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
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
//! Manually manage memory through raw pointers.
//!
//! *[See also the pointer primitive types](pointer).*
//!
//! # Safety
//!
//! Many functions in this module take raw pointers as arguments and read from
//! or write to them. For this to be safe, these pointers must be *valid*.
//! Whether a pointer is valid depends on the operation it is used for
//! (read or write), and the extent of the memory that is accessed (i.e.,
//! how many bytes are read/written). Most functions use `*mut T` and `*const T`
//! to access only a single value, in which case the documentation omits the size
//! and implicitly assumes it to be `size_of::<T>()` bytes.
//!
//! The precise rules for validity are not determined yet. The guarantees that are
//! provided at this point are very minimal:
//!
//! * A [null] pointer is *never* valid, not even for accesses of [size zero][zst].
//! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer
//!   be *dereferenceable*: the memory range of the given size starting at the pointer must all be
//!   within the bounds of a single allocated object. Note that in Rust,
//!   every (stack-allocated) variable is considered a separate allocated object.
//! * Even for operations of [size zero][zst], the pointer must not be pointing to deallocated
//!   memory, i.e., deallocation makes pointers invalid even for zero-sized operations. However,
//!   casting any non-zero integer *literal* to a pointer is valid for zero-sized accesses, even if
//!   some memory happens to exist at that address and gets deallocated. This corresponds to writing
//!   your own allocator: allocating zero-sized objects is not very hard. The canonical way to
//!   obtain a pointer that is valid for zero-sized accesses is [`NonNull::dangling`].
//FIXME: mention `ptr::invalid` above, once it is stable.
//! * All accesses performed by functions in this module are *non-atomic* in the sense
//!   of [atomic operations] used to synchronize between threads. This means it is
//!   undefined behavior to perform two concurrent accesses to the same location from different
//!   threads unless both accesses only read from memory. Notice that this explicitly
//!   includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
//!   be used for inter-thread synchronization.
//! * The result of casting a reference to a pointer is valid for as long as the
//!   underlying object is live and no reference (just raw pointers) is used to
//!   access the same memory.
//!
//! These axioms, along with careful use of [`offset`] for pointer arithmetic,
//! are enough to correctly implement many useful things in unsafe code. Stronger guarantees
//! will be provided eventually, as the [aliasing] rules are being determined. For more
//! information, see the [book] as well as the section in the reference devoted
//! to [undefined behavior][ub].
//!
//! ## Alignment
//!
//! Valid raw pointers as defined above are not necessarily properly aligned (where
//! "proper" alignment is defined by the pointee type, i.e., `*const T` must be
//! aligned to `mem::align_of::<T>()`). However, most functions require their
//! arguments to be properly aligned, and will explicitly state
//! this requirement in their documentation. Notable exceptions to this are
//! [`read_unaligned`] and [`write_unaligned`].
//!
//! When a function requires proper alignment, it does so even if the access
//! has size 0, i.e., even if memory is not actually touched. Consider using
//! [`NonNull::dangling`] in such cases.
//!
//! ## Allocated object
//!
//! For several operations, such as [`offset`] or field projections (`expr.field`), the notion of an
//! "allocated object" becomes relevant. An allocated object is a contiguous region of memory.
//! Common examples of allocated objects include stack-allocated variables (each variable is a
//! separate allocated object), heap allocations (each allocation created by the global allocator is
//! a separate allocated object), and `static` variables.
//!
//!
//! # Strict Provenance
//!
//! **The following text is non-normative, insufficiently formal, and is an extremely strict
//! interpretation of provenance. It's ok if your code doesn't strictly conform to it.**
//!
//! [Strict Provenance][] is an experimental set of APIs that help tools that try
//! to validate the memory-safety of your program's execution. Notably this includes [Miri][]
//! and [CHERI][], which can detect when you access out of bounds memory or otherwise violate
//! Rust's memory model.
//!
//! Provenance must exist in some form for any programming
//! language compiled for modern computer architectures, but specifying a model for provenance
//! in a way that is useful to both compilers and programmers is an ongoing challenge.
//! The [Strict Provenance][] experiment seeks to explore the question: *what if we just said you
//! couldn't do all the nasty operations that make provenance so messy?*
//!
//! What APIs would have to be removed? What APIs would have to be added? How much would code
//! have to change, and is it worse or better now? Would any patterns become truly inexpressible?
//! Could we carve out special exceptions for those patterns? Should we?
//!
//! A secondary goal of this project is to see if we can disambiguate the many functions of
//! pointer<->integer casts enough for the definition of `usize` to be loosened so that it
//! isn't *pointer*-sized but address-space/offset/allocation-sized (we'll probably continue
//! to conflate these notions). This would potentially make it possible to more efficiently
//! target platforms where pointers are larger than offsets, such as CHERI and maybe some
//! segmented architectures.
//!
//! ## Provenance
//!
//! **This section is *non-normative* and is part of the [Strict Provenance][] experiment.**
//!
//! Pointers are not *simply* an "integer" or "address". For instance, it's uncontroversial
//! to say that a Use After Free is clearly Undefined Behaviour, even if you "get lucky"
//! and the freed memory gets reallocated before your read/write (in fact this is the
//! worst-case scenario, UAFs would be much less concerning if this didn't happen!).
//! To rationalize this claim, pointers need to somehow be *more* than just their addresses:
//! they must have provenance.
//!
//! When an allocation is created, that allocation has a unique Original Pointer. For alloc
//! APIs this is literally the pointer the call returns, and for local variables and statics,
//! this is the name of the variable/static. This is mildly overloading the term "pointer"
//! for the sake of brevity/exposition.
//!
//! The Original Pointer for an allocation is guaranteed to have unique access to the entire
//! allocation and *only* that allocation. In this sense, an allocation can be thought of
//! as a "sandbox" that cannot be broken into or out of. *Provenance* is the permission
//! to access an allocation's sandbox and has both a *spatial* and *temporal* component:
//!
//! * Spatial: A range of bytes that the pointer is allowed to access.
//! * Temporal: The lifetime (of the allocation) that access to these bytes is tied to.
//!
//! Spatial provenance makes sure you don't go beyond your sandbox, while temporal provenance
//! makes sure that you can't "get lucky" after your permission to access some memory
//! has been revoked (either through deallocations or borrows expiring).
//!
//! Provenance is implicitly shared with all pointers transitively derived from
//! The Original Pointer through operations like [`offset`], borrowing, and pointer casts.
//! Some operations may *shrink* the derived provenance, limiting how much memory it can
//! access or how long it's valid for (i.e. borrowing a subfield and subslicing).
//!
//! Shrinking provenance cannot be undone: even if you "know" there is a larger allocation, you
//! can't derive a pointer with a larger provenance. Similarly, you cannot "recombine"
//! two contiguous provenances back into one (i.e. with a `fn merge(&[T], &[T]) -> &[T]`).
//!
//! A reference to a value always has provenance over exactly the memory that field occupies.
//! A reference to a slice always has provenance over exactly the range that slice describes.
//!
//! If an allocation is deallocated, all pointers with provenance to that allocation become
//! invalidated, and effectively lose their provenance.
//!
//! The strict provenance experiment is mostly only interested in exploring stricter *spatial*
//! provenance. In this sense it can be thought of as a subset of the more ambitious and
//! formal [Stacked Borrows][] research project, which is what tools like [Miri][] are based on.
//! In particular, Stacked Borrows is necessary to properly describe what borrows are allowed
//! to do and when they become invalidated. This necessarily involves much more complex
//! *temporal* reasoning than simply identifying allocations. Adjusting APIs and code
//! for the strict provenance experiment will also greatly help Stacked Borrows.
//!
//!
//! ## Pointer Vs Addresses
//!
//! **This section is *non-normative* and is part of the [Strict Provenance][] experiment.**
//!
//! One of the largest historical issues with trying to define provenance is that programmers
//! freely convert between pointers and integers. Once you allow for this, it generally becomes
//! impossible to accurately track and preserve provenance information, and you need to appeal
//! to very complex and unreliable heuristics. But of course, converting between pointers and
//! integers is very useful, so what can we do?
//!
//! Also did you know WASM is actually a "Harvard Architecture"? As in function pointers are
//! handled completely differently from data pointers? And we kind of just shipped Rust on WASM
//! without really addressing the fact that we let you freely convert between function pointers
//! and data pointers, because it mostly Just Works? Let's just put that on the "pointer casts
//! are dubious" pile.
//!
//! Strict Provenance attempts to square these circles by decoupling Rust's traditional conflation
//! of pointers and `usize` (and `isize`), and defining a pointer to semantically contain the
//! following information:
//!
//! * The **address-space** it is part of (e.g. "data" vs "code" in WASM).
//! * The **address** it points to, which can be represented by a `usize`.
//! * The **provenance** it has, defining the memory it has permission to access.
//!
//! Under Strict Provenance, a usize *cannot* accurately represent a pointer, and converting from
//! a pointer to a usize is generally an operation which *only* extracts the address. It is
//! therefore *impossible* to construct a valid pointer from a usize because there is no way
//! to restore the address-space and provenance. In other words, pointer-integer-pointer
//! roundtrips are not possible (in the sense that the resulting pointer is not dereferenceable).
//!
//! The key insight to making this model *at all* viable is the [`with_addr`][] method:
//!
//! ```text
//!     /// Creates a new pointer with the given address.
//!     ///
//!     /// This performs the same operation as an `addr as ptr` cast, but copies
//!     /// the *address-space* and *provenance* of `self` to the new pointer.
