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Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0728.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0728.md | 75 |
1 files changed, 75 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0728.md b/compiler/rustc_error_codes/src/error_codes/E0728.md new file mode 100644 index 000000000..f4968a4f0 --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0728.md @@ -0,0 +1,75 @@ +[`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 |