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
// Copyright 2013-2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use cmp::Ordering;
use ops::Try;

use super::{AlwaysOk, LoopState};
use super::{Chain, Cycle, Cloned, Enumerate, Filter, FilterMap, FlatMap, Fuse};
use super::{Inspect, Map, Peekable, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile, Rev};
use super::{Zip, Sum, Product};
use super::{ChainState, FromIterator, ZipImpl};

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

/// An interface 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]: index.html
/// [impl]: index.html#implementing-iterator
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
    on(
        _Self="&str",
        label="`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
    ),
    label="`{Self}` is not an iterator; maybe try calling `.iter()` or a similar method"
)]
#[doc(spotlight)]
pub trait Iterator {
    /// The type of the elements being iterated over.
    #[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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`Some(Item)`]: ../../std/option/enum.Option.html#variant.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());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn next(&mut self) -> Option<Self::Item>;

    /// 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 [`Option`]`<`[`usize`]`>`.
    /// 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 `(0, None)` which is correct for any
    /// iterator.
    ///
    /// [`usize`]: ../../std/primitive.usize.html
    /// [`Option`]: ../../std/option/enum.Option.html
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// let iter = a.iter();
    ///
    /// assert_eq!((3, Some(3)), iter.size_hint());
    /// ```
    ///
    /// A more complex example:
    ///
    /// ```
    /// // The even numbers from zero to ten.
    /// 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_value(), None), iter.size_hint());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }

    /// Consumes the iterator, counting the number of iterations and returning it.
    ///
    /// This method will evaluate the iterator until its [`next`] returns
    /// [`None`]. Once [`None`] is encountered, `count()` returns the number of
    /// times it called [`next`].
    ///
    /// [`next`]: #tymethod.next
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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.
    ///
    /// [`usize::MAX`]: ../../std/isize/constant.MAX.html
    ///
    /// # 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]
    #[rustc_inherit_overflow_checks]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn count(self) -> usize where Self: Sized {
        // Might overflow.
        self.fold(0, |cnt, _| cnt + 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn last(self) -> Option<Self::Item> where Self: Sized {
        let mut last = None;
        for x in self { last = Some(x); }
        last
    }

    /// 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
        for x in self {
            if n == 0 { return Some(x) }
            n -= 1;
        }
        None
    }

    /// Creates an iterator starting at the same point, but stepping by
    /// the given amount at each iteration.
    ///
    /// Note that it will always return the first element of the iterator,
    /// regardless of the step given.
    ///
    /// # Panics
    ///
    /// The method will panic if the given step is `0`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iterator_step_by)]
    /// let a = [0, 1, 2, 3, 4, 5];
    /// let mut iter = a.into_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]
    #[unstable(feature = "iterator_step_by",
               reason = "unstable replacement of Range::step_by",
               issue = "27741")]
    fn step_by(self, step: usize) -> StepBy<Self> where Self: Sized {
        assert!(step != 0);
        StepBy{iter: self, step: step - 1, first_take: true}
    }

    /// 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. 🔗
    ///
    /// # 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:
    ///
    /// [`IntoIterator`]: trait.IntoIterator.html
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// ```
    /// 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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> where
        Self: Sized, U: IntoIterator<Item=Self::Item>,
    {
        Chain{a: self, b: other.into_iter(), state: ChainState::Both}
    }

    /// '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.
    ///
    /// When either iterator returns [`None`], all further calls to [`next`]
    /// will return [`None`].
    ///
    /// # 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:
    ///
    /// [`IntoIterator`]: trait.IntoIterator.html
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// ```
    /// 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]);
    /// ```
    ///
    /// [`enumerate`]: trait.Iterator.html#method.enumerate
    /// [`next`]: ../../std/iter/trait.Iterator.html#tymethod.next
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where
        Self: Sized, U: IntoIterator
    {
        Zip::new(self, other.into_iter())
    }

    /// 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/first-edition/loops.html#for
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.into_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);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn map<B, F>(self, f: F) -> Map<Self, F> where
        Self: Sized, F: FnMut(Self::Item) -> B,
    {
        Map{iter: self, f: 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 adaptors like `Chain`.
    ///
    /// [`for`]: ../../book/first-edition/loops.html#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")]
    fn for_each<F>(self, mut f: F) where
        Self: Sized, F: FnMut(Self::Item),
    {
        self.fold((), move |(), item| f(item));
    }