//!     /// This allows us to dynamically preserve and propagate this important
//!     /// information in a way that is otherwise impossible with a unary cast.
//!     ///
//!     /// This is equivalent to using `wrapping_offset` to offset `self` to the
//!     /// given address, and therefore has all the same capabilities and restrictions.
//!     pub fn with_addr(self, addr: usize) -> Self;
//! ```
//!
//! So you're still able to drop down to the address representation and do whatever
//! clever bit tricks you want *as long as* you're able to keep around a pointer
//! into the allocation you care about that can "reconstitute" the other parts of the pointer.
//! Usually this is very easy, because you only are taking a pointer, messing with the address,
//! and then immediately converting back to a pointer. To make this use case more ergonomic,
//! we provide the [`map_addr`][] method.
//!
//! To help make it clear that code is "following" Strict Provenance semantics, we also provide an
//! [`addr`][] method which promises that the returned address is not part of a
//! pointer-usize-pointer roundtrip. In the future we may provide a lint for pointer<->integer
//! casts to help you audit if your code conforms to strict provenance.
//!
//!
//! ## Using Strict Provenance
//!
//! Most code needs no changes to conform to strict provenance, as the only really concerning
//! operation that *wasn't* obviously already Undefined Behaviour is casts from usize to a
//! pointer. For code which *does* cast a usize to a pointer, the scope of the change depends
//! on exactly what you're doing.
//!
//! In general you just need to make sure that if you want to convert a usize address to a
//! pointer and then use that pointer to read/write memory, you need to keep around a pointer
//! that has sufficient provenance to perform that read/write itself. In this way all of your
//! casts from an address to a pointer are essentially just applying offsets/indexing.
//!
//! This is generally trivial to do for simple cases like tagged pointers *as long as you
//! represent the tagged pointer as an actual pointer and not a usize*. For instance:
//!
//! ```
//! #![feature(strict_provenance)]
//!
//! unsafe {
//!     // A flag we want to pack into our pointer
//!     static HAS_DATA: usize = 0x1;
//!     static FLAG_MASK: usize = !HAS_DATA;
//!
//!     // Our value, which must have enough alignment to have spare least-significant-bits.
//!     let my_precious_data: u32 = 17;
//!     assert!(core::mem::align_of::<u32>() > 1);
//!
//!     // Create a tagged pointer
//!     let ptr = &my_precious_data as *const u32;
//!     let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
//!
//!     // Check the flag:
//!     if tagged.addr() & HAS_DATA != 0 {
//!         // Untag and read the pointer
//!         let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
//!         assert_eq!(data, 17);
//!     } else {
//!         unreachable!()
//!     }
//! }
//! ```
//!
//! (Yes, if you've been using AtomicUsize for pointers in concurrent datastructures, you should
//! be using AtomicPtr instead. If that messes up the way you atomically manipulate pointers,
//! we would like to know why, and what needs to be done to fix it.)
//!
//! Something more complicated and just generally *evil* like an XOR-List requires more significant
//! changes like allocating all nodes in a pre-allocated Vec or Arena and using a pointer
//! to the whole allocation to reconstitute the XORed addresses.
//!
//! Situations where a valid pointer *must* be created from just an address, such as baremetal code
//! accessing a memory-mapped interface at a fixed address, are an open question on how to support.
//! These situations *will* still be allowed, but we might require some kind of "I know what I'm
//! doing" annotation to explain the situation to the compiler. It's also possible they need no
//! special attention at all, because they're generally accessing memory outside the scope of
//! "the abstract machine", or already using "I know what I'm doing" annotations like "volatile".
//!
//! Under [Strict Provenance] it is Undefined Behaviour to:
//!
//! * Access memory through a pointer that does not have provenance over that memory.
//!
//! * [`offset`] a pointer to or from an address it doesn't have provenance over.
//!   This means it's always UB to offset a pointer derived from something deallocated,
//!   even if the offset is 0. Note that a pointer "one past the end" of its provenance
//!   is not actually outside its provenance, it just has 0 bytes it can load/store.
//!
//! But it *is* still sound to:
//!
//! * Create an invalid pointer from just an address (see [`ptr::invalid`][]). This can
//!   be used for sentinel values like `null` *or* to represent a tagged pointer that will
//!   never be dereferenceable. In general, it is always sound for an integer to pretend
//!   to be a pointer "for fun" as long as you don't use operations on it which require
//!   it to be valid (offset, read, write, etc).
//!
//! * Forge an allocation of size zero at any sufficiently aligned non-null address.
//!   i.e. the usual "ZSTs are fake, do what you want" rules apply *but* this only applies
//!   for actual forgery (integers cast to pointers). If you borrow some struct's field
//!   that *happens* to be zero-sized, the resulting pointer will have provenance tied to
//!   that allocation and it will still get invalidated if the allocation gets deallocated.
//!   In the future we may introduce an API to make such a forged allocation explicit.
//!
//! * [`wrapping_offset`][] a pointer outside its provenance. This includes invalid pointers
//!   which have "no" provenance. Unfortunately there may be practical limits on this for a
//!   particular platform, and it's an open question as to how to specify this (if at all).
//!   Notably, [CHERI][] relies on a compression scheme that can't handle a
//!   pointer getting offset "too far" out of bounds. If this happens, the address
//!   returned by `addr` will be the value you expect, but the provenance will get invalidated
//!   and using it to read/write will fault. The details of this are architecture-specific
//!   and based on alignment, but the buffer on either side of the pointer's range is pretty
//!   generous (think kilobytes, not bytes).
//!
//! * Compare arbitrary pointers by address. Addresses *are* just integers and so there is
//!   always a coherent answer, even if the pointers are invalid or from different
//!   address-spaces/provenances. Of course, comparing addresses from different address-spaces
//!   is generally going to be *meaningless*, but so is comparing Kilograms to Meters, and Rust
//!   doesn't prevent that either. Similarly, if you get "lucky" and notice that a pointer
//!   one-past-the-end is the "same" address as the start of an unrelated allocation, anything
//!   you do with that fact is *probably* going to be gibberish. The scope of that gibberish
//!   is kept under control by the fact that the two pointers *still* aren't allowed to access
//!   the other's allocation (bytes), because they still have different provenance.
//!
//! * Perform pointer tagging tricks. This falls out of [`wrapping_offset`] but is worth
//!   mentioning in more detail because of the limitations of [CHERI][]. Low-bit tagging
//!   is very robust, and often doesn't even go out of bounds because types ensure
//!   size >= align (and over-aligning actually gives CHERI more flexibility). Anything
//!   more complex than this rapidly enters "extremely platform-specific" territory as
//!   certain things may or may not be allowed based on specific supported operations.
//!   For instance, ARM explicitly supports high-bit tagging, and so CHERI on ARM inherits
//!   that and should support it.
//!
//! ## Pointer-usize-pointer roundtrips and 'exposed' provenance
//!
//! **This section is *non-normative* and is part of the [Strict Provenance] experiment.**
//!
//! As discussed above, pointer-usize-pointer roundtrips are not possible under [Strict Provenance].
//! However, there exists legacy Rust code that is full of such roundtrips, and legacy platform APIs
//! regularly assume that `usize` can capture all the information that makes up a pointer. There
//! also might be code that cannot be ported to Strict Provenance (which is something we would [like
//! to hear about][Strict Provenance]).
//!
//! For situations like this, there is a fallback plan, a way to 'opt out' of Strict Provenance.
//! However, note that this makes your code a lot harder to specify, and the code will not work
//! (well) with tools like [Miri] and [CHERI].
//!
//! This fallback plan is provided by the [`expose_addr`] and [`from_exposed_addr`] methods (which
//! are equivalent to `as` casts between pointers and integers). [`expose_addr`] is a lot like
//! [`addr`], but additionally adds the provenance of the pointer to a global list of 'exposed'
//! provenances. (This list is purely conceptual, it exists for the purpose of specifying Rust but
//! is not materialized in actual executions, except in tools like [Miri].) [`from_exposed_addr`]
//! can be used to construct a pointer with one of these previously 'exposed' provenances.
//! [`from_exposed_addr`] takes only `addr: usize` as arguments, so unlike in [`with_addr`] there is
//! no indication of what the correct provenance for the returned pointer is -- and that is exactly
//! what makes pointer-usize-pointer roundtrips so tricky to rigorously specify! There is no
//! algorithm that decides which provenance will be used. You can think of this as "guessing" the
//! right provenance, and the guess will be "maximally in your favor", in the sense that if there is
//! any way to avoid undefined behavior, then that is the guess that will be taken. However, if
//! there is *no* previously 'exposed' provenance that justifies the way the returned pointer will
//! be used, the program has undefined behavior.
//!
//! Using [`expose_addr`] or [`from_exposed_addr`] (or the equivalent `as` casts) means that code is
//! *not* following Strict Provenance rules. The goal of the Strict Provenance experiment is to
//! determine whether it is possible to use Rust without [`expose_addr`] and [`from_exposed_addr`].
//! If this is successful, it would be a major win for avoiding specification complexity and to
//! facilitate adoption of tools like [CHERI] and [Miri] that can be a big help in increasing the
//! confidence in (unsafe) Rust code.
//!
//! [aliasing]: ../../nomicon/aliasing.html
//! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
//! [ub]: ../../reference/behavior-considered-undefined.html
//! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
//! [atomic operations]: crate::sync::atomic
//! [`offset`]: pointer::offset
//! [`wrapping_offset`]: pointer::wrapping_offset
//! [`with_addr`]: pointer::with_addr
//! [`map_addr`]: pointer::map_addr
//! [`addr`]: pointer::addr
//! [`ptr::invalid`]: core::ptr::invalid
//! [`expose_addr`]: pointer::expose_addr
//! [`from_exposed_addr`]: from_exposed_addr
//! [Miri]: https://github.com/rust-lang/miri
//! [CHERI]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
//! [Strict Provenance]: https://github.com/rust-lang/rust/issues/95228
//! [Stacked Borrows]: https://plv.mpi-sws.org/rustbelt/stacked-borrows/

