E0728 - Error codes index
Description: Book listing all Rust error codes

# Error code E0728

`await` has been used outside `async` function or `async` block.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    use std::pin::Pin;
    use std::future::Future;
    use std::task::{Context, Poll};

    struct WakeOnceThenComplete(bool);

    fn wake_and_yield_once() -> WakeOnceThenComplete {
        WakeOnceThenComplete(false)
    }

    impl Future for WakeOnceThenComplete {
        type Output = ();
        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.0 {
                Poll::Ready(())
            } else {
                cx.waker().wake_by_ref();
                self.0 = true;
                Poll::Pending
            }
        }
    }

    fn foo() {
        wake_and_yield_once().await // `await` is used outside `async` context
    }
    }

`await` is used to suspend the current computation until the given future is ready to produce a value. So it is legal only within an `async` context, like an `async` function or an `async` block.

    #![allow(unused)]
    fn main() {
    use std::pin::Pin;
    use std::future::Future;
    use std::task::{Context, Poll};

    struct WakeOnceThenComplete(bool);

    fn wake_and_yield_once() -> WakeOnceThenComplete {
        WakeOnceThenComplete(false)
    }

    impl Future for WakeOnceThenComplete {
        type Output = ();
        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.0 {
                Poll::Ready(())
            } else {
                cx.waker().wake_by_ref();
                self.0 = true;
                Poll::Pending
            }
        }
    }

    async fn foo() {
        wake_and_yield_once().await // `await` is used within `async` function
    }

    fn bar(x: u8) -> impl Future<Output = u8> {
        async move {
            wake_and_yield_once().await; // `await` is used within `async` block
            x
        }
    }
    }