    /// Creates an iterator which uses a closure to determine if an element
    /// should be yielded.
    ///
    /// The closure must return `true` or `false`. `filter()` creates an
    /// iterator which calls this closure on each element. If the closure
    /// returns `true`, then the element is returned. If the closure returns
    /// `false`, it will try again, and call the closure on the next element,
    /// seeing if it passes the test.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [0i32, 1, 2];
    ///
    /// let mut iter = a.into_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.into_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.into_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.into_iter().filter(|&&x| x > 1); // two &s
    ///
    /// assert_eq!(iter.next(), Some(&2));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// of these layers.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn filter<P>(self, predicate: P) -> Filter<Self, P> where
        Self: Sized, P: FnMut(&Self::Item) -> bool,
    {
        Filter{iter: self, predicate: predicate}
    }

    /// Creates an iterator that both filters and maps.
    ///
    /// The closure must return an [`Option<T>`]. `filter_map` creates an
    /// iterator which calls this closure on each element. If the closure
    /// returns [`Some(element)`][`Some`], then that element is returned. If the
    /// closure returns [`None`], it will try again, and call the closure on the
    /// next element, seeing if it will return [`Some`].
    ///
    /// Why `filter_map` and not just [`filter`] and [`map`]? The key is in this
    /// part:
    ///
    /// [`filter`]: #method.filter
    /// [`map`]: #method.map
    ///
    /// > If the closure returns [`Some(element)`][`Some`], then that element is returned.
    ///
    /// In other words, it removes the [`Option<T>`] layer automatically. If your
    /// mapping is already returning an [`Option<T>`] and you want to skip over
    /// [`None`]s, then `filter_map` is much, much nicer to use.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = ["1", "lol", "3", "NaN", "5"];
    ///
    /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
    ///
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), Some(5));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// Here's the same example, but with [`filter`] and [`map`]:
    ///
    /// ```
    /// let a = ["1", "lol", "3", "NaN", "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(3));
    /// assert_eq!(iter.next(), Some(5));
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Some`]: ../../std/option/enum.Option.html#variant.Some
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
        Self: Sized, F: FnMut(Self::Item) -> Option<B>,
    {
        FilterMap { iter: self, f: 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`].
    ///
    /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
    /// [`usize`]: ../../std/primitive.usize.html
    /// [`zip`]: #method.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")]
    fn enumerate(self) -> Enumerate<Self> where Self: Sized {
        Enumerate { iter: self, count: 0 }
    }

    /// Creates an iterator which can use `peek` to look at the next element of
    /// the iterator without consuming it.
    ///
    /// Adds a [`peek`] method to an iterator. See its documentation for
    /// more information.
    ///
    /// Note that the underlying iterator is still advanced when [`peek`] is
    /// 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.
    ///
    /// [`peek`]: struct.Peekable.html#method.peek
    /// [`next`]: ../../std/iter/trait.Iterator.html#tymethod.next
    ///
    /// # 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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn peekable(self) -> Peekable<Self> where Self: Sized {
        Peekable{iter: self, peeked: None}
    }

    /// Creates an iterator that [`skip`]s elements based on a predicate.
    ///
    /// [`skip`]: #method.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.into_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 is a double reference:
    ///
    /// ```
    /// let a = [-1, 0, 1];
    ///
    /// let mut iter = a.into_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.into_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]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where
        Self: Sized, P: FnMut(&Self::Item) -> bool,
    {
        SkipWhile{iter: self, flag: false, predicate: 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.into_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.into_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.into_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.into_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 or
    /// some similar thing.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
        Self: Sized, P: FnMut(&Self::Item) -> bool,
    {
        TakeWhile{iter: self, flag: false, predicate: predicate}
    }

    /// Creates an iterator that skips the first `n` elements.
    ///
    /// After they have been consumed, the rest of the elements are yielded.
    ///
    /// # 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")]
    fn skip(self, n: usize) -> Skip<Self> where Self: Sized {
        Skip{iter: self, n: n}
    }

    /// Creates an iterator that yields its first `n` elements.
    ///
    /// # 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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn take(self, n: usize) -> Take<Self> where Self: Sized, {
        Take{iter: self, n: n}
    }

    /// An iterator adaptor similar to [`fold`] that holds internal state and
    /// produces a new iterator.
    ///
    /// [`fold`]: #method.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
    /// yielded by the iterator.
    ///
    /// [`Option`]: ../../std/option/enum.Option.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.iter().scan(1, |state, &x| {
    ///     // each iteration, we'll multiply the state by the element
    ///     *state = *state * x;
    ///
    ///     // the value passed on to the next iteration
    ///     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")]
    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{iter: self, f: f, state: initial_state}
    }