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

use crate::cmp::Ordering;
use crate::fmt;
use crate::hash;
use crate::intrinsics::{
    self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping,
};

use crate::mem::{self, MaybeUninit};

mod alignment;
#[unstable(feature = "ptr_alignment_type", issue = "102070")]
pub use alignment::Alignment;

#[stable(feature = "rust1", since = "1.0.0")]
#[doc(inline)]
pub use crate::intrinsics::copy_nonoverlapping;

#[stable(feature = "rust1", since = "1.0.0")]
#[doc(inline)]
pub use crate::intrinsics::copy;

#[stable(feature = "rust1", since = "1.0.0")]
#[doc(inline)]
pub use crate::intrinsics::write_bytes;

mod metadata;
pub(crate) use metadata::PtrRepr;
#[unstable(feature = "ptr_metadata", issue = "81513")]
pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};

mod non_null;
#[stable(feature = "nonnull", since = "1.25.0")]
pub use non_null::NonNull;

mod unique;
#[unstable(feature = "ptr_internals", issue = "none")]
pub use unique::Unique;

mod const_ptr;
mod mut_ptr;

/// Executes the destructor (if any) of the pointed-to value.
///
/// This is semantically equivalent to calling [`ptr::read`] and discarding
/// the result, but has the following advantages:
///
/// * It is *required* to use `drop_in_place` to drop unsized types like
///   trait objects, because they can't be read out onto the stack and
///   dropped normally.
///
/// * It is friendlier to the optimizer to do this over [`ptr::read`] when
///   dropping manually allocated memory (e.g., in the implementations of
///   `Box`/`Rc`/`Vec`), as the compiler doesn't need to prove that it's
///   sound to elide the copy.
///
/// * It can be used to drop [pinned] data when `T` is not `repr(packed)`
///   (pinned data must not be moved before it is dropped).
///
/// Unaligned values cannot be dropped in place, they must be copied to an aligned
/// location first using [`ptr::read_unaligned`]. For packed structs, this move is
/// done automatically by the compiler. This means the fields of packed structs
/// are not dropped in-place.
///
/// [`ptr::read`]: self::read
/// [`ptr::read_unaligned`]: self::read_unaligned
/// [pinned]: crate::pin
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `to_drop` must be [valid] for both reads and writes.
///
/// * `to_drop` must be properly aligned.
///
/// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
///   additional invariants - this is type-dependent.
///
/// Additionally, if `T` is not [`Copy`], using the pointed-to value after
/// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
/// foo` counts as a use because it will cause the value to be dropped
/// again. [`write()`] can be used to overwrite data without causing it to be
/// dropped.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// # Examples
///
/// Manually remove the last item from a vector:
///
/// ```
/// use std::ptr;
/// use std::rc::Rc;
///
/// let last = Rc::new(1);
/// let weak = Rc::downgrade(&last);
///
/// let mut v = vec![Rc::new(0), last];
///
/// unsafe {
///     // Get a raw pointer to the last element in `v`.
///     let ptr = &mut v[1] as *mut _;
///     // Shorten `v` to prevent the last item from being dropped. We do that first,
///     // to prevent issues if the `drop_in_place` below panics.
///     v.set_len(1);
///     // Without a call `drop_in_place`, the last item would never be dropped,
///     // and the memory it manages would be leaked.
///     ptr::drop_in_place(ptr);
/// }
///
/// assert_eq!(v, &[0.into()]);
///
/// // Ensure that the last item was dropped.
/// assert!(weak.upgrade().is_none());
/// ```
#[stable(feature = "drop_in_place", since = "1.8.0")]
#[lang = "drop_in_place"]
#[allow(unconditional_recursion)]
pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
    // Code here does not matter - this is replaced by the
    // real drop glue by the compiler.

    // SAFETY: see comment above
    unsafe { drop_in_place(to_drop) }
}

/// Creates a null raw pointer.
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let p: *const i32 = ptr::null();
/// assert!(p.is_null());
/// ```
#[inline(always)]
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_promotable]
#[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
#[rustc_allow_const_fn_unstable(ptr_metadata)]
#[rustc_diagnostic_item = "ptr_null"]
pub const fn null<T: ?Sized + Thin>() -> *const T {
    from_raw_parts(invalid(0), ())
}

/// Creates an invalid pointer with the given address.
///
/// This is different from `addr as *const T`, which creates a pointer that picks up a previously
/// exposed provenance. See [`from_exposed_addr`] for more details on that operation.
///
/// The module's top-level documentation discusses the precise meaning of an "invalid"
/// pointer but essentially this expresses that the pointer is not associated
/// with any actual allocation and is little more than a usize address in disguise.
///
/// This pointer will have no provenance associated with it and is therefore
/// UB to read/write/offset. This mostly exists to facilitate things
/// like `ptr::null` and `NonNull::dangling` which make invalid pointers.
///
/// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
/// may be desirable to give them their own API just to make that 100% clear.)
///
/// This API and its claimed semantics are part of the Strict Provenance experiment,
/// see the [module documentation][crate::ptr] for details.
#[inline(always)]
#[must_use]
#[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
#[unstable(feature = "strict_provenance", issue = "95228")]
pub const fn invalid<T>(addr: usize) -> *const T {
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    // We use transmute rather than a cast so tools like Miri can tell that this
    // is *not* the same as from_exposed_addr.
    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
    // pointer).
    unsafe { mem::transmute(addr) }
}

/// Creates an invalid mutable pointer with the given address.
///
/// This is different from `addr as *mut T`, which creates a pointer that picks up a previously
/// exposed provenance. See [`from_exposed_addr_mut`] for more details on that operation.
///
/// The module's top-level documentation discusses the precise meaning of an "invalid"
/// pointer but essentially this expresses that the pointer is not associated
/// with any actual allocation and is little more than a usize address in disguise.
///
/// This pointer will have no provenance associated with it and is therefore
/// UB to read/write/offset. This mostly exists to facilitate things
/// like `ptr::null` and `NonNull::dangling` which make invalid pointers.
///
/// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
/// may be desirable to give them their own API just to make that 100% clear.)
///
/// This API and its claimed semantics are part of the Strict Provenance experiment,
/// see the [module documentation][crate::ptr] for details.
#[inline(always)]
#[must_use]
#[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
#[unstable(feature = "strict_provenance", issue = "95228")]
pub const fn invalid_mut<T>(addr: usize) -> *mut T {
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    // We use transmute rather than a cast so tools like Miri can tell that this
    // is *not* the same as from_exposed_addr.
    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
    // pointer).
    unsafe { mem::transmute(addr) }
}

