std/thread/
mod.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
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
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
//! Native threads.
//!
//! ## The threading model
//!
//! An executing Rust program consists of a collection of native OS threads,
//! each with their own stack and local state. Threads can be named, and
//! provide some built-in support for low-level synchronization.
//!
//! Communication between threads can be done through
//! [channels], Rust's message-passing types, along with [other forms of thread
//! synchronization](../../std/sync/index.html) and shared-memory data
//! structures. In particular, types that are guaranteed to be
//! threadsafe are easily shared between threads using the
//! atomically-reference-counted container, [`Arc`].
//!
//! Fatal logic errors in Rust cause *thread panic*, during which
//! a thread will unwind the stack, running destructors and freeing
//! owned resources. While not meant as a 'try/catch' mechanism, panics
//! in Rust can nonetheless be caught (unless compiling with `panic=abort`) with
//! [`catch_unwind`](../../std/panic/fn.catch_unwind.html) and recovered
//! from, or alternatively be resumed with
//! [`resume_unwind`](../../std/panic/fn.resume_unwind.html). If the panic
//! is not caught the thread will exit, but the panic may optionally be
//! detected from a different thread with [`join`]. If the main thread panics
//! without the panic being caught, the application will exit with a
//! non-zero exit code.
//!
//! When the main thread of a Rust program terminates, the entire program shuts
//! down, even if other threads are still running. However, this module provides
//! convenient facilities for automatically waiting for the termination of a
//! thread (i.e., join).
//!
//! ## Spawning a thread
//!
//! A new thread can be spawned using the [`thread::spawn`][`spawn`] function:
//!
//! ```rust
//! use std::thread;
//!
//! thread::spawn(move || {
//!     // some work here
//! });
//! ```
//!
//! In this example, the spawned thread is "detached," which means that there is
//! no way for the program to learn when the spawned thread completes or otherwise
//! terminates.
//!
//! To learn when a thread completes, it is necessary to capture the [`JoinHandle`]
//! object that is returned by the call to [`spawn`], which provides
//! a `join` method that allows the caller to wait for the completion of the
//! spawned thread:
//!
//! ```rust
//! use std::thread;
//!
//! let thread_join_handle = thread::spawn(move || {
//!     // some work here
//! });
//! // some work here
//! let res = thread_join_handle.join();
//! ```
//!
//! The [`join`] method returns a [`thread::Result`] containing [`Ok`] of the final
//! value produced by the spawned thread, or [`Err`] of the value given to
//! a call to [`panic!`] if the thread panicked.
//!
//! Note that there is no parent/child relationship between a thread that spawns a
//! new thread and the thread being spawned.  In particular, the spawned thread may or
//! may not outlive the spawning thread, unless the spawning thread is the main thread.
//!
//! ## Configuring threads
//!
//! A new thread can be configured before it is spawned via the [`Builder`] type,
//! which currently allows you to set the name and stack size for the thread:
//!
//! ```rust
//! # #![allow(unused_must_use)]
//! use std::thread;
//!
//! thread::Builder::new().name("thread1".to_string()).spawn(move || {
//!     println!("Hello, world!");
//! });
//! ```
//!
//! ## The `Thread` type
//!
//! Threads are represented via the [`Thread`] type, which you can get in one of
//! two ways:
//!
//! * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
//!   function, and calling [`thread`][`JoinHandle::thread`] on the [`JoinHandle`].
//! * By requesting the current thread, using the [`thread::current`] function.
//!
//! The [`thread::current`] function is available even for threads not spawned
//! by the APIs of this module.
//!
//! ## Thread-local storage
//!
//! This module also provides an implementation of thread-local storage for Rust
//! programs. Thread-local storage is a method of storing data into a global
//! variable that each thread in the program will have its own copy of.
//! Threads do not share this data, so accesses do not need to be synchronized.
//!
//! A thread-local key owns the value it contains and will destroy the value when the
//! thread exits. It is created with the [`thread_local!`] macro and can contain any
//! value that is `'static` (no borrowed pointers). It provides an accessor function,
//! [`with`], that yields a shared reference to the value to the specified
//! closure. Thread-local keys allow only shared access to values, as there would be no
//! way to guarantee uniqueness if mutable borrows were allowed. Most values
//! will want to make use of some form of **interior mutability** through the
//! [`Cell`] or [`RefCell`] types.
//!
//! ## Naming threads
//!
//! Threads are able to have associated names for identification purposes. By default, spawned
//! threads are unnamed. To specify a name for a thread, build the thread with [`Builder`] and pass
//! the desired thread name to [`Builder::name`]. To retrieve the thread name from within the
//! thread, use [`Thread::name`]. A couple of examples where the name of a thread gets used:
//!
//! * If a panic occurs in a named thread, the thread name will be printed in the panic message.
//! * The thread name is provided to the OS where applicable (e.g., `pthread_setname_np` in
//!   unix-like platforms).
//!
//! ## Stack size
//!
//! The default stack size is platform-dependent and subject to change.
//! Currently, it is 2 MiB on all Tier-1 platforms.
//!
//! There are two ways to manually specify the stack size for spawned threads:
//!
//! * Build the thread with [`Builder`] and pass the desired stack size to [`Builder::stack_size`].
//! * Set the `RUST_MIN_STACK` environment variable to an integer representing the desired stack
//!   size (in bytes). Note that setting [`Builder::stack_size`] will override this. Be aware that
//!   changes to `RUST_MIN_STACK` may be ignored after program start.
//!
//! Note that the stack size of the main thread is *not* determined by Rust.
//!
//! [channels]: crate::sync::mpsc
//! [`join`]: JoinHandle::join
//! [`Result`]: crate::result::Result
//! [`Ok`]: crate::result::Result::Ok
//! [`Err`]: crate::result::Result::Err
//! [`thread::current`]: current
//! [`thread::Result`]: Result
//! [`unpark`]: Thread::unpark
//! [`thread::park_timeout`]: park_timeout
//! [`Cell`]: crate::cell::Cell
//! [`RefCell`]: crate::cell::RefCell
//! [`with`]: LocalKey::with
//! [`thread_local!`]: crate::thread_local

#![stable(feature = "rust1", since = "1.0.0")]
#![deny(unsafe_op_in_unsafe_fn)]
// Under `test`, `__FastLocalKeyInner` seems unused.
#![cfg_attr(test, allow(dead_code))]

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

use crate::any::Any;
use crate::cell::{Cell, OnceCell, UnsafeCell};
use crate::ffi::CStr;
use crate::marker::PhantomData;
use crate::mem::{self, forget, ManuallyDrop};
use crate::num::NonZero;
use crate::pin::Pin;
use crate::ptr::addr_of_mut;
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::sync::Arc;
use crate::sys::sync::Parker;
use crate::sys::thread as imp;
use crate::sys_common::{AsInner, IntoInner};
use crate::time::{Duration, Instant};
use crate::{env, fmt, io, panic, panicking, str};

#[stable(feature = "scoped_threads", since = "1.63.0")]
mod scoped;

#[stable(feature = "scoped_threads", since = "1.63.0")]
pub use scoped::{scope, Scope, ScopedJoinHandle};

////////////////////////////////////////////////////////////////////////////////
// Thread-local storage
////////////////////////////////////////////////////////////////////////////////

#[macro_use]
mod local;

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::local::{AccessError, LocalKey};

