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
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
use crate::array;
use crate::cmp::{self, Ordering};
use crate::num::NonZeroUsize;
use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};

use super::super::try_process;
use super::super::ByRefSized;
use super::super::TrustedRandomAccessNoCoerce;
use super::super::{ArrayChunks, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
use super::super::{FlatMap, Flatten};
use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
use super::super::{
    Inspect, Map, MapWhile, MapWindows, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take,
    TakeWhile,
};

fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}

/// A trait for dealing with iterators.
///
/// This is the main iterator trait. For more about the concept of iterators
/// generally, please see the [module-level documentation]. In particular, you
/// may want to know how to [implement `Iterator`][impl].
///
/// [module-level documentation]: crate::iter
/// [impl]: crate::iter#implementing-iterator
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
    on(
        _Self = "core::ops::range::RangeTo<Idx>",
        label = "if you meant to iterate until a value, add a starting value",
        note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
              bounded `Range`: `0..end`"
    ),
    on(
        _Self = "core::ops::range::RangeToInclusive<Idx>",
        label = "if you meant to iterate until a value (including it), add a starting value",
        note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
              to have a bounded `RangeInclusive`: `0..=end`"
    ),
    on(
        _Self = "[]",
        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
    ),
    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
    on(
        _Self = "alloc::vec::Vec<T, A>",
        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
    ),
    on(
        _Self = "&str",
        label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
    ),
    on(
        _Self = "alloc::string::String",
        label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
    ),
    on(
        _Self = "{integral}",
        note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
              syntax `start..end` or the inclusive range syntax `start..=end`"
    ),
    on(
        _Self = "{float}",
        note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
              syntax `start..end` or the inclusive range syntax `start..=end`"
    ),
    label = "`{Self}` is not an iterator",
    message = "`{Self}` is not an iterator"
)]
#[doc(notable_trait)]
#[lang = "iterator"]
#[rustc_diagnostic_item = "Iterator"]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub trait Iterator {
    /// The type of the elements being iterated over.
    #[rustc_diagnostic_item = "IteratorItem"]
    #[stable(feature = "rust1", since = "1.0.0")]
    type Item;

    /// Advances the iterator and returns the next value.
    ///
    /// Returns [`None`] when iteration is finished. Individual iterator
    /// implementations may choose to resume iteration, and so calling `next()`
    /// again may or may not eventually start returning [`Some(Item)`] again at some
    /// point.
    ///
    /// [`Some(Item)`]: Some
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// // A call to next() returns the next value...
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&2), iter.next());
    /// assert_eq!(Some(&3), iter.next());
    ///
    /// // ... and then None once it's over.
    /// assert_eq!(None, iter.next());
    ///
    /// // More calls may or may not return `None`. Here, they always will.
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next());
    /// ```
    #[lang = "next"]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn next(&mut self) -> Option<Self::Item>;

    /// Advances the iterator and returns an array containing the next `N` values.
    ///
    /// If there are not enough elements to fill the array then `Err` is returned
    /// containing an iterator over the remaining elements.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_next_chunk)]
    ///
    /// let mut iter = "lorem".chars();
    ///
    /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']);              // N is inferred as 2
    /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']);         // N is inferred as 3
    /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
    /// ```
    ///
    /// Split a string and get the first three items.
    ///
    /// ```
    /// #![feature(iter_next_chunk)]
    ///
    /// let quote = "not all those who wander are lost";
    /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
    /// assert_eq!(first, "not");
    /// assert_eq!(second, "all");
    /// assert_eq!(third, "those");
    /// ```
    #[inline]
    #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
    #[rustc_do_not_const_check]
    fn next_chunk<const N: usize>(
        &mut self,
    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
    where
        Self: Sized,
    {
        array::iter_next_chunk(self)
    }

    /// Returns the bounds on the remaining length of the iterator.
    ///
    /// Specifically, `size_hint()` returns a tuple where the first element
    /// is the lower bound, and the second element is the upper bound.
    ///
    /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
    /// A [`None`] here means that either there is no known upper bound, or the
    /// upper bound is larger than [`usize`].
    ///
    /// # Implementation notes
    ///
    /// It is not enforced that an iterator implementation yields the declared
    /// number of elements. A buggy iterator may yield less than the lower bound
    /// or more than the upper bound of elements.
    ///
    /// `size_hint()` is primarily intended to be used for optimizations such as
    /// reserving space for the elements of the iterator, but must not be
    /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
    /// implementation of `size_hint()` should not lead to memory safety
    /// violations.
    ///
    /// That said, the implementation should provide a correct estimation,
    /// because otherwise it would be a violation of the trait's protocol.
    ///
    /// The default implementation returns <code>(0, [None])</code> which is correct for any
    /// iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// let mut iter = a.iter();
    ///
    /// assert_eq!((3, Some(3)), iter.size_hint());
    /// let _ = iter.next();
    /// assert_eq!((2, Some(2)), iter.size_hint());
    /// ```
    ///
    /// A more complex example:
    ///
    /// ```
    /// // The even numbers in the range of zero to nine.
    /// let iter = (0..10).filter(|x| x % 2 == 0);
    ///
    /// // We might iterate from zero to ten times. Knowing that it's five
    /// // exactly wouldn't be possible without executing filter().
    /// assert_eq!((0, Some(10)), iter.size_hint());
    ///
    /// // Let's add five more numbers with chain()
    /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
    ///
    /// // now both bounds are increased by five
    /// assert_eq!((5, Some(15)), iter.size_hint());
    /// ```
    ///
    /// Returning `None` for an upper bound:
    ///
    /// ```
    /// // an infinite iterator has no upper bound
    /// // and the maximum possible lower bound
    /// let iter = 0..;
    ///
    /// assert_eq!((usize::MAX, None), iter.size_hint());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, None)
    }

    /// Consumes the iterator, counting the number of iterations and returning it.
    ///
    /// This method will call [`next`] repeatedly until [`None`] is encountered,
    /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
    /// called at least once even if the iterator does not have any elements.
    ///
    /// [`next`]: Iterator::next
    ///
    /// # Overflow Behavior
    ///
    /// The method does no guarding against overflows, so counting elements of
    /// an iterator with more than [`usize::MAX`] elements either produces the
    /// wrong result or panics. If debug assertions are enabled, a panic is
    /// guaranteed.
    ///
    /// # Panics
    ///
    /// This function might panic if the iterator has more than [`usize::MAX`]
    /// elements.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().count(), 3);
    ///
    /// let a = [1, 2, 3, 4, 5];
    /// assert_eq!(a.iter().count(), 5);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn count(self) -> usize
    where
        Self: Sized,
    {
        self.fold(
            0,
            #[rustc_inherit_overflow_checks]
            |count, _| count + 1,
        )
    }

    /// Consumes the iterator, returning the last element.
    ///
    /// This method will evaluate the iterator until it returns [`None`]. While
    /// doing so, it keeps track of the current element. After [`None`] is
    /// returned, `last()` will then return the last element it saw.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().last(), Some(&3));
    ///
    /// let a = [1, 2, 3, 4, 5];
    /// assert_eq!(a.iter().last(), Some(&5));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn last(self) -> Option<Self::Item>
    where
        Self: Sized,
    {
        #[inline]
        fn some<T>(_: Option<T>, x: T) -> Option<T> {
            Some(x)
        }

        self.fold(None, some)
    }

    /// Advances the iterator by `n` elements.
    ///
    /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
    /// times until [`None`] is encountered.
    ///
    /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
    /// `n` elements, or a `Err(NonZeroUsize)` with value `k` if [`None`] is encountered,
    /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
    /// Otherwise, `k` is always less than `n`.
    ///
    /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
    /// can advance its outer iterator until it finds an inner iterator that is not empty, which
    /// then often allows it to return a more accurate `size_hint()` than in its initial state.
    ///
    /// [`Flatten`]: crate::iter::Flatten
    /// [`next`]: Iterator::next
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_advance_by)]
    ///
    /// use std::num::NonZeroUsize;
    /// let a = [1, 2, 3, 4];
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.advance_by(2), Ok(()));
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.advance_by(0), Ok(()));
    /// assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(99).unwrap())); // only `&4` was skipped
    /// ```
    #[inline]
    #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
    #[rustc_do_not_const_check]
    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        for i in 0..n {
            if self.next().is_none() {
                // SAFETY: `i` is always less than `n`.
                return Err(unsafe { NonZeroUsize::new_unchecked(n - i) });
            }
        }
        Ok(())
    }

    /// Returns the `n`th element of the iterator.
    ///
    /// Like most indexing operations, the count starts from zero, so `nth(0)`
    /// returns the first value, `nth(1)` the second, and so on.
    ///
    /// Note that all preceding elements, as well as the returned element, will be
    /// consumed from the iterator. That means that the preceding elements will be
    /// discarded, and also that calling `nth(0)` multiple times on the same iterator
    /// will return different elements.
    ///
    /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
    /// iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth(1), Some(&2));
    /// ```
    ///
    /// Calling `nth()` multiple times doesn't rewind the iterator:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.nth(1), Some(&2));
    /// assert_eq!(iter.nth(1), None);
    /// ```
    ///
    /// Returning `None` if there are less than `n + 1` elements:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.iter().nth(10), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        self.advance_by(n).ok()?;
        self.next()
    }

    /// Creates an iterator starting at the same point, but stepping by
    /// the given amount at each iteration.
    ///
    /// Note 1: The first element of the iterator will always be returned,
    /// regardless of the step given.
    ///
    /// Note 2: The time at which ignored elements are pulled is not fixed.
    /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
    /// `self.nth(step-1)`, …, but is also free to behave like the sequence
    /// `advance_n_and_return_first(&mut self, step)`,
    /// `advance_n_and_return_first(&mut self, step)`, …
    /// Which way is used may change for some iterators for performance reasons.
    /// The second way will advance the iterator earlier and may consume more items.
    ///
    /// `advance_n_and_return_first` is the equivalent of:
    /// ```
    /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
    /// where
    ///     I: Iterator,
    /// {
    ///     let next = iter.next();
    ///     if n > 1 {
    ///         iter.nth(n - 2);
    ///     }
    ///     next
    /// }
    /// ```
    ///
    /// # Panics
    ///
    /// The method will panic if the given step is `0`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [0, 1, 2, 3, 4, 5];
    /// let mut iter = a.iter().step_by(2);
    ///
    /// assert_eq!(iter.next(), Some(&0));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), Some(&4));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    #[rustc_do_not_const_check]
    fn step_by(self, step: usize) -> StepBy<Self>
    where
        Self: Sized,
    {
        StepBy::new(self, step)
    }

    /// Takes two iterators and creates a new iterator over both in sequence.
    ///
    /// `chain()` will return a new iterator which will first iterate over
    /// values from the first iterator and then over values from the second
    /// iterator.
    ///
    /// In other words, it links two iterators together, in a chain. 🔗
    ///
    /// [`once`] is commonly used to adapt a single value into a chain of
    /// other kinds of iteration.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a1 = [1, 2, 3];
    /// let a2 = [4, 5, 6];
    ///
    /// let mut iter = a1.iter().chain(a2.iter());
    ///
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.next(), Some(&4));
    /// assert_eq!(iter.next(), Some(&5));
    /// assert_eq!(iter.next(), Some(&6));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
    /// anything that can be converted into an [`Iterator`], not just an
    /// [`Iterator`] itself. For example, slices (`&[T]`) implement
    /// [`IntoIterator`], and so can be passed to `chain()` directly:
    ///
    /// ```
    /// let s1 = &[1, 2, 3];
    /// let s2 = &[4, 5, 6];
    ///
    /// let mut iter = s1.iter().chain(s2);
    ///
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.next(), Some(&4));
    /// assert_eq!(iter.next(), Some(&5));
    /// assert_eq!(iter.next(), Some(&6));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
    ///
    /// ```
    /// #[cfg(windows)]
    /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
    ///     use std::os::windows::ffi::OsStrExt;
    ///     s.encode_wide().chain(std::iter::once(0)).collect()
    /// }
    /// ```
    ///
    /// [`once`]: crate::iter::once
    /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
    where
        Self: Sized,
        U: IntoIterator<Item = Self::Item>,
    {
        Chain::new(self, other.into_iter())
    }

