summaryrefslogtreecommitdiffstats
path: root/library/alloc/src/vec/mod.rs
blob: f2aa30f18fcf65d27d81adb237b35f4cd4b0c5c5 (plain)
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
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
//! A contiguous growable array type with heap-allocated contents, written
//! `Vec<T>`.
//!
//! Vectors have *O*(1) indexing, amortized *O*(1) push (to the end) and
//! *O*(1) pop (from the end).
//!
//! Vectors ensure they never allocate more than `isize::MAX` bytes.
//!
//! # Examples
//!
//! You can explicitly create a [`Vec`] with [`Vec::new`]:
//!
//! ```
//! let v: Vec<i32> = Vec::new();
//! ```
//!
//! ...or by using the [`vec!`] macro:
//!
//! ```
//! let v: Vec<i32> = vec![];
//!
//! let v = vec![1, 2, 3, 4, 5];
//!
//! let v = vec![0; 10]; // ten zeroes
//! ```
//!
//! You can [`push`] values onto the end of a vector (which will grow the vector
//! as needed):
//!
//! ```
//! let mut v = vec![1, 2];
//!
//! v.push(3);
//! ```
//!
//! Popping values works in much the same way:
//!
//! ```
//! let mut v = vec![1, 2];
//!
//! let two = v.pop();
//! ```
//!
//! Vectors also support indexing (through the [`Index`] and [`IndexMut`] traits):
//!
//! ```
//! let mut v = vec![1, 2, 3];
//! let three = v[2];
//! v[1] = v[1] + 5;
//! ```
//!
//! [`push`]: Vec::push

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

#[cfg(not(no_global_oom_handling))]
use core::cmp;
use core::cmp::Ordering;
use core::convert::TryFrom;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::intrinsics::assume;
use core::iter;
#[cfg(not(no_global_oom_handling))]
use core::iter::FromIterator;
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
use core::ops::{self, Index, IndexMut, Range, RangeBounds};
use core::ptr::{self, NonNull};
use core::slice::{self, SliceIndex};

use crate::alloc::{Allocator, Global};
use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::collections::TryReserveError;
use crate::raw_vec::RawVec;

#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
pub use self::drain_filter::DrainFilter;

mod drain_filter;

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "vec_splice", since = "1.21.0")]
pub use self::splice::Splice;

#[cfg(not(no_global_oom_handling))]
mod splice;

#[stable(feature = "drain", since = "1.6.0")]
pub use self::drain::Drain;

mod drain;

#[cfg(not(no_global_oom_handling))]
mod cow;

#[cfg(not(no_global_oom_handling))]
pub(crate) use self::in_place_collect::AsVecIntoIter;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::into_iter::IntoIter;

mod into_iter;

#[cfg(not(no_global_oom_handling))]
use self::is_zero::IsZero;

mod is_zero;

#[cfg(not(no_global_oom_handling))]
mod in_place_collect;

mod partial_eq;

#[cfg(not(no_global_oom_handling))]
use self::spec_from_elem::SpecFromElem;

#[cfg(not(no_global_oom_handling))]
mod spec_from_elem;

#[cfg(not(no_global_oom_handling))]
use self::set_len_on_drop::SetLenOnDrop;

#[cfg(not(no_global_oom_handling))]
mod set_len_on_drop;

#[cfg(not(no_global_oom_handling))]
use self::in_place_drop::{InPlaceDrop, InPlaceDstBufDrop};

#[cfg(not(no_global_oom_handling))]
mod in_place_drop;

#[cfg(not(no_global_oom_handling))]
use self::spec_from_iter_nested::SpecFromIterNested;

#[cfg(not(no_global_oom_handling))]
mod spec_from_iter_nested;

#[cfg(not(no_global_oom_handling))]
use self::spec_from_iter::SpecFromIter;

#[cfg(not(no_global_oom_handling))]
mod spec_from_iter;

#[cfg(not(no_global_oom_handling))]
use self::spec_extend::SpecExtend;

#[cfg(not(no_global_oom_handling))]
mod spec_extend;

/// A contiguous growable array type, written as `Vec<T>`, short for 'vector'.
///
/// # Examples
///
/// ```
/// let mut vec = Vec::new();
/// vec.push(1);
/// vec.push(2);
///
/// assert_eq!(vec.len(), 2);
/// assert_eq!(vec[0], 1);
///
/// assert_eq!(vec.pop(), Some(2));
/// assert_eq!(vec.len(), 1);
///
/// vec[0] = 7;
/// assert_eq!(vec[0], 7);
///
/// vec.extend([1, 2, 3]);
///
/// for x in &vec {
///     println!("{x}");
/// }
/// assert_eq!(vec, [7, 1, 2, 3]);
/// ```
///
/// The [`vec!`] macro is provided for convenient initialization:
///
/// ```
/// let mut vec1 = vec![1, 2, 3];
/// vec1.push(4);
/// let vec2 = Vec::from([1, 2, 3, 4]);
/// assert_eq!(vec1, vec2);
/// ```
///
/// It can also initialize each element of a `Vec<T>` with a given value.
/// This may be more efficient than performing allocation and initialization
/// in separate steps, especially when initializing a vector of zeros:
///
/// ```
/// let vec = vec![0; 5];
/// assert_eq!(vec, [0, 0, 0, 0, 0]);
///
/// // The following is equivalent, but potentially slower:
/// let mut vec = Vec::with_capacity(5);
/// vec.resize(5, 0);
/// assert_eq!(vec, [0, 0, 0, 0, 0]);
/// ```
///
/// For more information, see
/// [Capacity and Reallocation](#capacity-and-reallocation).
///
/// Use a `Vec<T>` as an efficient stack:
///
/// ```
/// let mut stack = Vec::new();
///
/// stack.push(1);
/// stack.push(2);
/// stack.push(3);
///
/// while let Some(top) = stack.pop() {
///     // Prints 3, 2, 1
///     println!("{top}");
/// }
/// ```
///
/// # Indexing
///
/// The `Vec` type allows to access values by index, because it implements the
/// [`Index`] trait. An example will be more explicit:
///
/// ```
/// let v = vec![0, 2, 4, 6];
/// println!("{}", v[1]); // it will display '2'
/// ```
///
/// However be careful: if you try to access an index which isn't in the `Vec`,
/// your software will panic! You cannot do this:
///
/// ```should_panic
/// let v = vec![0, 2, 4, 6];
/// println!("{}", v[6]); // it will panic!
/// ```
///
/// Use [`get`] and [`get_mut`] if you want to check whether the index is in
/// the `Vec`.
///
/// # Slicing
///
/// A `Vec` can be mutable. On the other hand, slices are read-only objects.
/// To get a [slice][prim@slice], use [`&`]. Example:
///
/// ```
/// fn read_slice(slice: &[usize]) {
///     // ...
/// }
///
/// let v = vec![0, 1];
/// read_slice(&v);
///
/// // ... and that's all!
/// // you can also do it like this:
/// let u: &[usize] = &v;
/// // or like this:
/// let u: &[_] = &v;
/// ```
///
/// In Rust, it's more common to pass slices as arguments rather than vectors
/// when you just want to provide read access. The same goes for [`String`] and
/// [`&str`].
///
/// # Capacity and reallocation
///
/// The capacity of a vector is the amount of space allocated for any future
/// elements that will be added onto the vector. This is not to be confused with
/// the *length* of a vector, which specifies the number of actual elements
/// within the vector. If a vector's length exceeds its capacity, its capacity
/// will automatically be increased, but its elements will have to be
/// reallocated.
///
/// For example, a vector with capacity 10 and length 0 would be an empty vector
/// with space for 10 more elements. Pushing 10 or fewer elements onto the
/// vector will not change its capacity or cause reallocation to occur. However,
/// if the vector's length is increased to 11, it will have to reallocate, which
/// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
/// whenever possible to specify how big the vector is expected to get.
///
/// # Guarantees
///
/// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
/// about its design. This ensures that it's as low-overhead as possible in
/// the general case, and can be correctly manipulated in primitive ways
/// by unsafe code. Note that these guarantees refer to an unqualified `Vec<T>`.
/// If additional type parameters are added (e.g., to support custom allocators),
/// overriding their defaults may change the behavior.
///
/// Most fundamentally, `Vec` is and always will be a (pointer, capacity, length)
/// triplet. No more, no less. The order of these fields is completely
/// unspecified, and you should use the appropriate methods to modify these.
/// The pointer will never be null, so this type is null-pointer-optimized.
///
/// However, the pointer might not actually point to allocated memory. In particular,
/// if you construct a `Vec` with capacity 0 via [`Vec::new`], [`vec![]`][`vec!`],
/// [`Vec::with_capacity(0)`][`Vec::with_capacity`], or by calling [`shrink_to_fit`]
/// on an empty Vec, it will not allocate memory. Similarly, if you store zero-sized
/// types inside a `Vec`, it will not allocate space for them. *Note that in this case
/// the `Vec` might not report a [`capacity`] of 0*. `Vec` will allocate if and only
/// if <code>[mem::size_of::\<T>]\() * [capacity]\() > 0</code>. In general, `Vec`'s allocation
/// details are very subtle --- if you intend to allocate memory using a `Vec`
/// and use it for something else (either to pass to unsafe code, or to build your
/// own memory-backed collection), be sure to deallocate this memory by using
/// `from_raw_parts` to recover the `Vec` and then dropping it.
///
/// If a `Vec` *has* allocated memory, then the memory it points to is on the heap
/// (as defined by the allocator Rust is configured to use by default), and its
/// pointer points to [`len`] initialized, contiguous elements in order (what
/// you would see if you coerced it to a slice), followed by <code>[capacity] - [len]</code>
/// logically uninitialized, contiguous elements.
///
/// A vector containing the elements `'a'` and `'b'` with capacity 4 can be
/// visualized as below. The top part is the `Vec` struct, it contains a
/// pointer to the head of the allocation in the heap, length and capacity.
/// The bottom part is the allocation on the heap, a contiguous memory block.
///
/// ```text
///             ptr      len  capacity
///        +--------+--------+--------+
///        | 0x0123 |      2 |      4 |
///        +--------+--------+--------+
///             |
///             v
/// Heap   +--------+--------+--------+--------+
///        |    'a' |    'b' | uninit | uninit |
///        +--------+--------+--------+--------+
/// ```
///
/// - **uninit** represents memory that is not initialized, see [`MaybeUninit`].
/// - Note: the ABI is not stable and `Vec` makes no guarantees about its memory
///   layout (including the order of fields).
///
/// `Vec` will never perform a "small optimization" where elements are actually
/// stored on the stack for two reasons:
///
/// * It would make it more difficult for unsafe code to correctly manipulate
///   a `Vec`. The contents of a `Vec` wouldn't have a stable address if it were
///   only moved, and it would be more difficult to determine if a `Vec` had
///   actually allocated memory.
///
/// * It would penalize the general case, incurring an additional branch
///   on every access.
///
/// `Vec` will never automatically shrink itself, even if completely empty. This
/// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
/// and then filling it back up to the same [`len`] should incur no calls to
/// the allocator. If you wish to free up unused memory, use
/// [`shrink_to_fit`] or [`shrink_to`].
///
/// [`push`] and [`insert`] will never (re)allocate if the reported capacity is
/// sufficient. [`push`] and [`insert`] *will* (re)allocate if
/// <code>[len] == [capacity]</code>. That is, the reported capacity is completely
/// accurate, and can be relied on. It can even be used to manually free the memory
/// allocated by a `Vec` if desired. Bulk insertion methods *may* reallocate, even
/// when not necessary.
///
/// `Vec` does not guarantee any particular growth strategy when reallocating
/// when full, nor when [`reserve`] is called. The current strategy is basic
/// and it may prove desirable to use a non-constant growth factor. Whatever
/// strategy is used will of course guarantee *O*(1) amortized [`push`].
///
/// `vec![x; n]`, `vec![a, b, c, d]`, and
/// [`Vec::with_capacity(n)`][`Vec::with_capacity`], will all produce a `Vec`
/// with exactly the requested capacity. If <code>[len] == [capacity]</code>,
/// (as is the case for the [`vec!`] macro), then a `Vec<T>` can be converted to
/// and from a [`Box<[T]>`][owned slice] without reallocating or moving the elements.
///
/// `Vec` will not specifically overwrite any data that is removed from it,
/// but also won't specifically preserve it. Its uninitialized memory is
/// scratch space that it may use however it wants. It will generally just do
/// whatever is most efficient or otherwise easy to implement. Do not rely on
/// removed data to be erased for security purposes. Even if you drop a `Vec`, its
/// buffer may simply be reused by another allocation. Even if you zero a `Vec`'s memory
/// first, that might not actually happen because the optimizer does not consider
/// this a side-effect that must be preserved. There is one case which we will
/// not break, however: using `unsafe` code to write to the excess capacity,
/// and then increasing the length to match, is always valid.
///
/// Currently, `Vec` does not guarantee the order in which elements are dropped.
/// The order has changed in the past and may change again.
///
/// [`get`]: slice::get
/// [`get_mut`]: slice::get_mut
/// [`String`]: crate::string::String
/// [`&str`]: type@str
/// [`shrink_to_fit`]: Vec::shrink_to_fit
/// [`shrink_to`]: Vec::shrink_to
/// [capacity]: Vec::capacity
/// [`capacity`]: Vec::capacity
/// [mem::size_of::\<T>]: core::mem::size_of
/// [len]: Vec::len
/// [`len`]: Vec::len
/// [`push`]: Vec::push
/// [`insert`]: Vec::insert
/// [`reserve`]: Vec::reserve
/// [`MaybeUninit`]: core::mem::MaybeUninit
/// [owned slice]: Box
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Vec")]
#[rustc_insignificant_dtor]
pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
    buf: RawVec<T, A>,
    len: usize,
}

