[Pin]\ 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 [Box]\ and [&mut] T allow
//! replacing and moving the values they contain: you can move out of a [Box]\,
//! or you can use [`mem::swap`]. [Pin]\ wraps a pointer type `P`, so
//! [Pin]<[Box]\> functions much like a regular [Box]\:
//! when a [Pin]<[Box]\> gets dropped, so do its contents, and the memory gets
//! deallocated. Similarly, [Pin]<[&mut] T> is a lot like [&mut] T.
//! However, [Pin]\ does not let clients actually obtain a [Box]\
//! or [&mut] T to pinned data, which implies that you cannot use operations such
//! as [`mem::swap`]:
//!
//! ```
//! use std::pin::Pin;
//! fn swap_pins[Pin]\ does *not* change the fact that a Rust
//! compiler considers all types movable. [`mem::swap`] remains callable for any `T`. Instead,
//! [Pin]\ prevents certain *values* (pointed to by pointers wrapped in
//! [Pin]\) from being moved by making it impossible to call methods that require
//! [&mut] T on them (like [`mem::swap`]).
//!
//! [Pin]\ can be used to wrap any pointer type `P`, and as such it interacts with
//! [`Deref`] and [`DerefMut`]. A [Pin]\ where P: [Deref] should be
//! considered as a "`P`-style pointer" to a pinned P::[Target] – so, a
//! [Pin]<[Box]\> is an owned pointer to a pinned `T`, and a
//! [Pin]<[Rc]\> is a reference-counted pointer to a pinned `T`.
//! For correctness, [Pin]\ 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 [Pin]\. For T: [Unpin],
//! [Pin]<[Box]\> and [Box]\ function identically, as do
//! [Pin]<[&mut] T> and [&mut] 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]\. For example,
//! whether or not [Box]\ is [`Unpin`] has no effect on the behavior of
//! [Pin]<[Box]\> (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 [Pin]\, we discuss some examples for how it might be used.
//! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee).
//!
//! ```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.
//! struct Unmovable {
//! data: String,
//! slice: NonNull[Some]\(v) 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`].
//!
//! 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 to not ever call [`drop`] on a pinned element (e.g., you can still
//! call [`mem::forget`] on a [Pin]<[Box]\> ). 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`]*.
//!
//! # `Drop` implementation
//!
//! If your type uses pinning (such as the two examples above), you have to be careful
//! when implementing [`Drop`][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 [`Pin::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]<[&]Self> or [Pin]<[&mut] Self>) has consequences for your
//! [`Drop`][Drop]implementation as well: if an element of your type could have been pinned,
//! you must treat [`Drop`][Drop] as implicitly taking [Pin]<[&mut] Self>.
//!
//! For example, you could implement [`Drop`][Drop] as follows:
//!
//! ```rust,no_run
//! # use std::pin::Pin;
//! # struct Type { }
//! impl Drop for Type {
//! fn drop(&mut self) {
//! // `new_unchecked` is okay because we know this value is never used
//! // again after being dropped.
//! inner_drop(unsafe { Pin::new_unchecked(self)});
//! fn inner_drop(this: Pin<&mut Type>) {
//! // Actual drop code goes here.
//! }
//! }
//! }
//! ```
//!
//! 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.
//!
//! Moreover, if your type is `#[repr(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 [Pin]<[&mut] Struct>.
//! The usual approach is to write helper methods (so called *projections*)
//! that turn [Pin]<[&mut] Struct> into a reference to the field, but what type should
//! that reference have? Is it [Pin]<[&mut] Field> or [&mut] Field?
//! The same question arises with the fields of an `enum`, and also when considering
//! container/wrapper types such as [Vec]\, [Box]\,
//! or [RefCell]\. (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 [Pin]<[&mut] Struct>
//! into [Pin]<[&mut] Field> or [&mut] Field. 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
//! "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`
//!
//! 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 [Pin]<[&mut] Field> 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.
//!
//! Fields without structural pinning may have a projection method that turns
//! [Pin]<[&mut] Struct> 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.
//! unsafe { &mut self.get_unchecked_mut().field }
//! }
//! }
//! ```
//!
