1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
//! An async multi-producer multi-consumer channel, where each message can be received by only
//! one of all existing consumers.
//!
//! There are two kinds of channels:
//!
//! 1. [Bounded][`bounded()`] channel with limited capacity.
//! 2. [Unbounded][`unbounded()`] channel with unlimited capacity.
//!
//! A channel has the [`Sender`] and [`Receiver`] side. Both sides are cloneable and can be shared
//! among multiple threads.
//!
//! When all [`Sender`]s or all [`Receiver`]s are dropped, the channel becomes closed. When a
//! channel is closed, no more messages can be sent, but remaining messages can still be received.
//!
//! The channel can also be closed manually by calling [`Sender::close()`] or
//! [`Receiver::close()`].
//!
//! # Examples
//!
//! ```
//! # futures_lite::future::block_on(async {
//! let (s, r) = async_channel::unbounded();
//!
//! assert_eq!(s.send("Hello").await, Ok(()));
//! assert_eq!(r.recv().await, Ok("Hello"));
//! # });
//! ```

#![cfg_attr(not(feature = "std"), no_std)]
#![forbid(unsafe_code)]
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
#![doc(
    html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]
#![doc(
    html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]

extern crate alloc;

use core::fmt;
use core::future::Future;
use core::marker::PhantomPinned;
use core::pin::Pin;
use core::sync::atomic::{AtomicUsize, Ordering};
use core::task::{Context, Poll};
use core::usize;

use alloc::sync::Arc;

use concurrent_queue::{ConcurrentQueue, PopError, PushError};
use event_listener::{Event, EventListener};
use event_listener_strategy::{easy_wrapper, EventListenerFuture, Strategy};
use futures_core::ready;
use futures_core::stream::Stream;
use pin_project_lite::pin_project;

struct Channel<T> {
    /// Inner message queue.
    queue: ConcurrentQueue<T>,

    /// Send operations waiting while the channel is full.
    send_ops: Event,

    /// Receive operations waiting while the channel is empty and not closed.
    recv_ops: Event,

    /// Stream operations while the channel is empty and not closed.
    stream_ops: Event,

    /// The number of currently active `Sender`s.
    sender_count: AtomicUsize,

    /// The number of currently active `Receivers`s.
    receiver_count: AtomicUsize,
}

impl<T> Channel<T> {
    /// Closes the channel and notifies all blocked operations.
    ///
    /// Returns `true` if this call has closed the channel and it was not closed already.
    fn close(&self) -> bool {
        if self.queue.close() {
            // Notify all send operations.
            self.send_ops.notify(usize::MAX);

            // Notify all receive and stream operations.
            self.recv_ops.notify(usize::MAX);
            self.stream_ops.notify(usize::MAX);

            true
        } else {
            false
        }
    }
}

/// Creates a bounded channel.
///
/// The created channel has space to hold at most `cap` messages at a time.
///
/// # Panics
///
/// Capacity must be a positive number. If `cap` is zero, this function will panic.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_channel::{bounded, TryRecvError, TrySendError};
///
/// let (s, r) = bounded(1);
///
/// assert_eq!(s.send(10).await, Ok(()));
/// assert_eq!(s.try_send(20), Err(TrySendError::Full(20)));
///
/// assert_eq!(r.recv().await, Ok(10));
/// assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
/// # });
/// ```
pub fn bounded<T>(cap: usize) -> (Sender<T>, Receiver<T>) {
    assert!(cap > 0, "capacity cannot be zero");

    let channel = Arc::new(Channel {
        queue: ConcurrentQueue::bounded(cap),
        send_ops: Event::new(),
        recv_ops: Event::new(),
        stream_ops: Event::new(),
        sender_count: AtomicUsize::new(1),
        receiver_count: AtomicUsize::new(1),
    });

    let s = Sender {
        channel: channel.clone(),
    };
    let r = Receiver {
        listener: None,
        channel,
        _pin: PhantomPinned,
    };
    (s, r)
}