////////////////////////////////////////////////////////////////////////////////
// Inherent methods
////////////////////////////////////////////////////////////////////////////////

impl<T> Vec<T> {
    /// Constructs a new, empty `Vec<T>`.
    ///
    /// The vector will not allocate until elements are pushed onto it.
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(unused_mut)]
    /// let mut vec: Vec<i32> = Vec::new();
    /// ```
    #[inline]
    #[rustc_const_stable(feature = "const_vec_new", since = "1.39.0")]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub const fn new() -> Self {
        Vec { buf: RawVec::NEW, len: 0 }
    }

    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
    ///
    /// The vector will be able to hold at least `capacity` elements without
    /// reallocating. This method is allowed to allocate for more elements than
    /// `capacity`. If `capacity` is 0, the vector will not allocate.
    ///
    /// It is important to note that although the returned vector has the
    /// minimum *capacity* specified, the vector will have a zero *length*. For
    /// an explanation of the difference between length and capacity, see
    /// *[Capacity and reallocation]*.
    ///
    /// If it is important to know the exact allocated capacity of a `Vec`,
    /// always use the [`capacity`] method after construction.
    ///
    /// For `Vec<T>` where `T` is a zero-sized type, there will be no allocation
    /// and the capacity will always be `usize::MAX`.
    ///
    /// [Capacity and reallocation]: #capacity-and-reallocation
    /// [`capacity`]: Vec::capacity
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = Vec::with_capacity(10);
    ///
    /// // The vector contains no items, even though it has capacity for more
    /// assert_eq!(vec.len(), 0);
    /// assert!(vec.capacity() >= 10);
    ///
    /// // These are all done without reallocating...
    /// for i in 0..10 {
    ///     vec.push(i);
    /// }
    /// assert_eq!(vec.len(), 10);
    /// assert!(vec.capacity() >= 10);
    ///
    /// // ...but this may make the vector reallocate
    /// vec.push(11);
    /// assert_eq!(vec.len(), 11);
    /// assert!(vec.capacity() >= 11);
    ///
    /// // A vector of a zero-sized type will always over-allocate, since no
    /// // allocation is necessary
    /// let vec_units = Vec::<()>::with_capacity(10);
    /// assert_eq!(vec_units.capacity(), usize::MAX);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn with_capacity(capacity: usize) -> Self {
        Self::with_capacity_in(capacity, Global)
    }

    /// Creates a `Vec<T>` directly from a pointer, a capacity, and a length.
    ///
    /// # Safety
    ///
    /// This is highly unsafe, due to the number of invariants that aren't
    /// checked:
    ///
    /// * `ptr` must have been allocated using the global allocator, such as via
    ///   the [`alloc::alloc`] function.
    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
    ///   (`T` having a less strict alignment is not sufficient, the alignment really
    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
    ///   allocated and deallocated with the same layout.)
    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
    ///   to be the same size as the pointer was allocated with. (Because similar to
    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
    /// * `length` needs to be less than or equal to `capacity`.
    /// * The first `length` values must be properly initialized values of type `T`.
    /// * `capacity` needs to be the capacity that the pointer was allocated with.
    /// * The allocated size in bytes must be no larger than `isize::MAX`.
    ///   See the safety documentation of [`pointer::offset`].
    ///
    /// These requirements are always upheld by any `ptr` that has been allocated
    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
    /// upheld.
    ///
    /// Violating these may cause problems like corrupting the allocator's
    /// internal data structures. For example it is normally **not** safe
    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
    /// `size_t`, doing so is only safe if the array was initially allocated by
    /// a `Vec` or `String`.
    /// It's also not safe to build one from a `Vec<u16>` and its length, because
    /// the allocator cares about the alignment, and these two types have different
    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
    /// these issues, it is often preferable to do casting/transmuting using
    /// [`slice::from_raw_parts`] instead.
    ///
    /// The ownership of `ptr` is effectively transferred to the
    /// `Vec<T>` which may then deallocate, reallocate or change the
    /// contents of memory pointed to by the pointer at will. Ensure
    /// that nothing else uses the pointer after calling this
    /// function.
    ///
    /// [`String`]: crate::string::String
    /// [`alloc::alloc`]: crate::alloc::alloc
    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    /// use std::mem;
    ///
    /// let v = vec![1, 2, 3];
    ///
    // FIXME Update this when vec_into_raw_parts is stabilized
    /// // Prevent running `v`'s destructor so we are in complete control
    /// // of the allocation.
    /// let mut v = mem::ManuallyDrop::new(v);
    ///
    /// // Pull out the various important pieces of information about `v`
    /// let p = v.as_mut_ptr();
    /// let len = v.len();
    /// let cap = v.capacity();
    ///
    /// unsafe {
    ///     // Overwrite memory with 4, 5, 6
    ///     for i in 0..len {
    ///         ptr::write(p.add(i), 4 + i);
    ///     }
    ///
    ///     // Put everything back together into a Vec
    ///     let rebuilt = Vec::from_raw_parts(p, len, cap);
    ///     assert_eq!(rebuilt, [4, 5, 6]);
    /// }
    /// ```
    ///
    /// Using memory that was allocated elsewhere:
    ///
    /// ```rust
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::{AllocError, Allocator, Global, Layout};
    ///
    /// fn main() {
    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
    ///
    ///     let vec = unsafe {
    ///         let mem = match Global.allocate(layout) {
    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
    ///             Err(AllocError) => return,
    ///         };
    ///
    ///         mem.write(1_000_000);
    ///
    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
    ///     };
    ///
    ///     assert_eq!(vec, &[1_000_000]);
    ///     assert_eq!(vec.capacity(), 16);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
        unsafe { Self::from_raw_parts_in(ptr, length, capacity, Global) }
    }
}

impl<T, A: Allocator> Vec<T, A> {
    /// Constructs a new, empty `Vec<T, A>`.
    ///
    /// The vector will not allocate until elements are pushed onto it.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// # #[allow(unused_mut)]
    /// let mut vec: Vec<i32, _> = Vec::new_in(System);
    /// ```
    #[inline]
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub const fn new_in(alloc: A) -> Self {
        Vec { buf: RawVec::new_in(alloc), len: 0 }
    }

    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
    /// with the provided allocator.
    ///
    /// The vector will be able to hold at least `capacity` elements without
    /// reallocating. This method is allowed to allocate for more elements than
    /// `capacity`. If `capacity` is 0, the vector will not allocate.
    ///
    /// It is important to note that although the returned vector has the
    /// minimum *capacity* specified, the vector will have a zero *length*. For
    /// an explanation of the difference between length and capacity, see
    /// *[Capacity and reallocation]*.
    ///
    /// If it is important to know the exact allocated capacity of a `Vec`,
    /// always use the [`capacity`] method after construction.
    ///
    /// For `Vec<T, A>` where `T` is a zero-sized type, there will be no allocation
    /// and the capacity will always be `usize::MAX`.
    ///
    /// [Capacity and reallocation]: #capacity-and-reallocation
    /// [`capacity`]: Vec::capacity
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut vec = Vec::with_capacity_in(10, System);
    ///
    /// // The vector contains no items, even though it has capacity for more
    /// assert_eq!(vec.len(), 0);
    /// assert_eq!(vec.capacity(), 10);
    ///
    /// // These are all done without reallocating...
    /// for i in 0..10 {
    ///     vec.push(i);
    /// }
    /// assert_eq!(vec.len(), 10);
    /// assert_eq!(vec.capacity(), 10);
    ///
    /// // ...but this may make the vector reallocate
    /// vec.push(11);
    /// assert_eq!(vec.len(), 11);
    /// assert!(vec.capacity() >= 11);
    ///
    /// // A vector of a zero-sized type will always over-allocate, since no
    /// // allocation is necessary
    /// let vec_units = Vec::<(), System>::with_capacity_in(10, System);
    /// assert_eq!(vec_units.capacity(), usize::MAX);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
        Vec { buf: RawVec::with_capacity_in(capacity, alloc), len: 0 }
    }

