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Add a stack-pin!-ning macro to the pin module.
Add a type annotation to improve error messages with type mismatches Add a link to the temporary-lifetime-extension section of the reference
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Daniel Henry-Mantilla
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b321742c6c
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ee9cd7bb6a
@ -909,3 +909,245 @@ impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}
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#[stable(feature = "pin", since = "1.33.0")]
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impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
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/// Constructs a <code>[Pin]<[&mut] T></code>, by pinning[^1] a `value: T` _locally_[^2]
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/// (≠ [in the heap][`Box::pin`]).
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///
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/// [^1]: If the (type `T` of the) given value does not implement [`Unpin`], then this
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/// effectively pins the `value` in memory, where it will be unable to be moved.
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/// Otherwise, <code>[Pin]<[&mut] T></code> behaves like <code>[&mut] T</code>, and operations such
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/// as [`mem::replace()`][crate::mem::replace] will allow extracting that value, and therefore,
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/// moving it.
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/// See [the `Unpin` section of the `pin` module][self#unpin] for more info.
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///
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/// [^2]: This is usually dubbed "stack"-pinning. And whilst local values are almost always located
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/// in the stack (_e.g._, when within the body of a non-`async` function), the truth is that inside
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/// the body of an `async fn` or block —more generally, the body of a generator— any locals crossing
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/// an `.await` point —a `yield` point— end up being part of the state captured by the `Future` —by
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/// the `Generator`—, and thus will be stored wherever that one is.
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///
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/// ## Examples
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///
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/// ### Basic usage
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///
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/// ```rust
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/// #![feature(pin_macro)]
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/// # use core::marker::PhantomPinned as Foo;
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/// use core::pin::{pin, Pin};
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///
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/// fn stuff(foo: Pin<&mut Foo>) {
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/// // …
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/// # let _ = foo;
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/// }
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///
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/// let pinned_foo = pin!(Foo { /* … */ });
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/// stuff(pinned_foo);
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/// // or, directly:
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/// stuff(pin!(Foo { /* … */ }));
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/// ```
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///
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/// ### Manually polling a `Future` (wihout `Unpin` bounds)
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///
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/// ```rust
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/// #![feature(pin_macro)]
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/// use std::{
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/// future::Future,
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/// pin::pin,
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/// task::{Context, Poll},
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/// thread,
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/// };
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/// # use std::{sync::Arc, task::Wake, thread::Thread};
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///
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/// # /// A waker that wakes up the current thread when called.
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/// # struct ThreadWaker(Thread);
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/// #
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/// # impl Wake for ThreadWaker {
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/// # fn wake(self: Arc<Self>) {
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/// # self.0.unpark();
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/// # }
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/// # }
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/// #
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/// /// Runs a future to completion.
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/// fn block_on<Fut: Future>(fut: Fut) -> Fut::Output {
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/// let waker_that_unparks_thread = // …
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/// # Arc::new(ThreadWaker(thread::current())).into();
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/// let mut cx = Context::from_waker(&waker_that_unparks_thread);
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/// // Pin the future so it can be polled.
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/// let mut pinned_fut = pin!(fut);
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/// loop {
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/// match pinned_fut.as_mut().poll(&mut cx) {
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/// Poll::Pending => thread::park(),
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/// Poll::Ready(res) => return res,
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/// }
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/// }
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/// }
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/// #
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/// # assert_eq!(42, block_on(async { 42 }));
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/// ```
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///
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/// ### With `Generator`s
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///
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/// ```rust
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/// #![feature(generators, generator_trait, pin_macro)]
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/// use core::{
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/// ops::{Generator, GeneratorState},
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/// pin::pin,
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/// };
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///
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/// fn generator_fn() -> impl Generator<Yield = usize, Return = ()> /* not Unpin */ {
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/// // Allow generator to be self-referential (not `Unpin`)
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/// // vvvvvv so that locals can cross yield points.
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/// static || {
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/// let foo = String::from("foo"); // --+
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/// yield 0; // | <- crosses yield point!
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/// println!("{}", &foo); // <----------+
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/// yield foo.len();
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/// }
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/// }
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///
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/// fn main() {
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/// let mut generator = pin!(generator_fn());
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/// match generator.as_mut().resume(()) {
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/// GeneratorState::Yielded(0) => {},
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/// _ => unreachable!(),
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/// }
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/// match generator.as_mut().resume(()) {
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/// GeneratorState::Yielded(3) => {},
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/// _ => unreachable!(),
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/// }
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/// match generator.resume(()) {
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/// GeneratorState::Yielded(_) => unreachable!(),
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/// GeneratorState::Complete(()) => {},
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/// }
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/// }
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/// ```
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///
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/// ## Remarks
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///
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/// Precisely because a value is pinned to local storage, the resulting <code>[Pin]<[&mut] T></code>
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/// reference ends up borrowing a local tied to that block: it can't escape it.
