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
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
//! Shareable mutable containers.
//!
//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
//! have one of the following:
//!
//! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
//! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
//!
//! This is enforced by the Rust compiler. However, there are situations where this rule is not
//! flexible enough. Sometimes it is required to have multiple references to an object and yet
//! mutate it.
//!
//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
//! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded
//! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
//! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to
//! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types.
//!
//! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
//! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
//! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
//! with typical Rust types that exhibit 'inherited mutability'.
//!
//! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
//! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
//! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
//! methods to retrieve and change the current interior value:
//!
//!  - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
//!    interior value.
//!  - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
//!    interior value with [`Default::default()`] and returns the replaced value.
//!  - For all types, the [`replace`](Cell::replace) method replaces the current interior value and
//!    returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the
//!    `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method
//!    replaces the interior value, dropping the replaced value.
//!
//! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
//! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
//! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
//! to borrow a value that is already mutably borrowed; when this happens it results in thread
//! panic.
//!
//! # When to choose interior mutability
//!
//! The more common inherited mutability, where one must have unique access to mutate a value, is
//! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
//! interior mutability is something of a last resort. Since cell types enable mutation where it
//! would otherwise be disallowed though, there are occasions when interior mutability might be
//! appropriate, or even *must* be used, e.g.
//!
//! * Introducing mutability 'inside' of something immutable
//! * Implementation details of logically-immutable methods.
//! * Mutating implementations of [`Clone`].
//!
//! ## Introducing mutability 'inside' of something immutable
//!
//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
//! be cloned and shared between multiple parties. Because the contained values may be
//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
//! impossible to mutate data inside of these smart pointers at all.
//!
//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
//! mutability:
//!
//! ```
//! use std::cell::{RefCell, RefMut};
//! use std::collections::HashMap;
//! use std::rc::Rc;
//!
//! fn main() {
//!     let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
//!     // Create a new block to limit the scope of the dynamic borrow
//!     {
//!         let mut map: RefMut<_> = shared_map.borrow_mut();
//!         map.insert("africa", 92388);
//!         map.insert("kyoto", 11837);
//!         map.insert("piccadilly", 11826);
//!         map.insert("marbles", 38);
//!     }
//!
//!     // Note that if we had not let the previous borrow of the cache fall out
//!     // of scope then the subsequent borrow would cause a dynamic thread panic.
//!     // This is the major hazard of using `RefCell`.
//!     let total: i32 = shared_map.borrow().values().sum();
//!     println!("{}", total);
//! }
//! ```
//!
//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
//! multi-threaded situation.
//!
//! ## Implementation details of logically-immutable methods
//!
//! Occasionally it may be desirable not to expose in an API that there is mutation happening
//! "under the hood". This may be because logically the operation is immutable, but e.g., caching
//! forces the implementation to perform mutation; or because you must employ mutation to implement
//! a trait method that was originally defined to take `&self`.
//!
//! ```
//! # #![allow(dead_code)]
//! use std::cell::RefCell;
//!
//! struct Graph {
//!     edges: Vec<(i32, i32)>,
//!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
//! }
//!
//! impl Graph {
//!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
//!         self.span_tree_cache.borrow_mut()
//!             .get_or_insert_with(|| self.calc_span_tree())
//!             .clone()
//!     }
//!
//!     fn calc_span_tree(&self) -> Vec<(i32, i32)> {
//!         // Expensive computation goes here
//!         vec![]
//!     }
//! }
//! ```
//!
//! ## Mutating implementations of `Clone`
//!
//! This is simply a special - but common - case of the previous: hiding mutability for operations
//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
//! reference counts within a `Cell<T>`.
//!
//! ```
//! use std::cell::Cell;
//! use std::ptr::NonNull;
//! use std::process::abort;
//! use std::marker::PhantomData;
//!
//! struct Rc<T: ?Sized> {
//!     ptr: NonNull<RcBox<T>>,
//!     phantom: PhantomData<RcBox<T>>,
//! }
//!
//! struct RcBox<T: ?Sized> {
//!     strong: Cell<usize>,
//!     refcount: Cell<usize>,
//!     value: T,
//! }
//!
//! impl<T: ?Sized> Clone for Rc<T> {
//!     fn clone(&self) -> Rc<T> {
//!         self.inc_strong();
//!         Rc {
//!             ptr: self.ptr,
//!             phantom: PhantomData,
//!         }
//!     }
//! }
//!
//! trait RcBoxPtr<T: ?Sized> {
//!
//!     fn inner(&self) -> &RcBox<T>;
//!
//!     fn strong(&self) -> usize {
//!         self.inner().strong.get()
//!     }
//!
//!     fn inc_strong(&self) {
//!         self.inner()
//!             .strong
//!             .set(self.strong()
//!                      .checked_add(1)
//!                      .unwrap_or_else(|| abort() ));
//!     }
//! }
//!
//! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
//!    fn inner(&self) -> &RcBox<T> {
//!        unsafe {
//!            self.ptr.as_ref()
//!        }
//!    }
//! }
//! ```
//!
//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
//! [`atomic`]: crate::sync::atomic

