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
use core::borrow::Borrow;
use core::cell::UnsafeCell;
use core::fmt;
use core::marker::{PhantomData, PhantomPinned};
use core::ops::{Deref, DerefMut};
use core::pin::Pin;
use core::task::Poll;
use core::usize;

use alloc::sync::Arc;

// We don't use loom::UnsafeCell as that doesn't work with the Mutex API.
use crate::sync::atomic::{AtomicUsize, Ordering};

#[cfg(all(feature = "std", not(target_family = "wasm")))]
use std::time::{Duration, Instant};

use event_listener::{Event, EventListener};
use event_listener_strategy::{easy_wrapper, EventListenerFuture};

/// An async mutex.
///
/// The locking mechanism uses eventual fairness to ensure locking will be fair on average without
/// sacrificing performance. This is done by forcing a fair lock whenever a lock operation is
/// starved for longer than 0.5 milliseconds.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_lock::Mutex;
///
/// let m = Mutex::new(1);
///
/// let mut guard = m.lock().await;
/// *guard = 2;
///
/// assert!(m.try_lock().is_none());
/// drop(guard);
/// assert_eq!(*m.try_lock().unwrap(), 2);
/// # })
/// ```
pub struct Mutex<T: ?Sized> {
    /// Current state of the mutex.
    ///
    /// The least significant bit is set to 1 if the mutex is locked.
    /// The other bits hold the number of starved lock operations.
    state: AtomicUsize,

    /// Lock operations waiting for the mutex to be released.
    lock_ops: Event,

    /// The value inside the mutex.
    data: UnsafeCell<T>,
}

unsafe impl<T: Send + ?Sized> Send for Mutex<T> {}
unsafe impl<T: Send + ?Sized> Sync for Mutex<T> {}

impl<T> Mutex<T> {
    const_fn! {
        const_if: #[cfg(not(loom))];
        /// Creates a new async mutex.
        ///
        /// # Examples
        ///
        /// ```
        /// use async_lock::Mutex;
        ///
        /// let mutex = Mutex::new(0);
        /// ```
        pub const fn new(data: T) -> Mutex<T> {
            Mutex {
                state: AtomicUsize::new(0),
                lock_ops: Event::new(),
                data: UnsafeCell::new(data),
            }
        }
    }

    /// Consumes the mutex, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// assert_eq!(mutex.into_inner(), 10);
    /// ```
    pub fn into_inner(self) -> T {
        self.data.into_inner()
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires the mutex.
    ///
    /// Returns a guard that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// let guard = mutex.lock().await;
    /// assert_eq!(*guard, 10);
    /// # })
    /// ```
    #[inline]
    pub fn lock(&self) -> Lock<'_, T> {
        Lock::_new(LockInner {
            mutex: self,
            acquire_slow: None,
        })
    }

    /// Acquires the mutex using the blocking strategy.
    ///
    /// Returns a guard that releases the mutex when dropped.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`lock`][Mutex::lock] method,
    /// this method will block the current thread until the lock is acquired.
    ///
    /// This method should not be used in an asynchronous context. It is intended to be
    /// used in a way that a mutex can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in a deadlock.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// let guard = mutex.lock_blocking();
    /// assert_eq!(*guard, 10);
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    #[inline]
    pub fn lock_blocking(&self) -> MutexGuard<'_, T> {
        self.lock().wait()
    }

    /// Attempts to acquire the mutex.
    ///
    /// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, a
    /// guard is returned that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// if let Some(guard) = mutex.try_lock() {
    ///     assert_eq!(*guard, 10);
    /// }
    /// # ;
    /// ```
    #[inline]
    pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
        if self
            .state
            .compare_exchange(0, 1, Ordering::Acquire, Ordering::Acquire)
            .is_ok()
        {
            Some(MutexGuard(self))
        } else {
            None
        }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the mutex mutably, no actual locking takes place -- the mutable
    /// borrow statically guarantees the mutex is not already acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut() = 10;
    /// assert_eq!(*mutex.lock().await, 10);
    /// # })
    /// ```
    pub fn get_mut(&mut self) -> &mut T {
        self.data.get_mut()
    }

    /// Unlocks the mutex directly.
    ///
    /// # Safety
    ///
    /// This function is intended to be used only in the case where the mutex is locked,
    /// and the guard is subsequently forgotten. Calling this while you don't hold a lock
    /// on the mutex will likely lead to UB.
    pub(crate) unsafe fn unlock_unchecked(&self) {
        // Remove the last bit and notify a waiting lock operation.
        self.state.fetch_sub(1, Ordering::Release);
        self.lock_ops.notify(1);
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires the mutex and clones a reference to it.
    ///
    /// Returns an owned guard that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::Mutex;
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10));
    /// let guard = mutex.lock_arc().await;
    /// assert_eq!(*guard, 10);
    /// # })
    /// ```
    #[inline]
    pub fn lock_arc(self: &Arc<Self>) -> LockArc<T> {
        LockArc::_new(LockArcInnards::Unpolled {
            mutex: Some(self.clone()),
        })
    }

