Rollup merge of #116129 - fu5ha:better-pin-docs-2, r=Amanieu

Rewrite `pin` module documentation to clarify usage and invariants

The documentation of `pin` today does not give a complete treatment of pinning from first principles, nor does it adequately help build intuition and understanding for how the different elements of the pinning story fit together.

This rewrite attempts to address these in a way that makes the concept more approachable while also making the documentation more normative.

This PR picks up where `@mcy` left off in #88500 (thanks to him for the original work and `@Manishearth` for mentioning it such that I originally found it). I've directly incorporated much of the feedback left on the original PR and have rewritten and changed some of the main conceits of the prose to better adhere to the feedback from the reviewers on that PR or just explain something in (hopefully) a better way.

6 files changed, 1104 insertions, 375 deletions

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.
-///
-/// Rust itself has no notion of immovable types, and considers moves (e.g.,
-/// through assignment or [`mem::replace`]) to always be safe.
-///
-/// 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` 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`].
-///
-/// `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.
+/// Types that do not require any pinning guarantees.
+///
+/// For information on what "pinning" is, see the [`pin` module] documentation.
+///
+/// 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.
+///
+/// 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.
+///
+/// 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.
-///
-/// [`mem::replace`]: crate::mem::replace
-/// [Pin]: crate::pin::Pin
-/// [`pin` module]: crate::pin
+/// 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 "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.
-//!
-//! 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.
-//!
-//! At a high level, a <code>[Pin]\<P></code> ensures that the pointee of any pointer type
-//! `P` has a stable location in memory, meaning it cannot be moved elsewhere
-//! and its memory cannot be deallocated until it gets dropped. We say that the
-//! pointee is "pinned". Things get more subtle when discussing types that
-//! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning)
-//! for more details.
-//!
-//! By default, all types in Rust are movable. Rust allows passing all types by-value,
-//! and common smart-pointer types such as <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`]:
+//! Types that pin data to a location in memory.
+//!
+//! 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
+//!
+//! 1. Not be *moved* out of its memory location
+//! 2. More generally, remain *valid* at that same memory location
+//!
+//! 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.
+//!
+//! ```
+//! #[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();
+//! ```
+//!
+//! 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.
+//!
+//! 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.
+//!
+//! # What is "pinning"?
+//! [what-is-pinning]: self#what-is-pinning
+//!
+//! 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.
+//!
+//! ## Address-sensitive values, AKA "when we need pinning"
+//! [address-sensitive-values]: self#address-sensitive-values-aka-when-we-need-pinning
+//!
+//! 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].
+//!
+//! [`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;
-//! 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`.
+//! 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();
 //! ```
 //!
-//! 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`]).
-//!
-//! <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.
-//!
-//! # `Unpin`
-//!
-//! 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>.
-//!
-//! 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).
-//!
-//! # Example: self-referential struct
-//!
-//! 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).
+//! 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 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 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!
+//!
+//! 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.
+//!
+//! 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:
+//!
+//! *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).
+//!
+//! 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.
 //!
-//! 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.
+//! 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.
 //!
-//! 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.
+//! Therefore, pinning data also comes with [the "`Drop` guarantee"][drop-guarantee].
 //!
-//! 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.
+//! # Subtle details and the `Drop` guarantee
+//! [subtle-details]: self#subtle-details-and-the-drop-guarantee
+//! [drop-guarantee]: self#subtle-details-and-the-drop-guarantee
 //!
-//! Therefore, pinning also comes with a [`drop`]-related guarantee.
+//! 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.
 //!
-//! # `Drop` guarantee
+//! 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:
 //!
-//! 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.
+//! 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.*
 //!
-//! 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`].
+//! There is some subtlety to this which we have not yet talked about in detail. The invariant
+//! described above means that, yes,
 //!
-//! This is exactly the kind of guarantee that the intrusive linked list from the previous
-//! section needs to function correctly.
+//! 1. The value must not be moved out of its location in memory
 //!
-//! 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`]*.
+//! but it also implies that,
 //!
-//! # `Drop` implementation
+//! 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)
+//! };
+//! ```
 //!
-//! 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`].
+//! 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>.
 //!
-//! 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
+//! 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.
-//!
-//! # Projections and Structural Pinning
-//!
-//! When working with pinned structs, the question arises how one can access the
-//! fields of that struct in a method that takes just <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 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!
-//!
-//! As the author of a data structure you get to decide for each field whether pinning
+//! pinning with a [`#[repr(packed)]`][packed] type.
+//!
+//! ### Implementing [`Drop`] for pointer types which will be used as [`Pin`]ning pointers
+//!
+//! 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.
+//!
+//! ## "Assigning" pinned data
+//!
+//! 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.
+//!
+//! 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.
 //!
-//! 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.
+//! ### 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:
-//!
-//! 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
+//! Structural pinning comes with a few extra requirements:
+//!
+//! 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.
+///
+/// [`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`].
+///
+/// *See the [`pin` module] documentation for a more thorough exploration of pinning.*
+///
+/// ## Pinning values with [`Pin<Ptr>`]
+///
+/// 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);
+/// ```
 ///
-/// 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`].
+/// There are similar pinning methods offered on the other standard library smart pointer types
+/// as well, like [`Rc`] and [`Arc`].
 ///
-/// `Pin<P>` is guaranteed to have the same memory layout and ABI as `P`.
+/// ### Pinning a value on the stack using [`pin!`]
 ///
-/// *See the [`pin` module] documentation for an explanation of pinning.*
+/// 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.
 ///
-/// [`pin` module]: self
+/// ## 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].
+    ///
+    /// 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 making a promise about the `P::Deref` and
-    /// `P::DerefMut` implementations, if they exist. Most importantly, they
+    /// 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