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
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
//! Multi-producer, single-consumer FIFO queue communication primitives.
//!
//! This module provides message-based communication over channels, concretely
//! defined among three types:
//!
//! * [`Sender`]
//! * [`SyncSender`]
//! * [`Receiver`]
//!
//! A [`Sender`] or [`SyncSender`] is used to send data to a [`Receiver`]. Both
//! senders are clone-able (multi-producer) such that many threads can send
//! simultaneously to one receiver (single-consumer).
//!
//! These channels come in two flavors:
//!
//! 1. An asynchronous, infinitely buffered channel. The [`channel`] function
//!    will return a `(Sender, Receiver)` tuple where all sends will be
//!    **asynchronous** (they never block). The channel conceptually has an
//!    infinite buffer.
//!
//! 2. A synchronous, bounded channel. The [`sync_channel`] function will
//!    return a `(SyncSender, Receiver)` tuple where the storage for pending
//!    messages is a pre-allocated buffer of a fixed size. All sends will be
//!    **synchronous** by blocking until there is buffer space available. Note
//!    that a bound of 0 is allowed, causing the channel to become a "rendezvous"
//!    channel where each sender atomically hands off a message to a receiver.
//!
//! [`send`]: Sender::send
//!
//! ## Disconnection
//!
//! The send and receive operations on channels will all return a [`Result`]
//! indicating whether the operation succeeded or not. An unsuccessful operation
//! is normally indicative of the other half of a channel having "hung up" by
//! being dropped in its corresponding thread.
//!
//! Once half of a channel has been deallocated, most operations can no longer
//! continue to make progress, so [`Err`] will be returned. Many applications
//! will continue to [`unwrap`] the results returned from this module,
//! instigating a propagation of failure among threads if one unexpectedly dies.
//!
//! [`unwrap`]: Result::unwrap
//!
//! # Examples
//!
//! Simple usage:
//!
//! ```
//! use std::thread;
//! use std::sync::mpsc::channel;
//!
//! // Create a simple streaming channel
//! let (tx, rx) = channel();
//! thread::spawn(move|| {
//!     tx.send(10).unwrap();
//! });
//! assert_eq!(rx.recv().unwrap(), 10);
//! ```
//!
//! Shared usage:
//!
//! ```
//! use std::thread;
//! use std::sync::mpsc::channel;
//!
//! // Create a shared channel that can be sent along from many threads
//! // where tx is the sending half (tx for transmission), and rx is the receiving
//! // half (rx for receiving).
//! let (tx, rx) = channel();
//! for i in 0..10 {
//!     let tx = tx.clone();
//!     thread::spawn(move|| {
//!         tx.send(i).unwrap();
//!     });
//! }
//!
//! for _ in 0..10 {
//!     let j = rx.recv().unwrap();
//!     assert!(0 <= j && j < 10);
//! }
//! ```
//!
//! Propagating panics:
//!
//! ```
//! use std::sync::mpsc::channel;
//!
//! // The call to recv() will return an error because the channel has already
//! // hung up (or been deallocated)
//! let (tx, rx) = channel::<i32>();
//! drop(tx);
//! assert!(rx.recv().is_err());
//! ```
//!
//! Synchronous channels:
//!
//! ```
//! use std::thread;
//! use std::sync::mpsc::sync_channel;
//!
//! let (tx, rx) = sync_channel::<i32>(0);
//! thread::spawn(move|| {
//!     // This will wait for the parent thread to start receiving
//!     tx.send(53).unwrap();
//! });
//! rx.recv().unwrap();
//! ```
//!
//! Unbounded receive loop:
//!
//! ```
//! use std::sync::mpsc::sync_channel;
//! use std::thread;
//!
//! let (tx, rx) = sync_channel(3);
//!
//! for _ in 0..3 {
//!     // It would be the same without thread and clone here
//!     // since there will still be one `tx` left.
//!     let tx = tx.clone();
//!     // cloned tx dropped within thread
//!     thread::spawn(move || tx.send("ok").unwrap());
//! }
//!
//! // Drop the last sender to stop `rx` waiting for message.
//! // The program will not complete if we comment this out.
//! // **All** `tx` needs to be dropped for `rx` to have `Err`.
//! drop(tx);
//!
//! // Unbounded receiver waiting for all senders to complete.
//! while let Ok(msg) = rx.recv() {
//!     println!("{}", msg);
//! }
//!
//! println!("completed");
//! ```

#![stable(feature = "rust1", since = "1.0.0")]

#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;

#[cfg(all(test, not(target_os = "emscripten")))]
mod sync_tests;