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

use crate::cmp::Ordering;
use crate::fmt::{self, Debug, Display};
use crate::marker::Unsize;
use crate::mem;
use crate::ops::{CoerceUnsized, Deref, DerefMut};
use crate::ptr;

/// A mutable memory location.
///
/// # Examples
///
/// In this example, you can see that `Cell<T>` enables mutation inside an
/// immutable struct. In other words, it enables "interior mutability".
///
/// ```
/// use std::cell::Cell;
///
/// struct SomeStruct {
///     regular_field: u8,
///     special_field: Cell<u8>,
/// }
///
/// let my_struct = SomeStruct {
///     regular_field: 0,
///     special_field: Cell::new(1),
/// };
///
/// let new_value = 100;
///
/// // ERROR: `my_struct` is immutable
/// // my_struct.regular_field = new_value;
///
/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
/// // which can always be mutated
/// my_struct.special_field.set(new_value);
/// assert_eq!(my_struct.special_field.get(), new_value);
/// ```
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
pub struct Cell<T: ?Sized> {
    value: UnsafeCell<T>,
}

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

// Note that this negative impl isn't strictly necessary for correctness,
// as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
// However, given how important `Cell`'s `!Sync`-ness is,
// having an explicit negative impl is nice for documentation purposes
// and results in nicer error messages.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for Cell<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Copy> Clone for Cell<T> {
    #[inline]
    fn clone(&self) -> Cell<T> {
        Cell::new(self.get())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Cell<T> {
    /// Creates a `Cell<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> Cell<T> {
        Cell::new(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialEq + Copy> PartialEq for Cell<T> {
    #[inline]
    fn eq(&self, other: &Cell<T>) -> bool {
        self.get() == other.get()
    }
}

#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: Eq + Copy> Eq for Cell<T> {}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
    #[inline]
    fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
        self.get().partial_cmp(&other.get())
    }

    #[inline]
    fn lt(&self, other: &Cell<T>) -> bool {
        self.get() < other.get()
    }

    #[inline]
    fn le(&self, other: &Cell<T>) -> bool {
        self.get() <= other.get()
    }

    #[inline]
    fn gt(&self, other: &Cell<T>) -> bool {
        self.get() > other.get()
    }

    #[inline]
    fn ge(&self, other: &Cell<T>) -> bool {
        self.get() >= other.get()
    }
}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: Ord + Copy> Ord for Cell<T> {
    #[inline]
    fn cmp(&self, other: &Cell<T>) -> Ordering {
        self.get().cmp(&other.get())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const From<T> for Cell<T> {
    fn from(t: T) -> Cell<T> {
        Cell::new(t)
    }
}

impl<T> Cell<T> {
    /// Creates a new `Cell` containing the given value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
    #[inline]
    pub const fn new(value: T) -> Cell<T> {
        Cell { value: UnsafeCell::new(value) }
    }

    /// Sets the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// c.set(10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn set(&self, val: T) {
        let old = self.replace(val);
        drop(old);
    }

    /// Swaps the values of two `Cell`s.
    /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c1 = Cell::new(5i32);
    /// let c2 = Cell::new(10i32);
    /// c1.swap(&c2);
    /// assert_eq!(10, c1.get());
    /// assert_eq!(5, c2.get());
    /// ```
    #[inline]
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn swap(&self, other: &Self) {
        if ptr::eq(self, other) {
            return;
        }
        // SAFETY: This can be risky if called from separate threads, but `Cell`
        // is `!Sync` so this won't happen. This also won't invalidate any
        // pointers since `Cell` makes sure nothing else will be pointing into
        // either of these `Cell`s.
        unsafe {
            ptr::swap(self.value.get(), other.value.get());
        }
    }

    /// Replaces the contained value with `val`, and returns the old contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let cell = Cell::new(5);
    /// assert_eq!(cell.get(), 5);
    /// assert_eq!(cell.replace(10), 5);
    /// assert_eq!(cell.get(), 10);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn replace(&self, val: T) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        mem::replace(unsafe { &mut *self.value.get() }, val)
    }

    /// Unwraps the value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let five = c.into_inner();
    ///
    /// assert_eq!(five, 5);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    pub const fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

impl<T: Copy> Cell<T> {
    /// Returns a copy of the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// let five = c.get();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get(&self) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        unsafe { *self.value.get() }
    }

