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
#![stable(feature = "futures_api", since = "1.36.0")]

use crate::mem::transmute;

use crate::fmt;
use crate::marker::PhantomData;
use crate::ptr;

/// A `RawWaker` allows the implementor of a task executor to create a [`Waker`]
/// or a [`LocalWaker`] which provides customized wakeup behavior.
///
/// It consists of a data pointer and a [virtual function pointer table (vtable)][vtable]
/// that customizes the behavior of the `RawWaker`.
///
/// `RawWaker`s are unsafe to use.
/// Implementing the [`Wake`] trait is a safe alternative that requires memory allocation.
///
/// [vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
/// [`Wake`]: ../../alloc/task/trait.Wake.html
#[derive(PartialEq, Debug)]
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct RawWaker {
    /// A data pointer, which can be used to store arbitrary data as required
    /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
    /// that is associated with the task.
    /// The value of this field gets passed to all functions that are part of
    /// the vtable as the first parameter.
    data: *const (),
    /// Virtual function pointer table that customizes the behavior of this waker.
    vtable: &'static RawWakerVTable,
}

impl RawWaker {
    /// Creates a new `RawWaker` from the provided `data` pointer and `vtable`.
    ///
    /// The `data` pointer can be used to store arbitrary data as required
    /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
    /// that is associated with the task.
    /// The value of this pointer will get passed to all functions that are part
    /// of the `vtable` as the first parameter.
    ///
    /// It is important to consider that the `data` pointer must point to a
    /// thread safe type such as an `[Arc]<T: Send + Sync>`
    /// when used to construct a [`Waker`]. This restriction is lifted when
    /// constructing a [`LocalWaker`], which allows using types that do not implement
    /// <code>[Send] + [Sync]</code> like `[Rc]<T>`.
    ///
    /// The `vtable` customizes the behavior of a `Waker` which gets created
    /// from a `RawWaker`. For each operation on the `Waker`, the associated
    /// function in the `vtable` of the underlying `RawWaker` will be called.
    ///
    /// [`Arc`]: std::sync::Arc
    /// [`Rc`]: std::rc::Rc
    #[inline]
    #[rustc_promotable]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
    #[must_use]
    pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
        RawWaker { data, vtable }
    }

    /// Get the `data` pointer used to create this `RawWaker`.
    #[inline]
    #[must_use]
    #[unstable(feature = "waker_getters", issue = "96992")]
    pub fn data(&self) -> *const () {
        self.data
    }

    /// Get the `vtable` pointer used to create this `RawWaker`.
    #[inline]
    #[must_use]
    #[unstable(feature = "waker_getters", issue = "96992")]
    pub fn vtable(&self) -> &'static RawWakerVTable {
        self.vtable
    }

    #[unstable(feature = "noop_waker", issue = "98286")]
    const NOOP: RawWaker = {
        const VTABLE: RawWakerVTable = RawWakerVTable::new(
            // Cloning just returns a new no-op raw waker
            |_| RawWaker::NOOP,
            // `wake` does nothing
            |_| {},
            // `wake_by_ref` does nothing
            |_| {},
            // Dropping does nothing as we don't allocate anything
            |_| {},
        );
        RawWaker::new(ptr::null(), &VTABLE)
    };
}

/// A virtual function pointer table (vtable) that specifies the behavior
/// of a [`RawWaker`].
///
/// The pointer passed to all functions inside the vtable is the `data` pointer
/// from the enclosing [`RawWaker`] object.
///
/// The functions inside this struct are only intended to be called on the `data`
/// pointer of a properly constructed [`RawWaker`] object from inside the
/// [`RawWaker`] implementation. Calling one of the contained functions using
/// any other `data` pointer will cause undefined behavior.
///
/// Note that while this type implements `PartialEq`, comparing function pointers, and hence
/// comparing structs like this that contain function pointers, is unreliable: pointers to the same
/// function can compare inequal (because functions are duplicated in multiple codegen units), and
/// pointers to *different* functions can compare equal (since identical functions can be
/// deduplicated within a codegen unit).
///
/// # Thread safety
/// If the [`RawWaker`] will be used to construct a [`Waker`] then
/// these functions must all be thread-safe (even though [`RawWaker`] is
/// <code>\![Send] + \![Sync]</code>). This is because [`Waker`] is <code>[Send] + [Sync]</code>,
/// and it may be moved to arbitrary threads or invoked by `&` reference. For example,
/// this means that if the `clone` and `drop` functions manage a reference count,
/// they must do so atomically.
///
/// However, if the [`RawWaker`] will be used to construct a [`LocalWaker`] instead, then
/// these functions don't need to be thread safe. This means that <code>\![Send] + \![Sync]</code>
///  data can be stored in the data pointer, and reference counting does not need any atomic
/// synchronization. This is because [`LocalWaker`] is not thread safe itself, so it cannot
/// be sent across threads.
#[stable(feature = "futures_api", since = "1.36.0")]
#[derive(PartialEq, Copy, Clone, Debug)]
pub struct RawWakerVTable {
    /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
    /// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
    ///
    /// The implementation of this function must retain all resources that are
    /// required for this additional instance of a [`RawWaker`] and associated
    /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
    /// of the same task that would have been awoken by the original [`RawWaker`].
    clone: unsafe fn(*const ()) -> RawWaker,

