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
//! 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
//! child 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" from the current
//! thread. This means that it can outlive its parent (the thread that spawned
//! it), unless this parent is the main thread.
//!
//! The parent thread can also wait on the completion of the child
//! thread; a call to [`spawn`] produces a [`JoinHandle`], which provides
//! a `join` method for waiting:
//!
//! ```rust
//! use std::thread;
//!
//! let child = thread::spawn(move || {
//!     // some work here
//! });
//! // some work here
//! let res = child.join();
//! ```
//!
//! The [`join`] method returns a [`thread::Result`] containing [`Ok`] of the final
//! value produced by the child thread, or [`Err`] of the value given to
//! a call to [`panic!`] if the child panicked.
//!
//! ## 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 child thread:
//!
//! ```rust
//! # #![allow(unused_must_use)]
//! use std::thread;
//!
//! thread::Builder::new().name("child1".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 examples of 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 for spawned threads is 2 MiB, though this particular stack size is
//! subject to change in the future. 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.
//!
//! 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::name`]: Thread::name
//! [`thread::park_timeout`]: park_timeout
//! [`Cell`]: crate::cell::Cell
//! [`RefCell`]: crate::cell::RefCell
//! [`with`]: LocalKey::with

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

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

use crate::any::Any;
use crate::cell::UnsafeCell;
use crate::ffi::{CStr, CString};
use crate::fmt;
use crate::io;
use crate::mem;
use crate::num::NonZeroU64;
use crate::panic;
use crate::panicking;
use crate::str;
use crate::sync::Arc;
use crate::sys::thread as imp;
use crate::sys_common::mutex;
use crate::sys_common::thread;
use crate::sys_common::thread_info;
use crate::sys_common::thread_parker::Parker;
use crate::sys_common::{AsInner, IntoInner};
use crate::time::Duration;

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

#[macro_use]
mod local;

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

// The types used by the thread_local! macro to access TLS keys. Note that there
// are two types, the "OS" type and the "fast" type. The OS thread local key
// type is accessed via platform-specific API calls and is slow, while the fast
// key type is accessed via code generated via LLVM, where TLS keys are set up
// by the elf linker. Note that the OS TLS type is always available: on macOS
// the standard library is compiled with support for older platform versions
// where fast TLS was not available; end-user code is compiled with fast TLS
// where available, but both are needed.

#[unstable(feature = "libstd_thread_internals", issue = "none")]
#[cfg(target_thread_local)]
#[doc(hidden)]
pub use self::local::fast::Key as __FastLocalKeyInner;
#[unstable(feature = "libstd_thread_internals", issue = "none")]
#[doc(hidden)]
pub use self::local::os::Key as __OsLocalKeyInner;
#[unstable(feature = "libstd_thread_internals", issue = "none")]
#[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))]
#[doc(hidden)]
pub use self::local::statik::Key as __StaticLocalKeyInner;

////////////////////////////////////////////////////////////////////////////////
// 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
#[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 child 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 child 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 no references in the supplied thread closure
    /// or its return type can outlive the spawned thread's lifetime. 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
    ///
    /// ```
    /// #![feature(thread_spawn_unchecked)]
    /// 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
    #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
    pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
    where
        F: FnOnce() -> T,
        F: Send + 'a,
        T: Send + 'a,
    {
        let Builder { name, stack_size } = self;

        let stack_size = stack_size.unwrap_or_else(thread::min_stack);

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

        let my_packet: Arc<UnsafeCell<Option<Result<T>>>> = Arc::new(UnsafeCell::new(None));
        let their_packet = my_packet.clone();

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

            // SAFETY: the stack guard passed is the one for the current thread.
            // This means the current thread's stack and the new thread's stack
            // are properly set and protected from each other.
            thread_info::set(unsafe { imp::guard::current() }, their_thread);
            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                crate::sys_common::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.get() = Some(try_result) };
        };

        Ok(JoinHandle(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 {
                Some(imp::Thread::new(
                    stack_size,
                    mem::transmute::<Box<dyn FnOnce() + 'a>, Box<dyn FnOnce() + 'static>>(
                        Box::new(main),
                    ),
                )?)
            },
            thread: my_thread,
            packet: Packet(my_packet),
        }))
    }
}

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

/// Spawns a new thread, returning a [`JoinHandle`] for it.
///
/// The join handle will implicitly *detach* the child thread upon being
/// dropped. In this case, the child thread may outlive the parent (unless
/// the parent thread is the main thread; the whole process is terminated when
/// the main thread finishes). Additionally, the join handle provides a [`join`]
/// method that can be used to join the child thread. If the child thread
/// panics, [`join`] will return an [`Err`] containing the argument given to
/// [`panic!`].
///
/// This 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 `detach` and 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")
}

/// 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();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn current() -> Thread {
    thread_info::current_thread().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 is used when the programmer knows that the thread will have nothing
/// to do for some time, and thus avoid wasting computing time.
///
/// For example when polling on a resource, it is common to check that it is
/// available, and if not to yield in order to avoid busy waiting.
///
/// Thus the pattern of `yield`ing after a failed poll is rather common when
/// implementing low-level shared resources or synchronization primitives.
///
/// However programmers will usually prefer to use [`channel`]s, [`Condvar`]s,
/// [`Mutex`]es or [`join`] for their synchronization routines, as they avoid
/// thinking about thread scheduling.
///
/// Note that [`channel`]s for example are implemented using this primitive.
/// Indeed when you call `send` or `recv`, which are blocking, they will yield
/// if the channel is not available.
///
/// # 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]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn panicking() -> bool {
    panicking::panicking()
}