    /// Updates the contained value using a function and returns the new value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_update)]
    ///
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let new = c.update(|x| x + 1);
    ///
    /// assert_eq!(new, 6);
    /// assert_eq!(c.get(), 6);
    /// ```
    #[inline]
    #[unstable(feature = "cell_update", issue = "50186")]
    pub fn update<F>(&self, f: F) -> T
    where
        F: FnOnce(T) -> T,
    {
        let old = self.get();
        let new = f(old);
        self.set(new);
        new
    }
}

impl<T: ?Sized> Cell<T> {
    /// Returns a raw pointer to the underlying data in this cell.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// let ptr = c.as_ptr();
    /// ```
    #[inline]
    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
    #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
    pub const fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows `Cell` mutably (at compile-time) which guarantees
    /// that we possess the only reference.
    ///
    /// However be cautious: this method expects `self` to be mutable, which is
    /// generally not the case when using a `Cell`. If you require interior
    /// mutability by reference, consider using `RefCell` which provides
    /// run-time checked mutable borrows through its [`borrow_mut`] method.
    ///
    /// [`borrow_mut`]: RefCell::borrow_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let mut c = Cell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(c.get(), 6);
    /// ```
    #[inline]
    #[stable(feature = "cell_get_mut", since = "1.11.0")]
    pub fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    /// Returns a `&Cell<T>` from a `&mut T`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let slice: &mut [i32] = &mut [1, 2, 3];
    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
    ///
    /// assert_eq!(slice_cell.len(), 3);
    /// ```
    #[inline]
    #[stable(feature = "as_cell", since = "1.37.0")]
    pub fn from_mut(t: &mut T) -> &Cell<T> {
        // SAFETY: `&mut` ensures unique access.
        unsafe { &*(t as *mut T as *const Cell<T>) }
    }
}

impl<T: Default> Cell<T> {
    /// Takes the value of the cell, leaving `Default::default()` in its place.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let five = c.take();
    ///
    /// assert_eq!(five, 5);
    /// assert_eq!(c.into_inner(), 0);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn take(&self) -> T {
        self.replace(Default::default())
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}

impl<T> Cell<[T]> {
    /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let slice: &mut [i32] = &mut [1, 2, 3];
    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
    ///
    /// assert_eq!(slice_cell.len(), 3);
    /// ```
    #[stable(feature = "as_cell", since = "1.37.0")]
    pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
        // SAFETY: `Cell<T>` has the same memory layout as `T`.
        unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
    }
}

impl<T, const N: usize> Cell<[T; N]> {
    /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(as_array_of_cells)]
    /// use std::cell::Cell;
    ///
    /// let mut array: [i32; 3] = [1, 2, 3];
    /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
    /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
    /// ```
    #[unstable(feature = "as_array_of_cells", issue = "88248")]
    pub fn as_array_of_cells(&self) -> &[Cell<T>; N] {
        // SAFETY: `Cell<T>` has the same memory layout as `T`.
        unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
    }
}

/// A mutable memory location with dynamically checked borrow rules
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowFlag>,
    // Stores the location of the earliest currently active borrow.
    // This gets updated whenever we go from having zero borrows
    // to having a single borrow. When a borrow occurs, this gets included
    // in the generated `BorrowError/`BorrowMutError`
    #[cfg(feature = "debug_refcell")]
    borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
    value: UnsafeCell<T>,
}

/// An error returned by [`RefCell::try_borrow`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowError {
    #[cfg(feature = "debug_refcell")]
    location: &'static crate::panic::Location<'static>,
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut builder = f.debug_struct("BorrowError");

        #[cfg(feature = "debug_refcell")]
        builder.field("location", self.location);

        builder.finish()
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Display::fmt("already mutably borrowed", f)
    }
}

/// An error returned by [`RefCell::try_borrow_mut`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowMutError {
    #[cfg(feature = "debug_refcell")]
    location: &'static crate::panic::Location<'static>,
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut builder = f.debug_struct("BorrowMutError");

        #[cfg(feature = "debug_refcell")]
        builder.field("location", self.location);

        builder.finish()
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Display::fmt("already borrowed", f)
    }
}

// Positive values represent the number of `Ref` active. Negative values
// represent the number of `RefMut` active. Multiple `RefMut`s can only be
// active at a time if they refer to distinct, nonoverlapping components of a
// `RefCell` (e.g., different ranges of a slice).
//
// `Ref` and `RefMut` are both two words in size, and so there will likely never
// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
// range. Thus, a `BorrowFlag` will probably never overflow or underflow.
// However, this is not a guarantee, as a pathological program could repeatedly
// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
// explicitly check for overflow and underflow in order to avoid unsafety, or at
// least behave correctly in the event that overflow or underflow happens (e.g.,
// see BorrowRef::new).
type BorrowFlag = isize;
const UNUSED: BorrowFlag = 0;

