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Auto merge of #58574 - RalfJung:pin, r=Mark-Simulacrum

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improve Pin documentation

Incorporates a bunch of the documentation-related comments that came up when discussing `Pin` stabilization.

Cc @alexcrichton @withoutboats @cramertj @jonhoo

Fixes https://github.com/rust-lang/rust/issues/58130
This commit is contained in:
bors 2019-02-22 04:11:23 +00:00
commit c6fd02772b
2 changed files with 310 additions and 49 deletions
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27
src/libcore/marker.rs
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@ -597,34 +597,43 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
/// Types which can be safely moved after being pinned.
///
/// Since Rust itself has no notion of immovable types, and will consider moves to always be safe,
/// Since Rust itself has no notion of immovable types, and considers moves
/// (e.g. through assignment or [`mem::replace`]) to always be safe,
/// this trait cannot prevent types from moving by itself.
///
/// Instead it can be used to prevent moves through the type system,
/// by controlling the behavior of pointers wrapped in the [`Pin`] wrapper,
/// Instead it is used to prevent moves through the type system,
/// by controlling the behavior of pointers `P` wrapped in the [`Pin<P>`] wrapper,
/// which "pin" the type in place by not allowing it to be moved out of them.
/// See the [`pin module`] documentation for more information on pinning.
///
/// Implementing this trait lifts the restrictions of pinning off a type,
/// which then allows it to move out with functions such as [`replace`].
/// which then allows it to move out 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>`] because you cannot get the
/// `&mut T` you need for that, and *that* is what makes this system work.
///
/// So this, for example, can only be done on types implementing `Unpin`:
///
/// ```rust
/// use std::mem::replace;
/// use std::mem;
/// use std::pin::Pin;
///
/// let mut string = "this".to_string();
/// let mut pinned_string = Pin::new(&mut string);
///
/// // dereferencing the pointer mutably is only possible because String implements Unpin
/// replace(&mut *pinned_string, "other".to_string());
/// // We need a mutable reference to call `mem::replace`.
/// // We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
/// // but that is only possible because `String` implements `Unpin`.
/// mem::replace(&mut *pinned_string, "other".to_string());
/// ```
///
/// This trait is automatically implemented for almost every type.
///
/// [`replace`]: ../../std/mem/fn.replace.html
/// [`Pin`]: ../pin/struct.Pin.html
/// [`mem::replace`]: ../../std/mem/fn.replace.html
/// [`Pin<P>`]: ../pin/struct.Pin.html
/// [`pin module`]: ../../std/pin/index.html
#[stable(feature = "pin", since = "1.33.0")]
#[cfg_attr(not(stage0), lang = "unpin")]

332
src/libcore/pin.rs
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@ -2,45 +2,63 @@
//!
//! It is sometimes useful to have objects that are guaranteed to not 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,
//! since moving an object with pointers to itself will invalidate them,
//! which could cause undefined behavior.
//!
//! 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.
//! A [`Pin<P>`] 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".
//!
//! 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.
//! By default, all types in Rust are movable. Rust allows passing all types by-value,
//! and common smart-pointer types such as `Box<T>` and `&mut T` allow replacing and
//! moving the values they contain: you can move out of a `Box<T>`, or you can use [`mem::swap`].
//! [`Pin<P>`] wraps a pointer type `P`, so `Pin<Box<T>>` functions much like a regular `Box<T>`:
//! when a `Pin<Box<T>>` gets dropped, so do its contents, and the memory gets deallocated.
//! Similarily, `Pin<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does not let clients
//! actually obtain a `Box<T>` or `&mut T` to pinned data, which implies that you cannot use
//! operations such as [`mem::swap`]:
//! ```
//! 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`.
//! }
//! ```
//!
//! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
//! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, `Pin<P>`
//! prevents certain *values* (pointed to by pointers wrapped in `Pin<P>`) from being
//! moved by making it impossible to call methods that require `&mut T` on them
//! (like [`mem::swap`]).
//!
//! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
//! [`Deref`] and [`DerefMut`]. A `Pin<P>` where `P: Deref` should be considered
//! as a "`P`-style pointer" to a pinned `P::Target` -- so, a `Pin<Box<T>>` is
//! an owned pointer to a pinned `T`, and a `Pin<Rc<T>>` is a reference-counted
//! pointer to a pinned `T`.
//! For correctness, [`Pin<P>`] relies on the [`Deref`] and [`DerefMut`] implementations
//! to not move out of their `self` parameter, and to only ever return a pointer
//! to pinned data when they are called on a pinned pointer.
//!
//! # `Unpin`
//!
