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    //! Temporal quantification.
    //!
    //! Example:
    //!
    //! ```
    //! use std::time::Duration;
    //!
    //! let five_seconds = Duration::new(5, 0);
    //! // both declarations are equivalent
    //! assert_eq!(Duration::new(5, 0), Duration::from_secs(5));
    //! ```

    #![stable(feature = "time", since = "1.3.0")]

    use crate::cmp;
    use crate::error::Error;
    use crate::fmt;
    use crate::ops::{Add, Sub, AddAssign, SubAssign};
    use crate::sys::time;
    use crate::sys_common::FromInner;
    use crate::sys_common::mutex::Mutex;

    #[stable(feature = "time", since = "1.3.0")]
    pub use core::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.
    ///
    /// Example:
    ///
    /// ```no_run
    /// use std::time::{Duration, Instant};
    /// use std::thread::sleep;
    ///
    /// fn main() {
    ///    let now = Instant::now();
    ///
    ///    // we sleep for 2 seconds
    ///    sleep(Duration::new(2, 0));
    ///    // it prints '2'
    ///    println!("{}", now.elapsed().as_secs());
    /// }
    /// ```
    #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
    #[stable(feature = "time2", since = "1.8.0")]
    pub struct Instant(time::Instant);

    /// A measurement of the system clock, useful for talking to
    /// external entities like the file system or other processes.
    ///
    /// Distinct from the [`Instant`] type, this time measurement **is not
    /// monotonic**. This means that you can save a file to the file system, then
    /// save another file to the file system, **and the second file has a
    /// `SystemTime` measurement earlier than the first**. In other words, an
    /// operation that happens after another operation in real time may have an
    /// earlier `SystemTime`!
    ///
    /// Consequently, comparing two `SystemTime` instances to learn about the
    /// duration between them returns a [`Result`] instead of an infallible [`Duration`]
    /// to indicate that this sort of time drift may happen and needs to be handled.
    ///
    /// Although a `SystemTime` cannot be directly inspected, the [`UNIX_EPOCH`]
    /// constant is provided in this module as an anchor in time to learn
    /// information about a `SystemTime`. By calculating the duration from this
    /// fixed point in time, a `SystemTime` can be converted to a human-readable time,
    /// or perhaps some other string representation.
    ///
    /// The size of a `SystemTime` struct may vary depending on the target operating
    /// system.
    ///
    /// [`Instant`]: ../../std/time/struct.Instant.html
    /// [`Result`]: ../../std/result/enum.Result.html
    /// [`Duration`]: ../../std/time/struct.Duration.html
    /// [`UNIX_EPOCH`]: ../../std/time/constant.UNIX_EPOCH.html
    ///
    /// Example:
    ///
    /// ```no_run
    /// use std::time::{Duration, SystemTime};
    /// use std::thread::sleep;
    ///
    /// fn main() {
    ///    let now = SystemTime::now();
    ///
    ///    // we sleep for 2 seconds
    ///    sleep(Duration::new(2, 0));
    ///    match now.elapsed() {
    ///        Ok(elapsed) => {
    ///            // it prints '2'
    ///            println!("{}", elapsed.as_secs());
    ///        }
    ///        Err(e) => {
    ///            // an error occurred!
    ///            println!("Error: {:?}", e);
    ///        }
    ///    }
    /// }
    /// ```
    #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
    #[stable(feature = "time2", since = "1.8.0")]
    pub struct SystemTime(time::SystemTime);

    /// An error returned from the `duration_since` and `elapsed` methods on
    /// `SystemTime`, used to learn how far in the opposite direction a system time
    /// lies.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::thread::sleep;
    /// use std::time::{Duration, SystemTime};
    ///
    /// let sys_time = SystemTime::now();
    /// sleep(Duration::from_secs(1));
    /// let new_sys_time = SystemTime::now();
    /// match sys_time.duration_since(new_sys_time) {
    ///     Ok(_) => {}
    ///     Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
    /// }
    /// ```
    #[derive(Clone, Debug)]
    #[stable(feature = "time2", since = "1.8.0")]
    pub struct SystemTimeError(Duration);

    impl Instant {
        /// Returns an instant corresponding to "now".
        ///
        /// # Examples
        ///
        /// ```
        /// use std::time::Instant;
        ///
        /// let now = Instant::now();
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn now() -> Instant {
            let os_now = time::Instant::now();