    /// This function will be called when `wake` is called on the [`Waker`].
    /// It must wake up the task associated with this [`RawWaker`].
    ///
    /// The implementation of this function must make sure to release any
    /// resources that are associated with this instance of a [`RawWaker`] and
    /// associated task.
    wake: unsafe fn(*const ()),

    /// This function will be called when `wake_by_ref` is called on the [`Waker`].
    /// It must wake up the task associated with this [`RawWaker`].
    ///
    /// This function is similar to `wake`, but must not consume the provided data
    /// pointer.
    wake_by_ref: unsafe fn(*const ()),

    /// This function gets called when a [`Waker`] gets dropped.
    ///
    /// The implementation of this function must make sure to release any
    /// resources that are associated with this instance of a [`RawWaker`] and
    /// associated task.
    drop: unsafe fn(*const ()),
}

impl RawWakerVTable {
    /// Creates a new `RawWakerVTable` from the provided `clone`, `wake`,
    /// `wake_by_ref`, and `drop` functions.
    ///
    /// If the [`RawWaker`] will be used to construct a [`Waker`] then
    /// these functions must all be thread-safe (even though [`RawWaker`] is
    /// <code>\![Send] + \![Sync]</code>). This is because [`Waker`] is <code>[Send] + [Sync]</code>,
    /// and it may be moved to arbitrary threads or invoked by `&` reference. For example,
    /// this means that if the `clone` and `drop` functions manage a reference count,
    /// they must do so atomically.
    ///
    /// However, if the [`RawWaker`] will be used to construct a [`LocalWaker`] instead, then
    /// these functions don't need to be thread safe. This means that <code>\![Send] + \![Sync]</code>
    /// data can be stored in the data pointer, and reference counting does not need any atomic
    /// synchronization. This is because [`LocalWaker`] is not thread safe itself, so it cannot
    /// be sent across threads.
    /// # `clone`
    ///
    /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
    /// the [`Waker`]/[`LocalWaker`] in which the [`RawWaker`] is stored gets cloned.
    ///
    /// The implementation of this function must retain all resources that are
    /// required for this additional instance of a [`RawWaker`] and associated
    /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
    /// of the same task that would have been awoken by the original [`RawWaker`].
    ///
    /// # `wake`
    ///
    /// This function will be called when `wake` is called on the [`Waker`].
    /// It must wake up the task associated with this [`RawWaker`].
    ///
    /// The implementation of this function must make sure to release any
    /// resources that are associated with this instance of a [`RawWaker`] and
    /// associated task.
    ///
    /// # `wake_by_ref`
    ///
    /// This function will be called when `wake_by_ref` is called on the [`Waker`].
    /// It must wake up the task associated with this [`RawWaker`].
    ///
    /// This function is similar to `wake`, but must not consume the provided data
    /// pointer.
    ///
    /// # `drop`
    ///
    /// This function gets called when a [`Waker`]/[`LocalWaker`] gets dropped.
    ///
    /// The implementation of this function must make sure to release any
    /// resources that are associated with this instance of a [`RawWaker`] and
    /// associated task.
    #[rustc_promotable]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
    pub const fn new(
        clone: unsafe fn(*const ()) -> RawWaker,
        wake: unsafe fn(*const ()),
        wake_by_ref: unsafe fn(*const ()),
        drop: unsafe fn(*const ()),
    ) -> Self {
        Self { clone, wake, wake_by_ref, drop }
    }
}

