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
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
#![stable(feature = "rust1", since = "1.0.0")]

//! Thread-safe reference-counting pointers.
//!
//! See the [`Arc<T>`][Arc] documentation for more details.
//!
//! **Note**: This module is only available on platforms that support atomic
//! loads and stores of pointers. This may be detected at compile time using
//! `#[cfg(target_has_atomic = "ptr")]`.

use core::any::Any;
use core::borrow;
use core::cmp::Ordering;
use core::convert::{From, TryFrom};
use core::fmt;
use core::hash::{Hash, Hasher};
use core::hint;
use core::intrinsics::abort;
#[cfg(not(no_global_oom_handling))]
use core::iter;
use core::marker::{PhantomData, Unpin, Unsize};
#[cfg(not(no_global_oom_handling))]
use core::mem::size_of_val;
use core::mem::{self, align_of_val_raw};
use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
use core::panic::{RefUnwindSafe, UnwindSafe};
use core::pin::Pin;
use core::ptr::{self, NonNull};
#[cfg(not(no_global_oom_handling))]
use core::slice::from_raw_parts_mut;
use core::sync::atomic;
use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};

#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
#[cfg(not(no_global_oom_handling))]
use crate::alloc::{box_free, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::rc::is_dangling;
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;

#[cfg(test)]
mod tests;

/// A soft limit on the amount of references that may be made to an `Arc`.
///
/// Going above this limit will abort your program (although not
/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

#[cfg(not(sanitize = "thread"))]
macro_rules! acquire {
    ($x:expr) => {
        atomic::fence(Acquire)
    };
}

// ThreadSanitizer does not support memory fences. To avoid false positive
// reports in Arc / Weak implementation use atomic loads for synchronization
// instead.
#[cfg(sanitize = "thread")]
macro_rules! acquire {
    ($x:expr) => {
        $x.load(Acquire)
    };
}

/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
/// Reference Counted'.
///
/// The type `Arc<T>` provides shared ownership of a value of type `T`,
/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
/// a new `Arc` instance, which points to the same allocation on the heap as the
/// source `Arc`, while increasing a reference count. When the last `Arc`
/// pointer to a given allocation is destroyed, the value stored in that allocation (often
/// referred to as "inner value") is also dropped.
///
/// Shared references in Rust disallow mutation by default, and `Arc` is no
/// exception: you cannot generally obtain a mutable reference to something
/// inside an `Arc`. If you need to mutate through an `Arc`, use
/// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
/// types.
///
/// **Note**: This type is only available on platforms that support atomic
/// loads and stores of pointers, which includes all platforms that support
/// the `std` crate but not all those which only support [`alloc`](crate).
/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
///
/// ## Thread Safety
///
/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
/// counting. This means that it is thread-safe. The disadvantage is that
/// atomic operations are more expensive than ordinary memory accesses. If you
/// are not sharing reference-counted allocations between threads, consider using
/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
/// However, a library might choose `Arc<T>` in order to give library consumers
/// more flexibility.
///
/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
/// data, but it  doesn't add thread safety to its data. Consider
/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
/// non-atomic operations.
///
/// In the end, this means that you may need to pair `Arc<T>` with some sort of
/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
///
/// ## Breaking cycles with `Weak`
///
/// The [`downgrade`][downgrade] method can be used to create a non-owning
/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
/// already been dropped. In other words, `Weak` pointers do not keep the value
/// inside the allocation alive; however, they *do* keep the allocation
/// (the backing store for the value) alive.
///
/// A cycle between `Arc` pointers will never be deallocated. For this reason,
/// [`Weak`] is used to break cycles. For example, a tree could have
/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
/// pointers from children back to their parents.
///
/// # Cloning references
///
/// Creating a new reference from an existing reference-counted pointer is done using the
/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
///
/// ```
/// use std::sync::Arc;
/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
/// // The two syntaxes below are equivalent.
/// let a = foo.clone();
/// let b = Arc::clone(&foo);
/// // a, b, and foo are all Arcs that point to the same memory location
/// ```
///
/// ## `Deref` behavior
///
/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
/// functions, called using [fully qualified syntax]:
///
/// ```
/// use std::sync::Arc;
///
/// let my_arc = Arc::new(());
/// let my_weak = Arc::downgrade(&my_arc);
/// ```
///
/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
/// fully qualified syntax. Some people prefer to use fully qualified syntax,
/// while others prefer using method-call syntax.
///
/// ```
/// use std::sync::Arc;
///
/// let arc = Arc::new(());
/// // Method-call syntax
/// let arc2 = arc.clone();
/// // Fully qualified syntax
/// let arc3 = Arc::clone(&arc);
/// ```
///
/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
/// already been dropped.
///
/// [`Rc<T>`]: crate::rc::Rc
/// [clone]: Clone::clone
/// [mutex]: ../../std/sync/struct.Mutex.html
/// [rwlock]: ../../std/sync/struct.RwLock.html
/// [atomic]: core::sync::atomic
/// [`Send`]: core::marker::Send
/// [`Sync`]: core::marker::Sync
/// [deref]: core::ops::Deref
/// [downgrade]: Arc::downgrade
/// [upgrade]: Weak::upgrade
/// [RefCell\<T>]: core::cell::RefCell
/// [`RefCell<T>`]: core::cell::RefCell
/// [`std::sync`]: ../../std/sync/index.html
/// [`Arc::clone(&from)`]: Arc::clone
/// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
///
/// # Examples
///
/// Sharing some immutable data between threads:
///
// Note that we **do not** run these tests here. The windows builders get super
// unhappy if a thread outlives the main thread and then exits at the same time
// (something deadlocks) so we just avoid this entirely by not running these
// tests.
/// ```no_run
/// use std::sync::Arc;
/// use std::thread;
///
/// let five = Arc::new(5);
///
/// for _ in 0..10 {
///     let five = Arc::clone(&five);
///
///     thread::spawn(move || {
///         println!("{five:?}");
///     });
/// }
/// ```
///
/// Sharing a mutable [`AtomicUsize`]:
///
/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
///
/// ```no_run
/// use std::sync::Arc;
/// use std::sync::atomic::{AtomicUsize, Ordering};
/// use std::thread;
///
/// let val = Arc::new(AtomicUsize::new(5));
///
/// for _ in 0..10 {
///     let val = Arc::clone(&val);
///
///     thread::spawn(move || {
///         let v = val.fetch_add(1, Ordering::SeqCst);
///         println!("{v:?}");
///     });
/// }
/// ```
///
/// See the [`rc` documentation][rc_examples] for more examples of reference
/// counting in general.
///
/// [rc_examples]: crate::rc#examples
#[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Arc<T: ?Sized> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
}

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

#[stable(feature = "catch_unwind", since = "1.9.0")]
impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}

#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}

impl<T: ?Sized> Arc<T> {
    unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
        Self { ptr, phantom: PhantomData }
    }

    unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
        unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
    }
}

/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
/// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
///
/// Since a `Weak` reference does not count towards ownership, it will not
/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
/// guarantees about the value still being present. Thus it may return [`None`]
/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
/// itself (the backing store) from being deallocated.
///
/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
/// prevent circular references between [`Arc`] pointers, since mutual owning references
/// would never allow either [`Arc`] to be dropped. For example, a tree could
/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
/// pointers from children back to their parents.
///
/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
///
/// [`upgrade`]: Weak::upgrade
#[stable(feature = "arc_weak", since = "1.4.0")]
pub struct Weak<T: ?Sized> {
    // This is a `NonNull` to allow optimizing the size of this type in enums,
    // but it is not necessarily a valid pointer.
    // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
    // to allocate space on the heap. That's not a value a real pointer
    // will ever have because RcBox has alignment at least 2.
    // This is only possible when `T: Sized`; unsized `T` never dangle.
    ptr: NonNull<ArcInner<T>>,
}

#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}

#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized> fmt::Debug for Weak<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "(Weak)")
    }
}

// This is repr(C) to future-proof against possible field-reordering, which
// would interfere with otherwise safe [into|from]_raw() of transmutable
// inner types.
#[repr(C)]
struct ArcInner<T: ?Sized> {
    strong: atomic::AtomicUsize,

    // the value usize::MAX acts as a sentinel for temporarily "locking" the
    // ability to upgrade weak pointers or downgrade strong ones; this is used
    // to avoid races in `make_mut` and `get_mut`.
    weak: atomic::AtomicUsize,

    data: T,
}

/// Calculate layout for `ArcInner<T>` using the inner value's layout
fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
    // Calculate layout using the given value layout.
    // Previously, layout was calculated on the expression
    // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
    // reference (see #54908).
    Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
}

unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}

impl<T> Arc<T> {
    /// Constructs a new `Arc<T>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(data: T) -> Arc<T> {
        // Start the weak pointer count as 1 which is the weak pointer that's
        // held by all the strong pointers (kinda), see std/rc.rs for more info
        let x: Box<_> = Box::new(ArcInner {
            strong: atomic::AtomicUsize::new(1),
            weak: atomic::AtomicUsize::new(1),
            data,
        });
        unsafe { Self::from_inner(Box::leak(x).into()) }
    }