// A description of how Rust's channel implementation works
//
// Channels are supposed to be the basic building block for all other
// concurrent primitives that are used in Rust. As a result, the channel type
// needs to be highly optimized, flexible, and broad enough for use everywhere.
//
// The choice of implementation of all channels is to be built on lock-free data
// structures. The channels themselves are then consequently also lock-free data
// structures. As always with lock-free code, this is a very "here be dragons"
// territory, especially because I'm unaware of any academic papers that have
// gone into great length about channels of these flavors.
//
// ## Flavors of channels
//
// From the perspective of a consumer of this library, there is only one flavor
// of channel. This channel can be used as a stream and cloned to allow multiple
// senders. Under the hood, however, there are actually three flavors of
// channels in play.
//
// * Flavor::Oneshots - these channels are highly optimized for the one-send use
//                      case. They contain as few atomics as possible and
//                      involve one and exactly one allocation.
// * Streams - these channels are optimized for the non-shared use case. They
//             use a different concurrent queue that is more tailored for this
//             use case. The initial allocation of this flavor of channel is not
//             optimized.
// * Shared - this is the most general form of channel that this module offers,
//            a channel with multiple senders. This type is as optimized as it
//            can be, but the previous two types mentioned are much faster for
//            their use-cases.
//
// ## Concurrent queues
//
// The basic idea of Rust's Sender/Receiver types is that send() never blocks,
// but recv() obviously blocks. This means that under the hood there must be
// some shared and concurrent queue holding all of the actual data.
//
// With two flavors of channels, two flavors of queues are also used. We have
// chosen to use queues from a well-known author that are abbreviated as SPSC
// and MPSC (single producer, single consumer and multiple producer, single
// consumer). SPSC queues are used for streams while MPSC queues are used for
// shared channels.
//
// ### SPSC optimizations
//
// The SPSC queue found online is essentially a linked list of nodes where one
// half of the nodes are the "queue of data" and the other half of nodes are a
// cache of unused nodes. The unused nodes are used such that an allocation is
// not required on every push() and a free doesn't need to happen on every
// pop().
//
// As found online, however, the cache of nodes is of an infinite size. This
// means that if a channel at one point in its life had 50k items in the queue,
// then the queue will always have the capacity for 50k items. I believed that
// this was an unnecessary limitation of the implementation, so I have altered
// the queue to optionally have a bound on the cache size.
//
// By default, streams will have an unbounded SPSC queue with a small-ish cache
// size. The hope is that the cache is still large enough to have very fast
// send() operations while not too large such that millions of channels can
// coexist at once.
//
// ### MPSC optimizations
//
// Right now the MPSC queue has not been optimized. Like the SPSC queue, it uses
// a linked list under the hood to earn its unboundedness, but I have not put
// forth much effort into having a cache of nodes similar to the SPSC queue.
//
// For now, I believe that this is "ok" because shared channels are not the most
// common type, but soon we may wish to revisit this queue choice and determine
// another candidate for backend storage of shared channels.
//
// ## Overview of the Implementation
//
// Now that there's a little background on the concurrent queues used, it's
// worth going into much more detail about the channels themselves. The basic
// pseudocode for a send/recv are:
//
//
//      send(t)                             recv()
//        queue.push(t)                       return if queue.pop()
//        if increment() == -1                deschedule {
//          wakeup()                            if decrement() > 0
//                                                cancel_deschedule()
//                                            }
//                                            queue.pop()
//
// As mentioned before, there are no locks in this implementation, only atomic
// instructions are used.
//
// ### The internal atomic counter
//
// Every channel has a shared counter with each half to keep track of the size
// of the queue. This counter is used to abort descheduling by the receiver and
// to know when to wake up on the sending side.
//
// As seen in the pseudocode, senders will increment this count and receivers
// will decrement the count. The theory behind this is that if a sender sees a
// -1 count, it will wake up the receiver, and if the receiver sees a 1+ count,
// then it doesn't need to block.
//
// The recv() method has a beginning call to pop(), and if successful, it needs
// to decrement the count. It is a crucial implementation detail that this
// decrement does *not* happen to the shared counter. If this were the case,
// then it would be possible for the counter to be very negative when there were
// no receivers waiting, in which case the senders would have to determine when
// it was actually appropriate to wake up a receiver.
//
// Instead, the "steal count" is kept track of separately (not atomically
// because it's only used by receivers), and then the decrement() call when
// descheduling will lump in all of the recent steals into one large decrement.
//
// The implication of this is that if a sender sees a -1 count, then there's
// guaranteed to be a waiter waiting!
//
// ## Native Implementation
//
// A major goal of these channels is to work seamlessly on and off the runtime.
// All of the previous race conditions have been worded in terms of
// scheduler-isms (which is obviously not available without the runtime).
//
// For now, native usage of channels (off the runtime) will fall back onto
// mutexes/cond vars for descheduling/atomic decisions. The no-contention path
// is still entirely lock-free, the "deschedule" blocks above are surrounded by
// a mutex and the "wakeup" blocks involve grabbing a mutex and signaling on a
// condition variable.
//
// ## Select
//
// Being able to support selection over channels has greatly influenced this
// design, and not only does selection need to work inside the runtime, but also
// outside the runtime.
//
// The implementation is fairly straightforward. The goal of select() is not to
// return some data, but only to return which channel can receive data without
// blocking. The implementation is essentially the entire blocking procedure
// followed by an increment as soon as its woken up. The cancellation procedure
// involves an increment and swapping out of to_wake to acquire ownership of the
// thread to unblock.
//
// Sadly this current implementation requires multiple allocations, so I have
// seen the throughput of select() be much worse than it should be. I do not
// believe that there is anything fundamental that needs to change about these
// channels, however, in order to support a more efficient select().
//
// FIXME: Select is now removed, so these factors are ready to be cleaned up!
//
// # Conclusion
//
// And now that you've seen all the races that I found and attempted to fix,
// here's the code for you to find some more!

use crate::cell::UnsafeCell;
use crate::error;
use crate::fmt;
use crate::mem;
use crate::sync::Arc;
use crate::time::{Duration, Instant};

mod blocking;
mod mpsc_queue;
mod oneshot;
mod shared;
mod spsc_queue;
mod stream;
mod sync;

mod cache_aligned;

/// The receiving half of Rust's [`channel`] (or [`sync_channel`]) type.
/// This half can only be owned by one thread.
///
/// Messages sent to the channel can be retrieved using [`recv`].
///
/// [`recv`]: Receiver::recv
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
/// use std::time::Duration;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send("Hello world!").unwrap();
///     thread::sleep(Duration::from_secs(2)); // block for two seconds
///     send.send("Delayed for 2 seconds").unwrap();
/// });
///
/// println!("{}", recv.recv().unwrap()); // Received immediately
/// println!("Waiting...");
/// println!("{}", recv.recv().unwrap()); // Received after 2 seconds
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Receiver")]
pub struct Receiver<T> {
    inner: UnsafeCell<Flavor<T>>,
}

// The receiver port can be sent from place to place, so long as it
// is not used to receive non-sendable things.
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for Receiver<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> !Sync for Receiver<T> {}