// Implementation details used by the thread_local!{} macro.
#[doc(hidden)]
#[unstable(feature = "thread_local_internals", issue = "none")]
pub mod local_impl {
    pub use crate::sys::thread_local::*;
}

////////////////////////////////////////////////////////////////////////////////
// Builder
////////////////////////////////////////////////////////////////////////////////

/// Thread factory, which can be used in order to configure the properties of
/// a new thread.
///
/// Methods can be chained on it in order to configure it.
///
/// The two configurations available are:
///
/// - [`name`]: specifies an [associated name for the thread][naming-threads]
/// - [`stack_size`]: specifies the [desired stack size for the thread][stack-size]
///
/// The [`spawn`] method will take ownership of the builder and create an
/// [`io::Result`] to the thread handle with the given configuration.
///
/// The [`thread::spawn`] free function uses a `Builder` with default
/// configuration and [`unwrap`]s its return value.
///
/// You may want to use [`spawn`] instead of [`thread::spawn`], when you want
/// to recover from a failure to launch a thread, indeed the free function will
/// panic where the `Builder` method will return a [`io::Result`].
///
/// # Examples
///
/// ```
/// use std::thread;
///
/// let builder = thread::Builder::new();
///
/// let handler = builder.spawn(|| {
///     // thread code
/// }).unwrap();
///
/// handler.join().unwrap();
/// ```
///
/// [`stack_size`]: Builder::stack_size
/// [`name`]: Builder::name
/// [`spawn`]: Builder::spawn
/// [`thread::spawn`]: spawn
/// [`io::Result`]: crate::io::Result
/// [`unwrap`]: crate::result::Result::unwrap
/// [naming-threads]: ./index.html#naming-threads
/// [stack-size]: ./index.html#stack-size
#[must_use = "must eventually spawn the thread"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Builder {
    // A name for the thread-to-be, for identification in panic messages
    name: Option<String>,
    // The size of the stack for the spawned thread in bytes
    stack_size: Option<usize>,
}

impl Builder {
    /// Generates the base configuration for spawning a thread, from which
    /// configuration methods can be chained.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new()
    ///                               .name("foo".into())
    ///                               .stack_size(32 * 1024);
    ///
    /// let handler = builder.spawn(|| {
    ///     // thread code
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> Builder {
        Builder { name: None, stack_size: None }
    }

    /// Names the thread-to-be. Currently the name is used for identification
    /// only in panic messages.
    ///
    /// The name must not contain null bytes (`\0`).
    ///
    /// For more information about named threads, see
    /// [this module-level documentation][naming-threads].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new()
    ///     .name("foo".into());
    ///
    /// let handler = builder.spawn(|| {
    ///     assert_eq!(thread::current().name(), Some("foo"))
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    ///
    /// [naming-threads]: ./index.html#naming-threads
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn name(mut self, name: String) -> Builder {
        self.name = Some(name);
        self
    }

    /// Sets the size of the stack (in bytes) for the new thread.
    ///
    /// The actual stack size may be greater than this value if
    /// the platform specifies a minimal stack size.
    ///
    /// For more information about the stack size for threads, see
    /// [this module-level documentation][stack-size].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new().stack_size(32 * 1024);
    /// ```
    ///
    /// [stack-size]: ./index.html#stack-size
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn stack_size(mut self, size: usize) -> Builder {
        self.stack_size = Some(size);
        self
    }

    /// Spawns a new thread by taking ownership of the `Builder`, and returns an
    /// [`io::Result`] to its [`JoinHandle`].
    ///
    /// The spawned thread may outlive the caller (unless the caller thread
    /// is the main thread; the whole process is terminated when the main
    /// thread finishes). The join handle can be used to block on
    /// termination of the spawned thread, including recovering its panics.
    ///
    /// For a more complete documentation see [`thread::spawn`][`spawn`].
    ///
    /// # Errors
    ///
    /// Unlike the [`spawn`] free function, this method yields an
    /// [`io::Result`] to capture any failure to create the thread at
    /// the OS level.
    ///
    /// [`io::Result`]: crate::io::Result
    ///
    /// # Panics
    ///
    /// Panics if a thread name was set and it contained null bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let handler = builder.spawn(|| {
    ///     // thread code
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn spawn<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
    where
        F: FnOnce() -> T,
        F: Send + 'static,
        T: Send + 'static,
    {
        unsafe { self.spawn_unchecked(f) }
    }

    /// Spawns a new thread without any lifetime restrictions by taking ownership
    /// of the `Builder`, and returns an [`io::Result`] to its [`JoinHandle`].
    ///
    /// The spawned thread may outlive the caller (unless the caller thread
    /// is the main thread; the whole process is terminated when the main
    /// thread finishes). The join handle can be used to block on
    /// termination of the spawned thread, including recovering its panics.
    ///
    /// This method is identical to [`thread::Builder::spawn`][`Builder::spawn`],
    /// except for the relaxed lifetime bounds, which render it unsafe.
    /// For a more complete documentation see [`thread::spawn`][`spawn`].
    ///
    /// # Errors
    ///
    /// Unlike the [`spawn`] free function, this method yields an
    /// [`io::Result`] to capture any failure to create the thread at
    /// the OS level.
    ///
    /// # Panics
    ///
    /// Panics if a thread name was set and it contained null bytes.
    ///
    /// # Safety
    ///
    /// The caller has to ensure that the spawned thread does not outlive any
    /// references in the supplied thread closure and its return type.
    /// This can be guaranteed in two ways:
    ///
    /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
    /// data is dropped
    /// - use only types with `'static` lifetime bounds, i.e., those with no or only
    /// `'static` references (both [`thread::Builder::spawn`][`Builder::spawn`]
    /// and [`thread::spawn`][`spawn`] enforce this property statically)
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let x = 1;
    /// let thread_x = &x;
    ///
    /// let handler = unsafe {
    ///     builder.spawn_unchecked(move || {
    ///         println!("x = {}", *thread_x);
    ///     }).unwrap()
    /// };
    ///
    /// // caller has to ensure `join()` is called, otherwise
    /// // it is possible to access freed memory if `x` gets
    /// // dropped before the thread closure is executed!
    /// handler.join().unwrap();
    /// ```
    ///
    /// [`io::Result`]: crate::io::Result
    #[stable(feature = "thread_spawn_unchecked", since = "1.82.0")]
    pub unsafe fn spawn_unchecked<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
    where
        F: FnOnce() -> T,
        F: Send,
        T: Send,
    {
        Ok(JoinHandle(unsafe { self.spawn_unchecked_(f, None) }?))
    }

    unsafe fn spawn_unchecked_<'scope, F, T>(
        self,
        f: F,
        scope_data: Option<Arc<scoped::ScopeData>>,
    ) -> io::Result<JoinInner<'scope, T>>
    where
        F: FnOnce() -> T,
        F: Send,
        T: Send,
    {
        let Builder { name, stack_size } = self;

        let stack_size = stack_size.unwrap_or_else(|| {
            static MIN: AtomicUsize = AtomicUsize::new(0);

            match MIN.load(Ordering::Relaxed) {
                0 => {}
                n => return n - 1,
            }

            let amt = env::var_os("RUST_MIN_STACK")
                .and_then(|s| s.to_str().and_then(|s| s.parse().ok()))
                .unwrap_or(imp::DEFAULT_MIN_STACK_SIZE);

