1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
//! 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());
/// }
/// ```
///
/// # Underlying System calls
/// Currently, the following system calls are being used to get the current time using `now()`:
///
/// |  Platform |               System call                                            |
/// |:---------:|:--------------------------------------------------------------------:|
/// | Cloud ABI | [clock_time_get (Monotonic Clock)]                                   |
/// | SGX       | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
/// | UNIX      | [clock_time_get (Monotonic Clock)]                                   |
/// | Darwin    | [mach_absolute_time]                                                 |
/// | VXWorks   | [clock_gettime (Monotonic Clock)]                                    |
/// | WASI      | [__wasi_clock_time_get (Monotonic Clock)]                            |
/// | Windows   | [QueryPerformanceCounter]                                            |
///
/// [QueryPerformanceCounter]: https://docs.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancecounter
/// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
/// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
/// [__wasi_clock_time_get (Monotonic Clock)]: https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-api.md#clock_time_get
/// [clock_gettime (Monotonic Clock)]: https://linux.die.net/man/3/clock_gettime
/// [mach_absolute_time]: https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/services/services.html
/// [clock_time_get (Monotonic Clock)]: https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt
///
/// **Disclaimer:** These system calls might change over time.
///
#[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);
///        }
///    }
/// }
/// ```
///
/// # Underlying System calls
/// Currently, the following system calls are being used to get the current time using `now()`:
///
/// |  Platform |               System call                                            |
/// |:---------:|:--------------------------------------------------------------------:|
/// | Cloud ABI | [clock_time_get (Realtime Clock)]                                    |
/// | SGX       | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
/// | UNIX      | [clock_gettime (Realtime Clock)]                                     |
/// | DARWIN    | [gettimeofday]                                                       |
/// | VXWorks   | [clock_gettime (Realtime Clock)]                                     |
/// | WASI      | [__wasi_clock_time_get (Realtime Clock)]                             |
/// | Windows   | [GetSystemTimeAsFileTime]                                            |
///
/// [clock_time_get (Realtime Clock)]: https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt
/// [gettimeofday]: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
/// [clock_gettime (Realtime Clock)]: https://linux.die.net/man/3/clock_gettime
/// [__wasi_clock_time_get (Realtime Clock)]: https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-api.md#clock_time_get
/// [GetSystemTimeAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimeasfiletime
///
/// **Disclaimer:** These system calls might change over time.
///
#[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 later than this one.
    ///
    /// # 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.checked_duration_since(now));
    /// println!("{:?}", now.checked_duration_since(new_now)); // None
    /// ```
    #[stable(feature = "checked_duration_since", since = "1.39.0")]
    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
    /// 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
    /// ```
    #[stable(feature = "checked_duration_since", since = "1.39.0")]
    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);
    }
}