/// Creates an unbounded channel.
///
/// The created channel can hold an unlimited number of messages.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_channel::{unbounded, TryRecvError};
///
/// let (s, r) = unbounded();
///
/// assert_eq!(s.send(10).await, Ok(()));
/// assert_eq!(s.send(20).await, Ok(()));
///
/// assert_eq!(r.recv().await, Ok(10));
/// assert_eq!(r.recv().await, Ok(20));
/// assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
/// # });
/// ```
pub fn unbounded<T>() -> (Sender<T>, Receiver<T>) {
    let channel = Arc::new(Channel {
        queue: ConcurrentQueue::unbounded(),
        send_ops: Event::new(),
        recv_ops: Event::new(),
        stream_ops: Event::new(),
        sender_count: AtomicUsize::new(1),
        receiver_count: AtomicUsize::new(1),
    });

    let s = Sender {
        channel: channel.clone(),
    };
    let r = Receiver {
        listener: None,
        channel,
        _pin: PhantomPinned,
    };
    (s, r)
}

/// The sending side of a channel.
///
/// Senders can be cloned and shared among threads. When all senders associated with a channel are
/// dropped, the channel becomes closed.
///
/// The channel can also be closed manually by calling [`Sender::close()`].
pub struct Sender<T> {
    /// Inner channel state.
    channel: Arc<Channel<T>>,
}

impl<T> Sender<T> {
    /// Attempts to send a message into the channel.
    ///
    /// If the channel is full or closed, this method returns an error.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_channel::{bounded, TrySendError};
    ///
    /// let (s, r) = bounded(1);
    ///
    /// assert_eq!(s.try_send(1), Ok(()));
    /// assert_eq!(s.try_send(2), Err(TrySendError::Full(2)));
    ///
    /// drop(r);
    /// assert_eq!(s.try_send(3), Err(TrySendError::Closed(3)));
    /// ```
    pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
        match self.channel.queue.push(msg) {
            Ok(()) => {
                // Notify a blocked receive operation. If the notified operation gets canceled,
                // it will notify another blocked receive operation.
                self.channel.recv_ops.notify_additional(1);

                // Notify all blocked streams.
                self.channel.stream_ops.notify(usize::MAX);

                Ok(())
            }
            Err(PushError::Full(msg)) => Err(TrySendError::Full(msg)),
            Err(PushError::Closed(msg)) => Err(TrySendError::Closed(msg)),
        }
    }

    /// Sends a message into the channel.
    ///
    /// If the channel is full, this method waits until there is space for a message.
    ///
    /// If the channel is closed, this method returns an error.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, SendError};
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert_eq!(s.send(1).await, Ok(()));
    /// drop(r);
    /// assert_eq!(s.send(2).await, Err(SendError(2)));
    /// # });
    /// ```
    pub fn send(&self, msg: T) -> Send<'_, T> {
        Send::_new(SendInner {
            sender: self,
            msg: Some(msg),
            listener: None,
            _pin: PhantomPinned,
        })
    }

    /// Sends a message into this channel using the blocking strategy.
    ///
    /// If the channel is full, this method will block until there is room.
    /// If the channel is closed, this method returns an error.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`send`](Self::send) method,
    /// this method will block the current thread until the message is sent.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a channel can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_channel::{unbounded, SendError};
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert_eq!(s.send_blocking(1), Ok(()));
    /// drop(r);
    /// assert_eq!(s.send_blocking(2), Err(SendError(2)));
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn send_blocking(&self, msg: T) -> Result<(), SendError<T>> {
        self.send(msg).wait()
    }

    /// Closes the channel.
    ///
    /// Returns `true` if this call has closed the channel and it was not closed already.
    ///
    /// The remaining messages can still be received.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded();
    /// assert_eq!(s.send(1).await, Ok(()));
    /// assert!(s.close());
    ///
    /// assert_eq!(r.recv().await, Ok(1));
    /// assert_eq!(r.recv().await, Err(RecvError));
    /// # });
    /// ```
    pub fn close(&self) -> bool {
        self.channel.close()
    }

    /// Returns `true` if the channel is closed.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert!(!s.is_closed());
    ///
    /// drop(r);
    /// assert!(s.is_closed());
    /// # });
    /// ```
    pub fn is_closed(&self) -> bool {
        self.channel.queue.is_closed()
    }