            // 0 is our sentinel value, so ensure that we'll never see 0 after
            // initialization has run
            MIN.store(amt + 1, Ordering::Relaxed);
            amt
        });

        let my_thread = name.map_or_else(Thread::new_unnamed, Thread::new);
        let their_thread = my_thread.clone();

        let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
            scope: scope_data,
            result: UnsafeCell::new(None),
            _marker: PhantomData,
        });
        let their_packet = my_packet.clone();

        let output_capture = crate::io::set_output_capture(None);
        crate::io::set_output_capture(output_capture.clone());

        // Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
        // See <https://github.com/rust-lang/rust/issues/101983> for more details.
        // To prevent leaks we use a wrapper that drops its contents.
        #[repr(transparent)]
        struct MaybeDangling<T>(mem::MaybeUninit<T>);
        impl<T> MaybeDangling<T> {
            fn new(x: T) -> Self {
                MaybeDangling(mem::MaybeUninit::new(x))
            }
            fn into_inner(self) -> T {
                // Make sure we don't drop.
                let this = ManuallyDrop::new(self);
                // SAFETY: we are always initialized.
                unsafe { this.0.assume_init_read() }
            }
        }
        impl<T> Drop for MaybeDangling<T> {
            fn drop(&mut self) {
                // SAFETY: we are always initialized.
                unsafe { self.0.assume_init_drop() };
            }
        }

        let f = MaybeDangling::new(f);
        let main = move || {
            if let Some(name) = their_thread.cname() {
                imp::Thread::set_name(name);
            }

            crate::io::set_output_capture(output_capture);

            let f = f.into_inner();
            set_current(their_thread);
            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                crate::sys::backtrace::__rust_begin_short_backtrace(f)
            }));
            // SAFETY: `their_packet` as been built just above and moved by the
            // closure (it is an Arc<...>) and `my_packet` will be stored in the
            // same `JoinInner` as this closure meaning the mutation will be
            // safe (not modify it and affect a value far away).
            unsafe { *their_packet.result.get() = Some(try_result) };
            // Here `their_packet` gets dropped, and if this is the last `Arc` for that packet that
            // will call `decrement_num_running_threads` and therefore signal that this thread is
            // done.
            drop(their_packet);
            // Here, the lifetime `'scope` can end. `main` keeps running for a bit
            // after that before returning itself.
        };

        if let Some(scope_data) = &my_packet.scope {
            scope_data.increment_num_running_threads();
        }

        let main = Box::new(main);
        // SAFETY: dynamic size and alignment of the Box remain the same. See below for why the
        // lifetime change is justified.
        let main =
            unsafe { Box::from_raw(Box::into_raw(main) as *mut (dyn FnOnce() + Send + 'static)) };

        Ok(JoinInner {
            // SAFETY:
            //
            // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
            // through FFI or otherwise used with low-level threading primitives that have no
            // notion of or way to enforce lifetimes.
            //
            // As mentioned in the `Safety` section of this function's documentation, the caller of
            // this function needs to guarantee that the passed-in lifetime is sufficiently long
            // for the lifetime of the thread.
            //
            // Similarly, the `sys` implementation must guarantee that no references to the closure
            // exist after the thread has terminated, which is signaled by `Thread::join`
            // returning.
            native: unsafe { imp::Thread::new(stack_size, main)? },
            thread: my_thread,
            packet: my_packet,
        })
    }
}

////////////////////////////////////////////////////////////////////////////////
// Free functions
////////////////////////////////////////////////////////////////////////////////

/// Spawns a new thread, returning a [`JoinHandle`] for it.
///
/// The join handle provides a [`join`] method that can be used to join the spawned
/// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
/// the argument given to [`panic!`].
///
/// If the join handle is dropped, the spawned thread will implicitly be *detached*.
/// In this case, the spawned thread may no longer be joined.
/// (It is the responsibility of the program to either eventually join threads it
/// creates or detach them; otherwise, a resource leak will result.)
///
/// This call will create a thread using default parameters of [`Builder`], if you
/// want to specify the stack size or the name of the thread, use this API
/// instead.
///
/// As you can see in the signature of `spawn` there are two constraints on
/// both the closure given to `spawn` and its return value, let's explain them:
///
/// - The `'static` constraint means that the closure and its return value
///   must have a lifetime of the whole program execution. The reason for this
///   is that threads can outlive the lifetime they have been created in.
///
///   Indeed if the thread, and by extension its return value, can outlive their
///   caller, we need to make sure that they will be valid afterwards, and since
///   we *can't* know when it will return we need to have them valid as long as
///   possible, that is until the end of the program, hence the `'static`
///   lifetime.
/// - The [`Send`] constraint is because the closure will need to be passed
///   *by value* from the thread where it is spawned to the new thread. Its
///   return value will need to be passed from the new thread to the thread
///   where it is `join`ed.
///   As a reminder, the [`Send`] marker trait expresses that it is safe to be
///   passed from thread to thread. [`Sync`] expresses that it is safe to have a
///   reference be passed from thread to thread.
///
/// # Panics
///
/// Panics if the OS fails to create a thread; use [`Builder::spawn`]
/// to recover from such errors.
///
/// # Examples
///
/// Creating a thread.
///
/// ```
/// use std::thread;
///
/// let handler = thread::spawn(|| {
///     // thread code
/// });
///
/// handler.join().unwrap();
/// ```
///
/// As mentioned in the module documentation, threads are usually made to
/// communicate using [`channels`], here is how it usually looks.
///
/// This example also shows how to use `move`, in order to give ownership
/// of values to a thread.
///
/// ```
/// use std::thread;
/// use std::sync::mpsc::channel;
///
/// let (tx, rx) = channel();
///
/// let sender = thread::spawn(move || {
///     tx.send("Hello, thread".to_owned())
///         .expect("Unable to send on channel");
/// });
///
/// let receiver = thread::spawn(move || {
///     let value = rx.recv().expect("Unable to receive from channel");
///     println!("{value}");
/// });
///
/// sender.join().expect("The sender thread has panicked");
/// receiver.join().expect("The receiver thread has panicked");
/// ```
///
/// A thread can also return a value through its [`JoinHandle`], you can use
/// this to make asynchronous computations (futures might be more appropriate
/// though).
///
/// ```
/// use std::thread;
///
/// let computation = thread::spawn(|| {
///     // Some expensive computation.
///     42
/// });
///
/// let result = computation.join().unwrap();
/// println!("{result}");
/// ```
///
/// [`channels`]: crate::sync::mpsc
/// [`join`]: JoinHandle::join
/// [`Err`]: crate::result::Result::Err
#[stable(feature = "rust1", since = "1.0.0")]
pub fn spawn<F, T>(f: F) -> JoinHandle<T>
where
    F: FnOnce() -> T,
    F: Send + 'static,
    T: Send + 'static,
{
    Builder::new().spawn(f).expect("failed to spawn thread")
}

thread_local! {
    // Invariant: `CURRENT` and `CURRENT_ID` will always be initialized together.
    // If `CURRENT` is initialized, then `CURRENT_ID` will hold the same value
    // as `CURRENT.id()`.
    static CURRENT: OnceCell<Thread> = const { OnceCell::new() };
    static CURRENT_ID: Cell<Option<ThreadId>> = const { Cell::new(None) };
}