    /// 'Zips up' two iterators into a single iterator of pairs.
    ///
    /// `zip()` returns a new iterator that will iterate over two other
    /// iterators, returning a tuple where the first element comes from the
    /// first iterator, and the second element comes from the second iterator.
    ///
    /// In other words, it zips two iterators together, into a single one.
    ///
    /// If either iterator returns [`None`], [`next`] from the zipped iterator
    /// will return [`None`].
    /// If the zipped iterator has no more elements to return then each further attempt to advance
    /// it will first try to advance the first iterator at most one time and if it still yielded an item
    /// try to advance the second iterator at most one time.
    ///
    /// To 'undo' the result of zipping up two iterators, see [`unzip`].
    ///
    /// [`unzip`]: Iterator::unzip
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a1 = [1, 2, 3];
    /// let a2 = [4, 5, 6];
    ///
    /// let mut iter = a1.iter().zip(a2.iter());
    ///
    /// assert_eq!(iter.next(), Some((&1, &4)));
    /// assert_eq!(iter.next(), Some((&2, &5)));
    /// assert_eq!(iter.next(), Some((&3, &6)));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
    /// anything that can be converted into an [`Iterator`], not just an
    /// [`Iterator`] itself. For example, slices (`&[T]`) implement
    /// [`IntoIterator`], and so can be passed to `zip()` directly:
    ///
    /// ```
    /// let s1 = &[1, 2, 3];
    /// let s2 = &[4, 5, 6];
    ///
    /// let mut iter = s1.iter().zip(s2);
    ///
    /// assert_eq!(iter.next(), Some((&1, &4)));
    /// assert_eq!(iter.next(), Some((&2, &5)));
    /// assert_eq!(iter.next(), Some((&3, &6)));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// `zip()` is often used to zip an infinite iterator to a finite one.
    /// This works because the finite iterator will eventually return [`None`],
    /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
    ///
    /// ```
    /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
    ///
    /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
    ///
    /// assert_eq!((0, 'f'), enumerate[0]);
    /// assert_eq!((0, 'f'), zipper[0]);
    ///
    /// assert_eq!((1, 'o'), enumerate[1]);
    /// assert_eq!((1, 'o'), zipper[1]);
    ///
    /// assert_eq!((2, 'o'), enumerate[2]);
    /// assert_eq!((2, 'o'), zipper[2]);
    /// ```
    ///
    /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
    ///
    /// ```
    /// use std::iter::zip;
    ///
    /// let a = [1, 2, 3];
    /// let b = [2, 3, 4];
    ///
    /// let mut zipped = zip(
    ///     a.into_iter().map(|x| x * 2).skip(1),
    ///     b.into_iter().map(|x| x * 2).skip(1),
    /// );
    ///
    /// assert_eq!(zipped.next(), Some((4, 6)));
    /// assert_eq!(zipped.next(), Some((6, 8)));
    /// assert_eq!(zipped.next(), None);
    /// ```
    ///
    /// compared to:
    ///
    /// ```
    /// # let a = [1, 2, 3];
    /// # let b = [2, 3, 4];
    /// #
    /// let mut zipped = a
    ///     .into_iter()
    ///     .map(|x| x * 2)
    ///     .skip(1)
    ///     .zip(b.into_iter().map(|x| x * 2).skip(1));
    /// #
    /// # assert_eq!(zipped.next(), Some((4, 6)));
    /// # assert_eq!(zipped.next(), Some((6, 8)));
    /// # assert_eq!(zipped.next(), None);
    /// ```
    ///
    /// [`enumerate`]: Iterator::enumerate
    /// [`next`]: Iterator::next
    /// [`zip`]: crate::iter::zip
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
    where
        Self: Sized,
        U: IntoIterator,
    {
        Zip::new(self, other.into_iter())
    }

    /// Creates a new iterator which places a copy of `separator` between adjacent
    /// items of the original iterator.
    ///
    /// In case `separator` does not implement [`Clone`] or needs to be
    /// computed every time, use [`intersperse_with`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_intersperse)]
    ///
    /// let mut a = [0, 1, 2].iter().intersperse(&100);
    /// assert_eq!(a.next(), Some(&0));   // The first element from `a`.
    /// assert_eq!(a.next(), Some(&100)); // The separator.
    /// assert_eq!(a.next(), Some(&1));   // The next element from `a`.
    /// assert_eq!(a.next(), Some(&100)); // The separator.
    /// assert_eq!(a.next(), Some(&2));   // The last element from `a`.
    /// assert_eq!(a.next(), None);       // The iterator is finished.
    /// ```
    ///
    /// `intersperse` can be very useful to join an iterator's items using a common element:
    /// ```
    /// #![feature(iter_intersperse)]
    ///
    /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
    /// assert_eq!(hello, "Hello World !");
    /// ```
    ///
    /// [`Clone`]: crate::clone::Clone
    /// [`intersperse_with`]: Iterator::intersperse_with
    #[inline]
    #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
    #[rustc_do_not_const_check]
    fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
    where
        Self: Sized,
        Self::Item: Clone,
    {
        Intersperse::new(self, separator)
    }

    /// Creates a new iterator which places an item generated by `separator`
    /// between adjacent items of the original iterator.
    ///
    /// The closure will be called exactly once each time an item is placed
    /// between two adjacent items from the underlying iterator; specifically,
    /// the closure is not called if the underlying iterator yields less than
    /// two items and after the last item is yielded.
    ///
    /// If the iterator's item implements [`Clone`], it may be easier to use
    /// [`intersperse`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_intersperse)]
    ///
    /// #[derive(PartialEq, Debug)]
    /// struct NotClone(usize);
    ///
    /// let v = [NotClone(0), NotClone(1), NotClone(2)];
    /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
    ///
    /// assert_eq!(it.next(), Some(NotClone(0)));  // The first element from `v`.
    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
    /// assert_eq!(it.next(), Some(NotClone(1)));  // The next element from `v`.
    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
    /// assert_eq!(it.next(), Some(NotClone(2)));  // The last element from `v`.
    /// assert_eq!(it.next(), None);               // The iterator is finished.
    /// ```
    ///
    /// `intersperse_with` can be used in situations where the separator needs
    /// to be computed:
    /// ```
    /// #![feature(iter_intersperse)]
    ///
    /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
    ///
    /// // The closure mutably borrows its context to generate an item.
    /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
    /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
    ///
    /// let result = src.intersperse_with(separator).collect::<String>();
    /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
    /// ```
    /// [`Clone`]: crate::clone::Clone
    /// [`intersperse`]: Iterator::intersperse
    #[inline]
    #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
    #[rustc_do_not_const_check]
    fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
    where
        Self: Sized,
        G: FnMut() -> Self::Item,
    {
        IntersperseWith::new(self, separator)
    }

    /// Takes a closure and creates an iterator which calls that closure on each
    /// element.
    ///
    /// `map()` transforms one iterator into another, by means of its argument:
    /// something that implements [`FnMut`]. It produces a new iterator which
    /// calls this closure on each element of the original iterator.
    ///
    /// If you are good at thinking in types, you can think of `map()` like this:
    /// If you have an iterator that gives you elements of some type `A`, and
    /// you want an iterator of some other type `B`, you can use `map()`,
    /// passing a closure that takes an `A` and returns a `B`.
    ///
    /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
    /// lazy, it is best used when you're already working with other iterators.
    /// If you're doing some sort of looping for a side effect, it's considered
    /// more idiomatic to use [`for`] than `map()`.
    ///
    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter().map(|x| 2 * x);
    ///
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(4));
    /// assert_eq!(iter.next(), Some(6));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
    ///
    /// ```
    /// # #![allow(unused_must_use)]
    /// // don't do this:
    /// (0..5).map(|x| println!("{x}"));
    ///
    /// // it won't even execute, as it is lazy. Rust will warn you about this.
    ///
    /// // Instead, use for:
    /// for x in 0..5 {
    ///     println!("{x}");
    /// }
    /// ```
    #[rustc_diagnostic_item = "IteratorMap"]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn map<B, F>(self, f: F) -> Map<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item) -> B,
    {
        Map::new(self, f)
    }

    /// Calls a closure on each element of an iterator.
    ///
    /// This is equivalent to using a [`for`] loop on the iterator, although
    /// `break` and `continue` are not possible from a closure. It's generally
    /// more idiomatic to use a `for` loop, but `for_each` may be more legible
    /// when processing items at the end of longer iterator chains. In some
    /// cases `for_each` may also be faster than a loop, because it will use
    /// internal iteration on adapters like `Chain`.
    ///
    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::sync::mpsc::channel;
    ///
    /// let (tx, rx) = channel();
    /// (0..5).map(|x| x * 2 + 1)
    ///       .for_each(move |x| tx.send(x).unwrap());
    ///
    /// let v: Vec<_> = rx.iter().collect();
    /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
    /// ```
    ///
    /// For such a small example, a `for` loop may be cleaner, but `for_each`
    /// might be preferable to keep a functional style with longer iterators:
    ///
    /// ```
    /// (0..5).flat_map(|x| x * 100 .. x * 110)
    ///       .enumerate()
    ///       .filter(|&(i, x)| (i + x) % 3 == 0)
    ///       .for_each(|(i, x)| println!("{i}:{x}"));
    /// ```
    #[inline]
    #[stable(feature = "iterator_for_each", since = "1.21.0")]
    #[rustc_do_not_const_check]
    fn for_each<F>(self, f: F)
    where
        Self: Sized,
        F: FnMut(Self::Item),
    {
        #[inline]
        fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
            move |(), item| f(item)
        }

        self.fold((), call(f));
    }

    /// Creates an iterator which uses a closure to determine if an element
    /// should be yielded.
    ///
    /// Given an element the closure must return `true` or `false`. The returned
    /// iterator will yield only the elements for which the closure returns
    /// true.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [0i32, 1, 2];
    ///
    /// let mut iter = a.iter().filter(|x| x.is_positive());
    ///
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Because the closure passed to `filter()` takes a reference, and many
    /// iterators iterate over references, this leads to a possibly confusing
    /// situation, where the type of the closure is a double reference:
    ///
    /// ```
    /// let a = [0, 1, 2];
    ///
    /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
    ///
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// It's common to instead use destructuring on the argument to strip away
    /// one:
    ///
    /// ```
    /// let a = [0, 1, 2];
    ///
    /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
    ///
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// or both:
    ///
    /// ```
    /// let a = [0, 1, 2];
    ///
    /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
    ///
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// of these layers.
    ///
    /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn filter<P>(self, predicate: P) -> Filter<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        Filter::new(self, predicate)
    }

    /// Creates an iterator that both filters and maps.
    ///
    /// The returned iterator yields only the `value`s for which the supplied
    /// closure returns `Some(value)`.
    ///
    /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
    /// concise. The example below shows how a `map().filter().map()` can be
    /// shortened to a single call to `filter_map`.
    ///
    /// [`filter`]: Iterator::filter
    /// [`map`]: Iterator::map
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = ["1", "two", "NaN", "four", "5"];
    ///
    /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
    ///
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(5));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Here's the same example, but with [`filter`] and [`map`]:
    ///
    /// ```
    /// let a = ["1", "two", "NaN", "four", "5"];
    /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(5));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item) -> Option<B>,
    {
        FilterMap::new(self, f)
    }

    /// Creates an iterator which gives the current iteration count as well as
    /// the next value.
    ///
    /// The iterator returned yields pairs `(i, val)`, where `i` is the
    /// current index of iteration and `val` is the value returned by the
    /// iterator.
    ///
    /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
    /// different sized integer, the [`zip`] function provides similar
    /// functionality.
    ///
    /// # Overflow Behavior
    ///
    /// The method does no guarding against overflows, so enumerating more than
    /// [`usize::MAX`] elements either produces the wrong result or panics. If
    /// debug assertions are enabled, a panic is guaranteed.
    ///
    /// # Panics
    ///
    /// The returned iterator might panic if the to-be-returned index would
    /// overflow a [`usize`].
    ///
    /// [`zip`]: Iterator::zip
    ///
    /// # Examples
    ///
    /// ```
    /// let a = ['a', 'b', 'c'];
    ///
    /// let mut iter = a.iter().enumerate();
    ///
    /// assert_eq!(iter.next(), Some((0, &'a')));
    /// assert_eq!(iter.next(), Some((1, &'b')));
    /// assert_eq!(iter.next(), Some((2, &'c')));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn enumerate(self) -> Enumerate<Self>
    where
        Self: Sized,
    {
        Enumerate::new(self)
    }