/// An iterator over messages on a [`Receiver`], created by [`iter`].
///
/// This iterator will block whenever [`next`] is called,
/// waiting for a new message, and [`None`] will be returned
/// when the corresponding channel has hung up.
///
/// [`iter`]: Receiver::iter
/// [`next`]: Iterator::next
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send(1u8).unwrap();
///     send.send(2u8).unwrap();
///     send.send(3u8).unwrap();
/// });
///
/// for x in recv.iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Iter<'a, T: 'a> {
    rx: &'a Receiver<T>,
}

/// An iterator that attempts to yield all pending values for a [`Receiver`],
/// created by [`try_iter`].
///
/// [`None`] will be returned when there are no pending values remaining or
/// if the corresponding channel has hung up.
///
/// This iterator will never block the caller in order to wait for data to
/// become available. Instead, it will return [`None`].
///
/// [`try_iter`]: Receiver::try_iter
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
/// use std::time::Duration;
///
/// let (sender, receiver) = channel();
///
/// // Nothing is in the buffer yet
/// assert!(receiver.try_iter().next().is_none());
/// println!("Nothing in the buffer...");
///
/// thread::spawn(move || {
///     sender.send(1).unwrap();
///     sender.send(2).unwrap();
///     sender.send(3).unwrap();
/// });
///
/// println!("Going to sleep...");
/// thread::sleep(Duration::from_secs(2)); // block for two seconds
///
/// for x in receiver.try_iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "receiver_try_iter", since = "1.15.0")]
#[derive(Debug)]
pub struct TryIter<'a, T: 'a> {
    rx: &'a Receiver<T>,
}

/// An owning iterator over messages on a [`Receiver`],
/// created by [`into_iter`].
///
/// This iterator will block whenever [`next`]
/// is called, waiting for a new message, and [`None`] will be
/// returned if the corresponding channel has hung up.
///
/// [`into_iter`]: Receiver::into_iter
/// [`next`]: Iterator::next
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send(1u8).unwrap();
///     send.send(2u8).unwrap();
///     send.send(3u8).unwrap();
/// });
///
/// for x in recv.into_iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "receiver_into_iter", since = "1.1.0")]
#[derive(Debug)]
pub struct IntoIter<T> {
    rx: Receiver<T>,
}

/// The sending-half of Rust's asynchronous [`channel`] type. This half can only be
/// owned by one thread, but it can be cloned to send to other threads.
///
/// Messages can be sent through this channel with [`send`].
///
/// Note: all senders (the original and the clones) need to be dropped for the receiver
/// to stop blocking to receive messages with [`Receiver::recv`].
///
/// [`send`]: Sender::send
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (sender, receiver) = channel();
/// let sender2 = sender.clone();
///
/// // First thread owns sender
/// thread::spawn(move || {
///     sender.send(1).unwrap();
/// });
///
/// // Second thread owns sender2
/// thread::spawn(move || {
///     sender2.send(2).unwrap();
/// });
///
/// let msg = receiver.recv().unwrap();
/// let msg2 = receiver.recv().unwrap();
///
/// assert_eq!(3, msg + msg2);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Sender<T> {
    inner: UnsafeCell<Flavor<T>>,
}

// The send port can be sent from place to place, so long as it
// is not used to send non-sendable things.
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for Sender<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> !Sync for Sender<T> {}

/// The sending-half of Rust's synchronous [`sync_channel`] type.
///
/// Messages can be sent through this channel with [`send`] or [`try_send`].
///
/// [`send`] will block if there is no space in the internal buffer.
///
/// [`send`]: SyncSender::send
/// [`try_send`]: SyncSender::try_send
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::sync_channel;
/// use std::thread;
///
/// // Create a sync_channel with buffer size 2
/// let (sync_sender, receiver) = sync_channel(2);
/// let sync_sender2 = sync_sender.clone();
///
/// // First thread owns sync_sender
/// thread::spawn(move || {
///     sync_sender.send(1).unwrap();
///     sync_sender.send(2).unwrap();
/// });
///
/// // Second thread owns sync_sender2
/// thread::spawn(move || {
///     sync_sender2.send(3).unwrap();
///     // thread will now block since the buffer is full
///     println!("Thread unblocked!");
/// });
///
/// let mut msg;
///
/// msg = receiver.recv().unwrap();
/// println!("message {} received", msg);
///
/// // "Thread unblocked!" will be printed now
///
/// msg = receiver.recv().unwrap();
/// println!("message {} received", msg);
///
/// msg = receiver.recv().unwrap();
///
/// println!("message {} received", msg);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SyncSender<T> {
    inner: Arc<sync::Packet<T>>,
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for SyncSender<T> {}

/// An error returned from the [`Sender::send`] or [`SyncSender::send`]
/// function on **channel**s.
///
/// A **send** operation can only fail if the receiving end of a channel is
/// disconnected, implying that the data could never be received. The error
/// contains the data being sent as a payload so it can be recovered.
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(PartialEq, Eq, Clone, Copy)]
pub struct SendError<T>(#[stable(feature = "rust1", since = "1.0.0")] pub T);

/// An error returned from the [`recv`] function on a [`Receiver`].
///
/// The [`recv`] operation can only fail if the sending half of a
/// [`channel`] (or [`sync_channel`]) is disconnected, implying that no further
/// messages will ever be received.
///
/// [`recv`]: Receiver::recv
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RecvError;

/// This enumeration is the list of the possible reasons that [`try_recv`] could
/// not return data when called. This can occur with both a [`channel`] and
/// a [`sync_channel`].
///
/// [`try_recv`]: Receiver::try_recv
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum TryRecvError {
    /// This **channel** is currently empty, but the **Sender**(s) have not yet
    /// disconnected, so data may yet become available.
    #[stable(feature = "rust1", since = "1.0.0")]
    Empty,