#[inline(always)]
fn is_writing(x: BorrowFlag) -> bool {
    x < UNUSED
}

#[inline(always)]
fn is_reading(x: BorrowFlag) -> bool {
    x > UNUSED
}

impl<T> RefCell<T> {
    /// Creates a new `RefCell` containing `value`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
    #[inline]
    pub const fn new(value: T) -> RefCell<T> {
        RefCell {
            value: UnsafeCell::new(value),
            borrow: Cell::new(UNUSED),
            #[cfg(feature = "debug_refcell")]
            borrowed_at: Cell::new(None),
        }
    }

    /// Consumes the `RefCell`, returning the wrapped value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let five = c.into_inner();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    #[inline]
    pub const fn into_inner(self) -> T {
        // Since this function takes `self` (the `RefCell`) by value, the
        // compiler statically verifies that it is not currently borrowed.
        self.value.into_inner()
    }

    /// Replaces the wrapped value with a new one, returning the old value,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace(6);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "refcell_replace", since = "1.24.0")]
    #[track_caller]
    pub fn replace(&self, t: T) -> T {
        mem::replace(&mut *self.borrow_mut(), t)
    }

    /// Replaces the wrapped value with a new one computed from `f`, returning
    /// the old value, without deinitializing either one.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace_with(|&mut old| old + 1);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
    #[track_caller]
    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
        let mut_borrow = &mut *self.borrow_mut();
        let replacement = f(mut_borrow);
        mem::replace(mut_borrow, replacement)
    }

    /// Swaps the wrapped value of `self` with the wrapped value of `other`,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
    ///
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let c = RefCell::new(5);
    /// let d = RefCell::new(6);
    /// c.swap(&d);
    /// assert_eq!(c, RefCell::new(6));
    /// assert_eq!(d, RefCell::new(5));
    /// ```
    #[inline]
    #[stable(feature = "refcell_swap", since = "1.24.0")]
    pub fn swap(&self, other: &Self) {
        mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
    }
}

impl<T: ?Sized> RefCell<T> {
    /// Immutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple
    /// immutable borrows can be taken out at the same time.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow`](#method.try_borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let borrowed_five = c.borrow();
    /// let borrowed_five2 = c.borrow();
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```should_panic
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let m = c.borrow_mut();
    /// let b = c.borrow(); // this causes a panic
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    #[track_caller]
    pub fn borrow(&self) -> Ref<'_, T> {
        self.try_borrow().expect("already mutably borrowed")
    }

    /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
    /// borrowed.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
    /// taken out at the same time.
    ///
    /// This is the non-panicking variant of [`borrow`](#method.borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow_mut();
    ///     assert!(c.try_borrow().is_err());
    /// }
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow().is_ok());
    /// }
    /// ```
    #[stable(feature = "try_borrow", since = "1.13.0")]
    #[inline]
    #[cfg_attr(feature = "debug_refcell", track_caller)]
    pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
        match BorrowRef::new(&self.borrow) {
            Some(b) => {
                #[cfg(feature = "debug_refcell")]
                {
                    // `borrowed_at` is always the *first* active borrow
                    if b.borrow.get() == 1 {
                        self.borrowed_at.set(Some(crate::panic::Location::caller()));
                    }
                }

                // SAFETY: `BorrowRef` ensures that there is only immutable access
                // to the value while borrowed.
                Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
            }
            None => Err(BorrowError {
                // If a borrow occurred, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            }),
        }
    }

    /// Mutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed. For a non-panicking variant, use
    /// [`try_borrow_mut`](#method.try_borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new("hello".to_owned());
    ///
    /// *c.borrow_mut() = "bonjour".to_owned();
    ///
    /// assert_eq!(&*c.borrow(), "bonjour");
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```should_panic
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// let m = c.borrow();
    ///
    /// let b = c.borrow_mut(); // this causes a panic
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    #[track_caller]
    pub fn borrow_mut(&self) -> RefMut<'_, T> {
        self.try_borrow_mut().expect("already borrowed")
    }

    /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow_mut().is_err());
    /// }
    ///
    /// assert!(c.try_borrow_mut().is_ok());
    /// ```
    #[stable(feature = "try_borrow", since = "1.13.0")]
    #[inline]
    #[cfg_attr(feature = "debug_refcell", track_caller)]
    pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
        match BorrowRefMut::new(&self.borrow) {
            Some(b) => {
                #[cfg(feature = "debug_refcell")]
                {
                    self.borrowed_at.set(Some(crate::panic::Location::caller()));
                }

                // SAFETY: `BorrowRef` guarantees unique access.
                Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
            }
            None => Err(BorrowMutError {
                // If a borrow occurred, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            }),
        }
    }

