Await expressions

Syntax
AwaitExpression :
   Expression . await

Await expressions are legal only within an async context, like an async fn or an async block. They operate on a future. Their effect is to suspend the current computation until the given future is ready to produce a value.

More specifically, an <expr>.await expression has the following effect.

  1. Evaluate <expr> to a future tmp;
  2. Pin tmp using Pin::new_unchecked;
  3. This pinned future is then polled by calling the Future::poll method and passing it the current task context;
  4. If the call to poll returns Poll::Pending, then the future returns Poll::Pending, suspending its state so that, when the surrounding async context is re-polled, execution returns to step 2;
  5. Otherwise the call to poll must have returned Poll::Ready, in which case the value contained in the Poll::Ready variant is used as the result of the await expression itself.

Edition differences: Await expressions are only available beginning with Rust 2018.

Task context

The task context refers to the Context which was supplied to the current async context when the async context itself was polled. Because await expressions are only legal in an async context, there must be some task context available.

Approximate desugaring

Effectively, an <expr>.await expression is roughly equivalent to the following (this desugaring is not normative):

match /* <expr> */ {
    mut pinned => loop {
        let mut pin = unsafe { Pin::new_unchecked(&mut pinned) };
        match Pin::future::poll(Pin::borrow(&mut pin), &mut current_context) {
            Poll::Ready(r) => break r,
            Poll::Pending => yield Poll::Pending,
        }
    }
}

where the yield pseudo-code returns Poll::Pending and, when re-invoked, resumes execution from that point. The variable current_context refers to the context taken from the async environment.