    /// Returns `true` if the channel is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert!(s.is_empty());
    /// s.send(1).await;
    /// assert!(!s.is_empty());
    /// # });
    /// ```
    pub fn is_empty(&self) -> bool {
        self.channel.queue.is_empty()
    }

    /// Returns `true` if the channel is full.
    ///
    /// Unbounded channels are never full.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::bounded;
    ///
    /// let (s, r) = bounded(1);
    ///
    /// assert!(!s.is_full());
    /// s.send(1).await;
    /// assert!(s.is_full());
    /// # });
    /// ```
    pub fn is_full(&self) -> bool {
        self.channel.queue.is_full()
    }

    /// Returns the number of messages in the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded();
    /// assert_eq!(s.len(), 0);
    ///
    /// s.send(1).await;
    /// s.send(2).await;
    /// assert_eq!(s.len(), 2);
    /// # });
    /// ```
    pub fn len(&self) -> usize {
        self.channel.queue.len()
    }

    /// Returns the channel capacity if it's bounded.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_channel::{bounded, unbounded};
    ///
    /// let (s, r) = bounded::<i32>(5);
    /// assert_eq!(s.capacity(), Some(5));
    ///
    /// let (s, r) = unbounded::<i32>();
    /// assert_eq!(s.capacity(), None);
    /// ```
    pub fn capacity(&self) -> Option<usize> {
        self.channel.queue.capacity()
    }

    /// Returns the number of receivers for the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert_eq!(s.receiver_count(), 1);
    ///
    /// let r2 = r.clone();
    /// assert_eq!(s.receiver_count(), 2);
    /// # });
    /// ```
    pub fn receiver_count(&self) -> usize {
        self.channel.receiver_count.load(Ordering::SeqCst)
    }

    /// Returns the number of senders for the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert_eq!(s.sender_count(), 1);
    ///
    /// let s2 = s.clone();
    /// assert_eq!(s.sender_count(), 2);
    /// # });
    /// ```
    pub fn sender_count(&self) -> usize {
        self.channel.sender_count.load(Ordering::SeqCst)
    }

    /// Downgrade the sender to a weak reference.
    pub fn downgrade(&self) -> WeakSender<T> {
        WeakSender {
            channel: self.channel.clone(),
        }
    }
}

impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        // Decrement the sender count and close the channel if it drops down to zero.
        if self.channel.sender_count.fetch_sub(1, Ordering::AcqRel) == 1 {
            self.channel.close();
        }
    }
}

impl<T> fmt::Debug for Sender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Sender {{ .. }}")
    }
}

impl<T> Clone for Sender<T> {
    fn clone(&self) -> Sender<T> {
        let count = self.channel.sender_count.fetch_add(1, Ordering::Relaxed);

        // Make sure the count never overflows, even if lots of sender clones are leaked.
        if count > usize::MAX / 2 {
            abort();
        }

        Sender {
            channel: self.channel.clone(),
        }
    }
}

pin_project! {
    /// The receiving side of a channel.
    ///
    /// Receivers can be cloned and shared among threads. When all receivers associated with a channel
    /// are dropped, the channel becomes closed.
    ///
    /// The channel can also be closed manually by calling [`Receiver::close()`].
    ///
    /// Receivers implement the [`Stream`] trait.
    pub struct Receiver<T> {
        // Inner channel state.
        channel: Arc<Channel<T>>,

        // Listens for a send or close event to unblock this stream.
        listener: Option<EventListener>,

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

    impl<T> PinnedDrop for Receiver<T> {
        fn drop(this: Pin<&mut Self>) {
            let this = this.project();

