async_lock/
once_cell.rs

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
use core::cell::UnsafeCell;
use core::convert::Infallible;
use core::fmt;
use core::future::Future;
use core::mem::{forget, MaybeUninit};
use core::ptr;

use crate::sync::atomic::{AtomicUsize, Ordering};

#[cfg(not(loom))]
use crate::sync::WithMut;

#[cfg(all(feature = "std", not(target_family = "wasm")))]
use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};

use event_listener::Event;
use event_listener_strategy::{NonBlocking, Strategy};

#[cfg(all(feature = "std", not(target_family = "wasm")))]
use event_listener::Listener;

/// The current state of the `OnceCell`.
#[derive(Copy, Clone, PartialEq, Eq)]
#[repr(usize)]
enum State {
    /// The `OnceCell` is uninitialized.
    Uninitialized = 0,
    /// The `OnceCell` is being initialized.
    Initializing = 1,
    /// The `OnceCell` is initialized.
    Initialized = 2,
}

impl From<usize> for State {
    fn from(val: usize) -> Self {
        match val {
            0 => State::Uninitialized,
            1 => State::Initializing,
            2 => State::Initialized,
            _ => unreachable!("Invalid state"),
        }
    }
}

impl From<State> for usize {
    fn from(val: State) -> Self {
        val as usize
    }
}

/// A memory location that can be written to at most once.
///
/// A `OnceCell` can be used to store a single value, and only once. However,
/// once the value is stored, it can be accessed directly through a reference
/// instead of needing an RAII guard like `Mutex` or `RwLock`.
///
/// # Examples
///
/// This structure is useful for a variety of patterns, most notably for one-time
/// initialization.
///
/// ```rust
/// use async_lock::OnceCell;
///
/// # struct Foobar;
///
/// async fn very_expensive_initialization() -> Foobar {
///     // Imagine this is very expensive to initialize,
///     // for instance, it requires a network request or
///     // a database call.
///     # Foobar
/// }
///
/// struct LazyFoobar {
///     inner: OnceCell<Foobar>,
/// }
///
/// impl LazyFoobar {
///     fn new() -> Self {
///         Self {
///             inner: OnceCell::new(),
///         }
///     }
///
///     async fn load(&self) -> &Foobar {
///         self.inner.get_or_init(|| async {
///             very_expensive_initialization().await
///         }).await
///     }
/// }
/// ```
pub struct OnceCell<T> {
    /// Listeners waiting for a chance to initialize the cell.
    ///
    /// These are the users of get_or_init() and similar functions.
    active_initializers: Event,

    /// Listeners waiting for the cell to be initialized.
    ///
    /// These are the users of wait().
    passive_waiters: Event,

    /// State associated with the cell.
    state: AtomicUsize,

    /// The value of the cell.
    value: UnsafeCell<MaybeUninit<T>>,
}

unsafe impl<T: Send> Send for OnceCell<T> {}
unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}

impl<T> OnceCell<T> {
    const_fn! {
        const_if: #[cfg(not(loom))];
        /// Create a new, uninitialized `OnceCell`.
        ///
        /// # Example
        ///
        /// ```rust
        /// use async_lock::OnceCell;
        ///
        /// let cell = OnceCell::new();
        /// # cell.set_blocking(1);
        /// ```
        pub const fn new() -> Self {
            Self {
                active_initializers: Event::new(),
                passive_waiters: Event::new(),
                state: AtomicUsize::new(State::Uninitialized as _),
                value: UnsafeCell::new(MaybeUninit::uninit()),
            }
        }
    }

    /// Tell whether or not the cell is initialized.
    ///
    /// This may not always be accurate. For instance, it is possible for
    /// another thread to initialize the cell between the time when this
    /// function is called and the time when the result is actually used.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    /// assert!(!cell.is_initialized());
    /// cell.set(1).await;
    /// assert!(cell.is_initialized());
    /// # });
    /// ```
    pub fn is_initialized(&self) -> bool {
        State::from(self.state.load(Ordering::Acquire)) == State::Initialized
    }