    /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
    /// to look at the next element of the iterator without consuming it. See
    /// their documentation for more information.
    ///
    /// Note that the underlying iterator is still advanced when [`peek`] or
    /// [`peek_mut`] are called for the first time: In order to retrieve the
    /// next element, [`next`] is called on the underlying iterator, hence any
    /// side effects (i.e. anything other than fetching the next value) of
    /// the [`next`] method will occur.
    ///
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let xs = [1, 2, 3];
    ///
    /// let mut iter = xs.iter().peekable();
    ///
    /// // peek() lets us see into the future
    /// assert_eq!(iter.peek(), Some(&&1));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// assert_eq!(iter.next(), Some(&2));
    ///
    /// // we can peek() multiple times, the iterator won't advance
    /// assert_eq!(iter.peek(), Some(&&3));
    /// assert_eq!(iter.peek(), Some(&&3));
    ///
    /// assert_eq!(iter.next(), Some(&3));
    ///
    /// // after the iterator is finished, so is peek()
    /// assert_eq!(iter.peek(), None);
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Using [`peek_mut`] to mutate the next item without advancing the
    /// iterator:
    ///
    /// ```
    /// let xs = [1, 2, 3];
    ///
    /// let mut iter = xs.iter().peekable();
    ///
    /// // `peek_mut()` lets us see into the future
    /// assert_eq!(iter.peek_mut(), Some(&mut &1));
    /// assert_eq!(iter.peek_mut(), Some(&mut &1));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// if let Some(mut p) = iter.peek_mut() {
    ///     assert_eq!(*p, &2);
    ///     // put a value into the iterator
    ///     *p = &1000;
    /// }
    ///
    /// // The value reappears as the iterator continues
    /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
    /// ```
    /// [`peek`]: Peekable::peek
    /// [`peek_mut`]: Peekable::peek_mut
    /// [`next`]: Iterator::next
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn peekable(self) -> Peekable<Self>
    where
        Self: Sized,
    {
        Peekable::new(self)
    }

    /// Creates an iterator that [`skip`]s elements based on a predicate.
    ///
    /// [`skip`]: Iterator::skip
    ///
    /// `skip_while()` takes a closure as an argument. It will call this
    /// closure on each element of the iterator, and ignore elements
    /// until it returns `false`.
    ///
    /// After `false` is returned, `skip_while()`'s job is over, and the
    /// rest of the elements are yielded.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [-1i32, 0, 1];
    ///
    /// let mut iter = a.iter().skip_while(|x| x.is_negative());
    ///
    /// assert_eq!(iter.next(), Some(&0));
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Because the closure passed to `skip_while()` takes a reference, and many
    /// iterators iterate over references, this leads to a possibly confusing
    /// situation, where the type of the closure argument is a double reference:
    ///
    /// ```
    /// let a = [-1, 0, 1];
    ///
    /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
    ///
    /// assert_eq!(iter.next(), Some(&0));
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Stopping after an initial `false`:
    ///
    /// ```
    /// let a = [-1, 0, 1, -2];
    ///
    /// let mut iter = a.iter().skip_while(|x| **x < 0);
    ///
    /// assert_eq!(iter.next(), Some(&0));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// // while this would have been false, since we already got a false,
    /// // skip_while() isn't used any more
    /// assert_eq!(iter.next(), Some(&-2));
    ///
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[doc(alias = "drop_while")]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        SkipWhile::new(self, predicate)
    }

    /// Creates an iterator that yields elements based on a predicate.
    ///
    /// `take_while()` takes a closure as an argument. It will call this
    /// closure on each element of the iterator, and yield elements
    /// while it returns `true`.
    ///
    /// After `false` is returned, `take_while()`'s job is over, and the
    /// rest of the elements are ignored.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [-1i32, 0, 1];
    ///
    /// let mut iter = a.iter().take_while(|x| x.is_negative());
    ///
    /// assert_eq!(iter.next(), Some(&-1));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Because the closure passed to `take_while()` takes a reference, and many
    /// iterators iterate over references, this leads to a possibly confusing
    /// situation, where the type of the closure is a double reference:
    ///
    /// ```
    /// let a = [-1, 0, 1];
    ///
    /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
    ///
    /// assert_eq!(iter.next(), Some(&-1));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Stopping after an initial `false`:
    ///
    /// ```
    /// let a = [-1, 0, 1, -2];
    ///
    /// let mut iter = a.iter().take_while(|x| **x < 0);
    ///
    /// assert_eq!(iter.next(), Some(&-1));
    ///
    /// // We have more elements that are less than zero, but since we already
    /// // got a false, take_while() isn't used any more
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Because `take_while()` needs to look at the value in order to see if it
    /// should be included or not, consuming iterators will see that it is
    /// removed:
    ///
    /// ```
    /// let a = [1, 2, 3, 4];
    /// let mut iter = a.iter();
    ///
    /// let result: Vec<i32> = iter.by_ref()
    ///                            .take_while(|n| **n != 3)
    ///                            .cloned()
    ///                            .collect();
    ///
    /// assert_eq!(result, &[1, 2]);
    ///
    /// let result: Vec<i32> = iter.cloned().collect();
    ///
    /// assert_eq!(result, &[4]);
    /// ```
    ///
    /// The `3` is no longer there, because it was consumed in order to see if
    /// the iteration should stop, but wasn't placed back into the iterator.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        TakeWhile::new(self, predicate)
    }

    /// Creates an iterator that both yields elements based on a predicate and maps.
    ///
    /// `map_while()` takes a closure as an argument. It will call this
    /// closure on each element of the iterator, and yield elements
    /// while it returns [`Some(_)`][`Some`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [-1i32, 4, 0, 1];
    ///
    /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
    ///
    /// assert_eq!(iter.next(), Some(-16));
    /// assert_eq!(iter.next(), Some(4));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Here's the same example, but with [`take_while`] and [`map`]:
    ///
    /// [`take_while`]: Iterator::take_while
    /// [`map`]: Iterator::map
    ///
    /// ```
    /// let a = [-1i32, 4, 0, 1];
    ///
    /// let mut iter = a.iter()
    ///                 .map(|x| 16i32.checked_div(*x))
    ///                 .take_while(|x| x.is_some())
    ///                 .map(|x| x.unwrap());
    ///
    /// assert_eq!(iter.next(), Some(-16));
    /// assert_eq!(iter.next(), Some(4));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Stopping after an initial [`None`]:
    ///
    /// ```
    /// let a = [0, 1, 2, -3, 4, 5, -6];
    ///
    /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
    /// let vec = iter.collect::<Vec<_>>();
    ///
    /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
    /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
    /// assert_eq!(vec, vec![0, 1, 2]);
    /// ```
    ///
    /// Because `map_while()` needs to look at the value in order to see if it
    /// should be included or not, consuming iterators will see that it is
    /// removed:
    ///
    /// ```
    /// let a = [1, 2, -3, 4];
    /// let mut iter = a.iter();
    ///
    /// let result: Vec<u32> = iter.by_ref()
    ///                            .map_while(|n| u32::try_from(*n).ok())
    ///                            .collect();
    ///
    /// assert_eq!(result, &[1, 2]);
    ///
    /// let result: Vec<i32> = iter.cloned().collect();
    ///
    /// assert_eq!(result, &[4]);
    /// ```
    ///
    /// The `-3` is no longer there, because it was consumed in order to see if
    /// the iteration should stop, but wasn't placed back into the iterator.
    ///
    /// Note that unlike [`take_while`] this iterator is **not** fused.
    /// It is also not specified what this iterator returns after the first [`None`] is returned.
    /// If you need fused iterator, use [`fuse`].
    ///
    /// [`fuse`]: Iterator::fuse
    #[inline]
    #[stable(feature = "iter_map_while", since = "1.57.0")]
    #[rustc_do_not_const_check]
    fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
    where
        Self: Sized,
        P: FnMut(Self::Item) -> Option<B>,
    {
        MapWhile::new(self, predicate)
    }

    /// Creates an iterator that skips the first `n` elements.
    ///
    /// `skip(n)` skips elements until `n` elements are skipped or the end of the
    /// iterator is reached (whichever happens first). After that, all the remaining
    /// elements are yielded. In particular, if the original iterator is too short,
    /// then the returned iterator is empty.
    ///
    /// Rather than overriding this method directly, instead override the `nth` method.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter().skip(2);
    ///
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn skip(self, n: usize) -> Skip<Self>
    where
        Self: Sized,
    {
        Skip::new(self, n)
    }

    /// Creates an iterator that yields the first `n` elements, or fewer
    /// if the underlying iterator ends sooner.
    ///
    /// `take(n)` yields elements until `n` elements are yielded or the end of
    /// the iterator is reached (whichever happens first).
    /// The returned iterator is a prefix of length `n` if the original iterator
    /// contains at least `n` elements, otherwise it contains all of the
    /// (fewer than `n`) elements of the original iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter().take(2);
    ///
    /// assert_eq!(iter.next(), Some(&1));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// `take()` is often used with an infinite iterator, to make it finite:
    ///
    /// ```
    /// let mut iter = (0..).take(3);
    ///
    /// assert_eq!(iter.next(), Some(0));
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// If less than `n` elements are available,
    /// `take` will limit itself to the size of the underlying iterator:
    ///
    /// ```
    /// let v = [1, 2];
    /// let mut iter = v.into_iter().take(5);
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn take(self, n: usize) -> Take<Self>
    where
        Self: Sized,
    {
        Take::new(self, n)
    }

    /// An iterator adapter which, like [`fold`], holds internal state, but
    /// unlike [`fold`], produces a new iterator.
    ///
    /// [`fold`]: Iterator::fold
    ///
    /// `scan()` takes two arguments: an initial value which seeds the internal
    /// state, and a closure with two arguments, the first being a mutable
    /// reference to the internal state and the second an iterator element.
    /// The closure can assign to the internal state to share state between
    /// iterations.
    ///
    /// On iteration, the closure will be applied to each element of the
    /// iterator and the return value from the closure, an [`Option`], is
    /// returned by the `next` method. Thus the closure can return
    /// `Some(value)` to yield `value`, or `None` to end the iteration.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3, 4];
    ///
    /// let mut iter = a.iter().scan(1, |state, &x| {
    ///     // each iteration, we'll multiply the state by the element ...
    ///     *state = *state * x;
    ///
    ///     // ... and terminate if the state exceeds 6
    ///     if *state > 6 {
    ///         return None;
    ///     }
    ///     // ... else yield the negation of the state
    ///     Some(-*state)
    /// });
    ///
    /// assert_eq!(iter.next(), Some(-1));
    /// assert_eq!(iter.next(), Some(-2));
    /// assert_eq!(iter.next(), Some(-6));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
    where
        Self: Sized,
        F: FnMut(&mut St, Self::Item) -> Option<B>,
    {
        Scan::new(self, initial_state, f)
    }

    /// Creates an iterator that works like map, but flattens nested structure.
    ///
    /// The [`map`] adapter is very useful, but only when the closure
    /// argument produces values. If it produces an iterator instead, there's
    /// an extra layer of indirection. `flat_map()` will remove this extra layer
    /// on its own.
    ///
    /// You can think of `flat_map(f)` as the semantic equivalent
    /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
    ///
    /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
    /// one item for each element, and `flat_map()`'s closure returns an
    /// iterator for each element.
    ///
    /// [`map`]: Iterator::map
    /// [`flatten`]: Iterator::flatten
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let words = ["alpha", "beta", "gamma"];
    ///
    /// // chars() returns an iterator
    /// let merged: String = words.iter()
    ///                           .flat_map(|s| s.chars())
    ///                           .collect();
    /// assert_eq!(merged, "alphabetagamma");
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
    where
        Self: Sized,
        U: IntoIterator,
        F: FnMut(Self::Item) -> U,
    {
        FlatMap::new(self, f)
    }

