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    #![cfg_attr(not(feature = "rt"), allow(dead_code))]

    //! Source of time abstraction.
    //!
    //! By default, `std::time::Instant::now()` is used. However, when the
    //! `test-util` feature flag is enabled, the values returned for `now()` are
    //! configurable.

    cfg_not_test_util! {
        use crate::time::{Duration, Instant};

        #[derive(Debug, Clone)]
        pub(crate) struct Clock {}

        pub(crate) fn now() -> Instant {
            Instant::from_std(std::time::Instant::now())
        }

        impl Clock {
            pub(crate) fn new(_enable_pausing: bool, _start_paused: bool) -> Clock {
                Clock {}
            }

            pub(crate) fn now(&self) -> Instant {
                now()
            }

            pub(crate) fn is_paused(&self) -> bool {
                false
            }

            pub(crate) fn advance(&self, _dur: Duration) {
                unreachable!();
            }
        }
    }

    cfg_test_util! {
        use crate::time::{Duration, Instant};
        use std::sync::{Arc, Mutex};

        cfg_rt! {
            fn clock() -> Option<Clock> {
                crate::runtime::context::clock()
            }
        }

        cfg_not_rt! {
            fn clock() -> Option<Clock> {
                None
            }
        }

        /// A handle to a source of time.
        #[derive(Debug, Clone)]
        pub(crate) struct Clock {
            inner: Arc<Mutex<Inner>>,
        }

        #[derive(Debug)]
        struct Inner {
            /// True if the ability to pause time is enabled.
            enable_pausing: bool,

            /// Instant to use as the clock's base instant.
            base: std::time::Instant,

            /// Instant at which the clock was last unfrozen
            unfrozen: Option<std::time::Instant>,
        }

        /// Pause time
        ///
        /// The current value of `Instant::now()` is saved and all subsequent calls
        /// to `Instant::now()` until the timer wheel is checked again will return
        /// the saved value. Once the timer wheel is checked, time will immediately
        /// advance to the next registered `Sleep`. This is useful for running tests
        /// that depend on time.
        ///
        /// Pausing time requires the `current_thread` Tokio runtime. This is the
        /// default runtime used by `#[tokio::test]`. The runtime can be initialized
        /// with time in a paused state using the `Builder::start_paused` method.
        ///
        /// # Panics
        ///
        /// Panics if time is already frozen or if called from outside of a
        /// `current_thread` Tokio runtime.
        pub fn pause() {
            let clock = clock().expect("time cannot be frozen from outside the Tokio runtime");
            clock.pause();
        }

        /// Resume time
        ///
        /// Clears the saved `Instant::now()` value. Subsequent calls to
        /// `Instant::now()` will return the value returned by the system call.
        ///
        /// # Panics
        ///
        /// Panics if time is not frozen or if called from outside of the Tokio
        /// runtime.
        pub fn resume() {
            let clock = clock().expect("time cannot be frozen from outside the Tokio runtime");
            let mut inner = clock.inner.lock().unwrap();

            if inner.unfrozen.is_some() {
                panic!("time is not frozen");
            }

            inner.unfrozen = Some(std::time::Instant::now());
        }

        /// Advance time
        ///
        /// Increments the saved `Instant::now()` value by `duration`. Subsequent
        /// calls to `Instant::now()` will return the result of the increment.
        ///
        /// # Panics
        ///
        /// Panics if time is not frozen or if called from outside of the Tokio
        /// runtime.
        pub async fn advance(duration: Duration) {
            use crate::future::poll_fn;
            use std::task::Poll;

            let clock = clock().expect("time cannot be frozen from outside the Tokio runtime");
            clock.advance(duration);

            let mut yielded = false;
            poll_fn(|cx| {
                if yielded {
                    Poll::Ready(())
                } else {
                    yielded = true;
                    cx.waker().wake_by_ref();
                    Poll::Pending
                }
            }).await;
        }

        /// Return the current instant, factoring in frozen time.
        pub(crate) fn now() -> Instant {
            if let Some(clock) = clock() {
                clock.now()
            } else {
                Instant::from_std(std::time::Instant::now())
            }
        }

        impl Clock {
            /// Return a new `Clock` instance that uses the current execution context's
            /// source of time.
            pub(crate) fn new(enable_pausing: bool, start_paused: bool) -> Clock {
                let now = std::time::Instant::now();

                let clock = Clock {
                    inner: Arc::new(Mutex::new(Inner {
                        enable_pausing,
                        base: now,
                        unfrozen: Some(now),
                    })),
                };

                if start_paused {
                    clock.pause();
                }

                clock
            }

            pub(crate) fn pause(&self) {
                let mut inner = self.inner.lock().unwrap();

                if !inner.enable_pausing {
                    drop(inner); // avoid poisoning the lock
                    panic!("`time::pause()` requires the `current_thread` Tokio runtime. \
                            This is the default Runtime used by `#[tokio::test].");
                }

                let elapsed = inner.unfrozen.as_ref().expect("time is already frozen").elapsed();
                inner.base += elapsed;
                inner.unfrozen = None;
            }

            pub(crate) fn is_paused(&self) -> bool {
                let inner = self.inner.lock().unwrap();
                inner.unfrozen.is_none()
            }

            pub(crate) fn advance(&self, duration: Duration) {
                let mut inner = self.inner.lock().unwrap();

                if inner.unfrozen.is_some() {
                    panic!("time is not frozen");
                }

                inner.base += duration;
            }

            pub(crate) fn now(&self) -> Instant {
                let inner = self.inner.lock().unwrap();

                let mut ret = inner.base;

                if let Some(unfrozen) = inner.unfrozen {
                    ret += unfrozen.elapsed();
                }

                Instant::from_std(ret)
            }
        }
    }