Add future::timeout()

Author: Stjepan Glavina

src/future/mod.rs

@@ -5,6 +5,8 @@ pub use std::future::Future;
 
 pub use pending::pending;
 pub use ready::ready;
+pub use timeout::{timeout, TimeoutError};
 
 mod pending;
 mod ready;
+mod timeout;

src/future/timeout.rs

@@ -0,0 +1,78 @@
+use std::error::Error;
+use std::fmt;
+use std::pin::Pin;
+use std::time::Duration;
+
+use futures_timer::Delay;
+use pin_utils::unsafe_pinned;
+
+use crate::future::Future;
+use crate::task::{Context, Poll};
+
+/// Awaits a future or times out after a duration of time.
+///
+/// # Examples
+///
+/// ```no_run
+/// # #![feature(async_await)]
+/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
+/// #
+/// use std::time::Duration;
+///
+/// use async_std::future;
+///
+/// let never = future::pending::<()>();
+/// let dur = Duration::from_secs(5);
+/// assert!(future::timeout(dur, never).await.is_err());
+/// #
+/// # Ok(()) }) }
+/// ```
+pub async fn timeout<F, T>(dur: Duration, f: F) -> Result<T, TimeoutError>
+where
+    F: Future<Output = T>,
+{
+    let f = TimeoutFuture {
+        future: f,
+        delay: Delay::new(dur),
+    };
+    f.await
+}
+
+/// A future that times out after a duration of time.
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+struct TimeoutFuture<F> {
+    future: F,
+    delay: Delay,
+}
+
+impl<F> TimeoutFuture<F> {
+    unsafe_pinned!(future: F);
+    unsafe_pinned!(delay: Delay);
+}
+
+impl<F: Future> Future for TimeoutFuture<F> {
+    type Output = Result<F::Output, TimeoutError>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.as_mut().future().poll(cx) {
+            Poll::Ready(v) => Poll::Ready(Ok(v)),
+            Poll::Pending => match self.delay().poll(cx) {
+                Poll::Ready(_) => Poll::Ready(Err(TimeoutError)),
+                Poll::Pending => Poll::Pending,
+            },
+        }
+    }
+}
+
+/// An error returned when a future times out.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct TimeoutError;
+
+impl Error for TimeoutError {}
+
+impl fmt::Display for TimeoutError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "future has timed out".fmt(f)
+    }
+}