    /// Acquires the mutex and clones a reference to it using the blocking strategy.
    ///
    /// Returns an owned guard that releases the mutex when dropped.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`lock_arc`][Mutex::lock_arc] method,
    /// this method will block the current thread until the lock is acquired.
    ///
    /// This method should not be used in an asynchronous context. It is intended to be
    /// used in a way that a mutex can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in a deadlock.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Mutex;
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10));
    /// let guard = mutex.lock_arc_blocking();
    /// assert_eq!(*guard, 10);
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    #[inline]
    pub fn lock_arc_blocking(self: &Arc<Self>) -> MutexGuardArc<T> {
        self.lock_arc().wait()
    }

    /// Attempts to acquire the mutex and clone a reference to it.
    ///
    /// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, an
    /// owned guard is returned that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Mutex;
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10));
    /// if let Some(guard) = mutex.try_lock() {
    ///     assert_eq!(*guard, 10);
    /// }
    /// # ;
    /// ```
    #[inline]
    pub fn try_lock_arc(self: &Arc<Self>) -> Option<MutexGuardArc<T>> {
        if self
            .state
            .compare_exchange(0, 1, Ordering::Acquire, Ordering::Acquire)
            .is_ok()
        {
            Some(MutexGuardArc(self.clone()))
        } else {
            None
        }
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct Locked;
        impl fmt::Debug for Locked {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.write_str("<locked>")
            }
        }

        match self.try_lock() {
            None => f.debug_struct("Mutex").field("data", &Locked).finish(),
            Some(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
        }
    }
}

impl<T> From<T> for Mutex<T> {
    fn from(val: T) -> Mutex<T> {
        Mutex::new(val)
    }
}

impl<T: Default + ?Sized> Default for Mutex<T> {
    fn default() -> Mutex<T> {
        Mutex::new(Default::default())
    }
}

easy_wrapper! {
    /// The future returned by [`Mutex::lock`].
    pub struct Lock<'a, T: ?Sized>(LockInner<'a, T> => MutexGuard<'a, T>);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project_lite::pin_project! {
    /// Inner future for acquiring the mutex.
    struct LockInner<'a, T: ?Sized> {
        // Reference to the mutex.
        mutex: &'a Mutex<T>,

        // The future that waits for the mutex to become available.
        #[pin]
        acquire_slow: Option<AcquireSlow<&'a Mutex<T>, T>>,
    }
}

unsafe impl<T: Send + ?Sized> Send for Lock<'_, T> {}
unsafe impl<T: Sync + ?Sized> Sync for Lock<'_, T> {}

impl<T: ?Sized> fmt::Debug for Lock<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("Lock { .. }")
    }
}

impl<'a, T: ?Sized> EventListenerFuture for LockInner<'a, T> {
    type Output = MutexGuard<'a, T>;

    #[inline]
    fn poll_with_strategy<'x, S: event_listener_strategy::Strategy<'x>>(
        self: Pin<&mut Self>,
        strategy: &mut S,
        context: &mut S::Context,
    ) -> Poll<Self::Output> {
        let mut this = self.project();

        // This may seem weird, but the borrow checker complains otherwise.
        if this.acquire_slow.is_none() {
            match this.mutex.try_lock() {
                Some(guard) => return Poll::Ready(guard),
                None => {
                    this.acquire_slow.set(Some(AcquireSlow::new(this.mutex)));
                }
            }
        }

        ready!(this
            .acquire_slow
            .as_pin_mut()
            .unwrap()
            .poll_with_strategy(strategy, context));
        Poll::Ready(MutexGuard(this.mutex))
    }
}

easy_wrapper! {
    /// The future returned by [`Mutex::lock_arc`].
    pub struct LockArc<T: ?Sized>(LockArcInnards<T> => MutexGuardArc<T>);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project_lite::pin_project! {
    #[project = LockArcInnardsProj]
    enum LockArcInnards<T: ?Sized> {
        /// We have not tried to poll the fast path yet.
        Unpolled { mutex: Option<Arc<Mutex<T>>> },

