rust allows passing all types by-value,
-//! and common smart-pointer types such as <code>[Box]\<T></code> and <code>[&mut] T</code> allow
-//! replacing and moving the values they contain: you can move out of a <code>[Box]\<T></code>,
-//! or you can use [`mem::swap`]. <code>[Pin]\<P></code> wraps a pointer type `P`, so
-//! <code>[Pin]<[Box]\<T>></code> functions much like a regular <code>[Box]\<T></code>:
-//! when a <code>[Pin]<[Box]\<T>></code> gets dropped, so do its contents, and the memory gets
-//! deallocated. Similarly, <code>[Pin]<[&mut] T></code> is a lot like <code>[&mut] T</code>.
-//! However, <code>[Pin]\<P></code> does not let clients actually obtain a <code>[Box]\<T></code>
-//! or <code>[&mut] T</code> to pinned data, which implies that you cannot use operations such
-//! as [`mem::swap`]:
+//! is called "pinning." We would say that a value which satisfies these guarantees has been
+//! "pinned," in that it has been permanently (until the end of its lifespan) attached to its
+//! location in memory, as though pinned to a pinboard. Pinning a value is an incredibly useful
+//! building block for [`unsafe`] code to be able to reason about whether a raw pointer to the
+//! pinned value is still valid. [As we'll see later][drop-guarantee], this is necessarily from the
+//! time the value is first pinned until the end of its lifespan. This concept of "pinning" is
+//! necessary to implement safe interfaces on top of things like self-referential types and
+//! intrusive data structures which cannot currently be modeled in fully safe Rust using only
+//! borrow-checked [references][reference].
+//!
+//! "Pinning" allows us to put a *value* which exists at some location in memory into a state where
+//! safe code cannot *move* that value to a different location in memory or otherwise invalidate it
+//! at its current location (unless it implements [`Unpin`], which we will
+//! [talk about below][self#unpin]). Anything that wants to interact with the pinned value in a way
+//! that has the potential to violate these guarantees must promise that it will not actually
+//! violate them, using the [`unsafe`] keyword to mark that such a promise is upheld by the user
+//! and not the compiler. In this way, we can allow other [`unsafe`] code to rely on any pointers
+//! that point to the pinned value to be valid to dereference while it is pinned.
+//!
+//! Note that as long as you don't use [`unsafe`], it's impossible to create or misuse a pinned
+//! value in a way that is unsound. See the documentation of [`Pin<Ptr>`] for more
+//! information on the practicalities of how to pin a value and how to use that pinned value from a
+//! user's perspective without using [`unsafe`].
+//!
+//! The rest of this documentation is intended to be the source of truth for users of [`Pin<Ptr>`]
+//! that are implementing the [`unsafe`] pieces of an interface that relies on pinning for validity;
+//! users of [`Pin<Ptr>`] in safe code do not need to read it in detail.
+//!
+//! There are several sections to this documentation:
+//!
+//! * [What is "*moving*"?][what-is-moving]
+//! * [What is "pinning"?][what-is-pinning]
+//! * [Address sensitivity, AKA "when do we need pinning?"][address-sensitive-values]
+//! * [Examples of types with address-sensitive states][address-sensitive-examples]
+//! * [Self-referential struct][self-ref]
+//! * [Intrusive, doubly-linked list][linked-list]
+//! * [Subtle details and the `Drop` guarantee][subtle-details]
+//!
+//! # What is "*moving*"?
+//! [what-is-moving]: self#what-is-moving
+//!
+//! When we say a value is *moved*, we mean that the compiler copies, byte-for-byte, the
+//! value from one location to another. In a purely mechanical sense, this is identical to
+//! [`Copy`]ing a value from one place in memory to another. In Rust, "move" carries with it the
+//! semantics of ownership transfer from one variable to another, which is the key difference
+//! between a [`Copy`] and a move. For the purposes of this module's documentation, however, when
+//! we write *move* in italics, we mean *specifically* that the value has *moved* in the mechanical
+//! sense of being located at a new place in memory.
+//!
+//! All values in Rust are trivially *moveable*. This means that the address at which a value is
+//! located is not necessarily stable in between borrows. The compiler is allowed to *move* a value
+//! to a new address without running any code to notify that value that its address
+//! has changed. Although the compiler will not insert memory *moves* where no semantic move has
+//! occurred, there are many places where a value *may* be moved. For example, when doing
+//! assignment or passing a value into a function.
//!
//! ```
-//! use std::pin::Pin;
-//! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
-//! // `mem::swap` needs `&mut T`, but we cannot get it.
-//! // We are stuck, we cannot swap the contents of these references.
-//! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
-//! // we are not allowed to use it for moving things out of the `Pin`.
+//! #[derive(Default)]
+//! struct AddrTracker(Option<usize>);
+//!
+//! impl AddrTracker {
+//! // If we haven't checked the addr of self yet, store the current
+//! // address. If we have, confirm that the current address is the same
+//! // as it was last time, or else panic.
+//! fn check_for_move(&mut self) {
+//! let current_addr = self as *mut Self as usize;
+//! match self.0 {
+//! None => self.0 = Some(current_addr),
+//! Some(prev_addr) => assert_eq!(prev_addr, current_addr),
+//! }
+//! }
//! }
+//!
+//! // Create a tracker and store the initial address
+//! let mut tracker = AddrTracker::default();
+//! tracker.check_for_move();
+//!
+//! // Here we shadow the variable. This carries a semantic move, and may therefore also
+//! // come with a mechanical memory *move*
+//! let mut tracker = tracker;
+//!
+//! // May panic!
+//! // tracker.check_for_move();
//! ```
//!
-//! It is worth reiterating that <code>[Pin]\<P></code> does *not* change the fact that a Rust
-//! compiler considers all types movable. [`mem::swap`] remains callable for any `T`. Instead,
-//! <code>[Pin]\<P></code> prevents certain *values* (pointed to by pointers wrapped in
-//! <code>[Pin]\<P></code>) from being moved by making it impossible to call methods that require
-//! <code>[&mut] T</code> on them (like [`mem::swap`]).
+//! In this sense, Rust does not guarantee that `check_for_move()` will never panic, because the
+//! compiler is permitted to *move* `tracker` in many situations.
//!
-//! <code>[Pin]\<P></code> can be used to wrap any pointer type `P`, and as such it interacts with
-//! [`Deref`] and [`DerefMut`]. A <code>[Pin]\<P></code> where <code>P: [Deref]</code> should be
-//! considered as a "`P`-style pointer" to a pinned <code>P::[Target]</code> – so, a
-//! <code>[Pin]<[Box]\<T>></code> is an owned pointer to a pinned `T`, and a
-//! <code>[Pin]<[Rc]\<T>></code> is a reference-counted pointer to a pinned `T`.
-//! For correctness, <code>[Pin]\<P></code> relies on the implementations of [`Deref`] and
-//! [`DerefMut`] not to move out of their `self` parameter, and only ever to
-//! return a pointer to pinned data when they are called on a pinned pointer.
+//! Common smart-pointer types such as [`Box<T>`] and [`&mut T`] also allow *moving* the underlying
+//! *value* they point at: you can move out of a [`Box<T>`], or you can use [`mem::replace`] to
+//! move a `T` out of a [`&mut T`]. Therefore, putting a value (such as `tracker` above) behind a
+//! pointer isn't enough on its own to ensure that its address does not change.
//!
-//! # `Unpin`
+//! # What is "pinning"?
+//! [what-is-pinning]: self#what-is-pinning
//!
-//! Many types are always freely movable, even when pinned, because they do not
-//! rely on having a stable address. This includes all the basic types (like
-//! [`bool`], [`i32`], and references) as well as types consisting solely of these
-//! types. Types that do not care about pinning implement the [`Unpin`]
-//! auto-trait, which cancels the effect of <code>[Pin]\<P></code>. For <code>T: [Unpin]</code>,
-//! <code>[Pin]<[Box]\<T>></code> and <code>[Box]\<T></code> function identically, as do
-//! <code>[Pin]<[&mut] T></code> and <code>[&mut] T</code>.
+//! We say that a value has been *pinned* when it has been put into a state where it is guaranteed
+//! to remain *located at the same place in memory* from the time it is pinned until its
+//! [`drop`] is called.
//!
-//! Note that pinning and [`Unpin`] only affect the pointed-to type <code>P::[Target]</code>,
-//! not the pointer type `P` itself that got wrapped in <code>[Pin]\<P></code>. For example,
-//! whether or not <code>[Box]\<T></code> is [`Unpin`] has no effect on the behavior of
-//! <code>[Pin]<[Box]\<T>></code> (here, `T` is the pointed-to type).
+//! ## Address-sensitive values, AKA "when we need pinning"
+//! [address-sensitive-values]: self#address-sensitive-values-aka-when-we-need-pinning
//!
-//! # Example: self-referential struct
+//! Most values in Rust are entirely okay with being *moved* around at-will.
+//! Types for which it is *always* the case that *any* value of that type can be
+//! *moved* at-will should implement [`Unpin`], which we will discuss more [below][self#unpin].
//!
-//! Before we go into more details to explain the guarantees and choices
-//! associated with <code>[Pin]\<P></code>, we discuss some examples for how it might be used.
-//! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee).
+//! [`Pin`] is specifically targeted at allowing the implementation of *safe interfaces* around
+//! types which have some state during which they become "address-sensitive." A value in such an
+//! "address-sensitive" state is *not* okay with being *moved* around at-will. Such a value must
+//! stay *un-moved* and valid during the address-sensitive portion of its lifespan because some
+//! interface is relying on those invariants to be true in order for its implementation to be sound.
+//!
+//! As a motivating example of a type which may become address-sensitive, consider a type which
+//! contains a pointer to another piece of its own data, *i.e.* a "self-referential" type. In order
+//! for such a type to be implemented soundly, the pointer which points into `self`'s data must be
+//! proven valid whenever it is accessed. But if that value is *moved*, the pointer will still
+//! point to the old address where the value was located and not into the new location of `self`,
+//! thus becoming invalid. A key example of such self-referential types are the state machines
+//! generated by the compiler to implement [`Future`] for `async fn`s.
+//!
+//! Such types that have an *address-sensitive* state usually follow a lifecycle
+//! that looks something like so:
+//!
+//! 1. A value is created which can be freely moved around.
+//! * e.g. calling an async function which returns a state machine implementing [`Future`]
+//! 2. An operation causes the value to depend on its own address not changing
+//! * e.g. calling [`poll`] for the first time on the produced [`Future`]
+//! 3. Further pieces of the safe interface of the type use internal [`unsafe`] operations which
+//! assume that the address of the value is stable
+//! * e.g. subsequent calls to [`poll`]
+//! 4. Before the value is invalidated (e.g. deallocated), it is *dropped*, giving it a chance to
+//! notify anything with pointers to itself that those pointers will be invalidated
+//! * e.g. [`drop`]ping the [`Future`] [^pin-drop-future]
+//!
+//! There are two possible ways to ensure the invariants required for 2. and 3. above (which
+//! apply to any address-sensitive type, not just self-referrential types) do not get broken.
+//!