    /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
    /// to allow you to construct a `T` which holds a weak pointer to itself.
    ///
    /// Generally, a structure circularly referencing itself, either directly or
    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
    /// Using this function, you get access to the weak pointer during the
    /// initialization of `T`, before the `Arc<T>` is created, such that you can
    /// clone and store it inside the `T`.
    ///
    /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
    /// then calls your closure, giving it a `Weak<T>` to this allocation,
    /// and only afterwards completes the construction of the `Arc<T>` by placing
    /// the `T` returned from your closure into the allocation.
    ///
    /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
    /// returns, calling [`upgrade`] on the weak reference inside your closure will
    /// fail and result in a `None` value.
    ///
    /// # Panics
    ///
    /// If `data_fn` panics, the panic is propagated to the caller, and the
    /// temporary [`Weak<T>`] is dropped normally.
    ///
    /// # Example
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// use std::sync::{Arc, Weak};
    ///
    /// struct Gadget {
    ///     me: Weak<Gadget>,
    /// }
    ///
    /// impl Gadget {
    ///     /// Construct a reference counted Gadget.
    ///     fn new() -> Arc<Self> {
    ///         // `me` is a `Weak<Gadget>` pointing at the new allocation of the
    ///         // `Arc` we're constructing.
    ///         Arc::new_cyclic(|me| {
    ///             // Create the actual struct here.
    ///             Gadget { me: me.clone() }
    ///         })
    ///     }
    ///
    ///     /// Return a reference counted pointer to Self.
    ///     fn me(&self) -> Arc<Self> {
    ///         self.me.upgrade().unwrap()
    ///     }
    /// }
    /// ```
    /// [`upgrade`]: Weak::upgrade
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
    pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
    where
        F: FnOnce(&Weak<T>) -> T,
    {
        // Construct the inner in the "uninitialized" state with a single
        // weak reference.
        let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner {
            strong: atomic::AtomicUsize::new(0),
            weak: atomic::AtomicUsize::new(1),
            data: mem::MaybeUninit::<T>::uninit(),
        }))
        .into();
        let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();

        let weak = Weak { ptr: init_ptr };

        // It's important we don't give up ownership of the weak pointer, or
        // else the memory might be freed by the time `data_fn` returns. If
        // we really wanted to pass ownership, we could create an additional
        // weak pointer for ourselves, but this would result in additional
        // updates to the weak reference count which might not be necessary
        // otherwise.
        let data = data_fn(&weak);

        // Now we can properly initialize the inner value and turn our weak
        // reference into a strong reference.
        let strong = unsafe {
            let inner = init_ptr.as_ptr();
            ptr::write(ptr::addr_of_mut!((*inner).data), data);

            // The above write to the data field must be visible to any threads which
            // observe a non-zero strong count. Therefore we need at least "Release" ordering
            // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
            //
            // "Acquire" ordering is not required. When considering the possible behaviours
            // of `data_fn` we only need to look at what it could do with a reference to a
            // non-upgradeable `Weak`:
            // - It can *clone* the `Weak`, increasing the weak reference count.
            // - It can drop those clones, decreasing the weak reference count (but never to zero).
            //
            // These side effects do not impact us in any way, and no other side effects are
            // possible with safe code alone.
            let prev_value = (*inner).strong.fetch_add(1, Release);
            debug_assert_eq!(prev_value, 0, "No prior strong references should exist");

            Arc::from_inner(init_ptr)
        };

        // Strong references should collectively own a shared weak reference,
        // so don't run the destructor for our old weak reference.
        mem::forget(weak);
        strong
    }

    /// Constructs a new `Arc` with uninitialized contents.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut five = Arc::<u32>::new_uninit();
    ///
    /// // Deferred initialization:
    /// Arc::get_mut(&mut five).unwrap().write(5);
    ///
    /// let five = unsafe { five.assume_init() };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
        unsafe {
            Arc::from_ptr(Arc::allocate_for_layout(
                Layout::new::<T>(),
                |layout| Global.allocate(layout),
                |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
            ))
        }
    }

    /// Constructs a new `Arc` with uninitialized contents, with the memory
    /// being filled with `0` bytes.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// use std::sync::Arc;
    ///
    /// let zero = Arc::<u32>::new_zeroed();
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0)
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
        unsafe {
            Arc::from_ptr(Arc::allocate_for_layout(
                Layout::new::<T>(),
                |layout| Global.allocate_zeroed(layout),
                |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
            ))
        }
    }

    /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
    /// `data` will be pinned in memory and unable to be moved.
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "pin", since = "1.33.0")]
    #[must_use]
    pub fn pin(data: T) -> Pin<Arc<T>> {
        unsafe { Pin::new_unchecked(Arc::new(data)) }
    }

    /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
        unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
    }

    /// Constructs a new `Arc<T>`, returning an error if allocation fails.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    /// use std::sync::Arc;
    ///
    /// let five = Arc::try_new(5)?;
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
        // Start the weak pointer count as 1 which is the weak pointer that's
        // held by all the strong pointers (kinda), see std/rc.rs for more info
        let x: Box<_> = Box::try_new(ArcInner {
            strong: atomic::AtomicUsize::new(1),
            weak: atomic::AtomicUsize::new(1),
            data,
        })?;
        unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
    }

    /// Constructs a new `Arc` with uninitialized contents, returning an error
    /// if allocation fails.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit, allocator_api)]
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut five = Arc::<u32>::try_new_uninit()?;
    ///
    /// // Deferred initialization:
    /// Arc::get_mut(&mut five).unwrap().write(5);
    ///
    /// let five = unsafe { five.assume_init() };
    ///
    /// assert_eq!(*five, 5);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
        unsafe {
            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
                Layout::new::<T>(),
                |layout| Global.allocate(layout),
                |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
            )?))
        }
    }

    /// Constructs a new `Arc` with uninitialized contents, with the memory
    /// being filled with `0` bytes, returning an error if allocation fails.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit, allocator_api)]
    ///
    /// use std::sync::Arc;
    ///
    /// let zero = Arc::<u32>::try_new_zeroed()?;
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
        unsafe {
            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
                Layout::new::<T>(),
                |layout| Global.allocate_zeroed(layout),
                |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
            )?))
        }
    }
    /// Returns the inner value, if the `Arc` has exactly one strong reference.
    ///
    /// Otherwise, an [`Err`] is returned with the same `Arc` that was
    /// passed in.
    ///
    /// This will succeed even if there are outstanding weak references.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let x = Arc::new(3);
    /// assert_eq!(Arc::try_unwrap(x), Ok(3));
    ///
    /// let x = Arc::new(4);
    /// let _y = Arc::clone(&x);
    /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
    /// ```
    #[inline]
    #[stable(feature = "arc_unique", since = "1.4.0")]
    pub fn try_unwrap(this: Self) -> Result<T, Self> {
        if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
            return Err(this);
        }

        acquire!(this.inner().strong);

        unsafe {
            let elem = ptr::read(&this.ptr.as_ref().data);

            // Make a weak pointer to clean up the implicit strong-weak reference
            let _weak = Weak { ptr: this.ptr };
            mem::forget(this);

            Ok(elem)
        }
    }
}

impl<T> Arc<[T]> {
    /// Constructs a new atomically reference-counted slice with uninitialized contents.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
    ///
    /// // Deferred initialization:
    /// let data = Arc::get_mut(&mut values).unwrap();
    /// data[0].write(1);
    /// data[1].write(2);
    /// data[2].write(3);
    ///
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
        unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
    }