            // Decrement the receiver count and close the channel if it drops down to zero.
            if this.channel.receiver_count.fetch_sub(1, Ordering::AcqRel) == 1 {
                this.channel.close();
            }
        }
    }
}

impl<T> Receiver<T> {
    /// Attempts to receive a message from the channel.
    ///
    /// If the channel is empty, or empty and closed, this method returns an error.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, TryRecvError};
    ///
    /// let (s, r) = unbounded();
    /// assert_eq!(s.send(1).await, Ok(()));
    ///
    /// assert_eq!(r.try_recv(), Ok(1));
    /// assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
    ///
    /// drop(s);
    /// assert_eq!(r.try_recv(), Err(TryRecvError::Closed));
    /// # });
    /// ```
    pub fn try_recv(&self) -> Result<T, TryRecvError> {
        match self.channel.queue.pop() {
            Ok(msg) => {
                // Notify a blocked send operation. If the notified operation gets canceled, it
                // will notify another blocked send operation.
                self.channel.send_ops.notify_additional(1);

                Ok(msg)
            }
            Err(PopError::Empty) => Err(TryRecvError::Empty),
            Err(PopError::Closed) => Err(TryRecvError::Closed),
        }
    }

    /// Receives a message from the channel.
    ///
    /// If the channel is empty, this method waits until there is a message.
    ///
    /// If the channel is closed, this method receives a message or returns an error if there are
    /// no more messages.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert_eq!(s.send(1).await, Ok(()));
    /// drop(s);
    ///
    /// assert_eq!(r.recv().await, Ok(1));
    /// assert_eq!(r.recv().await, Err(RecvError));
    /// # });
    /// ```
    pub fn recv(&self) -> Recv<'_, T> {
        Recv::_new(RecvInner {
            receiver: self,
            listener: None,
            _pin: PhantomPinned,
        })
    }

    /// Receives a message from the channel using the blocking strategy.
    ///
    /// If the channel is empty, this method waits until there is a message.
    /// If the channel is closed, this method receives a message or returns an error if there are
    /// no more messages.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`recv`](Self::recv) method,
    /// this method will block the current thread until the message is sent.
    ///
    /// This method should not be used in an asynchronous context. It is intended
    /// to be used such that a channel can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in deadlocks.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert_eq!(s.send_blocking(1), Ok(()));
    /// drop(s);
    ///
    /// assert_eq!(r.recv_blocking(), Ok(1));
    /// assert_eq!(r.recv_blocking(), Err(RecvError));
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub fn recv_blocking(&self) -> Result<T, RecvError> {
        self.recv().wait()
    }

    /// Closes the channel.
    ///
    /// Returns `true` if this call has closed the channel and it was not closed already.
    ///
    /// The remaining messages can still be received.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded();
    /// assert_eq!(s.send(1).await, Ok(()));
    ///
    /// assert!(r.close());
    /// assert_eq!(r.recv().await, Ok(1));
    /// assert_eq!(r.recv().await, Err(RecvError));
    /// # });
    /// ```
    pub fn close(&self) -> bool {
        self.channel.close()
    }

    /// Returns `true` if the channel is closed.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::{unbounded, RecvError};
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert!(!r.is_closed());
    ///
    /// drop(s);
    /// assert!(r.is_closed());
    /// # });
    /// ```
    pub fn is_closed(&self) -> bool {
        self.channel.queue.is_closed()
    }

    /// Returns `true` if the channel is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded();
    ///
    /// assert!(s.is_empty());
    /// s.send(1).await;
    /// assert!(!s.is_empty());
    /// # });
    /// ```
    pub fn is_empty(&self) -> bool {
        self.channel.queue.is_empty()
    }

    /// Returns `true` if the channel is full.
    ///
    /// Unbounded channels are never full.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::bounded;
    ///
    /// let (s, r) = bounded(1);
    ///
    /// assert!(!r.is_full());
    /// s.send(1).await;
    /// assert!(r.is_full());
    /// # });
    /// ```
    pub fn is_full(&self) -> bool {
        self.channel.queue.is_full()
    }

    /// Returns the number of messages in the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded();
    /// assert_eq!(r.len(), 0);
    ///
    /// s.send(1).await;
    /// s.send(2).await;
    /// assert_eq!(r.len(), 2);
    /// # });
    /// ```
    pub fn len(&self) -> usize {
        self.channel.queue.len()
    }

    /// Returns the channel capacity if it's bounded.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_channel::{bounded, unbounded};
    ///
    /// let (s, r) = bounded::<i32>(5);
    /// assert_eq!(r.capacity(), Some(5));
    ///
    /// let (s, r) = unbounded::<i32>();
    /// assert_eq!(r.capacity(), None);
    /// ```
    pub fn capacity(&self) -> Option<usize> {
        self.channel.queue.capacity()
    }