    /// The **channel**'s sending half has become disconnected, and there will
    /// never be any more data received on it.
    #[stable(feature = "rust1", since = "1.0.0")]
    Disconnected,
}

/// This enumeration is the list of possible errors that made [`recv_timeout`]
/// unable to return data when called. This can occur with both a [`channel`] and
/// a [`sync_channel`].
///
/// [`recv_timeout`]: Receiver::recv_timeout
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
#[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
pub enum RecvTimeoutError {
    /// This **channel** is currently empty, but the **Sender**(s) have not yet
    /// disconnected, so data may yet become available.
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    Timeout,
    /// The **channel**'s sending half has become disconnected, and there will
    /// never be any more data received on it.
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    Disconnected,
}

/// This enumeration is the list of the possible error outcomes for the
/// [`try_send`] method.
///
/// [`try_send`]: SyncSender::try_send
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(PartialEq, Eq, Clone, Copy)]
pub enum TrySendError<T> {
    /// The data could not be sent on the [`sync_channel`] because it would require that
    /// the callee block to send the data.
    ///
    /// If this is a buffered channel, then the buffer is full at this time. If
    /// this is not a buffered channel, then there is no [`Receiver`] available to
    /// acquire the data.
    #[stable(feature = "rust1", since = "1.0.0")]
    Full(#[stable(feature = "rust1", since = "1.0.0")] T),

    /// This [`sync_channel`]'s receiving half has disconnected, so the data could not be
    /// sent. The data is returned back to the callee in this case.
    #[stable(feature = "rust1", since = "1.0.0")]
    Disconnected(#[stable(feature = "rust1", since = "1.0.0")] T),
}

enum Flavor<T> {
    Oneshot(Arc<oneshot::Packet<T>>),
    Stream(Arc<stream::Packet<T>>),
    Shared(Arc<shared::Packet<T>>),
    Sync(Arc<sync::Packet<T>>),
}

#[doc(hidden)]
trait UnsafeFlavor<T> {
    fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>>;
    unsafe fn inner_mut(&self) -> &mut Flavor<T> {
        &mut *self.inner_unsafe().get()
    }
    unsafe fn inner(&self) -> &Flavor<T> {
        &*self.inner_unsafe().get()
    }
}
impl<T> UnsafeFlavor<T> for Sender<T> {
    fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> {
        &self.inner
    }
}
impl<T> UnsafeFlavor<T> for Receiver<T> {
    fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> {
        &self.inner
    }
}

/// Creates a new asynchronous channel, returning the sender/receiver halves.
/// All data sent on the [`Sender`] will become available on the [`Receiver`] in
/// the same order as it was sent, and no [`send`] will block the calling thread
/// (this channel has an "infinite buffer", unlike [`sync_channel`], which will
/// block after its buffer limit is reached). [`recv`] will block until a message
/// is available while there is at least one [`Sender`] alive (including clones).
///
/// The [`Sender`] can be cloned to [`send`] to the same channel multiple times, but
/// only one [`Receiver`] is supported.
///
/// If the [`Receiver`] is disconnected while trying to [`send`] with the
/// [`Sender`], the [`send`] method will return a [`SendError`]. Similarly, if the
/// [`Sender`] is disconnected while trying to [`recv`], the [`recv`] method will
/// return a [`RecvError`].
///
/// [`send`]: Sender::send
/// [`recv`]: Receiver::recv
///
/// # Examples
///
/// ```
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (sender, receiver) = channel();
///
/// // Spawn off an expensive computation
/// thread::spawn(move|| {
/// #   fn expensive_computation() {}
///     sender.send(expensive_computation()).unwrap();
/// });
///
/// // Do some useful work for awhile
///
/// // Let's see what that answer was
/// println!("{:?}", receiver.recv().unwrap());
/// ```
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
    let a = Arc::new(oneshot::Packet::new());
    (Sender::new(Flavor::Oneshot(a.clone())), Receiver::new(Flavor::Oneshot(a)))
}

/// Creates a new synchronous, bounded channel.
/// All data sent on the [`SyncSender`] will become available on the [`Receiver`]
/// in the same order as it was sent. Like asynchronous [`channel`]s, the
/// [`Receiver`] will block until a message becomes available. `sync_channel`
/// differs greatly in the semantics of the sender, however.
///
/// This channel has an internal buffer on which messages will be queued.
/// `bound` specifies the buffer size. When the internal buffer becomes full,
/// future sends will *block* waiting for the buffer to open up. Note that a
/// buffer size of 0 is valid, in which case this becomes "rendezvous channel"
/// where each [`send`] will not return until a [`recv`] is paired with it.
///
/// The [`SyncSender`] can be cloned to [`send`] to the same channel multiple
/// times, but only one [`Receiver`] is supported.
///
/// Like asynchronous channels, if the [`Receiver`] is disconnected while trying
/// to [`send`] with the [`SyncSender`], the [`send`] method will return a
/// [`SendError`]. Similarly, If the [`SyncSender`] is disconnected while trying
/// to [`recv`], the [`recv`] method will return a [`RecvError`].
///
/// [`send`]: SyncSender::send
/// [`recv`]: Receiver::recv
///
/// # Examples
///
/// ```
/// use std::sync::mpsc::sync_channel;
/// use std::thread;
///
/// let (sender, receiver) = sync_channel(1);
///
/// // this returns immediately
/// sender.send(1).unwrap();
///
/// thread::spawn(move|| {
///     // this will block until the previous message has been received
///     sender.send(2).unwrap();
/// });
///
/// assert_eq!(receiver.recv().unwrap(), 1);
/// assert_eq!(receiver.recv().unwrap(), 2);
/// ```
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) {
    let a = Arc::new(sync::Packet::new(bound));
    (SyncSender::new(a.clone()), Receiver::new(Flavor::Sync(a)))
}