//! You may also impl [Unpin] for Struct *even if* the type of `field`
//! is not [`Unpin`]. What that type thinks about pinning is not relevant
//! when no [Pin]<[&mut] Field> is ever created.
//!
//! ## Pinning *is* 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 [Pin]<[&mut] Field>, thus
//! witnessing that the field is pinned:
//!
//! ```rust,no_run
//! # use std::pin::Pin;
//! # type Field = i32;
//! # struct Struct { field: Field }
//! impl Struct {
//! fn pin_get_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
//! impl\ [Unpin] for Struct\ . (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
//! [&mut] self, 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 [VecDeque]\: the destructor of
//! [VecDeque]\ 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.
//! ([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 structural fields when your type is pinned. For example, if the struct contains an
//! [Option]\ and there is a [`take`][Option::take]-like operation with type
//! fn([Pin]<[&mut] Struct\>) -> [Option]\ ,
//! that operation can be used to move a `T` out of a pinned `Struct[RefCell]\ 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[RefCell]\ (using [RefCell]::get_pin_mut) and then move that
//! content using the mutable reference we got later.
//!
//! ## Examples
//!
//! For a type like [Vec]\, both possibilities (structural pinning or not) make
//! sense. A [Vec]\ 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 [Vec]\ 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 [Vec]\ without structural pinning could
//! impl\ [Unpin] for [Vec]\ , because the contents are never pinned
//! and the [Vec]\ 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 [Box]\: [Unpin]
//! holds for all `T`. It makes sense to do this for pointer types, because moving the
//! [Box]\ does not actually move the `T`: the [Box]\ can be freely
//! movable (aka [`Unpin`]) even if the `T` is not. In fact, even [Pin]<[Box]\> 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]\ and [Pin]<[Box]\> ,
//! 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,
//! you can make those fields not structural and hence freely access them with a
//! mutable reference even when you just have [Pin]<[&mut] Self> (such as in your own
//! [`poll`] implementation).
//!
//! [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"
//! [RefCell]: crate::cell::RefCell "cell::RefCell"
//! [`drop`]: Drop::drop
//! [VecDeque]: ../../std/collections/struct.VecDeque.html "collections::VecDeque"
//! [`ptr::write`]: crate::ptr::write "ptr::write"
//! [`Future`]: crate::future::Future "future::Future"
//! [drop-impl]: #drop-implementation
//! [drop-guarantee]: #drop-guarantee
//! [`poll`]: crate::future::Future::poll "future::Future::poll"
//! [&]: reference "shared reference"
//! [&mut]: reference "mutable reference"
//! [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe"
#![stable(feature = "pin", since = "1.33.0")]
use crate::cmp::{self, PartialEq, PartialOrd};
use crate::fmt;
use crate::hash::{Hash, Hasher};
use crate::marker::{Sized, Unpin};
use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
/// A pinned pointer.
///
/// This is a wrapper around a kind of pointer which makes that pointer "pin" its
/// value in place, preventing the value referenced by that pointer from being moved
/// unless it implements [`Unpin`].
///
/// *See the [`pin` module] documentation for an explanation of pinning.*
///
/// [`pin` module]: self
//
// Note: the `Clone` derive below causes unsoundness as it's possible to implement
// `Clone` for mutable references.
// See {
pointer: P,
}
// The following implementations aren't derived in order to avoid soundness
// issues. `&self.pointer` should not be accessible to untrusted trait
// implementations.
//
// See
where
P::Target: PartialEq {}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
impl
where
P::Target: PartialOrd {
fn cmp(&self, other: &Self) -> cmp::Ordering {
P::Target::cmp(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
impl {
fn hash {
/// Construct a new `Pin ` around a pointer to some data of a type that
/// implements [`Unpin`].
///
/// Unlike `Pin::new_unchecked`, this method is safe because the pointer
/// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
#[inline(always)]
#[rustc_const_unstable(feature = "const_pin", issue = "76654")]
#[stable(feature = "pin", since = "1.33.0")]
pub const fn new(pointer: P) -> Pin {
// SAFETY: the value pointed to is `Unpin`, and so has no requirements
// around pinning.
unsafe { Pin::new_unchecked(pointer) }
}
/// Unwraps this `Pin ` returning the underlying pointer.