    /// Returns the number of receivers for the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert_eq!(r.receiver_count(), 1);
    ///
    /// let r2 = r.clone();
    /// assert_eq!(r.receiver_count(), 2);
    /// # });
    /// ```
    pub fn receiver_count(&self) -> usize {
        self.channel.receiver_count.load(Ordering::SeqCst)
    }

    /// Returns the number of senders for the channel.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_channel::unbounded;
    ///
    /// let (s, r) = unbounded::<()>();
    /// assert_eq!(r.sender_count(), 1);
    ///
    /// let s2 = s.clone();
    /// assert_eq!(r.sender_count(), 2);
    /// # });
    /// ```
    pub fn sender_count(&self) -> usize {
        self.channel.sender_count.load(Ordering::SeqCst)
    }

    /// Downgrade the receiver to a weak reference.
    pub fn downgrade(&self) -> WeakReceiver<T> {
        WeakReceiver {
            channel: self.channel.clone(),
        }
    }
}

impl<T> fmt::Debug for Receiver<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Receiver {{ .. }}")
    }
}

impl<T> Clone for Receiver<T> {
    fn clone(&self) -> Receiver<T> {
        let count = self.channel.receiver_count.fetch_add(1, Ordering::Relaxed);

        // Make sure the count never overflows, even if lots of receiver clones are leaked.
        if count > usize::MAX / 2 {
            abort();
        }

        Receiver {
            channel: self.channel.clone(),
            listener: None,
            _pin: PhantomPinned,
        }
    }
}

impl<T> Stream for Receiver<T> {
    type Item = T;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        loop {
            // If this stream is listening for events, first wait for a notification.
            {
                let this = self.as_mut().project();
                if let Some(listener) = this.listener.as_mut() {
                    ready!(Pin::new(listener).poll(cx));
                    *this.listener = None;
                }
            }

            loop {
                // Attempt to receive a message.
                match self.try_recv() {
                    Ok(msg) => {
                        // The stream is not blocked on an event - drop the listener.
                        let this = self.as_mut().project();
                        *this.listener = None;
                        return Poll::Ready(Some(msg));
                    }
                    Err(TryRecvError::Closed) => {
                        // The stream is not blocked on an event - drop the listener.
                        let this = self.as_mut().project();
                        *this.listener = None;
                        return Poll::Ready(None);
                    }
                    Err(TryRecvError::Empty) => {}
                }

                // Receiving failed - now start listening for notifications or wait for one.
                let this = self.as_mut().project();
                if this.listener.is_some() {
                    // Go back to the outer loop to wait for a notification.
                    break;
                } else {
                    *this.listener = Some(this.channel.stream_ops.listen());
                }
            }
        }
    }
}

impl<T> futures_core::stream::FusedStream for Receiver<T> {
    fn is_terminated(&self) -> bool {
        self.channel.queue.is_closed() && self.channel.queue.is_empty()
    }
}

/// A [`Sender`] that prevents the channel from not being closed.
///
/// This is created through the [`Sender::downgrade`] method. In order to use it, it needs
/// to be upgraded into a [`Sender`] through the `upgrade` method.
pub struct WeakSender<T> {
    channel: Arc<Channel<T>>,
}

impl<T> WeakSender<T> {
    /// Upgrade the [`WeakSender`] into a [`Sender`].
    pub fn upgrade(&self) -> Option<Sender<T>> {
        if self.channel.queue.is_closed() {
            None
        } else {
            match self.channel.sender_count.fetch_update(
                Ordering::Relaxed,
                Ordering::Relaxed,
                |count| if count == 0 { None } else { Some(count + 1) },
            ) {
                Err(_) => None,
                Ok(new_value) if new_value > usize::MAX / 2 => {
                    // Make sure the count never overflows, even if lots of sender clones are leaked.
                    abort();
                }
                Ok(_) => Some(Sender {
                    channel: self.channel.clone(),
                }),
            }
        }
    }
}

impl<T> Clone for WeakSender<T> {
    fn clone(&self) -> Self {
        WeakSender {
            channel: self.channel.clone(),
        }
    }
}

impl<T> fmt::Debug for WeakSender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "WeakSender {{ .. }}")
    }
}

