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    //! Allows a future to execute for a maximum amount of time.
    //!
    //! See [`Timeout`] documentation for more details.
    //!
    //! [`Timeout`]: struct@Timeout

    use crate::time::{error::Elapsed, sleep_until, Duration, Instant, Sleep};

    use pin_project_lite::pin_project;
    use std::future::Future;
    use std::pin::Pin;
    use std::task::{self, Poll};

    /// Require a `Future` to complete before the specified duration has elapsed.
    ///
    /// If the future completes before the duration has elapsed, then the completed
    /// value is returned. Otherwise, an error is returned and the future is
    /// canceled.
    ///
    /// # Cancelation
    ///
    /// Cancelling a timeout is done by dropping the future. No additional cleanup
    /// or other work is required.
    ///
    /// The original future may be obtained by calling [`Timeout::into_inner`]. This
    /// consumes the `Timeout`.
    ///
    /// # Examples
    ///
    /// Create a new `Timeout` set to expire in 10 milliseconds.
    ///
    /// ```rust
    /// use tokio::time::timeout;
    /// use tokio::sync::oneshot;
    ///
    /// use std::time::Duration;
    ///
    /// # async fn dox() {
    /// let (tx, rx) = oneshot::channel();
    /// # tx.send(()).unwrap();
    ///
    /// // Wrap the future with a `Timeout` set to expire in 10 milliseconds.
    /// if let Err(_) = timeout(Duration::from_millis(10), rx).await {
    ///     println!("did not receive value within 10 ms");
    /// }
    /// # }
    /// ```
    pub fn timeout<T>(duration: Duration, future: T) -> Timeout<T>
    where
        T: Future,
    {
        let deadline = Instant::now().checked_add(duration);
        let delay = match deadline {
            Some(deadline) => Sleep::new_timeout(deadline),
            None => Sleep::far_future(),
        };
        Timeout::new_with_delay(future, delay)
    }

    /// Require a `Future` to complete before the specified instant in time.
    ///
    /// If the future completes before the instant is reached, then the completed
    /// value is returned. Otherwise, an error is returned.
    ///
    /// # Cancelation
    ///
    /// Cancelling a timeout is done by dropping the future. No additional cleanup
    /// or other work is required.
    ///
    /// The original future may be obtained by calling [`Timeout::into_inner`]. This
    /// consumes the `Timeout`.
    ///
    /// # Examples
    ///
    /// Create a new `Timeout` set to expire in 10 milliseconds.
    ///
    /// ```rust
    /// use tokio::time::{Instant, timeout_at};
    /// use tokio::sync::oneshot;
    ///
    /// use std::time::Duration;
    ///
    /// # async fn dox() {
    /// let (tx, rx) = oneshot::channel();
    /// # tx.send(()).unwrap();
    ///
    /// // Wrap the future with a `Timeout` set to expire 10 milliseconds into the
    /// // future.
    /// if let Err(_) = timeout_at(Instant::now() + Duration::from_millis(10), rx).await {
    ///     println!("did not receive value within 10 ms");
    /// }
    /// # }
    /// ```
    pub fn timeout_at<T>(deadline: Instant, future: T) -> Timeout<T>
    where
        T: Future,
    {
        let delay = sleep_until(deadline);

        Timeout {
            value: future,
            delay,
        }
    }

    pin_project! {
        /// Future returned by [`timeout`](timeout) and [`timeout_at`](timeout_at).
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        #[derive(Debug)]
        pub struct Timeout<T> {
            #[pin]
            value: T,
            #[pin]
            delay: Sleep,
        }
    }

    impl<T> Timeout<T> {
        pub(crate) fn new_with_delay(value: T, delay: Sleep) -> Timeout<T> {
            Timeout { value, delay }
        }

        /// Gets a reference to the underlying value in this timeout.
        pub fn get_ref(&self) -> &T {
            &self.value
        }

        /// Gets a mutable reference to the underlying value in this timeout.
        pub fn get_mut(&mut self) -> &mut T {
            &mut self.value
        }

        /// Consumes this timeout, returning the underlying value.
        pub fn into_inner(self) -> T {
            self.value
        }
    }

    impl<T> Future for Timeout<T>
    where
        T: Future,
    {
        type Output = Result<T::Output, Elapsed>;

        fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            let me = self.project();

            // First, try polling the future
            if let Poll::Ready(v) = me.value.poll(cx) {
                return Poll::Ready(Ok(v));
            }

            // Now check the timer
            match me.delay.poll(cx) {
                Poll::Ready(()) => Poll::Ready(Err(Elapsed::new())),
                Poll::Pending => Poll::Pending,
            }
        }
    }