////////////////////////////////////////////////////////////////////////////////
// Sender
////////////////////////////////////////////////////////////////////////////////

impl<T> Sender<T> {
    fn new(inner: Flavor<T>) -> Sender<T> {
        Sender { inner: UnsafeCell::new(inner) }
    }

    /// Attempts to send a value on this channel, returning it back if it could
    /// not be sent.
    ///
    /// A successful send occurs when it is determined that the other end of
    /// the channel has not hung up already. An unsuccessful send would be one
    /// where the corresponding receiver has already been deallocated. Note
    /// that a return value of [`Err`] means that the data will never be
    /// received, but a return value of [`Ok`] does *not* mean that the data
    /// will be received. It is possible for the corresponding receiver to
    /// hang up immediately after this function returns [`Ok`].
    ///
    /// This method will never block the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::mpsc::channel;
    ///
    /// let (tx, rx) = channel();
    ///
    /// // This send is always successful
    /// tx.send(1).unwrap();
    ///
    /// // This send will fail because the receiver is gone
    /// drop(rx);
    /// assert_eq!(tx.send(1).unwrap_err().0, 1);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn send(&self, t: T) -> Result<(), SendError<T>> {
        let (new_inner, ret) = match *unsafe { self.inner() } {
            Flavor::Oneshot(ref p) => {
                if !p.sent() {
                    return p.send(t).map_err(SendError);
                } else {
                    let a = Arc::new(stream::Packet::new());
                    let rx = Receiver::new(Flavor::Stream(a.clone()));
                    match p.upgrade(rx) {
                        oneshot::UpSuccess => {
                            let ret = a.send(t);
                            (a, ret)
                        }
                        oneshot::UpDisconnected => (a, Err(t)),
                        oneshot::UpWoke(token) => {
                            // This send cannot panic because the thread is
                            // asleep (we're looking at it), so the receiver
                            // can't go away.
                            a.send(t).ok().unwrap();
                            token.signal();
                            (a, Ok(()))
                        }
                    }
                }
            }
            Flavor::Stream(ref p) => return p.send(t).map_err(SendError),
            Flavor::Shared(ref p) => return p.send(t).map_err(SendError),
            Flavor::Sync(..) => unreachable!(),
        };

        unsafe {
            let tmp = Sender::new(Flavor::Stream(new_inner));
            mem::swap(self.inner_mut(), tmp.inner_mut());
        }
        ret.map_err(SendError)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Sender<T> {
    /// Clone a sender to send to other threads.
    ///
    /// Note, be aware of the lifetime of the sender because all senders
    /// (including the original) need to be dropped in order for
    /// [`Receiver::recv`] to stop blocking.
    fn clone(&self) -> Sender<T> {
        let packet = match *unsafe { self.inner() } {
            Flavor::Oneshot(ref p) => {
                let a = Arc::new(shared::Packet::new());
                {
                    let guard = a.postinit_lock();
                    let rx = Receiver::new(Flavor::Shared(a.clone()));
                    let sleeper = match p.upgrade(rx) {
                        oneshot::UpSuccess | oneshot::UpDisconnected => None,
                        oneshot::UpWoke(task) => Some(task),
                    };
                    a.inherit_blocker(sleeper, guard);
                }
                a
            }
            Flavor::Stream(ref p) => {
                let a = Arc::new(shared::Packet::new());
                {
                    let guard = a.postinit_lock();
                    let rx = Receiver::new(Flavor::Shared(a.clone()));
                    let sleeper = match p.upgrade(rx) {
                        stream::UpSuccess | stream::UpDisconnected => None,
                        stream::UpWoke(task) => Some(task),
                    };
                    a.inherit_blocker(sleeper, guard);
                }
                a
            }
            Flavor::Shared(ref p) => {
                p.clone_chan();
                return Sender::new(Flavor::Shared(p.clone()));
            }
            Flavor::Sync(..) => unreachable!(),
        };

        unsafe {
            let tmp = Sender::new(Flavor::Shared(packet.clone()));
            mem::swap(self.inner_mut(), tmp.inner_mut());
        }
        Sender::new(Flavor::Shared(packet))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        match *unsafe { self.inner() } {
            Flavor::Oneshot(ref p) => p.drop_chan(),
            Flavor::Stream(ref p) => p.drop_chan(),
            Flavor::Shared(ref p) => p.drop_chan(),
            Flavor::Sync(..) => unreachable!(),
        }
    }
}

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for Sender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Sender").finish_non_exhaustive()
    }
}

////////////////////////////////////////////////////////////////////////////////
// SyncSender
////////////////////////////////////////////////////////////////////////////////

impl<T> SyncSender<T> {
    fn new(inner: Arc<sync::Packet<T>>) -> SyncSender<T> {
        SyncSender { inner }
    }

    /// Sends a value on this synchronous channel.
    ///
    /// This function will *block* until space in the internal buffer becomes
    /// available or a receiver is available to hand off the message to.
    ///
    /// Note that a successful send does *not* guarantee that the receiver will
    /// ever see the data if there is a buffer on this channel. Items may be
    /// enqueued in the internal buffer for the receiver to receive at a later
    /// time. If the buffer size is 0, however, the channel becomes a rendezvous
    /// channel and it guarantees that the receiver has indeed received
    /// the data if this function returns success.
    ///
    /// This function will never panic, but it may return [`Err`] if the
    /// [`Receiver`] has disconnected and is no longer able to receive
    /// information.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::sync_channel;
    /// use std::thread;
    ///
    /// // Create a rendezvous sync_channel with buffer size 0
    /// let (sync_sender, receiver) = sync_channel(0);
    ///
    /// thread::spawn(move || {
    ///    println!("sending message...");
    ///    sync_sender.send(1).unwrap();
    ///    // Thread is now blocked until the message is received
    ///
    ///    println!("...message received!");
    /// });
    ///
    /// let msg = receiver.recv().unwrap();
    /// assert_eq!(1, msg);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn send(&self, t: T) -> Result<(), SendError<T>> {
        self.inner.send(t).map_err(SendError)
    }

