Auto merge of #55992 - cramertj:pin-docs, r=alexcrichton

Expand std::pin module docs and rename std::pin::Pinned to PhantomPinned

cc https://github.com/rust-lang/rust/issues/49150, https://github.com/rust-lang/rust/issues/55766

r? @withoutboats

src/libcore/marker.rs

@@ -640,15 +640,15 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 #[unstable(feature = "pin", issue = "49150")]
 pub auto trait Unpin {}
 
-/// A type which does not implement `Unpin`.
+/// A marker type which does not implement `Unpin`.
 ///
-/// If a type contains a `Pinned`, it will not implement `Unpin` by default.
+/// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
 #[unstable(feature = "pin", issue = "49150")]
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-pub struct Pinned;
+pub struct PhantomPinned;
 
 #[unstable(feature = "pin", issue = "49150")]
-impl !Unpin for Pinned {}
+impl !Unpin for PhantomPinned {}
 
 #[unstable(feature = "pin", issue = "49150")]
 impl<'a, T: ?Sized + 'a> Unpin for &'a T {}

src/libcore/pin.rs

@@ -7,23 +7,33 @@
 //! since moving an object with pointers to itself will invalidate them,
 //! which could cause undefined behavior.
 //!
-//! In order to prevent objects from moving, they must be pinned
-//! by wrapping a pointer to the data in the [`Pin`] type. A pointer wrapped
-//! in a `Pin` is otherwise equivalent to its normal version, e.g., `Pin<Box<T>>`
-//! and `Box<T>` work the same way except that the first is pinning the value
-//! of `T` in place.
+//! By default, all types in Rust are movable. Rust allows passing all types by-value,
+//! and common smart-pointer types such as `Box`, `Rc`, and `&mut` allow replacing and
+//! moving the values they contain. In order to prevent objects from moving, they must
+//! be pinned by wrapping a pointer to the data in the [`Pin`] type.
+//! Doing this prohibits moving the value behind the pointer.
+//! For example, `Pin<Box<T>>` functions much like a regular `Box<T>`,
+//! but doesn't allow moving `T`. The pointer value itself (the `Box`) can still be moved,
+//! but the value behind it cannot.
 //!
-//! First of all, these are pointer types because pinned data mustn't be passed around by value
-//! (that would change its location in memory).
-//! Secondly, since data can be moved out of `&mut` and `Box` with functions such as [`swap`],
-//! which causes their contents to swap places in memory,
-//! we need dedicated types that prohibit such operations.
+//! Since data can be moved out of `&mut` and `Box` with functions such as [`swap`],
+//! changing the location of the underlying data, [`Pin`] prohibits accessing the
+//! underlying pointer type (the `&mut` or `Box`) directly, and provides its own set of
+//! APIs for accessing and using the value. [`Pin`] also guarantees that no other
+//! functions will move the pointed-to value. This allows for the creation of
+//! self-references and other special behaviors that are only possible for unmovable
+//! values.
 //!
-//! However, these restrictions are usually not necessary,
-//! so most types implement the [`Unpin`] auto-trait,
-//! which indicates that the type can be moved out safely.
-//! Doing so removes the limitations of pinning types,
-//! making them the same as their non-pinning counterparts.
+//! However, these restrictions are usually not necessary. Many types are always freely
+//! movable. These types implement the [`Unpin`] auto-trait, which nullifies the affect
+//! of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function identically, as do
+//! `Pin<&mut T>` and `&mut T`.
+//!
+//! Note that pinning and `Unpin` only affect the pointed-to type. For example, whether
+//! or not `Box<T>` is `Unpin` has no affect on the behavior of `Pin<Box<T>>`. Similarly,
+//! `Pin<Box<T>>` and `Pin<&mut T>` are always `Unpin` themselves, even though the
+//! `T` underneath them isn't, because the pointers in `Pin<Box<_>>` and `Pin<&mut _>`
+//! are always freely movable, even if the data they point to isn't.
 //!
 //! [`Pin`]: struct.Pin.html
 //! [`Unpin`]: trait.Unpin.html
@@ -36,7 +46,7 @@
 //! #![feature(pin)]
 //!
 //! use std::pin::Pin;
-//! use std::marker::Pinned;
+//! use std::marker::PhantomPinned;
 //! use std::ptr::NonNull;
 //!
 //! // This is a self-referential struct since the slice field points to the data field.
@@ -47,7 +57,7 @@
 //! struct Unmovable {
 //!     data: String,
 //!     slice: NonNull<String>,
-//!     _pin: Pinned,
+//!     _pin: PhantomPinned,
 //! }
 //!
 //! impl Unmovable {
@@ -60,7 +70,7 @@
 //!             // we only create the pointer once the data is in place
 //!             // otherwise it will have already moved before we even started
 //!             slice: NonNull::dangling(),
-//!             _pin: Pinned,
+//!             _pin: PhantomPinned,
 //!         };
 //!         let mut boxed = Box::pinned(res);
 //!