    /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
    /// filled with `0` bytes.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
    /// incorrect usage of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// use std::sync::Arc;
    ///
    /// let values = Arc::<[u32]>::new_zeroed_slice(3);
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0])
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
        unsafe {
            Arc::from_ptr(Arc::allocate_for_layout(
                Layout::array::<T>(len).unwrap(),
                |layout| Global.allocate_zeroed(layout),
                |mem| {
                    ptr::slice_from_raw_parts_mut(mem as *mut T, len)
                        as *mut ArcInner<[mem::MaybeUninit<T>]>
                },
            ))
        }
    }
}

impl<T> Arc<mem::MaybeUninit<T>> {
    /// Converts to `Arc<T>`.
    ///
    /// # Safety
    ///
    /// As with [`MaybeUninit::assume_init`],
    /// it is up to the caller to guarantee that the inner value
    /// really is in an initialized state.
    /// Calling this when the content is not yet fully initialized
    /// causes immediate undefined behavior.
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut five = Arc::<u32>::new_uninit();
    ///
    /// // Deferred initialization:
    /// Arc::get_mut(&mut five).unwrap().write(5);
    ///
    /// let five = unsafe { five.assume_init() };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use = "`self` will be dropped if the result is not used"]
    #[inline]
    pub unsafe fn assume_init(self) -> Arc<T> {
        unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
    }
}

impl<T> Arc<[mem::MaybeUninit<T>]> {
    /// Converts to `Arc<[T]>`.
    ///
    /// # Safety
    ///
    /// As with [`MaybeUninit::assume_init`],
    /// it is up to the caller to guarantee that the inner value
    /// really is in an initialized state.
    /// Calling this when the content is not yet fully initialized
    /// causes immediate undefined behavior.
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
    ///
    /// // Deferred initialization:
    /// let data = Arc::get_mut(&mut values).unwrap();
    /// data[0].write(1);
    /// data[1].write(2);
    /// data[2].write(3);
    ///
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use = "`self` will be dropped if the result is not used"]
    #[inline]
    pub unsafe fn assume_init(self) -> Arc<[T]> {
        unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
    }
}

impl<T: ?Sized> Arc<T> {
    /// Consumes the `Arc`, returning the wrapped pointer.
    ///
    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
    /// [`Arc::from_raw`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let x = Arc::new("hello".to_owned());
    /// let x_ptr = Arc::into_raw(x);
    /// assert_eq!(unsafe { &*x_ptr }, "hello");
    /// ```
    #[must_use = "losing the pointer will leak memory"]
    #[stable(feature = "rc_raw", since = "1.17.0")]
    pub fn into_raw(this: Self) -> *const T {
        let ptr = Self::as_ptr(&this);
        mem::forget(this);
        ptr
    }

    /// Provides a raw pointer to the data.
    ///
    /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
    /// as long as there are strong counts in the `Arc`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let x = Arc::new("hello".to_owned());
    /// let y = Arc::clone(&x);
    /// let x_ptr = Arc::as_ptr(&x);
    /// assert_eq!(x_ptr, Arc::as_ptr(&y));
    /// assert_eq!(unsafe { &*x_ptr }, "hello");
    /// ```
    #[must_use]
    #[stable(feature = "rc_as_ptr", since = "1.45.0")]
    pub fn as_ptr(this: &Self) -> *const T {
        let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);

        // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
        // this is required to retain raw/mut provenance such that e.g. `get_mut` can
        // write through the pointer after the Rc is recovered through `from_raw`.
        unsafe { ptr::addr_of_mut!((*ptr).data) }
    }

    /// Constructs an `Arc<T>` from a raw pointer.
    ///
    /// The raw pointer must have been previously returned by a call to
    /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
    /// alignment as `T`. This is trivially true if `U` is `T`.
    /// Note that if `U` is not `T` but has the same size and alignment, this is
    /// basically like transmuting references of different types. See
    /// [`mem::transmute`][transmute] for more information on what
    /// restrictions apply in this case.
    ///
    /// The user of `from_raw` has to make sure a specific value of `T` is only
    /// dropped once.
    ///
    /// This function is unsafe because improper use may lead to memory unsafety,
    /// even if the returned `Arc<T>` is never accessed.
    ///
    /// [into_raw]: Arc::into_raw
    /// [transmute]: core::mem::transmute
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let x = Arc::new("hello".to_owned());
    /// let x_ptr = Arc::into_raw(x);
    ///
    /// unsafe {
    ///     // Convert back to an `Arc` to prevent leak.
    ///     let x = Arc::from_raw(x_ptr);
    ///     assert_eq!(&*x, "hello");
    ///
    ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
    /// }
    ///
    /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
    /// ```
    #[stable(feature = "rc_raw", since = "1.17.0")]
    pub unsafe fn from_raw(ptr: *const T) -> Self {
        unsafe {
            let offset = data_offset(ptr);

            // Reverse the offset to find the original ArcInner.
            let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;

            Self::from_ptr(arc_ptr)
        }
    }

    /// Creates a new [`Weak`] pointer to this allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// let weak_five = Arc::downgrade(&five);
    /// ```
    #[must_use = "this returns a new `Weak` pointer, \
                  without modifying the original `Arc`"]
    #[stable(feature = "arc_weak", since = "1.4.0")]
    pub fn downgrade(this: &Self) -> Weak<T> {
        // This Relaxed is OK because we're checking the value in the CAS
        // below.
        let mut cur = this.inner().weak.load(Relaxed);

        loop {
            // check if the weak counter is currently "locked"; if so, spin.
            if cur == usize::MAX {
                hint::spin_loop();
                cur = this.inner().weak.load(Relaxed);
                continue;
            }

            // NOTE: this code currently ignores the possibility of overflow
            // into usize::MAX; in general both Rc and Arc need to be adjusted
            // to deal with overflow.

            // Unlike with Clone(), we need this to be an Acquire read to
            // synchronize with the write coming from `is_unique`, so that the
            // events prior to that write happen before this read.
            match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
                Ok(_) => {
                    // Make sure we do not create a dangling Weak
                    debug_assert!(!is_dangling(this.ptr.as_ptr()));
                    return Weak { ptr: this.ptr };
                }
                Err(old) => cur = old,
            }
        }
    }

    /// Gets the number of [`Weak`] pointers to this allocation.
    ///
    /// # Safety
    ///
    /// This method by itself is safe, but using it correctly requires extra care.
    /// Another thread can change the weak count at any time,
    /// including potentially between calling this method and acting on the result.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    /// let _weak_five = Arc::downgrade(&five);
    ///
    /// // This assertion is deterministic because we haven't shared
    /// // the `Arc` or `Weak` between threads.
    /// assert_eq!(1, Arc::weak_count(&five));
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "arc_counts", since = "1.15.0")]
    pub fn weak_count(this: &Self) -> usize {
        let cnt = this.inner().weak.load(Acquire);
        // If the weak count is currently locked, the value of the
        // count was 0 just before taking the lock.
        if cnt == usize::MAX { 0 } else { cnt - 1 }
    }

    /// Gets the number of strong (`Arc`) pointers to this allocation.
    ///
    /// # Safety
    ///
    /// This method by itself is safe, but using it correctly requires extra care.
    /// Another thread can change the strong count at any time,
    /// including potentially between calling this method and acting on the result.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    /// let _also_five = Arc::clone(&five);
    ///
    /// // This assertion is deterministic because we haven't shared
    /// // the `Arc` between threads.
    /// assert_eq!(2, Arc::strong_count(&five));
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "arc_counts", since = "1.15.0")]
    pub fn strong_count(this: &Self) -> usize {
        this.inner().strong.load(Acquire)
    }

    /// Increments the strong reference count on the `Arc<T>` associated with the
    /// provided pointer by one.
    ///
    /// # Safety
    ///
    /// The pointer must have been obtained through `Arc::into_raw`, and the
    /// associated `Arc` instance must be valid (i.e. the strong count must be at
    /// least 1) for the duration of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// unsafe {
    ///     let ptr = Arc::into_raw(five);
    ///     Arc::increment_strong_count(ptr);
    ///
    ///     // This assertion is deterministic because we haven't shared
    ///     // the `Arc` between threads.
    ///     let five = Arc::from_raw(ptr);
    ///     assert_eq!(2, Arc::strong_count(&five));
    /// }
    /// ```
    #[inline]
    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
    pub unsafe fn increment_strong_count(ptr: *const T) {
        // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
        let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
        // Now increase refcount, but don't drop new refcount either
        let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
    }

    /// Decrements the strong reference count on the `Arc<T>` associated with the
    /// provided pointer by one.
    ///
    /// # Safety
    ///
    /// The pointer must have been obtained through `Arc::into_raw`, and the
    /// associated `Arc` instance must be valid (i.e. the strong count must be at
    /// least 1) when invoking this method. This method can be used to release the final
    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
    /// released.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// unsafe {
    ///     let ptr = Arc::into_raw(five);
    ///     Arc::increment_strong_count(ptr);
    ///
    ///     // Those assertions are deterministic because we haven't shared
    ///     // the `Arc` between threads.
    ///     let five = Arc::from_raw(ptr);
    ///     assert_eq!(2, Arc::strong_count(&five));
    ///     Arc::decrement_strong_count(ptr);
    ///     assert_eq!(1, Arc::strong_count(&five));
    /// }
    /// ```
    #[inline]
    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
    pub unsafe fn decrement_strong_count(ptr: *const T) {
        unsafe { mem::drop(Arc::from_raw(ptr)) };
    }