        /// We are acquiring the mutex through the slow path.
        AcquireSlow {
            #[pin]
            inner: AcquireSlow<Arc<Mutex<T>>, T>
        },
    }
}

unsafe impl<T: Send + ?Sized> Send for LockArc<T> {}
unsafe impl<T: Sync + ?Sized> Sync for LockArc<T> {}

impl<T: ?Sized> fmt::Debug for LockArcInnards<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("LockArc { .. }")
    }
}

impl<T: ?Sized> EventListenerFuture for LockArcInnards<T> {
    type Output = MutexGuardArc<T>;

    fn poll_with_strategy<'a, S: event_listener_strategy::Strategy<'a>>(
        mut self: Pin<&mut Self>,
        strategy: &mut S,
        context: &mut S::Context,
    ) -> Poll<Self::Output> {
        // Set the inner future if needed.
        if let LockArcInnardsProj::Unpolled { mutex } = self.as_mut().project() {
            let mutex = mutex.take().expect("mutex taken more than once");

            // Try the fast path before trying to register slowly.
            if let Some(guard) = mutex.try_lock_arc() {
                return Poll::Ready(guard);
            }

            // Set the inner future to the slow acquire path.
            self.as_mut().set(LockArcInnards::AcquireSlow {
                inner: AcquireSlow::new(mutex),
            });
        }

        // Poll the inner future.
        let value = match self.project() {
            LockArcInnardsProj::AcquireSlow { inner } => {
                ready!(inner.poll_with_strategy(strategy, context))
            }
            _ => unreachable!(),
        };

        Poll::Ready(MutexGuardArc(value))
    }
}

pin_project_lite::pin_project! {
    /// Future for acquiring the mutex slowly.
    struct AcquireSlow<B: Borrow<Mutex<T>>, T: ?Sized> {
        // Reference to the mutex.
        mutex: Option<B>,

        // The event listener waiting on the mutex.
        listener: Option<EventListener>,

        // The point at which the mutex lock was started.
        start: Start,

        // This lock operation is starving.
        starved: bool,

        // Capture the `T` lifetime.
        #[pin]
        _marker: PhantomData<T>,

        // Keeping this type `!Unpin` enables future optimizations.
        #[pin]
        _pin: PhantomPinned
    }

    impl<T: ?Sized, B: Borrow<Mutex<T>>> PinnedDrop for AcquireSlow<B, T> {
        fn drop(this: Pin<&mut Self>) {
            // Make sure the starvation counter is decremented.
            this.take_mutex();
        }
    }
}

/// `pin_project_lite` doesn't support `#[cfg]` yet, so we have to do this manually.
struct Start {
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    start: Option<Instant>,
}

impl<T: ?Sized, B: Borrow<Mutex<T>>> AcquireSlow<B, T> {
    /// Create a new `AcquireSlow` future.
    #[cold]
    fn new(mutex: B) -> Self {
        AcquireSlow {
            mutex: Some(mutex),
            listener: None,
            start: Start {
                #[cfg(all(feature = "std", not(target_family = "wasm")))]
                start: None,
            },
            starved: false,
            _marker: PhantomData,
            _pin: PhantomPinned,
        }
    }

    /// Take the mutex reference out, decrementing the counter if necessary.
    fn take_mutex(self: Pin<&mut Self>) -> Option<B> {
        let this = self.project();
        let mutex = this.mutex.take();

        if *this.starved {
            if let Some(mutex) = mutex.as_ref() {
                // Decrement this counter before we exit.
                mutex.borrow().state.fetch_sub(2, Ordering::Release);
            }
        }

        mutex
    }
}

impl<T: ?Sized, B: Unpin + Borrow<Mutex<T>>> EventListenerFuture for AcquireSlow<B, T> {
    type Output = B;

    #[cold]
    fn poll_with_strategy<'a, S: event_listener_strategy::Strategy<'a>>(
        mut self: Pin<&mut Self>,
        strategy: &mut S,
        context: &mut S::Context,
    ) -> Poll<Self::Output> {
        let this = self.as_mut().project();
        #[cfg(all(feature = "std", not(target_family = "wasm")))]
        let start = *this.start.start.get_or_insert_with(Instant::now);
        let mutex = Borrow::<Mutex<T>>::borrow(
            this.mutex.as_ref().expect("future polled after completion"),
        );

        // Only use this hot loop if we aren't currently starved.
        if !*this.starved {
            loop {
                // Start listening for events.
                if this.listener.is_none() {
                    *this.listener = Some(mutex.lock_ops.listen());

                    // Try locking if nobody is being starved.
                    match mutex
                        .state
                        .compare_exchange(0, 1, Ordering::Acquire, Ordering::Acquire)
                        .unwrap_or_else(|x| x)
                    {
                        // Lock acquired!
                        0 => return Poll::Ready(self.take_mutex().unwrap()),

                        // Lock is held and nobody is starved.
                        1 => {}

                        // Somebody is starved.
                        _ => break,
                    }
                } else {
                    ready!(strategy.poll(this.listener, context));