    /// Creates a `Vec<T, A>` directly from a pointer, a capacity, a length,
    /// and an allocator.
    ///
    /// # Safety
    ///
    /// This is highly unsafe, due to the number of invariants that aren't
    /// checked:
    ///
    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
    ///   (`T` having a less strict alignment is not sufficient, the alignment really
    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
    ///   allocated and deallocated with the same layout.)
    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
    ///   to be the same size as the pointer was allocated with. (Because similar to
    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
    /// * `length` needs to be less than or equal to `capacity`.
    /// * The first `length` values must be properly initialized values of type `T`.
    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
    /// * The allocated size in bytes must be no larger than `isize::MAX`.
    ///   See the safety documentation of [`pointer::offset`].
    ///
    /// These requirements are always upheld by any `ptr` that has been allocated
    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
    /// upheld.
    ///
    /// Violating these may cause problems like corrupting the allocator's
    /// internal data structures. For example it is **not** safe
    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
    /// It's also not safe to build one from a `Vec<u16>` and its length, because
    /// the allocator cares about the alignment, and these two types have different
    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
    ///
    /// The ownership of `ptr` is effectively transferred to the
    /// `Vec<T>` which may then deallocate, reallocate or change the
    /// contents of memory pointed to by the pointer at will. Ensure
    /// that nothing else uses the pointer after calling this
    /// function.
    ///
    /// [`String`]: crate::string::String
    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
    /// [*fit*]: crate::alloc::Allocator#memory-fitting
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// use std::ptr;
    /// use std::mem;
    ///
    /// let mut v = Vec::with_capacity_in(3, System);
    /// v.push(1);
    /// v.push(2);
    /// v.push(3);
    ///
    // FIXME Update this when vec_into_raw_parts is stabilized
    /// // Prevent running `v`'s destructor so we are in complete control
    /// // of the allocation.
    /// let mut v = mem::ManuallyDrop::new(v);
    ///
    /// // Pull out the various important pieces of information about `v`
    /// let p = v.as_mut_ptr();
    /// let len = v.len();
    /// let cap = v.capacity();
    /// let alloc = v.allocator();
    ///
    /// unsafe {
    ///     // Overwrite memory with 4, 5, 6
    ///     for i in 0..len {
    ///         ptr::write(p.add(i), 4 + i);
    ///     }
    ///
    ///     // Put everything back together into a Vec
    ///     let rebuilt = Vec::from_raw_parts_in(p, len, cap, alloc.clone());
    ///     assert_eq!(rebuilt, [4, 5, 6]);
    /// }
    /// ```
    ///
    /// Using memory that was allocated elsewhere:
    ///
    /// ```rust
    /// use std::alloc::{alloc, Layout};
    ///
    /// fn main() {
    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
    ///     let vec = unsafe {
    ///         let mem = alloc(layout).cast::<u32>();
    ///         if mem.is_null() {
    ///             return;
    ///         }
    ///
    ///         mem.write(1_000_000);
    ///
    ///         Vec::from_raw_parts(mem, 1, 16)
    ///     };
    ///
    ///     assert_eq!(vec, &[1_000_000]);
    ///     assert_eq!(vec.capacity(), 16);
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
        unsafe { Vec { buf: RawVec::from_raw_parts_in(ptr, capacity, alloc), len: length } }
    }

    /// Decomposes a `Vec<T>` into its raw components.
    ///
    /// Returns the raw pointer to the underlying data, the length of
    /// the vector (in elements), and the allocated capacity of the
    /// data (in elements). These are the same arguments in the same
    /// order as the arguments to [`from_raw_parts`].
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `Vec`. The only way to do
    /// this is to convert the raw pointer, length, and capacity back
    /// into a `Vec` with the [`from_raw_parts`] function, allowing
    /// the destructor to perform the cleanup.
    ///
    /// [`from_raw_parts`]: Vec::from_raw_parts
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(vec_into_raw_parts)]
    /// let v: Vec<i32> = vec![-1, 0, 1];
    ///
    /// let (ptr, len, cap) = v.into_raw_parts();
    ///
    /// let rebuilt = unsafe {
    ///     // We can now make changes to the components, such as
    ///     // transmuting the raw pointer to a compatible type.
    ///     let ptr = ptr as *mut u32;
    ///
    ///     Vec::from_raw_parts(ptr, len, cap)
    /// };
    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
    /// ```
    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
        let mut me = ManuallyDrop::new(self);
        (me.as_mut_ptr(), me.len(), me.capacity())
    }

    /// Decomposes a `Vec<T>` into its raw components.
    ///
    /// Returns the raw pointer to the underlying data, the length of the vector (in elements),
    /// the allocated capacity of the data (in elements), and the allocator. These are the same
    /// arguments in the same order as the arguments to [`from_raw_parts_in`].
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `Vec`. The only way to do
    /// this is to convert the raw pointer, length, and capacity back
    /// into a `Vec` with the [`from_raw_parts_in`] function, allowing
    /// the destructor to perform the cleanup.
    ///
    /// [`from_raw_parts_in`]: Vec::from_raw_parts_in
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, vec_into_raw_parts)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut v: Vec<i32, System> = Vec::new_in(System);
    /// v.push(-1);
    /// v.push(0);
    /// v.push(1);
    ///
    /// let (ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
    ///
    /// let rebuilt = unsafe {
    ///     // We can now make changes to the components, such as
    ///     // transmuting the raw pointer to a compatible type.
    ///     let ptr = ptr as *mut u32;
    ///
    ///     Vec::from_raw_parts_in(ptr, len, cap, alloc)
    /// };
    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
    pub fn into_raw_parts_with_alloc(self) -> (*mut T, usize, usize, A) {
        let mut me = ManuallyDrop::new(self);
        let len = me.len();
        let capacity = me.capacity();
        let ptr = me.as_mut_ptr();
        let alloc = unsafe { ptr::read(me.allocator()) };
        (ptr, len, capacity, alloc)
    }

    /// Returns the total number of elements the vector can hold without
    /// reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec: Vec<i32> = Vec::with_capacity(10);
    /// vec.push(42);
    /// assert_eq!(vec.capacity(), 10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize {
        self.buf.capacity()
    }

    /// Reserves capacity for at least `additional` more elements to be inserted
    /// in the given `Vec<T>`. The collection may reserve more space to
    /// speculatively avoid frequent reallocations. After calling `reserve`,
    /// capacity will be greater than or equal to `self.len() + additional`.
    /// Does nothing if capacity is already sufficient.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1];
    /// vec.reserve(10);
    /// assert!(vec.capacity() >= 11);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        self.buf.reserve(self.len, additional);
    }

    /// Reserves the minimum capacity for at least `additional` more elements to
    /// be inserted in the given `Vec<T>`. Unlike [`reserve`], this will not
    /// deliberately over-allocate to speculatively avoid frequent allocations.
    /// After calling `reserve_exact`, capacity will be greater than or equal to
    /// `self.len() + additional`. Does nothing if the capacity is already
    /// sufficient.
    ///
    /// Note that the allocator may give the collection more space than it
    /// requests. Therefore, capacity can not be relied upon to be precisely
    /// minimal. Prefer [`reserve`] if future insertions are expected.
    ///
    /// [`reserve`]: Vec::reserve
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1];
    /// vec.reserve_exact(10);
    /// assert!(vec.capacity() >= 11);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.buf.reserve_exact(self.len, additional);
    }

    /// Tries to reserve capacity for at least `additional` more elements to be inserted
    /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
    /// frequent reallocations. After calling `try_reserve`, capacity will be
    /// greater than or equal to `self.len() + additional` if it returns
    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
    /// preserves the contents even if an error occurs.
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    ///
    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
    ///     let mut output = Vec::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve(data.len())?;
    ///
    ///     // Now we know this can't OOM in the middle of our complex work
    ///     output.extend(data.iter().map(|&val| {
    ///         val * 2 + 5 // very complicated
    ///     }));
    ///
    ///     Ok(output)
    /// }
    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.buf.try_reserve(self.len, additional)
    }

    /// Tries to reserve the minimum capacity for at least `additional`
    /// elements to be inserted in the given `Vec<T>`. Unlike [`try_reserve`],
    /// this will not deliberately over-allocate to speculatively avoid frequent
    /// allocations. After calling `try_reserve_exact`, capacity will be greater
    /// than or equal to `self.len() + additional` if it returns `Ok(())`.
    /// Does nothing if the capacity is already sufficient.
    ///
    /// Note that the allocator may give the collection more space than it
    /// requests. Therefore, capacity can not be relied upon to be precisely
    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
    ///
    /// [`try_reserve`]: Vec::try_reserve
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    ///
    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
    ///     let mut output = Vec::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve_exact(data.len())?;
    ///
    ///     // Now we know this can't OOM in the middle of our complex work
    ///     output.extend(data.iter().map(|&val| {
    ///         val * 2 + 5 // very complicated
    ///     }));
    ///
    ///     Ok(output)
    /// }
    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.buf.try_reserve_exact(self.len, additional)
    }

    /// Shrinks the capacity of the vector as much as possible.
    ///
    /// It will drop down as close as possible to the length but the allocator
    /// may still inform the vector that there is space for a few more elements.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = Vec::with_capacity(10);
    /// vec.extend([1, 2, 3]);
    /// assert_eq!(vec.capacity(), 10);
    /// vec.shrink_to_fit();
    /// assert!(vec.capacity() >= 3);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn shrink_to_fit(&mut self) {
        // The capacity is never less than the length, and there's nothing to do when
        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
        // by only calling it with a greater capacity.
        if self.capacity() > self.len {
            self.buf.shrink_to_fit(self.len);
        }
    }

    /// Shrinks the capacity of the vector with a lower bound.
    ///
    /// The capacity will remain at least as large as both the length
    /// and the supplied value.
    ///
    /// If the current capacity is less than the lower limit, this is a no-op.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = Vec::with_capacity(10);
    /// vec.extend([1, 2, 3]);
    /// assert_eq!(vec.capacity(), 10);
    /// vec.shrink_to(4);
    /// assert!(vec.capacity() >= 4);
    /// vec.shrink_to(0);
    /// assert!(vec.capacity() >= 3);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "shrink_to", since = "1.56.0")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        if self.capacity() > min_capacity {
            self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
        }
    }

    /// Converts the vector into [`Box<[T]>`][owned slice].
    ///
    /// If the vector has excess capacity, its items will be moved into a
    /// newly-allocated buffer with exactly the right capacity.
    ///
    /// [owned slice]: Box
    ///
    /// # Examples
    ///
    /// ```
    /// let v = vec![1, 2, 3];
    ///
    /// let slice = v.into_boxed_slice();
    /// ```
    ///
    /// Any excess capacity is removed:
    ///
    /// ```
    /// let mut vec = Vec::with_capacity(10);
    /// vec.extend([1, 2, 3]);
    ///
    /// assert_eq!(vec.capacity(), 10);
    /// let slice = vec.into_boxed_slice();
    /// assert_eq!(slice.into_vec().capacity(), 3);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_boxed_slice(mut self) -> Box<[T], A> {
        unsafe {
            self.shrink_to_fit();
            let me = ManuallyDrop::new(self);
            let buf = ptr::read(&me.buf);
            let len = me.len();
            buf.into_box(len).assume_init()
        }
    }

    /// Shortens the vector, keeping the first `len` elements and dropping
    /// the rest.
    ///
    /// If `len` is greater than the vector's current length, this has no
    /// effect.
    ///
    /// The [`drain`] method can emulate `truncate`, but causes the excess
    /// elements to be returned instead of dropped.
    ///
    /// Note that this method has no effect on the allocated capacity
    /// of the vector.
    ///
    /// # Examples
    ///
    /// Truncating a five element vector to two elements:
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3, 4, 5];
    /// vec.truncate(2);
    /// assert_eq!(vec, [1, 2]);
    /// ```
    ///
    /// No truncation occurs when `len` is greater than the vector's current
    /// length:
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// vec.truncate(8);
    /// assert_eq!(vec, [1, 2, 3]);
    /// ```
    ///
    /// Truncating when `len == 0` is equivalent to calling the [`clear`]
    /// method.
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// vec.truncate(0);
    /// assert_eq!(vec, []);
    /// ```
    ///
    /// [`clear`]: Vec::clear
    /// [`drain`]: Vec::drain
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn truncate(&mut self, len: usize) {
        // This is safe because:
        //
        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
        //   case avoids creating an invalid slice, and
        // * the `len` of the vector is shrunk before calling `drop_in_place`,
        //   such that no value will be dropped twice in case `drop_in_place`
        //   were to panic once (if it panics twice, the program aborts).
        unsafe {
            // Note: It's intentional that this is `>` and not `>=`.
            //       Changing it to `>=` has negative performance
            //       implications in some cases. See #78884 for more.
            if len > self.len {
                return;
            }
            let remaining_len = self.len - len;
            let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
            self.len = len;
            ptr::drop_in_place(s);
        }
    }