+//! 1. Have the value detect when it is moved and update all the pointers that point to itself.
+//! 2. Guarantee that the address of the value does not change (and that memory is not re-used
+//! for anything else) during the time that the pointers to it are expected to be valid to
+//! dereference.
+//!
+//! Since, as we discussed, Rust can move values without notifying them that they have moved, the
+//! first option is ruled out.
+//!
+//! In order to implement the second option, we must in some way enforce its key invariant,
+//! *i.e.* prevent the value from being *moved* or otherwise invalidated (you may notice this
+//! sounds an awful lot like the definition of *pinning* a value). There a few ways one might be
+//! able to enforce this invariant in Rust:
+//!
+//! 1. Offer a wholly `unsafe` API to interact with the object, thus requiring every caller to
+//! uphold the invariant themselves
+//! 2. Store the value that must not be moved behind a carefully managed pointer internal to
+//! the object
+//! 3. Leverage the type system to encode and enforce this invariant by presenting a restricted
+//! API surface to interact with *any* object that requires these invariants
+//!
+//! The first option is quite obviously undesirable, as the [`unsafe`]ty of the interface will
+//! become viral throughout all code that interacts with the object.
+//!
+//! The second option is a viable solution to the problem for some use cases, in particular
+//! for self-referrential types. Under this model, any type that has an address sensitive state
+//! would ultimately store its data in something like a [`Box<T>`], carefully manage internal
+//! access to that data to ensure no *moves* or other invalidation occurs, and finally
+//! provide a safe interface on top.
+//!
+//! There are a couple of linked disadvantages to using this model. The most significant is that
+//! each individual object must assume it is *on its own* to ensure
+//! that its data does not become *moved* or otherwise invalidated. Since there is no shared
+//! contract between values of different types, an object cannot assume that others interacting
+//! with it will properly respect the invariants around interacting with its data and must
+//! therefore protect it from everyone. Because of this, *composition* of address-sensitive types
+//! requires at least a level of pointer indirection each time a new object is added to the mix
+//! (and, practically, a heap allocation).
+//!
+//! Although there were other reason as well, this issue of expensive composition is the key thing
+//! that drove Rust towards adopting a different model. It is particularly a problem
+//! when one considers, for exapmle, the implications of composing together the [`Future`]s which
+//! will eventaully make up an asynchronous task (including address-sensitive `async fn` state
+//! machines). It is plausible that there could be many layers of [`Future`]s composed together,
+//! including multiple layers of `async fn`s handling different parts of a task. It was deemed
+//! unacceptable to force indirection and allocation for each layer of composition in this case.
+//!
+//! [`Pin<Ptr>`] is an implementation of the third option. It allows us to solve the issues
+//! discussed with the second option by building a *shared contractual language* around the
+//! guarantees of "pinning" data.
+//!
+//! [^pin-drop-future]: Futures themselves do not ever need to notify other bits of code that
+//! they are being dropped, however data structures like stack-based intrusive linked lists do.
+//!
+//! ## Using [`Pin<Ptr>`] to pin values
+//!
+//! In order to pin a value, we wrap a *pointer to that value* (of some type `Ptr`) in a
+//! [`Pin<Ptr>`]. [`Pin<Ptr>`] can wrap any pointer type, forming a promise that the **pointee**
+//! will not be *moved* or [otherwise invalidated][subtle-details].
+//!
+//! We call such a [`Pin`]-wrapped pointer a **pinning pointer,** (or pinning reference, or pinning
+//! `Box`, etc.) because its existence is the thing that is conceptually pinning the underlying
+//! pointee in place: it is the metaphorical "pin" securing the data in place on the pinboard
+//! (in memory).
+//!
+//! Notice that the thing wrapped by [`Pin`] is not the value which we want to pin itself, but
+//! rather a pointer to that value! A [`Pin<Ptr>`] does not pin the `Ptr`; instead, it pins the
+//! pointer's ***pointee** value*.
+//!
+//! ### Pinning as a library contract
+//!
+//! Pinning does not require nor make use of any compiler "magic"[^noalias], only a specific
+//! contract between the [`unsafe`] parts of a library API and its users.
+//!
+//! It is important to stress this point as a user of the [`unsafe`] parts of the [`Pin`] API.
+//! Practically, this means that performing the mechanics of "pinning" a value by creating a
+//! [`Pin<Ptr>`] to it *does not* actually change the way the compiler behaves towards the
+//! inner value! It is possible to use incorrect [`unsafe`] code to create a [`Pin<Ptr>`] to a
+//! value which does not actually satisfy the invariants that a pinned value must satisfy, and in
+//! this way lead to undefined behavior even in (from that point) fully safe code. Similarly, using
+//! [`unsafe`], one may get access to a bare [`&mut T`] from a [`Pin<Ptr>`] and
+//! use that to invalidly *move* the pinned value out. It is the job of the user of the
+//! [`unsafe`] parts of the [`Pin`] API to ensure these invariants are not violated.
+//!
+//! This differs from e.g. [`UnsafeCell`] which changes the semantics of a program's compiled
+//! output. A [`Pin<Ptr>`] is a handle to a value which we have promised we will not move out of,
+//! but Rust still considers all values themselves to be fundamentally moveable through, *e.g.*
+//! assignment or [`mem::replace`].
+//!
+//! [^noalias]: There is a bit of nuance here that is still being decided about what the aliasing
+//! semantics of `Pin<&mut T>` should be, but this is true as of today.
+//!
+//! ### How [`Pin`] prevents misuse in safe code
+//!
+//! In order to accomplish the goal of pinning the pointee value, [`Pin<Ptr>`] restricts access to
+//! the wrapped `Ptr` type in safe code. Specifically, [`Pin`] disallows the ability to access
+//! the wrapped pointer in ways that would allow the user to *move* the underlying pointee value or
+//! otherwise re-use that memory for something else without using [`unsafe`]. For example, a
+//! [`Pin<&mut T>`] makes it impossible to obtain the wrapped <code>[&mut] T</code> safely because
+//! through that <code>[&mut] T</code> it would be possible to *move* the underlying value out of
+//! the pointer with [`mem::replace`], etc.
+//!
+//! As discussed above, this promise must be upheld manually by [`unsafe`] code which interacts
+//! with the [`Pin<Ptr>`] so that other [`unsafe`] code can rely on the pointee value being
+//! *un-moved* and valid. Interfaces that operate on values which are in an address-sensitive state
+//! accept an argument like <code>[Pin]<[&mut] T></code> or <code>[Pin]<[Box]\<T>></code> to
+//! indicate this contract to the caller.
+//!
+//! [As discussed below][drop-guarantee], opting in to using pinning guarantees in the interface
+//! of an address-sensitive type has consequences for the implementation of some safe traits on
+//! that type as well.
+//!
+//! ## Interaction between [`Deref`] and [`Pin<Ptr>`]
+//!
+//! Since [`Pin<Ptr>`] can wrap any pointer type, it uses [`Deref`] and [`DerefMut`] in
+//! order to identify the type of the pinned pointee data and provide (restricted) access to it.
+//!
+//! A [`Pin<Ptr>`] where [`Ptr: Deref`][Deref] is a "`Ptr`-style pinning pointer" to a pinned
+//! [`Ptr::Target`][Target] – so, a <code>[Pin]<[Box]\<T>></code> is an owned, pinning pointer to a
+//! pinned `T`, and a <code>[Pin]<[Rc]\<T>></code> is a reference-counted, pinning pointer to a
+//! pinned `T`.
+//!
+//! [`Pin<Ptr>`] also uses the [`<Ptr as Deref>::Target`][Target] type information to modify the
+//! interface it is allowed to provide for interacting with that data (for example, when a
+//! pinning pointer points at pinned data which implements [`Unpin`], as
+//! [discussed below][self#unpin]).
+//!
+//! [`Pin<Ptr>`] requires that implementations of [`Deref`] and [`DerefMut`] on `Ptr` return a
+//! pointer to the pinned data directly and do not *move* out of the `self` parameter during their
+//! implementation of [`DerefMut::deref_mut`]. It is unsound for [`unsafe`] code to wrap pointer
+//! types with such "malicious" implementations of [`Deref`]; see [`Pin<Ptr>::new_unchecked`] for
+//! details.
+//!
+//! ## Fixing `AddrTracker`
+//!
+//! The guarantee of a stable address is necessary to make our `AddrTracker` example work. When
+//! `check_for_move` sees a <code>[Pin]<&mut AddrTracker></code>, it can safely assume that value
+//! will exist at that same address until said value goes out of scope, and thus multiple calls
+//! to it *cannot* panic.
+//!
+//! ```
+//! use std::marker::PhantomPinned;
+//! use std::pin::Pin;
+//! use std::pin::pin;
+//!
+//! #[derive(Default)]
+//! struct AddrTracker {
+//! prev_addr: Option<usize>,
+//! // remove auto-implemented `Unpin` bound to mark this type as having some
+//! // address-sensitive state. This is essential for our expected pinning
+//! // guarantees to work, and is discussed more below.
+//! _pin: PhantomPinned,
+//! }
+//!
+//! impl AddrTracker {
+//! fn check_for_move(self: Pin<&mut Self>) {
+//! let current_addr = &*self as *const Self as usize;
+//! match self.prev_addr {
+//! None => {
+//! // SAFETY: we do not move out of self
+//! let self_data_mut = unsafe { self.get_unchecked_mut() };
+//! self_data_mut.prev_addr = Some(current_addr);
+//! },
+//! Some(prev_addr) => assert_eq!(prev_addr, current_addr),
+//! }
+//! }
+//! }
+//!
+//! // 1. Create the value, not yet in an address-sensitive state
+//! let tracker = AddrTracker::default();
+//!
+//! // 2. Pin the value by putting it behind a pinning pointer, thus putting
+//! // it into an address-sensitive state
+//! let mut ptr_to_pinned_tracker: Pin<&mut AddrTracker> = pin!(tracker);
+//! ptr_to_pinned_tracker.as_mut().check_for_move();
+//!
+//! // Trying to access `tracker` or pass `ptr_to_pinned_tracker` to anything that requires
+//! // mutable access to a non-pinned version of it will no longer compile
+//!
+//! // 3. We can now assume that the tracker value will never be moved, thus
+//! // this will never panic!
+//! ptr_to_pinned_tracker.as_mut().check_for_move();
+//! ```
+//!
+//! Note that this invariant is enforced by simply making it impossible to call code that would
+//! perform a move on the pinned value. This is the case since the only way to access that pinned
+//! value is through the pinning <code>[Pin]<[&mut] T>></code>, which in turn restricts our access.
+//!
+//! ## [`Unpin`]
+//!
+//! The vast majority of Rust types have no address-sensitive states. These types
+//! implement the [`Unpin`] auto-trait, which cancels the restrictive effects of
+//! [`Pin`] when the *pointee* type `T` is [`Unpin`]. When [`T: Unpin`][Unpin],
+//! <code>[Pin]<[Box]\<T>></code> functions identically to a non-pinning [`Box<T>`]; similarly,
+//! <code>[Pin]<[&mut] T></code> would impose no additional restrictions above a regular
+//! [`&mut T`].
+//!