            // And here we come upon a sad state of affairs. The whole point of
            // `Instant` is that it's monotonically increasing. We've found in the
            // wild, however, that it's not actually monotonically increasing for
            // one reason or another. These appear to be OS and hardware level bugs,
            // and there's not really a whole lot we can do about them. Here's a
            // taste of what we've found:
            //
            // * #48514 - OpenBSD, x86_64
            // * #49281 - linux arm64 and s390x
            // * #51648 - windows, x86
            // * #56560 - windows, x86_64, AWS
            // * #56612 - windows, x86, vm (?)
            // * #56940 - linux, arm64
            // * https://bugzilla.mozilla.org/show_bug.cgi?id=1487778 - a similar
            //   Firefox bug
            //
            // It simply seems that this it just happens so that a lot in the wild
            // we're seeing panics across various platforms where consecutive calls
            // to `Instant::now`, such as via the `elapsed` function, are panicking
            // as they're going backwards. Placed here is a last-ditch effort to try
            // to fix things up. We keep a global "latest now" instance which is
            // returned instead of what the OS says if the OS goes backwards.
            //
            // To hopefully mitigate the impact of this though a few platforms are
            // whitelisted as "these at least haven't gone backwards yet".
            if time::Instant::actually_monotonic() {
                return Instant(os_now)
            }

            static LOCK: Mutex = Mutex::new();
            static mut LAST_NOW: time::Instant = time::Instant::zero();
            unsafe {
                let _lock = LOCK.lock();
                let now = cmp::max(LAST_NOW, os_now);
                LAST_NOW = now;
                Instant(now)
            }
        }

        /// Returns the amount of time elapsed from another instant to this one.
        ///
        /// # Panics
        ///
        /// This function will panic if `earlier` is later than `self`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::time::{Duration, Instant};
        /// use std::thread::sleep;
        ///
        /// let now = Instant::now();
        /// sleep(Duration::new(1, 0));
        /// let new_now = Instant::now();
        /// println!("{:?}", new_now.duration_since(now));
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn duration_since(&self, earlier: Instant) -> Duration {
            self.0.checked_sub_instant(&earlier.0).expect("supplied instant is later than self")
        }

        /// Returns the amount of time elapsed from another instant to this one,
        /// or None if that instant is earlier than this one.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// #![feature(checked_duration_since)]
        /// use std::time::{Duration, Instant};
        /// use std::thread::sleep;
        ///
        /// let now = Instant::now();
        /// sleep(Duration::new(1, 0));
        /// let new_now = Instant::now();
        /// println!("{:?}", new_now.checked_duration_since(now));
        /// println!("{:?}", now.checked_duration_since(new_now)); // None
        /// ```
        #[unstable(feature = "checked_duration_since", issue = "58402")]
        pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
            self.0.checked_sub_instant(&earlier.0)
        }

        /// Returns the amount of time elapsed from another instant to this one,
        /// or zero duration if that instant is earlier than this one.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// #![feature(checked_duration_since)]
        /// use std::time::{Duration, Instant};
        /// use std::thread::sleep;
        ///
        /// let now = Instant::now();
        /// sleep(Duration::new(1, 0));
        /// let new_now = Instant::now();
        /// println!("{:?}", new_now.saturating_duration_since(now));
        /// println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
        /// ```
        #[unstable(feature = "checked_duration_since", issue = "58402")]
        pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
            self.checked_duration_since(earlier).unwrap_or(Duration::new(0, 0))
        }

        /// 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
        ///
        /// ```no_run
        /// use std::thread::sleep;
        /// use std::time::{Duration, Instant};
        ///
        /// let instant = Instant::now();
        /// let three_secs = Duration::from_secs(3);
        /// sleep(three_secs);
        /// assert!(instant.elapsed() >= three_secs);
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        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.
        #[stable(feature = "time_checked_add", since = "1.34.0")]
        pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
            self.0.checked_add_duration(&duration).map(Instant)
        }

        /// 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.
        #[stable(feature = "time_checked_add", since = "1.34.0")]
        pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
            self.0.checked_sub_duration(&duration).map(Instant)
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Add<Duration> for Instant {
        type Output = Instant;

        /// # Panics
        ///
        /// This function may panic if the resulting point in time cannot be represented by the
        /// underlying data structure. See [`checked_add`] for a version without panic.
        ///
        /// [`checked_add`]: ../../std/time/struct.Instant.html#method.checked_add
        fn add(self, other: Duration) -> Instant {
            self.checked_add(other)
                .expect("overflow when adding duration to instant")
        }
    }

