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
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;

use crate::cell::UnsafeCell;
use crate::fmt;
use crate::marker::PhantomData;
use crate::mem::ManuallyDrop;
use crate::ops::{Deref, DerefMut};
use crate::ptr::NonNull;
use crate::sync::{poison, LockResult, TryLockError, TryLockResult};
use crate::sys::sync as sys;

/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore blocking any threads waiting for the lock to
/// become available. An `RwLock` will allow any number of readers to acquire the
/// lock as long as a writer is not holding the lock.
///
/// The priority policy of the lock is dependent on the underlying operating
/// system's implementation, and this type does not guarantee that any
/// particular policy will be used. In particular, a writer which is waiting to
/// acquire the lock in `write` might or might not block concurrent calls to
/// `read`, e.g.:
///
/// <details><summary>Potential deadlock example</summary>
///
/// ```text
/// // Thread 1              |  // Thread 2
/// let _rg1 = lock.read();  |
///                          |  // will block
///                          |  let _wg = lock.write();
/// // may deadlock          |
/// let _rg2 = lock.read();  |
/// ```
///
/// </details>
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads and
/// [`Sync`] to allow concurrent access through readers. The RAII guards
/// returned from the locking methods implement [`Deref`] (and [`DerefMut`]
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Poisoning
///
/// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
/// that an `RwLock` may only be poisoned if a panic occurs while it is locked
/// exclusively (write mode). If a panic occurs in any reader, then the lock
/// will not be poisoned.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
///     let r1 = lock.read().unwrap();
///     let r2 = lock.read().unwrap();
///     assert_eq!(*r1, 5);
///     assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
///     let mut w = lock.write().unwrap();
///     *w += 1;
///     assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
///
/// [`Mutex`]: super::Mutex
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLock")]
pub struct RwLock<T: ?Sized> {
    inner: sys::RwLock,
    poison: poison::Flag,
    data: UnsafeCell<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] and [`try_read`] methods on
/// [`RwLock`].
///
/// [`read`]: RwLock::read
/// [`try_read`]: RwLock::try_read
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a RwLockReadGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Futures to not implement `Send`"]
#[stable(feature = "rust1", since = "1.0.0")]
#[clippy::has_significant_drop]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLockReadGuard")]
pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
    // `RwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
    // is preferable over `const* T` to allow for niche optimization.
    data: NonNull<T>,
    inner_lock: &'a sys::RwLock,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and [`try_write`] methods
/// on [`RwLock`].
///
/// [`write`]: RwLock::write
/// [`try_write`]: RwLock::try_write
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a RwLockWriteGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Future's to not implement `Send`"]
#[stable(feature = "rust1", since = "1.0.0")]
#[clippy::has_significant_drop]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLockWriteGuard")]
pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
    lock: &'a RwLock<T>,
    poison: poison::Guard,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}

/// RAII structure used to release the shared read access of a lock when
/// dropped, which can point to a subfield of the protected data.
///
/// This structure is created by the [`map`] and [`try_map`] methods
/// on [`RwLockReadGuard`].
///
/// [`map`]: RwLockReadGuard::map
/// [`try_map`]: RwLockReadGuard::try_map
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a MappedRwLockReadGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Futures to not implement `Send`"]
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
#[clippy::has_significant_drop]
pub struct MappedRwLockReadGuard<'a, T: ?Sized + 'a> {
    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
    // `MappedRwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
    // is preferable over `const* T` to allow for niche optimization.
    data: NonNull<T>,
    inner_lock: &'a sys::RwLock,
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> !Send for MappedRwLockReadGuard<'_, T> {}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockReadGuard<'_, T> {}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped, which can point to a subfield of the protected data.
///
/// This structure is created by the [`map`] and [`try_map`] methods
/// on [`RwLockWriteGuard`].
///
/// [`map`]: RwLockWriteGuard::map
/// [`try_map`]: RwLockWriteGuard::try_map
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a MappedRwLockWriteGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Future's to not implement `Send`"]
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
#[clippy::has_significant_drop]
pub struct MappedRwLockWriteGuard<'a, T: ?Sized + 'a> {
    // NB: we use a pointer instead of `&'a mut T` to avoid `noalias` violations, because a
    // `MappedRwLockWriteGuard` argument doesn't hold uniqueness for its whole scope, only until it drops.
    // `NonNull` is covariant over `T`, so we add a `PhantomData<&'a mut T>` field
    // below for the correct variance over `T` (invariance).
    data: NonNull<T>,
    inner_lock: &'a sys::RwLock,
    poison_flag: &'a poison::Flag,
    poison: poison::Guard,
    _variance: PhantomData<&'a mut T>,
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> !Send for MappedRwLockWriteGuard<'_, T> {}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockWriteGuard<'_, T> {}

