rwlock.rs.html -- source
Description: Source to the Rust file `src/libstd/sync/rwlock.rs`.
Keywords: rust, rustlang, rust-lang

      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

    use crate::cell::UnsafeCell;
    use crate::fmt;
    use crate::mem;
    use crate::ops::{Deref, DerefMut};
    use crate::ptr;
    use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
    use crate::sys_common::rwlock 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.
    ///
    /// 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
    /// ```
    ///
    /// [`Deref`]: ../../std/ops/trait.Deref.html
    /// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
    /// [`Send`]: ../../std/marker/trait.Send.html
    /// [`Sync`]: ../../std/marker/trait.Sync.html
    /// [`Mutex`]: struct.Mutex.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLock<T: ?Sized> {
        inner: Box<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`]: struct.RwLock.html#method.read
    /// [`try_read`]: struct.RwLock.html#method.try_read
    /// [`RwLock`]: struct.RwLock.html
    #[must_use = "if unused the RwLock will immediately unlock"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
        lock: &'a RwLock<T>,
    }

    #[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`]: struct.RwLock.html#method.write
    /// [`try_write`]: struct.RwLock.html#method.try_write
    /// [`RwLock`]: struct.RwLock.html
    #[must_use = "if unused the RwLock will immediately unlock"]
    #[stable(feature = "rust1", since = "1.0.0")]
    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> {}

    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")]
        pub fn new(t: T) -> RwLock<T> {
            RwLock {
                inner: box 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 = lock.clone();
        ///
        /// 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 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(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 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(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 = lock.clone();
        ///
        /// 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()
        }

        /// 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,
        {
            // We know statically that there are no outstanding references to
            // `self` so there's no need to lock the inner lock.
            //
            // To get the inner value, we'd like to call `data.into_inner()`,
            // but because `RwLock` impl-s `Drop`, we can't move out of it, so
            // we'll have to destructure it manually instead.
            unsafe {
                // Like `let RwLock { inner, poison, data } = self`.
                let (inner, poison, data) = {
                    let RwLock { ref inner, ref poison, ref data } = self;
                    (ptr::read(inner), ptr::read(poison), ptr::read(data))
                };
                mem::forget(self);
                inner.destroy(); // Keep in sync with the `Drop` impl.
                drop(inner);

                poison::map_result(poison.borrow(), |_| data.into_inner())
            }
        }

        /// 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> {
            // We know statically that there are no other references to `self`, so
            // there's no need to lock the inner lock.
            let data = unsafe { &mut *self.data.get() };
            poison::map_result(self.poison.borrow(), |_| data)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
        fn drop(&mut self) {
            // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
            unsafe { self.inner.destroy() }
        }
    }

    #[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 {
            match self.try_read() {
                Ok(guard) => f.debug_struct("RwLock").field("data", &&*guard).finish(),
                Err(TryLockError::Poisoned(err)) => {
                    f.debug_struct("RwLock").field("data", &&**err.get_ref()).finish()
                }
                Err(TryLockError::WouldBlock) => {
                    struct LockedPlaceholder;
                    impl fmt::Debug for LockedPlaceholder {
                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                            f.write_str("<locked>")
                        }
                    }

                    f.debug_struct("RwLock").field("data", &LockedPlaceholder).finish()
                }
            }
        }
    }

    #[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`].
        ///
        /// [`RwLock::new`]: ../../std/sync/struct.RwLock.html#method.new
        fn from(t: T) -> Self {
            RwLock::new(t)
        }
    }

    impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |_| RwLockReadGuard { lock })
        }
    }

    impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |guard| RwLockWriteGuard { lock, poison: guard })
        }
    }

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

    #[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: fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("RwLockWriteGuard").field("lock", &self.lock).finish()
        }
    }

    #[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)
        }
    }

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

        fn deref(&self) -> &T {
            unsafe { &*self.lock.data.get() }
        }
    }

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

        fn deref(&self) -> &T {
            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 {
            unsafe { &mut *self.lock.data.get() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
        fn drop(&mut self) {
            unsafe {
                self.lock.inner.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);
            unsafe {
                self.lock.inner.write_unlock();
            }
        }
    }

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use crate::sync::atomic::{AtomicUsize, Ordering};
        use crate::sync::mpsc::channel;
        use crate::sync::{Arc, RwLock, TryLockError};
        use crate::thread;
        use rand::{self, Rng};

        #[derive(Eq, PartialEq, Debug)]
        struct NonCopy(i32);

        #[test]
        fn smoke() {
            let l = RwLock::new(());
            drop(l.read().unwrap());
            drop(l.write().unwrap());
            drop((l.read().unwrap(), l.read().unwrap()));
            drop(l.write().unwrap());
        }

        #[test]
        fn frob() {
            const N: u32 = 10;
            const M: usize = 1000;

            let r = Arc::new(RwLock::new(()));

            let (tx, rx) = channel::<()>();
            for _ in 0..N {
                let tx = tx.clone();
                let r = r.clone();
                thread::spawn(move || {
                    let mut rng = rand::thread_rng();
                    for _ in 0..M {
                        if rng.gen_bool(1.0 / (N as f64)) {
                            drop(r.write().unwrap());
                        } else {
                            drop(r.read().unwrap());
                        }
                    }
                    drop(tx);
                });
            }
            drop(tx);
            let _ = rx.recv();
        }

