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
    }
}
}