    /// Extracts a slice containing the entire vector.
    ///
    /// Equivalent to `&s[..]`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{self, Write};
    /// let buffer = vec![1, 2, 3, 5, 8];
    /// io::sink().write(buffer.as_slice()).unwrap();
    /// ```
    #[inline]
    #[stable(feature = "vec_as_slice", since = "1.7.0")]
    pub fn as_slice(&self) -> &[T] {
        self
    }

    /// Extracts a mutable slice of the entire vector.
    ///
    /// Equivalent to `&mut s[..]`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{self, Read};
    /// let mut buffer = vec![0; 3];
    /// io::repeat(0b101).read_exact(buffer.as_mut_slice()).unwrap();
    /// ```
    #[inline]
    #[stable(feature = "vec_as_slice", since = "1.7.0")]
    pub fn as_mut_slice(&mut self) -> &mut [T] {
        self
    }

    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
    /// valid for zero sized reads if the vector didn't allocate.
    ///
    /// The caller must ensure that the vector outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    /// Modifying the vector may cause its buffer to be reallocated,
    /// which would also make any pointers to it invalid.
    ///
    /// The caller must also ensure that the memory the pointer (non-transitively) points to
    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
    ///
    /// # Examples
    ///
    /// ```
    /// let x = vec![1, 2, 4];
    /// let x_ptr = x.as_ptr();
    ///
    /// unsafe {
    ///     for i in 0..x.len() {
    ///         assert_eq!(*x_ptr.add(i), 1 << i);
    ///     }
    /// }
    /// ```
    ///
    /// [`as_mut_ptr`]: Vec::as_mut_ptr
    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
    #[inline]
    pub fn as_ptr(&self) -> *const T {
        // We shadow the slice method of the same name to avoid going through
        // `deref`, which creates an intermediate reference.
        let ptr = self.buf.ptr();
        unsafe {
            assume(!ptr.is_null());
        }
        ptr
    }

    /// Returns an unsafe mutable pointer to the vector's buffer, or a dangling
    /// raw pointer valid for zero sized reads if the vector didn't allocate.
    ///
    /// The caller must ensure that the vector outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    /// Modifying the vector may cause its buffer to be reallocated,
    /// which would also make any pointers to it invalid.
    ///
    /// # Examples
    ///
    /// ```
    /// // Allocate vector big enough for 4 elements.
    /// let size = 4;
    /// let mut x: Vec<i32> = Vec::with_capacity(size);
    /// let x_ptr = x.as_mut_ptr();
    ///
    /// // Initialize elements via raw pointer writes, then set length.
    /// unsafe {
    ///     for i in 0..size {
    ///         *x_ptr.add(i) = i as i32;
    ///     }
    ///     x.set_len(size);
    /// }
    /// assert_eq!(&*x, &[0, 1, 2, 3]);
    /// ```
    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
    #[inline]
    pub fn as_mut_ptr(&mut self) -> *mut T {
        // We shadow the slice method of the same name to avoid going through
        // `deref_mut`, which creates an intermediate reference.
        let ptr = self.buf.ptr();
        unsafe {
            assume(!ptr.is_null());
        }
        ptr
    }

    /// Returns a reference to the underlying allocator.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn allocator(&self) -> &A {
        self.buf.allocator()
    }

    /// Forces the length of the vector to `new_len`.
    ///
    /// This is a low-level operation that maintains none of the normal
    /// invariants of the type. Normally changing the length of a vector
    /// is done using one of the safe operations instead, such as
    /// [`truncate`], [`resize`], [`extend`], or [`clear`].
    ///
    /// [`truncate`]: Vec::truncate
    /// [`resize`]: Vec::resize
    /// [`extend`]: Extend::extend
    /// [`clear`]: Vec::clear
    ///
    /// # Safety
    ///
    /// - `new_len` must be less than or equal to [`capacity()`].
    /// - The elements at `old_len..new_len` must be initialized.
    ///
    /// [`capacity()`]: Vec::capacity
    ///
    /// # Examples
    ///
    /// This method can be useful for situations in which the vector
    /// is serving as a buffer for other code, particularly over FFI:
    ///
    /// ```no_run
    /// # #![allow(dead_code)]
    /// # // This is just a minimal skeleton for the doc example;
    /// # // don't use this as a starting point for a real library.
    /// # pub struct StreamWrapper { strm: *mut std::ffi::c_void }
    /// # const Z_OK: i32 = 0;
    /// # extern "C" {
    /// #     fn deflateGetDictionary(
    /// #         strm: *mut std::ffi::c_void,
    /// #         dictionary: *mut u8,
    /// #         dictLength: *mut usize,
    /// #     ) -> i32;
    /// # }
    /// # impl StreamWrapper {
    /// pub fn get_dictionary(&self) -> Option<Vec<u8>> {
    ///     // Per the FFI method's docs, "32768 bytes is always enough".
    ///     let mut dict = Vec::with_capacity(32_768);
    ///     let mut dict_length = 0;
    ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
    ///     // 1. `dict_length` elements were initialized.
    ///     // 2. `dict_length` <= the capacity (32_768)
    ///     // which makes `set_len` safe to call.
    ///     unsafe {
    ///         // Make the FFI call...
    ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
    ///         if r == Z_OK {
    ///             // ...and update the length to what was initialized.
    ///             dict.set_len(dict_length);
    ///             Some(dict)
    ///         } else {
    ///             None
    ///         }
    ///     }
    /// }
    /// # }
    /// ```
    ///
    /// While the following example is sound, there is a memory leak since
    /// the inner vectors were not freed prior to the `set_len` call:
    ///
    /// ```
    /// let mut vec = vec![vec![1, 0, 0],
    ///                    vec![0, 1, 0],
    ///                    vec![0, 0, 1]];
    /// // SAFETY:
    /// // 1. `old_len..0` is empty so no elements need to be initialized.
    /// // 2. `0 <= capacity` always holds whatever `capacity` is.
    /// unsafe {
    ///     vec.set_len(0);
    /// }
    /// ```
    ///
    /// Normally, here, one would use [`clear`] instead to correctly drop
    /// the contents and thus not leak memory.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn set_len(&mut self, new_len: usize) {
        debug_assert!(new_len <= self.capacity());

        self.len = new_len;
    }

    /// Removes an element from the vector and returns it.
    ///
    /// The removed element is replaced by the last element of the vector.
    ///
    /// This does not preserve ordering, but is *O*(1).
    /// If you need to preserve the element order, use [`remove`] instead.
    ///
    /// [`remove`]: Vec::remove
    ///
    /// # Panics
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = vec!["foo", "bar", "baz", "qux"];
    ///
    /// assert_eq!(v.swap_remove(1), "bar");
    /// assert_eq!(v, ["foo", "qux", "baz"]);
    ///
    /// assert_eq!(v.swap_remove(0), "foo");
    /// assert_eq!(v, ["baz", "qux"]);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn swap_remove(&mut self, index: usize) -> T {
        #[cold]
        #[inline(never)]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("swap_remove index (is {index}) should be < len (is {len})");
        }

        let len = self.len();
        if index >= len {
            assert_failed(index, len);
        }
        unsafe {
            // We replace self[index] with the last element. Note that if the
            // bounds check above succeeds there must be a last element (which
            // can be self[index] itself).
            let value = ptr::read(self.as_ptr().add(index));
            let base_ptr = self.as_mut_ptr();
            ptr::copy(base_ptr.add(len - 1), base_ptr.add(index), 1);
            self.set_len(len - 1);
            value
        }
    }

    /// Inserts an element at position `index` within the vector, shifting all
    /// elements after it to the right.
    ///
    /// # Panics
    ///
    /// Panics if `index > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// vec.insert(1, 4);
    /// assert_eq!(vec, [1, 4, 2, 3]);
    /// vec.insert(4, 5);
    /// assert_eq!(vec, [1, 4, 2, 3, 5]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn insert(&mut self, index: usize, element: T) {
        #[cold]
        #[inline(never)]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("insertion index (is {index}) should be <= len (is {len})");
        }

        let len = self.len();

        // space for the new element
        if len == self.buf.capacity() {
            self.reserve(1);
        }

        unsafe {
            // infallible
            // The spot to put the new value
            {
                let p = self.as_mut_ptr().add(index);
                if index < len {
                    // Shift everything over to make space. (Duplicating the
                    // `index`th element into two consecutive places.)
                    ptr::copy(p, p.add(1), len - index);
                } else if index == len {
                    // No elements need shifting.
                } else {
                    assert_failed(index, len);
                }
                // Write it in, overwriting the first copy of the `index`th
                // element.
                ptr::write(p, element);
            }
            self.set_len(len + 1);
        }
    }

    /// Removes and returns the element at position `index` within the vector,
    /// shifting all elements after it to the left.
    ///
    /// Note: Because this shifts over the remaining elements, it has a
    /// worst-case performance of *O*(*n*). If you don't need the order of elements
    /// to be preserved, use [`swap_remove`] instead. If you'd like to remove
    /// elements from the beginning of the `Vec`, consider using
    /// [`VecDeque::pop_front`] instead.
    ///
    /// [`swap_remove`]: Vec::swap_remove
    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
    ///
    /// # Panics
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    /// assert_eq!(v.remove(1), 2);
    /// assert_eq!(v, [1, 3]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[track_caller]
    pub fn remove(&mut self, index: usize) -> T {
        #[cold]
        #[inline(never)]
        #[track_caller]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("removal index (is {index}) should be < len (is {len})");
        }

        let len = self.len();
        if index >= len {
            assert_failed(index, len);
        }
        unsafe {
            // infallible
            let ret;
            {
                // the place we are taking from.
                let ptr = self.as_mut_ptr().add(index);
                // copy it out, unsafely having a copy of the value on
                // the stack and in the vector at the same time.
                ret = ptr::read(ptr);

                // Shift everything down to fill in that spot.
                ptr::copy(ptr.add(1), ptr, len - index - 1);
            }
            self.set_len(len - 1);
            ret
        }
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
    /// This method operates in place, visiting each element exactly once in the
    /// original order, and preserves the order of the retained elements.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3, 4];
    /// vec.retain(|&x| x % 2 == 0);
    /// assert_eq!(vec, [2, 4]);
    /// ```
    ///
    /// Because the elements are visited exactly once in the original order,
    /// external state may be used to decide which elements to keep.
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3, 4, 5];
    /// let keep = [false, true, true, false, true];
    /// let mut iter = keep.iter();
    /// vec.retain(|_| *iter.next().unwrap());
    /// assert_eq!(vec, [2, 3, 5]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&T) -> bool,
    {
        self.retain_mut(|elem| f(elem));
    }

    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
    ///
    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
    /// This method operates in place, visiting each element exactly once in the
    /// original order, and preserves the order of the retained elements.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3, 4];
    /// vec.retain_mut(|x| if *x <= 3 {
    ///     *x += 1;
    ///     true
    /// } else {
    ///     false
    /// });
    /// assert_eq!(vec, [2, 3, 4]);
    /// ```
    #[stable(feature = "vec_retain_mut", since = "1.61.0")]
    pub fn retain_mut<F>(&mut self, mut f: F)
    where
        F: FnMut(&mut T) -> bool,
    {
        let original_len = self.len();
        // Avoid double drop if the drop guard is not executed,
        // since we may make some holes during the process.
        unsafe { self.set_len(0) };