    /// Get a reference to the inner value, or `None` if the value
    /// is not yet initialized.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    /// assert!(cell.get().is_none());
    /// cell.set(1).await;
    /// assert_eq!(cell.get(), Some(&1));
    /// # });
    /// ```
    pub fn get(&self) -> Option<&T> {
        if self.is_initialized() {
            // SAFETY: We know that the value is initialized, so it is safe to
            // read it.
            Some(unsafe { self.get_unchecked() })
        } else {
            None
        }
    }

    /// Get a mutable reference to the inner value, or `None` if the value
    /// is not yet initialized.
    ///
    /// This function is useful for initializing the value inside the cell
    /// when we still have a mutable reference to the cell.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut cell = OnceCell::new();
    /// assert!(cell.get_mut().is_none());
    /// cell.set(1).await;
    /// assert_eq!(cell.get_mut(), Some(&mut 1));
    /// *cell.get_mut().unwrap() = 2;
    /// assert_eq!(cell.get(), Some(&2));
    /// # });
    /// ```
    pub fn get_mut(&mut self) -> Option<&mut T> {
        self.state.with_mut(|state| {
            if State::from(*state) == State::Initialized {
                // SAFETY: We know that the value is initialized, so it is safe to
                // read it.
                Some(unsafe { &mut *self.value.get().cast() })
            } else {
                None
            }
        })
    }

    /// Take the value out of this `OnceCell`, moving it back to the uninitialized
    /// state.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut cell = OnceCell::new();
    /// cell.set(1).await;
    /// assert_eq!(cell.take(), Some(1));
    /// assert!(!cell.is_initialized());
    /// # });
    /// ```
    pub fn take(&mut self) -> Option<T> {
        self.state.with_mut(|state| {
            if State::from(*state) == State::Initialized {
                // SAFETY: We know that the value is initialized, so it is safe to
                // read it.
                let value = unsafe { ptr::read(self.value.get().cast()) };
                *state = State::Uninitialized.into();
                Some(value)
            } else {
                None
            }
        })
    }

    /// Convert this `OnceCell` into the inner value, if it is initialized.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    /// cell.set(1).await;
    /// assert_eq!(cell.into_inner(), Some(1));
    /// # });
    /// ```
    pub fn into_inner(mut self) -> Option<T> {
        self.take()
    }

    /// Wait for the cell to be initialized, and then return a reference to the
    /// inner value.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    /// use std::sync::Arc;
    /// use std::time::Duration;
    /// use std::thread::{sleep, spawn};
    ///
    /// let cell = Arc::new(OnceCell::new());
    /// let cell2 = cell.clone();
    ///
    /// spawn(move || {
    ///    sleep(Duration::from_millis(5));
    ///    cell2.set_blocking(1);
    /// });
    ///
    /// # futures_lite::future::block_on(async {
    /// assert_eq!(cell.wait().await, &1);
    /// # });
    /// ```
    pub async fn wait(&self) -> &T {
        // Fast path: see if the value is already initialized.
        if let Some(value) = self.get() {
            return value;
        }

        // Slow path: wait for the value to be initialized.
        event_listener::listener!(self.passive_waiters => listener);

        // Try again.
        if let Some(value) = self.get() {
            return value;
        }

        listener.await;
        debug_assert!(self.is_initialized());

        // SAFETY: We know that the value is initialized, so it is safe to
        // read it.
        unsafe { self.get_unchecked() }
    }

    /// Wait for the cell to be initialized, and then return a reference to the
    /// inner value.
    ///
    /// # Blocking
    ///
    /// In contrast to the `wait` method, this method blocks the current thread of
    /// execution instead of awaiting.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a `OnceCell` can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    /// use std::sync::Arc;
    /// use std::time::Duration;
    /// use std::thread::{sleep, spawn};
    ///
    /// let cell = Arc::new(OnceCell::new());
    /// let cell2 = cell.clone();
    ///
    /// spawn(move || {
    ///    sleep(Duration::from_millis(5));
    ///    cell2.set_blocking(1);
    /// });
    ///
    /// assert_eq!(cell.wait_blocking(), &1);
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn wait_blocking(&self) -> &T {
        // Fast path: see if the value is already initialized.
        if let Some(value) = self.get() {
            return value;
        }

        // Slow path: wait for the value to be initialized.
        event_listener::listener!(self.passive_waiters => listener);