    /// 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.
    ///
    /// 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`]: #method.map
    ///
    /// # 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")]
    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
        where Self: Sized, U: IntoIterator, F: FnMut(Self::Item) -> U,
    {
        FlatMap{iter: self, f: f, frontiter: None, backiter: None }
    }

    /// 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`Some(T)`]: ../../std/option/enum.Option.html#variant.Some
    ///
    /// # 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")]
    fn fuse(self) -> Fuse<Self> where Self: Sized {
        Fuse{iter: self, done: false}
    }

    /// Do 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 much more common for `inspect()` to be used as a debugging tool
    /// than to exist in your final code, but never say never.
    ///
    /// # 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
    /// 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
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn inspect<F>(self, f: F) -> Inspect<Self, F> where
        Self: Sized, F: FnMut(&Self::Item),
    {
        Inspect{iter: self, f: f}
    }

    /// Borrows an iterator, rather than consuming it.
    ///
    /// This is useful to allow applying iterator adaptors while still
    /// retaining ownership of the original iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let iter = a.into_iter();
    ///
    /// let sum: i32 = iter.take(5)
    ///                    .fold(0, |acc, &i| acc + i );
    ///
    /// assert_eq!(sum, 6);
    ///
    /// // if we try to use iter again, it won't work. The following line
    /// // gives "error: use of moved value: `iter`
    /// // assert_eq!(iter.next(), None);
    ///
    /// // let's try that again
    /// let a = [1, 2, 3];
    ///
    /// let mut iter = a.into_iter();
    ///
    /// // instead, we add in a .by_ref()
    /// let sum: i32 = iter.by_ref()
    ///                    .take(2)
    ///                    .fold(0, |acc, &i| acc + i );
    ///
    /// assert_eq!(sum, 3);
    ///
    /// // now this is just fine:
    /// assert_eq!(iter.next(), Some(&3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    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.
    ///
    /// One of the keys to `collect()`'s power is that many things you might
    /// not think of as 'collections' actually are. For example, a [`String`]
    /// is a collection of [`char`]s. And a collection of
    /// [`Result<T, E>`][`Result`] can be thought of as single
    /// [`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`]: ../../std/iter/trait.Iterator.html#tymethod.next
    /// [`String`]: ../../std/string/struct.String.html
    /// [`char`]: ../../std/primitive.char.html
    /// [`Result`]: ../../std/result/enum.Result.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
        FromIterator::from_iter(self)
    }

    /// 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`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [1, 2, 3];
    ///
    /// let (even, odd): (Vec<i32>, Vec<i32>) = 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")]
    fn partition<B, F>(self, mut f: F) -> (B, B) where
        Self: Sized,
        B: Default + Extend<Self::Item>,
        F: FnMut(&Self::Item) -> bool
    {
        let mut left: B = Default::default();
        let mut right: B = Default::default();

        for x in self {
            if f(&x) {
                left.extend(Some(x))
            } else {
                right.extend(Some(x))
            }
        }

        (left, right)
    }

    /// 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
    ///
    /// Most 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 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:
    ///
    /// ```
    /// #![feature(iterator_try_fold)]
    /// 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:
    ///
    /// ```
    /// #![feature(iterator_try_fold)]
    /// 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));
    /// ```
    #[inline]
    #[unstable(feature = "iterator_try_fold", issue = "45594")]
    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<Ok=B>
    {
        let mut accum = init;
        while let Some(x) = self.next() {
            accum = f(accum, x)?;
        }
        Try::from_ok(accum)
    }

    /// An iterator method that applies a function, producing a single, final value.
    ///
    /// `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,
    /// may not terminate for infinite iterators, even on traits for which a
    /// result is determinable in finite time.
    ///
    /// # 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`.
    ///
    /// 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/first-edition/loops.html#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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn fold<B, F>(mut self, init: B, mut f: F) -> B where
        Self: Sized, F: FnMut(B, Self::Item) -> B,
    {
        self.try_fold(init, move |acc, x| AlwaysOk(f(acc, x))).0
    }

    /// 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")]
    fn all<F>(&mut self, mut f: F) -> bool where
        Self: Sized, F: FnMut(Self::Item) -> bool
    {
        self.try_fold((), move |(), x| {
            if f(x) { LoopState::Continue(()) }
            else { LoopState::Break(()) }
        }) == LoopState::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")]
    fn any<F>(&mut self, mut f: F) -> bool where
        Self: Sized,
        F: FnMut(Self::Item) -> bool
    {
        self.try_fold((), move |(), x| {
            if f(x) { LoopState::Break(()) }
            else { LoopState::Continue(()) }
        }) == LoopState::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`.
    ///
    /// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    {
        self.try_fold((), move |(), x| {
            if predicate(&x) { LoopState::Break(x) }
            else { LoopState::Continue(()) }
        }).break_value()
    }