+//! The idea of this trait is to alleviate the reduced ergonomics of APIs that require the use
+//! of [`Pin`] for soundness for some types, but which also want to be used by other types that
+//! don't care about pinning. The prime example of such an API is [`Future::poll`]. There are many
+//! [`Future`] types that don't care about pinning. These futures can implement [`Unpin`] and
+//! therefore get around the pinning related restrictions in the API, while still allowing the
+//! subset of [`Future`]s which *do* require pinning to be implemented soundly.
+//!
+//! Note that the interaction between a [`Pin<Ptr>`] and [`Unpin`] is through the type of the
+//! **pointee** value, [`<Ptr as Deref>::Target`][Target]. Whether the `Ptr` type itself
+//! implements [`Unpin`] does not affect the behavior of a [`Pin<Ptr>`]. For example, whether or not
+//! [`Box`] is [`Unpin`] has no effect on the behavior of <code>[Pin]<[Box]\<T>></code>, because
+//! `T` is the type of the pointee value, not [`Box`]. So, whether `T` implements [`Unpin`] is
+//! the thing that will affect the behavior of the <code>[Pin]<[Box]\<T>></code>.
+//!
+//! Builtin types that are [`Unpin`] include all of the primitive types, like [`bool`], [`i32`],
+//! and [`f32`], references (<code>[&]T</code> and <code>[&mut] T</code>), etc., as well as many
+//! core and standard library types like [`Box<T>`], [`String`], and more.
+//! These types are marked [`Unpin`] because they do not have an ddress-sensitive state like the
+//! ones we discussed above. If they did have such a state, those parts of their interface would be
+//! unsound without being expressed through pinning, and they would then need to not
+//! implement [`Unpin`].
+//!
+//! The compiler is free to take the conservative stance of marking types as [`Unpin`] so long as
+//! all of the types that compose its fields are also [`Unpin`]. This is because if a type
+//! implements [`Unpin`], then it is unsound for that type's implementation to rely on
+//! pinning-related guarantees for soundness, *even* when viewed through a "pinning" pointer! It is
+//! the responsibility of the implementor of a type that relies upon pinning for soundness to
+//! ensure that type is *not* marked as [`Unpin`] by adding [`PhantomPinned`] field. This is
+//! exactly what we did with our `AddrTracker` example above. Without doing this, you *must not*
+//! rely on pinning-related guarantees to apply to your type!
+//!
+//! If need to truly pin a value of a foreign or built-in type that implements [`Unpin`], you'll
+//! need to create your own wrapper type around the [`Unpin`] type you want to pin and then
+//! opts-out of [`Unpin`] using [`PhantomPinned`].
+//!
+//! Exposing access to the inner field which you want to remain pinned must then be carefully
+//! considered as well! Remember, exposing a method that gives access to a
+//! <code>[Pin]<[&mut] InnerT>></code> where `InnerT: [Unpin]` would allow safe code to trivially
+//! move the inner value out of that pinning pointer, which is precisely what you're seeking to
+//! prevent! Exposing a field of a pinned value through a pinning pointer is called "projecting"
+//! a pin, and the more general case of deciding in which cases a pin should be able to be
+//! projected or not is called "structural pinning." We will go into more detail about this
+//! [below][structural-pinning].
+//!
+//! # Examples of address-sensitive types
+//! [address-sensitive-examples]: #examples-of-address-sensitive-types
+//!
+//! ## A self-referential struct
+//! [self-ref]: #a-self-referential-struct
+//! [`Unmovable`]: #a-self-referential-struct
+//!
+//! Self-referential structs are the simplest kind of address-sensitive type.
+//!
+//! It is often useful for a struct to hold a pointer back into itself, which
+//! allows the program to efficiently track subsections of the struct.
+//! Below, the `slice` field is a pointer into the `data` field, which
+//! we could imagine being used to track a sliding window of `data` in parser
+//! code.
+//!
+//! As mentioned before, this pattern is also used extensively by compiler-generated
+//! [`Future`]s.
//!
//! ```rust
//! use std::pin::Pin;
//! use std::marker::PhantomPinned;
//! use std::ptr::NonNull;
//!
-//! // This is a self-referential struct because the slice field points to the data field.
-//! // We cannot inform the compiler about that with a normal reference,
-//! // as this pattern cannot be described with the usual borrowing rules.
-//! // Instead we use a raw pointer, though one which is known not to be null,
-//! // as we know it's pointing at the string.
+//! /// This is a self-referential struct because `self.slice` points into `self.data`.
//! struct Unmovable {
-//! data: String,
-//! slice: NonNull<String>,
+//! /// Backing buffer.
+//! data: [u8; 64],
+//! /// Points at `self.data` which we know is itself non-null. Raw pointer because we can't do
+//! /// this with a normal reference.
+//! slice: NonNull<[u8]>,
+//! /// Suppress `Unpin` so that this cannot be moved out of a `Pin` once constructed.
//! _pin: PhantomPinned,
//! }
//!
//! impl Unmovable {
-//! // To ensure the data doesn't move when the function returns,
-//! // we place it in the heap where it will stay for the lifetime of the object,
-//! // and the only way to access it would be through a pointer to it.
-//! fn new(data: String) -> Pin<Box<Self>> {
+//! /// Create a new `Unmovable`.
+//! ///
+//! /// To ensure the data doesn't move we place it on the heap behind a pinning Box.
+//! /// Note that the data is pinned, but the `Pin<Box<Self>>` which is pinning it can
+//! /// itself still be moved. This is important because it means we can return the pinning
+//! /// pointer from the function, which is itself a kind of move!
+//! fn new() -> Pin<Box<Self>> {
//! let res = Unmovable {
-//! data,
-//! // we only create the pointer once the data is in place
-//! // otherwise it will have already moved before we even started
-//! slice: NonNull::dangling(),
+//! data: [0; 64],
+//! // We only create the pointer once the data is in place
+//! // otherwise it will have already moved before we even started.
+//! slice: NonNull::from(&[]),
//! _pin: PhantomPinned,
//! };
-//! let mut boxed = Box::pin(res);
+//! // First we put the data in a box, which will be its final resting place
+//! let mut boxed = Box::new(res);
//!
-//! let slice = NonNull::from(&boxed.data);
-//! // we know this is safe because modifying a field doesn't move the whole struct
-//! unsafe {
-//! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
-//! Pin::get_unchecked_mut(mut_ref).slice = slice;
-//! }
-//! boxed
+//! // Then we make the slice field point to the proper part of that boxed data.
+//! // From now on we need to make sure we don't move the boxed data.
+//! boxed.slice = NonNull::from(&boxed.data);
+//!
+//! // To do that, we pin the data in place by pointing to it with a pinning
+//! // (`Pin`-wrapped) pointer.
+//! //
+//! // `Box::into_pin` makes existing `Box` pin the data in-place without moving it,
+//! // so we can safely do this now *after* inserting the slice pointer above, but we have
+//! // to take care that we haven't performed any other semantic moves of `res` in between.
+//! let pin = Box::into_pin(boxed);
+//!
+//! // Now we can return the pinned (through a pinning Box) data
+//! pin
//! }
//! }
//!
-//! let unmoved = Unmovable::new("hello".to_string());
-//! // The pointer should point to the correct location,
-//! // so long as the struct hasn't moved.
+//! let unmovable: Pin<Box<Unmovable>> = Unmovable::new();
+//!
+//! // The inner pointee `Unmovable` struct will now never be allowed to move.
//! // Meanwhile, we are free to move the pointer around.
//! # #[allow(unused_mut)]
-//! let mut still_unmoved = unmoved;
+//! let mut still_unmoved = unmovable;
//! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
//!
-//! // Since our type doesn't implement Unpin, this will fail to compile:
-//! // let mut new_unmoved = Unmovable::new("world".to_string());
+//! // We cannot mutably dereference a `Pin<Ptr>` unless the pointee is `Unpin` or we use unsafe.
+//! // Since our type doesn't implement `Unpin`, this will fail to compile.
+//! // let mut new_unmoved = Unmovable::new();
//! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
//! ```
//!
-//! # Example: intrusive doubly-linked list
+//! ## An intrusive, doubly-linked list
+//! [linked-list]: #an-intrusive-doubly-linked-list
//!
-//! In an intrusive doubly-linked list, the collection does not actually allocate
-//! the memory for the elements itself. Allocation is controlled by the clients,
-//! and elements can live on a stack frame that lives shorter than the collection does.
+//! In an intrusive doubly-linked list, the collection itself does not own the memory in which
+//! each of its elements is stored. Instead, each client is free to allocate space for elements it
+//! adds to the list in whichever manner it likes, including on the stack! Elements can live on a
+//! stack frame that lives shorter than the collection does provided the elements that live in a
+//! given stack frame are removed from the list before going out of scope.
//!
-//! To make this work, every element has pointers to its predecessor and successor in
-//! the list. Elements can only be added when they are pinned, because moving the elements
-//! around would invalidate the pointers. Moreover, the [`Drop`][Drop] implementation of a linked
-//! list element will patch the pointers of its predecessor and successor to remove itself
-//! from the list.
+//! To make such an intrusive data structure work, every element stores pointers to its predecessor
+//! and successor within its own data, rather than having the list structure itself managing those
+//! pointers. It is in this sense that the structure is "intrusive": the details of how an
+//! element is stored within the larger structure "intrudes" on the implementation of the element
+//! type itself!
//!
-//! Crucially, we have to be able to rely on [`drop`] being called. If an element
-//! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it
-//! from its neighboring elements would become invalid, which would break the data structure.
+//! The full implementation details of such a data structure are outside the scope of this
+//! documentation, but we will discuss how [`Pin`] can help to do so.
//!
-//! Therefore, pinning also comes with a [`drop`]-related guarantee.
+//! Using such an intrusive pattern, elements may only be added when they are pinned. If we think
+//! about the consequences of adding non-pinned values to such a list, this becomes clear:
//!
-//! # `Drop` guarantee
+//! *Moving* or otherwise invalidating an element's data would invalidate the pointers back to it
+//! which are stored in the elements ahead and behind it. Thus, in order to soundly dereference
+//! the pointers stored to the next and previous elements, we must satisfy the guarantee that
+//! nothing has invalidated those pointers (which point to data that we do not own).
//!
-//! The purpose of pinning is to be able to rely on the placement of some data in memory.
-//! To make this work, not just moving the data is restricted; deallocating, repurposing, or
-//! otherwise invalidating the memory used to store the data is restricted, too.
-//! Concretely, for pinned data you have to maintain the invariant
-//! that *its memory will not get invalidated or repurposed from the moment it gets pinned until
-//! when [`drop`] is called*. Only once [`drop`] returns or panics, the memory may be reused.
+//! Moreover, the [`Drop`][Drop] implementation of each element must in some way notify its
+//! predecessor and successor elements that it should be removed from the list before it is fully
+//! destroyed, otherwise the pointers back to it would again become invalidated.
//!
-//! Memory can be "invalidated" by deallocation, but also by
-//! replacing a <code>[Some]\(v)</code> by [`None`], or calling [`Vec::set_len`] to "kill" some
-//! elements off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without
-//! calling the destructor first. None of this is allowed for pinned data without calling [`drop`].