/// Sets the thread handle for the current thread.
///
/// Aborts if the handle has been set already to reduce code size.
pub(crate) fn set_current(thread: Thread) {
    let tid = thread.id();
    // Using `unwrap` here can add ~3kB to the binary size. We have complete
    // control over where this is called, so just abort if there is a bug.
    CURRENT.with(|current| match current.set(thread) {
        Ok(()) => CURRENT_ID.set(Some(tid)),
        Err(_) => rtabort!("thread::set_current should only be called once per thread"),
    });
}

/// Gets a handle to the thread that invokes it.
///
/// In contrast to the public `current` function, this will not panic if called
/// from inside a TLS destructor.
pub(crate) fn try_current() -> Option<Thread> {
    CURRENT
        .try_with(|current| {
            current
                .get_or_init(|| {
                    let thread = Thread::new_unnamed();
                    CURRENT_ID.set(Some(thread.id()));
                    thread
                })
                .clone()
        })
        .ok()
}

/// Gets the id of the thread that invokes it.
#[inline]
pub(crate) fn current_id() -> ThreadId {
    CURRENT_ID.get().unwrap_or_else(|| {
        // If `CURRENT_ID` isn't initialized yet, then `CURRENT` must also not be initialized.
        // `current()` will initialize both `CURRENT` and `CURRENT_ID` so subsequent calls to
        // `current_id()` will succeed immediately.
        current().id()
    })
}

/// Gets a handle to the thread that invokes it.
///
/// # Examples
///
/// Getting a handle to the current thread with `thread::current()`:
///
/// ```
/// use std::thread;
///
/// let handler = thread::Builder::new()
///     .name("named thread".into())
///     .spawn(|| {
///         let handle = thread::current();
///         assert_eq!(handle.name(), Some("named thread"));
///     })
///     .unwrap();
///
/// handler.join().unwrap();
/// ```
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn current() -> Thread {
    try_current().expect(
        "use of std::thread::current() is not possible \
         after the thread's local data has been destroyed",
    )
}

/// Cooperatively gives up a timeslice to the OS scheduler.
///
/// This calls the underlying OS scheduler's yield primitive, signaling
/// that the calling thread is willing to give up its remaining timeslice
/// so that the OS may schedule other threads on the CPU.
///
/// A drawback of yielding in a loop is that if the OS does not have any
/// other ready threads to run on the current CPU, the thread will effectively
/// busy-wait, which wastes CPU time and energy.
///
/// Therefore, when waiting for events of interest, a programmer's first
/// choice should be to use synchronization devices such as [`channel`]s,
/// [`Condvar`]s, [`Mutex`]es or [`join`] since these primitives are
/// implemented in a blocking manner, giving up the CPU until the event
/// of interest has occurred which avoids repeated yielding.
///
/// `yield_now` should thus be used only rarely, mostly in situations where
/// repeated polling is required because there is no other suitable way to
/// learn when an event of interest has occurred.
///
/// # Examples
///
/// ```
/// use std::thread;
///
/// thread::yield_now();
/// ```
///
/// [`channel`]: crate::sync::mpsc
/// [`join`]: JoinHandle::join
/// [`Condvar`]: crate::sync::Condvar
/// [`Mutex`]: crate::sync::Mutex
#[stable(feature = "rust1", since = "1.0.0")]
pub fn yield_now() {
    imp::Thread::yield_now()
}

/// Determines whether the current thread is unwinding because of panic.
///
/// A common use of this feature is to poison shared resources when writing
/// unsafe code, by checking `panicking` when the `drop` is called.
///
/// This is usually not needed when writing safe code, as [`Mutex`es][Mutex]
/// already poison themselves when a thread panics while holding the lock.
///
/// This can also be used in multithreaded applications, in order to send a
/// message to other threads warning that a thread has panicked (e.g., for
/// monitoring purposes).
///
/// # Examples
///
/// ```should_panic
/// use std::thread;
///
/// struct SomeStruct;
///
/// impl Drop for SomeStruct {
///     fn drop(&mut self) {
///         if thread::panicking() {
///             println!("dropped while unwinding");
///         } else {
///             println!("dropped while not unwinding");
///         }
///     }
/// }
///
/// {
///     print!("a: ");
///     let a = SomeStruct;
/// }
///
/// {
///     print!("b: ");
///     let b = SomeStruct;
///     panic!()
/// }
/// ```
///
/// [Mutex]: crate::sync::Mutex
#[inline]
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn panicking() -> bool {
    panicking::panicking()
}

/// Uses [`sleep`].
///
/// Puts the current thread to sleep for at least the specified amount of time.
///
/// The thread may sleep longer than the duration specified due to scheduling
/// specifics or platform-dependent functionality. It will never sleep less.
///
/// This function is blocking, and should not be used in `async` functions.
///
/// # Platform-specific behavior
///
/// On Unix platforms, the underlying syscall may be interrupted by a
/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
/// the specified duration, this function may invoke that system call multiple
/// times.
///
/// # Examples
///
/// ```no_run
/// use std::thread;
///
/// // Let's sleep for 2 seconds:
/// thread::sleep_ms(2000);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(since = "1.6.0", note = "replaced by `std::thread::sleep`")]
pub fn sleep_ms(ms: u32) {
    sleep(Duration::from_millis(ms as u64))
}

/// Puts the current thread to sleep for at least the specified amount of time.
///
/// The thread may sleep longer than the duration specified due to scheduling
/// specifics or platform-dependent functionality. It will never sleep less.
///
/// This function is blocking, and should not be used in `async` functions.
///
/// # Platform-specific behavior
///
/// On Unix platforms, the underlying syscall may be interrupted by a
/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
/// the specified duration, this function may invoke that system call multiple
/// times.
/// Platforms which do not support nanosecond precision for sleeping will
/// have `dur` rounded up to the nearest granularity of time they can sleep for.
///
/// Currently, specifying a zero duration on Unix platforms returns immediately
/// without invoking the underlying [`nanosleep`] syscall, whereas on Windows
/// platforms the underlying [`Sleep`] syscall is always invoked.
/// If the intention is to yield the current time-slice you may want to use
/// [`yield_now`] instead.
///
/// [`nanosleep`]: https://linux.die.net/man/2/nanosleep
/// [`Sleep`]: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-sleep
///
/// # Examples
///
/// ```no_run
/// use std::{thread, time};
///
/// let ten_millis = time::Duration::from_millis(10);
/// let now = time::Instant::now();
///
/// thread::sleep(ten_millis);
///
/// assert!(now.elapsed() >= ten_millis);
/// ```
#[stable(feature = "thread_sleep", since = "1.4.0")]
pub fn sleep(dur: Duration) {
    imp::Thread::sleep(dur)
}