        // Vec: [Kept, Kept, Hole, Hole, Hole, Hole, Unchecked, Unchecked]
        //      |<-              processed len   ->| ^- next to check
        //                  |<-  deleted cnt     ->|
        //      |<-              original_len                          ->|
        // Kept: Elements which predicate returns true on.
        // Hole: Moved or dropped element slot.
        // Unchecked: Unchecked valid elements.
        //
        // This drop guard will be invoked when predicate or `drop` of element panicked.
        // It shifts unchecked elements to cover holes and `set_len` to the correct length.
        // In cases when predicate and `drop` never panick, it will be optimized out.
        struct BackshiftOnDrop<'a, T, A: Allocator> {
            v: &'a mut Vec<T, A>,
            processed_len: usize,
            deleted_cnt: usize,
            original_len: usize,
        }

        impl<T, A: Allocator> Drop for BackshiftOnDrop<'_, T, A> {
            fn drop(&mut self) {
                if self.deleted_cnt > 0 {
                    // SAFETY: Trailing unchecked items must be valid since we never touch them.
                    unsafe {
                        ptr::copy(
                            self.v.as_ptr().add(self.processed_len),
                            self.v.as_mut_ptr().add(self.processed_len - self.deleted_cnt),
                            self.original_len - self.processed_len,
                        );
                    }
                }
                // SAFETY: After filling holes, all items are in contiguous memory.
                unsafe {
                    self.v.set_len(self.original_len - self.deleted_cnt);
                }
            }
        }

        let mut g = BackshiftOnDrop { v: self, processed_len: 0, deleted_cnt: 0, original_len };

        fn process_loop<F, T, A: Allocator, const DELETED: bool>(
            original_len: usize,
            f: &mut F,
            g: &mut BackshiftOnDrop<'_, T, A>,
        ) where
            F: FnMut(&mut T) -> bool,
        {
            while g.processed_len != original_len {
                // SAFETY: Unchecked element must be valid.
                let cur = unsafe { &mut *g.v.as_mut_ptr().add(g.processed_len) };
                if !f(cur) {
                    // Advance early to avoid double drop if `drop_in_place` panicked.
                    g.processed_len += 1;
                    g.deleted_cnt += 1;
                    // SAFETY: We never touch this element again after dropped.
                    unsafe { ptr::drop_in_place(cur) };
                    // We already advanced the counter.
                    if DELETED {
                        continue;
                    } else {
                        break;
                    }
                }
                if DELETED {
                    // SAFETY: `deleted_cnt` > 0, so the hole slot must not overlap with current element.
                    // We use copy for move, and never touch this element again.
                    unsafe {
                        let hole_slot = g.v.as_mut_ptr().add(g.processed_len - g.deleted_cnt);
                        ptr::copy_nonoverlapping(cur, hole_slot, 1);
                    }
                }
                g.processed_len += 1;
            }
        }

        // Stage 1: Nothing was deleted.
        process_loop::<F, T, A, false>(original_len, &mut f, &mut g);

        // Stage 2: Some elements were deleted.
        process_loop::<F, T, A, true>(original_len, &mut f, &mut g);

        // All item are processed. This can be optimized to `set_len` by LLVM.
        drop(g);
    }

    /// Removes all but the first of consecutive elements in the vector that resolve to the same
    /// key.
    ///
    /// If the vector is sorted, this removes all duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![10, 20, 21, 30, 20];
    ///
    /// vec.dedup_by_key(|i| *i / 10);
    ///
    /// assert_eq!(vec, [10, 20, 30, 20]);
    /// ```
    #[stable(feature = "dedup_by", since = "1.16.0")]
    #[inline]
    pub fn dedup_by_key<F, K>(&mut self, mut key: F)
    where
        F: FnMut(&mut T) -> K,
        K: PartialEq,
    {
        self.dedup_by(|a, b| key(a) == key(b))
    }

    /// Removes all but the first of consecutive elements in the vector satisfying a given equality
    /// relation.
    ///
    /// The `same_bucket` function is passed references to two elements from the vector and
    /// must determine if the elements compare equal. The elements are passed in opposite order
    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
    ///
    /// If the vector is sorted, this removes all duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec!["foo", "bar", "Bar", "baz", "bar"];
    ///
    /// vec.dedup_by(|a, b| a.eq_ignore_ascii_case(b));
    ///
    /// assert_eq!(vec, ["foo", "bar", "baz", "bar"]);
    /// ```
    #[stable(feature = "dedup_by", since = "1.16.0")]
    pub fn dedup_by<F>(&mut self, mut same_bucket: F)
    where
        F: FnMut(&mut T, &mut T) -> bool,
    {
        let len = self.len();
        if len <= 1 {
            return;
        }

        /* INVARIANT: vec.len() > read >= write > write-1 >= 0 */
        struct FillGapOnDrop<'a, T, A: core::alloc::Allocator> {
            /* Offset of the element we want to check if it is duplicate */
            read: usize,

            /* Offset of the place where we want to place the non-duplicate
             * when we find it. */
            write: usize,

            /* The Vec that would need correction if `same_bucket` panicked */
            vec: &'a mut Vec<T, A>,
        }

        impl<'a, T, A: core::alloc::Allocator> Drop for FillGapOnDrop<'a, T, A> {
            fn drop(&mut self) {
                /* This code gets executed when `same_bucket` panics */

                /* SAFETY: invariant guarantees that `read - write`
                 * and `len - read` never overflow and that the copy is always
                 * in-bounds. */
                unsafe {
                    let ptr = self.vec.as_mut_ptr();
                    let len = self.vec.len();

                    /* How many items were left when `same_bucket` panicked.
                     * Basically vec[read..].len() */
                    let items_left = len.wrapping_sub(self.read);

                    /* Pointer to first item in vec[write..write+items_left] slice */
                    let dropped_ptr = ptr.add(self.write);
                    /* Pointer to first item in vec[read..] slice */
                    let valid_ptr = ptr.add(self.read);

                    /* Copy `vec[read..]` to `vec[write..write+items_left]`.
                     * The slices can overlap, so `copy_nonoverlapping` cannot be used */
                    ptr::copy(valid_ptr, dropped_ptr, items_left);

                    /* How many items have been already dropped
                     * Basically vec[read..write].len() */
                    let dropped = self.read.wrapping_sub(self.write);

                    self.vec.set_len(len - dropped);
                }
            }
        }

        let mut gap = FillGapOnDrop { read: 1, write: 1, vec: self };
        let ptr = gap.vec.as_mut_ptr();

        /* Drop items while going through Vec, it should be more efficient than
         * doing slice partition_dedup + truncate */

        /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
         * are always in-bounds and read_ptr never aliases prev_ptr */
        unsafe {
            while gap.read < len {
                let read_ptr = ptr.add(gap.read);
                let prev_ptr = ptr.add(gap.write.wrapping_sub(1));

                if same_bucket(&mut *read_ptr, &mut *prev_ptr) {
                    // Increase `gap.read` now since the drop may panic.
                    gap.read += 1;
                    /* We have found duplicate, drop it in-place */
                    ptr::drop_in_place(read_ptr);
                } else {
                    let write_ptr = ptr.add(gap.write);

                    /* Because `read_ptr` can be equal to `write_ptr`, we either
                     * have to use `copy` or conditional `copy_nonoverlapping`.
                     * Looks like the first option is faster. */
                    ptr::copy(read_ptr, write_ptr, 1);

                    /* We have filled that place, so go further */
                    gap.write += 1;
                    gap.read += 1;
                }
            }

            /* Technically we could let `gap` clean up with its Drop, but
             * when `same_bucket` is guaranteed to not panic, this bloats a little
             * the codegen, so we just do it manually */
            gap.vec.set_len(gap.write);
            mem::forget(gap);
        }
    }

    /// Appends an element to the back of a collection.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2];
    /// vec.push(3);
    /// assert_eq!(vec, [1, 2, 3]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push(&mut self, value: T) {
        // This will panic or abort if we would allocate > isize::MAX bytes
        // or if the length increment would overflow for zero-sized types.
        if self.len == self.buf.capacity() {
            self.buf.reserve_for_push(self.len);
        }
        unsafe {
            let end = self.as_mut_ptr().add(self.len);
            ptr::write(end, value);
            self.len += 1;
        }
    }

    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
    /// with the element.
    ///
    /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
    ///
    /// [`push`]: Vec::push
    /// [`reserve`]: Vec::reserve
    /// [`try_reserve`]: Vec::try_reserve
    ///
    /// # Examples
    ///
    /// A manual, panic-free alternative to [`FromIterator`]:
    ///
    /// ```
    /// #![feature(vec_push_within_capacity)]
    ///
    /// use std::collections::TryReserveError;
    /// fn from_iter_fallible<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
    ///     let mut vec = Vec::new();
    ///     for value in iter {
    ///         if let Err(value) = vec.push_within_capacity(value) {
    ///             vec.try_reserve(1)?;
    ///             // this cannot fail, the previous line either returned or added at least 1 free slot
    ///             let _ = vec.push_within_capacity(value);
    ///         }
    ///     }
    ///     Ok(vec)
    /// }
    /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
    /// ```
    #[inline]
    #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
        if self.len == self.buf.capacity() {
            return Err(value);
        }
        unsafe {
            let end = self.as_mut_ptr().add(self.len);
            ptr::write(end, value);
            self.len += 1;
        }
        Ok(())
    }

    /// Removes the last element from a vector and returns it, or [`None`] if it
    /// is empty.
    ///
    /// If you'd like to pop the first element, consider using
    /// [`VecDeque::pop_front`] instead.
    ///
    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// assert_eq!(vec.pop(), Some(3));
    /// assert_eq!(vec, [1, 2]);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop(&mut self) -> Option<T> {
        if self.len == 0 {
            None
        } else {
            unsafe {
                self.len -= 1;
                Some(ptr::read(self.as_ptr().add(self.len())))
            }
        }
    }

    /// Moves all the elements of `other` into `self`, leaving `other` empty.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// let mut vec2 = vec![4, 5, 6];
    /// vec.append(&mut vec2);
    /// assert_eq!(vec, [1, 2, 3, 4, 5, 6]);
    /// assert_eq!(vec2, []);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "append", since = "1.4.0")]
    pub fn append(&mut self, other: &mut Self) {
        unsafe {
            self.append_elements(other.as_slice() as _);
            other.set_len(0);
        }
    }

    /// Appends elements to `self` from other buffer.
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    unsafe fn append_elements(&mut self, other: *const [T]) {
        let count = unsafe { (*other).len() };
        self.reserve(count);
        let len = self.len();
        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
        self.len += count;
    }

    /// Removes the specified range from the vector in bulk, returning all
    /// removed elements as an iterator. If the iterator is dropped before
    /// being fully consumed, it drops the remaining removed elements.
    ///
    /// The returned iterator keeps a mutable borrow on the vector to optimize
    /// its implementation.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Leaking
    ///
    /// If the returned iterator goes out of scope without being dropped (due to
    /// [`mem::forget`], for example), the vector may have lost and leaked
    /// elements arbitrarily, including elements outside the range.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    /// let u: Vec<_> = v.drain(1..).collect();
    /// assert_eq!(v, &[1]);
    /// assert_eq!(u, &[2, 3]);
    ///
    /// // A full range clears the vector, like `clear()` does
    /// v.drain(..);
    /// assert_eq!(v, &[]);
    /// ```
    #[stable(feature = "drain", since = "1.6.0")]
    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
    where
        R: RangeBounds<usize>,
    {
        // Memory safety
        //
        // When the Drain is first created, it shortens the length of
        // the source vector to make sure no uninitialized or moved-from elements
        // are accessible at all if the Drain's destructor never gets to run.
        //
        // Drain will ptr::read out the values to remove.
        // When finished, remaining tail of the vec is copied back to cover
        // the hole, and the vector length is restored to the new length.
        //
        let len = self.len();
        let Range { start, end } = slice::range(range, ..len);

        unsafe {
            // set self.vec length's to start, to be safe in case Drain is leaked
            self.set_len(start);
            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
            Drain {
                tail_start: end,
                tail_len: len - end,
                iter: range_slice.iter(),
                vec: NonNull::from(self),
            }
        }
    }