    /// Returns a raw pointer to the underlying data in this cell.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let ptr = c.as_ptr();
    /// ```
    #[inline]
    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
    pub fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows `RefCell` mutably (at compile-time) so there is no
    /// need for dynamic checks.
    ///
    /// However be cautious: this method expects `self` to be mutable, which is
    /// generally not the case when using a `RefCell`. Take a look at the
    /// [`borrow_mut`] method instead if `self` isn't mutable.
    ///
    /// Also, please be aware that this method is only for special circumstances and is usually
    /// not what you want. In case of doubt, use [`borrow_mut`] instead.
    ///
    /// [`borrow_mut`]: RefCell::borrow_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let mut c = RefCell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(c, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "cell_get_mut", since = "1.11.0")]
    pub fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
    ///
    /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
    /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
    /// if some `Ref` or `RefMut` borrows have been leaked.
    ///
    /// [`get_mut`]: RefCell::get_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::RefCell;
    ///
    /// let mut c = RefCell::new(0);
    /// std::mem::forget(c.borrow_mut());
    ///
    /// assert!(c.try_borrow().is_err());
    /// c.undo_leak();
    /// assert!(c.try_borrow().is_ok());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn undo_leak(&mut self) -> &mut T {
        *self.borrow.get_mut() = UNUSED;
        self.get_mut()
    }

    /// Immutably borrows the wrapped value, returning an error if the value is
    /// currently mutably borrowed.
    ///
    /// # Safety
    ///
    /// Unlike `RefCell::borrow`, this method is unsafe because it does not
    /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
    /// borrowing the `RefCell` while the reference returned by this method
    /// is alive is undefined behaviour.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow_mut();
    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_err());
    /// }
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
    /// }
    /// ```
    #[stable(feature = "borrow_state", since = "1.37.0")]
    #[inline]
    pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
        if !is_writing(self.borrow.get()) {
            // SAFETY: We check that nobody is actively writing now, but it is
            // the caller's responsibility to ensure that nobody writes until
            // the returned reference is no longer in use.
            // Also, `self.value.get()` refers to the value owned by `self`
            // and is thus guaranteed to be valid for the lifetime of `self`.
            Ok(unsafe { &*self.value.get() })
        } else {
            Err(BorrowError {
                // If a borrow occurred, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            })
        }
    }
}

impl<T: Default> RefCell<T> {
    /// Takes the wrapped value, leaving `Default::default()` in its place.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// let five = c.take();
    ///
    /// assert_eq!(five, 5);
    /// assert_eq!(c.into_inner(), 0);
    /// ```
    #[stable(feature = "refcell_take", since = "1.50.0")]
    pub fn take(&self) -> T {
        self.replace(Default::default())
    }
}

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

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> Clone for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed.
    #[inline]
    #[track_caller]
    fn clone(&self) -> RefCell<T> {
        RefCell::new(self.borrow().clone())
    }

    /// # Panics
    ///
    /// Panics if `other` is currently mutably borrowed.
    #[inline]
    #[track_caller]
    fn clone_from(&mut self, other: &Self) {
        self.get_mut().clone_from(&other.borrow())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for RefCell<T> {
    /// Creates a `RefCell<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> RefCell<T> {
        RefCell::new(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn eq(&self, other: &RefCell<T>) -> bool {
        *self.borrow() == *other.borrow()
    }
}

#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: ?Sized + Eq> Eq for RefCell<T> {}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
        self.borrow().partial_cmp(&*other.borrow())
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn lt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() < *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn le(&self, other: &RefCell<T>) -> bool {
        *self.borrow() <= *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn gt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() > *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn ge(&self, other: &RefCell<T>) -> bool {
        *self.borrow() >= *other.borrow()
    }
}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + Ord> Ord for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn cmp(&self, other: &RefCell<T>) -> Ordering {
        self.borrow().cmp(&*other.borrow())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const From<T> for RefCell<T> {
    fn from(t: T) -> RefCell<T> {
        RefCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}

struct BorrowRef<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl<'b> BorrowRef<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
        let b = borrow.get().wrapping_add(1);
        if !is_reading(b) {
            // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
            // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
            //    due to Rust's reference aliasing rules
            // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
            //    into isize::MIN (the max amount of writing borrows) so we can't allow
            //    an additional read borrow because isize can't represent so many read borrows
            //    (this can only happen if you mem::forget more than a small constant amount of
            //    `Ref`s, which is not good practice)
            None
        } else {
            // Incrementing borrow can result in a reading value (> 0) in these cases:
            // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
            // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
            //    is large enough to represent having one more read borrow
            borrow.set(b);
            Some(BorrowRef { borrow })
        }
    }
}

impl Drop for BorrowRef<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        self.borrow.set(borrow - 1);
    }
}

impl Clone for BorrowRef<'_> {
    #[inline]
    fn clone(&self) -> Self {
        // Since this Ref exists, we know the borrow flag
        // is a reading borrow.
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        // Prevent the borrow counter from overflowing into
        // a writing borrow.
        assert!(borrow != isize::MAX);
        self.borrow.set(borrow + 1);
        BorrowRef { borrow: self.borrow }
    }
}