/// Convert an address back to a pointer, picking up a previously 'exposed' provenance.
///
/// This is equivalent to `addr as *const T`. The provenance of the returned pointer is that of *any*
/// pointer that was previously passed to [`expose_addr`][pointer::expose_addr] or a `ptr as usize`
/// cast. If there is no previously 'exposed' provenance that justifies the way this pointer will be
/// used, the program has undefined behavior. Note that there is no algorithm that decides which
/// provenance will be used. You can think of this as "guessing" the right provenance, and the guess
/// will be "maximally in your favor", in the sense that if there is any way to avoid undefined
/// behavior, then that is the guess that will be taken.
///
/// On platforms with multiple address spaces, it is your responsibility to ensure that the
/// address makes sense in the address space that this pointer will be used with.
///
/// Using this method means that code is *not* following strict provenance rules. "Guessing" a
/// suitable provenance complicates specification and reasoning and may not be supported by
/// tools that help you to stay conformant with the Rust memory model, so it is recommended to
/// use [`with_addr`][pointer::with_addr] wherever possible.
///
/// On most platforms this will produce a value with the same bytes as the address. Platforms
/// which need to store additional information in a pointer may not support this operation,
/// since it is generally not possible to actually *compute* which provenance the returned
/// pointer has to pick up.
///
/// This API and its claimed semantics are part of the Strict Provenance experiment, see the
/// [module documentation][crate::ptr] for details.
#[must_use]
#[inline]
#[unstable(feature = "strict_provenance", issue = "95228")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub fn from_exposed_addr<T>(addr: usize) -> *const T
where
    T: Sized,
{
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    addr as *const T
}

/// Convert an address back to a mutable pointer, picking up a previously 'exposed' provenance.
///
/// This is equivalent to `addr as *mut T`. The provenance of the returned pointer is that of *any*
/// pointer that was previously passed to [`expose_addr`][pointer::expose_addr] or a `ptr as usize`
/// cast. If there is no previously 'exposed' provenance that justifies the way this pointer will be
/// used, the program has undefined behavior. Note that there is no algorithm that decides which
/// provenance will be used. You can think of this as "guessing" the right provenance, and the guess
/// will be "maximally in your favor", in the sense that if there is any way to avoid undefined
/// behavior, then that is the guess that will be taken.
///
/// On platforms with multiple address spaces, it is your responsibility to ensure that the
/// address makes sense in the address space that this pointer will be used with.
///
/// Using this method means that code is *not* following strict provenance rules. "Guessing" a
/// suitable provenance complicates specification and reasoning and may not be supported by
/// tools that help you to stay conformant with the Rust memory model, so it is recommended to
/// use [`with_addr`][pointer::with_addr] wherever possible.
///
/// On most platforms this will produce a value with the same bytes as the address. Platforms
/// which need to store additional information in a pointer may not support this operation,
/// since it is generally not possible to actually *compute* which provenance the returned
/// pointer has to pick up.
///
/// This API and its claimed semantics are part of the Strict Provenance experiment, see the
/// [module documentation][crate::ptr] for details.
#[must_use]
#[inline]
#[unstable(feature = "strict_provenance", issue = "95228")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub fn from_exposed_addr_mut<T>(addr: usize) -> *mut T
where
    T: Sized,
{
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    addr as *mut T
}

/// Creates a null mutable raw pointer.
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let p: *mut i32 = ptr::null_mut();
/// assert!(p.is_null());
/// ```
#[inline(always)]
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_promotable]
#[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
#[rustc_allow_const_fn_unstable(ptr_metadata)]
#[rustc_diagnostic_item = "ptr_null_mut"]
pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
    from_raw_parts_mut(invalid_mut(0), ())
}

/// Forms a raw slice from a pointer and a length.
///
/// The `len` argument is the number of **elements**, not the number of bytes.
///
/// This function is safe, but actually using the return value is unsafe.
/// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
///
/// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
///
/// # Examples
///
/// ```rust
/// use std::ptr;
///
/// // create a slice pointer when starting out with a pointer to the first element
/// let x = [5, 6, 7];
/// let raw_pointer = x.as_ptr();
/// let slice = ptr::slice_from_raw_parts(raw_pointer, 3);
/// assert_eq!(unsafe { &*slice }[2], 7);
/// ```
#[inline]
#[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
#[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
#[rustc_allow_const_fn_unstable(ptr_metadata)]
pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
    from_raw_parts(data.cast(), len)
}

/// Performs the same functionality as [`slice_from_raw_parts`], except that a
/// raw mutable slice is returned, as opposed to a raw immutable slice.
///
/// See the documentation of [`slice_from_raw_parts`] for more details.
///
/// This function is safe, but actually using the return value is unsafe.
/// See the documentation of [`slice::from_raw_parts_mut`] for slice safety requirements.
///
/// [`slice::from_raw_parts_mut`]: crate::slice::from_raw_parts_mut
///
/// # Examples
///
/// ```rust
/// use std::ptr;
///
/// let x = &mut [5, 6, 7];
/// let raw_pointer = x.as_mut_ptr();
/// let slice = ptr::slice_from_raw_parts_mut(raw_pointer, 3);
///
/// unsafe {
///     (*slice)[2] = 99; // assign a value at an index in the slice
/// };
///
/// assert_eq!(unsafe { &*slice }[2], 99);
/// ```
#[inline]
#[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
#[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
    from_raw_parts_mut(data.cast(), len)
}

/// Swaps the values at two mutable locations of the same type, without
/// deinitializing either.
///
/// But for the following exceptions, this function is semantically
/// equivalent to [`mem::swap`]:
///
/// * It operates on raw pointers instead of references. When references are
///   available, [`mem::swap`] should be preferred.
///
/// * The two pointed-to values may overlap. If the values do overlap, then the
///   overlapping region of memory from `x` will be used. This is demonstrated
///   in the second example below.
///
/// * The operation is "untyped" in the sense that data may be uninitialized or otherwise violate
///   the requirements of `T`. The initialization state is preserved exactly.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * Both `x` and `y` must be [valid] for both reads and writes.
///
/// * Both `x` and `y` must be properly aligned.
///
/// Note that even if `T` has size `0`, the pointers must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// # Examples
///
/// Swapping two non-overlapping regions:
///
/// ```
/// use std::ptr;
///
/// let mut array = [0, 1, 2, 3];
///
/// let (x, y) = array.split_at_mut(2);
/// let x = x.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[0..2]`
/// let y = y.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[2..4]`
///
/// unsafe {
///     ptr::swap(x, y);
///     assert_eq!([2, 3, 0, 1], array);
/// }
/// ```
///
/// Swapping two overlapping regions:
///
/// ```
/// use std::ptr;
///
/// let mut array: [i32; 4] = [0, 1, 2, 3];
///
/// let array_ptr: *mut i32 = array.as_mut_ptr();
///
/// let x = array_ptr as *mut [i32; 3]; // this is `array[0..3]`
/// let y = unsafe { array_ptr.add(1) } as *mut [i32; 3]; // this is `array[1..4]`
///
/// unsafe {
///     ptr::swap(x, y);
///     // The indices `1..3` of the slice overlap between `x` and `y`.
///     // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
///     // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
///     // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
///     // This implementation is defined to make the latter choice.
///     assert_eq!([1, 0, 1, 2], array);
/// }
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_swap", issue = "83163")]
pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
    // Give ourselves some scratch space to work with.
    // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
    let mut tmp = MaybeUninit::<T>::uninit();

    // Perform the swap
    // SAFETY: the caller must guarantee that `x` and `y` are
    // valid for writes and properly aligned. `tmp` cannot be
    // overlapping either `x` or `y` because `tmp` was just allocated
    // on the stack as a separate allocated object.
    unsafe {
        copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
        copy(y, x, 1); // `x` and `y` may overlap
        copy_nonoverlapping(tmp.as_ptr(), y, 1);
    }
}

/// Swaps `count * size_of::<T>()` bytes between the two regions of memory
/// beginning at `x` and `y`. The two regions must *not* overlap.
///
/// The operation is "untyped" in the sense that data may be uninitialized or otherwise violate the
/// requirements of `T`. The initialization state is preserved exactly.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * Both `x` and `y` must be [valid] for both reads and writes of `count *
///   size_of::<T>()` bytes.
///
/// * Both `x` and `y` must be properly aligned.
///
/// * The region of memory beginning at `x` with a size of `count *
///   size_of::<T>()` bytes must *not* overlap with the region of memory
///   beginning at `y` with the same size.
///
/// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
/// the pointers must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::ptr;
///
/// let mut x = [1, 2, 3, 4];
/// let mut y = [7, 8, 9];
///
/// unsafe {
///     ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
/// }
///
/// assert_eq!(x, [7, 8, 3, 4]);
/// assert_eq!(y, [1, 2, 9]);
/// ```
#[inline]
#[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
#[rustc_const_unstable(feature = "const_swap", issue = "83163")]
pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
    #[allow(unused)]
    macro_rules! attempt_swap_as_chunks {
        ($ChunkTy:ty) => {
            if mem::align_of::<T>() >= mem::align_of::<$ChunkTy>()
                && mem::size_of::<T>() % mem::size_of::<$ChunkTy>() == 0
            {
                let x: *mut $ChunkTy = x.cast();
                let y: *mut $ChunkTy = y.cast();
                let count = count * (mem::size_of::<T>() / mem::size_of::<$ChunkTy>());
                // SAFETY: these are the same bytes that the caller promised were
                // ok, just typed as `MaybeUninit<ChunkTy>`s instead of as `T`s.
                // The `if` condition above ensures that we're not violating
                // alignment requirements, and that the division is exact so
                // that we don't lose any bytes off the end.
                return unsafe { swap_nonoverlapping_simple_untyped(x, y, count) };
            }
        };
    }