    /// Clears the vector, removing all values.
    ///
    /// Note that this method has no effect on the allocated capacity
    /// of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    ///
    /// v.clear();
    ///
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        let elems: *mut [T] = self.as_mut_slice();

        // SAFETY:
        // - `elems` comes directly from `as_mut_slice` and is therefore valid.
        // - Setting `self.len` before calling `drop_in_place` means that,
        //   if an element's `Drop` impl panics, the vector's `Drop` impl will
        //   do nothing (leaking the rest of the elements) instead of dropping
        //   some twice.
        unsafe {
            self.len = 0;
            ptr::drop_in_place(elems);
        }
    }

    /// Returns the number of elements in the vector, also referred to
    /// as its 'length'.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = vec![1, 2, 3];
    /// assert_eq!(a.len(), 3);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize {
        self.len
    }

    /// Returns `true` if the vector contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = Vec::new();
    /// assert!(v.is_empty());
    ///
    /// v.push(1);
    /// assert!(!v.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Splits the collection into two at the given index.
    ///
    /// Returns a newly allocated vector containing the elements in the range
    /// `[at, len)`. After the call, the original vector will be left containing
    /// the elements `[0, at)` with its previous capacity unchanged.
    ///
    /// # Panics
    ///
    /// Panics if `at > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// let vec2 = vec.split_off(1);
    /// assert_eq!(vec, [1]);
    /// assert_eq!(vec2, [2, 3]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[must_use = "use `.truncate()` if you don't need the other half"]
    #[stable(feature = "split_off", since = "1.4.0")]
    pub fn split_off(&mut self, at: usize) -> Self
    where
        A: Clone,
    {
        #[cold]
        #[inline(never)]
        fn assert_failed(at: usize, len: usize) -> ! {
            panic!("`at` split index (is {at}) should be <= len (is {len})");
        }

        if at > self.len() {
            assert_failed(at, self.len());
        }

        if at == 0 {
            // the new vector can take over the original buffer and avoid the copy
            return mem::replace(
                self,
                Vec::with_capacity_in(self.capacity(), self.allocator().clone()),
            );
        }

        let other_len = self.len - at;
        let mut other = Vec::with_capacity_in(other_len, self.allocator().clone());

        // Unsafely `set_len` and copy items to `other`.
        unsafe {
            self.set_len(at);
            other.set_len(other_len);

            ptr::copy_nonoverlapping(self.as_ptr().add(at), other.as_mut_ptr(), other.len());
        }
        other
    }

    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `Vec` is extended by the
    /// difference, with each additional slot filled with the result of
    /// calling the closure `f`. The return values from `f` will end up
    /// in the `Vec` in the order they have been generated.
    ///
    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
    ///
    /// This method uses a closure to create new values on every push. If
    /// you'd rather [`Clone`] a given value, use [`Vec::resize`]. If you
    /// want to use the [`Default`] trait to generate values, you can
    /// pass [`Default::default`] as the second argument.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// vec.resize_with(5, Default::default);
    /// assert_eq!(vec, [1, 2, 3, 0, 0]);
    ///
    /// let mut vec = vec![];
    /// let mut p = 1;
    /// vec.resize_with(4, || { p *= 2; p });
    /// assert_eq!(vec, [2, 4, 8, 16]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "vec_resize_with", since = "1.33.0")]
    pub fn resize_with<F>(&mut self, new_len: usize, f: F)
    where
        F: FnMut() -> T,
    {
        let len = self.len();
        if new_len > len {
            self.extend_trusted(iter::repeat_with(f).take(new_len - len));
        } else {
            self.truncate(new_len);
        }
    }

    /// Consumes and leaks the `Vec`, returning a mutable reference to the contents,
    /// `&'a mut [T]`. Note that the type `T` must outlive the chosen lifetime
    /// `'a`. If the type has only static references, or none at all, then this
    /// may be chosen to be `'static`.
    ///
    /// As of Rust 1.57, this method does not reallocate or shrink the `Vec`,
    /// so the leaked allocation may include unused capacity that is not part
    /// of the returned slice.
    ///
    /// This function is mainly useful for data that lives for the remainder of
    /// the program's life. Dropping the returned reference will cause a memory
    /// leak.
    ///
    /// # Examples
    ///
    /// Simple usage:
    ///
    /// ```
    /// let x = vec![1, 2, 3];
    /// let static_ref: &'static mut [usize] = x.leak();
    /// static_ref[0] += 1;
    /// assert_eq!(static_ref, &[2, 2, 3]);
    /// ```
    #[stable(feature = "vec_leak", since = "1.47.0")]
    #[inline]
    pub fn leak<'a>(self) -> &'a mut [T]
    where
        A: 'a,
    {
        let mut me = ManuallyDrop::new(self);
        unsafe { slice::from_raw_parts_mut(me.as_mut_ptr(), me.len) }
    }

    /// Returns the remaining spare capacity of the vector as a slice of
    /// `MaybeUninit<T>`.
    ///
    /// The returned slice can be used to fill the vector with data (e.g. by
    /// reading from a file) before marking the data as initialized using the
    /// [`set_len`] method.
    ///
    /// [`set_len`]: Vec::set_len
    ///
    /// # Examples
    ///
    /// ```
    /// // Allocate vector big enough for 10 elements.
    /// let mut v = Vec::with_capacity(10);
    ///
    /// // Fill in the first 3 elements.
    /// let uninit = v.spare_capacity_mut();
    /// uninit[0].write(0);
    /// uninit[1].write(1);
    /// uninit[2].write(2);
    ///
    /// // Mark the first 3 elements of the vector as being initialized.
    /// unsafe {
    ///     v.set_len(3);
    /// }
    ///
    /// assert_eq!(&v, &[0, 1, 2]);
    /// ```
    #[stable(feature = "vec_spare_capacity", since = "1.60.0")]
    #[inline]
    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
        // Note:
        // This method is not implemented in terms of `split_at_spare_mut`,
        // to prevent invalidation of pointers to the buffer.
        unsafe {
            slice::from_raw_parts_mut(
                self.as_mut_ptr().add(self.len) as *mut MaybeUninit<T>,
                self.buf.capacity() - self.len,
            )
        }
    }

    /// Returns vector content as a slice of `T`, along with the remaining spare
    /// capacity of the vector as a slice of `MaybeUninit<T>`.
    ///
    /// The returned spare capacity slice can be used to fill the vector with data
    /// (e.g. by reading from a file) before marking the data as initialized using
    /// the [`set_len`] method.
    ///
    /// [`set_len`]: Vec::set_len
    ///
    /// Note that this is a low-level API, which should be used with care for
    /// optimization purposes. If you need to append data to a `Vec`
    /// you can use [`push`], [`extend`], [`extend_from_slice`],
    /// [`extend_from_within`], [`insert`], [`append`], [`resize`] or
    /// [`resize_with`], depending on your exact needs.
    ///
    /// [`push`]: Vec::push
    /// [`extend`]: Vec::extend
    /// [`extend_from_slice`]: Vec::extend_from_slice
    /// [`extend_from_within`]: Vec::extend_from_within
    /// [`insert`]: Vec::insert
    /// [`append`]: Vec::append
    /// [`resize`]: Vec::resize
    /// [`resize_with`]: Vec::resize_with
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(vec_split_at_spare)]
    ///
    /// let mut v = vec![1, 1, 2];
    ///
    /// // Reserve additional space big enough for 10 elements.
    /// v.reserve(10);
    ///
    /// let (init, uninit) = v.split_at_spare_mut();
    /// let sum = init.iter().copied().sum::<u32>();
    ///
    /// // Fill in the next 4 elements.
    /// uninit[0].write(sum);
    /// uninit[1].write(sum * 2);
    /// uninit[2].write(sum * 3);
    /// uninit[3].write(sum * 4);
    ///
    /// // Mark the 4 elements of the vector as being initialized.
    /// unsafe {
    ///     let len = v.len();
    ///     v.set_len(len + 4);
    /// }
    ///
    /// assert_eq!(&v, &[1, 1, 2, 4, 8, 12, 16]);
    /// ```
    #[unstable(feature = "vec_split_at_spare", issue = "81944")]
    #[inline]
    pub fn split_at_spare_mut(&mut self) -> (&mut [T], &mut [MaybeUninit<T>]) {
        // SAFETY:
        // - len is ignored and so never changed
        let (init, spare, _) = unsafe { self.split_at_spare_mut_with_len() };
        (init, spare)
    }

    /// Safety: changing returned .2 (&mut usize) is considered the same as calling `.set_len(_)`.
    ///
    /// This method provides unique access to all vec parts at once in `extend_from_within`.
    unsafe fn split_at_spare_mut_with_len(
        &mut self,
    ) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
        let ptr = self.as_mut_ptr();
        // SAFETY:
        // - `ptr` is guaranteed to be valid for `self.len` elements
        // - but the allocation extends out to `self.buf.capacity()` elements, possibly
        // uninitialized
        let spare_ptr = unsafe { ptr.add(self.len) };
        let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
        let spare_len = self.buf.capacity() - self.len;

        // SAFETY:
        // - `ptr` is guaranteed to be valid for `self.len` elements
        // - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
        unsafe {
            let initialized = slice::from_raw_parts_mut(ptr, self.len);
            let spare = slice::from_raw_parts_mut(spare_ptr, spare_len);

            (initialized, spare, &mut self.len)
        }
    }
}

impl<T: Clone, A: Allocator> Vec<T, A> {
    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `Vec` is extended by the
    /// difference, with each additional slot filled with `value`.
    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
    ///
    /// This method requires `T` to implement [`Clone`],
    /// in order to be able to clone the passed value.
    /// If you need more flexibility (or want to rely on [`Default`] instead of
    /// [`Clone`]), use [`Vec::resize_with`].
    /// If you only need to resize to a smaller size, use [`Vec::truncate`].
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec!["hello"];
    /// vec.resize(3, "world");
    /// assert_eq!(vec, ["hello", "world", "world"]);
    ///
    /// let mut vec = vec![1, 2, 3, 4];
    /// vec.resize(2, 0);
    /// assert_eq!(vec, [1, 2]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "vec_resize", since = "1.5.0")]
    pub fn resize(&mut self, new_len: usize, value: T) {
        let len = self.len();

        if new_len > len {
            self.extend_with(new_len - len, ExtendElement(value))
        } else {
            self.truncate(new_len);
        }
    }

    /// Clones and appends all elements in a slice to the `Vec`.
    ///
    /// Iterates over the slice `other`, clones each element, and then appends
    /// it to this `Vec`. The `other` slice is traversed in-order.
    ///
    /// Note that this function is same as [`extend`] except that it is
    /// specialized to work with slices instead. If and when Rust gets
    /// specialization this function will likely be deprecated (but still
    /// available).
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1];
    /// vec.extend_from_slice(&[2, 3, 4]);
    /// assert_eq!(vec, [1, 2, 3, 4]);
    /// ```
    ///
    /// [`extend`]: Vec::extend
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
    pub fn extend_from_slice(&mut self, other: &[T]) {
        self.spec_extend(other.iter())
    }