    #[inline]
    fn inner(&self) -> &ArcInner<T> {
        // This unsafety is ok because while this arc is alive we're guaranteed
        // that the inner pointer is valid. Furthermore, we know that the
        // `ArcInner` structure itself is `Sync` because the inner data is
        // `Sync` as well, so we're ok loaning out an immutable pointer to these
        // contents.
        unsafe { self.ptr.as_ref() }
    }

    // Non-inlined part of `drop`.
    #[inline(never)]
    unsafe fn drop_slow(&mut self) {
        // Destroy the data at this time, even though we must not free the box
        // allocation itself (there might still be weak pointers lying around).
        unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };

        // Drop the weak ref collectively held by all strong references
        drop(Weak { ptr: self.ptr });
    }

    /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
    /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    /// let same_five = Arc::clone(&five);
    /// let other_five = Arc::new(5);
    ///
    /// assert!(Arc::ptr_eq(&five, &same_five));
    /// assert!(!Arc::ptr_eq(&five, &other_five));
    /// ```
    ///
    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
    #[inline]
    #[must_use]
    #[stable(feature = "ptr_eq", since = "1.17.0")]
    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
        this.ptr.as_ptr() == other.ptr.as_ptr()
    }
}

impl<T: ?Sized> Arc<T> {
    /// Allocates an `ArcInner<T>` with sufficient space for
    /// a possibly-unsized inner value where the value has the layout provided.
    ///
    /// The function `mem_to_arcinner` is called with the data pointer
    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
    #[cfg(not(no_global_oom_handling))]
    unsafe fn allocate_for_layout(
        value_layout: Layout,
        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
    ) -> *mut ArcInner<T> {
        let layout = arcinner_layout_for_value_layout(value_layout);
        unsafe {
            Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
                .unwrap_or_else(|_| handle_alloc_error(layout))
        }
    }

    /// Allocates an `ArcInner<T>` with sufficient space for
    /// a possibly-unsized inner value where the value has the layout provided,
    /// returning an error if allocation fails.
    ///
    /// The function `mem_to_arcinner` is called with the data pointer
    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
    unsafe fn try_allocate_for_layout(
        value_layout: Layout,
        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
    ) -> Result<*mut ArcInner<T>, AllocError> {
        let layout = arcinner_layout_for_value_layout(value_layout);

        let ptr = allocate(layout)?;

        // Initialize the ArcInner
        let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
        debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);

        unsafe {
            ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
            ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
        }

        Ok(inner)
    }

    /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
    #[cfg(not(no_global_oom_handling))]
    unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
        // Allocate for the `ArcInner<T>` using the given value.
        unsafe {
            Self::allocate_for_layout(
                Layout::for_value(&*ptr),
                |layout| Global.allocate(layout),
                |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
            )
        }
    }

    #[cfg(not(no_global_oom_handling))]
    fn from_box(v: Box<T>) -> Arc<T> {
        unsafe {
            let (box_unique, alloc) = Box::into_unique(v);
            let bptr = box_unique.as_ptr();

            let value_size = size_of_val(&*bptr);
            let ptr = Self::allocate_for_ptr(bptr);

            // Copy value as bytes
            ptr::copy_nonoverlapping(
                bptr as *const T as *const u8,
                &mut (*ptr).data as *mut _ as *mut u8,
                value_size,
            );

            // Free the allocation without dropping its contents
            box_free(box_unique, alloc);

            Self::from_ptr(ptr)
        }
    }
}

impl<T> Arc<[T]> {
    /// Allocates an `ArcInner<[T]>` with the given length.
    #[cfg(not(no_global_oom_handling))]
    unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
        unsafe {
            Self::allocate_for_layout(
                Layout::array::<T>(len).unwrap(),
                |layout| Global.allocate(layout),
                |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
            )
        }
    }

    /// Copy elements from slice into newly allocated `Arc<[T]>`
    ///
    /// Unsafe because the caller must either take ownership or bind `T: Copy`.
    #[cfg(not(no_global_oom_handling))]
    unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
        unsafe {
            let ptr = Self::allocate_for_slice(v.len());

            ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());

            Self::from_ptr(ptr)
        }
    }

    /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
    ///
    /// Behavior is undefined should the size be wrong.
    #[cfg(not(no_global_oom_handling))]
    unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
        // Panic guard while cloning T elements.
        // In the event of a panic, elements that have been written
        // into the new ArcInner will be dropped, then the memory freed.
        struct Guard<T> {
            mem: NonNull<u8>,
            elems: *mut T,
            layout: Layout,
            n_elems: usize,
        }

        impl<T> Drop for Guard<T> {
            fn drop(&mut self) {
                unsafe {
                    let slice = from_raw_parts_mut(self.elems, self.n_elems);
                    ptr::drop_in_place(slice);

                    Global.deallocate(self.mem, self.layout);
                }
            }
        }

        unsafe {
            let ptr = Self::allocate_for_slice(len);

            let mem = ptr as *mut _ as *mut u8;
            let layout = Layout::for_value(&*ptr);

            // Pointer to first element
            let elems = &mut (*ptr).data as *mut [T] as *mut T;

            let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };

            for (i, item) in iter.enumerate() {
                ptr::write(elems.add(i), item);
                guard.n_elems += 1;
            }

            // All clear. Forget the guard so it doesn't free the new ArcInner.
            mem::forget(guard);

            Self::from_ptr(ptr)
        }
    }
}

/// Specialization trait used for `From<&[T]>`.
#[cfg(not(no_global_oom_handling))]
trait ArcFromSlice<T> {
    fn from_slice(slice: &[T]) -> Self;
}

#[cfg(not(no_global_oom_handling))]
impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
    #[inline]
    default fn from_slice(v: &[T]) -> Self {
        unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
    }
}

#[cfg(not(no_global_oom_handling))]
impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
    #[inline]
    fn from_slice(v: &[T]) -> Self {
        unsafe { Arc::copy_from_slice(v) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for Arc<T> {
    /// Makes a clone of the `Arc` pointer.
    ///
    /// This creates another pointer to the same allocation, increasing the
    /// strong reference count.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// let _ = Arc::clone(&five);
    /// ```
    #[inline]
    fn clone(&self) -> Arc<T> {
        // Using a relaxed ordering is alright here, as knowledge of the
        // original reference prevents other threads from erroneously deleting
        // the object.
        //
        // As explained in the [Boost documentation][1], Increasing the
        // reference counter can always be done with memory_order_relaxed: New
        // references to an object can only be formed from an existing
        // reference, and passing an existing reference from one thread to
        // another must already provide any required synchronization.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        let old_size = self.inner().strong.fetch_add(1, Relaxed);

        // However we need to guard against massive refcounts in case someone is `mem::forget`ing
        // Arcs. If we don't do this the count can overflow and users will use-after free. This
        // branch will never be taken in any realistic program. We abort because such a program is
        // incredibly degenerate, and we don't care to support it.
        //
        // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
        // But we do that check *after* having done the increment, so there is a chance here that
        // the worst already happened and we actually do overflow the `usize` counter. However, that
        // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
        // above and the `abort` below, which seems exceedingly unlikely.
        if old_size > MAX_REFCOUNT {
            abort();
        }

        unsafe { Self::from_inner(self.ptr) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Arc<T> {
    type Target = T;

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

#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized> Receiver for Arc<T> {}

impl<T: Clone> Arc<T> {
    /// Makes a mutable reference into the given `Arc`.
    ///
    /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
    /// [`clone`] the inner value to a new allocation to ensure unique ownership.  This is also
    /// referred to as clone-on-write.
    ///
    /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
    /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
    /// be cloned.
    ///
    /// See also [`get_mut`], which will fail rather than cloning the inner value
    /// or dissociating [`Weak`] pointers.
    ///
    /// [`clone`]: Clone::clone
    /// [`get_mut`]: Arc::get_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let mut data = Arc::new(5);
    ///
    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
    /// let mut other_data = Arc::clone(&data); // Won't clone inner data
    /// *Arc::make_mut(&mut data) += 1;         // Clones inner data
    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
    /// *Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything
    ///
    /// // Now `data` and `other_data` point to different allocations.
    /// assert_eq!(*data, 8);
    /// assert_eq!(*other_data, 12);
    /// ```
    ///
    /// [`Weak`] pointers will be dissociated:
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let mut data = Arc::new(75);
    /// let weak = Arc::downgrade(&data);
    ///
    /// assert!(75 == *data);
    /// assert!(75 == *weak.upgrade().unwrap());
    ///
    /// *Arc::make_mut(&mut data) += 1;
    ///
    /// assert!(76 == *data);
    /// assert!(weak.upgrade().is_none());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "arc_unique", since = "1.4.0")]
    pub fn make_mut(this: &mut Self) -> &mut T {
        // Note that we hold both a strong reference and a weak reference.
        // Thus, releasing our strong reference only will not, by itself, cause
        // the memory to be deallocated.
        //
        // Use Acquire to ensure that we see any writes to `weak` that happen
        // before release writes (i.e., decrements) to `strong`. Since we hold a
        // weak count, there's no chance the ArcInner itself could be
        // deallocated.
        if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
            // Another strong pointer exists, so we must clone.
            // Pre-allocate memory to allow writing the cloned value directly.
            let mut arc = Self::new_uninit();
            unsafe {
                let data = Arc::get_mut_unchecked(&mut arc);
                (**this).write_clone_into_raw(data.as_mut_ptr());
                *this = arc.assume_init();
            }
        } else if this.inner().weak.load(Relaxed) != 1 {
            // Relaxed suffices in the above because this is fundamentally an
            // optimization: we are always racing with weak pointers being
            // dropped. Worst case, we end up allocated a new Arc unnecessarily.