//! However, these restrictions are usually not necessary. Many types are always freely
//! movable. These types implement the [`Unpin`] auto-trait, which nullifies the effect
//! of [`Pin`]. For `T: Unpin`, `Pin<Box<T>>` and `Box<T>` function identically, as do
//! `Pin<&mut T>` and `&mut T`.
//! movable, even when pinned, because they do not rely on having a stable address.
//! This includes all the basic types (like `bool`, `i32`, 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 [`Pin<P>`]. 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.
//! Note that pinning and `Unpin` only affect the pointed-to type `P::Target`, not the pointer
//! type `P` itself that got wrapped in `Pin<P>`. For example, whether or not `Box<T>` is
//! `Unpin` has no effect on the behavior of `Pin<Box<T>>` (here, `T` is the
//! pointed-to type).
//!
//! [`Pin`]: struct.Pin.html
//! [`Unpin`]: ../../std/marker/trait.Unpin.html
//! [`swap`]: ../../std/mem/fn.swap.html
//! [`Box`]: ../../std/boxed/struct.Box.html
//!
//! # Examples
//! # Example: self-referential struct
//!
//! ```rust
//! use std::pin::Pin;
@ -94,6 +112,150 @@
//! // let new_unmoved = Unmovable::new("world".to_string());
//! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
//! ```
//!
//! # Example: intrusive doubly-linked list
//!
//! In an intrusive doubly-linked list, the collection does not actually allocate
//! the memory for the elements itself. Allocation is controlled by the clients,
//! and elements can live on a stack frame that lives shorter than the collection does.
//!
//! 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` implementation of a linked
//! list element will patch the pointers of its predecessor and successor to remove itself
//! from the list.
//!
//! 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 neighbouring elements would become invalid, which would break the data structure.
//!
//! Therefore, pinning also comes with a `drop`-related guarantee.
//!
//! # `Drop` guarantee
//!
//! 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 from the moment it gets pinned until
//! when `drop` is called*. Memory can be invalidated by deallocation, but also by
//! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
//! off of a vector.
//!
//! This is exactly the kind of guarantee that the intrusive linked list from the previous
//! section needs to function correctly.
//!
//! Notice that this guarantee does *not* mean that memory does not leak! It is still
//! completely okay not to ever call `drop` on a pinned element (e.g., you can still
//! call [`mem::forget`] on a `Pin<Box<T>>`). In the example of the doubly-linked
//! list, that element would just stay in the list. However you may not free or reuse the storage
//! *without calling `drop`*.
//!
//! # `Drop` implementation
//!
//! If your type uses pinning (such as the two examples above), you have to be careful
//! when implementing `Drop`. The `drop` function takes `&mut self`, but this
//! is called *even if your type was previously pinned*! It is as if the
//! compiler automatically called `get_unchecked_mut`.
//!
//! 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 `Pin<&[mut] Self>`)
//! has consequences for your `Drop` implementation as well: if an element
//! of your type could have been pinned, you must treat Drop as implicitly taking
//! `Pin<&mut Self>`.
//!
//! In particular, if your type is `#[repr(packed)]`, the compiler will automatically
//! move fields around to be able to drop them. As a consequence, you cannot use
//! pinning with a `#[repr(packed)]` type.
//!
//! # Projections and Structural Pinning
//!
//! One interesting question arises when considering the interaction of pinning and
//! the fields of a struct. When can a struct have a "pinning projection", i.e.,
//! an operation with type `fn(Pin<&[mut] Struct>) -> Pin<&[mut] Field>`?
//! In a similar vein, when can a generic wrapper type (such as `Vec<T>`, `Box<T>`, or `RefCell<T>`)
//! have an operation with type `fn(Pin<&[mut] Wrapper<T>>) -> Pin<&[mut] T>`?
//!
//! Having a pinning projection for some field means that pinning is "structural":
//! when the wrapper is pinned, the field must be considered pinned, too.
//! After all, the pinning projection lets us get a `Pin<&[mut] Field>`.
//!
//! However, structural pinning comes with a few extra requirements, so not all
//! wrappers can be structural and hence not all wrappers can offer pinning projections:
//!
//! 1. The wrapper 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 wrapper it is your responsibility *not* to add something like
//! `impl<T> Unpin for Wrapper<T>`. (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 wrapper 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
//! `&mut self`, but the wrapper (and hence its fields) might have been pinned before.
//! You have to guarantee that you do not move a field inside your `Drop` implementation.
//! In particular, as explained previously, this means that your wrapper type must *not*
//! be `#[repr(packed)]`.