    /// Attempts to send a value on this channel without blocking.
    ///
    /// This method differs from [`send`] by returning immediately if the
    /// channel's buffer is full or no receiver is waiting to acquire some
    /// data. Compared with [`send`], this function has two failure cases
    /// instead of one (one for disconnection, one for a full buffer).
    ///
    /// See [`send`] for notes about guarantees of whether the
    /// receiver has received the data or not if this function is successful.
    ///
    /// [`send`]: Self::send
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::sync_channel;
    /// use std::thread;
    ///
    /// // Create a sync_channel with buffer size 1
    /// let (sync_sender, receiver) = sync_channel(1);
    /// let sync_sender2 = sync_sender.clone();
    ///
    /// // First thread owns sync_sender
    /// thread::spawn(move || {
    ///     sync_sender.send(1).unwrap();
    ///     sync_sender.send(2).unwrap();
    ///     // Thread blocked
    /// });
    ///
    /// // Second thread owns sync_sender2
    /// thread::spawn(move || {
    ///     // This will return an error and send
    ///     // no message if the buffer is full
    ///     let _ = sync_sender2.try_send(3);
    /// });
    ///
    /// let mut msg;
    /// msg = receiver.recv().unwrap();
    /// println!("message {} received", msg);
    ///
    /// msg = receiver.recv().unwrap();
    /// println!("message {} received", msg);
    ///
    /// // Third message may have never been sent
    /// match receiver.try_recv() {
    ///     Ok(msg) => println!("message {} received", msg),
    ///     Err(_) => println!("the third message was never sent"),
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
        self.inner.try_send(t)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for SyncSender<T> {
    fn clone(&self) -> SyncSender<T> {
        self.inner.clone_chan();
        SyncSender::new(self.inner.clone())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Drop for SyncSender<T> {
    fn drop(&mut self) {
        self.inner.drop_chan();
    }
}

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for SyncSender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SyncSender").finish_non_exhaustive()
    }
}

////////////////////////////////////////////////////////////////////////////////
// Receiver
////////////////////////////////////////////////////////////////////////////////

impl<T> Receiver<T> {
    fn new(inner: Flavor<T>) -> Receiver<T> {
        Receiver { inner: UnsafeCell::new(inner) }
    }

    /// Attempts to return a pending value on this receiver without blocking.
    ///
    /// This method will never block the caller in order to wait for data to
    /// become available. Instead, this will always return immediately with a
    /// possible option of pending data on the channel.
    ///
    /// This is useful for a flavor of "optimistic check" before deciding to
    /// block on a receiver.
    ///
    /// Compared with [`recv`], this function has two failure cases instead of one
    /// (one for disconnection, one for an empty buffer).
    ///
    /// [`recv`]: Self::recv
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::{Receiver, channel};
    ///
    /// let (_, receiver): (_, Receiver<i32>) = channel();
    ///
    /// assert!(receiver.try_recv().is_err());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_recv(&self) -> Result<T, TryRecvError> {
        loop {
            let new_port = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => match p.try_recv() {
                    Ok(t) => return Ok(t),
                    Err(oneshot::Empty) => return Err(TryRecvError::Empty),
                    Err(oneshot::Disconnected) => return Err(TryRecvError::Disconnected),
                    Err(oneshot::Upgraded(rx)) => rx,
                },
                Flavor::Stream(ref p) => match p.try_recv() {
                    Ok(t) => return Ok(t),
                    Err(stream::Empty) => return Err(TryRecvError::Empty),
                    Err(stream::Disconnected) => return Err(TryRecvError::Disconnected),
                    Err(stream::Upgraded(rx)) => rx,
                },
                Flavor::Shared(ref p) => match p.try_recv() {
                    Ok(t) => return Ok(t),
                    Err(shared::Empty) => return Err(TryRecvError::Empty),
                    Err(shared::Disconnected) => return Err(TryRecvError::Disconnected),
                },
                Flavor::Sync(ref p) => match p.try_recv() {
                    Ok(t) => return Ok(t),
                    Err(sync::Empty) => return Err(TryRecvError::Empty),
                    Err(sync::Disconnected) => return Err(TryRecvError::Disconnected),
                },
            };
            unsafe {
                mem::swap(self.inner_mut(), new_port.inner_mut());
            }
        }
    }

    /// Attempts to wait for a value on this receiver, returning an error if the
    /// corresponding channel has hung up.
    ///
    /// This function will always block the current thread if there is no data
    /// available and it's possible for more data to be sent (at least one sender
    /// still exists). Once a message is sent to the corresponding [`Sender`]
    /// (or [`SyncSender`]), this receiver will wake up and return that
    /// message.
    ///
    /// If the corresponding [`Sender`] has disconnected, or it disconnects while
    /// this call is blocking, this call will wake up and return [`Err`] to
    /// indicate that no more messages can ever be received on this channel.
    /// However, since channels are buffered, messages sent before the disconnect
    /// will still be properly received.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::mpsc;
    /// use std::thread;
    ///
    /// let (send, recv) = mpsc::channel();
    /// let handle = thread::spawn(move || {
    ///     send.send(1u8).unwrap();
    /// });
    ///
    /// handle.join().unwrap();
    ///
    /// assert_eq!(Ok(1), recv.recv());
    /// ```
    ///
    /// Buffering behavior:
    ///
    /// ```
    /// use std::sync::mpsc;
    /// use std::thread;
    /// use std::sync::mpsc::RecvError;
    ///
    /// let (send, recv) = mpsc::channel();
    /// let handle = thread::spawn(move || {
    ///     send.send(1u8).unwrap();
    ///     send.send(2).unwrap();
    ///     send.send(3).unwrap();
    ///     drop(send);
    /// });
    ///
    /// // wait for the thread to join so we ensure the sender is dropped
    /// handle.join().unwrap();
    ///
    /// assert_eq!(Ok(1), recv.recv());
    /// assert_eq!(Ok(2), recv.recv());
    /// assert_eq!(Ok(3), recv.recv());
    /// assert_eq!(Err(RecvError), recv.recv());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn recv(&self) -> Result<T, RecvError> {
        loop {
            let new_port = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => match p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(oneshot::Disconnected) => return Err(RecvError),
                    Err(oneshot::Upgraded(rx)) => rx,
                    Err(oneshot::Empty) => unreachable!(),
                },
                Flavor::Stream(ref p) => match p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(stream::Disconnected) => return Err(RecvError),
                    Err(stream::Upgraded(rx)) => rx,
                    Err(stream::Empty) => unreachable!(),
                },
                Flavor::Shared(ref p) => match p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(shared::Disconnected) => return Err(RecvError),
                    Err(shared::Empty) => unreachable!(),
                },
                Flavor::Sync(ref p) => return p.recv(None).map_err(|_| RecvError),
            };
            unsafe {
                mem::swap(self.inner_mut(), new_port.inner_mut());
            }
        }
    }