    #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
    impl AddAssign<Duration> for Instant {
        fn add_assign(&mut self, other: Duration) {
            *self = *self + other;
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Sub<Duration> for Instant {
        type Output = Instant;

        fn sub(self, other: Duration) -> Instant {
            self.checked_sub(other)
                .expect("overflow when subtracting duration from instant")
        }
    }

    #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
    impl SubAssign<Duration> for Instant {
        fn sub_assign(&mut self, other: Duration) {
            *self = *self - other;
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Sub<Instant> for Instant {
        type Output = Duration;

        fn sub(self, other: Instant) -> Duration {
            self.duration_since(other)
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl fmt::Debug for Instant {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.0.fmt(f)
        }
    }

    impl SystemTime {
        /// An anchor in time which can be used to create new `SystemTime` instances or
        /// learn about where in time a `SystemTime` lies.
        ///
        /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
        /// respect to the system clock. Using `duration_since` on an existing
        /// `SystemTime` instance can tell how far away from this point in time a
        /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
        /// `SystemTime` instance to represent another fixed point in time.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::time::SystemTime;
        ///
        /// match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
        ///     Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
        ///     Err(_) => panic!("SystemTime before UNIX EPOCH!"),
        /// }
        /// ```
        #[stable(feature = "assoc_unix_epoch", since = "1.28.0")]
        pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;

        /// Returns the system time corresponding to "now".
        ///
        /// # Examples
        ///
        /// ```
        /// use std::time::SystemTime;
        ///
        /// let sys_time = SystemTime::now();
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn now() -> SystemTime {
            SystemTime(time::SystemTime::now())
        }

        /// Returns the amount of time elapsed from an earlier point in time.
        ///
        /// This function may fail because measurements taken earlier are not
        /// guaranteed to always be before later measurements (due to anomalies such
        /// as the system clock being adjusted either forwards or backwards).
        /// [`Instant`] can be used to measure elapsed time without this risk of failure.
        ///
        /// If successful, [`Ok`]`(`[`Duration`]`)` is returned where the duration represents
        /// the amount of time elapsed from the specified measurement to this one.
        ///
        /// Returns an [`Err`] if `earlier` is later than `self`, and the error
        /// contains how far from `self` the time is.
        ///
        /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
        /// [`Duration`]: ../../std/time/struct.Duration.html
        /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
        /// [`Instant`]: ../../std/time/struct.Instant.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::time::SystemTime;
        ///
        /// let sys_time = SystemTime::now();
        /// let difference = sys_time.duration_since(sys_time)
        ///                          .expect("Clock may have gone backwards");
        /// println!("{:?}", difference);
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn duration_since(&self, earlier: SystemTime)
                              -> Result<Duration, SystemTimeError> {
            self.0.sub_time(&earlier.0).map_err(SystemTimeError)
        }

        /// Returns the difference between the clock time when this
        /// system time was created, and the current clock time.
        ///
        /// This function may fail as the underlying system clock is susceptible to
        /// drift and updates (e.g., the system clock could go backwards), so this
        /// function may not always succeed. If successful, [`Ok`]`(`[`Duration`]`)` is
        /// returned where the duration represents the amount of time elapsed from
        /// this time measurement to the current time.
        ///
        /// To measure elapsed time reliably, use [`Instant`] instead.
        ///
        /// Returns an [`Err`] if `self` is later than the current system time, and
        /// the error contains how far from the current system time `self` is.
        ///
        /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
        /// [`Duration`]: ../../std/time/struct.Duration.html
        /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
        /// [`Instant`]: ../../std/time/struct.Instant.html
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::thread::sleep;
        /// use std::time::{Duration, SystemTime};
        ///
        /// let sys_time = SystemTime::now();
        /// let one_sec = Duration::from_secs(1);
        /// sleep(one_sec);
        /// assert!(sys_time.elapsed().unwrap() >= one_sec);
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn elapsed(&self) -> Result<Duration, SystemTimeError> {
            SystemTime::now().duration_since(*self)
        }

        /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
        /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
        /// otherwise.
        #[stable(feature = "time_checked_add", since = "1.34.0")]
        pub fn checked_add(&self, duration: Duration) -> Option<SystemTime> {
            self.0.checked_add_duration(&duration).map(SystemTime)
        }

        /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
        /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
        /// otherwise.
        #[stable(feature = "time_checked_add", since = "1.34.0")]
        pub fn checked_sub(&self, duration: Duration) -> Option<SystemTime> {
            self.0.checked_sub_duration(&duration).map(SystemTime)
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Add<Duration> for SystemTime {
        type Output = SystemTime;