+//! Crucially, this means we have to be able to rely on [`drop`] always being called before an
+//! element is invalidated. If an element could be deallocated or otherwise invalidated without
+//! calling [`drop`], the pointers to it stored in its neighboring elements would
+//! become invalid, which would break the data structure.
//!
-//! This is exactly the kind of guarantee that the intrusive linked list from the previous
-//! section needs to function correctly.
+//! Therefore, pinning data also comes with [the "`Drop` guarantee"][drop-guarantee].
//!
-//! Notice that this guarantee does *not* mean that memory does not leak! It is still
-//! completely okay to not ever call [`drop`] on a pinned element (e.g., you can still
-//! call [`mem::forget`] on a <code>[Pin]<[Box]\<T>></code>). In the example of the doubly-linked
-//! list, that element would just stay in the list. However you must not free or reuse the storage
-//! *without calling [`drop`]*.
+//! # Subtle details and the `Drop` guarantee
+//! [subtle-details]: self#subtle-details-and-the-drop-guarantee
+//! [drop-guarantee]: self#subtle-details-and-the-drop-guarantee
//!
-//! # `Drop` implementation
+//! The purpose of pinning is not *just* to prevent a value from being *moved*, but more
+//! generally to be able to rely on the pinned value *remaining valid **at a specific place*** in
+//! memory.
//!
-//! If your type uses pinning (such as the two examples above), you have to be careful
-//! when implementing [`Drop`][Drop]. The [`drop`] function takes <code>[&mut] self</code>, but this
-//! is called *even if your type was previously pinned*! It is as if the
-//! compiler automatically called [`Pin::get_unchecked_mut`].
+//! To do so, pinning a value adds an *additional* invariant that must be upheld in order for use
+//! of the pinned data to be valid, on top of the ones that must be upheld for a non-pinned value
+//! of the same type to be valid:
//!
-//! This can never cause a problem in safe code because implementing a type that
-//! relies on pinning requires unsafe code, but be aware that deciding to make
-//! use of pinning in your type (for example by implementing some operation on
-//! <code>[Pin]<[&]Self></code> or <code>[Pin]<[&mut] Self></code>) has consequences for your
+//! From the moment a value is pinned by constructing a [`Pin`]ning pointer to it, that value
+//! must *remain, **valid***, at that same address in memory, *until its [`drop`] handler is
+//! called.*
+//!
+//! There is some subtlety to this which we have not yet talked about in detail. The invariant
+//! described above means that, yes,
+//!
+//! 1. The value must not be moved out of its location in memory
+//!
+//! but it also implies that,
+//!
+//! 2. The memory location that stores the value must not get invalidated or otherwise repurposed
+//! during the lifespan of the pinned value until its [`drop`] returns or panics
+//!
+//! This point is subtle but required for intrusive data structures to be implemented soundly.
+//!
+//! ## `Drop` guarantee
+//!
+//! There needs to be a way for a pinned value to notify any code that is relying on its pinned
+//! status that it is about to be destroyed. In this way, the dependent code can remove the
+//! pinned value's address from its data structures or otherwise change its behavior with the
+//! knowledge that it can no longer rely on that value existing at the location it was pinned to.
+//!
+//! Thus, in any situation where we may want to overwrite a pinned value, that value's [`drop`] must
+//! be called beforehand (unless the pinned value implements [`Unpin`], in which case we can ignore
+//! all of [`Pin`]'s guarantees, as usual).
+//!
+//! The most common storage-reuse situations occur when a value on the stack is destroyed as part
+//! of a function return and when heap storage is freed. In both cases, [`drop`] gets run for us
+//! by Rust when using standard safe code. However, for manual heap allocations or otherwise
+//! custom-allocated storage, [`unsafe`] code must make sure to call [`ptr::drop_in_place`] before
+//! deallocating and re-using said storage.
+//!
+//! In addition, storage "re-use"/invalidation can happen even if no storage is (de-)allocated.
+//! For example, if we had an [`Option`] which contained a `Some(v)` where `v` is pinned, then `v`
+//! would be invalidated by setting that option to `None`.
+//!
+//! Similarly, if a [`Vec`] was used to store pinned values and [`Vec::set_len`] was used to
+//! manually "kill" some elements of a vector, all of the items "killed" would become invalidated,
+//! which would be *undefined behavior* if those items were pinned.
+//!
+//! Both of these cases are somewhat contrived, but it is crucial to remember that [`Pin`]ned data
+//! *must* be [`drop`]ped before it is invalidated; not just to prevent memory leaks, but as a
+//! matter of soundness. As a corollary, the following code can *never* be made safe:
+//!
+//! ```rust
+//! # use std::mem::ManuallyDrop;
+//! # use std::pin::Pin;
+//! # struct Type;
+//! // Pin something inside a `ManuallyDrop`. This is fine on its own.
+//! let mut pin: Pin<Box<ManuallyDrop<Type>>> = Box::pin(ManuallyDrop::new(Type));
+//!
+//! // However, creating a pinning mutable reference to the type *inside*
+//! // the `ManuallyDrop` is not!
+//! let inner: Pin<&mut Type> = unsafe {
+//! Pin::map_unchecked_mut(pin.as_mut(), |x| &mut **x)
+//! };
+//! ```
+//!
+//! Because [`mem::ManuallyDrop`] inhibits the destructor of `Type`, it won't get run when the
+//! <code>[Box]<[ManuallyDrop]\<Type>></code> is dropped, thus violating the drop guarantee of the
+//! <code>[Pin]<[&mut] Type>></code>.
+//!
+//! Of course, *leaking* memory in such a way that its underlying storage will never get invalidated
+//! or re-used is still fine: [`mem::forget`]ing a [`Box<T>`] prevents its storage from ever getting
+//! re-used, so the [`drop`] guarantee is still satisfied.
+//!
+//! # Implementing an address-sensitive type.
+//!
+//! This section goes into detail on important considerations for implementing your own
+//! address-sensitive types, which are different from merely using [`Pin<Ptr>`] in a generic
+//! way.
+//!
+//! ## Implementing [`Drop`] for types with address-sensitive states
+//! [drop-impl]: self#implementing-drop-for-types-with-address-sensitive-states
+//!
+//! The [`drop`] function takes [`&mut self`], but this is called *even if that `self` has been
+//! pinned*! Implementing [`Drop`] for a type with address-sensitive states, because if `self` was
+//! indeed in an address-sensitive state before [`drop`] was called, it is as if the compiler
+//! automatically called [`Pin::get_unchecked_mut`].
+//!
+//! This can never cause a problem in purely safe code because creating a pinning pointer to
+//! a type which has an address-sensitive (thus does not implement `Unpin`) requires `unsafe`,
+//! but it is important to note that choosing to take advantage of pinning-related guarantees
+//! to justify validity in the implementation of your type has consequences for that type's
//! [`Drop`][Drop] implementation as well: if an element of your type could have been pinned,
-//! you must treat [`Drop`][Drop] as implicitly taking <code>[Pin]<[&mut] Self></code>.
+//! you must treat [`Drop`][Drop] as implicitly taking <code>self: [Pin]<[&mut] Self></code>.
//!
-//! For example, you could implement [`Drop`][Drop] as follows:
+//! You should implement [`Drop`] as follows:
//!
//! ```rust,no_run
//! # use std::pin::Pin;
-//! # struct Type { }
+//! # struct Type;
//! impl Drop for Type {
//! fn drop(&mut self) {
//! // `new_unchecked` is okay because we know this value is never used
@@ -195,72 +623,157 @@
//! }
//! ```
//!
-//! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that
-//! you do not accidentally use `self`/`this` in a way that is in conflict with pinning.
+//! The function `inner_drop` has the signature that [`drop`] *should* have in this situation.
+//! This makes sure that you do not accidentally use `self`/`this` in a way that is in conflict
+//! with pinning's invariants.
//!
-//! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically
+//! Moreover, if your type is [`#[repr(packed)]`][packed], the compiler will automatically
//! move fields around to be able to drop them. It might even do
//! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use
-//! pinning with a `#[repr(packed)]` type.
+//! pinning with a [`#[repr(packed)]`][packed] type.
//!
-//! # Projections and Structural Pinning
+//! ### Implementing [`Drop`] for pointer types which will be used as [`Pin`]ning pointers
//!
-//! When working with pinned structs, the question arises how one can access the
-//! fields of that struct in a method that takes just <code>[Pin]<[&mut] Struct></code>.
-//! The usual approach is to write helper methods (so called *projections*)
-//! that turn <code>[Pin]<[&mut] Struct></code> into a reference to the field, but what type should
-//! that reference have? Is it <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>?
-//! The same question arises with the fields of an `enum`, and also when considering
-//! container/wrapper types such as <code>[Vec]\<T></code>, <code>[Box]\<T></code>,
-//! or <code>[RefCell]\<T></code>. (This question applies to both mutable and shared references,
-//! we just use the more common case of mutable references here for illustration.)
+//! It should further be noted that creating a pinning pointer of some type `Ptr` *also* carries
+//! with it implications on the way that `Ptr` type must implement [`Drop`]
+//! (as well as [`Deref`] and [`DerefMut`])! When implementing a pointer type that may be used as
+//! a pinning pointer, you must also take the same care described above not to *move* out of or
+//! otherwise invalidate the pointee during [`Drop`], [`Deref`], or [`DerefMut`]
+//! implementations.
//!
-//! It turns out that it is actually up to the author of the data structure to decide whether
-//! the pinned projection for a particular field turns <code>[Pin]<[&mut] Struct></code>
-//! into <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>. There are some
-//! constraints though, and the most important constraint is *consistency*:
-//! every field can be *either* projected to a pinned reference, *or* have
-//! pinning removed as part of the projection. If both are done for the same field,
-//! that will likely be unsound!
+//! ## "Assigning" pinned data
//!
-//! As the author of a data structure you get to decide for each field whether pinning
+//! Although in general it is not valid to swap data or assign through a [`Pin<Ptr>`] for the same
+//! reason that reusing a pinned object's memory is invalid, it is possible to do validly when
+//! implemented with special care for the needs of the exact data structure which is being
+//! modified. For example, the assigning function must know how to update all uses of the pinned
+//! address (and any other invariants necessary to satisfy validity for that type). For
+//! [`Unmovable`] (from the example above), we could write an assignment function like so:
+//!
+//! ```
+//! # use std::pin::Pin;
+//! # use std::marker::PhantomPinned;
+//! # use std::ptr::NonNull;
+//! # struct Unmovable {
+//! # data: [u8; 64],
+//! # slice: NonNull<[u8]>,
+//! # _pin: PhantomPinned,
+//! # }
+//! #
+//! impl Unmovable {
+//! // Copies the contents of `src` into `self`, fixing up the self-pointer
+//! // in the process.
+//! fn assign(self: Pin<&mut Self>, src: Pin<&mut Self>) {
+//! unsafe {
+//! let unpinned_self = Pin::into_inner_unchecked(self);
+//! let unpinned_src = Pin::into_inner_unchecked(src);
+//! *unpinned_self = Self {
+//! data: unpinned_src.data,
+//! slice: NonNull::from(&mut []),
+//! _pin: PhantomPinned,
+//! };
+//!