    /// Creates an iterator that flattens nested structure.
    ///
    /// This is useful when you have an iterator of iterators or an iterator of
    /// things that can be turned into iterators and you want to remove one
    /// level of indirection.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
    /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
    /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
    /// ```
    ///
    /// Mapping and then flattening:
    ///
    /// ```
    /// let words = ["alpha", "beta", "gamma"];
    ///
    /// // chars() returns an iterator
    /// let merged: String = words.iter()
    ///                           .map(|s| s.chars())
    ///                           .flatten()
    ///                           .collect();
    /// assert_eq!(merged, "alphabetagamma");
    /// ```
    ///
    /// You can also rewrite this in terms of [`flat_map()`], which is preferable
    /// in this case since it conveys intent more clearly:
    ///
    /// ```
    /// let words = ["alpha", "beta", "gamma"];
    ///
    /// // chars() returns an iterator
    /// let merged: String = words.iter()
    ///                           .flat_map(|s| s.chars())
    ///                           .collect();
    /// assert_eq!(merged, "alphabetagamma");
    /// ```
    ///
    /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
    ///
    /// ```
    /// let options = vec![Some(123), Some(321), None, Some(231)];
    /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
    /// assert_eq!(flattened_options, vec![123, 321, 231]);
    ///
    /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
    /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
    /// assert_eq!(flattened_results, vec![123, 321, 231]);
    /// ```
    ///
    /// Flattening only removes one level of nesting at a time:
    ///
    /// ```
    /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
    ///
    /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
    /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
    ///
    /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
    /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
    /// ```
    ///
    /// Here we see that `flatten()` does not perform a "deep" flatten.
    /// Instead, only one level of nesting is removed. That is, if you
    /// `flatten()` a three-dimensional array, the result will be
    /// two-dimensional and not one-dimensional. To get a one-dimensional
    /// structure, you have to `flatten()` again.
    ///
    /// [`flat_map()`]: Iterator::flat_map
    #[inline]
    #[stable(feature = "iterator_flatten", since = "1.29.0")]
    #[rustc_do_not_const_check]
    fn flatten(self) -> Flatten<Self>
    where
        Self: Sized,
        Self::Item: IntoIterator,
    {
        Flatten::new(self)
    }

    /// Calls the given function `f` for each contiguous window of size `N` over
    /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
    /// the windows during mapping overlap as well.
    ///
    /// In the following example, the closure is called three times with the
    /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
    ///
    /// ```
    /// #![feature(iter_map_windows)]
    ///
    /// let strings = "abcd".chars()
    ///     .map_windows(|[x, y]| format!("{}+{}", x, y))
    ///     .collect::<Vec<String>>();
    ///
    /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
    /// ```
    ///
    /// Note that the const parameter `N` is usually inferred by the
    /// destructured argument in the closure.
    ///
    /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
    /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
    /// empty iterator.
    ///
    /// The returned iterator implements [`FusedIterator`], because once `self`
    /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
    /// we cannot put it into a contigious array buffer, and thus the returned iterator
    /// should be fused.
    ///
    /// [`slice::windows()`]: slice::windows
    /// [`FusedIterator`]: crate::iter::FusedIterator
    ///
    /// # Panics
    ///
    /// Panics if `N` is 0. This check will most probably get changed to a
    /// compile time error before this method gets stabilized.
    ///
    /// ```should_panic
    /// #![feature(iter_map_windows)]
    ///
    /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
    /// ```
    ///
    /// # Examples
    ///
    /// Building the sums of neighboring numbers.
    ///
    /// ```
    /// #![feature(iter_map_windows)]
    ///
    /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
    /// assert_eq!(it.next(), Some(4));  // 1 + 3
    /// assert_eq!(it.next(), Some(11)); // 3 + 8
    /// assert_eq!(it.next(), Some(9));  // 8 + 1
    /// assert_eq!(it.next(), None);
    /// ```
    ///
    /// Since the elements in the following example implement `Copy`, we can
    /// just copy the array and get an iterator over the windows.
    ///
    /// ```
    /// #![feature(iter_map_windows)]
    ///
    /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
    /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
    /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
    /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
    /// assert_eq!(it.next(), Some(['r', 'i', 's']));
    /// assert_eq!(it.next(), None);
    /// ```
    ///
    /// You can also use this function to check the sortedness of an iterator.
    /// For the simple case, rather use [`Iterator::is_sorted`].
    ///
    /// ```
    /// #![feature(iter_map_windows)]
    ///
    /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
    ///     .map_windows(|[a, b]| a <= b);
    ///
    /// assert_eq!(it.next(), Some(true));  // 0.5 <= 1.0
    /// assert_eq!(it.next(), Some(true));  // 1.0 <= 3.5
    /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
    /// assert_eq!(it.next(), Some(true));  // 3.0 <= 8.5
    /// assert_eq!(it.next(), Some(true));  // 8.5 <= 8.5
    /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
    /// assert_eq!(it.next(), None);
    /// ```
    ///
    /// For non-fused iterators, they are fused after `map_windows`.
    ///
    /// ```
    /// #![feature(iter_map_windows)]
    ///
    /// #[derive(Default)]
    /// struct NonFusedIterator {
    ///     state: i32,
    /// }
    ///
    /// impl Iterator for NonFusedIterator {
    ///     type Item = i32;
    ///
    ///     fn next(&mut self) -> Option<i32> {
    ///         let val = self.state;
    ///         self.state = self.state + 1;
    ///
    ///         // yields `0..5` first, then only even numbers since `6..`.
    ///         if val < 5 || val % 2 == 0 {
    ///             Some(val)
    ///         } else {
    ///             None
    ///         }
    ///     }
    /// }
    ///
    ///
    /// let mut iter = NonFusedIterator::default();
    ///
    /// // yields 0..5 first.
    /// assert_eq!(iter.next(), Some(0));
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), Some(4));
    /// // then we can see our iterator going back and forth
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), Some(6));
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), Some(8));
    /// assert_eq!(iter.next(), None);
    ///
    /// // however, with `.map_windows()`, it is fused.
    /// let mut iter = NonFusedIterator::default()
    ///     .map_windows(|arr: &[_; 2]| *arr);
    ///
    /// assert_eq!(iter.next(), Some([0, 1]));
    /// assert_eq!(iter.next(), Some([1, 2]));
    /// assert_eq!(iter.next(), Some([2, 3]));
    /// assert_eq!(iter.next(), Some([3, 4]));
    /// assert_eq!(iter.next(), None);
    ///
    /// // it will always return `None` after the first time.
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
    #[rustc_do_not_const_check]
    fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
    where
        Self: Sized,
        F: FnMut(&[Self::Item; N]) -> R,
    {
        MapWindows::new(self, f)
    }

    /// Creates an iterator which ends after the first [`None`].
    ///
    /// After an iterator returns [`None`], future calls may or may not yield
    /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
    /// [`None`] is given, it will always return [`None`] forever.
    ///
    /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
    /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
    /// if the [`FusedIterator`] trait is improperly implemented.
    ///
    /// [`Some(T)`]: Some
    /// [`FusedIterator`]: crate::iter::FusedIterator
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // an iterator which alternates between Some and None
    /// struct Alternate {
    ///     state: i32,
    /// }
    ///
    /// impl Iterator for Alternate {
    ///     type Item = i32;
    ///
    ///     fn next(&mut self) -> Option<i32> {
    ///         let val = self.state;
    ///         self.state = self.state + 1;
    ///
    ///         // if it's even, Some(i32), else None
    ///         if val % 2 == 0 {
    ///             Some(val)
    ///         } else {
    ///             None
    ///         }
    ///     }
    /// }
    ///
    /// let mut iter = Alternate { state: 0 };
    ///
    /// // we can see our iterator going back and forth
    /// assert_eq!(iter.next(), Some(0));
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), None);
    ///
    /// // however, once we fuse it...
    /// let mut iter = iter.fuse();
    ///
    /// assert_eq!(iter.next(), Some(4));
    /// assert_eq!(iter.next(), None);
    ///
    /// // it will always return `None` after the first time.
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn fuse(self) -> Fuse<Self>
    where
        Self: Sized,
    {
        Fuse::new(self)
    }

    /// Does something with each element of an iterator, passing the value on.
    ///
    /// When using iterators, you'll often chain several of them together.
    /// While working on such code, you might want to check out what's
    /// happening at various parts in the pipeline. To do that, insert
    /// a call to `inspect()`.
    ///
    /// It's more common for `inspect()` to be used as a debugging tool than to
    /// exist in your final code, but applications may find it useful in certain
    /// situations when errors need to be logged before being discarded.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 4, 2, 3];
    ///
    /// // this iterator sequence is complex.
    /// let sum = a.iter()
    ///     .cloned()
    ///     .filter(|x| x % 2 == 0)
    ///     .fold(0, |sum, i| sum + i);
    ///
    /// println!("{sum}");
    ///
    /// // let's add some inspect() calls to investigate what's happening
    /// let sum = a.iter()
    ///     .cloned()
    ///     .inspect(|x| println!("about to filter: {x}"))
    ///     .filter(|x| x % 2 == 0)
    ///     .inspect(|x| println!("made it through filter: {x}"))
    ///     .fold(0, |sum, i| sum + i);
    ///
    /// println!("{sum}");
    /// ```
    ///
    /// This will print:
    ///
    /// ```text
    /// 6
    /// about to filter: 1
    /// about to filter: 4
    /// made it through filter: 4
    /// about to filter: 2
    /// made it through filter: 2
    /// about to filter: 3
    /// 6
    /// ```
    ///
    /// Logging errors before discarding them:
    ///
    /// ```
    /// let lines = ["1", "2", "a"];
    ///
    /// let sum: i32 = lines
    ///     .iter()
    ///     .map(|line| line.parse::<i32>())
    ///     .inspect(|num| {
    ///         if let Err(ref e) = *num {
    ///             println!("Parsing error: {e}");
    ///         }
    ///     })
    ///     .filter_map(Result::ok)
    ///     .sum();
    ///
    /// println!("Sum: {sum}");
    /// ```
    ///
    /// This will print:
    ///
    /// ```text
    /// Parsing error: invalid digit found in string
    /// Sum: 3
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn inspect<F>(self, f: F) -> Inspect<Self, F>
    where
        Self: Sized,
        F: FnMut(&Self::Item),
    {
        Inspect::new(self, f)
    }

    /// Borrows an iterator, rather than consuming it.
    ///
    /// This is useful to allow applying iterator adapters while still
    /// retaining ownership of the original iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
    ///
    /// // Take the first two words.
    /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
    /// assert_eq!(hello_world, vec!["hello", "world"]);
    ///
    /// // Collect the rest of the words.
    /// // We can only do this because we used `by_ref` earlier.
    /// let of_rust: Vec<_> = words.collect();
    /// assert_eq!(of_rust, vec!["of", "Rust"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    {
        self
    }

    /// Transforms an iterator into a collection.
    ///
    /// `collect()` can take anything iterable, and turn it into a relevant
    /// collection. This is one of the more powerful methods in the standard
    /// library, used in a variety of contexts.
    ///
    /// The most basic pattern in which `collect()` is used is to turn one
    /// collection into another. You take a collection, call [`iter`] on it,
    /// do a bunch of transformations, and then `collect()` at the end.
    ///
    /// `collect()` can also create instances of types that are not typical
    /// collections. For example, a [`String`] can be built from [`char`]s,
    /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
    /// into `Result<Collection<T>, E>`. See the examples below for more.
    ///
    /// Because `collect()` is so general, it can cause problems with type
    /// inference. As such, `collect()` is one of the few times you'll see
    /// the syntax affectionately known as the 'turbofish': `::<>`. This
    /// helps the inference algorithm understand specifically which collection
    /// you're trying to collect into.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let doubled: Vec<i32> = a.iter()
    ///                          .map(|&x| x * 2)
    ///                          .collect();
    ///
    /// assert_eq!(vec![2, 4, 6], doubled);
    /// ```
    ///
    /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
    /// we could collect into, for example, a [`VecDeque<T>`] instead:
    ///
    /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let a = [1, 2, 3];
    ///
    /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
    ///
    /// assert_eq!(2, doubled[0]);
    /// assert_eq!(4, doubled[1]);
    /// assert_eq!(6, doubled[2]);
    /// ```
    ///
    /// Using the 'turbofish' instead of annotating `doubled`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
    ///
    /// assert_eq!(vec![2, 4, 6], doubled);
    /// ```
    ///
    /// Because `collect()` only cares about what you're collecting into, you can
    /// still use a partial type hint, `_`, with the turbofish:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
    ///
    /// assert_eq!(vec![2, 4, 6], doubled);
    /// ```
    ///
    /// Using `collect()` to make a [`String`]:
    ///
    /// ```
    /// let chars = ['g', 'd', 'k', 'k', 'n'];
    ///
    /// let hello: String = chars.iter()
    ///     .map(|&x| x as u8)
    ///     .map(|x| (x + 1) as char)
    ///     .collect();
    ///
    /// assert_eq!("hello", hello);
    /// ```
    ///
    /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
    /// see if any of them failed:
    ///
    /// ```
    /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
    ///
    /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
    ///
    /// // gives us the first error
    /// assert_eq!(Err("nope"), result);
    ///
    /// let results = [Ok(1), Ok(3)];
    ///
    /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
    ///
    /// // gives us the list of answers
    /// assert_eq!(Ok(vec![1, 3]), result);
    /// ```
    ///
    /// [`iter`]: Iterator::next
    /// [`String`]: ../../std/string/struct.String.html
    /// [`char`]: type@char
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
    #[cfg_attr(not(test), rustc_diagnostic_item = "iterator_collect_fn")]
    #[rustc_do_not_const_check]
    fn collect<B: FromIterator<Self::Item>>(self) -> B
    where
        Self: Sized,
    {
        FromIterator::from_iter(self)
    }