                    // Try locking if nobody is being starved.
                    match mutex
                        .state
                        .compare_exchange(0, 1, Ordering::Acquire, Ordering::Acquire)
                        .unwrap_or_else(|x| x)
                    {
                        // Lock acquired!
                        0 => return Poll::Ready(self.take_mutex().unwrap()),

                        // Lock is held and nobody is starved.
                        1 => {}

                        // Somebody is starved.
                        _ => {
                            // Notify the first listener in line because we probably received a
                            // notification that was meant for a starved task.
                            mutex.lock_ops.notify(1);
                            break;
                        }
                    }

                    // If waiting for too long, fall back to a fairer locking strategy that will prevent
                    // newer lock operations from starving us forever.
                    #[cfg(all(feature = "std", not(target_family = "wasm")))]
                    if start.elapsed() > Duration::from_micros(500) {
                        break;
                    }
                }
            }

            // Increment the number of starved lock operations.
            if mutex.state.fetch_add(2, Ordering::Release) > usize::MAX / 2 {
                // In case of potential overflow, abort.
                crate::abort();
            }

            // Indicate that we are now starving and will use a fairer locking strategy.
            *this.starved = true;
        }

        // Fairer locking loop.
        loop {
            if this.listener.is_none() {
                // Start listening for events.
                *this.listener = Some(mutex.lock_ops.listen());

                // Try locking if nobody else is being starved.
                match mutex
                    .state
                    .compare_exchange(2, 2 | 1, Ordering::Acquire, Ordering::Acquire)
                    .unwrap_or_else(|x| x)
                {
                    // Lock acquired!
                    2 => return Poll::Ready(self.take_mutex().unwrap()),

                    // Lock is held by someone.
                    s if s % 2 == 1 => {}

                    // Lock is available.
                    _ => {
                        // Be fair: notify the first listener and then go wait in line.
                        mutex.lock_ops.notify(1);
                    }
                }
            } else {
                // Wait for a notification.
                ready!(strategy.poll(this.listener, context));

                // Try acquiring the lock without waiting for others.
                if mutex.state.fetch_or(1, Ordering::Acquire) % 2 == 0 {
                    return Poll::Ready(self.take_mutex().unwrap());
                }
            }
        }
    }
}

/// A guard that releases the mutex when dropped.
#[clippy::has_significant_drop]
pub struct MutexGuard<'a, T: ?Sized>(&'a Mutex<T>);

unsafe impl<T: Send + ?Sized> Send for MutexGuard<'_, T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuard<'_, T> {}

impl<'a, T: ?Sized> MutexGuard<'a, T> {
    /// Returns a reference to the mutex a guard came from.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::{Mutex, MutexGuard};
    ///
    /// let mutex = Mutex::new(10i32);
    /// let guard = mutex.lock().await;
    /// dbg!(MutexGuard::source(&guard));
    /// # })
    /// ```
    pub fn source(guard: &MutexGuard<'a, T>) -> &'a Mutex<T> {
        guard.0
    }
}

impl<T: ?Sized> Drop for MutexGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        // SAFETY: we are dropping the mutex guard, therefore unlocking the mutex.
        unsafe {
            self.0.unlock_unchecked();
        }
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: ?Sized> Deref for MutexGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.0.data.get() }
    }
}

impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.0.data.get() }
    }
}

/// An owned guard that releases the mutex when dropped.
#[clippy::has_significant_drop]
pub struct MutexGuardArc<T: ?Sized>(Arc<Mutex<T>>);

unsafe impl<T: Send + ?Sized> Send for MutexGuardArc<T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuardArc<T> {}

impl<T: ?Sized> MutexGuardArc<T> {
    /// Returns a reference to the mutex a guard came from.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::{Mutex, MutexGuardArc};
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10i32));
    /// let guard = mutex.lock_arc().await;
    /// dbg!(MutexGuardArc::source(&guard));
    /// # })
    /// ```
    pub fn source(guard: &Self) -> &Arc<Mutex<T>>
    where
        // Required because `MutexGuardArc` implements `Sync` regardless of whether `T` is `Send`,
        // but this method allows dropping `T` from a different thead than it was created in.
        T: Send,
    {
        &guard.0
    }
}

impl<T: ?Sized> Drop for MutexGuardArc<T> {
    #[inline]
    fn drop(&mut self) {
        // SAFETY: we are dropping the mutex guard, therefore unlocking the mutex.
        unsafe {
            self.0.unlock_unchecked();
        }
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuardArc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuardArc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: ?Sized> Deref for MutexGuardArc<T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.0.data.get() }
    }
}

impl<T: ?Sized> DerefMut for MutexGuardArc<T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.0.data.get() }
    }
}