impl<T> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
    #[inline]
    pub const fn new(t: T) -> RwLock<T> {
        RwLock { inner: sys::RwLock::new(), poison: poison::Flag::new(), data: UnsafeCell::new(t) }
    }
}

impl<T: ?Sized> RwLock<T> {
    /// Locks this `RwLock` with shared read access, blocking the current thread
    /// until it can be acquired.
    ///
    /// The calling thread will be blocked until there are no more writers which
    /// hold the lock. There may be other readers currently inside the lock when
    /// this method returns. This method does not provide any guarantees with
    /// respect to the ordering of whether contentious readers or writers will
    /// acquire the lock first.
    ///
    /// Returns an RAII guard which will release this thread's shared access
    /// once it is dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the `RwLock` is poisoned. An
    /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
    /// lock. The failure will occur immediately after the lock has been
    /// acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(1));
    /// let c_lock = Arc::clone(&lock);
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// thread::spawn(move || {
    ///     let r = c_lock.read();
    ///     assert!(r.is_ok());
    /// }).join().unwrap();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn read(&self) -> LockResult<RwLockReadGuard<'_, T>> {
        unsafe {
            self.inner.read();
            RwLockReadGuard::new(self)
        }
    }

    /// Attempts to acquire this `RwLock` with shared read access.
    ///
    /// If the access could not be granted at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the shared access
    /// when it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return the [`Poisoned`] error if the `RwLock` is
    /// poisoned. An `RwLock` is poisoned whenever a writer panics while holding
    /// an exclusive lock. `Poisoned` will only be returned if the lock would
    /// have otherwise been acquired.
    ///
    /// This function will return the [`WouldBlock`] error if the `RwLock` could
    /// not be acquired because it was already locked exclusively.
    ///
    /// [`Poisoned`]: TryLockError::Poisoned
    /// [`WouldBlock`]: TryLockError::WouldBlock
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// match lock.try_read() {
    ///     Ok(n) => assert_eq!(*n, 1),
    ///     Err(_) => unreachable!(),
    /// };
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
        unsafe {
            if self.inner.try_read() {
                Ok(RwLockReadGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Locks this `RwLock` with exclusive write access, blocking the current
    /// thread until it can be acquired.
    ///
    /// This function will not return while other writers or other readers
    /// currently have access to the lock.
    ///
    /// Returns an RAII guard which will drop the write access of this `RwLock`
    /// when dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the `RwLock` is poisoned. An
    /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
    /// lock. An error will be returned when the lock is acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let mut n = lock.write().unwrap();
    /// *n = 2;
    ///
    /// assert!(lock.try_read().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn write(&self) -> LockResult<RwLockWriteGuard<'_, T>> {
        unsafe {
            self.inner.write();
            RwLockWriteGuard::new(self)
        }
    }

    /// Attempts to lock this `RwLock` with exclusive write access.
    ///
    /// If the lock could not be acquired at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the lock when
    /// it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return the [`Poisoned`] error if the `RwLock` is
    /// poisoned. An `RwLock` is poisoned whenever a writer panics while holding
    /// an exclusive lock. `Poisoned` will only be returned if the lock would
    /// have otherwise been acquired.
    ///
    /// This function will return the [`WouldBlock`] error if the `RwLock` could
    /// not be acquired because it was already locked exclusively.
    ///
    /// [`Poisoned`]: TryLockError::Poisoned
    /// [`WouldBlock`]: TryLockError::WouldBlock
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// assert!(lock.try_write().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
        unsafe {
            if self.inner.try_write() {
                Ok(RwLockWriteGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Determines whether the lock is poisoned.
    ///
    /// If another thread is active, the lock can still become poisoned at any
    /// time. You should not trust a `false` value for program correctness
    /// without additional synchronization.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(0));
    /// let c_lock = Arc::clone(&lock);
    ///
    /// let _ = thread::spawn(move || {
    ///     let _lock = c_lock.write().unwrap();
    ///     panic!(); // the lock gets poisoned
    /// }).join();
    /// assert_eq!(lock.is_poisoned(), true);
    /// ```
    #[inline]
    #[stable(feature = "sync_poison", since = "1.2.0")]
    pub fn is_poisoned(&self) -> bool {
        self.poison.get()
    }