/// Wraps a borrowed reference to a value in a `RefCell` box.
/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(
    not(bootstrap),
    must_not_suspend = "holding a Ref across suspend \
                      points can cause BorrowErrors"
)]
pub struct Ref<'b, T: ?Sized + 'b> {
    value: &'b T,
    borrow: BorrowRef<'b>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Ref<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        self.value
    }
}

impl<'b, T: ?Sized> Ref<'b, T> {
    /// Copies a `Ref`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
    /// with the widespread use of `r.borrow().clone()` to clone the contents of
    /// a `RefCell`.
    #[stable(feature = "cell_extras", since = "1.15.0")]
    #[must_use]
    #[inline]
    pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
        Ref { value: orig.value, borrow: orig.borrow.clone() }
    }

    /// Makes a new `Ref` for a component of the borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as `Ref::map(...)`.
    /// A method would interfere with methods of the same name on the contents
    /// of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, Ref};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// let b1: Ref<(u32, char)> = c.borrow();
    /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
    /// assert_eq!(*b2, 5)
    /// ```
    #[stable(feature = "cell_map", since = "1.8.0")]
    #[inline]
    pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
    where
        F: FnOnce(&T) -> &U,
    {
        Ref { value: f(orig.value), borrow: orig.borrow }
    }

    /// Makes a new `Ref` for an optional component of the borrowed data. The
    /// original guard is returned as an `Err(..)` if the closure returns
    /// `None`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::filter_map(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_filter_map)]
    ///
    /// use std::cell::{RefCell, Ref};
    ///
    /// let c = RefCell::new(vec![1, 2, 3]);
    /// let b1: Ref<Vec<u32>> = c.borrow();
    /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
    /// assert_eq!(*b2.unwrap(), 2);
    /// ```
    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
    #[inline]
    pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
    where
        F: FnOnce(&T) -> Option<&U>,
    {
        match f(orig.value) {
            Some(value) => Ok(Ref { value, borrow: orig.borrow }),
            None => Err(orig),
        }
    }

    /// Splits a `Ref` into multiple `Ref`s for different components of the
    /// borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::map_split(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{Ref, RefCell};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow();
    /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// ```
    #[stable(feature = "refcell_map_split", since = "1.35.0")]
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
    where
        F: FnOnce(&T) -> (&U, &V),
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
    }

    /// Convert into a reference to the underlying data.
    ///
    /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
    /// already immutably borrowed. It is not a good idea to leak more than a constant number of
    /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
    /// have occurred in total.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::leak(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::{RefCell, Ref};
    /// let cell = RefCell::new(0);
    ///
    /// let value = Ref::leak(cell.borrow());
    /// assert_eq!(*value, 0);
    ///
    /// assert!(cell.try_borrow().is_ok());
    /// assert!(cell.try_borrow_mut().is_err());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn leak(orig: Ref<'b, T>) -> &'b T {
        // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
        // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
        // unique reference to the borrowed RefCell. No further mutable references can be created
        // from the original cell.
        mem::forget(orig.borrow);
        orig.value
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.value.fmt(f)
    }
}

impl<'b, T: ?Sized> RefMut<'b, T> {
    /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
    /// variant.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// {
    ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
    ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
    ///     assert_eq!(*b2, 5);
    ///     *b2 = 42;
    /// }
    /// assert_eq!(*c.borrow(), (42, 'b'));
    /// ```
    #[stable(feature = "cell_map", since = "1.8.0")]
    #[inline]
    pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
    where
        F: FnOnce(&mut T) -> &mut U,
    {
        // FIXME(nll-rfc#40): fix borrow-check
        let RefMut { value, borrow } = orig;
        RefMut { value: f(value), borrow }
    }

    /// Makes a new `RefMut` for an optional component of the borrowed data. The
    /// original guard is returned as an `Err(..)` if the closure returns
    /// `None`.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::filter_map(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_filter_map)]
    ///
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let c = RefCell::new(vec![1, 2, 3]);
    ///
    /// {
    ///     let b1: RefMut<Vec<u32>> = c.borrow_mut();
    ///     let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
    ///
    ///     if let Ok(mut b2) = b2 {
    ///         *b2 += 2;
    ///     }
    /// }
    ///
    /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
    /// ```
    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
    #[inline]
    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
    {
        // FIXME(nll-rfc#40): fix borrow-check
        let RefMut { value, borrow } = orig;
        let value = value as *mut T;
        // SAFETY: function holds onto an exclusive reference for the duration
        // of its call through `orig`, and the pointer is only de-referenced
        // inside of the function call never allowing the exclusive reference to
        // escape.
        match f(unsafe { &mut *value }) {
            Some(value) => Ok(RefMut { value, borrow }),
            None => {
                // SAFETY: same as above.
                Err(RefMut { value: unsafe { &mut *value }, borrow })
            }
        }
    }