/// The context of an asynchronous task.
///
/// Currently, `Context` only serves to provide access to a [`&Waker`](Waker)
/// which can be used to wake the current task.
#[stable(feature = "futures_api", since = "1.36.0")]
#[lang = "Context"]
pub struct Context<'a> {
    waker: &'a Waker,
    local_waker: &'a LocalWaker,
    // Ensure we future-proof against variance changes by forcing
    // the lifetime to be invariant (argument-position lifetimes
    // are contravariant while return-position lifetimes are
    // covariant).
    _marker: PhantomData<fn(&'a ()) -> &'a ()>,
    // Ensure `Context` is `!Send` and `!Sync` in order to allow
    // for future `!Send` and / or `!Sync` fields.
    _marker2: PhantomData<*mut ()>,
}

impl<'a> Context<'a> {
    /// Create a new `Context` from a [`&Waker`](Waker).
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    #[must_use]
    #[inline]
    pub const fn from_waker(waker: &'a Waker) -> Self {
        ContextBuilder::from_waker(waker).build()
    }

    /// Returns a reference to the [`Waker`] for the current task.
    #[inline]
    #[must_use]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const fn waker(&self) -> &'a Waker {
        &self.waker
    }
    /// Returns a reference to the [`LocalWaker`] for the current task.
    #[inline]
    #[unstable(feature = "local_waker", issue = "118959")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const fn local_waker(&self) -> &'a LocalWaker {
        &self.local_waker
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Context<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Context").field("waker", &self.waker).finish()
    }
}

/// A Builder used to construct a `Context` instance
/// with support for `LocalWaker`.
///
/// # Examples
/// ```
/// #![feature(local_waker)]
/// #![feature(noop_waker)]
/// use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
/// use std::future::Future;
///
/// let local_waker = LocalWaker::noop();
/// let waker = Waker::noop();
///
/// let mut cx = ContextBuilder::from_waker(&waker)
///     .local_waker(&local_waker)
///     .build();
///
/// let mut future = std::pin::pin!(async { 20 });
/// let poll = future.as_mut().poll(&mut cx);
/// assert_eq!(poll, Poll::Ready(20));
///
/// ```
#[unstable(feature = "local_waker", issue = "118959")]
#[derive(Debug)]
pub struct ContextBuilder<'a> {
    waker: &'a Waker,
    local_waker: &'a LocalWaker,
    // Ensure we future-proof against variance changes by forcing
    // the lifetime to be invariant (argument-position lifetimes
    // are contravariant while return-position lifetimes are
    // covariant).
    _marker: PhantomData<fn(&'a ()) -> &'a ()>,
    // Ensure `Context` is `!Send` and `!Sync` in order to allow
    // for future `!Send` and / or `!Sync` fields.
    _marker2: PhantomData<*mut ()>,
}

impl<'a> ContextBuilder<'a> {
    /// Create a ContextBuilder from a Waker.
    #[inline]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    #[unstable(feature = "local_waker", issue = "118959")]
    pub const fn from_waker(waker: &'a Waker) -> Self {
        // SAFETY: LocalWaker is just Waker without thread safety
        let local_waker = unsafe { transmute(waker) };
        Self { waker: waker, local_waker, _marker: PhantomData, _marker2: PhantomData }
    }

    /// This method is used to set the value for the local waker on `Context`.
    #[inline]
    #[unstable(feature = "local_waker", issue = "118959")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const fn local_waker(self, local_waker: &'a LocalWaker) -> Self {
        Self { local_waker, ..self }
    }

    /// Builds the `Context`.
    #[inline]
    #[unstable(feature = "local_waker", issue = "118959")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const fn build(self) -> Context<'a> {
        let ContextBuilder { waker, local_waker, _marker, _marker2 } = self;
        Context { waker, local_waker, _marker, _marker2 }
    }
}

/// A `Waker` is a handle for waking up a task by notifying its executor that it
/// is ready to be run.
///
/// This handle encapsulates a [`RawWaker`] instance, which defines the
/// executor-specific wakeup behavior.
///
/// The typical life of a `Waker` is that it is constructed by an executor, wrapped in a
/// [`Context`], then passed to [`Future::poll()`]. Then, if the future chooses to return
/// [`Poll::Pending`], it must also store the waker somehow and call [`Waker::wake()`] when
/// the future should be polled again.
///
/// Implements [`Clone`], [`Send`], and [`Sync`]; therefore, a waker may be invoked
/// from any thread, including ones not in any way managed by the executor. For example,
/// this might be done to wake a future when a blocking function call completes on another
/// thread.
///
/// Note that it is preferable to use `waker.clone_from(&new_waker)` instead
/// of `*waker = new_waker.clone()`, as the former will avoid cloning the waker
/// unnecessarily if the two wakers [wake the same task](Self::will_wake).
///
/// Constructing a `Waker` from a [`RawWaker`] is unsafe.
/// Implementing the [`Wake`] trait is a safe alternative that requires memory allocation.
///
/// [`Future::poll()`]: core::future::Future::poll
/// [`Poll::Pending`]: core::task::Poll::Pending
/// [`Wake`]: ../../alloc/task/trait.Wake.html
#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/66401
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct Waker {
    waker: RawWaker,
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl Unpin for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Send for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Sync for Waker {}