        #[test]
        fn test_rw_arc_poison_wr() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move || {
                let _lock = arc2.write().unwrap();
                panic!();
            })
            .join();
            assert!(arc.read().is_err());
        }

        #[test]
        fn test_rw_arc_poison_ww() {
            let arc = Arc::new(RwLock::new(1));
            assert!(!arc.is_poisoned());
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move || {
                let _lock = arc2.write().unwrap();
                panic!();
            })
            .join();
            assert!(arc.write().is_err());
            assert!(arc.is_poisoned());
        }

        #[test]
        fn test_rw_arc_no_poison_rr() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move || {
                let _lock = arc2.read().unwrap();
                panic!();
            })
            .join();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 1);
        }
        #[test]
        fn test_rw_arc_no_poison_rw() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move || {
                let _lock = arc2.read().unwrap();
                panic!()
            })
            .join();
            let lock = arc.write().unwrap();
            assert_eq!(*lock, 1);
        }

        #[test]
        fn test_rw_arc() {
            let arc = Arc::new(RwLock::new(0));
            let arc2 = arc.clone();
            let (tx, rx) = channel();

            thread::spawn(move || {
                let mut lock = arc2.write().unwrap();
                for _ in 0..10 {
                    let tmp = *lock;
                    *lock = -1;
                    thread::yield_now();
                    *lock = tmp + 1;
                }
                tx.send(()).unwrap();
            });

            // Readers try to catch the writer in the act
            let mut children = Vec::new();
            for _ in 0..5 {
                let arc3 = arc.clone();
                children.push(thread::spawn(move || {
                    let lock = arc3.read().unwrap();
                    assert!(*lock >= 0);
                }));
            }

            // Wait for children to pass their asserts
            for r in children {
                assert!(r.join().is_ok());
            }

            // Wait for writer to finish
            rx.recv().unwrap();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 10);
        }

        #[test]
        fn test_rw_arc_access_in_unwind() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _ = thread::spawn(move || -> () {
                struct Unwinder {
                    i: Arc<RwLock<isize>>,
                }
                impl Drop for Unwinder {
                    fn drop(&mut self) {
                        let mut lock = self.i.write().unwrap();
                        *lock += 1;
                    }
                }
                let _u = Unwinder { i: arc2 };
                panic!();
            })
            .join();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 2);
        }

        #[test]
        fn test_rwlock_unsized() {
            let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
            {
                let b = &mut *rw.write().unwrap();
                b[0] = 4;
                b[2] = 5;
            }
            let comp: &[i32] = &[4, 2, 5];
            assert_eq!(&*rw.read().unwrap(), comp);
        }

        #[test]
        fn test_rwlock_try_write() {
            let lock = RwLock::new(0isize);
            let read_guard = lock.read().unwrap();

            let write_result = lock.try_write();
            match write_result {
                Err(TryLockError::WouldBlock) => (),
                Ok(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
                Err(_) => assert!(false, "unexpected error"),
            }

            drop(read_guard);
        }

        #[test]
        fn test_into_inner() {
            let m = RwLock::new(NonCopy(10));
            assert_eq!(m.into_inner().unwrap(), NonCopy(10));
        }

        #[test]
        fn test_into_inner_drop() {
            struct Foo(Arc<AtomicUsize>);
            impl Drop for Foo {
                fn drop(&mut self) {
                    self.0.fetch_add(1, Ordering::SeqCst);
                }
            }
            let num_drops = Arc::new(AtomicUsize::new(0));
            let m = RwLock::new(Foo(num_drops.clone()));
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            {
                let _inner = m.into_inner().unwrap();
                assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            }
            assert_eq!(num_drops.load(Ordering::SeqCst), 1);
        }

        #[test]
        fn test_into_inner_poison() {
            let m = Arc::new(RwLock::new(NonCopy(10)));
            let m2 = m.clone();
            let _ = thread::spawn(move || {
                let _lock = m2.write().unwrap();
                panic!("test panic in inner thread to poison RwLock");
            })
            .join();

            assert!(m.is_poisoned());
            match Arc::try_unwrap(m).unwrap().into_inner() {
                Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
                Ok(x) => panic!("into_inner of poisoned RwLock is Ok: {:?}", x),
            }
        }

        #[test]
        fn test_get_mut() {
            let mut m = RwLock::new(NonCopy(10));
            *m.get_mut().unwrap() = NonCopy(20);
            assert_eq!(m.into_inner().unwrap(), NonCopy(20));
        }

        #[test]
        fn test_get_mut_poison() {
            let m = Arc::new(RwLock::new(NonCopy(10)));
            let m2 = m.clone();
            let _ = thread::spawn(move || {
                let _lock = m2.write().unwrap();
                panic!("test panic in inner thread to poison RwLock");
            })
            .join();

            assert!(m.is_poisoned());
            match Arc::try_unwrap(m).unwrap().get_mut() {
                Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
                Ok(x) => panic!("get_mut of poisoned RwLock is Ok: {:?}", x),
            }
        }
    }