            // We removed the last strong ref, but there are additional weak
            // refs remaining. We'll move the contents to a new Arc, and
            // invalidate the other weak refs.

            // Note that it is not possible for the read of `weak` to yield
            // usize::MAX (i.e., locked), since the weak count can only be
            // locked by a thread with a strong reference.

            // Materialize our own implicit weak pointer, so that it can clean
            // up the ArcInner as needed.
            let _weak = Weak { ptr: this.ptr };

            // Can just steal the data, all that's left is Weaks
            let mut arc = Self::new_uninit();
            unsafe {
                let data = Arc::get_mut_unchecked(&mut arc);
                data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
                ptr::write(this, arc.assume_init());
            }
        } else {
            // We were the sole reference of either kind; bump back up the
            // strong ref count.
            this.inner().strong.store(1, Release);
        }

        // As with `get_mut()`, the unsafety is ok because our reference was
        // either unique to begin with, or became one upon cloning the contents.
        unsafe { Self::get_mut_unchecked(this) }
    }

    /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
    /// clone.
    ///
    /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
    /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(arc_unwrap_or_clone)]
    /// # use std::{ptr, sync::Arc};
    /// let inner = String::from("test");
    /// let ptr = inner.as_ptr();
    ///
    /// let arc = Arc::new(inner);
    /// let inner = Arc::unwrap_or_clone(arc);
    /// // The inner value was not cloned
    /// assert!(ptr::eq(ptr, inner.as_ptr()));
    ///
    /// let arc = Arc::new(inner);
    /// let arc2 = arc.clone();
    /// let inner = Arc::unwrap_or_clone(arc);
    /// // Because there were 2 references, we had to clone the inner value.
    /// assert!(!ptr::eq(ptr, inner.as_ptr()));
    /// // `arc2` is the last reference, so when we unwrap it we get back
    /// // the original `String`.
    /// let inner = Arc::unwrap_or_clone(arc2);
    /// assert!(ptr::eq(ptr, inner.as_ptr()));
    /// ```
    #[inline]
    #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
    pub fn unwrap_or_clone(this: Self) -> T {
        Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
    }
}

impl<T: ?Sized> Arc<T> {
    /// Returns a mutable reference into the given `Arc`, if there are
    /// no other `Arc` or [`Weak`] pointers to the same allocation.
    ///
    /// Returns [`None`] otherwise, because it is not safe to
    /// mutate a shared value.
    ///
    /// See also [`make_mut`][make_mut], which will [`clone`][clone]
    /// the inner value when there are other `Arc` pointers.
    ///
    /// [make_mut]: Arc::make_mut
    /// [clone]: Clone::clone
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let mut x = Arc::new(3);
    /// *Arc::get_mut(&mut x).unwrap() = 4;
    /// assert_eq!(*x, 4);
    ///
    /// let _y = Arc::clone(&x);
    /// assert!(Arc::get_mut(&mut x).is_none());
    /// ```
    #[inline]
    #[stable(feature = "arc_unique", since = "1.4.0")]
    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
        if this.is_unique() {
            // This unsafety is ok because we're guaranteed that the pointer
            // returned is the *only* pointer that will ever be returned to T. Our
            // reference count is guaranteed to be 1 at this point, and we required
            // the Arc itself to be `mut`, so we're returning the only possible
            // reference to the inner data.
            unsafe { Some(Arc::get_mut_unchecked(this)) }
        } else {
            None
        }
    }

    /// Returns a mutable reference into the given `Arc`,
    /// without any check.
    ///
    /// See also [`get_mut`], which is safe and does appropriate checks.
    ///
    /// [`get_mut`]: Arc::get_mut
    ///
    /// # Safety
    ///
    /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
    /// they must be must not be dereferenced or have active borrows for the duration
    /// of the returned borrow, and their inner type must be exactly the same as the
    /// inner type of this Rc (including lifetimes). This is trivially the case if no
    /// such pointers exist, for example immediately after `Arc::new`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let mut x = Arc::new(String::new());
    /// unsafe {
    ///     Arc::get_mut_unchecked(&mut x).push_str("foo")
    /// }
    /// assert_eq!(*x, "foo");
    /// ```
    /// Other `Arc` pointers to the same allocation must be to the same type.
    /// ```no_run
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let x: Arc<str> = Arc::from("Hello, world!");
    /// let mut y: Arc<[u8]> = x.clone().into();
    /// unsafe {
    ///     // this is Undefined Behavior, because x's inner type is str, not [u8]
    ///     Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
    /// }
    /// println!("{}", &*x); // Invalid UTF-8 in a str
    /// ```
    /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
    /// ```no_run
    /// #![feature(get_mut_unchecked)]
    ///
    /// use std::sync::Arc;
    ///
    /// let x: Arc<&str> = Arc::new("Hello, world!");
    /// {
    ///     let s = String::from("Oh, no!");
    ///     let mut y: Arc<&str> = x.clone().into();
    ///     unsafe {
    ///         // this is Undefined Behavior, because x's inner type
    ///         // is &'long str, not &'short str
    ///         *Arc::get_mut_unchecked(&mut y) = &s;
    ///     }
    /// }
    /// println!("{}", &*x); // Use-after-free
    /// ```
    #[inline]
    #[unstable(feature = "get_mut_unchecked", issue = "63292")]
    pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
        // We are careful to *not* create a reference covering the "count" fields, as
        // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
        unsafe { &mut (*this.ptr.as_ptr()).data }
    }

    /// Determine whether this is the unique reference (including weak refs) to
    /// the underlying data.
    ///
    /// Note that this requires locking the weak ref count.
    fn is_unique(&mut self) -> bool {
        // lock the weak pointer count if we appear to be the sole weak pointer
        // holder.
        //
        // The acquire label here ensures a happens-before relationship with any
        // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
        // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
        // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
        if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
            // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
            // counter in `drop` -- the only access that happens when any but the last reference
            // is being dropped.
            let unique = self.inner().strong.load(Acquire) == 1;

            // The release write here synchronizes with a read in `downgrade`,
            // effectively preventing the above read of `strong` from happening
            // after the write.
            self.inner().weak.store(1, Release); // release the lock
            unique
        } else {
            false
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
    /// Drops the `Arc`.
    ///
    /// This will decrement the strong reference count. If the strong reference
    /// count reaches zero then the only other references (if any) are
    /// [`Weak`], so we `drop` the inner value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// struct Foo;
    ///
    /// impl Drop for Foo {
    ///     fn drop(&mut self) {
    ///         println!("dropped!");
    ///     }
    /// }
    ///
    /// let foo  = Arc::new(Foo);
    /// let foo2 = Arc::clone(&foo);
    ///
    /// drop(foo);    // Doesn't print anything
    /// drop(foo2);   // Prints "dropped!"
    /// ```
    #[inline]
    fn drop(&mut self) {
        // Because `fetch_sub` is already atomic, we do not need to synchronize
        // with other threads unless we are going to delete the object. This
        // same logic applies to the below `fetch_sub` to the `weak` count.
        if self.inner().strong.fetch_sub(1, Release) != 1 {
            return;
        }

