[`await`] has been used outside [`async`] function or block.
Erroneous code examples:
```edition2018,compile_fail,E0728
# 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 fn` or an `async` block.
```edition2018
# 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
}
}
```
[`async`]: https://doc.rust-lang.org/std/keyword.async.html
[`await`]: https://doc.rust-lang.org/std/keyword.await.html