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 } } }