impl Waker {
    /// Wake up the task associated with this `Waker`.
    ///
    /// As long as the executor keeps running and the task is not finished, it is
    /// guaranteed that each invocation of [`wake()`](Self::wake) (or
    /// [`wake_by_ref()`](Self::wake_by_ref)) will be followed by at least one
    /// [`poll()`] of the task to which this `Waker` belongs. This makes
    /// it possible to temporarily yield to other tasks while running potentially
    /// unbounded processing loops.
    ///
    /// Note that the above implies that multiple wake-ups may be coalesced into a
    /// single [`poll()`] invocation by the runtime.
    ///
    /// Also note that yielding to competing tasks is not guaranteed: it is the
    /// executor’s choice which task to run and the executor may choose to run the
    /// current task again.
    ///
    /// [`poll()`]: crate::future::Future::poll
    #[inline]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn wake(self) {
        // The actual wakeup call is delegated through a virtual function call
        // to the implementation which is defined by the executor.
        let wake = self.waker.vtable.wake;
        let data = self.waker.data;

        // Don't call `drop` -- the waker will be consumed by `wake`.
        crate::mem::forget(self);

        // SAFETY: This is safe because `Waker::from_raw` is the only way
        // to initialize `wake` and `data` requiring the user to acknowledge
        // that the contract of `RawWaker` is upheld.
        unsafe { (wake)(data) };
    }

    /// Wake up the task associated with this `Waker` without consuming the `Waker`.
    ///
    /// This is similar to [`wake()`](Self::wake), but may be slightly less efficient in
    /// the case where an owned `Waker` is available. This method should be preferred to
    /// calling `waker.clone().wake()`.
    #[inline]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn wake_by_ref(&self) {
        // The actual wakeup call is delegated through a virtual function call
        // to the implementation which is defined by the executor.

        // SAFETY: see `wake`
        unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
    }

    /// Returns `true` if this `Waker` and another `Waker` would awake the same task.
    ///
    /// This function works on a best-effort basis, and may return false even
    /// when the `Waker`s would awaken the same task. However, if this function
    /// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
    ///
    /// This function is primarily used for optimization purposes — for example,
    /// this type's [`clone_from`](Self::clone_from) implementation uses it to
    /// avoid cloning the waker when they would wake the same task anyway.
    #[inline]
    #[must_use]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn will_wake(&self, other: &Waker) -> bool {
        let RawWaker { data: a_data, vtable: a_vtable } = self.waker;
        let RawWaker { data: b_data, vtable: b_vtable } = other.waker;
        a_data == b_data && ptr::eq(a_vtable, b_vtable)
    }

    /// Creates a new `Waker` from [`RawWaker`].
    ///
    /// # Safety
    ///
    /// The behavior of the returned `Waker` is undefined if the contract defined
    /// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
    ///
    /// (Authors wishing to avoid unsafe code may implement the [`Wake`] trait instead, at the
    /// cost of a required heap allocation.)
    ///
    /// [`Wake`]: ../../alloc/task/trait.Wake.html
    #[inline]
    #[must_use]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const unsafe fn from_raw(waker: RawWaker) -> Waker {
        Waker { waker }
    }

    /// Returns a reference to a `Waker` that does nothing when used.
    ///
    /// This is mostly useful for writing tests that need a [`Context`] to poll
    /// some futures, but are not expecting those futures to wake the waker or
    /// do not need to do anything specific if it happens.
    ///
    /// If an owned `Waker` is needed, `clone()` this one.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(noop_waker)]
    ///
    /// use std::future::Future;
    /// use std::task;
    ///
    /// let mut cx = task::Context::from_waker(task::Waker::noop());
    ///
    /// let mut future = Box::pin(async { 10 });
    /// assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "noop_waker", issue = "98286")]
    pub const fn noop() -> &'static Waker {
        const WAKER: &Waker = &Waker { waker: RawWaker::NOOP };
        WAKER
    }