    /// Splits a `RefMut` into multiple `RefMut`s for different components of the
    /// borrowed data.
    ///
    /// The underlying `RefCell` will remain mutably borrowed until both
    /// returned `RefMut`s go out of scope.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map_split(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow_mut();
    /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// begin.copy_from_slice(&[4, 3]);
    /// end.copy_from_slice(&[2, 1]);
    /// ```
    #[stable(feature = "refcell_map_split", since = "1.35.0")]
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(
        orig: RefMut<'b, T>,
        f: F,
    ) -> (RefMut<'b, U>, RefMut<'b, V>)
    where
        F: FnOnce(&mut T) -> (&mut U, &mut V),
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
    }

    /// Convert into a mutable reference to the underlying data.
    ///
    /// The underlying `RefCell` can not be borrowed from again and will always appear already
    /// mutably borrowed, making the returned reference the only to the interior.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::leak(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::{RefCell, RefMut};
    /// let cell = RefCell::new(0);
    ///
    /// let value = RefMut::leak(cell.borrow_mut());
    /// assert_eq!(*value, 0);
    /// *value = 1;
    ///
    /// assert!(cell.try_borrow_mut().is_err());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
        // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
        // require a unique reference to the borrowed RefCell. No further references can be created
        // from the original cell within that lifetime, making the current borrow the only
        // reference for the remaining lifetime.
        mem::forget(orig.borrow);
        orig.value
    }
}

struct BorrowRefMut<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl Drop for BorrowRefMut<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        self.borrow.set(borrow + 1);
    }
}

impl<'b> BorrowRefMut<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
        // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
        // mutable reference, and so there must currently be no existing
        // references. Thus, while clone increments the mutable refcount, here
        // we explicitly only allow going from UNUSED to UNUSED - 1.
        match borrow.get() {
            UNUSED => {
                borrow.set(UNUSED - 1);
                Some(BorrowRefMut { borrow })
            }
            _ => None,
        }
    }

    // Clones a `BorrowRefMut`.
    //
    // This is only valid if each `BorrowRefMut` is used to track a mutable
    // reference to a distinct, nonoverlapping range of the original object.
    // This isn't in a Clone impl so that code doesn't call this implicitly.
    #[inline]
    fn clone(&self) -> BorrowRefMut<'b> {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        // Prevent the borrow counter from underflowing.
        assert!(borrow != isize::MIN);
        self.borrow.set(borrow - 1);
        BorrowRefMut { borrow: self.borrow }
    }
}

/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(
    not(bootstrap),
    must_not_suspend = "holding a RefMut across suspend \
                      points can cause BorrowErrors"
)]
pub struct RefMut<'b, T: ?Sized + 'b> {
    value: &'b mut T,
    borrow: BorrowRefMut<'b>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RefMut<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        self.value
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RefMut<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        self.value
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.value.fmt(f)
    }
}