        // Try again.
        if let Some(value) = self.get() {
            return value;
        }

        listener.wait();
        debug_assert!(self.is_initialized());

        // SAFETY: We know that the value is initialized, so it is safe to
        // read it.
        unsafe { self.get_unchecked() }
    }

    /// Either get the value or initialize it with the given closure.
    ///
    /// The cell will not be initialized if the closure returns an error.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    /// #
    /// # // Prevent explicit value errors.
    /// # fn _explicit(_: &Result<&i32, ()>) {}
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    ///
    /// let result = cell.get_or_try_init(|| async { Err(()) }).await;
    /// assert!(result.is_err());
    ///
    /// let result = cell.get_or_try_init(|| async { Ok(1) }).await;
    /// # _explicit(&result);
    /// assert_eq!(result.unwrap(), &1);
    ///
    /// let result = cell.get_or_try_init(|| async { Err(()) }).await;
    ///
    /// assert_eq!(result.unwrap(), &1);
    /// # });
    /// ```
    pub async fn get_or_try_init<E, Fut: Future<Output = Result<T, E>>>(
        &self,
        closure: impl FnOnce() -> Fut,
    ) -> Result<&T, E> {
        // Fast path: see if the value is already initialized.
        if let Some(value) = self.get() {
            return Ok(value);
        }

        // Slow path: initialize the value.
        self.initialize_or_wait(closure, &mut NonBlocking::default())
            .await?;
        debug_assert!(self.is_initialized());

        // SAFETY: We know that the value is initialized, so it is safe to
        // read it.
        Ok(unsafe { self.get_unchecked() })
    }

    /// Either get the value or initialize it with the given closure.
    ///
    /// The cell will not be initialized if the closure returns an error.
    ///
    /// # Blocking
    ///
    /// In contrast to the `get_or_try_init` method, this method blocks the current thread of
    /// execution instead of awaiting.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a `OnceCell` can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    /// #
    /// # // Prevent explicit type errors.
    /// # fn _explicit(_: &Result<&i32, ()>) {}
    ///
    /// let cell = OnceCell::new();
    ///
    /// let result = cell.get_or_try_init_blocking(|| Err(()));
    /// assert!(result.is_err());
    ///
    /// let result = cell.get_or_try_init_blocking(|| Ok(1));
    /// # _explicit(&result);
    /// assert_eq!(result.unwrap(), &1);
    ///
    /// let result = cell.get_or_try_init_blocking(|| Err(()));
    ///
    /// assert_eq!(result.unwrap(), &1);
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn get_or_try_init_blocking<E>(
        &self,
        closure: impl FnOnce() -> Result<T, E>,
    ) -> Result<&T, E> {
        // Fast path: see if the value is already initialized.
        if let Some(value) = self.get() {
            return Ok(value);
        }

        // Slow path: initialize the value.
        // The futures provided should never block, so we can use `now_or_never`.
        now_or_never(self.initialize_or_wait(
            move || core::future::ready(closure()),
            &mut event_listener_strategy::Blocking::default(),
        ))?;
        debug_assert!(self.is_initialized());

        // SAFETY: We know that the value is initialized, so it is safe to
        // read it.
        Ok(unsafe { self.get_unchecked() })
    }

    /// Either get the value or initialize it with the given closure.
    ///
    /// Many tasks may call this function, but the value will only be set once
    /// and only one closure will be invoked.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    /// assert_eq!(cell.get_or_init(|| async { 1 }).await, &1);
    /// assert_eq!(cell.get_or_init(|| async { 2 }).await, &1);
    /// # });
    /// ```
    pub async fn get_or_init<Fut: Future<Output = T>>(&self, closure: impl FnOnce() -> Fut) -> &T {
        match self
            .get_or_try_init(move || async move {
                let result: Result<T, Infallible> = Ok(closure().await);
                result
            })
            .await
        {
            Ok(value) => value,
            Err(infallible) => match infallible {},
        }
    }