+//! let data_ptr = unpinned_src.data.as_ptr() as *const u8;
+//! let slice_ptr = unpinned_src.slice.as_ptr() as *const u8;
+//! let offset = slice_ptr.offset_from(data_ptr) as usize;
+//! let len = (*unpinned_src.slice.as_ptr()).len();
+//!
+//! unpinned_self.slice = NonNull::from(&mut unpinned_self.data[offset..offset+len]);
+//! }
+//! }
+//! }
+//! ```
+//!
+//! Even though we can't have the compiler do the assignment for us, it's possible to write
+//! such specialized functions for types that might need it.
+//!
+//! Note that it _is_ possible to assign generically through a [`Pin<Ptr>`] by way of [`Pin::set()`].
+//! This does not violate any guarantees, since it will run [`drop`] on the pointee value before
+//! assigning the new value. Thus, the [`drop`] implementation still has a chance to perform the
+//! necessary notifications to dependent values before the memory location of the original pinned
+//! value is overwritten.
+//!
+//! ## Projections and Structural Pinning
+//! [structural-pinning]: self#projections-and-structural-pinning
+//!
+//! With ordinary structs, it is natural that we want to add *projection* methods that allow
+//! borrowing one or more of the inner fields of a struct when the caller has access to a
+//! borrow of the whole struct:
+//!
+//! ```
+//! # struct Field;
+//! struct Struct {
+//! field: Field,
+//! // ...
+//! }
+//!
+//! impl Struct {
+//! fn field(&mut self) -> &mut Field { &mut self.field }
+//! }
+//! ```
+//!
+//! When working with address-sensitive types, it's not obvious what the signature of these
+//! functions should be. If `field` takes <code>self: [Pin]<[&mut Struct][&mut]></code>, should it
+//! return [`&mut Field`] or <code>[Pin]<[`&mut Field`]></code>? This question also arises with
+//! `enum`s and wrapper types like [`Vec<T>`], [`Box<T>`], and [`RefCell<T>`]. (This question
+//! applies just as well to shared references, but we'll examine the more common case of mutable
+//! references for illustration)
+//!
+//! It turns out that it's up to the author of `Struct` to decide which type the "projection"
+//! should produce. The choice must be *consistent* though: if a pin is projected to a field
+//! in one place, then it should very likely not be exposed elsewhere without projecting the
+//! pin.
+//!
+//! As the author of a data structure, you get to decide for each field whether pinning
//! "propagates" to this field or not. Pinning that propagates is also called "structural",
//! because it follows the structure of the type.
-//! In the following subsections, we describe the considerations that have to be made
-//! for either choice.
//!
-//! ## Pinning *is not* structural for `field`
+//! This choice depends on what guarantees you need from the field for your [`unsafe`] code to work.
+//! If the field is itself address-sensitive, or participates in the parent struct's address
+//! sensitivity, it will need to be structurally pinned.
//!
-//! It may seem counter-intuitive that the field of a pinned struct might not be pinned,
-//! but that is actually the easiest choice: if a <code>[Pin]<[&mut] Field></code> is never created,
-//! nothing can go wrong! So, if you decide that some field does not have structural pinning,
-//! all you have to ensure is that you never create a pinned reference to that field.
+//! A useful test is if [`unsafe`] code that consumes <code>[Pin]\<[&mut Struct][&mut]></code>
+//! also needs to take note of the address of the field itself, it may be evidence that that field
+//! is structurally pinned. Unfortunately, there are no hard-and-fast rules.
+//!
+//! ### Choosing pinning *not to be* structural for `field`...
+//!
+//! While counter-intuitive, it's often the easier choice: if you do not expose a
+//! <code>[Pin]<[&mut] Field></code>, you do not need to be careful about other code
+//! moving out of that field, you just have to ensure is that you never create pinning
+//! reference to that field. This does of course also mean that if you decide a field does not
+//! have structural pinning, you must not write [`unsafe`] code that assumes (invalidly) that the
+//! field *is* structurally pinned!
//!
//! Fields without structural pinning may have a projection method that turns
-//! <code>[Pin]<[&mut] Struct></code> into <code>[&mut] Field</code>:
+//! <code>[Pin]<[&mut] Struct></code> into [`&mut Field`]:
//!
//! ```rust,no_run
//! # use std::pin::Pin;
//! # type Field = i32;
//! # struct Struct { field: Field }
//! impl Struct {
-//! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field {
-//! // This is okay because `field` is never considered pinned.
+//! fn field(self: Pin<&mut Self>) -> &mut Field {
+//! // This is okay because `field` is never considered pinned, therefore we do not
+//! // need to uphold any pinning guarantees for this field in particular. Of course,
+//! // we must not elsewhere assume this field *is* pinned if we choose to expose
+//! // such a method!
//! unsafe { &mut self.get_unchecked_mut().field }
//! }
//! }
//! ```
//!
-//! You may also <code>impl [Unpin] for Struct</code> *even if* the type of `field`
-//! is not [`Unpin`]. What that type thinks about pinning is not relevant
-//! when no <code>[Pin]<[&mut] Field></code> is ever created.
+//! You may also in this situation <code>impl [Unpin] for Struct {}</code> *even if* the type of
+//! `field` is not [`Unpin`]. Since we have explicitly chosen not to care about pinning guarantees
+//! for `field`, the way `field`'s type interacts with pinning is no longer relevant in the
+//! context of its use in `Struct`.
//!
-//! ## Pinning *is* structural for `field`
+//! ### Choosing pinning *to be* structural for `field`...
//!
//! The other option is to decide that pinning is "structural" for `field`,
//! meaning that if the struct is pinned then so is the field.
//!
-//! This allows writing a projection that creates a <code>[Pin]<[&mut] Field></code>, thus
+//! This allows writing a projection that creates a <code>[Pin]<[`&mut Field`]></code>, thus
//! witnessing that the field is pinned:
//!
//! ```rust,no_run
@@ -268,108 +781,117 @@
//! # type Field = i32;
//! # struct Struct { field: Field }
//! impl Struct {
-//! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> {
+//! fn field(self: Pin<&mut Self>) -> Pin<&mut Field> {
//! // This is okay because `field` is pinned when `self` is.
//! unsafe { self.map_unchecked_mut(|s| &mut s.field) }
//! }
//! }
//! ```
//!
-//! However, structural pinning comes with a few extra requirements:
+//! Structural pinning comes with a few extra requirements:
//!
-//! 1. The struct must only be [`Unpin`] if all the structural fields are
-//! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of
-//! the struct it is your responsibility *not* to add something like
-//! <code>impl\<T> [Unpin] for Struct\<T></code>. (Notice that adding a projection operation
-//! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break
-//! the principle that you only have to worry about any of this if you use [`unsafe`].)
-//! 2. The destructor of the struct must not move structural fields out of its argument. This
-//! is the exact point that was raised in the [previous section][drop-impl]: [`drop`] takes
-//! <code>[&mut] self</code>, but the struct (and hence its fields) might have been pinned
-//! before. You have to guarantee that you do not move a field inside your [`Drop`][Drop]
-//! implementation. In particular, as explained previously, this means that your struct
-//! must *not* be `#[repr(packed)]`.
-//! See that section for how to write [`drop`] in a way that the compiler can help you
-//! not accidentally break pinning.
-//! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
-//! once your struct is pinned, the memory that contains the
-//! content is not overwritten or deallocated without calling the content's destructors.
-//! This can be tricky, as witnessed by <code>[VecDeque]\<T></code>: the destructor of
-//! <code>[VecDeque]\<T></code> can fail to call [`drop`] on all elements if one of the
-//! destructors panics. This violates the [`Drop`][Drop] guarantee, because it can lead to
-//! elements being deallocated without their destructor being called.
-//! (<code>[VecDeque]\<T></code> has no pinning projections, so this
-//! does not cause unsoundness.)
-//! 4. You must not offer any other operations that could lead to data being moved out of
+//! 1. *Structural [`Unpin`].* A struct can be [`Unpin`] only if all of its
+//! structurally-pinned fields are, too. This is [`Unpin`]'s behavior by default.
+//! However, as a libray author, it is your responsibility not to write something like
+//! <code>impl\<T> [Unpin] for Struct\<T> {}</code> and then offer a method that provides
+//! structural pinning to an inner field of `T`, which may not be [`Unpin`]! (Adding *any*
+//! projection operation requires unsafe code, so the fact that [`Unpin`] is a safe trait does
+//! not break the principle that you only have to worry about any of this if you use
+//! [`unsafe`])
+//!
+//! 2. *Pinned Destruction.* As discussed [above][drop-impl], [`drop`] takes
+//! [`&mut self`], but the struct (and hence its fields) might have been pinned
+//! before. The destructor must be written as if its argument was
+//! <code>self: [Pin]\<[`&mut Self`]></code>, instead.
+//!
+//! As a consequence, the struct *must not* be [`#[repr(packed)]`][packed].
+//!
+//! 3. *Structural Notice of Destruction.* You must uphold the the
+//! [`Drop` guarantee][drop-guarantee]: once your struct is pinned, the struct's storage cannot
+//! be re-used without calling the structurally-pinned fields' destructors, as well.
+//!
+//! This can be tricky, as witnessed by [`VecDeque<T>`]: the destructor of [`VecDeque<T>`]
+//! can fail to call [`drop`] on all elements if one of the destructors panics. This violates
+//! the [`Drop` guarantee][drop-guarantee], because it can lead to elements being deallocated
+//! without their destructor being called.
+//!
+//! [`VecDeque<T>`] has no pinning projections, so its destructor is sound. If it wanted
+//! to provide such structural pinning, its destructor would need to abort the process if any
+//! of the destructors panicked.
+//!
+//! 4. You must not offer any other operations that could lead to data being *moved* out of
//! the structural fields when your type is pinned. For example, if the struct contains an
-//! <code>[Option]\<T></code> and there is a [`take`][Option::take]-like operation with type
-//! <code>fn([Pin]<[&mut] Struct\<T>>) -> [Option]\<T></code>,
-//! that operation can be used to move a `T` out of a pinned `Struct<T>` – which means
-//! pinning cannot be structural for the field holding this data.
+//! [`Option<T>`] and there is a [`take`][Option::take]-like operation with type
+//! <code>fn([Pin]<[&mut Struct\<T>][&mut]>) -> [`Option<T>`]</code>,
+//! then that operation can be used to move a `T` out of a pinned `Struct<T>` – which
+//! means pinning cannot be structural for the field holding this data.
//!
//! For a more complex example of moving data out of a pinned type,
-//! imagine if <code>[RefCell]\<T></code> had a method
-//! <code>fn get_pin_mut(self: [Pin]<[&mut] Self>) -> [Pin]<[&mut] T></code>.
+//! imagine if [`RefCell<T>`] had a method
+//! <code>fn get_pin_mut(self: [Pin]<[`&mut Self`]>) -> [Pin]<[`&mut T`]></code>.