    /// Copies elements from `src` range to the end of the vector.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![0, 1, 2, 3, 4];
    ///
    /// vec.extend_from_within(2..);
    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4]);
    ///
    /// vec.extend_from_within(..2);
    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1]);
    ///
    /// vec.extend_from_within(4..8);
    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1, 4, 2, 3, 4]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "vec_extend_from_within", since = "1.53.0")]
    pub fn extend_from_within<R>(&mut self, src: R)
    where
        R: RangeBounds<usize>,
    {
        let range = slice::range(src, ..self.len());
        self.reserve(range.len());

        // SAFETY:
        // - `slice::range` guarantees that the given range is valid for indexing self
        unsafe {
            self.spec_extend_from_within(range);
        }
    }
}

impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
    /// Takes a `Vec<[T; N]>` and flattens it into a `Vec<T>`.
    ///
    /// # Panics
    ///
    /// Panics if the length of the resulting vector would overflow a `usize`.
    ///
    /// This is only possible when flattening a vector of arrays of zero-sized
    /// types, and thus tends to be irrelevant in practice. If
    /// `size_of::<T>() > 0`, this will never panic.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_flatten)]
    ///
    /// let mut vec = vec![[1, 2, 3], [4, 5, 6], [7, 8, 9]];
    /// assert_eq!(vec.pop(), Some([7, 8, 9]));
    ///
    /// let mut flattened = vec.into_flattened();
    /// assert_eq!(flattened.pop(), Some(6));
    /// ```
    #[unstable(feature = "slice_flatten", issue = "95629")]
    pub fn into_flattened(self) -> Vec<T, A> {
        let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
        let (new_len, new_cap) = if T::IS_ZST {
            (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
        } else {
            // SAFETY:
            // - `cap * N` cannot overflow because the allocation is already in
            // the address space.
            // - Each `[T; N]` has `N` valid elements, so there are `len * N`
            // valid elements in the allocation.
            unsafe { (len.unchecked_mul(N), cap.unchecked_mul(N)) }
        };
        // SAFETY:
        // - `ptr` was allocated by `self`
        // - `ptr` is well-aligned because `[T; N]` has the same alignment as `T`.
        // - `new_cap` refers to the same sized allocation as `cap` because
        // `new_cap * size_of::<T>()` == `cap * size_of::<[T; N]>()`
        // - `len` <= `cap`, so `len * N` <= `cap * N`.
        unsafe { Vec::<T, A>::from_raw_parts_in(ptr.cast(), new_len, new_cap, alloc) }
    }
}

// This code generalizes `extend_with_{element,default}`.
trait ExtendWith<T> {
    fn next(&mut self) -> T;
    fn last(self) -> T;
}

struct ExtendElement<T>(T);
impl<T: Clone> ExtendWith<T> for ExtendElement<T> {
    fn next(&mut self) -> T {
        self.0.clone()
    }
    fn last(self) -> T {
        self.0
    }
}

impl<T, A: Allocator> Vec<T, A> {
    #[cfg(not(no_global_oom_handling))]
    /// Extend the vector by `n` values, using the given generator.
    fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, mut value: E) {
        self.reserve(n);

        unsafe {
            let mut ptr = self.as_mut_ptr().add(self.len());
            // Use SetLenOnDrop to work around bug where compiler
            // might not realize the store through `ptr` through self.set_len()
            // don't alias.
            let mut local_len = SetLenOnDrop::new(&mut self.len);

            // Write all elements except the last one
            for _ in 1..n {
                ptr::write(ptr, value.next());
                ptr = ptr.add(1);
                // Increment the length in every step in case next() panics
                local_len.increment_len(1);
            }

            if n > 0 {
                // We can write the last element directly without cloning needlessly
                ptr::write(ptr, value.last());
                local_len.increment_len(1);
            }

            // len set by scope guard
        }
    }
}

impl<T: PartialEq, A: Allocator> Vec<T, A> {
    /// Removes consecutive repeated elements in the vector according to the
    /// [`PartialEq`] trait implementation.
    ///
    /// If the vector is sorted, this removes all duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec![1, 2, 2, 3, 2];
    ///
    /// vec.dedup();
    ///
    /// assert_eq!(vec, [1, 2, 3, 2]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn dedup(&mut self) {
        self.dedup_by(|a, b| a == b)
    }
}

////////////////////////////////////////////////////////////////////////////////
// Internal methods and functions
////////////////////////////////////////////////////////////////////////////////

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> {
    <T as SpecFromElem>::from_elem(elem, n, Global)
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn from_elem_in<T: Clone, A: Allocator>(elem: T, n: usize, alloc: A) -> Vec<T, A> {
    <T as SpecFromElem>::from_elem(elem, n, alloc)
}

trait ExtendFromWithinSpec {
    /// # Safety
    ///
    /// - `src` needs to be valid index
    /// - `self.capacity() - self.len()` must be `>= src.len()`
    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>);
}

impl<T: Clone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
    default unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
        // SAFETY:
        // - len is increased only after initializing elements
        let (this, spare, len) = unsafe { self.split_at_spare_mut_with_len() };

        // SAFETY:
        // - caller guarantees that src is a valid index
        let to_clone = unsafe { this.get_unchecked(src) };

        iter::zip(to_clone, spare)
            .map(|(src, dst)| dst.write(src.clone()))
            // Note:
            // - Element was just initialized with `MaybeUninit::write`, so it's ok to increase len
            // - len is increased after each element to prevent leaks (see issue #82533)
            .for_each(|_| *len += 1);
    }
}

impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
        let count = src.len();
        {
            let (init, spare) = self.split_at_spare_mut();

            // SAFETY:
            // - caller guarantees that `src` is a valid index
            let source = unsafe { init.get_unchecked(src) };

            // SAFETY:
            // - Both pointers are created from unique slice references (`&mut [_]`)
            //   so they are valid and do not overlap.
            // - Elements are :Copy so it's OK to copy them, without doing
            //   anything with the original values
            // - `count` is equal to the len of `source`, so source is valid for
            //   `count` reads
            // - `.reserve(count)` guarantees that `spare.len() >= count` so spare
            //   is valid for `count` writes
            unsafe { ptr::copy_nonoverlapping(source.as_ptr(), spare.as_mut_ptr() as _, count) };
        }

        // SAFETY:
        // - The elements were just initialized by `copy_nonoverlapping`
        self.len += count;
    }
}

////////////////////////////////////////////////////////////////////////////////
// Common trait implementations for Vec
////////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> ops::Deref for Vec<T, A> {
    type Target = [T];

    #[inline]
    fn deref(&self) -> &[T] {
        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
    #[inline]
    fn deref_mut(&mut self) -> &mut [T] {
        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Vec<T, A> {
    #[cfg(not(test))]
    fn clone(&self) -> Self {
        let alloc = self.allocator().clone();
        <[T]>::to_vec_in(&**self, alloc)
    }

    // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
    // required for this method definition, is not available. Instead use the
    // `slice::to_vec` function which is only available with cfg(test)
    // NB see the slice::hack module in slice.rs for more information
    #[cfg(test)]
    fn clone(&self) -> Self {
        let alloc = self.allocator().clone();
        crate::slice::to_vec(&**self, alloc)
    }

    fn clone_from(&mut self, other: &Self) {
        crate::slice::SpecCloneIntoVec::clone_into(other.as_slice(), self);
    }
}

/// The hash of a vector is the same as that of the corresponding slice,
/// as required by the `core::borrow::Borrow` implementation.
///
/// ```
/// #![feature(build_hasher_simple_hash_one)]
/// use std::hash::BuildHasher;
///
/// let b = std::collections::hash_map::RandomState::new();
/// let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
/// assert_eq!(b.hash_one(v), b.hash_one(s));
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Hash, A: Allocator> Hash for Vec<T, A> {
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        Hash::hash(&**self, state)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
    message = "vector indices are of type `usize` or ranges of `usize`",
    label = "vector indices are of type `usize` or ranges of `usize`"
)]
impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
    type Output = I::Output;

    #[inline]
    fn index(&self, index: I) -> &Self::Output {
        Index::index(&**self, index)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
    message = "vector indices are of type `usize` or ranges of `usize`",
    label = "vector indices are of type `usize` or ranges of `usize`"
)]
impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut Self::Output {
        IndexMut::index_mut(&mut **self, index)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> FromIterator<T> for Vec<T> {
    #[inline]
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
        <Self as SpecFromIter<T, I::IntoIter>>::from_iter(iter.into_iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> IntoIterator for Vec<T, A> {
    type Item = T;
    type IntoIter = IntoIter<T, A>;

    /// Creates a consuming iterator, that is, one that moves each value out of
    /// the vector (from start to end). The vector cannot be used after calling
    /// this.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = vec!["a".to_string(), "b".to_string()];
    /// let mut v_iter = v.into_iter();
    ///
    /// let first_element: Option<String> = v_iter.next();
    ///
    /// assert_eq!(first_element, Some("a".to_string()));
    /// assert_eq!(v_iter.next(), Some("b".to_string()));
    /// assert_eq!(v_iter.next(), None);
    /// ```
    #[inline]
    fn into_iter(self) -> Self::IntoIter {
        unsafe {
            let mut me = ManuallyDrop::new(self);
            let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
            let begin = me.as_mut_ptr();
            let end = if T::IS_ZST {
                begin.wrapping_byte_add(me.len())
            } else {
                begin.add(me.len()) as *const T
            };
            let cap = me.buf.capacity();
            IntoIter {
                buf: NonNull::new_unchecked(begin),
                phantom: PhantomData,
                cap,
                alloc,
                ptr: begin,
                end,
            }
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, A: Allocator> IntoIterator for &'a Vec<T, A> {
    type Item = &'a T;
    type IntoIter = slice::Iter<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, A: Allocator> IntoIterator for &'a mut Vec<T, A> {
    type Item = &'a mut T;
    type IntoIter = slice::IterMut<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter_mut()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> Extend<T> for Vec<T, A> {
    #[inline]
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
    }

    #[inline]
    fn extend_one(&mut self, item: T) {
        self.push(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

impl<T, A: Allocator> Vec<T, A> {
    // leaf method to which various SpecFrom/SpecExtend implementations delegate when
    // they have no further optimizations to apply
    #[cfg(not(no_global_oom_handling))]
    fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
        // This is the case for a general iterator.
        //
        // This function should be the moral equivalent of:
        //
        //      for item in iterator {
        //          self.push(item);
        //      }
        while let Some(element) = iterator.next() {
            let len = self.len();
            if len == self.capacity() {
                let (lower, _) = iterator.size_hint();
                self.reserve(lower.saturating_add(1));
            }
            unsafe {
                ptr::write(self.as_mut_ptr().add(len), element);
                // Since next() executes user code which can panic we have to bump the length
                // after each step.
                // NB can't overflow since we would have had to alloc the address space
                self.set_len(len + 1);
            }
        }
    }