        /// # Panics
        ///
        /// This function may panic if the resulting point in time cannot be represented by the
        /// underlying data structure. See [`checked_add`] for a version without panic.
        ///
        /// [`checked_add`]: ../../std/time/struct.SystemTime.html#method.checked_add
        fn add(self, dur: Duration) -> SystemTime {
            self.checked_add(dur)
                .expect("overflow when adding duration to instant")
        }
    }

    #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
    impl AddAssign<Duration> for SystemTime {
        fn add_assign(&mut self, other: Duration) {
            *self = *self + other;
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Sub<Duration> for SystemTime {
        type Output = SystemTime;

        fn sub(self, dur: Duration) -> SystemTime {
            self.checked_sub(dur)
                .expect("overflow when subtracting duration from instant")
        }
    }

    #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
    impl SubAssign<Duration> for SystemTime {
        fn sub_assign(&mut self, other: Duration) {
            *self = *self - other;
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl fmt::Debug for SystemTime {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.0.fmt(f)
        }
    }

    /// An anchor in time which can be used to create new `SystemTime` instances or
    /// learn about where in time a `SystemTime` lies.
    ///
    /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
    /// respect to the system clock. Using `duration_since` on an existing
    /// [`SystemTime`] instance can tell how far away from this point in time a
    /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
    /// [`SystemTime`] instance to represent another fixed point in time.
    ///
    /// [`SystemTime`]: ../../std/time/struct.SystemTime.html
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::time::{SystemTime, UNIX_EPOCH};
    ///
    /// match SystemTime::now().duration_since(UNIX_EPOCH) {
    ///     Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
    ///     Err(_) => panic!("SystemTime before UNIX EPOCH!"),
    /// }
    /// ```
    #[stable(feature = "time2", since = "1.8.0")]
    pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);

    impl SystemTimeError {
        /// Returns the positive duration which represents how far forward the
        /// second system time was from the first.
        ///
        /// A `SystemTimeError` is returned from the [`duration_since`] and [`elapsed`]
        /// methods of [`SystemTime`] whenever the second system time represents a point later
        /// in time than the `self` of the method call.
        ///
        /// [`duration_since`]: ../../std/time/struct.SystemTime.html#method.duration_since
        /// [`elapsed`]: ../../std/time/struct.SystemTime.html#method.elapsed
        /// [`SystemTime`]: ../../std/time/struct.SystemTime.html
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::thread::sleep;
        /// use std::time::{Duration, SystemTime};
        ///
        /// let sys_time = SystemTime::now();
        /// sleep(Duration::from_secs(1));
        /// let new_sys_time = SystemTime::now();
        /// match sys_time.duration_since(new_sys_time) {
        ///     Ok(_) => {}
        ///     Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
        /// }
        /// ```
        #[stable(feature = "time2", since = "1.8.0")]
        pub fn duration(&self) -> Duration {
            self.0
        }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl Error for SystemTimeError {
        fn description(&self) -> &str { "other time was not earlier than self" }
    }

    #[stable(feature = "time2", since = "1.8.0")]
    impl fmt::Display for SystemTimeError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "second time provided was later than self")
        }
    }

    impl FromInner<time::SystemTime> for SystemTime {
        fn from_inner(time: time::SystemTime) -> SystemTime {
            SystemTime(time)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::{Instant, SystemTime, Duration, UNIX_EPOCH};

        macro_rules! assert_almost_eq {
            ($a:expr, $b:expr) => ({
                let (a, b) = ($a, $b);
                if a != b {
                    let (a, b) = if a > b {(a, b)} else {(b, a)};
                    assert!(a - Duration::new(0, 1000) <= b,
                            "{:?} is not almost equal to {:?}", a, b);
                }
            })
        }

        #[test]
        fn instant_monotonic() {
            let a = Instant::now();
            let b = Instant::now();
            assert!(b >= a);
        }

        #[test]
        fn instant_elapsed() {
            let a = Instant::now();
            a.elapsed();
        }

        #[test]
        fn instant_math() {
            let a = Instant::now();
            let b = Instant::now();
            println!("a: {:?}", a);
            println!("b: {:?}", b);
            let dur = b.duration_since(a);
            println!("dur: {:?}", dur);
            assert_almost_eq!(b - dur, a);
            assert_almost_eq!(a + dur, b);

            let second = Duration::new(1, 0);
            assert_almost_eq!(a - second + second, a);
            assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);