    /// Clear the poisoned state from a lock.
    ///
    /// If the lock is poisoned, it will remain poisoned until this function is called. This allows
    /// recovering from a poisoned state and marking that it has recovered. For example, if the
    /// value is overwritten by a known-good value, then the lock can be marked as un-poisoned. Or
    /// possibly, the value could be inspected to determine if it is in a consistent state, and if
    /// so the poison is removed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(0));
    /// let c_lock = Arc::clone(&lock);
    ///
    /// let _ = thread::spawn(move || {
    ///     let _lock = c_lock.write().unwrap();
    ///     panic!(); // the lock gets poisoned
    /// }).join();
    ///
    /// assert_eq!(lock.is_poisoned(), true);
    /// let guard = lock.write().unwrap_or_else(|mut e| {
    ///     **e.get_mut() = 1;
    ///     lock.clear_poison();
    ///     e.into_inner()
    /// });
    /// assert_eq!(lock.is_poisoned(), false);
    /// assert_eq!(*guard, 1);
    /// ```
    #[inline]
    #[stable(feature = "mutex_unpoison", since = "1.77.0")]
    pub fn clear_poison(&self) {
        self.poison.clear();
    }

    /// Consumes this `RwLock`, returning the underlying data.
    ///
    /// # Errors
    ///
    /// This function will return an error if the `RwLock` is poisoned. An
    /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
    /// lock. An error will only be returned if the lock would have otherwise
    /// been acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(String::new());
    /// {
    ///     let mut s = lock.write().unwrap();
    ///     *s = "modified".to_owned();
    /// }
    /// assert_eq!(lock.into_inner().unwrap(), "modified");
    /// ```
    #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
    pub fn into_inner(self) -> LockResult<T>
    where
        T: Sized,
    {
        let data = self.data.into_inner();
        poison::map_result(self.poison.borrow(), |()| data)
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
    /// take place -- the mutable borrow statically guarantees no locks exist.
    ///
    /// # Errors
    ///
    /// This function will return an error if the `RwLock` is poisoned. An
    /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
    /// lock. An error will only be returned if the lock would have otherwise
    /// been acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let mut lock = RwLock::new(0);
    /// *lock.get_mut().unwrap() = 10;
    /// assert_eq!(*lock.read().unwrap(), 10);
    /// ```
    #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
    pub fn get_mut(&mut self) -> LockResult<&mut T> {
        let data = self.data.get_mut();
        poison::map_result(self.poison.borrow(), |()| data)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut d = f.debug_struct("RwLock");
        match self.try_read() {
            Ok(guard) => {
                d.field("data", &&*guard);
            }
            Err(TryLockError::Poisoned(err)) => {
                d.field("data", &&**err.get_ref());
            }
            Err(TryLockError::WouldBlock) => {
                d.field("data", &format_args!("<locked>"));
            }
        }
        d.field("poisoned", &self.poison.get());
        d.finish_non_exhaustive()
    }
}

#[stable(feature = "rw_lock_default", since = "1.10.0")]
impl<T: Default> Default for RwLock<T> {
    /// Creates a new `RwLock<T>`, with the `Default` value for T.
    fn default() -> RwLock<T> {
        RwLock::new(Default::default())
    }
}

#[stable(feature = "rw_lock_from", since = "1.24.0")]
impl<T> From<T> for RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    /// This is equivalent to [`RwLock::new`].
    fn from(t: T) -> Self {
        RwLock::new(t)
    }
}

impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
    /// Create a new instance of `RwLockReadGuard<T>` from a `RwLock<T>`.
    // SAFETY: if and only if `lock.inner.read()` (or `lock.inner.try_read()`) has been
    // successfully called from the same thread before instantiating this object.
    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
        poison::map_result(lock.poison.borrow(), |()| RwLockReadGuard {
            data: NonNull::new_unchecked(lock.data.get()),
            inner_lock: &lock.inner,
        })
    }
}

impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
    /// Create a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
    // SAFETY: if and only if `lock.inner.write()` (or `lock.inner.try_write()`) has been
    // successfully called from the same thread before instantiating this object.
    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
        unsafe { self.data.as_ref() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
        unsafe { &*self.lock.data.get() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
        unsafe { &mut *self.lock.data.get() }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        unsafe { self.data.as_ref() }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        unsafe { self.data.as_ref() }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        unsafe { self.data.as_mut() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
    fn drop(&mut self) {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
        unsafe {
            self.inner_lock.read_unlock();
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
    fn drop(&mut self) {
        self.lock.poison.done(&self.poison);
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
        unsafe {
            self.lock.inner.write_unlock();
        }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
    fn drop(&mut self) {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        unsafe {
            self.inner_lock.read_unlock();
        }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
    fn drop(&mut self) {
        self.poison_flag.done(&self.poison);
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        unsafe {
            self.inner_lock.write_unlock();
        }
    }
}

impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data, e.g.
    /// an enum variant.
    ///
    /// The `RwLock` is already locked for reading, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RwLockReadGuard::map(...)`. A method would interfere with methods of
    /// the same name on the contents of the `RwLockReadGuard` used through
    /// `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
    where
        F: FnOnce(&T) -> &U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
        let orig = ManuallyDrop::new(orig);
        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
    }

    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data. The
    /// original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `RwLock` is already locked for reading, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RwLockReadGuard::try_map(...)`. A method would interfere with methods
    /// of the same name on the contents of the `RwLockReadGuard` used through
    /// `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
    #[doc(alias = "filter_map")]
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn try_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
    where
        F: FnOnce(&T) -> Option<&U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { orig.data.as_ref() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
            }
            None => Err(orig),
        }
    }
}

impl<'a, T: ?Sized> MappedRwLockReadGuard<'a, T> {
    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
    /// e.g. an enum variant.
    ///
    /// The `RwLock` is already locked for reading, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
    /// used through `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
    where
        F: FnOnce(&T) -> &U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
        let orig = ManuallyDrop::new(orig);
        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
    }

    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
    /// The original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `RwLock` is already locked for reading, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedRwLockReadGuard::try_map(...)`. A method would interfere with
    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
    /// used through `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
    #[doc(alias = "filter_map")]
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn try_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
    where
        F: FnOnce(&T) -> Option<&U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { orig.data.as_ref() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
            }
            None => Err(orig),
        }
    }
}

impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data, e.g.
    /// an enum variant.
    ///
    /// The `RwLock` is already locked for writing, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RwLockWriteGuard::map(...)`. A method would interfere with methods of
    /// the same name on the contents of the `RwLockWriteGuard` used through
    /// `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
    where
        F: FnOnce(&mut T) -> &mut U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
        let orig = ManuallyDrop::new(orig);
        MappedRwLockWriteGuard {
            data,
            inner_lock: &orig.lock.inner,
            poison_flag: &orig.lock.poison,
            poison: orig.poison.clone(),
            _variance: PhantomData,
        }
    }

    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data. The
    /// original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `RwLock` is already locked for writing, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RwLockWriteGuard::try_map(...)`. A method would interfere with methods
    /// of the same name on the contents of the `RwLockWriteGuard` used through
    /// `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
    #[doc(alias = "filter_map")]
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn try_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { &mut *orig.lock.data.get() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedRwLockWriteGuard {
                    data,
                    inner_lock: &orig.lock.inner,
                    poison_flag: &orig.lock.poison,
                    poison: orig.poison.clone(),
                    _variance: PhantomData,
                })
            }
            None => Err(orig),
        }
    }
}

impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data,
    /// e.g. an enum variant.
    ///
    /// The `RwLock` is already locked for writing, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedRwLockWriteGuard::map(...)`. A method would interfere with
    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
    /// used through `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
    where
        F: FnOnce(&mut T) -> &mut U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
        let orig = ManuallyDrop::new(orig);
        MappedRwLockWriteGuard {
            data,
            inner_lock: orig.inner_lock,
            poison_flag: orig.poison_flag,
            poison: orig.poison.clone(),
            _variance: PhantomData,
        }
    }

    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data.
    /// The original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `RwLock` is already locked for writing, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedRwLockWriteGuard::try_map(...)`. A method would interfere with
    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
    /// used through `Deref`.
    ///
    /// # Panics
    ///
    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
    #[doc(alias = "filter_map")]
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    pub fn try_map<U, F>(mut orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `try_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { orig.data.as_mut() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedRwLockWriteGuard {
                    data,
                    inner_lock: orig.inner_lock,
                    poison_flag: orig.poison_flag,
                    poison: orig.poison.clone(),
                    _variance: PhantomData,
                })
            }
            None => Err(orig),
        }
    }
}