    /// Either get the value or initialize it with the given closure.
    ///
    /// Many tasks may call this function, but the value will only be set once
    /// and only one closure will be invoked.
    ///
    /// # Blocking
    ///
    /// In contrast to the `get_or_init` method, this method blocks the current thread of
    /// execution instead of awaiting.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a `OnceCell` can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// let cell = OnceCell::new();
    /// assert_eq!(cell.get_or_init_blocking(|| 1), &1);
    /// assert_eq!(cell.get_or_init_blocking(|| 2), &1);
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn get_or_init_blocking(&self, closure: impl FnOnce() -> T + Unpin) -> &T {
        let result = self.get_or_try_init_blocking(move || {
            let result: Result<T, Infallible> = Ok(closure());
            result
        });
        match result {
            Ok(value) => value,
            Err(infallible) => match infallible {},
        }
    }

    /// Try to set the value of the cell.
    ///
    /// If the cell is already initialized, this method returns the original
    /// value back.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    ///
    /// assert_eq!(cell.set(1).await, Ok(&1));
    /// assert_eq!(cell.get(), Some(&1));
    /// assert_eq!(cell.set(2).await, Err(2));
    /// # });
    /// ```
    pub async fn set(&self, value: T) -> Result<&T, T> {
        let mut value = Some(value);
        self.get_or_init(|| async { value.take().unwrap() }).await;

        match value {
            Some(value) => Err(value),
            None => {
                // SAFETY: value was taken, so we are initialized
                Ok(unsafe { self.get_unchecked() })
            }
        }
    }

    /// Try to set the value of the cell.
    ///
    /// If the cell is already initialized, this method returns the original
    /// value back.
    ///
    /// # Blocking
    ///
    /// In contrast to the `set` method, this method blocks the current thread of
    /// execution instead of awaiting.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a `OnceCell` can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// let cell = OnceCell::new();
    ///
    /// assert_eq!(cell.set_blocking(1), Ok(&1));
    /// assert_eq!(cell.get(), Some(&1));
    /// assert_eq!(cell.set_blocking(2), Err(2));
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn set_blocking(&self, value: T) -> Result<&T, T> {
        let mut value = Some(value);
        self.get_or_init_blocking(|| value.take().unwrap());

        match value {
            Some(value) => Err(value),
            None => {
                // SAFETY: value was taken, so we are initialized
                Ok(unsafe { self.get_unchecked() })
            }
        }
    }

    /// Wait for the cell to be initialized, optionally using a closure
    /// to initialize the cell if it is not initialized yet.
    #[cold]
    async fn initialize_or_wait<E, Fut: Future<Output = Result<T, E>>, F: FnOnce() -> Fut>(
        &self,
        closure: F,
        strategy: &mut impl for<'a> Strategy<'a>,
    ) -> Result<(), E> {
        // The event listener we're currently waiting on.
        let mut event_listener = None;

        let mut closure = Some(closure);

        loop {
            // Check the current state of the cell.
            let state = self.state.load(Ordering::Acquire);

            // Determine what we should do based on our state.
            match state.into() {
                State::Initialized => {
                    // The cell is initialized now, so we can return.
                    return Ok(());
                }
                State::Initializing => {
                    // The cell is currently initializing, or the cell is uninitialized
                    // but we do not have the ability to initialize it.
                    //
                    // We need to wait the initialization to complete.
                    if let Some(listener) = event_listener.take() {
                        strategy.wait(listener).await;
                    } else {
                        event_listener = Some(self.active_initializers.listen());
                    }
                }
                State::Uninitialized => {
                    // Try to move the cell into the initializing state.
                    if self
                        .state
                        .compare_exchange(
                            State::Uninitialized.into(),
                            State::Initializing.into(),
                            Ordering::AcqRel,
                            Ordering::Acquire,
                        )
                        .is_err()
                    {
                        // The cell was initialized while we were trying to
                        // initialize it.
                        continue;
                    }

                    // Now that we have an exclusive lock on the cell's value,
                    // we can try to initialize it.
                    let _guard = Guard(self);
                    let initializer = closure.take().unwrap();
                    match (initializer)().await {
                        Ok(value) => {
                            // Write the value into the cell and update the state.
                            unsafe {
                                ptr::write(self.value.get().cast(), value);
                            }
                            forget(_guard);
                            self.state
                                .store(State::Initialized.into(), Ordering::Release);

                            // Notify the listeners that the value is initialized.
                            self.active_initializers.notify_additional(core::usize::MAX);
                            self.passive_waiters.notify_additional(core::usize::MAX);