        // This fence is needed to prevent reordering of use of the data and
        // deletion of the data. Because it is marked `Release`, the decreasing
        // of the reference count synchronizes with this `Acquire` fence. This
        // means that use of the data happens before decreasing the reference
        // count, which happens before this fence, which happens before the
        // deletion of the data.
        //
        // As explained in the [Boost documentation][1],
        //
        // > It is important to enforce any possible access to the object in one
        // > thread (through an existing reference) to *happen before* deleting
        // > the object in a different thread. This is achieved by a "release"
        // > operation after dropping a reference (any access to the object
        // > through this reference must obviously happened before), and an
        // > "acquire" operation before deleting the object.
        //
        // In particular, while the contents of an Arc are usually immutable, it's
        // possible to have interior writes to something like a Mutex<T>. Since a
        // Mutex is not acquired when it is deleted, we can't rely on its
        // synchronization logic to make writes in thread A visible to a destructor
        // running in thread B.
        //
        // Also note that the Acquire fence here could probably be replaced with an
        // Acquire load, which could improve performance in highly-contended
        // situations. See [2].
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        // [2]: (https://github.com/rust-lang/rust/pull/41714)
        acquire!(self.inner().strong);

        unsafe {
            self.drop_slow();
        }
    }
}

impl Arc<dyn Any + Send + Sync> {
    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    /// use std::sync::Arc;
    ///
    /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Arc::new(my_string));
    /// print_if_string(Arc::new(0i8));
    /// ```
    #[inline]
    #[stable(feature = "rc_downcast", since = "1.29.0")]
    pub fn downcast<T>(self) -> Result<Arc<T>, Self>
    where
        T: Any + Send + Sync,
    {
        if (*self).is::<T>() {
            unsafe {
                let ptr = self.ptr.cast::<ArcInner<T>>();
                mem::forget(self);
                Ok(Arc::from_inner(ptr))
            }
        } else {
            Err(self)
        }
    }

    /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
    ///
    /// For a safe alternative see [`downcast`].
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(downcast_unchecked)]
    ///
    /// use std::any::Any;
    /// use std::sync::Arc;
    ///
    /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
    ///
    /// unsafe {
    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The contained value must be of type `T`. Calling this method
    /// with the incorrect type is *undefined behavior*.
    ///
    ///
    /// [`downcast`]: Self::downcast
    #[inline]
    #[unstable(feature = "downcast_unchecked", issue = "90850")]
    pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
    where
        T: Any + Send + Sync,
    {
        unsafe {
            let ptr = self.ptr.cast::<ArcInner<T>>();
            mem::forget(self);
            Arc::from_inner(ptr)
        }
    }
}

impl<T> Weak<T> {
    /// Constructs a new `Weak<T>`, without allocating any memory.
    /// Calling [`upgrade`] on the return value always gives [`None`].
    ///
    /// [`upgrade`]: Weak::upgrade
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Weak;
    ///
    /// let empty: Weak<i64> = Weak::new();
    /// assert!(empty.upgrade().is_none());
    /// ```
    #[stable(feature = "downgraded_weak", since = "1.10.0")]
    #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
    #[must_use]
    pub const fn new() -> Weak<T> {
        Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) } }
    }
}

/// Helper type to allow accessing the reference counts without
/// making any assertions about the data field.
struct WeakInner<'a> {
    weak: &'a atomic::AtomicUsize,
    strong: &'a atomic::AtomicUsize,
}

impl<T: ?Sized> Weak<T> {
    /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
    ///
    /// The pointer is valid only if there are some strong references. The pointer may be dangling,
    /// unaligned or even [`null`] otherwise.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use std::ptr;
    ///
    /// let strong = Arc::new("hello".to_owned());
    /// let weak = Arc::downgrade(&strong);
    /// // Both point to the same object
    /// assert!(ptr::eq(&*strong, weak.as_ptr()));
    /// // The strong here keeps it alive, so we can still access the object.
    /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
    ///
    /// drop(strong);
    /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
    /// // undefined behaviour.
    /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
    /// ```
    ///
    /// [`null`]: core::ptr::null "ptr::null"
    #[must_use]
    #[stable(feature = "weak_into_raw", since = "1.45.0")]
    pub fn as_ptr(&self) -> *const T {
        let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);

        if is_dangling(ptr) {
            // If the pointer is dangling, we return the sentinel directly. This cannot be
            // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
            ptr as *const T
        } else {
            // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
            // The payload may be dropped at this point, and we have to maintain provenance,
            // so use raw pointer manipulation.
            unsafe { ptr::addr_of_mut!((*ptr).data) }
        }
    }

    /// Consumes the `Weak<T>` and turns it into a raw pointer.
    ///
    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
    /// one weak reference (the weak count is not modified by this operation). It can be turned
    /// back into the `Weak<T>` with [`from_raw`].
    ///
    /// The same restrictions of accessing the target of the pointer as with
    /// [`as_ptr`] apply.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Weak};
    ///
    /// let strong = Arc::new("hello".to_owned());
    /// let weak = Arc::downgrade(&strong);
    /// let raw = weak.into_raw();
    ///
    /// assert_eq!(1, Arc::weak_count(&strong));
    /// assert_eq!("hello", unsafe { &*raw });
    ///
    /// drop(unsafe { Weak::from_raw(raw) });
    /// assert_eq!(0, Arc::weak_count(&strong));
    /// ```
    ///
    /// [`from_raw`]: Weak::from_raw
    /// [`as_ptr`]: Weak::as_ptr
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "weak_into_raw", since = "1.45.0")]
    pub fn into_raw(self) -> *const T {
        let result = self.as_ptr();
        mem::forget(self);
        result
    }

    /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
    ///
    /// This can be used to safely get a strong reference (by calling [`upgrade`]
    /// later) or to deallocate the weak count by dropping the `Weak<T>`.
    ///
    /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
    /// as these don't own anything; the method still works on them).
    ///
    /// # Safety
    ///
    /// The pointer must have originated from the [`into_raw`] and must still own its potential
    /// weak reference.
    ///
    /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
    /// takes ownership of one weak reference currently represented as a raw pointer (the weak
    /// count is not modified by this operation) and therefore it must be paired with a previous
    /// call to [`into_raw`].
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Weak};
    ///
    /// let strong = Arc::new("hello".to_owned());
    ///
    /// let raw_1 = Arc::downgrade(&strong).into_raw();
    /// let raw_2 = Arc::downgrade(&strong).into_raw();
    ///
    /// assert_eq!(2, Arc::weak_count(&strong));
    ///
    /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
    /// assert_eq!(1, Arc::weak_count(&strong));
    ///
    /// drop(strong);
    ///
    /// // Decrement the last weak count.
    /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
    /// ```
    ///
    /// [`new`]: Weak::new
    /// [`into_raw`]: Weak::into_raw
    /// [`upgrade`]: Weak::upgrade
    #[stable(feature = "weak_into_raw", since = "1.45.0")]
    pub unsafe fn from_raw(ptr: *const T) -> Self {
        // See Weak::as_ptr for context on how the input pointer is derived.

        let ptr = if is_dangling(ptr as *mut T) {
            // This is a dangling Weak.
            ptr as *mut ArcInner<T>
        } else {
            // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
            // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
            let offset = unsafe { data_offset(ptr) };
            // Thus, we reverse the offset to get the whole RcBox.
            // SAFETY: the pointer originated from a Weak, so this offset is safe.
            unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
        };

        // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
        Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
    }
}

impl<T: ?Sized> Weak<T> {
    /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
    /// dropping of the inner value if successful.
    ///
    /// Returns [`None`] if the inner value has since been dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// let weak_five = Arc::downgrade(&five);
    ///
    /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
    /// assert!(strong_five.is_some());
    ///
    /// // Destroy all strong pointers.
    /// drop(strong_five);
    /// drop(five);
    ///
    /// assert!(weak_five.upgrade().is_none());
    /// ```
    #[must_use = "this returns a new `Arc`, \
                  without modifying the original weak pointer"]
    #[stable(feature = "arc_weak", since = "1.4.0")]
    pub fn upgrade(&self) -> Option<Arc<T>> {
        // We use a CAS loop to increment the strong count instead of a
        // fetch_add as this function should never take the reference count
        // from zero to one.
        self.inner()?
            .strong
            // Relaxed is fine for the failure case because we don't have any expectations about the new state.
            // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
            // value can be initialized after `Weak` references have already been created. In that case, we
            // expect to observe the fully initialized value.
            .fetch_update(Acquire, Relaxed, |n| {
                // Any write of 0 we can observe leaves the field in permanently zero state.
                if n == 0 {
                    return None;
                }
                // See comments in `Arc::clone` for why we do this (for `mem::forget`).
                if n > MAX_REFCOUNT {
                    abort();
                }
                Some(n + 1)
            })
            .ok()
            // null checked above
            .map(|_| unsafe { Arc::from_inner(self.ptr) })
    }

    /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
    ///
    /// If `self` was created using [`Weak::new`], this will return 0.
    #[must_use]
    #[stable(feature = "weak_counts", since = "1.41.0")]
    pub fn strong_count(&self) -> usize {
        if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
    }