    /// Fallibly transforms an iterator into a collection, short circuiting if
    /// a failure is encountered.
    ///
    /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
    /// conversions during collection. Its main use case is simplifying conversions from
    /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
    /// types (e.g. [`Result`]).
    ///
    /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
    /// only the inner type produced on `Try::Output` must implement it. Concretely,
    /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
    /// [`FromIterator`], even though [`ControlFlow`] doesn't.
    ///
    /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
    /// may continue to be used, in which case it will continue iterating starting after the element that
    /// triggered the failure. See the last example below for an example of how this works.
    ///
    /// # Examples
    /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
    /// ```
    /// #![feature(iterator_try_collect)]
    ///
    /// let u = vec![Some(1), Some(2), Some(3)];
    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    /// assert_eq!(v, Some(vec![1, 2, 3]));
    /// ```
    ///
    /// Failing to collect in the same way:
    /// ```
    /// #![feature(iterator_try_collect)]
    ///
    /// let u = vec![Some(1), Some(2), None, Some(3)];
    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    /// assert_eq!(v, None);
    /// ```
    ///
    /// A similar example, but with `Result`:
    /// ```
    /// #![feature(iterator_try_collect)]
    ///
    /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    /// assert_eq!(v, Ok(vec![1, 2, 3]));
    ///
    /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    /// assert_eq!(v, Err(()));
    /// ```
    ///
    /// Finally, even [`ControlFlow`] works, despite the fact that it
    /// doesn't implement [`FromIterator`]. Note also that the iterator can
    /// continue to be used, even if a failure is encountered:
    ///
    /// ```
    /// #![feature(iterator_try_collect)]
    ///
    /// use core::ops::ControlFlow::{Break, Continue};
    ///
    /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
    /// let mut it = u.into_iter();
    ///
    /// let v = it.try_collect::<Vec<_>>();
    /// assert_eq!(v, Break(3));
    ///
    /// let v = it.try_collect::<Vec<_>>();
    /// assert_eq!(v, Continue(vec![4, 5]));
    /// ```
    ///
    /// [`collect`]: Iterator::collect
    #[inline]
    #[unstable(feature = "iterator_try_collect", issue = "94047")]
    #[rustc_do_not_const_check]
    fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
    where
        Self: Sized,
        <Self as Iterator>::Item: Try,
        <<Self as Iterator>::Item as Try>::Residual: Residual<B>,
        B: FromIterator<<Self::Item as Try>::Output>,
    {
        try_process(ByRefSized(self), |i| i.collect())
    }

    /// Collects all the items from an iterator into a collection.
    ///
    /// This method consumes the iterator and adds all its items to the
    /// passed collection. The collection is then returned, so the call chain
    /// can be continued.
    ///
    /// This is useful when you already have a collection and want to add
    /// the iterator items to it.
    ///
    /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
    /// but instead of being called on a collection, it's called on an iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_collect_into)]
    ///
    /// let a = [1, 2, 3];
    /// let mut vec: Vec::<i32> = vec![0, 1];
    ///
    /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
    /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
    ///
    /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
    /// ```
    ///
    /// `Vec` can have a manual set capacity to avoid reallocating it:
    ///
    /// ```
    /// #![feature(iter_collect_into)]
    ///
    /// let a = [1, 2, 3];
    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
    ///
    /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
    /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
    ///
    /// assert_eq!(6, vec.capacity());
    /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
    /// ```
    ///
    /// The returned mutable reference can be used to continue the call chain:
    ///
    /// ```
    /// #![feature(iter_collect_into)]
    ///
    /// let a = [1, 2, 3];
    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
    ///
    /// let count = a.iter().collect_into(&mut vec).iter().count();
    ///
    /// assert_eq!(count, vec.len());
    /// assert_eq!(vec, vec![1, 2, 3]);
    ///
    /// let count = a.iter().collect_into(&mut vec).iter().count();
    ///
    /// assert_eq!(count, vec.len());
    /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
    /// ```
    #[inline]
    #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
    #[rustc_do_not_const_check]
    fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
    where
        Self: Sized,
    {
        collection.extend(self);
        collection
    }

    /// Consumes an iterator, creating two collections from it.
    ///
    /// The predicate passed to `partition()` can return `true`, or `false`.
    /// `partition()` returns a pair, all of the elements for which it returned
    /// `true`, and all of the elements for which it returned `false`.
    ///
    /// See also [`is_partitioned()`] and [`partition_in_place()`].
    ///
    /// [`is_partitioned()`]: Iterator::is_partitioned
    /// [`partition_in_place()`]: Iterator::partition_in_place
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let (even, odd): (Vec<_>, Vec<_>) = a
    ///     .into_iter()
    ///     .partition(|n| n % 2 == 0);
    ///
    /// assert_eq!(even, vec![2]);
    /// assert_eq!(odd, vec![1, 3]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn partition<B, F>(self, f: F) -> (B, B)
    where
        Self: Sized,
        B: Default + Extend<Self::Item>,
        F: FnMut(&Self::Item) -> bool,
    {
        #[inline]
        fn extend<'a, T, B: Extend<T>>(
            mut f: impl FnMut(&T) -> bool + 'a,
            left: &'a mut B,
            right: &'a mut B,
        ) -> impl FnMut((), T) + 'a {
            move |(), x| {
                if f(&x) {
                    left.extend_one(x);
                } else {
                    right.extend_one(x);
                }
            }
        }

        let mut left: B = Default::default();
        let mut right: B = Default::default();

        self.fold((), extend(f, &mut left, &mut right));

        (left, right)
    }

    /// Reorders the elements of this iterator *in-place* according to the given predicate,
    /// such that all those that return `true` precede all those that return `false`.
    /// Returns the number of `true` elements found.
    ///
    /// The relative order of partitioned items is not maintained.
    ///
    /// # Current implementation
    ///
    /// The current algorithm tries to find the first element for which the predicate evaluates
    /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
    ///
    /// Time complexity: *O*(*n*)
    ///
    /// See also [`is_partitioned()`] and [`partition()`].
    ///
    /// [`is_partitioned()`]: Iterator::is_partitioned
    /// [`partition()`]: Iterator::partition
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(iter_partition_in_place)]
    ///
    /// let mut a = [1, 2, 3, 4, 5, 6, 7];
    ///
    /// // Partition in-place between evens and odds
    /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
    ///
    /// assert_eq!(i, 3);
    /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
    /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
    /// ```
    #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
    #[rustc_do_not_const_check]
    fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
    where
        Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
        P: FnMut(&T) -> bool,
    {
        // FIXME: should we worry about the count overflowing? The only way to have more than
        // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...

        // These closure "factory" functions exist to avoid genericity in `Self`.

        #[inline]
        fn is_false<'a, T>(
            predicate: &'a mut impl FnMut(&T) -> bool,
            true_count: &'a mut usize,
        ) -> impl FnMut(&&mut T) -> bool + 'a {
            move |x| {
                let p = predicate(&**x);
                *true_count += p as usize;
                !p
            }
        }

        #[inline]
        fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
            move |x| predicate(&**x)
        }

        // Repeatedly find the first `false` and swap it with the last `true`.
        let mut true_count = 0;
        while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
            if let Some(tail) = self.rfind(is_true(predicate)) {
                crate::mem::swap(head, tail);
                true_count += 1;
            } else {
                break;
            }
        }
        true_count
    }

    /// Checks if the elements of this iterator are partitioned according to the given predicate,
    /// such that all those that return `true` precede all those that return `false`.
    ///
    /// See also [`partition()`] and [`partition_in_place()`].
    ///
    /// [`partition()`]: Iterator::partition
    /// [`partition_in_place()`]: Iterator::partition_in_place
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(iter_is_partitioned)]
    ///
    /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
    /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
    /// ```
    #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
    #[rustc_do_not_const_check]
    fn is_partitioned<P>(mut self, mut predicate: P) -> bool
    where
        Self: Sized,
        P: FnMut(Self::Item) -> bool,
    {
        // Either all items test `true`, or the first clause stops at `false`
        // and we check that there are no more `true` items after that.
        self.all(&mut predicate) || !self.any(predicate)
    }

    /// An iterator method that applies a function as long as it returns
    /// successfully, producing a single, final value.
    ///
    /// `try_fold()` takes two arguments: an initial value, and a closure with
    /// two arguments: an 'accumulator', and an element. The closure either
    /// returns successfully, with the value that the accumulator should have
    /// for the next iteration, or it returns failure, with an error value that
    /// is propagated back to the caller immediately (short-circuiting).
    ///
    /// The initial value is the value the accumulator will have on the first
    /// call. If applying the closure succeeded against every element of the
    /// iterator, `try_fold()` returns the final accumulator as success.
    ///
    /// Folding is useful whenever you have a collection of something, and want
    /// to produce a single value from it.
    ///
    /// # Note to Implementors
    ///
    /// Several of the other (forward) methods have default implementations in
    /// terms of this one, so try to implement this explicitly if it can
    /// do something better than the default `for` loop implementation.
    ///
    /// In particular, try to have this call `try_fold()` on the internal parts
    /// from which this iterator is composed. If multiple calls are needed,
    /// the `?` operator may be convenient for chaining the accumulator value
    /// along, but beware any invariants that need to be upheld before those
    /// early returns. This is a `&mut self` method, so iteration needs to be
    /// resumable after hitting an error here.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// // the checked sum of all of the elements of the array
    /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
    ///
    /// assert_eq!(sum, Some(6));
    /// ```
    ///
    /// Short-circuiting:
    ///
    /// ```
    /// let a = [10, 20, 30, 100, 40, 50];
    /// let mut it = a.iter();
    ///
    /// // This sum overflows when adding the 100 element
    /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
    /// assert_eq!(sum, None);
    ///
    /// // Because it short-circuited, the remaining elements are still
    /// // available through the iterator.
    /// assert_eq!(it.len(), 2);
    /// assert_eq!(it.next(), Some(&40));
    /// ```
    ///
    /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
    /// a similar idea:
    ///
    /// ```
    /// use std::ops::ControlFlow;
    ///
    /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
    ///     if let Some(next) = prev.checked_add(x) {
    ///         ControlFlow::Continue(next)
    ///     } else {
    ///         ControlFlow::Break(prev)
    ///     }
    /// });
    /// assert_eq!(triangular, ControlFlow::Break(120));
    ///
    /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
    ///     if let Some(next) = prev.checked_add(x) {
    ///         ControlFlow::Continue(next)
    ///     } else {
    ///         ControlFlow::Break(prev)
    ///     }
    /// });
    /// assert_eq!(triangular, ControlFlow::Continue(435));
    /// ```
    #[inline]
    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    #[rustc_do_not_const_check]
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        let mut accum = init;
        while let Some(x) = self.next() {
            accum = f(accum, x)?;
        }
        try { accum }
    }