//! Then we could do the following:
//! ```compile_fail
+//! # use std::cell::RefCell;
+//! # use std::pin::Pin;
//! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
-//! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
-//! let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
-//! let b = rc_shr.borrow_mut();
-//! let content = &mut *b; // And here we have `&mut T` to the same data.
+//! // Here we get pinned access to the `T`.
+//! let _: Pin<&mut T> = rc.as_mut().get_pin_mut();
+//!
+//! // And here we have `&mut T` to the same data.
+//! let shared: &RefCell<T> = rc.into_ref().get_ref();
+//! let borrow = shared.borrow_mut();
+//! let content = &mut *borrow;
//! }
//! ```
-//! This is catastrophic, it means we can first pin the content of the
-//! <code>[RefCell]\<T></code> (using <code>[RefCell]::get_pin_mut</code>) and then move that
+//! This is catastrophic: it means we can first pin the content of the
+//! [`RefCell<T>`] (using <code>[RefCell]::get_pin_mut</code>) and then move that
//! content using the mutable reference we got later.
//!
-//! ## Examples
+//! ### Structural Pinning examples
//!
-//! For a type like <code>[Vec]\<T></code>, both possibilities (structural pinning or not) make
-//! sense. A <code>[Vec]\<T></code> with structural pinning could have `get_pin`/`get_pin_mut`
-//! methods to get pinned references to elements. However, it could *not* allow calling
-//! [`pop`][Vec::pop] on a pinned <code>[Vec]\<T></code> because that would move the (structurally
+//! For a type like [`Vec<T>`], both possibilities (structural pinning or not) make
+//! sense. A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut`
+//! methods to get pinning references to elements. However, it could *not* allow calling
+//! [`pop`][Vec::pop] on a pinned [`Vec<T>`] because that would move the (structurally
//! pinned) contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also
//! move the contents.
//!
-//! A <code>[Vec]\<T></code> without structural pinning could
-//! <code>impl\<T> [Unpin] for [Vec]\<T></code>, because the contents are never pinned
-//! and the <code>[Vec]\<T></code> itself is fine with being moved as well.
+//! A [`Vec<T>`] without structural pinning could
+//! <code>impl\<T> [Unpin] for [`Vec<T>`]</code>, because the contents are never pinned
+//! and the [`Vec<T>`] itself is fine with being moved as well.
//! At that point pinning just has no effect on the vector at all.
//!
//! In the standard library, pointer types generally do not have structural pinning,
-//! and thus they do not offer pinning projections. This is why <code>[Box]\<T>: [Unpin]</code>
+//! and thus they do not offer pinning projections. This is why <code>[`Box<T>`]: [Unpin]</code>
//! holds for all `T`. It makes sense to do this for pointer types, because moving the
-//! <code>[Box]\<T></code> does not actually move the `T`: the <code>[Box]\<T></code> can be freely
-//! movable (aka [`Unpin`]) even if the `T` is not. In fact, even <code>[Pin]<[Box]\<T>></code> and
-//! <code>[Pin]<[&mut] T></code> are always [`Unpin`] themselves, for the same reason:
+//! [`Box<T>`] does not actually move the `T`: the [`Box<T>`] can be freely
+//! movable (aka [`Unpin`]) even if the `T` is not. In fact, even <code>[Pin]<[`Box<T>`]></code> and
+//! <code>[Pin]<[`&mut T`]></code> are always [`Unpin`] themselves, for the same reason:
//! their contents (the `T`) are pinned, but the pointers themselves can be moved without moving
-//! the pinned data. For both <code>[Box]\<T></code> and <code>[Pin]<[Box]\<T>></code>,
+//! the pinned data. For both [`Box<T>`] and <code>[Pin]<[`Box<T>`]></code>,
//! whether the content is pinned is entirely independent of whether the
//! pointer is pinned, meaning pinning is *not* structural.
//!
//! When implementing a [`Future`] combinator, you will usually need structural pinning
-//! for the nested futures, as you need to get pinned references to them to call [`poll`].
-//! But if your combinator contains any other data that does not need to be pinned,
+//! for the nested futures, as you need to get pinning ([`Pin`]-wrapped) references to them to
+//! call [`poll`]. But if your combinator contains any other data that does not need to be pinned,
//! you can make those fields not structural and hence freely access them with a
-//! mutable reference even when you just have <code>[Pin]<[&mut] Self></code> (such as in your own
-//! [`poll`] implementation).
+//! mutable reference even when you just have <code>[Pin]<[`&mut Self`]></code>
+//! (such as in your own [`poll`] implementation).
//!
+//! [`&mut T`]: &mut
+//! [`&mut self`]: &mut
+//! [`&mut Self`]: &mut
+//! [`&mut Field`]: &mut
//! [Deref]: crate::ops::Deref "ops::Deref"
//! [`Deref`]: crate::ops::Deref "ops::Deref"
//! [Target]: crate::ops::Deref::Target "ops::Deref::Target"
//! [`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"
+//! [ManuallyDrop]: crate::mem::ManuallyDrop "ManuallyDrop"
//! [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
@@ -378,6 +900,23 @@
//! [&]: reference "shared reference"
//! [&mut]: reference "mutable reference"
//! [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
+//! [packed]: https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked
+//! [`std::alloc`]: ../../std/alloc/index.html
+//! [`Box<T>`]: ../../std/boxed/struct.Box.html
+//! [Box]: ../../std/boxed/struct.Box.html "Box"
+//! [`Box`]: ../../std/boxed/struct.Box.html "Box"
+//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
+//! [Rc]: ../../std/rc/struct.Rc.html "rc::Rc"
+//! [`Vec<T>`]: ../../std/vec/struct.Vec.html
+//! [Vec]: ../../std/vec/struct.Vec.html "Vec"
+//! [`Vec`]: ../../std/vec/struct.Vec.html "Vec"
+//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len "Vec::set_len"
+//! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop "Vec::pop"
+//! [Vec::push]: ../../std/vec/struct.Vec.html#method.push "Vec::push"
+//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
+//! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
+//! [VecDeque]: ../../std/collections/struct.VecDeque.html "collections::VecDeque"
+//! [`String`]: ../../std/string/struct.String.html "String"
#![stable(feature = "pin", since = "1.33.0")]
@@ -386,17 +925,159 @@ use crate::fmt;
use crate::hash::{Hash, Hasher};
use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
-/// A pinned pointer.
+#[allow(unused_imports)]
+use crate::{
+ cell::{RefCell, UnsafeCell},
+ future::Future,
+ marker::PhantomPinned,
+ mem, ptr,
+};
+
+/// A pointer which pins its pointee in place.
///
-/// 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`].
+/// [`Pin`] is a wrapper around some kind of pointer `Ptr` which makes that pointer "pin" its
+/// pointee value in place, thus preventing the value referenced by that pointer from being moved
+/// or otherwise invalidated at that place in memory unless it implements [`Unpin`].
///
-/// `Pin<P>` is guaranteed to have the same memory layout and ABI as `P`.
+/// *See the [`pin` module] documentation for a more thorough exploration of pinning.*
///
-/// *See the [`pin` module] documentation for an explanation of pinning.*
+/// ## Pinning values with [`Pin<Ptr>`]
///
-/// [`pin` module]: self
+/// In order to pin a value, we wrap a *pointer to that value* (of some type `Ptr`) in a
+/// [`Pin<Ptr>`]. [`Pin<Ptr>`] can wrap any pointer type, forming a promise that the **pointee**
+/// will not be *moved* or [otherwise invalidated][subtle-details]. If the pointee value's type
+/// implements [`Unpin`], we are free to disregard these requirements entirely and can wrap any
+/// pointer to that value in [`Pin`] directly via [`Pin::new`]. If the pointee value's type does
+/// not implement [`Unpin`], then Rust will not let us use the [`Pin::new`] function directly and
+/// we'll need to construct a [`Pin`]-wrapped pointer in one of the more specialized manners
+/// discussed below.
+///
+/// We call such a [`Pin`]-wrapped pointer a **pinning pointer** (or pinning ref, or pinning
+/// [`Box`], etc.) because its existince is the thing that is pinning the underlying pointee in
+/// place: it is the metaphorical "pin" securing the data in place on the pinboard (in memory).
+///
+/// It is important to stress that the thing in the [`Pin`] is not the value which we want to pin
+/// itself, but rather a pointer to that value! A [`Pin<Ptr>`] does not pin the `Ptr` but rather
+/// the pointer's ***pointee** value*.
+///
+/// The most common set of types which require pinning related guarantees for soundness are the
+/// compiler-generated state machines that implement [`Future`] for the return value of
+/// `async fn`s. These compiler-generated [`Future`]s may contain self-referrential pointers, one
+/// of the most common use cases for [`Pin`]. More details on this point are provided in the
+/// [`pin` module] docs, but suffice it to say they require the guarantees provided by pinning to
+/// be implemented soundly.
+///
+/// This requirement for the implementation of `async fn`s means that the [`Future`] trait
+/// requires all calls to [`poll`] to use a <code>self: [Pin]\<&mut Self></code> parameter instead
+/// of the usual `&mut self`. Therefore, when manually polling a future, you will need to pin it
+/// first.
+///
+/// You may notice that `async fn`-sourced [`Future`]s are only a small percentage of all
+/// [`Future`]s that exist, yet we had to modify the signature of [`poll`] for all [`Future`]s
+/// to accommodate them. This is unfortunate, but there is a way that the language attempts to
+/// alleviate the extra friction that this API choice incurs: the [`Unpin`] trait.
+///
+/// The vast majority of Rust types have no reason to ever care about being pinned. These
+/// types implement the [`Unpin`] trait, which entirely opts all values of that type out of
+/// pinning-related guarantees. For values of these types, pinning a value by pointing to it with a
+/// [`Pin<Ptr>`] will have no actual effect.
+///
+/// The reason this distinction exists is exactly to allow APIs like [`Future::poll`] to take a
+/// [`Pin<Ptr>`] as an argument for all types while only forcing [`Future`] types that actually
+/// care about pinning guarantees pay the ergonomics cost. For the majority of [`Future`] types
+/// that don't have a reason to care about being pinned and therefore implement [`Unpin`], the
+/// <code>[Pin]\<&mut Self></code> will act exactly like a regular `&mut Self`, allowing direct
+/// access to the underlying value. Only types that *don't* implement [`Unpin`] will be restricted.
+///
+/// ### Pinning a value of a type that implements [`Unpin`]
+///
+/// If the type of the value you need to "pin" implements [`Unpin`], you can trivially wrap any
+/// pointer to that value in a [`Pin`] by calling [`Pin::new`].
+///
+/// ```
+/// use std::pin::Pin;
+///
+/// // Create a value of a type that implements `Unpin`
+/// let mut unpin_future = std::future::ready(5);
+///
+/// // Pin it by creating a pinning mutable reference to it (ready to be `poll`ed!)
+/// let my_pinned_unpin_future: Pin<&mut _> = Pin::new(&mut unpin_future);
+/// ```
+///
+/// ### Pinning a value inside a [`Box`]
+///
+/// The simplest and most flexible way to pin a value that does not implement [`Unpin`] is to put
+/// that value inside a [`Box`] and then turn that [`Box`] into a "pinning [`Box`]" by wrapping it
+/// in a [`Pin`]. You can do both of these in a single step using [`Box::pin`]. Let's see an
+/// example of using this flow to pin a [`Future`] returned from calling an `async fn`, a common
+/// use case as described above.