/// Puts the current thread to sleep until the specified deadline has passed.
///
/// The thread may still be asleep after the deadline specified due to
/// scheduling specifics or platform-dependent functionality. It will never
/// wake before.
///
/// This function is blocking, and should not be used in `async` functions.
///
/// # Platform-specific behavior
///
/// This function uses [`sleep`] internally, see its platform-specific behaviour.
///
///
/// # Examples
///
/// A simple game loop that limits the game to 60 frames per second.
///
/// ```no_run
/// #![feature(thread_sleep_until)]
/// # use std::time::{Duration, Instant};
/// # use std::thread;
/// #
/// # fn update() {}
/// # fn render() {}
/// #
/// let max_fps = 60.0;
/// let frame_time = Duration::from_secs_f32(1.0/max_fps);
/// let mut next_frame = Instant::now();
/// loop {
///     thread::sleep_until(next_frame);
///     next_frame += frame_time;
///     update();
///     render();
/// }
/// ```
///
/// A slow api we must not call too fast and which takes a few
/// tries before succeeding. By using `sleep_until` the time the
/// api call takes does not influence when we retry or when we give up
///
/// ```no_run
/// #![feature(thread_sleep_until)]
/// # use std::time::{Duration, Instant};
/// # use std::thread;
/// #
/// # enum Status {
/// #     Ready(usize),
/// #     Waiting,
/// # }
/// # fn slow_web_api_call() -> Status { Status::Ready(42) }
/// #
/// # const MAX_DURATION: Duration = Duration::from_secs(10);
/// #
/// # fn try_api_call() -> Result<usize, ()> {
/// let deadline = Instant::now() + MAX_DURATION;
/// let delay = Duration::from_millis(250);
/// let mut next_attempt = Instant::now();
/// loop {
///     if Instant::now() > deadline {
///         break Err(());
///     }
///     if let Status::Ready(data) = slow_web_api_call() {
///         break Ok(data);
///     }
///
///     next_attempt = deadline.min(next_attempt + delay);
///     thread::sleep_until(next_attempt);
/// }
/// # }
/// # let _data = try_api_call();
/// ```
#[unstable(feature = "thread_sleep_until", issue = "113752")]
pub fn sleep_until(deadline: Instant) {
    let now = Instant::now();

    if let Some(delay) = deadline.checked_duration_since(now) {
        sleep(delay);
    }
}

/// Used to ensure that `park` and `park_timeout` do not unwind, as that can
/// cause undefined behaviour if not handled correctly (see #102398 for context).
struct PanicGuard;

impl Drop for PanicGuard {
    fn drop(&mut self) {
        rtabort!("an irrecoverable error occurred while synchronizing threads")
    }
}

/// Blocks unless or until the current thread's token is made available.
///
/// A call to `park` does not guarantee that the thread will remain parked
/// forever, and callers should be prepared for this possibility. However,
/// it is guaranteed that this function will not panic (it may abort the
/// process if the implementation encounters some rare errors).
///
/// # `park` and `unpark`
///
/// Every thread is equipped with some basic low-level blocking support, via the
/// [`thread::park`][`park`] function and [`thread::Thread::unpark`][`unpark`]
/// method. [`park`] blocks the current thread, which can then be resumed from
/// another thread by calling the [`unpark`] method on the blocked thread's
/// handle.
///
/// Conceptually, each [`Thread`] handle has an associated token, which is
/// initially not present:
///
/// * The [`thread::park`][`park`] function blocks the current thread unless or
///   until the token is available for its thread handle, at which point it
///   atomically consumes the token. It may also return *spuriously*, without
///   consuming the token. [`thread::park_timeout`] does the same, but allows
///   specifying a maximum time to block the thread for.
///
/// * The [`unpark`] method on a [`Thread`] atomically makes the token available
///   if it wasn't already. Because the token is initially absent, [`unpark`]
///   followed by [`park`] will result in the second call returning immediately.
///
/// The API is typically used by acquiring a handle to the current thread,
/// placing that handle in a shared data structure so that other threads can
/// find it, and then `park`ing in a loop. When some desired condition is met, another
/// thread calls [`unpark`] on the handle.
///
/// The motivation for this design is twofold:
///
/// * It avoids the need to allocate mutexes and condvars when building new
///   synchronization primitives; the threads already provide basic
///   blocking/signaling.
///
/// * It can be implemented very efficiently on many platforms.
///
/// # Memory Ordering
///
/// Calls to `park` _synchronize-with_ calls to `unpark`, meaning that memory
/// operations performed before a call to `unpark` are made visible to the thread that
/// consumes the token and returns from `park`. Note that all `park` and `unpark`
/// operations for a given thread form a total order and `park` synchronizes-with
/// _all_ prior `unpark` operations.
///
/// In atomic ordering terms, `unpark` performs a `Release` operation and `park`
/// performs the corresponding `Acquire` operation. Calls to `unpark` for the same
/// thread form a [release sequence].
///
/// Note that being unblocked does not imply a call was made to `unpark`, because
/// wakeups can also be spurious. For example, a valid, but inefficient,
/// implementation could have `park` and `unpark` return immediately without doing anything,
/// making *all* wakeups spurious.
///
/// # Examples
///
/// ```
/// use std::thread;
/// use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
/// use std::time::Duration;
///
/// let flag = Arc::new(AtomicBool::new(false));
/// let flag2 = Arc::clone(&flag);
///
/// let parked_thread = thread::spawn(move || {
///     // We want to wait until the flag is set. We *could* just spin, but using
///     // park/unpark is more efficient.
///     while !flag2.load(Ordering::Relaxed) {
///         println!("Parking thread");
///         thread::park();
///         // We *could* get here spuriously, i.e., way before the 10ms below are over!
///         // But that is no problem, we are in a loop until the flag is set anyway.
///         println!("Thread unparked");
///     }
///     println!("Flag received");
/// });
///
/// // Let some time pass for the thread to be spawned.
/// thread::sleep(Duration::from_millis(10));
///
/// // Set the flag, and let the thread wake up.
/// // There is no race condition here, if `unpark`
/// // happens first, `park` will return immediately.
/// // Hence there is no risk of a deadlock.
/// flag.store(true, Ordering::Relaxed);
/// println!("Unpark the thread");
/// parked_thread.thread().unpark();
///
/// parked_thread.join().unwrap();
/// ```
///
/// [`unpark`]: Thread::unpark
/// [`thread::park_timeout`]: park_timeout
/// [release sequence]: https://en.cppreference.com/w/cpp/atomic/memory_order#Release_sequence
#[stable(feature = "rust1", since = "1.0.0")]
pub fn park() {
    let guard = PanicGuard;
    // SAFETY: park_timeout is called on the parker owned by this thread.
    unsafe {
        current().park();
    }
    // No panic occurred, do not abort.
    forget(guard);
}

/// Uses [`park_timeout`].
///
/// Blocks unless or until the current thread's token is made available or
/// the specified duration has been reached (may wake spuriously).
///
/// The semantics of this function are equivalent to [`park`] except
/// that the thread will be blocked for roughly no longer than `dur`. This
/// method should not be used for precise timing due to anomalies such as
/// preemption or platform differences that might not cause the maximum
/// amount of time waited to be precisely `ms` long.
///
/// See the [park documentation][`park`] for more detail.
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(since = "1.6.0", note = "replaced by `std::thread::park_timeout`")]
pub fn park_timeout_ms(ms: u32) {
    park_timeout(Duration::from_millis(ms as u64))
}