///
/// This requires that the data inside this `Pin` is [`Unpin`] so that we
/// can ignore the pinning invariants when unwrapping it.
#[inline(always)]
#[rustc_const_unstable(feature = "const_pin", issue = "76654")]
#[stable(feature = "pin_into_inner", since = "1.39.0")]
pub const fn into_inner(pin: Pin ) -> P {
pin.pointer
}
}
impl {
/// Construct a new `Pin ` around a reference to some data of a type that
/// may or may not implement `Unpin`.
///
/// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
/// instead.
///
/// # Safety
///
/// This constructor is unsafe because we cannot guarantee that the data
/// pointed to by `pointer` is pinned, meaning that the data will not be moved or
/// its storage invalidated until it gets dropped. If the constructed `Pin ` does
/// not guarantee that the data `P` points to is pinned, that is a violation of
/// the API contract and may lead to undefined behavior in later (safe) operations.
///
/// By using this method, you are making a promise about the `P::Deref` and
/// `P::DerefMut` implementations, if they exist. Most importantly, they
/// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
/// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
/// and expect these methods to uphold the pinning invariants.
/// Moreover, by calling this method you promise that the reference `P`
/// dereferences to will not be moved out of again; in particular, it
/// must not be possible to obtain a `&mut P::Target` and then
/// move out of that reference (using, for example [`mem::swap`]).
///
/// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
/// while you are able to pin it for the given lifetime `'a`, you have no control
/// over whether it is kept pinned once `'a` ends:
/// ```
/// use std::mem;
/// use std::pin::Pin;
///
/// fn move_pinned_ref {
Pin { pointer }
}
/// Gets a pinned shared reference from this pinned pointer.
///
/// This is a generic method to go from `&Pin ` returning the underlying pointer.
///
/// # Safety
///
/// This function is unsafe. You must guarantee that you will continue to
/// treat the pointer `P` as pinned after you call this function, so that
/// the invariants on the `Pin` type can be upheld. If the code using the
/// resulting `P` does not continue to maintain the pinning invariants that
/// is a violation of the API contract and may lead to undefined behavior in
/// later (safe) operations.
///
/// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used
/// instead.
#[inline(always)]
#[rustc_const_unstable(feature = "const_pin", issue = "76654")]
#[stable(feature = "pin_into_inner", since = "1.39.0")]
pub const unsafe fn into_inner_unchecked(pin: Pin ) -> P {
pin.pointer
}
}
impl {
/// Gets a pinned mutable reference from this pinned pointer.
///
/// This is a generic method to go from `&mut Pin > {
/// Gets a pinned mutable reference from this nested pinned pointer.
///
/// This is a generic method to go from `Pin<&mut Pin >
//
// to
//
// Pin<&mut P::Target>
//
// is safe.
//
// We need to ensure that two things hold for that to be the case:
//
// 1) Once we give out a `Pin<&mut P::Target>`, an `&mut P::Target` will not be given out.
// 2) By giving out a `Pin<&mut P::Target>`, we do not risk of violating `Pin<&mut Pin >`
//
// The existence of `Pin ` is sufficient to guarantee #1: since we already have a
// `Pin `, it must already uphold the pinning guarantees, which must mean that
// `Pin<&mut P::Target>` does as well, since `Pin::as_mut` is safe. We do not have to rely
// on the fact that P is _also_ pinned.
//
// For #2, we need to ensure that code given a `Pin<&mut P::Target>` cannot cause the
// `Pin ` to move? That is not possible, since `Pin<&mut P::Target>` no longer retains
// any access to the `P` itself, much less the `Pin `.