    /// 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)`]: ../../std/option/enum.Option.html#variant.Some
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
    ///
    /// # 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]
    #[rustc_inherit_overflow_checks]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
        Self: Sized,
        P: FnMut(Self::Item) -> bool,
    {
        // The addition might panic on overflow
        self.try_fold(0, move |i, x| {
            if predicate(x) { LoopState::Break(i) }
            else { LoopState::Continue(i + 1) }
        }).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)`]: ../../std/option/enum.Option.html#variant.Some
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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];
    ///
    /// let mut iter = a.iter();
    ///
    /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
    ///
    /// // we can still use `iter`, as there are more elements.
    /// assert_eq!(iter.next(), Some(&1));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn rposition<P>(&mut self, mut 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`.
        let n = self.len();
        self.try_rfold(n, move |i, x| {
            let i = i - 1;
            if predicate(x) { LoopState::Break(i) }
            else { LoopState::Continue(i) }
        }).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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn max(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
    {
        select_fold1(self,
                     |_| (),
                     // switch to y even if it is only equal, to preserve
                     // stability.
                     |_, x, _, y| *x <= *y)
            .map(|(_, x)| x)
    }

    /// 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn min(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
    {
        select_fold1(self,
                     |_| (),
                     // only switch to y if it is strictly smaller, to
                     // preserve stability.
                     |_, x, _, y| *x > *y)
            .map(|(_, x)| x)
    }

    /// 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
        where Self: Sized, F: FnMut(&Self::Item) -> B,
    {
        select_fold1(self,
                     f,
                     // switch to y even if it is only equal, to preserve
                     // stability.
                     |x_p, _, y_p, _| x_p <= y_p)
            .map(|(_, x)| 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn max_by<F>(self, mut compare: F) -> Option<Self::Item>
        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    {
        select_fold1(self,
                     |_| (),
                     // switch to y even if it is only equal, to preserve
                     // stability.
                     |_, x, _, y| Ordering::Greater != compare(x, y))
            .map(|(_, x)| x)
    }

    /// 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [-3_i32, 0, 1, 5, -10];
    /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
    /// ```
    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
    fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
        where Self: Sized, F: FnMut(&Self::Item) -> B,
    {
        select_fold1(self,
                     f,
                     // only switch to y if it is strictly smaller, to
                     // preserve stability.
                     |x_p, _, y_p, _| x_p > y_p)
            .map(|(_, x)| 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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")]
    fn min_by<F>(self, mut compare: F) -> Option<Self::Item>
        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    {
        select_fold1(self,
                     |_| (),
                     // switch to y even if it is strictly smaller, to
                     // preserve stability.
                     |_, x, _, y| Ordering::Greater == compare(x, y))
            .map(|(_, x)| x)
    }


    /// 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.
    ///
    /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
    ///
    /// # 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]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn rev(self) -> Rev<Self> where Self: Sized + DoubleEndedIterator {
        Rev{iter: 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`]: #method.zip
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let a = [(1, 2), (3, 4)];
    ///
    /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
    ///
    /// assert_eq!(left, [1, 3]);
    /// assert_eq!(right, [2, 4]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    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 ts: FromA = Default::default();
        let mut us: FromB = Default::default();

        self.for_each(|(t, u)| {
            ts.extend(Some(t));
            us.extend(Some(u));
        });

        (ts, us)
    }

    /// 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`.
    ///
    /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.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]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn cloned<'a, T: 'a>(self) -> Cloned<Self>
        where Self: Sized + Iterator<Item=&'a T>, T: Clone
    {
        Cloned { it: 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.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    ///
    /// # 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]
    fn cycle(self) -> Cycle<Self> where Self: Sized + Clone {
        Cycle{orig: self.clone(), iter: 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.
    ///
    /// # 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")]
    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.
    ///
    /// # 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..).take_while(|&i| i <= 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")]
    fn product<P>(self) -> P
        where Self: Sized,
              P: Product<Self::Item>,
    {
        Product::product(self)
    }

    /// Lexicographically compares the elements of this `Iterator` with those
    /// of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn cmp<I>(mut self, other: I) -> Ordering where
        I: IntoIterator<Item = Self::Item>,
        Self::Item: Ord,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => if other.next().is_none() {
                    return Ordering::Equal
                } else {
                    return Ordering::Less
                },
                Some(val) => val,
            };

            let y = match other.next() {
                None => return Ordering::Greater,
                Some(val) => val,
            };

            match x.cmp(&y) {
                Ordering::Equal => (),
                non_eq => return non_eq,
            }
        }
    }