    // specific extend for `TrustedLen` iterators, called both by the specializations
    // and internal places where resolving specialization makes compilation slower
    #[cfg(not(no_global_oom_handling))]
    fn extend_trusted(&mut self, iterator: impl iter::TrustedLen<Item = T>) {
        let (low, high) = iterator.size_hint();
        if let Some(additional) = high {
            debug_assert_eq!(
                low,
                additional,
                "TrustedLen iterator's size hint is not exact: {:?}",
                (low, high)
            );
            self.reserve(additional);
            unsafe {
                let ptr = self.as_mut_ptr();
                let mut local_len = SetLenOnDrop::new(&mut self.len);
                iterator.for_each(move |element| {
                    ptr::write(ptr.add(local_len.current_len()), element);
                    // Since the loop executes user code which can panic we have to update
                    // the length every step to correctly drop what we've written.
                    // NB can't overflow since we would have had to alloc the address space
                    local_len.increment_len(1);
                });
            }
        } else {
            // Per TrustedLen contract a `None` upper bound means that the iterator length
            // truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
            // Since the other branch already panics eagerly (via `reserve()`) we do the same here.
            // This avoids additional codegen for a fallback code path which would eventually
            // panic anyway.
            panic!("capacity overflow");
        }
    }

    /// Creates a splicing iterator that replaces the specified range in the vector
    /// with the given `replace_with` iterator and yields the removed items.
    /// `replace_with` does not need to be the same length as `range`.
    ///
    /// `range` is removed even if the iterator is not consumed until the end.
    ///
    /// It is unspecified how many elements are removed from the vector
    /// if the `Splice` value is leaked.
    ///
    /// The input iterator `replace_with` is only consumed when the `Splice` value is dropped.
    ///
    /// This is optimal if:
    ///
    /// * The tail (elements in the vector after `range`) is empty,
    /// * or `replace_with` yields fewer or equal elements than `range`’s length
    /// * or the lower bound of its `size_hint()` is exact.
    ///
    /// Otherwise, a temporary vector is allocated and the tail is moved twice.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = vec![1, 2, 3, 4];
    /// let new = [7, 8, 9];
    /// let u: Vec<_> = v.splice(1..3, new).collect();
    /// assert_eq!(v, &[1, 7, 8, 9, 4]);
    /// assert_eq!(u, &[2, 3]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "vec_splice", since = "1.21.0")]
    pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<'_, I::IntoIter, A>
    where
        R: RangeBounds<usize>,
        I: IntoIterator<Item = T>,
    {
        Splice { drain: self.drain(range), replace_with: replace_with.into_iter() }
    }

    /// Creates an iterator which uses a closure to determine if an element should be removed.
    ///
    /// If the closure returns true, then the element is removed and yielded.
    /// If the closure returns false, the element will remain in the vector and will not be yielded
    /// by the iterator.
    ///
    /// Using this method is equivalent to the following code:
    ///
    /// ```
    /// # let some_predicate = |x: &mut i32| { *x == 2 || *x == 3 || *x == 6 };
    /// # let mut vec = vec![1, 2, 3, 4, 5, 6];
    /// let mut i = 0;
    /// while i < vec.len() {
    ///     if some_predicate(&mut vec[i]) {
    ///         let val = vec.remove(i);
    ///         // your code here
    ///     } else {
    ///         i += 1;
    ///     }
    /// }
    ///
    /// # assert_eq!(vec, vec![1, 4, 5]);
    /// ```
    ///
    /// But `drain_filter` is easier to use. `drain_filter` is also more efficient,
    /// because it can backshift the elements of the array in bulk.
    ///
    /// Note that `drain_filter` also lets you mutate every element in the filter closure,
    /// regardless of whether you choose to keep or remove it.
    ///
    /// # Examples
    ///
    /// Splitting an array into evens and odds, reusing the original allocation:
    ///
    /// ```
    /// #![feature(drain_filter)]
    /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
    ///
    /// let evens = numbers.drain_filter(|x| *x % 2 == 0).collect::<Vec<_>>();
    /// let odds = numbers;
    ///
    /// assert_eq!(evens, vec![2, 4, 6, 8, 14]);
    /// assert_eq!(odds, vec![1, 3, 5, 9, 11, 13, 15]);
    /// ```
    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<'_, T, F, A>
    where
        F: FnMut(&mut T) -> bool,
    {
        let old_len = self.len();

        // Guard against us getting leaked (leak amplification)
        unsafe {
            self.set_len(0);
        }

        DrainFilter { vec: self, idx: 0, del: 0, old_len, pred: filter, panic_flag: false }
    }
}

/// Extend implementation that copies elements out of references before pushing them onto the Vec.
///
/// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
/// append the entire slice at once.
///
/// [`copy_from_slice`]: slice::copy_from_slice
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "extend_ref", since = "1.2.0")]
impl<'a, T: Copy + 'a, A: Allocator + 'a> Extend<&'a T> for Vec<T, A> {
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.spec_extend(iter.into_iter())
    }

    #[inline]
    fn extend_one(&mut self, &item: &'a T) {
        self.push(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

/// Implements comparison of vectors, [lexicographically](core::cmp::Ord#lexicographical-comparison).
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd, A: Allocator> PartialOrd for Vec<T, A> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        PartialOrd::partial_cmp(&**self, &**other)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Eq, A: Allocator> Eq for Vec<T, A> {}

/// Implements ordering of vectors, [lexicographically](core::cmp::Ord#lexicographical-comparison).
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord, A: Allocator> Ord for Vec<T, A> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        Ord::cmp(&**self, &**other)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
    fn drop(&mut self) {
        unsafe {
            // use drop for [T]
            // use a raw slice to refer to the elements of the vector as weakest necessary type;
            // could avoid questions of validity in certain cases
            ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
        }
        // RawVec handles deallocation
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for Vec<T> {
    /// Creates an empty `Vec<T>`.
    ///
    /// The vector will not allocate until elements are pushed onto it.
    fn default() -> Vec<T> {
        Vec::new()
    }
}

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> AsRef<Vec<T, A>> for Vec<T, A> {
    fn as_ref(&self) -> &Vec<T, A> {
        self
    }
}

#[stable(feature = "vec_as_mut", since = "1.5.0")]
impl<T, A: Allocator> AsMut<Vec<T, A>> for Vec<T, A> {
    fn as_mut(&mut self) -> &mut Vec<T, A> {
        self
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> AsRef<[T]> for Vec<T, A> {
    fn as_ref(&self) -> &[T] {
        self
    }
}

#[stable(feature = "vec_as_mut", since = "1.5.0")]
impl<T, A: Allocator> AsMut<[T]> for Vec<T, A> {
    fn as_mut(&mut self) -> &mut [T] {
        self
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> From<&[T]> for Vec<T> {
    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Vec::from(&[1, 2, 3][..]), vec![1, 2, 3]);
    /// ```
    #[cfg(not(test))]
    fn from(s: &[T]) -> Vec<T> {
        s.to_vec()
    }
    #[cfg(test)]
    fn from(s: &[T]) -> Vec<T> {
        crate::slice::to_vec(s, Global)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "vec_from_mut", since = "1.19.0")]
impl<T: Clone> From<&mut [T]> for Vec<T> {
    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Vec::from(&mut [1, 2, 3][..]), vec![1, 2, 3]);
    /// ```
    #[cfg(not(test))]
    fn from(s: &mut [T]) -> Vec<T> {
        s.to_vec()
    }
    #[cfg(test)]
    fn from(s: &mut [T]) -> Vec<T> {
        crate::slice::to_vec(s, Global)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "vec_from_array", since = "1.44.0")]
impl<T, const N: usize> From<[T; N]> for Vec<T> {
    /// Allocate a `Vec<T>` and move `s`'s items into it.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Vec::from([1, 2, 3]), vec![1, 2, 3]);
    /// ```
    #[cfg(not(test))]
    fn from(s: [T; N]) -> Vec<T> {
        <[T]>::into_vec(Box::new(s))
    }

    #[cfg(test)]
    fn from(s: [T; N]) -> Vec<T> {
        crate::slice::into_vec(Box::new(s))
    }
}

#[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
where
    [T]: ToOwned<Owned = Vec<T>>,
{
    /// Convert a clone-on-write slice into a vector.
    ///
    /// If `s` already owns a `Vec<T>`, it will be returned directly.
    /// If `s` is borrowing a slice, a new `Vec<T>` will be allocated and
    /// filled by cloning `s`'s items into it.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// let o: Cow<[i32]> = Cow::Owned(vec![1, 2, 3]);
    /// let b: Cow<[i32]> = Cow::Borrowed(&[1, 2, 3]);
    /// assert_eq!(Vec::from(o), Vec::from(b));
    /// ```
    fn from(s: Cow<'a, [T]>) -> Vec<T> {
        s.into_owned()
    }
}

// note: test pulls in std, which causes errors here
#[cfg(not(test))]
#[stable(feature = "vec_from_box", since = "1.18.0")]
impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
    /// Convert a boxed slice into a vector by transferring ownership of
    /// the existing heap allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
    /// ```
    fn from(s: Box<[T], A>) -> Self {
        s.into_vec()
    }
}

// note: test pulls in std, which causes errors here
#[cfg(not(no_global_oom_handling))]
#[cfg(not(test))]
#[stable(feature = "box_from_vec", since = "1.20.0")]
impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
    /// Convert a vector into a boxed slice.
    ///
    /// If `v` has excess capacity, its items will be moved into a
    /// newly-allocated buffer with exactly the right capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
    /// ```
    ///
    /// Any excess capacity is removed:
    /// ```
    /// let mut vec = Vec::with_capacity(10);
    /// vec.extend([1, 2, 3]);
    ///
    /// assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());
    /// ```
    fn from(v: Vec<T, A>) -> Self {
        v.into_boxed_slice()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl From<&str> for Vec<u8> {
    /// Allocate a `Vec<u8>` and fill it with a UTF-8 string.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);
    /// ```
    fn from(s: &str) -> Vec<u8> {
        From::from(s.as_bytes())
    }
}

#[stable(feature = "array_try_from_vec", since = "1.48.0")]
impl<T, A: Allocator, const N: usize> TryFrom<Vec<T, A>> for [T; N] {
    type Error = Vec<T, A>;

    /// Gets the entire contents of the `Vec<T>` as an array,
    /// if its size exactly matches that of the requested array.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(vec![1, 2, 3].try_into(), Ok([1, 2, 3]));
    /// assert_eq!(<Vec<i32>>::new().try_into(), Ok([]));
    /// ```
    ///
    /// If the length doesn't match, the input comes back in `Err`:
    /// ```
    /// let r: Result<[i32; 4], _> = (0..10).collect::<Vec<_>>().try_into();
    /// assert_eq!(r, Err(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
    /// ```
    ///
    /// If you're fine with just getting a prefix of the `Vec<T>`,
    /// you can call [`.truncate(N)`](Vec::truncate) first.
    /// ```
    /// let mut v = String::from("hello world").into_bytes();
    /// v.sort();
    /// v.truncate(2);
    /// let [a, b]: [_; 2] = v.try_into().unwrap();
    /// assert_eq!(a, b' ');
    /// assert_eq!(b, b'd');
    /// ```
    fn try_from(mut vec: Vec<T, A>) -> Result<[T; N], Vec<T, A>> {
        if vec.len() != N {
            return Err(vec);
        }

        // SAFETY: `.set_len(0)` is always sound.
        unsafe { vec.set_len(0) };

        // SAFETY: A `Vec`'s pointer is always aligned properly, and
        // the alignment the array needs is the same as the items.
        // We checked earlier that we have sufficient items.
        // The items will not double-drop as the `set_len`
        // tells the `Vec` not to also drop them.
        let array = unsafe { ptr::read(vec.as_ptr() as *const [T; N]) };
        Ok(array)
    }
}