    // SAFETY: the caller must guarantee that `x` and `y` are
    // valid for writes and properly aligned.
    unsafe {
        assert_unsafe_precondition!([T](x: *mut T, y: *mut T, count: usize) =>
            is_aligned_and_not_null(x)
                && is_aligned_and_not_null(y)
                && is_nonoverlapping(x, y, count)
        );
    }

    // NOTE(scottmcm) Miri is disabled here as reading in smaller units is a
    // pessimization for it.  Also, if the type contains any unaligned pointers,
    // copying those over multiple reads is difficult to support.
    #[cfg(not(miri))]
    {
        // Split up the slice into small power-of-two-sized chunks that LLVM is able
        // to vectorize (unless it's a special type with more-than-pointer alignment,
        // because we don't want to pessimize things like slices of SIMD vectors.)
        if mem::align_of::<T>() <= mem::size_of::<usize>()
            && (!mem::size_of::<T>().is_power_of_two()
                || mem::size_of::<T>() > mem::size_of::<usize>() * 2)
        {
            attempt_swap_as_chunks!(usize);
            attempt_swap_as_chunks!(u8);
        }
    }

    // SAFETY: Same preconditions as this function
    unsafe { swap_nonoverlapping_simple_untyped(x, y, count) }
}

/// Same behaviour and safety conditions as [`swap_nonoverlapping`]
///
/// LLVM can vectorize this (at least it can for the power-of-two-sized types
/// `swap_nonoverlapping` tries to use) so no need to manually SIMD it.
#[inline]
#[rustc_const_unstable(feature = "const_swap", issue = "83163")]
const unsafe fn swap_nonoverlapping_simple_untyped<T>(x: *mut T, y: *mut T, count: usize) {
    let x = x.cast::<MaybeUninit<T>>();
    let y = y.cast::<MaybeUninit<T>>();
    let mut i = 0;
    while i < count {
        // SAFETY: By precondition, `i` is in-bounds because it's below `n`
        let x = unsafe { &mut *x.add(i) };
        // SAFETY: By precondition, `i` is in-bounds because it's below `n`
        // and it's distinct from `x` since the ranges are non-overlapping
        let y = unsafe { &mut *y.add(i) };
        mem::swap_simple::<MaybeUninit<T>>(x, y);

        i += 1;
    }
}

/// Moves `src` into the pointed `dst`, returning the previous `dst` value.
///
/// Neither value is dropped.
///
/// This function is semantically equivalent to [`mem::replace`] except that it
/// operates on raw pointers instead of references. When references are
/// available, [`mem::replace`] should be preferred.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `dst` must be [valid] for both reads and writes.
///
/// * `dst` must be properly aligned.
///
/// * `dst` must point to a properly initialized value of type `T`.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let mut rust = vec!['b', 'u', 's', 't'];
///
/// // `mem::replace` would have the same effect without requiring the unsafe
/// // block.
/// let b = unsafe {
///     ptr::replace(&mut rust[0], 'r')
/// };
///
/// assert_eq!(b, 'b');
/// assert_eq!(rust, &['r', 'u', 's', 't']);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
    // SAFETY: the caller must guarantee that `dst` is valid to be
    // cast to a mutable reference (valid for writes, aligned, initialized),
    // and cannot overlap `src` since `dst` must point to a distinct
    // allocated object.
    unsafe {
        assert_unsafe_precondition!([T](dst: *mut T) => is_aligned_and_not_null(dst));
        mem::swap(&mut *dst, &mut src); // cannot overlap
    }
    src
}

/// Reads the value from `src` without moving it. This leaves the
/// memory in `src` unchanged.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `src` must be [valid] for reads.
///
/// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
///   case.
///
/// * `src` must point to a properly initialized value of type `T`.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let x = 12;
/// let y = &x as *const i32;
///
/// unsafe {
///     assert_eq!(std::ptr::read(y), 12);
/// }
/// ```
///
/// Manually implement [`mem::swap`]:
///
/// ```
/// use std::ptr;
///
/// fn swap<T>(a: &mut T, b: &mut T) {
///     unsafe {
///         // Create a bitwise copy of the value at `a` in `tmp`.
///         let tmp = ptr::read(a);
///
///         // Exiting at this point (either by explicitly returning or by
///         // calling a function which panics) would cause the value in `tmp` to
///         // be dropped while the same value is still referenced by `a`. This
///         // could trigger undefined behavior if `T` is not `Copy`.
///
///         // Create a bitwise copy of the value at `b` in `a`.
///         // This is safe because mutable references cannot alias.
///         ptr::copy_nonoverlapping(b, a, 1);
///
///         // As above, exiting here could trigger undefined behavior because
///         // the same value is referenced by `a` and `b`.
///
///         // Move `tmp` into `b`.
///         ptr::write(b, tmp);
///
///         // `tmp` has been moved (`write` takes ownership of its second argument),
///         // so nothing is dropped implicitly here.
///     }
/// }
///
/// let mut foo = "foo".to_owned();
/// let mut bar = "bar".to_owned();
///
/// swap(&mut foo, &mut bar);
///
/// assert_eq!(foo, "bar");
/// assert_eq!(bar, "foo");
/// ```
///
/// ## Ownership of the Returned Value
///
/// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
/// If `T` is not [`Copy`], using both the returned value and the value at
/// `*src` can violate memory safety. Note that assigning to `*src` counts as a
/// use because it will attempt to drop the value at `*src`.
///
/// [`write()`] can be used to overwrite data without causing it to be dropped.
///
/// ```
/// use std::ptr;
///
/// let mut s = String::from("foo");
/// unsafe {
///     // `s2` now points to the same underlying memory as `s`.
///     let mut s2: String = ptr::read(&s);
///
///     assert_eq!(s2, "foo");
///
///     // Assigning to `s2` causes its original value to be dropped. Beyond
///     // this point, `s` must no longer be used, as the underlying memory has
///     // been freed.
///     s2 = String::default();
///     assert_eq!(s2, "");
///
///     // Assigning to `s` would cause the old value to be dropped again,
///     // resulting in undefined behavior.
///     // s = String::from("bar"); // ERROR
///
///     // `ptr::write` can be used to overwrite a value without dropping it.
///     ptr::write(&mut s, String::from("bar"));
/// }
///
/// assert_eq!(s, "bar");
/// ```
///
/// [valid]: self#safety
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub const unsafe fn read<T>(src: *const T) -> T {
    // We are calling the intrinsics directly to avoid function calls in the generated code
    // as `intrinsics::copy_nonoverlapping` is a wrapper function.
    extern "rust-intrinsic" {
        #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
    }

    let mut tmp = MaybeUninit::<T>::uninit();
    // SAFETY: the caller must guarantee that `src` is valid for reads.
    // `src` cannot overlap `tmp` because `tmp` was just allocated on
    // the stack as a separate allocated object.
    //
    // Also, since we just wrote a valid value into `tmp`, it is guaranteed
    // to be properly initialized.
    unsafe {
        assert_unsafe_precondition!([T](src: *const T) => is_aligned_and_not_null(src));
        copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
        tmp.assume_init()
    }
}