//! 3. You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
//! once your wrapper 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 `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, because it can lead to elements being deallocated without
//! their destructor being called. (`VecDeque` 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
//! the fields when your type is pinned. For example, if the wrapper contains an
//! `Option<T>` and there is a `take`-like operation with type
//! `fn(Pin<&mut Wrapper<T>>) -> Option<T>`,
//! that operation can be used to move a `T` out of a pinned `Wrapper<T>` -- which means
//! pinning cannot be structural.
//!
//! For a more complex example of moving data out of a pinned type, imagine if `RefCell<T>`
//! had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
//! Then we could do the following:
//! ```compile_fail
//! 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.
//! }
//! ```
//! This is catastrophic, it means we can first pin the content of the `RefCell<T>`
//! (using `RefCell::get_pin_mut`) and then move that content using the mutable
//! reference we got later.
//!
//! For a type like `Vec<T>`, both possibilites (structural pinning or not) make sense,
//! and the choice is up to the author. A `Vec<T>` with structural pinning could
//! have `get_pin`/`get_pin_mut` projections. However, it could *not* allow calling
//! `pop` on a pinned `Vec<T>` because that would move the (structurally pinned) contents!
//! Nor could it allow `push`, which might reallocate and thus also move the contents.
//! A `Vec<T>` without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
//! are never pinned and the `Vec<T>` itself is fine with being moved as well.
//!
//! In the standard library, pointer types generally do not have structural pinning,
//! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
//! It makes sense to do this for pointer types, because moving the `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 `Pin<Box<T>>` and `Pin<&mut T>` 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 `Box<T>` and `Pin<Box<T>>`,
//! whether the content is pinned is entirely independent of whether the pointer is
//! pinned, meaning pinning is *not* structural.
//!
//! [`Pin<P>`]: struct.Pin.html
//! [`Unpin`]: ../../std/marker/trait.Unpin.html
//! [`Deref`]: ../../std/ops/trait.Deref.html
//! [`DerefMut`]: ../../std/ops/trait.DerefMut.html
//! [`mem::swap`]: ../../std/mem/fn.swap.html
//! [`mem::forget`]: ../../std/mem/fn.forget.html
//! [`Box<T>`]: ../../std/boxed/struct.Box.html
//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
//! [`None`]: ../../std/option/enum.Option.html#variant.None
//! [`Some(v)`]: ../../std/option/enum.Option.html#variant.Some
//! [drop-impl]: #drop-implementation
//! [drop-guarantee]: #drop-guarantee
#![stable(feature = "pin", since = "1.33.0")]
@ -169,8 +331,13 @@ impl<P: Deref> Pin<P>
where
P::Target: Unpin,
{
/// Construct a new `Pin` around a pointer to some data of a type that
/// implements `Unpin`.
/// Construct a new `Pin<P>` around a pointer to some data of a type that
/// implements [`Unpin`].
///
/// Unlike `Pin::new_unchecked`, this method is safe because the pointer
/// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
///
/// [`Unpin`]: ../../std/marker/trait.Unpin.html
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn new(pointer: P) -> Pin<P> {
@ -181,18 +348,70 @@ where
}
impl<P: Deref> Pin<P> {
/// Construct a new `Pin` around a reference to some data of a type that
/// Construct a new `Pin<P>` around a reference to some data of a type that
/// may or may not implement `Unpin`.
///
/// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
/// instead.
///
/// # Safety
///
/// This constructor is unsafe because we cannot guarantee that the data
/// pointed to by `pointer` is pinned. If the constructed `Pin<P>` does
/// not guarantee that the data `P` points to is pinned, constructing a
/// `Pin<P>` is undefined behavior.
/// 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.
///
/// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
/// instead.
/// By using this method, you are making a promise about the `P::Deref` and
/// `P::DerefMut` implementations, if they exist. Most importantly, they
/// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
/// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
/// and expect these methods to uphold the pinning invariants.
/// Moreover, by calling this method you promise that the reference `P`
/// dereferences to will not be moved out of again; in particular, it
/// must not be possible to obtain a `&mut P::Target` and then
/// move out of that reference (using, for example [`mem::swap`]).
///
/// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
/// while you are able to pin it for the given lifetime `'a`, you have no control
/// over whether it is kept pinned once `'a` ends:
/// ```
/// use std::mem;
/// use std::pin::Pin;
///
/// fn move_pinned_ref<T>(mut a: T, mut b: T) {
/// unsafe {
/// let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
/// // This should mean the pointee `a` can never move again.
/// }
/// mem::swap(&mut a, &mut b);
/// // The address of `a` changed to `b`'s stack slot, so `a` got moved even
/// // though we have previously pinned it! We have violated the pinning API contract.