            // checked_add_duration will not panic on overflow
            let mut maybe_t = Some(Instant::now());
            let max_duration = Duration::from_secs(u64::max_value());
            // in case `Instant` can store `>= now + max_duration`.
            for _ in 0..2 {
                maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
            }
            assert_eq!(maybe_t, None);

            // checked_add_duration calculates the right time and will work for another year
            let year = Duration::from_secs(60 * 60 * 24 * 365);
            assert_eq!(a + year, a.checked_add(year).unwrap());
        }

        #[test]
        fn instant_math_is_associative() {
            let now = Instant::now();
            let offset = Duration::from_millis(5);
            // Changing the order of instant math shouldn't change the results,
            // especially when the expression reduces to X + identity.
            assert_eq!((now + offset) - now, (now - now) + offset);
        }

        #[test]
        #[should_panic]
        fn instant_duration_since_panic() {
            let a = Instant::now();
            (a - Duration::new(1, 0)).duration_since(a);
        }

        #[test]
        fn instant_checked_duration_since_nopanic() {
            let now = Instant::now();
            let earlier = now - Duration::new(1, 0);
            let later = now + Duration::new(1, 0);
            assert_eq!(earlier.checked_duration_since(now), None);
            assert_eq!(later.checked_duration_since(now), Some(Duration::new(1, 0)));
            assert_eq!(now.checked_duration_since(now), Some(Duration::new(0, 0)));
        }

        #[test]
        fn instant_saturating_duration_since_nopanic() {
            let a = Instant::now();
            let ret = (a - Duration::new(1, 0)).saturating_duration_since(a);
            assert_eq!(ret, Duration::new(0,0));
        }

        #[test]
        fn system_time_math() {
            let a = SystemTime::now();
            let b = SystemTime::now();
            match b.duration_since(a) {
                Ok(dur) if dur == Duration::new(0, 0) => {
                    assert_almost_eq!(a, b);
                }
                Ok(dur) => {
                    assert!(b > a);
                    assert_almost_eq!(b - dur, a);
                    assert_almost_eq!(a + dur, b);
                }
                Err(dur) => {
                    let dur = dur.duration();
                    assert!(a > b);
                    assert_almost_eq!(b + dur, a);
                    assert_almost_eq!(a - dur, b);
                }
            }

            let second = Duration::new(1, 0);
            assert_almost_eq!(a.duration_since(a - second).unwrap(), second);
            assert_almost_eq!(a.duration_since(a + second).unwrap_err()
                               .duration(), second);

            assert_almost_eq!(a - second + second, a);
            assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);

            let one_second_from_epoch = UNIX_EPOCH + Duration::new(1, 0);
            let one_second_from_epoch2 = UNIX_EPOCH + Duration::new(0, 500_000_000)
                + Duration::new(0, 500_000_000);
            assert_eq!(one_second_from_epoch, one_second_from_epoch2);

            // checked_add_duration will not panic on overflow
            let mut maybe_t = Some(SystemTime::UNIX_EPOCH);
            let max_duration = Duration::from_secs(u64::max_value());
            // in case `SystemTime` can store `>= UNIX_EPOCH + max_duration`.
            for _ in 0..2 {
                maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
            }
            assert_eq!(maybe_t, None);

            // checked_add_duration calculates the right time and will work for another year
            let year = Duration::from_secs(60 * 60 * 24 * 365);
            assert_eq!(a + year, a.checked_add(year).unwrap());
        }

        #[test]
        fn system_time_elapsed() {
            let a = SystemTime::now();
            drop(a.elapsed());
        }

        #[test]
        fn since_epoch() {
            let ts = SystemTime::now();
            let a = ts.duration_since(UNIX_EPOCH + Duration::new(1, 0)).unwrap();
            let b = ts.duration_since(UNIX_EPOCH).unwrap();
            assert!(b > a);
            assert_eq!(b - a, Duration::new(1, 0));

            let thirty_years = Duration::new(1, 0) * 60 * 60 * 24 * 365 * 30;

            // Right now for CI this test is run in an emulator, and apparently the
            // aarch64 emulator's sense of time is that we're still living in the
            // 70s.
            //
            // Otherwise let's assume that we're all running computers later than
            // 2000.
            if !cfg!(target_arch = "aarch64") {
                assert!(a > thirty_years);
            }

            // let's assume that we're all running computers earlier than 2090.
            // Should give us ~70 years to fix this!
            let hundred_twenty_years = thirty_years * 4;
            assert!(a < hundred_twenty_years);
        }
    }

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