Add a stack-pin!-ning macro to the pin module.

Author: danielhenrymantilla

library/core/src/pin.rs

@@ -909,3 +909,242 @@ impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}
 
 #[stable(feature = "pin", since = "1.33.0")]
 impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
+
+/// Constructs a <code>[Pin]<[&mut] T></code>, 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, <code>[Pin]<[&mut] T></code> behaves like <code>[&mut] T</code>, 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<Self>) {
+/// #         self.0.unpark();
+/// #     }
+/// # }
+/// #
+/// /// Runs a future to completion.
+/// fn block_on<Fut: Future>(fut: Fut) -> Fut::Output {
+///     let waker_that_unparks_thread = // …
+///         # Arc::new(ThreadWaker(thread::current())).into();
+///     let mut cx = Context::from_waker(&waker_that_unparks_thread);
+///     // Pin the future so it can be polled.
+///     let mut pinned_fut = pin!(fut);
+///     loop {
+///         match pinned_fut.as_mut().poll(&mut cx) {
+///             Poll::Pending => thread::park(),
+///             Poll::Ready(res) => return res,
+///         }
+///     }
+/// }
+/// #
+/// # assert_eq!(42, block_on(async { 42 }));
+/// ```
+///
+/// ### With `Generator`s
+///
+/// ```rust
+/// #![feature(generators, generator_trait, pin_macro)]
+/// use core::{
+///     ops::{Generator, GeneratorState},
+///     pin::pin,
+/// };
+///
+/// fn generator_fn() -> impl Generator<Yield = usize, Return = ()> /* not Unpin */ {
+///  // Allow generator to be self-referential (not `Unpin`)
+///  // vvvvvv        so that locals can cross yield points.
+///     static || {
+///         let foo = String::from("foo"); // --+
+///         yield 0;                         // | <- crosses yield point!
+///         println!("{}", &foo); // <----------+
+///         yield foo.len();
+///     }
+/// }
+///
+/// fn main() {
+///     let mut generator = pin!(generator_fn());
+///     match generator.as_mut().resume(()) {
+///         GeneratorState::Yielded(0) => {},
+///         _ => unreachable!(),
+///     }
+///     match generator.as_mut().resume(()) {
+///         GeneratorState::Yielded(3) => {},
+///         _ => unreachable!(),
+///     }
+///     match generator.resume(()) {
+///         GeneratorState::Yielded(_) => unreachable!(),
+///         GeneratorState::Complete(()) => {},
+///     }
+/// }
+/// ```
+///
+/// ## Remarks
+///
+/// Precisely because a value is pinned to local storage, the resulting <code>[Pin]<[&mut] T></code>
+/// 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
+/// ```
+///
+/// <details><summary>Error message</summary>
+///
+/// ```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
+/// # };
+/// ```
+///
+/// </details>
+///
+/// This makes [`pin!`] **unsuitable to pin values when intending to _return_ them**. Instead, the
+/// value is expected to be passed around _unpinned_ until the point where it is to be consumed,
+/// where it is then useful and even sensible to pin the value locally using [`pin!`].
+///
+/// If you really need to return a pinned value, consider using [`Box::pin`] instead.
+///
+/// On the other hand, pinning to the stack[<sup>2</sup>](#fn2) using [`pin!`] is likely to be
+/// cheaper than pinning into a fresh heap allocation using [`Box::pin`]. Moreover, by virtue of not
+/// even needing an allocator, [`pin!`] is the main non-`unsafe` `#![no_std]`-compatible [`Pin`]
+/// constructor.
+///
+/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
+#[unstable(feature = "pin_macro", issue = "93178")]
+pub macro pin($value:expr $(,)?) {
+    // This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
+    // review such a hypothetical macro (that any user-code could define):
+    //
+    // ```rust
+    // macro_rules! pin {( $value:expr ) => (
+    //     match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block.
+    //         $crate::pin::Pin::<&mut _>::new_unchecked(at_value)
+    //     }}
+    // )}
+    // ```
+    //
+    // Safety:
+    //   - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls
+    //     that would break `Pin`'s invariants.
+    //   - `{ $value }` is braced, making it a _block expression_, thus **moving**
+    //     the given `$value`, and making it _become an **anonymous** temporary_.
+    //     By virtue of being anonynomous, it can no longer be accessed, thus
+    //     preventing any attemps to `mem::replace` it or `mem::forget` it, _etc._
+    //
+    // This gives us a `pin!` definition that is sound, and which works, but only
+    // in certain scenarios:
+    //   - If the `pin!(value)` expression is _directly_ fed to a function call:
+    //     `let poll = pin!(fut).poll(cx);`
+    //   - If the `pin!(value)` expression is part of a scrutinee:
+    //     ```rust
+    //     match pin!(fut) { pinned_fut => {
+    //         pinned_fut.as_mut().poll(...);
+    //         pinned_fut.as_mut().poll(...);
+    //     }} // <- `fut` is dropped here.
+    //     ```
+    // Alas, it doesn't work for the more straight-forward use-case: `let` bindings.
+    // ```rust
+    // let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement
+    // pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed
+    //                      // note: consider using a `let` binding to create a longer lived value
+    // ```
+    //   - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66
+    //
+    // This makes such a macro incredibly unergonomic in practice, and the reason most macros
+    // out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`)
+    // instead of featuring the more intuitive ergonomics of an expression macro.
+    //
+    // Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a
+    // temporary is dropped at the end of its enclosing statement when it is part of the parameters
+    // given to function call, which has precisely been the case with our `Pin::new_unchecked()`!
+    // For instance,
+    // ```rust
+    // let p = Pin::new_unchecked(&mut <temporary>);
+    // ```
+    // becomes:
+    // ```rust
+    // let p = { let mut anon = <temporary>; &mut anon };
+    // ```
+    //
+    // However, when using a literal braced struct to construct the value, references to temporaries
+    // can then be taken. This makes Rust change the lifespan of such temporaries so that they are,
+    // instead, dropped _at the end of the enscoping block_.
+    // For instance,
+    // ```rust
+    // let p = Pin { pointer: &mut <temporary> };
+    // ```
+    // becomes:
+    // ```rust
+    // let mut anon = <temporary>;
+    // let p = Pin { pointer: &mut anon };
+    // ```
+    // which is *exactly* what we want.
+    //
+    // Finally, we don't hit problems _w.r.t._ the privacy of the `pointer` field, or the
+    // unqualified `Pin` name, thanks to `decl_macro`s being _fully_ hygienic (`def_site` hygiene).
+    Pin { pointer: &mut { $value } }
+}