    /// Get a reference to the underlying [`RawWaker`].
    #[inline]
    #[must_use]
    #[unstable(feature = "waker_getters", issue = "96992")]
    pub fn as_raw(&self) -> &RawWaker {
        &self.waker
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl Clone for Waker {
    #[inline]
    fn clone(&self) -> Self {
        Waker {
            // SAFETY: This is safe because `Waker::from_raw` is the only way
            // to initialize `clone` and `data` requiring the user to acknowledge
            // that the contract of [`RawWaker`] is upheld.
            waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
        }
    }

    #[inline]
    fn clone_from(&mut self, source: &Self) {
        if !self.will_wake(source) {
            *self = source.clone();
        }
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl Drop for Waker {
    #[inline]
    fn drop(&mut self) {
        // SAFETY: This is safe because `Waker::from_raw` is the only way
        // to initialize `drop` and `data` requiring the user to acknowledge
        // that the contract of `RawWaker` is upheld.
        unsafe { (self.waker.vtable.drop)(self.waker.data) }
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Waker {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
        f.debug_struct("Waker")
            .field("data", &self.waker.data)
            .field("vtable", &vtable_ptr)
            .finish()
    }
}

/// A `LocalWaker` is analogous to a [`Waker`], but it does not implement [`Send`] or [`Sync`].
///
/// This handle encapsulates a [`RawWaker`] instance, which defines the
/// executor-specific wakeup behavior.
///
/// Local wakers can be requested from a `Context` with the [`local_waker`] method.
///
/// The typical life of a `LocalWaker` is that it is constructed by an executor, wrapped in a
/// [`Context`] using [`ContextBuilder`], then passed to [`Future::poll()`]. Then, if the future chooses to return
/// [`Poll::Pending`], it must also store the waker somehow and call [`LocalWaker::wake()`] when
/// the future should be polled again.
///
/// Implements [`Clone`], but neither [`Send`] nor [`Sync`]; therefore, a local waker may
/// not be moved to other threads. In general, when deciding to use wakers or local wakers,
/// local wakers are preferable unless the waker needs to be sent across threads. This is because
/// wakers can incur in additional cost related to memory synchronization.
///
/// Note that it is preferable to use `local_waker.clone_from(&new_waker)` instead
/// of `*local_waker = new_waker.clone()`, as the former will avoid cloning the waker
/// unnecessarily if the two wakers [wake the same task](Self::will_wake).
///
/// # Examples
/// Usage of a local waker to implement a future analogous to `std::thread::yield_now()`.
/// ```
/// #![feature(local_waker)]
/// use std::future::{Future, poll_fn};
/// use std::task::Poll;
///
/// // a future that returns pending once.
/// fn yield_now() -> impl Future<Output=()> + Unpin {
///     let mut yielded = false;
///     poll_fn(move |cx| {
///         if !yielded {
///             yielded = true;
///             cx.local_waker().wake_by_ref();
///             return Poll::Pending;
///         }
///         return Poll::Ready(())
///     })
/// }
///
/// # async fn __() {
/// yield_now().await;
/// # }
/// ```
///
/// [`Future::poll()`]: core::future::Future::poll
/// [`Poll::Pending`]: core::task::Poll::Pending
/// [`local_waker`]: core::task::Context::local_waker
#[unstable(feature = "local_waker", issue = "118959")]
#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/66401
pub struct LocalWaker {
    waker: RawWaker,
}

#[unstable(feature = "local_waker", issue = "118959")]
impl Unpin for LocalWaker {}

impl LocalWaker {
    /// Wake up the task associated with this `LocalWaker`.
    ///
    /// As long as the executor keeps running and the task is not finished, it is
    /// guaranteed that each invocation of [`wake()`](Self::wake) (or
    /// [`wake_by_ref()`](Self::wake_by_ref)) will be followed by at least one
    /// [`poll()`] of the task to which this `LocalWaker` belongs. This makes
    /// it possible to temporarily yield to other tasks while running potentially
    /// unbounded processing loops.
    ///
    /// Note that the above implies that multiple wake-ups may be coalesced into a
    /// single [`poll()`] invocation by the runtime.
    ///
    /// Also note that yielding to competing tasks is not guaranteed: it is the
    /// executor’s choice which task to run and the executor may choose to run the
    /// current task again.
    ///
    /// [`poll()`]: crate::future::Future::poll
    #[inline]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn wake(self) {
        // The actual wakeup call is delegated through a virtual function call
        // to the implementation which is defined by the executor.
        let wake = self.waker.vtable.wake;
        let data = self.waker.data;