    /// An iterator method that applies a fallible function to each item in the
    /// iterator, stopping at the first error and returning that error.
    ///
    /// This can also be thought of as the fallible form of [`for_each()`]
    /// or as the stateless version of [`try_fold()`].
    ///
    /// [`for_each()`]: Iterator::for_each
    /// [`try_fold()`]: Iterator::try_fold
    ///
    /// # Examples
    ///
    /// ```
    /// use std::fs::rename;
    /// use std::io::{stdout, Write};
    /// use std::path::Path;
    ///
    /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
    ///
    /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
    /// assert!(res.is_ok());
    ///
    /// let mut it = data.iter().cloned();
    /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
    /// assert!(res.is_err());
    /// // It short-circuited, so the remaining items are still in the iterator:
    /// assert_eq!(it.next(), Some("stale_bread.json"));
    /// ```
    ///
    /// The [`ControlFlow`] type can be used with this method for the situations
    /// in which you'd use `break` and `continue` in a normal loop:
    ///
    /// ```
    /// use std::ops::ControlFlow;
    ///
    /// let r = (2..100).try_for_each(|x| {
    ///     if 323 % x == 0 {
    ///         return ControlFlow::Break(x)
    ///     }
    ///
    ///     ControlFlow::Continue(())
    /// });
    /// assert_eq!(r, ControlFlow::Break(17));
    /// ```
    #[inline]
    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    #[rustc_do_not_const_check]
    fn try_for_each<F, R>(&mut self, f: F) -> R
    where
        Self: Sized,
        F: FnMut(Self::Item) -> R,
        R: Try<Output = ()>,
    {
        #[inline]
        fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
            move |(), x| f(x)
        }

        self.try_fold((), call(f))
    }

    /// Folds every element into an accumulator by applying an operation,
    /// returning the final result.
    ///
    /// `fold()` takes two arguments: an initial value, and a closure with two
    /// arguments: an 'accumulator', and an element. The closure returns the value that
    /// the accumulator should have for the next iteration.
    ///
    /// The initial value is the value the accumulator will have on the first
    /// call.
    ///
    /// After applying this closure to every element of the iterator, `fold()`
    /// returns the accumulator.
    ///
    /// This operation is sometimes called 'reduce' or 'inject'.
    ///
    /// Folding is useful whenever you have a collection of something, and want
    /// to produce a single value from it.
    ///
    /// Note: `fold()`, and similar methods that traverse the entire iterator,
    /// might not terminate for infinite iterators, even on traits for which a
    /// result is determinable in finite time.
    ///
    /// Note: [`reduce()`] can be used to use the first element as the initial
    /// value, if the accumulator type and item type is the same.
    ///
    /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
    /// operators like `+`, the order the elements are combined in is not important, but for non-associative
    /// operators like `-` the order will affect the final result.
    /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
    ///
    /// # Note to Implementors
    ///
    /// Several of the other (forward) methods have default implementations in
    /// terms of this one, so try to implement this explicitly if it can
    /// do something better than the default `for` loop implementation.
    ///
    /// In particular, try to have this call `fold()` on the internal parts
    /// from which this iterator is composed.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// // the sum of all of the elements of the array
    /// let sum = a.iter().fold(0, |acc, x| acc + x);
    ///
    /// assert_eq!(sum, 6);
    /// ```
    ///
    /// Let's walk through each step of the iteration here:
    ///
    /// | element | acc | x | result |
    /// |---------|-----|---|--------|
    /// |         | 0   |   |        |
    /// | 1       | 0   | 1 | 1      |
    /// | 2       | 1   | 2 | 3      |
    /// | 3       | 3   | 3 | 6      |
    ///
    /// And so, our final result, `6`.
    ///
    /// This example demonstrates the left-associative nature of `fold()`:
    /// it builds a string, starting with an initial value
    /// and continuing with each element from the front until the back:
    ///
    /// ```
    /// let numbers = [1, 2, 3, 4, 5];
    ///
    /// let zero = "0".to_string();
    ///
    /// let result = numbers.iter().fold(zero, |acc, &x| {
    ///     format!("({acc} + {x})")
    /// });
    ///
    /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
    /// ```
    /// It's common for people who haven't used iterators a lot to
    /// use a `for` loop with a list of things to build up a result. Those
    /// can be turned into `fold()`s:
    ///
    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    ///
    /// ```
    /// let numbers = [1, 2, 3, 4, 5];
    ///
    /// let mut result = 0;
    ///
    /// // for loop:
    /// for i in &numbers {
    ///     result = result + i;
    /// }
    ///
    /// // fold:
    /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
    ///
    /// // they're the same
    /// assert_eq!(result, result2);
    /// ```
    ///
    /// [`reduce()`]: Iterator::reduce
    #[doc(alias = "inject", alias = "foldl")]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn fold<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        while let Some(x) = self.next() {
            accum = f(accum, x);
        }
        accum
    }

    /// Reduces the elements to a single one, by repeatedly applying a reducing
    /// operation.
    ///
    /// If the iterator is empty, returns [`None`]; otherwise, returns the
    /// result of the reduction.
    ///
    /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
    /// For iterators with at least one element, this is the same as [`fold()`]
    /// with the first element of the iterator as the initial accumulator value, folding
    /// every subsequent element into it.
    ///
    /// [`fold()`]: Iterator::fold
    ///
    /// # Example
    ///
    /// ```
    /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap();
    /// assert_eq!(reduced, 45);
    ///
    /// // Which is equivalent to doing it with `fold`:
    /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
    /// assert_eq!(reduced, folded);
    /// ```
    #[inline]
    #[stable(feature = "iterator_fold_self", since = "1.51.0")]
    #[rustc_do_not_const_check]
    fn reduce<F>(mut self, f: F) -> Option<Self::Item>
    where
        Self: Sized,
        F: FnMut(Self::Item, Self::Item) -> Self::Item,
    {
        let first = self.next()?;
        Some(self.fold(first, f))
    }

    /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
    /// closure returns a failure, the failure is propagated back to the caller immediately.
    ///
    /// The return type of this method depends on the return type of the closure. If the closure
    /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
    /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
    /// `Option<Option<Self::Item>>`.
    ///
    /// When called on an empty iterator, this function will return either `Some(None)` or
    /// `Ok(None)` depending on the type of the provided closure.
    ///
    /// For iterators with at least one element, this is essentially the same as calling
    /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
    ///
    /// [`try_fold()`]: Iterator::try_fold
    ///
    /// # Examples
    ///
    /// Safely calculate the sum of a series of numbers:
    ///
    /// ```
    /// #![feature(iterator_try_reduce)]
    ///
    /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    /// assert_eq!(sum, Some(Some(58)));
    /// ```
    ///
    /// Determine when a reduction short circuited:
    ///
    /// ```
    /// #![feature(iterator_try_reduce)]
    ///
    /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    /// assert_eq!(sum, None);
    /// ```
    ///
    /// Determine when a reduction was not performed because there are no elements:
    ///
    /// ```
    /// #![feature(iterator_try_reduce)]
    ///
    /// let numbers: Vec<usize> = Vec::new();
    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    /// assert_eq!(sum, Some(None));
    /// ```
    ///
    /// Use a [`Result`] instead of an [`Option`]:
    ///
    /// ```
    /// #![feature(iterator_try_reduce)]
    ///
    /// let numbers = vec!["1", "2", "3", "4", "5"];
    /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
    ///     numbers.into_iter().try_reduce(|x, y| {
    ///         if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
    ///     });
    /// assert_eq!(max, Ok(Some("5")));
    /// ```
    #[inline]
    #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
    #[rustc_do_not_const_check]
    fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
    where
        Self: Sized,
        F: FnMut(Self::Item, Self::Item) -> R,
        R: Try<Output = Self::Item>,
        R::Residual: Residual<Option<Self::Item>>,
    {
        let first = match self.next() {
            Some(i) => i,
            None => return Try::from_output(None),
        };

        match self.try_fold(first, f).branch() {
            ControlFlow::Break(r) => FromResidual::from_residual(r),
            ControlFlow::Continue(i) => Try::from_output(Some(i)),
        }
    }

    /// Tests if every element of the iterator matches a predicate.
    ///
    /// `all()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, and if they all return
    /// `true`, then so does `all()`. If any of them return `false`, it
    /// returns `false`.
    ///
    /// `all()` is short-circuiting; in other words, it will stop processing
    /// as soon as it finds a `false`, given that no matter what else happens,
    /// the result will also be `false`.
    ///
    /// An empty iterator returns `true`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert!(a.iter().all(|&x| x > 0));
    ///
    /// assert!(!a.iter().all(|&x| x > 2));
    /// ```
    ///
    /// Stopping at the first `false`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert!(!iter.all(|&x| x != 2));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&3));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn all<F>(&mut self, f: F) -> bool
    where
        Self: Sized,
        F: FnMut(Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
            move |(), x| {
                if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
            }
        }
        self.try_fold((), check(f)) == ControlFlow::Continue(())
    }

    /// Tests if any element of the iterator matches a predicate.
    ///
    /// `any()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, and if any of them return
    /// `true`, then so does `any()`. If they all return `false`, it
    /// returns `false`.
    ///
    /// `any()` is short-circuiting; in other words, it will stop processing
    /// as soon as it finds a `true`, given that no matter what else happens,
    /// the result will also be `true`.
    ///
    /// An empty iterator returns `false`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert!(a.iter().any(|&x| x > 0));
    ///
    /// assert!(!a.iter().any(|&x| x > 5));
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert!(iter.any(|&x| x != 2));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&2));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn any<F>(&mut self, f: F) -> bool
    where
        Self: Sized,
        F: FnMut(Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
            move |(), x| {
                if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
            }
        }

        self.try_fold((), check(f)) == ControlFlow::Break(())
    }

    /// Searches for an element of an iterator that satisfies a predicate.
    ///
    /// `find()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, and if any of them return
    /// `true`, then `find()` returns [`Some(element)`]. If they all return
    /// `false`, it returns [`None`].
    ///
    /// `find()` is short-circuiting; in other words, it will stop processing
    /// as soon as the closure returns `true`.
    ///
    /// Because `find()` takes a reference, and many iterators iterate over
    /// references, this leads to a possibly confusing situation where the
    /// argument is a double reference. You can see this effect in the
    /// examples below, with `&&x`.
    ///
    /// If you need the index of the element, see [`position()`].
    ///
    /// [`Some(element)`]: Some
    /// [`position()`]: Iterator::position
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
    ///
    /// assert_eq!(a.iter().find(|&&x| x == 5), None);
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&3));
    /// ```
    ///
    /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
            move |(), x| {
                if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
            }
        }

        self.try_fold((), check(predicate)).break_value()
    }

    /// Applies function to the elements of iterator and returns
    /// the first non-none result.
    ///
    /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = ["lol", "NaN", "2", "5"];
    ///
    /// let first_number = a.iter().find_map(|s| s.parse().ok());
    ///
    /// assert_eq!(first_number, Some(2));
    /// ```
    #[inline]
    #[stable(feature = "iterator_find_map", since = "1.30.0")]
    #[rustc_do_not_const_check]
    fn find_map<B, F>(&mut self, f: F) -> Option<B>
    where
        Self: Sized,
        F: FnMut(Self::Item) -> Option<B>,
    {
        #[inline]
        fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
            move |(), x| match f(x) {
                Some(x) => ControlFlow::Break(x),
                None => ControlFlow::Continue(()),
            }
        }

        self.try_fold((), check(f)).break_value()
    }

    /// Applies function to the elements of iterator and returns
    /// the first true result or the first error.
    ///
    /// The return type of this method depends on the return type of the closure.
    /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
    /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_find)]
    ///
    /// let a = ["1", "2", "lol", "NaN", "5"];
    ///
    /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
    ///     Ok(s.parse::<i32>()?  == search)
    /// };
    ///
    /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
    /// assert_eq!(result, Ok(Some(&"2")));
    ///
    /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
    /// assert!(result.is_err());
    /// ```
    ///
    /// This also supports other types which implement `Try`, not just `Result`.
    /// ```
    /// #![feature(try_find)]
    ///
    /// use std::num::NonZeroU32;
    /// let a = [3, 5, 7, 4, 9, 0, 11];
    /// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
    /// assert_eq!(result, Some(Some(&4)));
    /// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
    /// assert_eq!(result, Some(None));
    /// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
    /// assert_eq!(result, None);
    /// ```
    #[inline]
    #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
    #[rustc_do_not_const_check]
    fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>>
    where
        Self: Sized,
        F: FnMut(&Self::Item) -> R,
        R: Try<Output = bool>,
        R::Residual: Residual<Option<Self::Item>>,
    {
        #[inline]
        fn check<I, V, R>(
            mut f: impl FnMut(&I) -> V,
        ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
        where
            V: Try<Output = bool, Residual = R>,
            R: Residual<Option<I>>,
        {
            move |(), x| match f(&x).branch() {
                ControlFlow::Continue(false) => ControlFlow::Continue(()),
                ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
                ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
            }
        }

        match self.try_fold((), check(f)) {
            ControlFlow::Break(x) => x,
            ControlFlow::Continue(()) => Try::from_output(None),
        }
    }

