use std::time::Instant; pub trait SystemClock: Send + Sync {    /// Returns the current date and time in UTC.    fn utc_now(&self) -> Instant;} pub struct RealSystemClock; impl SystemClock for RealSystemClock {    fn utc_now(&self) -> Instant {        Instant::now()    }} #[cfg(any(test, feature = "test-support"))]pub struct FakeSystemClockState {    now: Instant,} #[cfg(any(test, feature = "test-support"))]pub struct FakeSystemClock {    // Use an unfair lock to ensure tests are deterministic.    state: parking_lot::Mutex<FakeSystemClockState>,} #[cfg(any(test, feature = "test-support"))]impl FakeSystemClock {    pub fn new() -> Self {        let state = FakeSystemClockState {            now: Instant::now(),        };         Self {            state: parking_lot::Mutex::new(state),        }    }     pub fn set_now(&self, now: Instant) {        self.state.lock().now = now;    }     pub fn advance(&self, duration: std::time::Duration) {        self.state.lock().now += duration;    }} #[cfg(any(test, feature = "test-support"))]impl SystemClock for FakeSystemClock {    fn utc_now(&self) -> Instant {        self.state.lock().now    }}