/// Reads the value from `src` without moving it. This leaves the
/// memory in `src` unchanged.
///
/// Unlike [`read`], `read_unaligned` works with unaligned pointers.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `src` must be [valid] for reads.
///
/// * `src` must point to a properly initialized value of type `T`.
///
/// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
/// value and the value at `*src` can [violate memory safety][read-ownership].
///
/// Note that even if `T` has size `0`, the pointer must be non-null.
///
/// [read-ownership]: read#ownership-of-the-returned-value
/// [valid]: self#safety
///
/// ## On `packed` structs
///
/// Attempting to create a raw pointer to an `unaligned` struct field with
/// an expression such as `&packed.unaligned as *const FieldType` creates an
/// intermediate unaligned reference before converting that to a raw pointer.
/// That this reference is temporary and immediately cast is inconsequential
/// as the compiler always expects references to be properly aligned.
/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
/// *undefined behavior* in your program.
///
/// Instead you must use the [`ptr::addr_of!`](addr_of) macro to
/// create the pointer. You may use that returned pointer together with this
/// function.
///
/// An example of what not to do and how this relates to `read_unaligned` is:
///
/// ```
/// #[repr(packed, C)]
/// struct Packed {
///     _padding: u8,
///     unaligned: u32,
/// }
///
/// let packed = Packed {
///     _padding: 0x00,
///     unaligned: 0x01020304,
/// };
///
/// // Take the address of a 32-bit integer which is not aligned.
/// // In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
/// let unaligned = std::ptr::addr_of!(packed.unaligned);
///
/// let v = unsafe { std::ptr::read_unaligned(unaligned) };
/// assert_eq!(v, 0x01020304);
/// ```
///
/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
///
/// # Examples
///
/// Read a usize value from a byte buffer:
///
/// ```
/// use std::mem;
///
/// fn read_usize(x: &[u8]) -> usize {
///     assert!(x.len() >= mem::size_of::<usize>());
///
///     let ptr = x.as_ptr() as *const usize;
///
///     unsafe { ptr.read_unaligned() }
/// }
/// ```
#[inline]
#[stable(feature = "ptr_unaligned", since = "1.17.0")]
#[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
    let mut tmp = MaybeUninit::<T>::uninit();
    // SAFETY: the caller must guarantee that `src` is valid for reads.
    // `src` cannot overlap `tmp` because `tmp` was just allocated on
    // the stack as a separate allocated object.
    //
    // Also, since we just wrote a valid value into `tmp`, it is guaranteed
    // to be properly initialized.
    unsafe {
        copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
        tmp.assume_init()
    }
}

/// Overwrites a memory location with the given value without reading or
/// dropping the old value.
///
/// `write` does not drop the contents of `dst`. This is safe, but it could leak
/// allocations or resources, so care should be taken not to overwrite an object
/// that should be dropped.
///
/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
/// location pointed to by `dst`.
///
/// This is appropriate for initializing uninitialized memory, or overwriting
/// memory that has previously been [`read`] from.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `dst` must be [valid] for writes.
///
/// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
///   case.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let mut x = 0;
/// let y = &mut x as *mut i32;
/// let z = 12;
///
/// unsafe {
///     std::ptr::write(y, z);
///     assert_eq!(std::ptr::read(y), 12);
/// }
/// ```
///
/// Manually implement [`mem::swap`]:
///
/// ```
/// use std::ptr;
///
/// fn swap<T>(a: &mut T, b: &mut T) {
///     unsafe {
///         // Create a bitwise copy of the value at `a` in `tmp`.
///         let tmp = ptr::read(a);
///
///         // Exiting at this point (either by explicitly returning or by
///         // calling a function which panics) would cause the value in `tmp` to
///         // be dropped while the same value is still referenced by `a`. This
///         // could trigger undefined behavior if `T` is not `Copy`.
///
///         // Create a bitwise copy of the value at `b` in `a`.
///         // This is safe because mutable references cannot alias.
///         ptr::copy_nonoverlapping(b, a, 1);
///
///         // As above, exiting here could trigger undefined behavior because
///         // the same value is referenced by `a` and `b`.
///
///         // Move `tmp` into `b`.
///         ptr::write(b, tmp);
///
///         // `tmp` has been moved (`write` takes ownership of its second argument),
///         // so nothing is dropped implicitly here.
///     }
/// }
///
/// let mut foo = "foo".to_owned();
/// let mut bar = "bar".to_owned();
///
/// swap(&mut foo, &mut bar);
///
/// assert_eq!(foo, "bar");
/// assert_eq!(bar, "foo");
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub const unsafe fn write<T>(dst: *mut T, src: T) {
    // We are calling the intrinsics directly to avoid function calls in the generated code
    // as `intrinsics::copy_nonoverlapping` is a wrapper function.
    extern "rust-intrinsic" {
        #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
    }

    // SAFETY: the caller must guarantee that `dst` is valid for writes.
    // `dst` cannot overlap `src` because the caller has mutable access
    // to `dst` while `src` is owned by this function.
    unsafe {
        assert_unsafe_precondition!([T](dst: *mut T) => is_aligned_and_not_null(dst));
        copy_nonoverlapping(&src as *const T, dst, 1);
        intrinsics::forget(src);
    }
}

/// Overwrites a memory location with the given value without reading or
/// dropping the old value.
///
/// Unlike [`write()`], the pointer may be unaligned.
///
/// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
/// could leak allocations or resources, so care should be taken not to overwrite
/// an object that should be dropped.
///
/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
/// location pointed to by `dst`.
///
/// This is appropriate for initializing uninitialized memory, or overwriting
/// memory that has previously been read with [`read_unaligned`].
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `dst` must be [valid] for writes.
///
/// Note that even if `T` has size `0`, the pointer must be non-null.
///
/// [valid]: self#safety
///
/// ## On `packed` structs
///
/// Attempting to create a raw pointer to an `unaligned` struct field with
/// an expression such as `&packed.unaligned as *const FieldType` creates an
/// intermediate unaligned reference before converting that to a raw pointer.
/// That this reference is temporary and immediately cast is inconsequential
/// as the compiler always expects references to be properly aligned.
/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
/// *undefined behavior* in your program.
///
/// Instead you must use the [`ptr::addr_of_mut!`](addr_of_mut)
/// macro to create the pointer. You may use that returned pointer together with
/// this function.
///
/// An example of how to do it and how this relates to `write_unaligned` is:
///
/// ```
/// #[repr(packed, C)]
/// struct Packed {
///     _padding: u8,
///     unaligned: u32,
/// }
///
/// let mut packed: Packed = unsafe { std::mem::zeroed() };
///
/// // Take the address of a 32-bit integer which is not aligned.
/// // In contrast to `&packed.unaligned as *mut _`, this has no undefined behavior.
/// let unaligned = std::ptr::addr_of_mut!(packed.unaligned);
///
/// unsafe { std::ptr::write_unaligned(unaligned, 42) };
///
/// assert_eq!({packed.unaligned}, 42); // `{...}` forces copying the field instead of creating a reference.
/// ```
///
/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however
/// (as can be seen in the `assert_eq!` above).
///
/// # Examples
///
/// Write a usize value to a byte buffer:
///
/// ```
/// use std::mem;
///
/// fn write_usize(x: &mut [u8], val: usize) {
///     assert!(x.len() >= mem::size_of::<usize>());
///
///     let ptr = x.as_mut_ptr() as *mut usize;
///
///     unsafe { ptr.write_unaligned(val) }
/// }
/// ```
#[inline]
#[stable(feature = "ptr_unaligned", since = "1.17.0")]
#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
    // SAFETY: the caller must guarantee that `dst` is valid for writes.
    // `dst` cannot overlap `src` because the caller has mutable access
    // to `dst` while `src` is owned by this function.
    unsafe {
        copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
        // We are calling the intrinsic directly to avoid function calls in the generated code.
        intrinsics::forget(src);
    }
}

/// Performs a volatile read of the value from `src` without moving it. This
/// leaves the memory in `src` unchanged.
///
/// Volatile operations are intended to act on I/O memory, and are guaranteed
/// to not be elided or reordered by the compiler across other volatile
/// operations.
///
/// # Notes
///
/// Rust does not currently have a rigorously and formally defined memory model,
/// so the precise semantics of what "volatile" means here is subject to change
/// over time. That being said, the semantics will almost always end up pretty
/// similar to [C11's definition of volatile][c11].
///
/// The compiler shouldn't change the relative order or number of volatile
/// memory operations. However, volatile memory operations on zero-sized types
/// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
/// and may be ignored.
///
/// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `src` must be [valid] for reads.
///
/// * `src` must be properly aligned.
///
/// * `src` must point to a properly initialized value of type `T`.
///
/// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
/// value and the value at `*src` can [violate memory safety][read-ownership].
/// However, storing non-[`Copy`] types in volatile memory is almost certainly
/// incorrect.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// [valid]: self#safety
/// [read-ownership]: read#ownership-of-the-returned-value
///
/// Just like in C, whether an operation is volatile has no bearing whatsoever
/// on questions involving concurrent access from multiple threads. Volatile
/// accesses behave exactly like non-atomic accesses in that regard. In particular,
/// a race between a `read_volatile` and any write operation to the same location
/// is undefined behavior.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let x = 12;
/// let y = &x as *const i32;
///
/// unsafe {
///     assert_eq!(std::ptr::read_volatile(y), 12);
/// }
/// ```
#[inline]
#[stable(feature = "volatile", since = "1.9.0")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn read_volatile<T>(src: *const T) -> T {
    // SAFETY: the caller must uphold the safety contract for `volatile_load`.
    unsafe {
        assert_unsafe_precondition!([T](src: *const T) => is_aligned_and_not_null(src));
        intrinsics::volatile_load(src)
    }
}