    /// Searches for an element in an iterator, returning its index.
    ///
    /// `position()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, and if one of them
    /// returns `true`, then `position()` returns [`Some(index)`]. If all of
    /// them return `false`, it returns [`None`].
    ///
    /// `position()` is short-circuiting; in other words, it will stop
    /// processing as soon as it finds a `true`.
    ///
    /// # Overflow Behavior
    ///
    /// The method does no guarding against overflows, so if there are more
    /// than [`usize::MAX`] non-matching elements, it either produces the wrong
    /// result or panics. If debug assertions are enabled, a panic is
    /// guaranteed.
    ///
    /// # Panics
    ///
    /// This function might panic if the iterator has more than `usize::MAX`
    /// non-matching elements.
    ///
    /// [`Some(index)`]: Some
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
    ///
    /// assert_eq!(a.iter().position(|&x| x == 5), None);
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [1, 2, 3, 4];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&3));
    ///
    /// // The returned index depends on iterator state
    /// assert_eq!(iter.position(|&x| x == 4), Some(0));
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn position<P>(&mut self, predicate: P) -> Option<usize>
    where
        Self: Sized,
        P: FnMut(Self::Item) -> bool,
    {
        #[inline]
        fn check<T>(
            mut predicate: impl FnMut(T) -> bool,
        ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
            #[rustc_inherit_overflow_checks]
            move |i, x| {
                if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
            }
        }

        self.try_fold(0, check(predicate)).break_value()
    }

    /// Searches for an element in an iterator from the right, returning its
    /// index.
    ///
    /// `rposition()` takes a closure that returns `true` or `false`. It applies
    /// this closure to each element of the iterator, starting from the end,
    /// and if one of them returns `true`, then `rposition()` returns
    /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
    ///
    /// `rposition()` is short-circuiting; in other words, it will stop
    /// processing as soon as it finds a `true`.
    ///
    /// [`Some(index)`]: Some
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
    ///
    /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
    /// ```
    ///
    /// Stopping at the first `true`:
    ///
    /// ```
    /// let a = [-1, 2, 3, 4];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&-1));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
        Self: Sized + ExactSizeIterator + DoubleEndedIterator,
    {
        // No need for an overflow check here, because `ExactSizeIterator`
        // implies that the number of elements fits into a `usize`.
        #[inline]
        fn check<T>(
            mut predicate: impl FnMut(T) -> bool,
        ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
            move |i, x| {
                let i = i - 1;
                if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
            }
        }

        let n = self.len();
        self.try_rfold(n, check(predicate)).break_value()
    }

    /// Returns the maximum element of an iterator.
    ///
    /// If several elements are equally maximum, the last element is
    /// returned. If the iterator is empty, [`None`] is returned.
    ///
    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
    /// incomparable. You can work around this by using [`Iterator::reduce`]:
    /// ```
    /// assert_eq!(
    ///     [2.4, f32::NAN, 1.3]
    ///         .into_iter()
    ///         .reduce(f32::max)
    ///         .unwrap(),
    ///     2.4
    /// );
    /// ```
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// let b: Vec<u32> = Vec::new();
    ///
    /// assert_eq!(a.iter().max(), Some(&3));
    /// assert_eq!(b.iter().max(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn max(self) -> Option<Self::Item>
    where
        Self: Sized,
        Self::Item: Ord,
    {
        self.max_by(Ord::cmp)
    }

    /// Returns the minimum element of an iterator.
    ///
    /// If several elements are equally minimum, the first element is returned.
    /// If the iterator is empty, [`None`] is returned.
    ///
    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
    /// incomparable. You can work around this by using [`Iterator::reduce`]:
    /// ```
    /// assert_eq!(
    ///     [2.4, f32::NAN, 1.3]
    ///         .into_iter()
    ///         .reduce(f32::min)
    ///         .unwrap(),
    ///     1.3
    /// );
    /// ```
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// let b: Vec<u32> = Vec::new();
    ///
    /// assert_eq!(a.iter().min(), Some(&1));
    /// assert_eq!(b.iter().min(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn min(self) -> Option<Self::Item>
    where
        Self: Sized,
        Self::Item: Ord,
    {
        self.min_by(Ord::cmp)
    }

    /// Returns the element that gives the maximum value from the
    /// specified function.
    ///
    /// If several elements are equally maximum, the last element is
    /// returned. If the iterator is empty, [`None`] is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [-3_i32, 0, 1, 5, -10];
    /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
    /// ```
    #[inline]
    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
    #[rustc_do_not_const_check]
    fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item) -> B,
    {
        #[inline]
        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
            move |x| (f(&x), x)
        }

        #[inline]
        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
            x_p.cmp(y_p)
        }

        let (_, x) = self.map(key(f)).max_by(compare)?;
        Some(x)
    }

    /// Returns the element that gives the maximum value with respect to the
    /// specified comparison function.
    ///
    /// If several elements are equally maximum, the last element is
    /// returned. If the iterator is empty, [`None`] is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [-3_i32, 0, 1, 5, -10];
    /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
    /// ```
    #[inline]
    #[stable(feature = "iter_max_by", since = "1.15.0")]
    #[rustc_do_not_const_check]
    fn max_by<F>(self, compare: F) -> Option<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    {
        #[inline]
        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
            move |x, y| cmp::max_by(x, y, &mut compare)
        }

        self.reduce(fold(compare))
    }

    /// Returns the element that gives the minimum value from the
    /// specified function.
    ///
    /// If several elements are equally minimum, the first element is
    /// returned. If the iterator is empty, [`None`] is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [-3_i32, 0, 1, 5, -10];
    /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
    /// ```
    #[inline]
    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
    #[rustc_do_not_const_check]
    fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item) -> B,
    {
        #[inline]
        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
            move |x| (f(&x), x)
        }

        #[inline]
        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
            x_p.cmp(y_p)
        }

        let (_, x) = self.map(key(f)).min_by(compare)?;
        Some(x)
    }

    /// Returns the element that gives the minimum value with respect to the
    /// specified comparison function.
    ///
    /// If several elements are equally minimum, the first element is
    /// returned. If the iterator is empty, [`None`] is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [-3_i32, 0, 1, 5, -10];
    /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
    /// ```
    #[inline]
    #[stable(feature = "iter_min_by", since = "1.15.0")]
    #[rustc_do_not_const_check]
    fn min_by<F>(self, compare: F) -> Option<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    {
        #[inline]
        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
            move |x, y| cmp::min_by(x, y, &mut compare)
        }

        self.reduce(fold(compare))
    }

    /// Reverses an iterator's direction.
    ///
    /// Usually, iterators iterate from left to right. After using `rev()`,
    /// an iterator will instead iterate from right to left.
    ///
    /// This is only possible if the iterator has an end, so `rev()` only
    /// works on [`DoubleEndedIterator`]s.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter().rev();
    ///
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[doc(alias = "reverse")]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn rev(self) -> Rev<Self>
    where
        Self: Sized + DoubleEndedIterator,
    {
        Rev::new(self)
    }

    /// Converts an iterator of pairs into a pair of containers.
    ///
    /// `unzip()` consumes an entire iterator of pairs, producing two
    /// collections: one from the left elements of the pairs, and one
    /// from the right elements.
    ///
    /// This function is, in some sense, the opposite of [`zip`].
    ///
    /// [`zip`]: Iterator::zip
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [(1, 2), (3, 4), (5, 6)];
    ///
    /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
    ///
    /// assert_eq!(left, [1, 3, 5]);
    /// assert_eq!(right, [2, 4, 6]);
    ///
    /// // you can also unzip multiple nested tuples at once
    /// let a = [(1, (2, 3)), (4, (5, 6))];
    ///
    /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
    /// assert_eq!(x, [1, 4]);
    /// assert_eq!(y, [2, 5]);
    /// assert_eq!(z, [3, 6]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
    where
        FromA: Default + Extend<A>,
        FromB: Default + Extend<B>,
        Self: Sized + Iterator<Item = (A, B)>,
    {
        let mut unzipped: (FromA, FromB) = Default::default();
        unzipped.extend(self);
        unzipped
    }

    /// Creates an iterator which copies all of its elements.
    ///
    /// This is useful when you have an iterator over `&T`, but you need an
    /// iterator over `T`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let v_copied: Vec<_> = a.iter().copied().collect();
    ///
    /// // copied is the same as .map(|&x| x)
    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
    ///
    /// assert_eq!(v_copied, vec![1, 2, 3]);
    /// assert_eq!(v_map, vec![1, 2, 3]);
    /// ```
    #[stable(feature = "iter_copied", since = "1.36.0")]
    #[rustc_do_not_const_check]
    fn copied<'a, T: 'a>(self) -> Copied<Self>
    where
        Self: Sized + Iterator<Item = &'a T>,
        T: Copy,
    {
        Copied::new(self)
    }

    /// Creates an iterator which [`clone`]s all of its elements.
    ///
    /// This is useful when you have an iterator over `&T`, but you need an
    /// iterator over `T`.
    ///
    /// There is no guarantee whatsoever about the `clone` method actually
    /// being called *or* optimized away. So code should not depend on
    /// either.
    ///
    /// [`clone`]: Clone::clone
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let v_cloned: Vec<_> = a.iter().cloned().collect();
    ///
    /// // cloned is the same as .map(|&x| x), for integers
    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
    ///
    /// assert_eq!(v_cloned, vec![1, 2, 3]);
    /// assert_eq!(v_map, vec![1, 2, 3]);
    /// ```
    ///
    /// To get the best performance, try to clone late:
    ///
    /// ```
    /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
    /// // don't do this:
    /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
    /// assert_eq!(&[vec![23]], &slower[..]);
    /// // instead call `cloned` late
    /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
    /// assert_eq!(&[vec![23]], &faster[..]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_do_not_const_check]
    fn cloned<'a, T: 'a>(self) -> Cloned<Self>
    where
        Self: Sized + Iterator<Item = &'a T>,
        T: Clone,
    {
        Cloned::new(self)
    }

    /// Repeats an iterator endlessly.
    ///
    /// Instead of stopping at [`None`], the iterator will instead start again,
    /// from the beginning. After iterating again, it will start at the
    /// beginning again. And again. And again. Forever. Note that in case the
    /// original iterator is empty, the resulting iterator will also be empty.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut it = a.iter().cycle();
    ///
    /// assert_eq!(it.next(), Some(&1));
    /// assert_eq!(it.next(), Some(&2));
    /// assert_eq!(it.next(), Some(&3));
    /// assert_eq!(it.next(), Some(&1));
    /// assert_eq!(it.next(), Some(&2));
    /// assert_eq!(it.next(), Some(&3));
    /// assert_eq!(it.next(), Some(&1));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    #[rustc_do_not_const_check]
    fn cycle(self) -> Cycle<Self>
    where
        Self: Sized + Clone,
    {
        Cycle::new(self)
    }