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///
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/// The following, for instance, fails to compile:
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///
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/// ```rust,compile_fail
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/// #![feature(pin_macro)]
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/// use core::pin::{pin, Pin};
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/// # use core::{marker::PhantomPinned as Foo, mem::drop as stuff};
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///
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/// let x: Pin<&mut Foo> = {
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/// let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
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/// x
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/// }; // <- Foo is dropped
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/// stuff(x); // Error: use of dropped value
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/// ```
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///
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/// <details><summary>Error message</summary>
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///
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/// ```rust
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/// # const _IGNORE: &str = stringify! {
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/// error[E0716]: temporary value dropped while borrowed
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/// --> src/main.rs:9:28
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/// |
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/// 8 | let x: Pin<&mut Foo> = {
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/// | - borrow later stored here
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/// 9 | let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
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/// | ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
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/// 10 | x
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/// 11 | }; // <- Foo is dropped
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/// | - temporary value is freed at the end of this statement
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/// |
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/// = note: consider using a let binding to create a longer lived value
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/// # };
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/// ```
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///
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/// </details>
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///
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/// This makes [`pin!`] **unsuitable to pin values when intending to _return_ them**. Instead, the
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/// value is expected to be passed around _unpinned_ until the point where it is to be consumed,
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/// where it is then useful and even sensible to pin the value locally using [`pin!`].
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///
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/// If you really need to return a pinned value, consider using [`Box::pin`] instead.
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///
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/// On the other hand, pinning to the stack[<sup>2</sup>](#fn2) using [`pin!`] is likely to be
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/// cheaper than pinning into a fresh heap allocation using [`Box::pin`]. Moreover, by virtue of not
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/// even needing an allocator, [`pin!`] is the main non-`unsafe` `#![no_std]`-compatible [`Pin`]
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/// constructor.
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///
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/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
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#[unstable(feature = "pin_macro", issue = "93178")]
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pub macro pin($value:expr $(,)?) {
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// This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
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// review such a hypothetical macro (that any user-code could define):
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//
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// ```rust
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// macro_rules! pin {( $value:expr ) => (
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// match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block.
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// $crate::pin::Pin::<&mut _>::new_unchecked(at_value)
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// }}
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// )}
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// ```
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//
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// Safety:
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// - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls
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// that would break `Pin`'s invariants.
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// - `{ $value }` is braced, making it a _block expression_, thus **moving**
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// the given `$value`, and making it _become an **anonymous** temporary_.
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// By virtue of being anonynomous, it can no longer be accessed, thus
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// preventing any attemps to `mem::replace` it or `mem::forget` it, _etc._
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//
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// This gives us a `pin!` definition that is sound, and which works, but only
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// in certain scenarios:
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// - If the `pin!(value)` expression is _directly_ fed to a function call:
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// `let poll = pin!(fut).poll(cx);`
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// - If the `pin!(value)` expression is part of a scrutinee:
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// ```rust
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// match pin!(fut) { pinned_fut => {
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// pinned_fut.as_mut().poll(...);
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// pinned_fut.as_mut().poll(...);
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// }} // <- `fut` is dropped here.
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// ```
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// Alas, it doesn't work for the more straight-forward use-case: `let` bindings.
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// ```rust
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// let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement
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// pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed
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// // note: consider using a `let` binding to create a longer lived value
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// ```
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// - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66
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//
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// This makes such a macro incredibly unergonomic in practice, and the reason most macros
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// out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`)
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// instead of featuring the more intuitive ergonomics of an expression macro.
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//
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// Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a
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// temporary is dropped at the end of its enclosing statement when it is part of the parameters
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// given to function call, which has precisely been the case with our `Pin::new_unchecked()`!
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// For instance,
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// ```rust
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// let p = Pin::new_unchecked(&mut <temporary>);
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// ```
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// becomes:
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// ```rust
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// let p = { let mut anon = <temporary>; &mut anon };
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// ```
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//
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// However, when using a literal braced struct to construct the value, references to temporaries
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// can then be taken. This makes Rust change the lifespan of such temporaries so that they are,
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// instead, dropped _at the end of the enscoping block_.
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// For instance,
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// ```rust
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// let p = Pin { pointer: &mut <temporary> };
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// ```
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// becomes:
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// ```rust
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// let mut anon = <temporary>;
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// let p = Pin { pointer: &mut anon };
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// ```
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// which is *exactly* what we want.
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//
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// See https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension
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// for more info.
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//
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// Finally, we don't hit problems _w.r.t._ the privacy of the `pointer` field, or the
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// unqualified `Pin` name, thanks to `decl_macro`s being _fully_ hygienic (`def_site` hygiene).
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Pin::<&mut _> { pointer: &mut { $value } }
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}
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