                            return Ok(());
                        }
                        Err(err) => {
                            // Update the state to indicate that the value is
                            // uninitialized.
                            drop(_guard);

                            return Err(err);
                        }
                    }
                }
            }
        }

        /// Set the cell's state back to `UNINITIALIZED on drop.
        ///
        /// If the closure panics, this ensures that the cell's state is set back to
        /// `UNINITIALIZED` and that the next listener is notified.
        struct Guard<'a, T>(&'a OnceCell<T>);

        impl<'a, T> Drop for Guard<'a, T> {
            fn drop(&mut self) {
                self.0
                    .state
                    .store(State::Uninitialized.into(), Ordering::Release);

                // Notify the next initializer that it's their turn.
                self.0.active_initializers.notify(1);
            }
        }
    }

    /// Get a reference to the inner value.
    ///
    /// # Safety
    ///
    /// The caller must ensure that the cell is initialized.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// # futures_lite::future::block_on(async {
    /// let cell = OnceCell::new();
    /// cell.set(1).await;
    ///
    /// // SAFETY: We know that the value is initialized, so it is safe to
    /// // read it.
    /// assert_eq!(unsafe { cell.get_unchecked() }, &1);
    /// # });
    /// ```
    pub unsafe fn get_unchecked(&self) -> &T {
        // SAFETY: The caller asserts that the value is initialized
        &*self.value.get().cast()
    }
}

impl<T> From<T> for OnceCell<T> {
    /// Create a new, initialized `OnceCell` from an existing value.
    ///
    /// # Example
    ///
    /// ```rust
    /// use async_lock::OnceCell;
    ///
    /// let cell = OnceCell::from(42);
    /// assert_eq!(cell.get(), Some(&42));
    /// ```
    fn from(value: T) -> Self {
        Self {
            active_initializers: Event::new(),
            passive_waiters: Event::new(),
            state: AtomicUsize::new(State::Initialized.into()),
            value: UnsafeCell::new(MaybeUninit::new(value)),
        }
    }
}

impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct Inner<'a, T>(&'a OnceCell<T>);

        impl<T: fmt::Debug> fmt::Debug for Inner<'_, T> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                match self.0.state.load(Ordering::Acquire).into() {
                    State::Uninitialized => f.write_str("<uninitialized>"),
                    State::Initializing => f.write_str("<initializing>"),
                    State::Initialized => {
                        // SAFETY: "value" is initialized.
                        let value = unsafe { self.0.get_unchecked() };
                        fmt::Debug::fmt(value, f)
                    }
                }
            }
        }

        f.debug_tuple("OnceCell").field(&Inner(self)).finish()
    }
}

impl<T> Drop for OnceCell<T> {
    fn drop(&mut self) {
        self.state.with_mut(|state| {
            if State::from(*state) == State::Initialized {
                // SAFETY: We know that the value is initialized, so it is safe to
                // drop it.
                unsafe { self.value.get().cast::<T>().drop_in_place() }
            }
        });
    }
}

impl<T> Default for OnceCell<T> {
    // Calls `OnceCell::new`.
    #[inline]
    fn default() -> Self {
        Self::new()
    }
}

/// Either return the result of a future now, or panic.
#[cfg(all(feature = "std", not(target_family = "wasm")))]
fn now_or_never<T>(f: impl Future<Output = T>) -> T {
    const NOOP_WAKER: RawWakerVTable = RawWakerVTable::new(clone, wake, wake_by_ref, drop);

    unsafe fn wake(_: *const ()) {}
    unsafe fn wake_by_ref(_: *const ()) {}
    unsafe fn clone(_: *const ()) -> RawWaker {
        RawWaker::new(ptr::null(), &NOOP_WAKER)
    }
    unsafe fn drop(_: *const ()) {}

    pin!(f);

    let waker = unsafe { Waker::from_raw(RawWaker::new(ptr::null(), &NOOP_WAKER)) };

    // Poll the future exactly once.
    let mut cx = Context::from_waker(&waker);

    match f.poll(&mut cx) {
        Poll::Ready(value) => value,
        Poll::Pending => unreachable!("future not ready"),
    }
}