    /// Gets an approximation of the number of `Weak` pointers pointing to this
    /// allocation.
    ///
    /// If `self` was created using [`Weak::new`], or if there are no remaining
    /// strong pointers, this will return 0.
    ///
    /// # Accuracy
    ///
    /// Due to implementation details, the returned value can be off by 1 in
    /// either direction when other threads are manipulating any `Arc`s or
    /// `Weak`s pointing to the same allocation.
    #[must_use]
    #[stable(feature = "weak_counts", since = "1.41.0")]
    pub fn weak_count(&self) -> usize {
        self.inner()
            .map(|inner| {
                let weak = inner.weak.load(Acquire);
                let strong = inner.strong.load(Acquire);
                if strong == 0 {
                    0
                } else {
                    // Since we observed that there was at least one strong pointer
                    // after reading the weak count, we know that the implicit weak
                    // reference (present whenever any strong references are alive)
                    // was still around when we observed the weak count, and can
                    // therefore safely subtract it.
                    weak - 1
                }
            })
            .unwrap_or(0)
    }

    /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
    /// (i.e., when this `Weak` was created by `Weak::new`).
    #[inline]
    fn inner(&self) -> Option<WeakInner<'_>> {
        if is_dangling(self.ptr.as_ptr()) {
            None
        } else {
            // We are careful to *not* create a reference covering the "data" field, as
            // the field may be mutated concurrently (for example, if the last `Arc`
            // is dropped, the data field will be dropped in-place).
            Some(unsafe {
                let ptr = self.ptr.as_ptr();
                WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
            })
        }
    }

    /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
    /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
    /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
    ///
    /// # Notes
    ///
    /// Since this compares pointers it means that `Weak::new()` will equal each
    /// other, even though they don't point to any allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let first_rc = Arc::new(5);
    /// let first = Arc::downgrade(&first_rc);
    /// let second = Arc::downgrade(&first_rc);
    ///
    /// assert!(first.ptr_eq(&second));
    ///
    /// let third_rc = Arc::new(5);
    /// let third = Arc::downgrade(&third_rc);
    ///
    /// assert!(!first.ptr_eq(&third));
    /// ```
    ///
    /// Comparing `Weak::new`.
    ///
    /// ```
    /// use std::sync::{Arc, Weak};
    ///
    /// let first = Weak::new();
    /// let second = Weak::new();
    /// assert!(first.ptr_eq(&second));
    ///
    /// let third_rc = Arc::new(());
    /// let third = Arc::downgrade(&third_rc);
    /// assert!(!first.ptr_eq(&third));
    /// ```
    ///
    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
    #[inline]
    #[must_use]
    #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
    pub fn ptr_eq(&self, other: &Self) -> bool {
        self.ptr.as_ptr() == other.ptr.as_ptr()
    }
}

#[stable(feature = "arc_weak", since = "1.4.0")]
impl<T: ?Sized> Clone for Weak<T> {
    /// Makes a clone of the `Weak` pointer that points to the same allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Weak};
    ///
    /// let weak_five = Arc::downgrade(&Arc::new(5));
    ///
    /// let _ = Weak::clone(&weak_five);
    /// ```
    #[inline]
    fn clone(&self) -> Weak<T> {
        let inner = if let Some(inner) = self.inner() {
            inner
        } else {
            return Weak { ptr: self.ptr };
        };
        // See comments in Arc::clone() for why this is relaxed. This can use a
        // fetch_add (ignoring the lock) because the weak count is only locked
        // where are *no other* weak pointers in existence. (So we can't be
        // running this code in that case).
        let old_size = inner.weak.fetch_add(1, Relaxed);

        // See comments in Arc::clone() for why we do this (for mem::forget).
        if old_size > MAX_REFCOUNT {
            abort();
        }

        Weak { ptr: self.ptr }
    }
}

#[stable(feature = "downgraded_weak", since = "1.10.0")]
impl<T> Default for Weak<T> {
    /// Constructs a new `Weak<T>`, without allocating memory.
    /// Calling [`upgrade`] on the return value always
    /// gives [`None`].
    ///
    /// [`upgrade`]: Weak::upgrade
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Weak;
    ///
    /// let empty: Weak<i64> = Default::default();
    /// assert!(empty.upgrade().is_none());
    /// ```
    fn default() -> Weak<T> {
        Weak::new()
    }
}

#[stable(feature = "arc_weak", since = "1.4.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
    /// Drops the `Weak` pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Weak};
    ///
    /// struct Foo;
    ///
    /// impl Drop for Foo {
    ///     fn drop(&mut self) {
    ///         println!("dropped!");
    ///     }
    /// }
    ///
    /// let foo = Arc::new(Foo);
    /// let weak_foo = Arc::downgrade(&foo);
    /// let other_weak_foo = Weak::clone(&weak_foo);
    ///
    /// drop(weak_foo);   // Doesn't print anything
    /// drop(foo);        // Prints "dropped!"
    ///
    /// assert!(other_weak_foo.upgrade().is_none());
    /// ```
    fn drop(&mut self) {
        // If we find out that we were the last weak pointer, then its time to
        // deallocate the data entirely. See the discussion in Arc::drop() about
        // the memory orderings
        //
        // It's not necessary to check for the locked state here, because the
        // weak count can only be locked if there was precisely one weak ref,
        // meaning that drop could only subsequently run ON that remaining weak
        // ref, which can only happen after the lock is released.
        let inner = if let Some(inner) = self.inner() { inner } else { return };

        if inner.weak.fetch_sub(1, Release) == 1 {
            acquire!(inner.weak);
            unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
trait ArcEqIdent<T: ?Sized + PartialEq> {
    fn eq(&self, other: &Arc<T>) -> bool;
    fn ne(&self, other: &Arc<T>) -> bool;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
    #[inline]
    default fn eq(&self, other: &Arc<T>) -> bool {
        **self == **other
    }
    #[inline]
    default fn ne(&self, other: &Arc<T>) -> bool {
        **self != **other
    }
}

/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
/// store large values, that are slow to clone, but also heavy to check for equality, causing this
/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
/// the same value, than two `&T`s.
///
/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
    #[inline]
    fn eq(&self, other: &Arc<T>) -> bool {
        Arc::ptr_eq(self, other) || **self == **other
    }

    #[inline]
    fn ne(&self, other: &Arc<T>) -> bool {
        !Arc::ptr_eq(self, other) && **self != **other
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
    /// Equality for two `Arc`s.
    ///
    /// Two `Arc`s are equal if their inner values are equal, even if they are
    /// stored in different allocation.
    ///
    /// If `T` also implements `Eq` (implying reflexivity of equality),
    /// two `Arc`s that point to the same allocation are always equal.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five == Arc::new(5));
    /// ```
    #[inline]
    fn eq(&self, other: &Arc<T>) -> bool {
        ArcEqIdent::eq(self, other)
    }

    /// Inequality for two `Arc`s.
    ///
    /// Two `Arc`s are unequal if their inner values are unequal.
    ///
    /// If `T` also implements `Eq` (implying reflexivity of equality),
    /// two `Arc`s that point to the same value are never unequal.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five != Arc::new(6));
    /// ```
    #[inline]
    fn ne(&self, other: &Arc<T>) -> bool {
        ArcEqIdent::ne(self, other)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
    /// Partial comparison for two `Arc`s.
    ///
    /// The two are compared by calling `partial_cmp()` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use std::cmp::Ordering;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
    /// ```
    fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
        (**self).partial_cmp(&**other)
    }

    /// Less-than comparison for two `Arc`s.
    ///
    /// The two are compared by calling `<` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five < Arc::new(6));
    /// ```
    fn lt(&self, other: &Arc<T>) -> bool {
        *(*self) < *(*other)
    }

    /// 'Less than or equal to' comparison for two `Arc`s.
    ///
    /// The two are compared by calling `<=` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five <= Arc::new(5));
    /// ```
    fn le(&self, other: &Arc<T>) -> bool {
        *(*self) <= *(*other)
    }

    /// Greater-than comparison for two `Arc`s.
    ///
    /// The two are compared by calling `>` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five > Arc::new(4));
    /// ```
    fn gt(&self, other: &Arc<T>) -> bool {
        *(*self) > *(*other)
    }