/// Blocks unless or until the current thread's token is made available or
/// the specified duration has been reached (may wake spuriously).
///
/// The semantics of this function are equivalent to [`park`][park] except
/// that the thread will be blocked for roughly no longer than `dur`. This
/// method should not be used for precise timing due to anomalies such as
/// preemption or platform differences that might not cause the maximum
/// amount of time waited to be precisely `dur` long.
///
/// See the [park documentation][park] for more details.
///
/// # Platform-specific behavior
///
/// Platforms which do not support nanosecond precision for sleeping will have
/// `dur` rounded up to the nearest granularity of time they can sleep for.
///
/// # Examples
///
/// Waiting for the complete expiration of the timeout:
///
/// ```rust,no_run
/// use std::thread::park_timeout;
/// use std::time::{Instant, Duration};
///
/// let timeout = Duration::from_secs(2);
/// let beginning_park = Instant::now();
///
/// let mut timeout_remaining = timeout;
/// loop {
///     park_timeout(timeout_remaining);
///     let elapsed = beginning_park.elapsed();
///     if elapsed >= timeout {
///         break;
///     }
///     println!("restarting park_timeout after {elapsed:?}");
///     timeout_remaining = timeout - elapsed;
/// }
/// ```
#[stable(feature = "park_timeout", since = "1.4.0")]
pub fn park_timeout(dur: Duration) {
    let guard = PanicGuard;
    // SAFETY: park_timeout is called on the parker owned by this thread.
    unsafe {
        current().inner.as_ref().parker().park_timeout(dur);
    }
    // No panic occurred, do not abort.
    forget(guard);
}

////////////////////////////////////////////////////////////////////////////////
// ThreadId
////////////////////////////////////////////////////////////////////////////////

/// A unique identifier for a running thread.
///
/// A `ThreadId` is an opaque object that uniquely identifies each thread
/// created during the lifetime of a process. `ThreadId`s are guaranteed not to
/// be reused, even when a thread terminates. `ThreadId`s are under the control
/// of Rust's standard library and there may not be any relationship between
/// `ThreadId` and the underlying platform's notion of a thread identifier --
/// the two concepts cannot, therefore, be used interchangeably. A `ThreadId`
/// can be retrieved from the [`id`] method on a [`Thread`].
///
/// # Examples
///
/// ```
/// use std::thread;
///
/// let other_thread = thread::spawn(|| {
///     thread::current().id()
/// });
///
/// let other_thread_id = other_thread.join().unwrap();
/// assert!(thread::current().id() != other_thread_id);
/// ```
///
/// [`id`]: Thread::id
#[stable(feature = "thread_id", since = "1.19.0")]
#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
pub struct ThreadId(NonZero<u64>);

impl ThreadId {
    // Generate a new unique thread ID.
    fn new() -> ThreadId {
        #[cold]
        fn exhausted() -> ! {
            panic!("failed to generate unique thread ID: bitspace exhausted")
        }

        cfg_if::cfg_if! {
            if #[cfg(target_has_atomic = "64")] {
                use crate::sync::atomic::AtomicU64;

                static COUNTER: AtomicU64 = AtomicU64::new(0);

                let mut last = COUNTER.load(Ordering::Relaxed);
                loop {
                    let Some(id) = last.checked_add(1) else {
                        exhausted();
                    };

                    match COUNTER.compare_exchange_weak(last, id, Ordering::Relaxed, Ordering::Relaxed) {
                        Ok(_) => return ThreadId(NonZero::new(id).unwrap()),
                        Err(id) => last = id,
                    }
                }
            } else {
                use crate::sync::{Mutex, PoisonError};

                static COUNTER: Mutex<u64> = Mutex::new(0);

                let mut counter = COUNTER.lock().unwrap_or_else(PoisonError::into_inner);
                let Some(id) = counter.checked_add(1) else {
                    // in case the panic handler ends up calling `ThreadId::new()`,
                    // avoid reentrant lock acquire.
                    drop(counter);
                    exhausted();
                };

                *counter = id;
                drop(counter);
                ThreadId(NonZero::new(id).unwrap())
            }
        }
    }

    /// This returns a numeric identifier for the thread identified by this
    /// `ThreadId`.
    ///
    /// As noted in the documentation for the type itself, it is essentially an
    /// opaque ID, but is guaranteed to be unique for each thread. The returned
    /// value is entirely opaque -- only equality testing is stable. Note that
    /// it is not guaranteed which values new threads will return, and this may
    /// change across Rust versions.
    #[must_use]
    #[unstable(feature = "thread_id_value", issue = "67939")]
    pub fn as_u64(&self) -> NonZero<u64> {
        self.0
    }
}

////////////////////////////////////////////////////////////////////////////////
// Thread
////////////////////////////////////////////////////////////////////////////////

/// The internal representation of a `Thread`'s name.
enum ThreadName {
    Main,
    Other(ThreadNameString),
    Unnamed,
}

// This module ensures private fields are kept private, which is necessary to enforce the safety requirements.
mod thread_name_string {
    use core::str;

    use super::ThreadName;
    use crate::ffi::{CStr, CString};

    /// Like a `String` it's guaranteed UTF-8 and like a `CString` it's null terminated.
    pub(crate) struct ThreadNameString {
        inner: CString,
    }
    impl core::ops::Deref for ThreadNameString {
        type Target = CStr;
        fn deref(&self) -> &CStr {
            &self.inner
        }
    }
    impl From<String> for ThreadNameString {
        fn from(s: String) -> Self {
            Self {
                inner: CString::new(s).expect("thread name may not contain interior null bytes"),
            }
        }
    }
    impl ThreadName {
        pub fn as_cstr(&self) -> Option<&CStr> {
            match self {
                ThreadName::Main => Some(c"main"),
                ThreadName::Other(other) => Some(other),
                ThreadName::Unnamed => None,
            }
        }

        pub fn as_str(&self) -> Option<&str> {
            // SAFETY: `as_cstr` can only return `Some` for a fixed CStr or a `ThreadNameString`,
            // which is guaranteed to be UTF-8.
            self.as_cstr().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
        }
    }
}
pub(crate) use thread_name_string::ThreadNameString;

/// The internal representation of a `Thread` handle
struct Inner {
    name: ThreadName, // Guaranteed to be UTF-8
    id: ThreadId,
    parker: Parker,
}

impl Inner {
    fn parker(self: Pin<&Self>) -> Pin<&Parker> {
        unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
    }
}

#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
/// A handle to a thread.
///
/// Threads are represented via the `Thread` type, which you can get in one of
/// two ways:
///
/// * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
///   function, and calling [`thread`][`JoinHandle::thread`] on the
///   [`JoinHandle`].
/// * By requesting the current thread, using the [`thread::current`] function.
///
/// The [`thread::current`] function is available even for threads not spawned
/// by the APIs of this module.
///
/// There is usually no need to create a `Thread` struct yourself, one
/// should instead use a function like `spawn` to create new threads, see the
/// docs of [`Builder`] and [`spawn`] for more details.
///
/// [`thread::current`]: current
pub struct Thread {
    inner: Pin<Arc<Inner>>,
}

impl Thread {
    /// Used only internally to construct a thread object without spawning.
    pub(crate) fn new(name: String) -> Thread {
        Self::new_inner(ThreadName::Other(name.into()))
    }

    pub(crate) fn new_unnamed() -> Thread {
        Self::new_inner(ThreadName::Unnamed)
    }

    // Used in runtime to construct main thread
    pub(crate) fn new_main() -> Thread {
        Self::new_inner(ThreadName::Main)
    }

    fn new_inner(name: ThreadName) -> Thread {
        // We have to use `unsafe` here to construct the `Parker` in-place,
        // which is required for the UNIX implementation.
        //
        // SAFETY: We pin the Arc immediately after creation, so its address never
        // changes.
        let inner = unsafe {
            let mut arc = Arc::<Inner>::new_uninit();
            let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
            addr_of_mut!((*ptr).name).write(name);
            addr_of_mut!((*ptr).id).write(ThreadId::new());
            Parker::new_in_place(addr_of_mut!((*ptr).parker));
            Pin::new_unchecked(arc.assume_init())
        };