    /// Lexicographically compares the elements of this `Iterator` with those
    /// of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn partial_cmp<I>(mut self, other: I) -> Option<Ordering> where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => if other.next().is_none() {
                    return Some(Ordering::Equal)
                } else {
                    return Some(Ordering::Less)
                },
                Some(val) => val,
            };

            let y = match other.next() {
                None => return Some(Ordering::Greater),
                Some(val) => val,
            };

            match x.partial_cmp(&y) {
                Some(Ordering::Equal) => (),
                non_eq => return non_eq,
            }
        }
    }

    /// Determines if the elements of this `Iterator` are equal to those of
    /// another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn eq<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialEq<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => return other.next().is_none(),
                Some(val) => val,
            };

            let y = match other.next() {
                None => return false,
                Some(val) => val,
            };

            if x != y { return false }
        }
    }

    /// Determines if the elements of this `Iterator` are unequal to those of
    /// another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn ne<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialEq<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => return other.next().is_some(),
                Some(val) => val,
            };

            let y = match other.next() {
                None => return true,
                Some(val) => val,
            };

            if x != y { return true }
        }
    }

    /// Determines if the elements of this `Iterator` are lexicographically
    /// less than those of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn lt<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => return other.next().is_some(),
                Some(val) => val,
            };

            let y = match other.next() {
                None => return false,
                Some(val) => val,
            };

            match x.partial_cmp(&y) {
                Some(Ordering::Less) => return true,
                Some(Ordering::Equal) => (),
                Some(Ordering::Greater) => return false,
                None => return false,
            }
        }
    }

    /// Determines if the elements of this `Iterator` are lexicographically
    /// less or equal to those of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn le<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => { other.next(); return true; },
                Some(val) => val,
            };

            let y = match other.next() {
                None => return false,
                Some(val) => val,
            };

            match x.partial_cmp(&y) {
                Some(Ordering::Less) => return true,
                Some(Ordering::Equal) => (),
                Some(Ordering::Greater) => return false,
                None => return false,
            }
        }
    }

    /// Determines if the elements of this `Iterator` are lexicographically
    /// greater than those of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn gt<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => { other.next(); return false; },
                Some(val) => val,
            };

            let y = match other.next() {
                None => return true,
                Some(val) => val,
            };

            match x.partial_cmp(&y) {
                Some(Ordering::Less) => return false,
                Some(Ordering::Equal) => (),
                Some(Ordering::Greater) => return true,
                None => return false,
            }
        }
    }

    /// Determines if the elements of this `Iterator` are lexicographically
    /// greater than or equal to those of another.
    #[stable(feature = "iter_order", since = "1.5.0")]
    fn ge<I>(mut self, other: I) -> bool where
        I: IntoIterator,
        Self::Item: PartialOrd<I::Item>,
        Self: Sized,
    {
        let mut other = other.into_iter();

        loop {
            let x = match self.next() {
                None => return other.next().is_none(),
                Some(val) => val,
            };

            let y = match other.next() {
                None => return true,
                Some(val) => val,
            };

            match x.partial_cmp(&y) {
                Some(Ordering::Less) => return false,
                Some(Ordering::Equal) => (),
                Some(Ordering::Greater) => return true,
                None => return false,
            }
        }
    }
}

/// Select an element from an iterator based on the given "projection"
/// and "comparison" function.
///
/// This is an idiosyncratic helper to try to factor out the
/// commonalities of {max,min}{,_by}. In particular, this avoids
/// having to implement optimizations several times.
#[inline]
fn select_fold1<I, B, FProj, FCmp>(mut it: I,
                                   mut f_proj: FProj,
                                   mut f_cmp: FCmp) -> Option<(B, I::Item)>
    where I: Iterator,
          FProj: FnMut(&I::Item) -> B,
          FCmp: FnMut(&B, &I::Item, &B, &I::Item) -> bool
{
    // start with the first element as our selection. This avoids
    // having to use `Option`s inside the loop, translating to a
    // sizeable performance gain (6x in one case).
    it.next().map(|first| {
        let first_p = f_proj(&first);

        it.fold((first_p, first), |(sel_p, sel), x| {
            let x_p = f_proj(&x);
            if f_cmp(&sel_p, &sel, &x_p, &x) {
                (x_p, x)
            } else {
                (sel_p, sel)
            }
        })
    })
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, I: Iterator + ?Sized> Iterator for &'a mut I {
    type Item = I::Item;
    fn next(&mut self) -> Option<I::Item> { (**self).next() }
    fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        (**self).nth(n)
    }
}