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    use crate::time::driver::{Handle, TimerEntry};
    use crate::time::{error::Error, Duration, Instant};

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

    /// Waits until `deadline` is reached.
    ///
    /// No work is performed while awaiting on the sleep future to complete. `Sleep`
    /// operates at millisecond granularity and should not be used for tasks that
    /// require high-resolution timers.
    ///
    /// # Cancellation
    ///
    /// Canceling a sleep instance is done by dropping the returned future. No additional
    /// cleanup work is required.
    // Alias for old name in 0.x
    #[cfg_attr(docsrs, doc(alias = "delay_until"))]
    pub fn sleep_until(deadline: Instant) -> Sleep {
        Sleep::new_timeout(deadline)
    }

    /// Waits until `duration` has elapsed.
    ///
    /// Equivalent to `sleep_until(Instant::now() + duration)`. An asynchronous
    /// analog to `std::thread::sleep`.
    ///
    /// No work is performed while awaiting on the sleep future to complete. `Sleep`
    /// operates at millisecond granularity and should not be used for tasks that
    /// require high-resolution timers.
    ///
    /// To run something regularly on a schedule, see [`interval`].
    ///
    /// The maximum duration for a sleep is 68719476734 milliseconds (approximately 2.2 years).
    ///
    /// # Cancellation
    ///
    /// Canceling a sleep instance is done by dropping the returned future. No additional
    /// cleanup work is required.
    ///
    /// # Examples
    ///
    /// Wait 100ms and print "100 ms have elapsed".
    ///
    /// ```
    /// use tokio::time::{sleep, Duration};
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     sleep(Duration::from_millis(100)).await;
    ///     println!("100 ms have elapsed");
    /// }
    /// ```
    ///
    /// [`interval`]: crate::time::interval()
    // Alias for old name in 0.x
    #[cfg_attr(docsrs, doc(alias = "delay_for"))]
    pub fn sleep(duration: Duration) -> Sleep {
        match Instant::now().checked_add(duration) {
            Some(deadline) => sleep_until(deadline),
            None => sleep_until(Instant::far_future()),
        }
    }

    pin_project! {
        /// Future returned by [`sleep`](sleep) and [`sleep_until`](sleep_until).
        ///
        /// This type does not implement the `Unpin` trait, which means that if you
        /// use it with [`select!`] or by calling `poll`, you have to pin it first.
        /// If you use it with `.await`, this does not apply.
        ///
        /// # Examples
        ///
        /// Wait 100ms and print "100 ms have elapsed".
        ///
        /// ```
        /// use tokio::time::{sleep, Duration};
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     sleep(Duration::from_millis(100)).await;
        ///     println!("100 ms have elapsed");
        /// }
        /// ```
        ///
        /// Use with [`select!`]. Pinning the `Sleep` with [`tokio::pin!`] is
        /// necessary when the same `Sleep` is selected on multiple times.
        /// ```no_run
        /// use tokio::time::{self, Duration, Instant};
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let sleep = time::sleep(Duration::from_millis(10));
        ///     tokio::pin!(sleep);
        ///
        ///     loop {
        ///         tokio::select! {
        ///             () = &mut sleep => {
        ///                 println!("timer elapsed");
        ///                 sleep.as_mut().reset(Instant::now() + Duration::from_millis(50));
        ///             },
        ///         }
        ///     }
        /// }
        /// ```
        /// Use in a struct with boxing. By pinning the `Sleep` with a `Box`, the
        /// `HasSleep` struct implements `Unpin`, even though `Sleep` does not.
        /// ```
        /// use std::future::Future;
        /// use std::pin::Pin;
        /// use std::task::{Context, Poll};
        /// use tokio::time::Sleep;
        ///
        /// struct HasSleep {
        ///     sleep: Pin<Box<Sleep>>,
        /// }
        ///
        /// impl Future for HasSleep {
        ///     type Output = ();
        ///
        ///     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        ///         self.sleep.as_mut().poll(cx)
        ///     }
        /// }
        /// ```
        /// Use in a struct with pin projection. This method avoids the `Box`, but
        /// the `HasSleep` struct will not be `Unpin` as a consequence.
        /// ```
        /// use std::future::Future;
        /// use std::pin::Pin;
        /// use std::task::{Context, Poll};
        /// use tokio::time::Sleep;
        /// use pin_project_lite::pin_project;
        ///
        /// pin_project! {
        ///     struct HasSleep {
        ///         #[pin]
        ///         sleep: Sleep,
        ///     }
        /// }
        ///
        /// impl Future for HasSleep {
        ///     type Output = ();
        ///
        ///     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        ///         self.project().sleep.poll(cx)
        ///     }
        /// }
        /// ```
        ///
        /// [`select!`]: ../macro.select.html
        /// [`tokio::pin!`]: ../macro.pin.html
        #[derive(Debug)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct Sleep {
            deadline: Instant,

            // The link between the `Sleep` instance and the timer that drives it.
            #[pin]
            entry: TimerEntry,
        }
    }

    impl Sleep {
        pub(crate) fn new_timeout(deadline: Instant) -> Sleep {
            let handle = Handle::current();
            let entry = TimerEntry::new(&handle, deadline);

            Sleep { deadline, entry }
        }

        pub(crate) fn far_future() -> Sleep {
            Self::new_timeout(Instant::far_future())
        }

        /// Returns the instant at which the future will complete.
        pub fn deadline(&self) -> Instant {
            self.deadline
        }

        /// Returns `true` if `Sleep` has elapsed.
        ///
        /// A `Sleep` instance is elapsed when the requested duration has elapsed.
        pub fn is_elapsed(&self) -> bool {
            self.entry.is_elapsed()
        }

        /// Resets the `Sleep` instance to a new deadline.
        ///
        /// Calling this function allows changing the instant at which the `Sleep`
        /// future completes without having to create new associated state.
        ///
        /// This function can be called both before and after the future has
        /// completed.
        ///
        /// To call this method, you will usually combine the call with
        /// [`Pin::as_mut`], which lets you call the method without consuming the
        /// `Sleep` itself.
        ///
        /// # Example
        ///
        /// ```
        /// use tokio::time::{Duration, Instant};
        ///
        /// # #[tokio::main(flavor = "current_thread")]
        /// # async fn main() {
        /// let sleep = tokio::time::sleep(Duration::from_millis(10));
        /// tokio::pin!(sleep);
        ///
        /// sleep.as_mut().reset(Instant::now() + Duration::from_millis(20));
        /// # }
        /// ```
        ///
        /// [`Pin::as_mut`]: fn@std::pin::Pin::as_mut
        pub fn reset(self: Pin<&mut Self>, deadline: Instant) {
            let me = self.project();
            me.entry.reset(deadline);
            *me.deadline = deadline;
        }

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

            // Keep track of task budget
            let coop = ready!(crate::coop::poll_proceed(cx));

            me.entry.poll_elapsed(cx).map(move |r| {
                coop.made_progress();
                r
            })
        }
    }

    impl Future for Sleep {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            // `poll_elapsed` can return an error in two cases:
            //
            // - AtCapacity: this is a pathological case where far too many
            //   sleep instances have been scheduled.
            // - Shutdown: No timer has been setup, which is a mis-use error.
            //
            // Both cases are extremely rare, and pretty accurately fit into
            // "logic errors", so we just panic in this case. A user couldn't
            // really do much better if we passed the error onwards.
            match ready!(self.as_mut().poll_elapsed(cx)) {
                Ok(()) => Poll::Ready(()),
                Err(e) => panic!("timer error: {}", e),
            }
        }
    }