+///
+/// ```
+/// use std::pin::Pin;
+///
+/// async fn add_one(x: u32) -> u32 {
+/// x + 1
+/// }
+///
+/// // Call the async function to get a future back
+/// let fut = add_one(42);
+///
+/// // Pin the future inside a pinning box
+/// let pinned_fut: Pin<Box<_>> = Box::pin(fut);
+/// ```
+///
+/// If you have a value which is already boxed, for example a [`Box<dyn Future>`][Box], you can pin
+/// that value in-place at its current memory address using [`Box::into_pin`].
+///
+/// ```
+/// use std::pin::Pin;
+/// use std::future::Future;
+///
+/// async fn add_one(x: u32) -> u32 {
+/// x + 1
+/// }
+///
+/// fn boxed_add_one(x: u32) -> Box<dyn Future<Output = u32>> {
+/// Box::new(add_one(x))
+/// }
+///
+/// let boxed_fut = boxed_add_one(42);
+///
+/// // Pin the future inside the existing box
+/// let pinned_fut: Pin<Box<_>> = Box::into_pin(boxed_fut);
+/// ```
+///
+/// There are similar pinning methods offered on the other standard library smart pointer types
+/// as well, like [`Rc`] and [`Arc`].
+///
+/// ### Pinning a value on the stack using [`pin!`]
+///
+/// There are some situations where it is desirable or even required (for example, in a `#[no_std]`
+/// context where you don't have access to the standard library or allocation in general) to
+/// pin a value which does not implement [`Unpin`] to its location on the stack. Doing so is
+/// possible using the [`pin!`] macro. See its documentation for more.
+///
+/// ## Layout and ABI
+///
+/// [`Pin<Ptr>`] is guaranteed to have the same memory layout and ABI[^noalias] as `Ptr`.
+///
+/// [^noalias]: There is a bit of nuance here that is still being decided about whether the
+/// aliasing semantics of `Pin<&mut T>` should be different than `&mut T`, but this is true as of
+/// today.
+///
+/// [`pin!`]: crate::pin::pin "pin!"
+/// [`Future`]: crate::future::Future "Future"
+/// [`poll`]: crate::future::Future::poll "Future::poll"
+/// [`Future::poll`]: crate::future::Future::poll "Future::poll"
+/// [`pin` module]: self "pin module"
+/// [`Rc`]: ../../std/rc/struct.Rc.html "Rc"
+/// [`Arc`]: ../../std/sync/struct.Arc.html "Arc"
+/// [Box]: ../../std/boxed/struct.Box.html "Box"
+/// [`Box`]: ../../std/boxed/struct.Box.html "Box"
+/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin "Box::pin"
+/// [`Box::into_pin`]: ../../std/boxed/struct.Box.html#method.into_pin "Box::into_pin"
+/// [subtle-details]: self#subtle-details-and-the-drop-guarantee "pin subtle details"
+/// [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
//
// Note: the `Clone` derive below causes unsoundness as it's possible to implement
// `Clone` for mutable references.
@@ -406,7 +1087,7 @@ use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
#[fundamental]
#[repr(transparent)]
#[derive(Copy, Clone)]
-pub struct Pin<P> {
+pub struct Pin<Ptr> {
// FIXME(#93176): this field is made `#[unstable] #[doc(hidden)] pub` to:
// - deter downstream users from accessing it (which would be unsound!),
// - let the `pin!` macro access it (such a macro requires using struct
@@ -414,7 +1095,7 @@ pub struct Pin<P> {
// Long-term, `unsafe` fields or macro hygiene are expected to offer more robust alternatives.
#[unstable(feature = "unsafe_pin_internals", issue = "none")]
#[doc(hidden)]
- pub pointer: P,
+ pub pointer: Ptr,
}
// The following implementations aren't derived in order to avoid soundness
@@ -424,68 +1105,68 @@ pub struct Pin<P> {
// 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>
+impl<Ptr: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<Ptr>
where
- P::Target: PartialEq<Q::Target>,
+ Ptr::Target: PartialEq<Q::Target>,
{
fn eq(&self, other: &Pin<Q>) -> bool {
- P::Target::eq(self, other)
+ Ptr::Target::eq(self, other)
}
fn ne(&self, other: &Pin<Q>) -> bool {
- P::Target::ne(self, other)
+ Ptr::Target::ne(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
-impl<P: Deref<Target: Eq>> Eq for Pin<P> {}
+impl<Ptr: Deref<Target: Eq>> Eq for Pin<Ptr> {}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
-impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P>
+impl<Ptr: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<Ptr>
where
- P::Target: PartialOrd<Q::Target>,
+ Ptr::Target: PartialOrd<Q::Target>,
{
fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
- P::Target::partial_cmp(self, other)
+ Ptr::Target::partial_cmp(self, other)
}
fn lt(&self, other: &Pin<Q>) -> bool {
- P::Target::lt(self, other)
+ Ptr::Target::lt(self, other)
}
fn le(&self, other: &Pin<Q>) -> bool {
- P::Target::le(self, other)
+ Ptr::Target::le(self, other)
}
fn gt(&self, other: &Pin<Q>) -> bool {
- P::Target::gt(self, other)
+ Ptr::Target::gt(self, other)
}
fn ge(&self, other: &Pin<Q>) -> bool {
- P::Target::ge(self, other)
+ Ptr::Target::ge(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
-impl<P: Deref<Target: Ord>> Ord for Pin<P> {
+impl<Ptr: Deref<Target: Ord>> Ord for Pin<Ptr> {
fn cmp(&self, other: &Self) -> cmp::Ordering {
- P::Target::cmp(self, other)
+ Ptr::Target::cmp(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
-impl<P: Deref<Target: Hash>> Hash for Pin<P> {
+impl<Ptr: Deref<Target: Hash>> Hash for Pin<Ptr> {
fn hash<H: Hasher>(&self, state: &mut H) {
- P::Target::hash(self, state);
+ Ptr::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
+impl<Ptr: Deref<Target: Unpin>> Pin<Ptr> {
+ /// Construct a new `Pin<Ptr>` 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.
+ /// `Ptr` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
///
/// # Examples
///
@@ -493,22 +1174,25 @@ impl<P: Deref<Target: Unpin>> Pin<P> {
/// use std::pin::Pin;
///
/// let mut val: u8 = 5;
- /// // We can pin the value, since it doesn't care about being moved
+ ///
+ /// // Since `val` doesn't care about being moved, we can safely create a "facade" `Pin`
+ /// // which will allow `val` to participate in `Pin`-bound apis without checking that
+ /// // pinning guarantees are actually upheld.
/// let mut pinned: Pin<&mut u8> = Pin::new(&mut val);
/// ```
#[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> {
+ pub const fn new(pointer: Ptr) -> Pin<Ptr> {
// 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.
+ /// Unwraps this `Pin<Ptr>`, returning the underlying pointer.
///
- /// This requires that the data inside this `Pin` implements [`Unpin`] so that we
- /// can ignore the pinning invariants when unwrapping it.
+ /// Doing this operation safely requires that the data pointed at by this pinning pointer
+ /// implemts [`Unpin`] so that we can ignore the pinning invariants when unwrapping it.
///
/// # Examples
///
@@ -517,46 +1201,54 @@ impl<P: Deref<Target: Unpin>> Pin<P> {
///
/// let mut val: u8 = 5;
/// let pinned: Pin<&mut u8> = Pin::new(&mut val);
- /// // Unwrap the pin to get a reference to the value
+ ///
+ /// // Unwrap the pin to get the underlying mutable reference to the value. We can do
+ /// // this because `val` doesn't care about being moved, so the `Pin` was just
+ /// // a "facade" anyway.
/// let r = Pin::into_inner(pinned);
/// assert_eq!(*r, 5);
/// ```
#[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 {
+ pub const fn into_inner(pin: Pin<Ptr>) -> Ptr {
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`.
+impl<Ptr: Deref> Pin<Ptr> {
+ /// Construct a new `Pin<Ptr>` 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
+ /// 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.
+ /// pointed to by `pointer` is pinned. At its core, pinning a value means making the
+ /// guarantee that the value's data will not be moved nor have its storage invalidated until
+ /// it gets dropped. For a more thorough explanation of pinning, see the [`pin` module docs].
///
- /// By using this method, you are making a promise about the `P::Deref` and
- /// `P::DerefMut` implementations, if they exist. Most importantly, they
+ /// If the caller that is constructing this `Pin<Ptr>` does not ensure that the data `Ptr`
+ /// points to is pinned, that is a violation of the API contract and may lead to undefined
+ /// behavior in later (even safe) operations.
+ ///
+ /// By using this method, you are also making a promise about the [`Deref`] and
+ /// [`DerefMut`] implementations of `Ptr`, 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*
+ /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pointer type `Ptr`*
/// and expect these methods to uphold the pinning invariants.
- /// Moreover, by calling this method you promise that the reference `P`
+ /// Moreover, by calling this method you promise that the reference `Ptr`
/// dereferences to will not be moved out of again; in particular, it
- /// must not be possible to obtain a `&mut P::Target` and then
+ /// must not be possible to obtain a `&mut Ptr::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:
+ /// over whether it is kept pinned once `'a` ends, and therefore cannot uphold the
+ /// guarantee that a value, once pinned, remains pinned until it is dropped:
+ ///
/// ```
/// use std::mem;
/// use std::pin::Pin;
@@ -583,12 +1275,14 @@ impl<P: Deref> Pin<P> {
/// use std::pin::Pin;
///
/// fn move_pinned_rc<T>(mut x: Rc<T>) {
- /// let pinned = unsafe { Pin::new_unchecked(Rc::clone(&x)) };
+ /// // This should mean the pointee can never move again.
+ /// let pin = unsafe { Pin::new_unchecked(Rc::clone(&x)) };
/// {
- /// let p: Pin<&T> = pinned.as_ref();
- /// // This should mean the pointee can never move again.
+ /// let p: Pin<&T> = pin.as_ref();
+ /// // ...
/// }
- /// drop(pinned);
+ /// drop(pin);
+ ///
/// let content = Rc::get_mut(&mut x).unwrap(); // Potential UB down the road ⚠️
/// // 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
@@ -649,15 +1343,16 @@ impl<P: Deref> Pin<P> {
/// ```
///
/// [`mem::swap`]: crate::mem::swap
+ /// [`pin` module docs]: self
#[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> {
+ pub const unsafe fn new_unchecked(pointer: Ptr) -> Pin<Ptr> {
Pin { pointer }
}
- /// Gets a pinned shared reference from this pinned pointer.
+ /// Gets a shared reference to the pinned value this [`Pin`] points to.
///
/// 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`,
@@ -666,34 +1361,39 @@ impl<P: Deref> Pin<P> {
/// 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> {
+ pub fn as_ref(&self) -> Pin<&Ptr::Target> {
// SAFETY: see documentation on this function
unsafe { Pin::new_unchecked(&*self.pointer) }
}
- /// Unwraps this `Pin<P>` returning the underlying pointer.