        Thread { inner }
    }

    /// Like the public [`park`], but callable on any handle. This is used to
    /// allow parking in TLS destructors.
    ///
    /// # Safety
    /// May only be called from the thread to which this handle belongs.
    pub(crate) unsafe fn park(&self) {
        unsafe { self.inner.as_ref().parker().park() }
    }

    /// Atomically makes the handle's token available if it is not already.
    ///
    /// Every thread is equipped with some basic low-level blocking support, via
    /// the [`park`][park] function and the `unpark()` method. These can be
    /// used as a more CPU-efficient implementation of a spinlock.
    ///
    /// See the [park documentation][park] for more details.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let parked_thread = thread::Builder::new()
    ///     .spawn(|| {
    ///         println!("Parking thread");
    ///         thread::park();
    ///         println!("Thread unparked");
    ///     })
    ///     .unwrap();
    ///
    /// // Let some time pass for the thread to be spawned.
    /// thread::sleep(Duration::from_millis(10));
    ///
    /// println!("Unpark the thread");
    /// parked_thread.thread().unpark();
    ///
    /// parked_thread.join().unwrap();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn unpark(&self) {
        self.inner.as_ref().parker().unpark();
    }

    /// Gets the thread's unique identifier.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let other_thread = thread::spawn(|| {
    ///     thread::current().id()
    /// });
    ///
    /// let other_thread_id = other_thread.join().unwrap();
    /// assert!(thread::current().id() != other_thread_id);
    /// ```
    #[stable(feature = "thread_id", since = "1.19.0")]
    #[must_use]
    pub fn id(&self) -> ThreadId {
        self.inner.id
    }

    /// Gets the thread's name.
    ///
    /// For more information about named threads, see
    /// [this module-level documentation][naming-threads].
    ///
    /// # Examples
    ///
    /// Threads by default have no name specified:
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let handler = builder.spawn(|| {
    ///     assert!(thread::current().name().is_none());
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    ///
    /// Thread with a specified name:
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new()
    ///     .name("foo".into());
    ///
    /// let handler = builder.spawn(|| {
    ///     assert_eq!(thread::current().name(), Some("foo"))
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    ///
    /// [naming-threads]: ./index.html#naming-threads
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn name(&self) -> Option<&str> {
        self.inner.name.as_str()
    }

    fn cname(&self) -> Option<&CStr> {
        self.inner.name.as_cstr()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Debug for Thread {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Thread")
            .field("id", &self.id())
            .field("name", &self.name())
            .finish_non_exhaustive()
    }
}

////////////////////////////////////////////////////////////////////////////////
// JoinHandle
////////////////////////////////////////////////////////////////////////////////

/// A specialized [`Result`] type for threads.
///
/// Indicates the manner in which a thread exited.
///
/// The value contained in the `Result::Err` variant
/// is the value the thread panicked with;
/// that is, the argument the `panic!` macro was called with.
/// Unlike with normal errors, this value doesn't implement
/// the [`Error`](crate::error::Error) trait.
///
/// Thus, a sensible way to handle a thread panic is to either:
///
/// 1. propagate the panic with [`std::panic::resume_unwind`]
/// 2. or in case the thread is intended to be a subsystem boundary
/// that is supposed to isolate system-level failures,
/// match on the `Err` variant and handle the panic in an appropriate way
///
/// A thread that completes without panicking is considered to exit successfully.
///
/// # Examples
///
/// Matching on the result of a joined thread:
///
/// ```no_run
/// use std::{fs, thread, panic};
///
/// fn copy_in_thread() -> thread::Result<()> {
///     thread::spawn(|| {
///         fs::copy("foo.txt", "bar.txt").unwrap();
///     }).join()
/// }
///
/// fn main() {
///     match copy_in_thread() {
///         Ok(_) => println!("copy succeeded"),
///         Err(e) => panic::resume_unwind(e),
///     }
/// }
/// ```
///
/// [`Result`]: crate::result::Result
/// [`std::panic::resume_unwind`]: crate::panic::resume_unwind
#[stable(feature = "rust1", since = "1.0.0")]
pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;

// This packet is used to communicate the return value between the spawned
// thread and the rest of the program. It is shared through an `Arc` and
// there's no need for a mutex here because synchronization happens with `join()`
// (the caller will never read this packet until the thread has exited).
//
// An Arc to the packet is stored into a `JoinInner` which in turns is placed
// in `JoinHandle`.
struct Packet<'scope, T> {
    scope: Option<Arc<scoped::ScopeData>>,
    result: UnsafeCell<Option<Result<T>>>,
    _marker: PhantomData<Option<&'scope scoped::ScopeData>>,
}

// Due to the usage of `UnsafeCell` we need to manually implement Sync.
// The type `T` should already always be Send (otherwise the thread could not
// have been created) and the Packet is Sync because all access to the
// `UnsafeCell` synchronized (by the `join()` boundary), and `ScopeData` is Sync.
unsafe impl<'scope, T: Send> Sync for Packet<'scope, T> {}

impl<'scope, T> Drop for Packet<'scope, T> {
    fn drop(&mut self) {
        // If this packet was for a thread that ran in a scope, the thread
        // panicked, and nobody consumed the panic payload, we make sure
        // the scope function will panic.
        let unhandled_panic = matches!(self.result.get_mut(), Some(Err(_)));
        // Drop the result without causing unwinding.
        // This is only relevant for threads that aren't join()ed, as
        // join() will take the `result` and set it to None, such that
        // there is nothing left to drop here.
        // If this panics, we should handle that, because we're outside the
        // outermost `catch_unwind` of our thread.
        // We just abort in that case, since there's nothing else we can do.
        // (And even if we tried to handle it somehow, we'd also need to handle
        // the case where the panic payload we get out of it also panics on
        // drop, and so on. See issue #86027.)
        if let Err(_) = panic::catch_unwind(panic::AssertUnwindSafe(|| {
            *self.result.get_mut() = None;
        })) {
            rtabort!("thread result panicked on drop");
        }
        // Book-keeping so the scope knows when it's done.
        if let Some(scope) = &self.scope {
            // Now that there will be no more user code running on this thread
            // that can use 'scope, mark the thread as 'finished'.
            // It's important we only do this after the `result` has been dropped,
            // since dropping it might still use things it borrowed from 'scope.
            scope.decrement_num_running_threads(unhandled_panic);
        }
    }
}

/// Inner representation for JoinHandle
struct JoinInner<'scope, T> {
    native: imp::Thread,
    thread: Thread,
    packet: Arc<Packet<'scope, T>>,
}

impl<'scope, T> JoinInner<'scope, T> {
    fn join(mut self) -> Result<T> {
        self.native.join();
        Arc::get_mut(&mut self.packet).unwrap().result.get_mut().take().unwrap()
    }
}