/// A [`Receiver`] that prevents the channel from not being closed.
///
/// This is created through the [`Receiver::downgrade`] method. In order to use it, it needs
/// to be upgraded into a [`Receiver`] through the `upgrade` method.
pub struct WeakReceiver<T> {
    channel: Arc<Channel<T>>,
}

impl<T> WeakReceiver<T> {
    /// Upgrade the [`WeakReceiver`] into a [`Receiver`].
    pub fn upgrade(&self) -> Option<Receiver<T>> {
        if self.channel.queue.is_closed() {
            None
        } else {
            match self.channel.receiver_count.fetch_update(
                Ordering::Relaxed,
                Ordering::Relaxed,
                |count| if count == 0 { None } else { Some(count + 1) },
            ) {
                Err(_) => None,
                Ok(new_value) if new_value > usize::MAX / 2 => {
                    // Make sure the count never overflows, even if lots of receiver clones are leaked.
                    abort();
                }
                Ok(_) => Some(Receiver {
                    channel: self.channel.clone(),
                    listener: None,
                    _pin: PhantomPinned,
                }),
            }
        }
    }
}

impl<T> Clone for WeakReceiver<T> {
    fn clone(&self) -> Self {
        WeakReceiver {
            channel: self.channel.clone(),
        }
    }
}

impl<T> fmt::Debug for WeakReceiver<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "WeakReceiver {{ .. }}")
    }
}

/// An error returned from [`Sender::send()`].
///
/// Received because the channel is closed.
#[derive(PartialEq, Eq, Clone, Copy)]
pub struct SendError<T>(pub T);

impl<T> SendError<T> {
    /// Unwraps the message that couldn't be sent.
    pub fn into_inner(self) -> T {
        self.0
    }
}

#[cfg(feature = "std")]
impl<T> std::error::Error for SendError<T> {}

impl<T> fmt::Debug for SendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "SendError(..)")
    }
}

impl<T> fmt::Display for SendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "sending into a closed channel")
    }
}

/// An error returned from [`Sender::try_send()`].
#[derive(PartialEq, Eq, Clone, Copy)]
pub enum TrySendError<T> {
    /// The channel is full but not closed.
    Full(T),

    /// The channel is closed.
    Closed(T),
}

impl<T> TrySendError<T> {
    /// Unwraps the message that couldn't be sent.
    pub fn into_inner(self) -> T {
        match self {
            TrySendError::Full(t) => t,
            TrySendError::Closed(t) => t,
        }
    }

    /// Returns `true` if the channel is full but not closed.
    pub fn is_full(&self) -> bool {
        match self {
            TrySendError::Full(_) => true,
            TrySendError::Closed(_) => false,
        }
    }

    /// Returns `true` if the channel is closed.
    pub fn is_closed(&self) -> bool {
        match self {
            TrySendError::Full(_) => false,
            TrySendError::Closed(_) => true,
        }
    }
}

#[cfg(feature = "std")]
impl<T> std::error::Error for TrySendError<T> {}

impl<T> fmt::Debug for TrySendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TrySendError::Full(..) => write!(f, "Full(..)"),
            TrySendError::Closed(..) => write!(f, "Closed(..)"),
        }
    }
}

impl<T> fmt::Display for TrySendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TrySendError::Full(..) => write!(f, "sending into a full channel"),
            TrySendError::Closed(..) => write!(f, "sending into a closed channel"),
        }
    }
}

/// An error returned from [`Receiver::recv()`].
///
/// Received because the channel is empty and closed.
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
pub struct RecvError;

#[cfg(feature = "std")]
impl std::error::Error for RecvError {}

impl fmt::Display for RecvError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "receiving from an empty and closed channel")
    }
}

/// An error returned from [`Receiver::try_recv()`].
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
pub enum TryRecvError {
    /// The channel is empty but not closed.
    Empty,

