async and await

async turns a function into a Future. await, which is “valid only within an async context”, composes Futures.

Examples

async

• An async-prefixed function foo as defined in its original form:

  async fn foo(v: usize) -> usize {
      // ...
  }

• The pseudocode mimics what the compiler generates:

  fn foo(v: usize) -> impl Future<Output = usize> {
      // ...
  }

await

• A Future “awaits” in its original form:

  // ...
  // within an async context
  // ...
  let v = future.await;
  // ...

• The pseudocode mimics what the compiler generates (the keyword yield ties to a Coroutine):

  // ...
  let mut future = pin!(future);
  let v = loop {
      match future.as_mut().poll(cx) {
          Poll::Ready(output) => break output,
          Poll::Pending => yield,
      }
  };
  // ...

Closure and block

async and await can also be applied to a closure or a block.