mod.rs.html -- source
Description: Source to the Rust file `library/std/src/sync/mpsc/mod.rs`.
Keywords: rust, rustlang, rust-lang

       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

    //! 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();
    //! ```

    #![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")]
    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 **Receiver::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.
    ///
    /// [`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`].
    ///
    /// [`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.
    ///
    /// [`Sender::send`]: Sender::send
    /// [`SyncSender::send`]: SyncSender::send
    #[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.
    ///
    /// 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());
    /// ```
    #[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);
    /// ```
    #[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> {
        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()
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // 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()
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // 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. 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
        ///
        /// ```
        /// 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. 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.
        ///
        /// # 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()
        }
    }

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

    #[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,
            }
        }
    }