Macro pin

pub macro pin($value:expr $(,)?) {
    ...
}

Constructs a Pin<&mut T>, by pinning a value: T locally.

Unlike Box::pin, this does not create a new heap allocation. As explained below, the element might still end up on the heap however.

The local pinning performed by this macro is usually dubbed “stack”-pinning. Outside of async contexts locals do indeed get stored on the stack. In async functions or blocks however, any locals crossing an .await point are part of the state captured by the Future, and will use the storage of those. That storage can either be on the heap or on the stack. Therefore, local pinning is a more accurate term.

If the type of the given value does not implement Unpin, then this macro pins the value in memory in a way that prevents moves. On the other hand, if the type does implement Unpin, Pin<&mut T> behaves like &mut T, and operations such as mem::replace() or mem::take() will allow moves of the value. See the Unpin section of the pin module for details.

Examples

Basic usage

use core::pin::{pin, Pin};

fn stuff(foo: Pin<&mut Foo>) {
    // …
}

let pinned_foo = pin!(Foo { /* … */ });
stuff(pinned_foo);
// or, directly:
stuff(pin!(Foo { /* … */ }));

Manually polling a Future (without Unpin bounds)

use std::{
    future::Future,
    pin::pin,
    task::{Context, Poll},
    thread,
};

/// Runs a future to completion.
fn block_on<Fut: Future>(fut: Fut) -> Fut::Output {
    let waker_that_unparks_thread = // …
    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,
        }
    }
}

With Coroutines

#![feature(coroutines)]
#![feature(coroutine_trait)]
use core::{
    ops::{Coroutine, CoroutineState},
    pin::pin,
};

fn coroutine_fn() -> impl Coroutine<Yield = usize, Return = ()> /* not Unpin */ {
 // Allow coroutine to be self-referential (not `Unpin`)
 // vvvvvv        so that locals can cross yield points.
    #[coroutine] static || {
        let foo = String::from("foo");
        let foo_ref = &foo; // ------+
        yield 0;                  // | <- crosses yield point!
        println!("{foo_ref}"); // <--+
        yield foo.len();
    }
}

fn main() {
    let mut coroutine = pin!(coroutine_fn());
    match coroutine.as_mut().resume(()) {
        CoroutineState::Yielded(0) => {},
        _ => unreachable!(),
    }
    match coroutine.as_mut().resume(()) {
        CoroutineState::Yielded(3) => {},
        _ => unreachable!(),
    }
    match coroutine.resume(()) {
        CoroutineState::Yielded(_) => unreachable!(),
        CoroutineState::Complete(()) => {},
    }
}

Remarks

Precisely because a value is pinned to local storage, the resulting 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:

use core::pin::{pin, Pin};

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

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 value 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

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, local pinning using pin! is likely to be cheaper than pinning into a fresh heap allocation using Box::pin. Moreover, by virtue of not requiring an allocator, pin! is the main non-unsafe #![no_std]-compatible Pin constructor.