Await expressions - The Rust Reference (original) (raw)

The Rust Reference

Await expressions

Syntax
AwaitExpression :
Expression . await

An await expression is a syntactic construct for suspending a computation provided by an implementation of std::future::IntoFuture until the given future is ready to produce a value.

The syntax for an await expression is an expression with a type that implements the IntoFuture trait, called the future operand, then the token ., and then the await keyword.

Await expressions are legal only within an async context, like an async fn, async closure, or async block.

More specifically, an await expression has the following effect.

1. Create a future by calling IntoFuture::into_future on the future operand.
2. Evaluate the future to a future tmp;
3. Pin tmp using Pin::new_unchecked;
4. This pinned future is then polled by calling the Future::poll method and passing it the current task context;
5. 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 3;
6. 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 await expression is roughly equivalent to the following non-normative desugaring:

match operand.into_future() {
    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.