/// Performs a volatile write of a memory location with the given value without
/// reading or dropping the old value.
///
/// Volatile operations are intended to act on I/O memory, and are guaranteed
/// to not be elided or reordered by the compiler across other volatile
/// operations.
///
/// `write_volatile` does not drop the contents of `dst`. This is safe, but it
/// could leak allocations or resources, so care should be taken not to overwrite
/// an object that should be dropped.
///
/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
/// location pointed to by `dst`.
///
/// # Notes
///
/// Rust does not currently have a rigorously and formally defined memory model,
/// so the precise semantics of what "volatile" means here is subject to change
/// over time. That being said, the semantics will almost always end up pretty
/// similar to [C11's definition of volatile][c11].
///
/// The compiler shouldn't change the relative order or number of volatile
/// memory operations. However, volatile memory operations on zero-sized types
/// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
/// and may be ignored.
///
/// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `dst` must be [valid] for writes.
///
/// * `dst` must be properly aligned.
///
/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
///
/// [valid]: self#safety
///
/// Just like in C, whether an operation is volatile has no bearing whatsoever
/// on questions involving concurrent access from multiple threads. Volatile
/// accesses behave exactly like non-atomic accesses in that regard. In particular,
/// a race between a `write_volatile` and any other operation (reading or writing)
/// on the same location is undefined behavior.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let mut x = 0;
/// let y = &mut x as *mut i32;
/// let z = 12;
///
/// unsafe {
///     std::ptr::write_volatile(y, z);
///     assert_eq!(std::ptr::read_volatile(y), 12);
/// }
/// ```
#[inline]
#[stable(feature = "volatile", since = "1.9.0")]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
    // SAFETY: the caller must uphold the safety contract for `volatile_store`.
    unsafe {
        assert_unsafe_precondition!([T](dst: *mut T) => is_aligned_and_not_null(dst));
        intrinsics::volatile_store(dst, src);
    }
}

/// Align pointer `p`.
///
/// Calculate offset (in terms of elements of `stride` stride) that has to be applied
/// to pointer `p` so that pointer `p` would get aligned to `a`.
///
/// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
/// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
/// constants.
///
/// If we ever decide to make it possible to call the intrinsic with `a` that is not a
/// power-of-two, it will probably be more prudent to just change to a naive implementation rather
/// than trying to adapt this to accommodate that change.
///
/// Any questions go to @nagisa.
#[lang = "align_offset"]
pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
    // FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
    // 1, where the method versions of these operations are not inlined.
    use intrinsics::{
        cttz_nonzero, exact_div, unchecked_rem, unchecked_shl, unchecked_shr, unchecked_sub,
        wrapping_add, wrapping_mul, wrapping_sub,
    };

    /// Calculate multiplicative modular inverse of `x` modulo `m`.
    ///
    /// This implementation is tailored for `align_offset` and has following preconditions:
    ///
    /// * `m` is a power-of-two;
    /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
    ///
    /// Implementation of this function shall not panic. Ever.
    #[inline]
    unsafe fn mod_inv(x: usize, m: usize) -> usize {
        /// Multiplicative modular inverse table modulo 2⁴ = 16.
        ///
        /// Note, that this table does not contain values where inverse does not exist (i.e., for
        /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
        const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
        /// Modulo for which the `INV_TABLE_MOD_16` is intended.
        const INV_TABLE_MOD: usize = 16;
        /// INV_TABLE_MOD²
        const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;

        let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
        // SAFETY: `m` is required to be a power-of-two, hence non-zero.
        let m_minus_one = unsafe { unchecked_sub(m, 1) };
        if m <= INV_TABLE_MOD {
            table_inverse & m_minus_one
        } else {
            // We iterate "up" using the following formula:
            //
            // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
            //
            // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
            let mut inverse = table_inverse;
            let mut going_mod = INV_TABLE_MOD_SQUARED;
            loop {
                // y = y * (2 - xy) mod n
                //
                // Note, that we use wrapping operations here intentionally – the original formula
                // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
                // usize::MAX` instead, because we take the result `mod n` at the end
                // anyway.
                inverse = wrapping_mul(inverse, wrapping_sub(2usize, wrapping_mul(x, inverse)));
                if going_mod >= m {
                    return inverse & m_minus_one;
                }
                going_mod = wrapping_mul(going_mod, going_mod);
            }
        }
    }

    let addr = p.addr();
    let stride = mem::size_of::<T>();
    // SAFETY: `a` is a power-of-two, therefore non-zero.
    let a_minus_one = unsafe { unchecked_sub(a, 1) };

    if stride == 0 {
        // SPECIAL_CASE: handle 0-sized types. No matter how many times we step, the address will
        // stay the same, so no offset will be able to align the pointer unless it is already
        // aligned. This branch _will_ be optimized out as `stride` is known at compile-time.
        let p_mod_a = addr & a_minus_one;
        return if p_mod_a == 0 { 0 } else { usize::MAX };
    }

    // SAFETY: `stride == 0` case has been handled by the special case above.
    let a_mod_stride = unsafe { unchecked_rem(a, stride) };
    if a_mod_stride == 0 {
        // SPECIAL_CASE: In cases where the `a` is divisible by `stride`, byte offset to align a
        // pointer can be computed more simply through `-p (mod a)`. In the off-chance the byte
        // offset is not a multiple of `stride`, the input pointer was misaligned and no pointer
        // offset will be able to produce a `p` aligned to the specified `a`.
        //
        // The naive `-p (mod a)` equation  inhibits LLVM's ability to select instructions
        // like `lea`. We compute `(round_up_to_next_alignment(p, a) - p)` instead. This
        // redistributes operations around the load-bearing, but pessimizing `and` instruction
        // sufficiently for LLVM to be able to utilize the various optimizations it knows about.
        //
        // LLVM handles the branch here particularly nicely. If this branch needs to be evaluated
        // at runtime, it will produce a mask `if addr_mod_stride == 0 { 0 } else { usize::MAX }`
        // in a branch-free way and then bitwise-OR it with whatever result the `-p mod a`
        // computation produces.

        // SAFETY: `stride == 0` case has been handled by the special case above.
        let addr_mod_stride = unsafe { unchecked_rem(addr, stride) };

        return if addr_mod_stride == 0 {
            let aligned_address = wrapping_add(addr, a_minus_one) & wrapping_sub(0, a);
            let byte_offset = wrapping_sub(aligned_address, addr);
            // SAFETY: `stride` is non-zero. This is guaranteed to divide exactly as well, because
            // addr has been verified to be aligned to the original type’s alignment requirements.
            unsafe { exact_div(byte_offset, stride) }
        } else {
            usize::MAX
        };
    }

    // GENERAL_CASE: From here on we’re handling the very general case where `addr` may be
    // misaligned, there isn’t an obvious relationship between `stride` and `a` that we can take an
    // advantage of, etc. This case produces machine code that isn’t particularly high quality,
    // compared to the special cases above. The code produced here is still within the realm of
    // miracles, given the situations this case has to deal with.

    // SAFETY: a is power-of-two hence non-zero. stride == 0 case is handled above.
    let gcdpow = unsafe { cttz_nonzero(stride).min(cttz_nonzero(a)) };
    // SAFETY: gcdpow has an upper-bound that’s at most the number of bits in a usize.
    let gcd = unsafe { unchecked_shl(1usize, gcdpow) };
    // SAFETY: gcd is always greater or equal to 1.
    if addr & unsafe { unchecked_sub(gcd, 1) } == 0 {
        // This branch solves for the following linear congruence equation:
        //
        // ` p + so = 0 mod a `
        //
        // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
        // requested alignment.
        //
        // With `g = gcd(a, s)`, and the above condition asserting that `p` is also divisible by
        // `g`, we can denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
        //
        // ` p' + s'o = 0 mod a' `
        // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
        //
        // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the
        // second term is "how does incrementing `p` by `s` bytes change the relative alignment of
        // `p`" (again divided by `g`). Division by `g` is necessary to make the inverse well
        // formed if `a` and `s` are not co-prime.
        //
        // Furthermore, the result produced by this solution is not "minimal", so it is necessary
        // to take the result `o mod lcm(s, a)`. This `lcm(s, a)` is the same as `a'`.