    /// Attempts to wait for a value on this receiver, returning an error if the
    /// corresponding channel has hung up, or if it waits more than `timeout`.
    ///
    /// This function will always block the current thread if there is no data
    /// available and it's possible for more data to be sent (at least one sender
    /// still exists). Once a message is sent to the corresponding [`Sender`]
    /// (or [`SyncSender`]), this receiver will wake up and return that
    /// message.
    ///
    /// If the corresponding [`Sender`] has disconnected, or it disconnects while
    /// this call is blocking, this call will wake up and return [`Err`] to
    /// indicate that no more messages can ever be received on this channel.
    /// However, since channels are buffered, messages sent before the disconnect
    /// will still be properly received.
    ///
    /// # Known Issues
    ///
    /// There is currently a known issue (see [`#39364`]) that causes `recv_timeout`
    /// to panic unexpectedly with the following example:
    ///
    /// ```no_run
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let (tx, rx) = channel::<String>();
    ///
    /// thread::spawn(move || {
    ///     let d = Duration::from_millis(10);
    ///     loop {
    ///         println!("recv");
    ///         let _r = rx.recv_timeout(d);
    ///     }
    /// });
    ///
    /// thread::sleep(Duration::from_millis(100));
    /// let _c1 = tx.clone();
    ///
    /// thread::sleep(Duration::from_secs(1));
    /// ```
    ///
    /// [`#39364`]: https://github.com/rust-lang/rust/issues/39364
    ///
    /// # Examples
    ///
    /// Successfully receiving value before encountering timeout:
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::time::Duration;
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_timeout(Duration::from_millis(400)),
    ///     Ok('a')
    /// );
    /// ```
    ///
    /// Receiving an error upon reaching timeout:
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::time::Duration;
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(800));
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_timeout(Duration::from_millis(400)),
    ///     Err(mpsc::RecvTimeoutError::Timeout)
    /// );
    /// ```
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> {
        // Do an optimistic try_recv to avoid the performance impact of
        // Instant::now() in the full-channel case.
        match self.try_recv() {
            Ok(result) => Ok(result),
            Err(TryRecvError::Disconnected) => Err(RecvTimeoutError::Disconnected),
            Err(TryRecvError::Empty) => match Instant::now().checked_add(timeout) {
                Some(deadline) => self.recv_deadline(deadline),
                // So far in the future that it's practically the same as waiting indefinitely.
                None => self.recv().map_err(RecvTimeoutError::from),
            },
        }
    }

    /// Attempts to wait for a value on this receiver, returning an error if the
    /// corresponding channel has hung up, or if `deadline` is reached.
    ///
    /// This function will always block the current thread if there is no data
    /// available and it's possible for more data to be sent. Once a message is
    /// sent to the corresponding [`Sender`] (or [`SyncSender`]), then this
    /// receiver will wake up and return that message.
    ///
    /// If the corresponding [`Sender`] has disconnected, or it disconnects while
    /// this call is blocking, this call will wake up and return [`Err`] to
    /// indicate that no more messages can ever be received on this channel.
    /// However, since channels are buffered, messages sent before the disconnect
    /// will still be properly received.
    ///
    /// # Examples
    ///
    /// Successfully receiving value before reaching deadline:
    ///
    /// ```no_run
    /// #![feature(deadline_api)]
    /// use std::thread;
    /// use std::time::{Duration, Instant};
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_deadline(Instant::now() + Duration::from_millis(400)),
    ///     Ok('a')
    /// );
    /// ```
    ///
    /// Receiving an error upon reaching deadline:
    ///
    /// ```no_run
    /// #![feature(deadline_api)]
    /// use std::thread;
    /// use std::time::{Duration, Instant};
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(800));
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_deadline(Instant::now() + Duration::from_millis(400)),
    ///     Err(mpsc::RecvTimeoutError::Timeout)
    /// );
    /// ```
    #[unstable(feature = "deadline_api", issue = "46316")]
    pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> {
        use self::RecvTimeoutError::*;

        loop {
            let port_or_empty = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(oneshot::Disconnected) => return Err(Disconnected),
                    Err(oneshot::Upgraded(rx)) => Some(rx),
                    Err(oneshot::Empty) => None,
                },
                Flavor::Stream(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(stream::Disconnected) => return Err(Disconnected),
                    Err(stream::Upgraded(rx)) => Some(rx),
                    Err(stream::Empty) => None,
                },
                Flavor::Shared(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(shared::Disconnected) => return Err(Disconnected),
                    Err(shared::Empty) => None,
                },
                Flavor::Sync(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(sync::Disconnected) => return Err(Disconnected),
                    Err(sync::Empty) => None,
                },
            };

            if let Some(new_port) = port_or_empty {
                unsafe {
                    mem::swap(self.inner_mut(), new_port.inner_mut());
                }
            }

