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    #![allow(clippy::trivially_copy_pass_by_ref)]

    use std::fmt;
    use std::ops;
    use std::time::Duration;

    /// A measurement of a monotonically nondecreasing clock.
    /// Opaque and useful only with `Duration`.
    ///
    /// Instants are always guaranteed to be no less than any previously measured
    /// instant when created, and are often useful for tasks such as measuring
    /// benchmarks or timing how long an operation takes.
    ///
    /// Note, however, that instants are not guaranteed to be **steady**. In other
    /// words, each tick of the underlying clock may not be the same length (e.g.
    /// some seconds may be longer than others). An instant may jump forwards or
    /// experience time dilation (slow down or speed up), but it will never go
    /// backwards.
    ///
    /// Instants are opaque types that can only be compared to one another. There is
    /// no method to get "the number of seconds" from an instant. Instead, it only
    /// allows measuring the duration between two instants (or comparing two
    /// instants).
    ///
    /// The size of an `Instant` struct may vary depending on the target operating
    /// system.
    ///
    /// # Note
    ///
    /// This type wraps the inner `std` variant and is used to align the Tokio
    /// clock for uses of `now()`. This can be useful for testing where you can
    /// take advantage of `time::pause()` and `time::advance()`.
    #[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Hash)]
    pub struct Instant {
        std: std::time::Instant,
    }

    impl Instant {
        /// Returns an instant corresponding to "now".
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::time::Instant;
        ///
        /// let now = Instant::now();
        /// ```
        pub fn now() -> Instant {
            variant::now()
        }

        /// Create a `tokio::time::Instant` from a `std::time::Instant`.
        pub fn from_std(std: std::time::Instant) -> Instant {
            Instant { std }
        }

        pub(crate) fn far_future() -> Instant {
            // Roughly 30 years from now.
            // API does not provide a way to obtain max `Instant`
            // or convert specific date in the future to instant.
            // 1000 years overflows on macOS, 100 years overflows on FreeBSD.
            Self::now() + Duration::from_secs(86400 * 365 * 30)
        }

        /// Convert the value into a `std::time::Instant`.
        pub fn into_std(self) -> std::time::Instant {
            self.std
        }

        /// Returns the amount of time elapsed from another instant to this one.
        ///
        /// # Panics
        ///
        /// This function will panic if `earlier` is later than `self`.
        pub fn duration_since(&self, earlier: Instant) -> Duration {
            self.std.duration_since(earlier.std)
        }

        /// Returns the amount of time elapsed from another instant to this one, or
        /// None if that instant is later than this one.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::time::{Duration, Instant, sleep};
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let now = Instant::now();
        ///     sleep(Duration::new(1, 0)).await;
        ///     let new_now = Instant::now();
        ///     println!("{:?}", new_now.checked_duration_since(now));
        ///     println!("{:?}", now.checked_duration_since(new_now)); // None
        /// }
        /// ```
        pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
            self.std.checked_duration_since(earlier.std)
        }

        /// Returns the amount of time elapsed from another instant to this one, or
        /// zero duration if that instant is earlier than this one.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::time::{Duration, Instant, sleep};
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let now = Instant::now();
        ///     sleep(Duration::new(1, 0)).await;
        ///     let new_now = Instant::now();
        ///     println!("{:?}", new_now.saturating_duration_since(now));
        ///     println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
        /// }
        /// ```
        pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
            self.std.saturating_duration_since(earlier.std)
        }

        /// Returns the amount of time elapsed since this instant was created.
        ///
        /// # Panics
        ///
        /// This function may panic if the current time is earlier than this
        /// instant, which is something that can happen if an `Instant` is
        /// produced synthetically.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::time::{Duration, Instant, sleep};
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let instant = Instant::now();
        ///     let three_secs = Duration::from_secs(3);
        ///     sleep(three_secs).await;
        ///     assert!(instant.elapsed() >= three_secs);
        /// }
        /// ```
        pub fn elapsed(&self) -> Duration {
            Instant::now() - *self
        }

        /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be
        /// represented as `Instant` (which means it's inside the bounds of the
        /// underlying data structure), `None` otherwise.
        pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
            self.std.checked_add(duration).map(Instant::from_std)
        }

        /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be
        /// represented as `Instant` (which means it's inside the bounds of the
        /// underlying data structure), `None` otherwise.
        pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
            self.std.checked_sub(duration).map(Instant::from_std)
        }
    }

    impl From<std::time::Instant> for Instant {
        fn from(time: std::time::Instant) -> Instant {
            Instant::from_std(time)
        }
    }

    impl From<Instant> for std::time::Instant {
        fn from(time: Instant) -> std::time::Instant {
            time.into_std()
        }
    }

    impl ops::Add<Duration> for Instant {
        type Output = Instant;

        fn add(self, other: Duration) -> Instant {
            Instant::from_std(self.std + other)
        }
    }

    impl ops::AddAssign<Duration> for Instant {
        fn add_assign(&mut self, rhs: Duration) {
            *self = *self + rhs;
        }
    }

    impl ops::Sub for Instant {
        type Output = Duration;

        fn sub(self, rhs: Instant) -> Duration {
            self.std - rhs.std
        }
    }

    impl ops::Sub<Duration> for Instant {
        type Output = Instant;

        fn sub(self, rhs: Duration) -> Instant {
            Instant::from_std(self.std - rhs)
        }
    }

    impl ops::SubAssign<Duration> for Instant {
        fn sub_assign(&mut self, rhs: Duration) {
            *self = *self - rhs;
        }
    }

    impl fmt::Debug for Instant {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.std.fmt(fmt)
        }
    }

    #[cfg(not(feature = "test-util"))]
    mod variant {
        use super::Instant;

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

    #[cfg(feature = "test-util")]
    mod variant {
        use super::Instant;

        pub(super) fn now() -> Instant {
            crate::time::clock::now()
        }
    }