        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
        // `a`.
        let a2 = unsafe { unchecked_shr(a, gcdpow) };
        // SAFETY: `a2` is non-zero. Shifting `a` by `gcdpow` cannot shift out any of the set bits
        // in `a` (of which it has exactly one).
        let a2minus1 = unsafe { unchecked_sub(a2, 1) };
        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
        // `a`.
        let s2 = unsafe { unchecked_shr(stride & a_minus_one, gcdpow) };
        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
        // `a`. Furthermore, the subtraction cannot overflow, because `a2 = a >> gcdpow` will
        // always be strictly greater than `(p % a) >> gcdpow`.
        let minusp2 = unsafe { unchecked_sub(a2, unchecked_shr(addr & a_minus_one, gcdpow)) };
        // SAFETY: `a2` is a power-of-two, as proven above. `s2` is strictly less than `a2`
        // because `(s % a) >> gcdpow` is strictly less than `a >> gcdpow`.
        return wrapping_mul(minusp2, unsafe { mod_inv(s2, a2) }) & a2minus1;
    }

    // Cannot be aligned at all.
    usize::MAX
}

/// Compares raw pointers for equality.
///
/// This is the same as using the `==` operator, but less generic:
/// the arguments have to be `*const T` raw pointers,
/// not anything that implements `PartialEq`.
///
/// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
/// by their address rather than comparing the values they point to
/// (which is what the `PartialEq for &T` implementation does).
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let five = 5;
/// let other_five = 5;
/// let five_ref = &five;
/// let same_five_ref = &five;
/// let other_five_ref = &other_five;
///
/// assert!(five_ref == same_five_ref);
/// assert!(ptr::eq(five_ref, same_five_ref));
///
/// assert!(five_ref == other_five_ref);
/// assert!(!ptr::eq(five_ref, other_five_ref));
/// ```
///
/// Slices are also compared by their length (fat pointers):
///
/// ```
/// let a = [1, 2, 3];
/// assert!(std::ptr::eq(&a[..3], &a[..3]));
/// assert!(!std::ptr::eq(&a[..2], &a[..3]));
/// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
/// ```
///
/// Traits are also compared by their implementation:
///
/// ```
/// #[repr(transparent)]
/// struct Wrapper { member: i32 }
///
/// trait Trait {}
/// impl Trait for Wrapper {}
/// impl Trait for i32 {}
///
/// let wrapper = Wrapper { member: 10 };
///
/// // Pointers have equal addresses.
/// assert!(std::ptr::eq(
///     &wrapper as *const Wrapper as *const u8,
///     &wrapper.member as *const i32 as *const u8
/// ));
///
/// // Objects have equal addresses, but `Trait` has different implementations.
/// assert!(!std::ptr::eq(
///     &wrapper as &dyn Trait,
///     &wrapper.member as &dyn Trait,
/// ));
/// assert!(!std::ptr::eq(
///     &wrapper as &dyn Trait as *const dyn Trait,
///     &wrapper.member as &dyn Trait as *const dyn Trait,
/// ));
///
/// // Converting the reference to a `*const u8` compares by address.
/// assert!(std::ptr::eq(
///     &wrapper as &dyn Trait as *const dyn Trait as *const u8,
///     &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
/// ));
/// ```
#[stable(feature = "ptr_eq", since = "1.17.0")]
#[inline]
pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
    a == b
}

/// Hash a raw pointer.
///
/// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
/// by its address rather than the value it points to
/// (which is what the `Hash for &T` implementation does).
///
/// # Examples
///
/// ```
/// use std::collections::hash_map::DefaultHasher;
/// use std::hash::{Hash, Hasher};
/// use std::ptr;
///
/// let five = 5;
/// let five_ref = &five;
///
/// let mut hasher = DefaultHasher::new();
/// ptr::hash(five_ref, &mut hasher);
/// let actual = hasher.finish();
///
/// let mut hasher = DefaultHasher::new();
/// (five_ref as *const i32).hash(&mut hasher);
/// let expected = hasher.finish();
///
/// assert_eq!(actual, expected);
/// ```
#[stable(feature = "ptr_hash", since = "1.35.0")]
pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
    use crate::hash::Hash;
    hashee.hash(into);
}

// If this is a unary fn pointer, it adds a doc comment.
// Otherwise, it hides the docs entirely.
macro_rules! maybe_fnptr_doc {
    (@ #[$meta:meta] $item:item) => {
        #[doc(hidden)]
        #[$meta]
        $item
    };
    ($a:ident @ #[$meta:meta] $item:item) => {
        #[doc(fake_variadic)]
        #[doc = "This trait is implemented for function pointers with up to twelve arguments."]
        #[$meta]
        $item
    };
    ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
        #[doc(hidden)]
        #[$meta]
        $item
    };
}

// FIXME(strict_provenance_magic): function pointers have buggy codegen that
// necessitates casting to a usize to get the backend to do the right thing.
// for now I will break AVR to silence *a billion* lints. We should probably
// have a proper "opaque function pointer type" to handle this kind of thing.

// Impls for function pointers
macro_rules! fnptr_impls_safety_abi {
    ($FnTy: ty, $($Arg: ident),*) => {
        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> PartialEq for $FnTy {
                #[inline]
                fn eq(&self, other: &Self) -> bool {
                    *self as usize == *other as usize
                }
            }
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> Eq for $FnTy {}
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> PartialOrd for $FnTy {
                #[inline]
                fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
                    (*self as usize).partial_cmp(&(*other as usize))
                }
            }
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> Ord for $FnTy {
                #[inline]
                fn cmp(&self, other: &Self) -> Ordering {
                    (*self as usize).cmp(&(*other as usize))
                }
            }
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> hash::Hash for $FnTy {
                fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
                    state.write_usize(*self as usize)
                }
            }
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    fmt::pointer_fmt_inner(*self as usize, f)
                }
            }
        }

        maybe_fnptr_doc! {
            $($Arg)* @
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    fmt::pointer_fmt_inner(*self as usize, f)
                }
            }
        }
    }
}

macro_rules! fnptr_impls_args {
    ($($Arg: ident),+) => {
        fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
        fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
        fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
        fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
        fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
        fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
    };
    () => {
        // No variadic functions with 0 parameters
        fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
        fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
        fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
        fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
    };
}

fnptr_impls_args! {}
fnptr_impls_args! { T }
fnptr_impls_args! { A, B }
fnptr_impls_args! { A, B, C }
fnptr_impls_args! { A, B, C, D }
fnptr_impls_args! { A, B, C, D, E }
fnptr_impls_args! { A, B, C, D, E, F }
fnptr_impls_args! { A, B, C, D, E, F, G }
fnptr_impls_args! { A, B, C, D, E, F, G, H }
fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }

/// Create a `const` raw pointer to a place, without creating an intermediate reference.
///
/// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
/// and points to initialized data. For cases where those requirements do not hold,
/// raw pointers should be used instead. However, `&expr as *const _` creates a reference
/// before casting it to a raw pointer, and that reference is subject to the same rules
/// as all other references. This macro can create a raw pointer *without* creating
/// a reference first.
///
/// Note, however, that the `expr` in `addr_of!(expr)` is still subject to all
/// the usual rules. In particular, `addr_of!(*ptr::null())` is Undefined
/// Behavior because it dereferences a null pointer.
///
/// # Example
///
/// ```
/// use std::ptr;
///
/// #[repr(packed)]
/// struct Packed {
///     f1: u8,
///     f2: u16,
/// }
///
/// let packed = Packed { f1: 1, f2: 2 };
/// // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
/// let raw_f2 = ptr::addr_of!(packed.f2);
/// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
/// ```
///
/// See [`addr_of_mut`] for how to create a pointer to unininitialized data.
/// Doing that with `addr_of` would not make much sense since one could only
/// read the data, and that would be Undefined Behavior.
#[stable(feature = "raw_ref_macros", since = "1.51.0")]
#[rustc_macro_transparency = "semitransparent"]
#[allow_internal_unstable(raw_ref_op)]
pub macro addr_of($place:expr) {
    &raw const $place
}

/// Create a `mut` raw pointer to a place, without creating an intermediate reference.
///
/// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
/// and points to initialized data. For cases where those requirements do not hold,
/// raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference
/// before casting it to a raw pointer, and that reference is subject to the same rules
/// as all other references. This macro can create a raw pointer *without* creating
/// a reference first.
///
/// Note, however, that the `expr` in `addr_of_mut!(expr)` is still subject to all
/// the usual rules. In particular, `addr_of_mut!(*ptr::null_mut())` is Undefined
/// Behavior because it dereferences a null pointer.
///
/// # Examples
///
/// **Creating a pointer to unaligned data:**
///
/// ```
/// use std::ptr;
///
/// #[repr(packed)]
/// struct Packed {
///     f1: u8,
///     f2: u16,
/// }
///
/// let mut packed = Packed { f1: 1, f2: 2 };
/// // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
/// let raw_f2 = ptr::addr_of_mut!(packed.f2);
/// unsafe { raw_f2.write_unaligned(42); }
/// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
/// ```
///
/// **Creating a pointer to uninitialized data:**
///
/// ```rust
/// use std::{ptr, mem::MaybeUninit};
///
/// struct Demo {
///     field: bool,
/// }
///
/// let mut uninit = MaybeUninit::<Demo>::uninit();
/// // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
/// // and thus be Undefined Behavior!
/// let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
/// unsafe { f1_ptr.write(true); }
/// let init = unsafe { uninit.assume_init() };
/// ```
#[stable(feature = "raw_ref_macros", since = "1.51.0")]
#[rustc_macro_transparency = "semitransparent"]
#[allow_internal_unstable(raw_ref_op)]
pub macro addr_of_mut($place:expr) {
    &raw mut $place
}