            // If we're already passed the deadline, and we're here without
            // data, return a timeout, else try again.
            if Instant::now() >= deadline {
                return Err(Timeout);
            }
        }
    }

    /// Returns an iterator that will block waiting for messages, but never
    /// [`panic!`]. It will return [`None`] when the channel has hung up.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    ///
    /// let (send, recv) = channel();
    ///
    /// thread::spawn(move || {
    ///     send.send(1).unwrap();
    ///     send.send(2).unwrap();
    ///     send.send(3).unwrap();
    /// });
    ///
    /// let mut iter = recv.iter();
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { rx: self }
    }

    /// Returns an iterator that will attempt to yield all pending values.
    /// It will return `None` if there are no more pending values or if the
    /// channel has hung up. The iterator will never [`panic!`] or block the
    /// user by waiting for values.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let (sender, receiver) = channel();
    ///
    /// // nothing is in the buffer yet
    /// assert!(receiver.try_iter().next().is_none());
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_secs(1));
    ///     sender.send(1).unwrap();
    ///     sender.send(2).unwrap();
    ///     sender.send(3).unwrap();
    /// });
    ///
    /// // nothing is in the buffer yet
    /// assert!(receiver.try_iter().next().is_none());
    ///
    /// // block for two seconds
    /// thread::sleep(Duration::from_secs(2));
    ///
    /// let mut iter = receiver.try_iter();
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[stable(feature = "receiver_try_iter", since = "1.15.0")]
    pub fn try_iter(&self) -> TryIter<'_, T> {
        TryIter { rx: self }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for Iter<'a, T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        self.rx.recv().ok()
    }
}

#[stable(feature = "receiver_try_iter", since = "1.15.0")]
impl<'a, T> Iterator for TryIter<'a, T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        self.rx.try_recv().ok()
    }
}

#[stable(feature = "receiver_into_iter", since = "1.1.0")]
impl<'a, T> IntoIterator for &'a Receiver<T> {
    type Item = T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

#[stable(feature = "receiver_into_iter", since = "1.1.0")]
impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<T> {
        self.rx.recv().ok()
    }
}

#[stable(feature = "receiver_into_iter", since = "1.1.0")]
impl<T> IntoIterator for Receiver<T> {
    type Item = T;
    type IntoIter = IntoIter<T>;

    fn into_iter(self) -> IntoIter<T> {
        IntoIter { rx: self }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        match *unsafe { self.inner() } {
            Flavor::Oneshot(ref p) => p.drop_port(),
            Flavor::Stream(ref p) => p.drop_port(),
            Flavor::Shared(ref p) => p.drop_port(),
            Flavor::Sync(ref p) => p.drop_port(),
        }
    }
}

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for Receiver<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Receiver").finish_non_exhaustive()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> fmt::Debug for SendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SendError").finish_non_exhaustive()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> fmt::Display for SendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        "sending on a closed channel".fmt(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Send> error::Error for SendError<T> {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "sending on a closed channel"
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> fmt::Debug for TrySendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TrySendError::Full(..) => "Full(..)".fmt(f),
            TrySendError::Disconnected(..) => "Disconnected(..)".fmt(f),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> fmt::Display for TrySendError<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TrySendError::Full(..) => "sending on a full channel".fmt(f),
            TrySendError::Disconnected(..) => "sending on a closed channel".fmt(f),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Send> error::Error for TrySendError<T> {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        match *self {
            TrySendError::Full(..) => "sending on a full channel",
            TrySendError::Disconnected(..) => "sending on a closed channel",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl<T> From<SendError<T>> for TrySendError<T> {
    /// Converts a `SendError<T>` into a `TrySendError<T>`.
    ///
    /// This conversion always returns a `TrySendError::Disconnected` containing the data in the `SendError<T>`.
    ///
    /// No data is allocated on the heap.
    fn from(err: SendError<T>) -> TrySendError<T> {
        match err {
            SendError(t) => TrySendError::Disconnected(t),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for RecvError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        "receiving on a closed channel".fmt(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl error::Error for RecvError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "receiving on a closed channel"
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for TryRecvError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            TryRecvError::Empty => "receiving on an empty channel".fmt(f),
            TryRecvError::Disconnected => "receiving on a closed channel".fmt(f),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl error::Error for TryRecvError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        match *self {
            TryRecvError::Empty => "receiving on an empty channel",
            TryRecvError::Disconnected => "receiving on a closed channel",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl From<RecvError> for TryRecvError {
    /// Converts a `RecvError` into a `TryRecvError`.
    ///
    /// This conversion always returns `TryRecvError::Disconnected`.
    ///
    /// No data is allocated on the heap.
    fn from(err: RecvError) -> TryRecvError {
        match err {
            RecvError => TryRecvError::Disconnected,
        }
    }
}

#[stable(feature = "mpsc_recv_timeout_error", since = "1.15.0")]
impl fmt::Display for RecvTimeoutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            RecvTimeoutError::Timeout => "timed out waiting on channel".fmt(f),
            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed".fmt(f),
        }
    }
}

#[stable(feature = "mpsc_recv_timeout_error", since = "1.15.0")]
impl error::Error for RecvTimeoutError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        match *self {
            RecvTimeoutError::Timeout => "timed out waiting on channel",
            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl From<RecvError> for RecvTimeoutError {
    /// Converts a `RecvError` into a `RecvTimeoutError`.
    ///
    /// This conversion always returns `RecvTimeoutError::Disconnected`.
    ///
    /// No data is allocated on the heap.
    fn from(err: RecvError) -> RecvTimeoutError {
        match err {
            RecvError => RecvTimeoutError::Disconnected,
        }
    }
}