/// }
/// ```
/// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
///
/// Similarily, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
/// aliases to the same data that are not subject to the pinning restrictions:
/// ```
/// use std::rc::Rc;
/// use std::pin::Pin;
///
/// fn move_pinned_rc<T>(mut x: Rc<T>) {
/// let pinned = unsafe { Pin::new_unchecked(x.clone()) };
/// {
/// let p: Pin<&T> = pinned.as_ref();
/// // This should mean the pointee can never move again.
/// }
/// drop(pinned);
/// let content = Rc::get_mut(&mut x).unwrap();
/// // Now, if `x` was the only reference, we have a mutable reference to
/// // data that we pinned above, which we could use to move it as we have
/// // seen in the previous example. We have violated the pinning API contract.
/// }
/// ```
///
/// [`mem::swap`]: ../../std/mem/fn.swap.html
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
@ -200,6 +419,12 @@ impl<P: Deref> Pin<P> {
}
/// Gets a pinned shared reference from this pinned pointer.
///
/// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
/// It is safe because, as part of the contract of `Pin::new_unchecked`,
/// the pointee cannot move after `Pin<Pointer<T>>` got created.
/// "Malicious" implementations of `Pointer::Deref` are likewise
/// ruled out by the contract of `Pin::new_unchecked`.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
@ -209,13 +434,22 @@ impl<P: Deref> Pin<P> {
impl<P: DerefMut> Pin<P> {
/// Gets a pinned mutable reference from this pinned pointer.
///
/// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
/// It is safe because, as part of the contract of `Pin::new_unchecked`,
/// the pointee cannot move after `Pin<Pointer<T>>` got created.
/// "Malicious" implementations of `Pointer::DerefMut` are likewise
/// ruled out by the contract of `Pin::new_unchecked`.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
unsafe { Pin::new_unchecked(&mut *self.pointer) }
}
/// Assign a new value to the memory behind the pinned reference.
/// Assigns a new value to the memory behind the pinned reference.
///
/// This overwrites pinned data, but that is okay: its destructor gets
/// run before being overwritten, so no pinning guarantee is violated.
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn set(self: &mut Pin<P>, value: P::Target)
@ -227,10 +461,12 @@ impl<P: DerefMut> Pin<P> {
}
impl<'a, T: ?Sized> Pin<&'a T> {
/// Construct a new pin by mapping the interior value.
/// Constructs a new pin by mapping the interior value.
///
/// For example, if you wanted to get a `Pin` of a field of something,
/// you could use this to get access to that field in one line of code.
/// However, there are several gotchas with these "pinning projections";
/// see the [`pin` module] documentation for further details on that topic.
///
/// # Safety
///
@ -238,6 +474,8 @@ impl<'a, T: ?Sized> Pin<&'a T> {
/// will not move so long as the argument value does not move (for example,
/// because it is one of the fields of that value), and also that you do
/// not move out of the argument you receive to the interior function.
///
/// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
pub unsafe fn map_unchecked<U, F>(self: Pin<&'a T>, func: F) -> Pin<&'a U> where
F: FnOnce(&T) -> &U,
@ -249,11 +487,21 @@ impl<'a, T: ?Sized> Pin<&'a T> {
/// Gets a shared reference out of a pin.
///
/// This is safe because it is not possible to move out of a shared reference.
/// It may seem like there is an issue here with interior mutability: in fact,
/// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
/// not a problem as long as there does not also exist a `Pin<&T>` pointing
/// to the same data, and `RefCell<T>` does not let you create a pinned reference
/// to its contents. See the discussion on ["pinning projections"] for further
/// details.
///
/// Note: `Pin` also implements `Deref` to the target, which can be used
/// to access the inner value. However, `Deref` only provides a reference
/// that lives for as long as the borrow of the `Pin`, not the lifetime of
/// the `Pin` itself. This method allows turning the `Pin` into a reference
/// with the same lifetime as the original `Pin`.
///
/// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
#[inline(always)]
pub fn get_ref(self: Pin<&'a T>) -> &'a T {
@ -306,6 +554,8 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
///
/// For example, if you wanted to get a `Pin` of a field of something,
/// you could use this to get access to that field in one line of code.
/// However, there are several gotchas with these "pinning projections";
/// see the [`pin` module] documentation for further details on that topic.
///
/// # Safety
///
@ -313,6 +563,8 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
/// will not move so long as the argument value does not move (for example,
/// because it is one of the fields of that value), and also that you do
/// not move out of the argument you receive to the interior function.
///
/// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
#[stable(feature = "pin", since = "1.33.0")]
pub unsafe fn map_unchecked_mut<U, F>(self: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
F: FnOnce(&mut T) -> &mut U,