/// 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")]
#[rustc_deprecated(since = "1.6.0", reason = "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.
///
/// # 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)
}

/// 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.
///
/// # 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.
///
/// In other words, each [`Thread`] acts a bit like a spinlock that can be
/// locked and unlocked using `park` and `unpark`.
///
/// Notice that being unblocked does not imply any synchronization with someone
/// that unparked this thread, it could also be spurious.
/// For example, it would be a valid, but inefficient, implementation to make both [`park`] and
/// [`unpark`] return immediately without doing anything.
///
/// 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.
///
/// # 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::Acquire) {
///         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::Release);
/// println!("Unpark the thread");
/// parked_thread.thread().unpark();
///
/// parked_thread.join().unwrap();
/// ```
///
/// [`unpark`]: Thread::unpark
/// [`thread::park_timeout`]: park_timeout
#[stable(feature = "rust1", since = "1.0.0")]
pub fn park() {
    // SAFETY: park_timeout is called on the parker owned by this thread.
    unsafe {
        current().inner.parker.park();
    }
}

/// Use [`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 may 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")]
#[rustc_deprecated(since = "1.6.0", reason = "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 may 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) {
    // SAFETY: park_timeout is called on the parker owned by this thread.
    unsafe {
        current().inner.parker.park_timeout(dur);
    }
}

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

/// A unique identifier for a running thread.
///
/// A `ThreadId` is an opaque object that has a unique value for each thread
/// that creates one. `ThreadId`s are not guaranteed to correspond to a thread's
/// system-designated identifier. 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(NonZeroU64);

impl ThreadId {
    // Generate a new unique thread ID.
    fn new() -> ThreadId {
        // It is UB to attempt to acquire this mutex reentrantly!
        static GUARD: mutex::StaticMutex = mutex::StaticMutex::new();
        static mut COUNTER: u64 = 1;

        unsafe {
            let _guard = GUARD.lock();

            // If we somehow use up all our bits, panic so that we're not
            // covering up subtle bugs of IDs being reused.
            if COUNTER == u64::MAX {
                panic!("failed to generate unique thread ID: bitspace exhausted");
            }

            let id = COUNTER;
            COUNTER += 1;

            ThreadId(NonZeroU64::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.
    #[unstable(feature = "thread_id_value", issue = "67939")]
    pub fn as_u64(&self) -> NonZeroU64 {
        self.0
    }
}

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

/// The internal representation of a `Thread` handle
struct Inner {
    name: Option<CString>, // Guaranteed to be UTF-8
    id: ThreadId,
    parker: 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: Arc<Inner>,
}

impl Thread {
    // Used only internally to construct a thread object without spawning
    // Panics if the name contains nuls.
    pub(crate) fn new(name: Option<String>) -> Thread {
        let cname =
            name.map(|n| CString::new(n).expect("thread name may not contain interior null bytes"));
        Thread {
            inner: Arc::new(Inner { name: cname, id: ThreadId::new(), parker: Parker::new() }),
        }
    }

    /// 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.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")]
    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")]
    pub fn name(&self) -> Option<&str> {
        self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
    }

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

#[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()
    }
}

////////////////////////////////////////////////////////////////////////////////
// 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. `unwrap` the `Result<T>`, propagating the panic
/// 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
///
/// ```no_run
/// use std::thread;
/// use std::fs;
///
/// fn copy_in_thread() -> thread::Result<()> {
///     thread::spawn(move || { fs::copy("foo.txt", "bar.txt").unwrap(); }).join()
/// }
///
/// fn main() {
///     match copy_in_thread() {
///         Ok(_) => println!("this is fine"),
///         Err(_) => println!("thread panicked"),
///     }
/// }
/// ```
///
/// [`Result`]: crate::result::Result
#[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 child thread
// and the parent thread. Memory is shared through the `Arc` within and there's
// no need for a mutex here because synchronization happens with `join()` (the
// parent thread never reads this packet until the child has exited).
//
// This packet itself is then stored into a `JoinInner` which in turns is placed
// in `JoinHandle` and `JoinGuard`. Due to the usage of `UnsafeCell` we need to
// manually worry about impls like Send and Sync. The type `T` should
// already always be Send (otherwise the thread could not have been created) and
// this type is inherently Sync because no methods take &self. Regardless,
// however, we add inheriting impls for Send/Sync to this type to ensure it's
// Send/Sync and that future modifications will still appropriately classify it.
struct Packet<T>(Arc<UnsafeCell<Option<Result<T>>>>);

unsafe impl<T: Send> Send for Packet<T> {}
unsafe impl<T: Sync> Sync for Packet<T> {}

/// Inner representation for JoinHandle
struct JoinInner<T> {
    native: Option<imp::Thread>,
    thread: Thread,
    packet: Packet<T>,
}

impl<T> JoinInner<T> {
    fn join(&mut self) -> Result<T> {
        self.native.take().unwrap().join();
        unsafe { (*self.packet.0.get()).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 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();
/// ```
///
/// Child being detached and outliving its parent:
///
/// ```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")]
pub struct JoinHandle<T>(JoinInner<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")]
    pub fn thread(&self) -> &Thread {
        &self.0.thread
    }

    /// Waits for the associated thread to finish.
    ///
    /// 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 are ordered before all
    /// operations that happen after `join` returns.
    ///
    /// If the child 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(mut self) -> Result<T> {
        self.0.join()
    }
}

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

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

#[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.pad("JoinHandle { .. }")
    }
}

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