+ /// Unwraps this `Pin<Ptr>`, returning the underlying `Ptr`.
///
/// # 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
+ /// treat the pointer `Ptr` 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
+ /// resulting `Ptr` 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.
///
+ /// Note that you must be able to guarantee that the data pointed to by `Ptr`
+ /// will be treated as pinned all the way until its `drop` handler is complete!
+ ///
+ /// *For more information, see the [`pin` module docs][self]*
+ ///
/// 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 {
+ pub const unsafe fn into_inner_unchecked(pin: Pin<Ptr>) -> Ptr {
pin.pointer
}
}
-impl<P: DerefMut> Pin<P> {
- /// Gets a pinned mutable reference from this pinned pointer.
+impl<Ptr: DerefMut> Pin<Ptr> {
+ /// Gets a mutable reference to the pinned value this `Pin<Ptr>` points to.
///
/// 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`,
@@ -701,7 +1401,8 @@ impl<P: DerefMut> Pin<P> {
/// "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.
+ /// This method is useful when doing multiple calls to functions that consume the
+ /// pinning pointer.
///
/// # Example
///
@@ -723,15 +1424,17 @@ impl<P: DerefMut> Pin<P> {
/// ```
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
- pub fn as_mut(&mut self) -> Pin<&mut P::Target> {
+ pub fn as_mut(&mut self) -> Pin<&mut Ptr::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.
+ /// Assigns a new value to the memory location pointed to by the `Pin<Ptr>`.
///
- /// This overwrites pinned data, but that is okay: its destructor gets
- /// run before being overwritten, so no pinning guarantee is violated.
+ /// This overwrites pinned data, but that is okay: the original pinned value's destructor gets
+ /// run before being overwritten and the new value is also a valid value of the same type, so
+ /// no pinning invariant is violated. See [the `pin` module documentation][subtle-details]
+ /// for more information on how this upholds the pinning invariants.
///
/// # Example
///
@@ -741,14 +1444,16 @@ impl<P: DerefMut> Pin<P> {
/// let mut val: u8 = 5;
/// let mut pinned: Pin<&mut u8> = Pin::new(&mut val);
/// println!("{}", pinned); // 5
- /// pinned.as_mut().set(10);
+ /// pinned.set(10);
/// println!("{}", pinned); // 10
/// ```
+ ///
+ /// [subtle-details]: self#subtle-details-and-the-drop-guarantee
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
- pub fn set(&mut self, value: P::Target)
+ pub fn set(&mut self, value: Ptr::Target)
where
- P::Target: Sized,
+ Ptr::Target: Sized,
{
*(self.pointer) = value;
}
@@ -790,15 +1495,15 @@ impl<'a, T: ?Sized> Pin<&'a T> {
/// 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.
+ /// to the inner `T` inside the `RefCell`, and `RefCell<T>` does not let you get a
+ /// `Pin<&T>` pointer 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`.
+ /// the reference contained in the `Pin`. This method allows turning the `Pin` into a reference
+ /// with the same lifetime as the reference it wraps.
///
/// ["pinning projections"]: self#projections-and-structural-pinning
#[inline(always)]
@@ -891,9 +1596,9 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
}
impl<T: ?Sized> Pin<&'static T> {
- /// Get a pinned reference from a static reference.
+ /// Get a pinning reference from a `&'static` reference.
///
- /// This is safe, because `T` is borrowed for the `'static` lifetime, which
+ /// This is safe because `T` is borrowed immutably for the `'static` lifetime, which
/// never ends.
#[stable(feature = "pin_static_ref", since = "1.61.0")]
#[rustc_const_unstable(feature = "const_pin", issue = "76654")]
@@ -904,49 +1609,50 @@ impl<T: ?Sized> Pin<&'static T> {
}
}
-impl<'a, P: DerefMut> Pin<&'a mut Pin<P>> {
- /// Gets a pinned mutable reference from this nested pinned pointer.
+impl<'a, Ptr: DerefMut> Pin<&'a mut Pin<Ptr>> {
+ /// Gets `Pin<&mut T>` to the underlying pinned value from this nested `Pin`-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
+ /// implementations of `Ptr::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> {
+ pub fn as_deref_mut(self) -> Pin<&'a mut Ptr::Target> {
// SAFETY: What we're asserting here is that going from
//
- // Pin<&mut Pin<P>>
+ // Pin<&mut Pin<Ptr>>
//
// to
//
- // Pin<&mut P::Target>
+ // Pin<&mut Ptr::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>>`
+ // 1) Once we give out a `Pin<&mut Ptr::Target>`, an `&mut Ptr::Target` will not be given out.
+ // 2) By giving out a `Pin<&mut Ptr::Target>`, we do not risk of violating
+ // `Pin<&mut Pin<Ptr>>`
//
- // 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.
+ // The existence of `Pin<Ptr>` is sufficient to guarantee #1: since we already have a
+ // `Pin<Ptr>`, it must already uphold the pinning guarantees, which must mean that
+ // `Pin<&mut Ptr::Target>` does as well, since `Pin::as_mut` is safe. We do not have to rely
+ // on the fact that `Ptr` 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>`.
+ // For #2, we need to ensure that code given a `Pin<&mut Ptr::Target>` cannot cause the
+ // `Pin<Ptr>` to move? That is not possible, since `Pin<&mut Ptr::Target>` no longer retains
+ // any access to the `Ptr` itself, much less the `Pin<Ptr>`.
unsafe { self.get_unchecked_mut() }.as_mut()
}
}
impl<T: ?Sized> Pin<&'static mut T> {
- /// Get a pinned mutable reference from a static mutable reference.
+ /// Get a pinning mutable reference from a static mutable reference.
///
- /// This is safe, because `T` is borrowed for the `'static` lifetime, which
+ /// This is safe because `T` is borrowed for the `'static` lifetime, which
/// never ends.
#[stable(feature = "pin_static_ref", since = "1.61.0")]
#[rustc_const_unstable(feature = "const_pin", issue = "76654")]
@@ -958,39 +1664,39 @@ impl<T: ?Sized> Pin<&'static mut T> {
}
#[stable(feature = "pin", since = "1.33.0")]
-impl<P: Deref> Deref for Pin<P> {
- type Target = P::Target;
- fn deref(&self) -> &P::Target {
+impl<Ptr: Deref> Deref for Pin<Ptr> {
+ type Target = Ptr::Target;
+ fn deref(&self) -> &Ptr::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 {
+impl<Ptr: DerefMut<Target: Unpin>> DerefMut for Pin<Ptr> {
+ fn deref_mut(&mut self) -> &mut Ptr::Target {
Pin::get_mut(Pin::as_mut(self))
}
}
#[unstable(feature = "receiver_trait", issue = "none")]
-impl<P: Receiver> Receiver for Pin<P> {}
+impl<Ptr: Receiver> Receiver for Pin<Ptr> {}
#[stable(feature = "pin", since = "1.33.0")]
-impl<P: fmt::Debug> fmt::Debug for Pin<P> {
+impl<Ptr: fmt::Debug> fmt::Debug for Pin<Ptr> {
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> {
+impl<Ptr: fmt::Display> fmt::Display for Pin<Ptr> {
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> {
+impl<Ptr: fmt::Pointer> fmt::Pointer for Pin<Ptr> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&self.pointer, f)
}
@@ -1002,10 +1708,10 @@ impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
// 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> {}
+impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr> where Ptr: CoerceUnsized<U> {}
#[stable(feature = "pin", since = "1.33.0")]
-impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
+impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr> where Ptr: DispatchFromDyn<U> {}
/// Constructs a <code>[Pin]<[&mut] T></code>, by pinning a `value: T` locally.
///
diff --git a/tests/ui/async-await/pin-needed-to-poll-2.stderr b/tests/ui/async-await/pin-needed-to-poll-2.stderr
index 9c1ad32cc2c..8eb671531e7 100644
--- a/tests/ui/async-await/pin-needed-to-poll-2.stderr
+++ b/tests/ui/async-await/pin-needed-to-poll-2.stderr
@@ -13,7 +13,7 @@ note: required because it appears within the type `Sleep`
|
LL | struct Sleep(std::marker::PhantomPinned);
| ^^^^^
-note: required by a bound in `Pin::<P>::new`
+note: required by a bound in `Pin::<Ptr>::new`
--> $SRC_DIR/core/src/pin.rs:LL:COL
error: aborting due to 1 previous error
diff --git a/tests/ui/closures/coerce-unsafe-closure-to-unsafe-fn-ptr.stderr b/tests/ui/closures/coerce-unsafe-closure-to-unsafe-fn-ptr.stderr
index f5cb3e2b5f8..48fc8461882 100644
--- a/tests/ui/closures/coerce-unsafe-closure-to-unsafe-fn-ptr.stderr
+++ b/tests/ui/closures/coerce-unsafe-closure-to-unsafe-fn-ptr.stderr
@@ -1,4 +1,4 @@
-error[E0133]: call to unsafe function `Pin::<P>::new_unchecked` is unsafe and requires unsafe function or block
+error[E0133]: call to unsafe function `Pin::<Ptr>::new_unchecked` is unsafe and requires unsafe function or block
--> $DIR/coerce-unsafe-closure-to-unsafe-fn-ptr.rs:2:31
|
LL | let _: unsafe fn() = || { ::std::pin::Pin::new_unchecked(&0_u8); };
diff --git a/tests/ui/self/arbitrary_self_types_pin_needing_borrow.stderr b/tests/ui/self/arbitrary_self_types_pin_needing_borrow.stderr
index ec985b254b3..1811cd6753f 100644
--- a/tests/ui/self/arbitrary_self_types_pin_needing_borrow.stderr
+++ b/tests/ui/self/arbitrary_self_types_pin_needing_borrow.stderr
@@ -6,7 +6,7 @@ LL | Pin::new(S).x();
| |
| required by a bound introduced by this call
|
-note: required by a bound in `Pin::<P>::new`
+note: required by a bound in `Pin::<Ptr>::new`
--> $SRC_DIR/core/src/pin.rs:LL:COL
help: consider borrowing here
|
diff --git a/tests/ui/suggestions/expected-boxed-future-isnt-pinned.stderr b/tests/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
index 7c81825e576..60ab392f55d 100644
--- a/tests/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
+++ b/tests/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
@@ -52,7 +52,7 @@ LL | Pin::new(x)
|
= note: consider using the `pin!` macro
consider using `Box::pin` if you need to access the pinned value outside of the current scope
-note: required by a bound in `Pin::<P>::new`
+note: required by a bound in `Pin::<Ptr>::new`
--> $SRC_DIR/core/src/pin.rs:LL:COL
error[E0277]: `dyn Future<Output = i32> + Send` cannot be unpinned
@@ -65,7 +65,7 @@ LL | Pin::new(Box::new(x))
|
= note: consider using the `pin!` macro
consider using `Box::pin` if you need to access the pinned value outside of the current scope
-note: required by a bound in `Pin::<P>::new`
+note: required by a bound in `Pin::<Ptr>::new`
--> $SRC_DIR/core/src/pin.rs:LL:COL
error[E0308]: mismatched types