    /// 'Greater than or equal to' comparison for two `Arc`s.
    ///
    /// The two are compared by calling `>=` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert!(five >= Arc::new(5));
    /// ```
    fn ge(&self, other: &Arc<T>) -> bool {
        *(*self) >= *(*other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord> Ord for Arc<T> {
    /// Comparison for two `Arc`s.
    ///
    /// The two are compared by calling `cmp()` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use std::cmp::Ordering;
    ///
    /// let five = Arc::new(5);
    ///
    /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
    /// ```
    fn cmp(&self, other: &Arc<T>) -> Ordering {
        (**self).cmp(&**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq> Eq for Arc<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> fmt::Pointer for Arc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Pointer::fmt(&(&**self as *const T), f)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Arc<T> {
    /// Creates a new `Arc<T>`, with the `Default` value for `T`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    ///
    /// let x: Arc<i32> = Default::default();
    /// assert_eq!(*x, 0);
    /// ```
    fn default() -> Arc<T> {
        Arc::new(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash> Hash for Arc<T> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (**self).hash(state)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_for_ptrs", since = "1.6.0")]
impl<T> From<T> for Arc<T> {
    /// Converts a `T` into an `Arc<T>`
    ///
    /// The conversion moves the value into a
    /// newly allocated `Arc`. It is equivalent to
    /// calling `Arc::new(t)`.
    ///
    /// # Example
    /// ```rust
    /// # use std::sync::Arc;
    /// let x = 5;
    /// let arc = Arc::new(5);
    ///
    /// assert_eq!(Arc::from(x), arc);
    /// ```
    fn from(t: T) -> Self {
        Arc::new(t)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: Clone> From<&[T]> for Arc<[T]> {
    /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let original: &[i32] = &[1, 2, 3];
    /// let shared: Arc<[i32]> = Arc::from(original);
    /// assert_eq!(&[1, 2, 3], &shared[..]);
    /// ```
    #[inline]
    fn from(v: &[T]) -> Arc<[T]> {
        <Self as ArcFromSlice<T>>::from_slice(v)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<&str> for Arc<str> {
    /// Allocate a reference-counted `str` and copy `v` into it.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let shared: Arc<str> = Arc::from("eggplant");
    /// assert_eq!("eggplant", &shared[..]);
    /// ```
    #[inline]
    fn from(v: &str) -> Arc<str> {
        let arc = Arc::<[u8]>::from(v.as_bytes());
        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<String> for Arc<str> {
    /// Allocate a reference-counted `str` and copy `v` into it.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let unique: String = "eggplant".to_owned();
    /// let shared: Arc<str> = Arc::from(unique);
    /// assert_eq!("eggplant", &shared[..]);
    /// ```
    #[inline]
    fn from(v: String) -> Arc<str> {
        Arc::from(&v[..])
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: ?Sized> From<Box<T>> for Arc<T> {
    /// Move a boxed object to a new, reference-counted allocation.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let unique: Box<str> = Box::from("eggplant");
    /// let shared: Arc<str> = Arc::from(unique);
    /// assert_eq!("eggplant", &shared[..]);
    /// ```
    #[inline]
    fn from(v: Box<T>) -> Arc<T> {
        Arc::from_box(v)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T> From<Vec<T>> for Arc<[T]> {
    /// Allocate a reference-counted slice and move `v`'s items into it.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let unique: Vec<i32> = vec![1, 2, 3];
    /// let shared: Arc<[i32]> = Arc::from(unique);
    /// assert_eq!(&[1, 2, 3], &shared[..]);
    /// ```
    #[inline]
    fn from(mut v: Vec<T>) -> Arc<[T]> {
        unsafe {
            let rc = Arc::copy_from_slice(&v);
            // Allow the Vec to free its memory, but not destroy its contents
            v.set_len(0);
            rc
        }
    }
}

#[stable(feature = "shared_from_cow", since = "1.45.0")]
impl<'a, B> From<Cow<'a, B>> for Arc<B>
where
    B: ToOwned + ?Sized,
    Arc<B>: From<&'a B> + From<B::Owned>,
{
    /// Create an atomically reference-counted pointer from
    /// a clone-on-write pointer by copying its content.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use std::sync::Arc;
    /// # use std::borrow::Cow;
    /// let cow: Cow<str> = Cow::Borrowed("eggplant");
    /// let shared: Arc<str> = Arc::from(cow);
    /// assert_eq!("eggplant", &shared[..]);
    /// ```
    #[inline]
    fn from(cow: Cow<'a, B>) -> Arc<B> {
        match cow {
            Cow::Borrowed(s) => Arc::from(s),
            Cow::Owned(s) => Arc::from(s),
        }
    }
}

#[stable(feature = "shared_from_str", since = "1.62.0")]
impl From<Arc<str>> for Arc<[u8]> {
    /// Converts an atomically reference-counted string slice into a byte slice.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::sync::Arc;
    /// let string: Arc<str> = Arc::from("eggplant");
    /// let bytes: Arc<[u8]> = Arc::from(string);
    /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
    /// ```
    #[inline]
    fn from(rc: Arc<str>) -> Self {
        // SAFETY: `str` has the same layout as `[u8]`.
        unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
    }
}

#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
    type Error = Arc<[T]>;

    fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
        if boxed_slice.len() == N {
            Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
        } else {
            Err(boxed_slice)
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_iter", since = "1.37.0")]
impl<T> iter::FromIterator<T> for Arc<[T]> {
    /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
    ///
    /// # Performance characteristics
    ///
    /// ## The general case
    ///
    /// In the general case, collecting into `Arc<[T]>` is done by first
    /// collecting into a `Vec<T>`. That is, when writing the following:
    ///
    /// ```rust
    /// # use std::sync::Arc;
    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
    /// ```
    ///
    /// this behaves as if we wrote:
    ///
    /// ```rust
    /// # use std::sync::Arc;
    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
    ///     .collect::<Vec<_>>() // The first set of allocations happens here.
    ///     .into(); // A second allocation for `Arc<[T]>` happens here.
    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
    /// ```
    ///
    /// This will allocate as many times as needed for constructing the `Vec<T>`
    /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
    ///
    /// ## Iterators of known length
    ///
    /// When your `Iterator` implements `TrustedLen` and is of an exact size,
    /// a single allocation will be made for the `Arc<[T]>`. For example:
    ///
    /// ```rust
    /// # use std::sync::Arc;
    /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
    /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
    /// ```
    fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
        ToArcSlice::to_arc_slice(iter.into_iter())
    }
}

/// Specialization trait used for collecting into `Arc<[T]>`.
trait ToArcSlice<T>: Iterator<Item = T> + Sized {
    fn to_arc_slice(self) -> Arc<[T]>;
}

#[cfg(not(no_global_oom_handling))]
impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
    default fn to_arc_slice(self) -> Arc<[T]> {
        self.collect::<Vec<T>>().into()
    }
}

#[cfg(not(no_global_oom_handling))]
impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
    fn to_arc_slice(self) -> Arc<[T]> {
        // This is the case for a `TrustedLen` iterator.
        let (low, high) = self.size_hint();
        if let Some(high) = high {
            debug_assert_eq!(
                low,
                high,
                "TrustedLen iterator's size hint is not exact: {:?}",
                (low, high)
            );

            unsafe {
                // SAFETY: We need to ensure that the iterator has an exact length and we have.
                Arc::from_iter_exact(self, low)
            }
        } else {
            // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
            // length exceeding `usize::MAX`.
            // The default implementation would collect into a vec which would panic.
            // Thus we panic here immediately without invoking `Vec` code.
            panic!("capacity overflow");
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
    fn borrow(&self) -> &T {
        &**self
    }
}

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized> AsRef<T> for Arc<T> {
    fn as_ref(&self) -> &T {
        &**self
    }
}

#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Arc<T> {}

/// Get the offset within an `ArcInner` for the payload behind a pointer.
///
/// # Safety
///
/// The pointer must point to (and have valid metadata for) a previously
/// valid instance of T, but the T is allowed to be dropped.
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
    // Align the unsized value to the end of the ArcInner.
    // Because RcBox is repr(C), it will always be the last field in memory.
    // SAFETY: since the only unsized types possible are slices, trait objects,
    // and extern types, the input safety requirement is currently enough to
    // satisfy the requirements of align_of_val_raw; this is an implementation
    // detail of the language that must not be relied upon outside of std.
    unsafe { data_offset_align(align_of_val_raw(ptr)) }
}

#[inline]
fn data_offset_align(align: usize) -> usize {
    let layout = Layout::new::<ArcInner<()>>();
    layout.size() + layout.padding_needed_for(align)
}

#[stable(feature = "arc_error", since = "1.52.0")]
impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
    #[allow(deprecated, deprecated_in_future)]
    fn description(&self) -> &str {
        core::error::Error::description(&**self)
    }

    #[allow(deprecated)]
    fn cause(&self) -> Option<&dyn core::error::Error> {
        core::error::Error::cause(&**self)
    }

    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
        core::error::Error::source(&**self)
    }

    fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
        core::error::Error::provide(&**self, req);
    }
}