/// An owned permission to join on a thread (block on its termination).
///
/// A `JoinHandle` *detaches* the associated thread when it is dropped, which
/// means that there is no longer any handle to the thread and no way to `join`
/// on it.
///
/// Due to platform restrictions, it is not possible to [`Clone`] this
/// handle: the ability to join a thread is a uniquely-owned permission.
///
/// This `struct` is created by the [`thread::spawn`] function and the
/// [`thread::Builder::spawn`] method.
///
/// # Examples
///
/// Creation from [`thread::spawn`]:
///
/// ```
/// use std::thread;
///
/// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
///     // some work here
/// });
/// ```
///
/// Creation from [`thread::Builder::spawn`]:
///
/// ```
/// use std::thread;
///
/// let builder = thread::Builder::new();
///
/// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
///     // some work here
/// }).unwrap();
/// ```
///
/// A thread being detached and outliving the thread that spawned it:
///
/// ```no_run
/// use std::thread;
/// use std::time::Duration;
///
/// let original_thread = thread::spawn(|| {
///     let _detached_thread = thread::spawn(|| {
///         // Here we sleep to make sure that the first thread returns before.
///         thread::sleep(Duration::from_millis(10));
///         // This will be called, even though the JoinHandle is dropped.
///         println!("♫ Still alive ♫");
///     });
/// });
///
/// original_thread.join().expect("The thread being joined has panicked");
/// println!("Original thread is joined.");
///
/// // We make sure that the new thread has time to run, before the main
/// // thread returns.
///
/// thread::sleep(Duration::from_millis(1000));
/// ```
///
/// [`thread::Builder::spawn`]: Builder::spawn
/// [`thread::spawn`]: spawn
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(target_os = "teeos", must_use)]
pub struct JoinHandle<T>(JoinInner<'static, T>);

#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
unsafe impl<T> Send for JoinHandle<T> {}
#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
unsafe impl<T> Sync for JoinHandle<T> {}

impl<T> JoinHandle<T> {
    /// Extracts a handle to the underlying thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
    ///     // some work here
    /// }).unwrap();
    ///
    /// let thread = join_handle.thread();
    /// println!("thread id: {:?}", thread.id());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn thread(&self) -> &Thread {
        &self.0.thread
    }

    /// Waits for the associated thread to finish.
    ///
    /// This function will return immediately if the associated thread has already finished.
    ///
    /// In terms of [atomic memory orderings],  the completion of the associated
    /// thread synchronizes with this function returning. In other words, all
    /// operations performed by that thread [happen
    /// before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses) all
    /// operations that happen after `join` returns.
    ///
    /// If the associated thread panics, [`Err`] is returned with the parameter given
    /// to [`panic!`].
    ///
    /// [`Err`]: crate::result::Result::Err
    /// [atomic memory orderings]: crate::sync::atomic
    ///
    /// # Panics
    ///
    /// This function may panic on some platforms if a thread attempts to join
    /// itself or otherwise may create a deadlock with joining threads.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
    ///     // some work here
    /// }).unwrap();
    /// join_handle.join().expect("Couldn't join on the associated thread");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn join(self) -> Result<T> {
        self.0.join()
    }

    /// Checks if the associated thread has finished running its main function.
    ///
    /// `is_finished` supports implementing a non-blocking join operation, by checking
    /// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
    /// block while waiting on the thread to finish, use [`join`][Self::join].
    ///
    /// This might return `true` for a brief moment after the thread's main
    /// function has returned, but before the thread itself has stopped running.
    /// However, once this returns `true`, [`join`][Self::join] can be expected
    /// to return quickly, without blocking for any significant amount of time.
    #[stable(feature = "thread_is_running", since = "1.61.0")]
    pub fn is_finished(&self) -> bool {
        Arc::strong_count(&self.0.packet) == 1
    }
}

impl<T> AsInner<imp::Thread> for JoinHandle<T> {
    fn as_inner(&self) -> &imp::Thread {
        &self.0.native
    }
}

impl<T> IntoInner<imp::Thread> for JoinHandle<T> {
    fn into_inner(self) -> imp::Thread {
        self.0.native
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T> fmt::Debug for JoinHandle<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("JoinHandle").finish_non_exhaustive()
    }
}

fn _assert_sync_and_send() {
    fn _assert_both<T: Send + Sync>() {}
    _assert_both::<JoinHandle<()>>();
    _assert_both::<Thread>();
}

/// Returns an estimate of the default amount of parallelism a program should use.
///
/// Parallelism is a resource. A given machine provides a certain capacity for
/// parallelism, i.e., a bound on the number of computations it can perform
/// simultaneously. This number often corresponds to the amount of CPUs a
/// computer has, but it may diverge in various cases.
///
/// Host environments such as VMs or container orchestrators may want to
/// restrict the amount of parallelism made available to programs in them. This
/// is often done to limit the potential impact of (unintentionally)
/// resource-intensive programs on other programs running on the same machine.
///
/// # Limitations
///
/// The purpose of this API is to provide an easy and portable way to query
/// the default amount of parallelism the program should use. Among other things it
/// does not expose information on NUMA regions, does not account for
/// differences in (co)processor capabilities or current system load,
/// and will not modify the program's global state in order to more accurately
/// query the amount of available parallelism.
///
/// Where both fixed steady-state and burst limits are available the steady-state
/// capacity will be used to ensure more predictable latencies.
///
/// Resource limits can be changed during the runtime of a program, therefore the value is
/// not cached and instead recomputed every time this function is called. It should not be
/// called from hot code.
///
/// The value returned by this function should be considered a simplified
/// approximation of the actual amount of parallelism available at any given
/// time. To get a more detailed or precise overview of the amount of
/// parallelism available to the program, you may wish to use
/// platform-specific APIs as well. The following platform limitations currently
/// apply to `available_parallelism`:
///
/// On Windows:
/// - It may undercount the amount of parallelism available on systems with more
///   than 64 logical CPUs. However, programs typically need specific support to
///   take advantage of more than 64 logical CPUs, and in the absence of such
///   support, the number returned by this function accurately reflects the
///   number of logical CPUs the program can use by default.
/// - It may overcount the amount of parallelism available on systems limited by
///   process-wide affinity masks, or job object limitations.
///
/// On Linux:
/// - It may overcount the amount of parallelism available when limited by a
///   process-wide affinity mask or cgroup quotas and `sched_getaffinity()` or cgroup fs can't be
///   queried, e.g. due to sandboxing.
/// - It may undercount the amount of parallelism if the current thread's affinity mask
///   does not reflect the process' cpuset, e.g. due to pinned threads.
/// - If the process is in a cgroup v1 cpu controller, this may need to
///   scan mountpoints to find the corresponding cgroup v1 controller,
///   which may take time on systems with large numbers of mountpoints.
///   (This does not apply to cgroup v2, or to processes not in a
///   cgroup.)
///
/// On all targets:
/// - It may overcount the amount of parallelism available when running in a VM
/// with CPU usage limits (e.g. an overcommitted host).
///
/// # Errors
///
/// This function will, but is not limited to, return errors in the following
/// cases:
///
/// - If the amount of parallelism is not known for the target platform.
/// - If the program lacks permission to query the amount of parallelism made
///   available to it.
///
/// # Examples
///
/// ```
/// # #![allow(dead_code)]
/// use std::{io, thread};
///
/// fn main() -> io::Result<()> {
///     let count = thread::available_parallelism()?.get();
///     assert!(count >= 1_usize);
///     Ok(())
/// }
/// ```
#[doc(alias = "available_concurrency")] // Alias for a previous name we gave this API on unstable.
#[doc(alias = "hardware_concurrency")] // Alias for C++ `std::thread::hardware_concurrency`.
#[doc(alias = "num_cpus")] // Alias for a popular ecosystem crate which provides similar functionality.
#[stable(feature = "available_parallelism", since = "1.59.0")]
pub fn available_parallelism() -> io::Result<NonZero<usize>> {
    imp::available_parallelism()
}