blob: f4968a4f00e38f4da4ea6376faeff7c039d5f63f (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
|
[`await`] has been used outside [`async`] function or [`async`] block.
Erroneous code example:
```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` function 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
|