        // Don't call `drop` -- the waker will be consumed by `wake`.
        crate::mem::forget(self);

        // SAFETY: This is safe because `Waker::from_raw` is the only way
        // to initialize `wake` and `data` requiring the user to acknowledge
        // that the contract of `RawWaker` is upheld.
        unsafe { (wake)(data) };
    }

    /// Wake up the task associated with this `LocalWaker` without consuming the `LocalWaker`.
    ///
    /// This is similar to [`wake()`](Self::wake), but may be slightly less efficient in
    /// the case where an owned `Waker` is available. This method should be preferred to
    /// calling `waker.clone().wake()`.
    #[inline]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn wake_by_ref(&self) {
        // The actual wakeup call is delegated through a virtual function call
        // to the implementation which is defined by the executor.

        // SAFETY: see `wake`
        unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
    }

    /// Returns `true` if this `LocalWaker` and another `LocalWaker` would awake the same task.
    ///
    /// This function works on a best-effort basis, and may return false even
    /// when the `Waker`s would awaken the same task. However, if this function
    /// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
    ///
    /// This function is primarily used for optimization purposes — for example,
    /// this type's [`clone_from`](Self::clone_from) implementation uses it to
    /// avoid cloning the waker when they would wake the same task anyway.
    #[inline]
    #[must_use]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub fn will_wake(&self, other: &LocalWaker) -> bool {
        self.waker == other.waker
    }

    /// Creates a new `LocalWaker` from [`RawWaker`].
    ///
    /// The behavior of the returned `LocalWaker` is undefined if the contract defined
    /// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
    /// Therefore this method is unsafe.
    #[inline]
    #[must_use]
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
    pub const unsafe fn from_raw(waker: RawWaker) -> LocalWaker {
        Self { waker }
    }

    /// Creates a new `LocalWaker` that does nothing when `wake` is called.
    ///
    /// This is mostly useful for writing tests that need a [`Context`] to poll
    /// some futures, but are not expecting those futures to wake the waker or
    /// do not need to do anything specific if it happens.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(local_waker)]
    /// #![feature(noop_waker)]
    ///
    /// use std::future::Future;
    /// use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
    ///
    /// let mut cx = ContextBuilder::from_waker(Waker::noop())
    ///     .local_waker(LocalWaker::noop())
    ///     .build();
    ///
    /// let mut future = Box::pin(async { 10 });
    /// assert_eq!(future.as_mut().poll(&mut cx), Poll::Ready(10));
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "noop_waker", issue = "98286")]
    pub const fn noop() -> &'static LocalWaker {
        const WAKER: &LocalWaker = &LocalWaker { waker: RawWaker::NOOP };
        WAKER
    }

    /// Get a reference to the underlying [`RawWaker`].
    #[inline]
    #[must_use]
    #[unstable(feature = "waker_getters", issue = "96992")]
    pub fn as_raw(&self) -> &RawWaker {
        &self.waker
    }
}
#[unstable(feature = "local_waker", issue = "118959")]
impl Clone for LocalWaker {
    #[inline]
    fn clone(&self) -> Self {
        LocalWaker {
            // SAFETY: This is safe because `Waker::from_raw` is the only way
            // to initialize `clone` and `data` requiring the user to acknowledge
            // that the contract of [`RawWaker`] is upheld.
            waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
        }
    }

    #[inline]
    fn clone_from(&mut self, source: &Self) {
        if !self.will_wake(source) {
            *self = source.clone();
        }
    }
}

#[unstable(feature = "local_waker", issue = "118959")]
impl AsRef<LocalWaker> for Waker {
    fn as_ref(&self) -> &LocalWaker {
        // SAFETY: LocalWaker is just Waker without thread safety
        unsafe { transmute(self) }
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for LocalWaker {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
        f.debug_struct("LocalWaker")
            .field("data", &self.waker.data)
            .field("vtable", &vtable_ptr)
            .finish()
    }
}

#[unstable(feature = "local_waker", issue = "118959")]
impl !Send for LocalWaker {}
#[unstable(feature = "local_waker", issue = "118959")]
impl !Sync for LocalWaker {}