/// The core primitive for interior mutability in Rust.
///
/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
/// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
///
/// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
/// use `UnsafeCell` to wrap their data.
///
/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
/// aliasing `&mut`, not even with `UnsafeCell<T>`.
///
/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
/// correctly.
///
/// [`.get()`]: `UnsafeCell::get`
///
/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
///
/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
/// reference) that is accessible by safe code (for example, because you returned it),
/// then you must not access the data in any way that contradicts that reference for the
/// remainder of `'a`. For example, this means that if you take the `*mut T` from an
/// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
/// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
/// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
/// safe code, then you must not access the data within the `UnsafeCell` until that
/// reference expires.
///
/// - At all times, you must avoid data races. If multiple threads have access to
/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
/// accesses (or use atomics).
///
/// To assist with proper design, the following scenarios are explicitly declared legal
/// for single-threaded code:
///
/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
/// references, but not with a `&mut T`
///
/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
/// co-exist with it. A `&mut T` must always be unique.
///
/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
/// `&UnsafeCell<T>` references alias the cell) is
/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
/// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
/// may be aliased for the duration of that `&mut` borrow.
/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
/// a `&mut T`.
///
/// [`.get_mut()`]: `UnsafeCell::get_mut`
///
/// # Examples
///
/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
/// there being multiple references aliasing the cell:
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let x: UnsafeCell<i32> = 42.into();
/// // Get multiple / concurrent / shared references to the same `x`.
/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
///
/// unsafe {
///     // SAFETY: within this scope there are no other references to `x`'s contents,
///     // so ours is effectively unique.
///     let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
///     *p1_exclusive += 27; //                                     |
/// } // <---------- cannot go beyond this point -------------------+
///
/// unsafe {
///     // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
///     // so we can have multiple shared accesses concurrently.
///     let p2_shared: &i32 = &*p2.get();
///     assert_eq!(*p2_shared, 42 + 27);
///     let p1_shared: &i32 = &*p1.get();
///     assert_eq!(*p1_shared, *p2_shared);
/// }
/// ```
///
/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
/// implies exclusive access to its `T`:
///
/// ```rust
/// #![forbid(unsafe_code)] // with exclusive accesses,
///                         // `UnsafeCell` is a transparent no-op wrapper,
///                         // so no need for `unsafe` here.
/// use std::cell::UnsafeCell;
///
/// let mut x: UnsafeCell<i32> = 42.into();
///
/// // Get a compile-time-checked unique reference to `x`.
/// let p_unique: &mut UnsafeCell<i32> = &mut x;
/// // With an exclusive reference, we can mutate the contents for free.
/// *p_unique.get_mut() = 0;
/// // Or, equivalently:
/// x = UnsafeCell::new(0);
///
/// // When we own the value, we can extract the contents for free.
/// let contents: i32 = x.into_inner();
/// assert_eq!(contents, 0);
/// ```
#[lang = "unsafe_cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[repr(no_niche)] // rust-lang/rust#68303.
pub struct UnsafeCell<T: ?Sized> {
    value: T,
}

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

impl<T> UnsafeCell<T> {
    /// Constructs a new instance of `UnsafeCell` which will wrap the specified
    /// value.
    ///
    /// All access to the inner value through methods is `unsafe`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
    #[inline(always)]
    pub const fn new(value: T) -> UnsafeCell<T> {
        UnsafeCell { value }
    }

    /// Unwraps the value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    ///
    /// let five = uc.into_inner();
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    pub const fn into_inner(self) -> T {
        self.value
    }
}

impl<T: ?Sized> UnsafeCell<T> {
    /// Gets a mutable pointer to the wrapped value.
    ///
    /// This can be cast to a pointer of any kind.
    /// Ensure that the access is unique (no active references, mutable or not)
    /// when casting to `&mut T`, and ensure that there are no mutations
    /// or mutable aliases going on when casting to `&T`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    ///
    /// let five = uc.get();
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
    pub const fn get(&self) -> *mut T {
        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
        // #[repr(transparent)]. This exploits libstd's special status, there is
        // no guarantee for user code that this will work in future versions of the compiler!
        self as *const UnsafeCell<T> as *const T as *mut T
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows the `UnsafeCell` mutably (at compile-time) which
    /// guarantees that we possess the only reference.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let mut c = UnsafeCell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(*c.get_mut(), 6);
    /// ```
    #[inline(always)]
    #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
    #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
    pub const fn get_mut(&mut self) -> &mut T {
        &mut self.value
    }

    /// Gets a mutable pointer to the wrapped value.
    /// The difference from [`get`] is that this function accepts a raw pointer,
    /// which is useful to avoid the creation of temporary references.
    ///
    /// The result can be cast to a pointer of any kind.
    /// Ensure that the access is unique (no active references, mutable or not)
    /// when casting to `&mut T`, and ensure that there are no mutations
    /// or mutable aliases going on when casting to `&T`.
    ///
    /// [`get`]: UnsafeCell::get()
    ///
    /// # Examples
    ///
    /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
    /// calling `get` would require creating a reference to uninitialized data:
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    /// use std::mem::MaybeUninit;
    ///
    /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
    /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
    /// let uc = unsafe { m.assume_init() };
    ///
    /// assert_eq!(uc.into_inner(), 5);
    /// ```
    #[inline(always)]
    #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
    pub const fn raw_get(this: *const Self) -> *mut T {
        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
        // #[repr(transparent)]. This exploits libstd's special status, there is
        // no guarantee for user code that this will work in future versions of the compiler!
        this as *const T as *mut T
    }
}

#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
impl<T: Default> Default for UnsafeCell<T> {
    /// Creates an `UnsafeCell`, with the `Default` value for T.
    fn default() -> UnsafeCell<T> {
        UnsafeCell::new(Default::default())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const From<T> for UnsafeCell<T> {
    fn from(t: T) -> UnsafeCell<T> {
        UnsafeCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}

#[allow(unused)]
fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
    let _: UnsafeCell<&dyn Send> = a;
    let _: Cell<&dyn Send> = b;
    let _: RefCell<&dyn Send> = c;
}