    /// The channel is empty and closed.
    Closed,
}

impl TryRecvError {
    /// Returns `true` if the channel is empty but not closed.
    pub fn is_empty(&self) -> bool {
        match self {
            TryRecvError::Empty => true,
            TryRecvError::Closed => false,
        }
    }

    /// Returns `true` if the channel is empty and closed.
    pub fn is_closed(&self) -> bool {
        match self {
            TryRecvError::Empty => false,
            TryRecvError::Closed => true,
        }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for TryRecvError {}

impl fmt::Display for TryRecvError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TryRecvError::Empty => write!(f, "receiving from an empty channel"),
            TryRecvError::Closed => write!(f, "receiving from an empty and closed channel"),
        }
    }
}

easy_wrapper! {
    /// A future returned by [`Sender::send()`].
    #[derive(Debug)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct Send<'a, T>(SendInner<'a, T> => Result<(), SendError<T>>);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project! {
    #[derive(Debug)]
    #[project(!Unpin)]
    struct SendInner<'a, T> {
        // Reference to the original sender.
        sender: &'a Sender<T>,

        // The message to send.
        msg: Option<T>,

        // Listener waiting on the channel.
        listener: Option<EventListener>,

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

impl<'a, T> EventListenerFuture for SendInner<'a, T> {
    type Output = Result<(), SendError<T>>;

    /// Run this future with the given `Strategy`.
    fn poll_with_strategy<'x, S: Strategy<'x>>(
        self: Pin<&mut Self>,
        strategy: &mut S,
        context: &mut S::Context,
    ) -> Poll<Result<(), SendError<T>>> {
        let this = self.project();

        loop {
            let msg = this.msg.take().unwrap();
            // Attempt to send a message.
            match this.sender.try_send(msg) {
                Ok(()) => return Poll::Ready(Ok(())),
                Err(TrySendError::Closed(msg)) => return Poll::Ready(Err(SendError(msg))),
                Err(TrySendError::Full(m)) => *this.msg = Some(m),
            }

            // Sending failed - now start listening for notifications or wait for one.
            if this.listener.is_some() {
                // Poll using the given strategy
                ready!(S::poll(strategy, &mut *this.listener, context));
            } else {
                *this.listener = Some(this.sender.channel.send_ops.listen());
            }
        }
    }
}

easy_wrapper! {
    /// A future returned by [`Receiver::recv()`].
    #[derive(Debug)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct Recv<'a, T>(RecvInner<'a, T> => Result<T, RecvError>);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project! {
    #[derive(Debug)]
    #[project(!Unpin)]
    struct RecvInner<'a, T> {
        // Reference to the receiver.
        receiver: &'a Receiver<T>,

        // Listener waiting on the channel.
        listener: Option<EventListener>,

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

impl<'a, T> EventListenerFuture for RecvInner<'a, T> {
    type Output = Result<T, RecvError>;

    /// Run this future with the given `Strategy`.
    fn poll_with_strategy<'x, S: Strategy<'x>>(
        self: Pin<&mut Self>,
        strategy: &mut S,
        cx: &mut S::Context,
    ) -> Poll<Result<T, RecvError>> {
        let this = self.project();

        loop {
            // Attempt to receive a message.
            match this.receiver.try_recv() {
                Ok(msg) => return Poll::Ready(Ok(msg)),
                Err(TryRecvError::Closed) => return Poll::Ready(Err(RecvError)),
                Err(TryRecvError::Empty) => {}
            }

            // Receiving failed - now start listening for notifications or wait for one.
            if this.listener.is_some() {
                // Poll using the given strategy
                ready!(S::poll(strategy, &mut *this.listener, cx));
            } else {
                *this.listener = Some(this.receiver.channel.recv_ops.listen());
            }
        }
    }
}

#[cfg(feature = "std")]
use std::process::abort;

#[cfg(not(feature = "std"))]
fn abort() -> ! {
    struct PanicOnDrop;

    impl Drop for PanicOnDrop {
        fn drop(&mut self) {
            panic!("Panic while panicking to abort");
        }
    }

    let _bomb = PanicOnDrop;
    panic!("Panic while panicking to abort")
}