unsafe { self.get_unchecked_mut() }.as_mut()
}
}
impl {
type Target = P::Target;
fn deref(&self) -> &P::Target {
Pin::get_ref(Pin::as_ref(self))
}
}
#[stable(feature = "pin", since = "1.33.0")]
impl {
fn deref_mut(&mut self) -> &mut P::Target {
Pin::get_mut(Pin::as_mut(self))
}
}
#[unstable(feature = "receiver_trait", issue = "none")]
impl {}
#[stable(feature = "pin", since = "1.33.0")]
impl {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&self.pointer, f)
}
}
#[stable(feature = "pin", since = "1.33.0")]
impl {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&self.pointer, f)
}
}
#[stable(feature = "pin", since = "1.33.0")]
impl {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&self.pointer, f)
}
}
// Note: this means that any impl of `CoerceUnsized` that allows coercing from
// a type that impls `Deref where P: CoerceUnsized {}
#[stable(feature = "pin", since = "1.33.0")]
impl where P: DispatchFromDyn {}
/// Constructs a ) -> bool {
P::Target::eq(self, other)
}
fn ne(&self, other: &Pin
) -> bool {
P::Target::ne(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
impl
) -> Option
) -> bool {
P::Target::lt(self, other)
}
fn le(&self, other: &Pin
) -> bool {
P::Target::le(self, other)
}
fn gt(&self, other: &Pin
) -> bool {
P::Target::gt(self, other)
}
fn ge(&self, other: &Pin
) -> bool {
P::Target::ge(self, other)
}
}
#[stable(feature = "pin_trait_impls", since = "1.41.0")]
impl
[Pin]<[&mut] T>, by pinning[^1] a `value: T` _locally_[^2]
/// (≠ [in the heap][`Box::pin`]).
///
/// [^1]: If the (type `T` of the) given value does not implement [`Unpin`], then this
/// effectively pins the `value` in memory, where it will be unable to be moved.
/// Otherwise, [Pin]<[&mut] T> behaves like [&mut] T, and operations such
/// as [`mem::replace()`][crate::mem::replace] will allow extracting that value, and therefore,
/// moving it.
/// See [the `Unpin` section of the `pin` module][self#unpin] for more info.
///
/// [^2]: This is usually dubbed "stack"-pinning. And whilst local values are almost always located
/// in the stack (_e.g._, when within the body of a non-`async` function), the truth is that inside
/// the body of an `async fn` or block —more generally, the body of a generator— any locals crossing
/// an `.await` point —a `yield` point— end up being part of the state captured by the `Future` —by
/// the `Generator`—, and thus will be stored wherever that one is.
///
/// ## Examples
///
/// ### Basic usage
///
/// ```rust
/// #![feature(pin_macro)]
/// # use core::marker::PhantomPinned as Foo;
/// use core::pin::{pin, Pin};
///
/// fn stuff(foo: Pin<&mut Foo>) {
/// // …
/// # let _ = foo;
/// }
///
/// let pinned_foo = pin!(Foo { /* … */ });
/// stuff(pinned_foo);
/// // or, directly:
/// stuff(pin!(Foo { /* … */ }));
/// ```
///
/// ### Manually polling a `Future` (wihout `Unpin` bounds)
///
/// ```rust
/// #![feature(pin_macro)]
/// use std::{
/// future::Future,
/// pin::pin,
/// task::{Context, Poll},
/// thread,
/// };
/// # use std::{sync::Arc, task::Wake, thread::Thread};
///
/// # /// A waker that wakes up the current thread when called.
/// # struct ThreadWaker(Thread);
/// #
/// # impl Wake for ThreadWaker {
/// # fn wake(self: Arc[Pin]<[&mut] T>
/// reference ends up borrowing a local tied to that block: it can't escape it.
///
/// The following, for instance, fails to compile:
///
/// ```rust,compile_fail
/// #![feature(pin_macro)]
/// use core::pin::{pin, Pin};
/// # use core::{marker::PhantomPinned as Foo, mem::drop as stuff};
///
/// let x: Pin<&mut Foo> = {
/// let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
/// x
/// }; // <- Foo is dropped
/// stuff(x); // Error: use of dropped value
/// ```
///
/// Error message
///
/// ```rust
/// # const _IGNORE: &str = stringify! {
/// error[E0716]: temporary value dropped while borrowed
/// --> src/main.rs:9:28
/// |
/// 8 | let x: Pin<&mut Foo> = {
/// | - borrow later stored here
/// 9 | let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
/// | ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
/// 10 | x
/// 11 | }; // <- Foo is dropped
/// | - temporary value is freed at the end of this statement
/// |
/// = note: consider using a let binding to create a longer lived value
/// # };
/// ```
///
///