    /// Returns an iterator over `N` elements of the iterator at a time.
    ///
    /// The chunks do not overlap. If `N` does not divide the length of the
    /// iterator, then the last up to `N-1` elements will be omitted and can be
    /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
    /// function of the iterator.
    ///
    /// # Panics
    ///
    /// Panics if `N` is 0.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_array_chunks)]
    ///
    /// let mut iter = "lorem".chars().array_chunks();
    /// assert_eq!(iter.next(), Some(['l', 'o']));
    /// assert_eq!(iter.next(), Some(['r', 'e']));
    /// assert_eq!(iter.next(), None);
    /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
    /// ```
    ///
    /// ```
    /// #![feature(iter_array_chunks)]
    ///
    /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
    /// //          ^-----^  ^------^
    /// for [x, y, z] in data.iter().array_chunks() {
    ///     assert_eq!(x + y + z, 4);
    /// }
    /// ```
    #[track_caller]
    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
    #[rustc_do_not_const_check]
    fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
    where
        Self: Sized,
    {
        ArrayChunks::new(self)
    }

    /// Sums the elements of an iterator.
    ///
    /// Takes each element, adds them together, and returns the result.
    ///
    /// An empty iterator returns the zero value of the type.
    ///
    /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
    /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
    ///
    /// # Panics
    ///
    /// When calling `sum()` and a primitive integer type is being returned, this
    /// method will panic if the computation overflows and debug assertions are
    /// enabled.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// let sum: i32 = a.iter().sum();
    ///
    /// assert_eq!(sum, 6);
    /// ```
    #[stable(feature = "iter_arith", since = "1.11.0")]
    #[rustc_do_not_const_check]
    fn sum<S>(self) -> S
    where
        Self: Sized,
        S: Sum<Self::Item>,
    {
        Sum::sum(self)
    }

    /// Iterates over the entire iterator, multiplying all the elements
    ///
    /// An empty iterator returns the one value of the type.
    ///
    /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
    /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
    ///
    /// # Panics
    ///
    /// When calling `product()` and a primitive integer type is being returned,
    /// method will panic if the computation overflows and debug assertions are
    /// enabled.
    ///
    /// # Examples
    ///
    /// ```
    /// fn factorial(n: u32) -> u32 {
    ///     (1..=n).product()
    /// }
    /// assert_eq!(factorial(0), 1);
    /// assert_eq!(factorial(1), 1);
    /// assert_eq!(factorial(5), 120);
    /// ```
    #[stable(feature = "iter_arith", since = "1.11.0")]
    #[rustc_do_not_const_check]
    fn product<P>(self) -> P
    where
        Self: Sized,
        P: Product<Self::Item>,
    {
        Product::product(self)
    }

    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    /// of another.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cmp::Ordering;
    ///
    /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
    /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
    /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn cmp<I>(self, other: I) -> Ordering
    where
        I: IntoIterator<Item = Self::Item>,
        Self::Item: Ord,
        Self: Sized,
    {
        self.cmp_by(other, |x, y| x.cmp(&y))
    }

    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    /// of another with respect to the specified comparison function.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_order_by)]
    ///
    /// use std::cmp::Ordering;
    ///
    /// let xs = [1, 2, 3, 4];
    /// let ys = [1, 4, 9, 16];
    ///
    /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
    /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
    /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
    /// ```
    #[unstable(feature = "iter_order_by", issue = "64295")]
    #[rustc_do_not_const_check]
    fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
    where
        Self: Sized,
        I: IntoIterator,
        F: FnMut(Self::Item, I::Item) -> Ordering,
    {
        #[inline]
        fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
        where
            F: FnMut(X, Y) -> Ordering,
        {
            move |x, y| match cmp(x, y) {
                Ordering::Equal => ControlFlow::Continue(()),
                non_eq => ControlFlow::Break(non_eq),
            }
        }

        match iter_compare(self, other.into_iter(), compare(cmp)) {
            ControlFlow::Continue(ord) => ord,
            ControlFlow::Break(ord) => ord,
        }
    }

    /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
    /// this [`Iterator`] with those of another. The comparison works like short-circuit
    /// evaluation, returning a result without comparing the remaining elements.
    /// As soon as an order can be determined, the evaluation stops and a result is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cmp::Ordering;
    ///
    /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
    /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
    /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
    /// ```
    ///
    /// For floating-point numbers, NaN does not have a total order and will result
    /// in `None` when compared:
    ///
    /// ```
    /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
    /// ```
    ///
    /// The results are determined by the order of evaluation.
    ///
    /// ```
    /// use std::cmp::Ordering;
    ///
    /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
    /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
    /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
    /// ```
    ///
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn partial_cmp<I>(self, other: I) -> Option<Ordering>
    where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
    }

    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    /// of another with respect to the specified comparison function.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_order_by)]
    ///
    /// use std::cmp::Ordering;
    ///
    /// let xs = [1.0, 2.0, 3.0, 4.0];
    /// let ys = [1.0, 4.0, 9.0, 16.0];
    ///
    /// assert_eq!(
    ///     xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
    ///     Some(Ordering::Less)
    /// );
    /// assert_eq!(
    ///     xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
    ///     Some(Ordering::Equal)
    /// );
    /// assert_eq!(
    ///     xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
    ///     Some(Ordering::Greater)
    /// );
    /// ```
    #[unstable(feature = "iter_order_by", issue = "64295")]
    #[rustc_do_not_const_check]
    fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
    where
        Self: Sized,
        I: IntoIterator,
        F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
    {
        #[inline]
        fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
        where
            F: FnMut(X, Y) -> Option<Ordering>,
        {
            move |x, y| match partial_cmp(x, y) {
                Some(Ordering::Equal) => ControlFlow::Continue(()),
                non_eq => ControlFlow::Break(non_eq),
            }
        }

        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
            ControlFlow::Continue(ord) => Some(ord),
            ControlFlow::Break(ord) => ord,
        }
    }

    /// Determines if the elements of this [`Iterator`] are equal to those of
    /// another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().eq([1].iter()), true);
    /// assert_eq!([1].iter().eq([1, 2].iter()), false);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn eq<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialEq<I::Item>,
        Self: Sized,
    {
        self.eq_by(other, |x, y| x == y)
    }

    /// Determines if the elements of this [`Iterator`] are equal to those of
    /// another with respect to the specified equality function.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_order_by)]
    ///
    /// let xs = [1, 2, 3, 4];
    /// let ys = [1, 4, 9, 16];
    ///
    /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
    /// ```
    #[unstable(feature = "iter_order_by", issue = "64295")]
    #[rustc_do_not_const_check]
    fn eq_by<I, F>(self, other: I, eq: F) -> bool
    where
        Self: Sized,
        I: IntoIterator,
        F: FnMut(Self::Item, I::Item) -> bool,
    {
        #[inline]
        fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
        where
            F: FnMut(X, Y) -> bool,
        {
            move |x, y| {
                if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
            }
        }

        match iter_compare(self, other.into_iter(), compare(eq)) {
            ControlFlow::Continue(ord) => ord == Ordering::Equal,
            ControlFlow::Break(()) => false,
        }
    }

    /// Determines if the elements of this [`Iterator`] are not equal to those of
    /// another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().ne([1].iter()), false);
    /// assert_eq!([1].iter().ne([1, 2].iter()), true);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn ne<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialEq<I::Item>,
        Self: Sized,
    {
        !self.eq(other)
    }

    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    /// less than those of another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().lt([1].iter()), false);
    /// assert_eq!([1].iter().lt([1, 2].iter()), true);
    /// assert_eq!([1, 2].iter().lt([1].iter()), false);
    /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn lt<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        self.partial_cmp(other) == Some(Ordering::Less)
    }

    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    /// less or equal to those of another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().le([1].iter()), true);
    /// assert_eq!([1].iter().le([1, 2].iter()), true);
    /// assert_eq!([1, 2].iter().le([1].iter()), false);
    /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn le<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
    }

    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    /// greater than those of another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().gt([1].iter()), false);
    /// assert_eq!([1].iter().gt([1, 2].iter()), false);
    /// assert_eq!([1, 2].iter().gt([1].iter()), true);
    /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn gt<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        self.partial_cmp(other) == Some(Ordering::Greater)
    }

    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    /// greater than or equal to those of another.
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!([1].iter().ge([1].iter()), true);
    /// assert_eq!([1].iter().ge([1, 2].iter()), false);
    /// assert_eq!([1, 2].iter().ge([1].iter()), true);
    /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
    /// ```
    #[stable(feature = "iter_order", since = "1.5.0")]
    #[rustc_do_not_const_check]
    fn ge<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
    }

    /// Checks if the elements of this iterator are sorted.
    ///
    /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
    /// iterator yields exactly zero or one element, `true` is returned.
    ///
    /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
    /// implies that this function returns `false` if any two consecutive items are not
    /// comparable.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_sorted)]
    ///
    /// assert!([1, 2, 2, 9].iter().is_sorted());
    /// assert!(![1, 3, 2, 4].iter().is_sorted());
    /// assert!([0].iter().is_sorted());
    /// assert!(std::iter::empty::<i32>().is_sorted());
    /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
    /// ```
    #[inline]
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    #[rustc_do_not_const_check]
    fn is_sorted(self) -> bool
    where
        Self: Sized,
        Self::Item: PartialOrd,
    {
        self.is_sorted_by(PartialOrd::partial_cmp)
    }

    /// Checks if the elements of this iterator are sorted using the given comparator function.
    ///
    /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
    /// function to determine the ordering of two elements. Apart from that, it's equivalent to
    /// [`is_sorted`]; see its documentation for more information.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_sorted)]
    ///
    /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
    /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
    /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
    /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
    /// ```
    ///
    /// [`is_sorted`]: Iterator::is_sorted
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    #[rustc_do_not_const_check]
    fn is_sorted_by<F>(mut self, compare: F) -> bool
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
    {
        #[inline]
        fn check<'a, T>(
            last: &'a mut T,
            mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
        ) -> impl FnMut(T) -> bool + 'a {
            move |curr| {
                if let Some(Ordering::Greater) | None = compare(&last, &curr) {
                    return false;
                }
                *last = curr;
                true
            }
        }

        let mut last = match self.next() {
            Some(e) => e,
            None => return true,
        };

        self.all(check(&mut last, compare))
    }

    /// Checks if the elements of this iterator are sorted using the given key extraction
    /// function.
    ///
    /// Instead of comparing the iterator's elements directly, this function compares the keys of
    /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
    /// its documentation for more information.
    ///
    /// [`is_sorted`]: Iterator::is_sorted
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_sorted)]
    ///
    /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
    /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
    /// ```
    #[inline]
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    #[rustc_do_not_const_check]
    fn is_sorted_by_key<F, K>(self, f: F) -> bool
    where
        Self: Sized,
        F: FnMut(Self::Item) -> K,
        K: PartialOrd,
    {
        self.map(f).is_sorted()
    }

    /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
    // The unusual name is to avoid name collisions in method resolution
    // see #76479.
    #[inline]
    #[doc(hidden)]
    #[unstable(feature = "trusted_random_access", issue = "none")]
    #[rustc_do_not_const_check]
    unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        unreachable!("Always specialized");
    }
}

/// Compares two iterators element-wise using the given function.
///
/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
/// the iterators.
///
/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
#[inline]
fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
where
    A: Iterator,
    B: Iterator,
    F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
{
    #[inline]
    fn compare<'a, B, X, T>(
        b: &'a mut B,
        mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
    ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
    where
        B: Iterator,
    {
        move |x| match b.next() {
            None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
            Some(y) => f(x, y).map_break(ControlFlow::Break),
        }
    }

    match a.try_for_each(compare(&mut b, f)) {
        ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
            None => Ordering::Equal,
            Some(_) => Ordering::Less,
        }),
        ControlFlow::Break(x) => x,
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator + ?Sized> Iterator for &mut I {
    type Item = I::Item;
    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        (**self).next()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        (**self).advance_by(n)
    }
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        (**self).nth(n)
    }
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        self.spec_fold(init, f)
    }
    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        self.spec_try_fold(init, f)
    }
}

/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
trait IteratorRefSpec: Iterator {
    fn spec_fold<B, F>(self, init: B, f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B;

    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>;
}

impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
    default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        let mut accum = init;
        while let Some(x) = self.next() {
            accum = f(accum, x);
        }
        accum
    }

    default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        let mut accum = init;
        while let Some(x) = self.next() {
            accum = f(accum, x)?;
        }
        try { accum }
    }
}

impl<I: Iterator> IteratorRefSpec for &mut I {
    impl_fold_via_try_fold! { spec_fold -> spec_try_fold }

    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        (**self).try_fold(init, f)
    }
}