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
//! Optional values.
//!
//! Type [`Option`] represents an optional value: every [`Option`]
//! is either [`Some`] and contains a value, or [`None`], and
//! does not. [`Option`] types are very common in Rust code, as
//! they have a number of uses:
//!
//! * Initial values
//! * Return values for functions that are not defined
//!   over their entire input range (partial functions)
//! * Return value for otherwise reporting simple errors, where [`None`] is
//!   returned on error
//! * Optional struct fields
//! * Struct fields that can be loaned or "taken"
//! * Optional function arguments
//! * Nullable pointers
//! * Swapping things out of difficult situations
//!
//! [`Option`]s are commonly paired with pattern matching to query the presence
//! of a value and take action, always accounting for the [`None`] case.
//!
//! ```
//! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
//!     if denominator == 0.0 {
//!         None
//!     } else {
//!         Some(numerator / denominator)
//!     }
//! }
//!
//! // The return value of the function is an option
//! let result = divide(2.0, 3.0);
//!
//! // Pattern match to retrieve the value
//! match result {
//!     // The division was valid
//!     Some(x) => println!("Result: {x}"),
//!     // The division was invalid
//!     None    => println!("Cannot divide by 0"),
//! }
//! ```
//!
//
// FIXME: Show how `Option` is used in practice, with lots of methods
//
//! # Options and pointers ("nullable" pointers)
//!
//! Rust's pointer types must always point to a valid location; there are
//! no "null" references. Instead, Rust has *optional* pointers, like
//! the optional owned box, <code>[Option]<[Box\<T>]></code>.
//!
//! [Box\<T>]: ../../std/boxed/struct.Box.html
//!
//! The following example uses [`Option`] to create an optional box of
//! [`i32`]. Notice that in order to use the inner [`i32`] value, the
//! `check_optional` function first needs to use pattern matching to
//! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
//! not ([`None`]).
//!
//! ```
//! let optional = None;
//! check_optional(optional);
//!
//! let optional = Some(Box::new(9000));
//! check_optional(optional);
//!
//! fn check_optional(optional: Option<Box<i32>>) {
//!     match optional {
//!         Some(p) => println!("has value {p}"),
//!         None => println!("has no value"),
//!     }
//! }
//! ```
//!
//! # Representation
//!
//! Rust guarantees to optimize the following types `T` such that
//! [`Option<T>`] has the same size as `T`:
//!
//! * [`Box<U>`]
//! * `&U`
//! * `&mut U`
//! * `fn`, `extern "C" fn`[^extern_fn]
//! * [`num::NonZero*`]
//! * [`ptr::NonNull<U>`]
//! * `#[repr(transparent)]` struct around one of the types in this list.
//!
//! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
//!
//! [`Box<U>`]: ../../std/boxed/struct.Box.html
//! [`num::NonZero*`]: crate::num
//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
//!
//! This is called the "null pointer optimization" or NPO.
//!
//! It is further guaranteed that, for the cases above, one can
//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
//! is undefined behaviour).
//!
//! # Method overview
//!
//! In addition to working with pattern matching, [`Option`] provides a wide
//! variety of different methods.
//!
//! ## Querying the variant
//!
//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
//! is [`Some`] or [`None`], respectively.
//!
//! [`is_none`]: Option::is_none
//! [`is_some`]: Option::is_some
//!
//! ## Adapters for working with references
//!
//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
//!   <code>[Option]<[&]T::[Target]></code>
//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
//!   <code>[Option]<[&mut] T::[Target]></code>
//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
//!   <code>[Option]<[Pin]<[&]T>></code>
//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
//!   <code>[Option]<[Pin]<[&mut] T>></code>
//!
//! [&]: reference "shared reference"
//! [&mut]: reference "mutable reference"
//! [Target]: Deref::Target "ops::Deref::Target"
//! [`as_deref`]: Option::as_deref
//! [`as_deref_mut`]: Option::as_deref_mut
//! [`as_mut`]: Option::as_mut
//! [`as_pin_mut`]: Option::as_pin_mut
//! [`as_pin_ref`]: Option::as_pin_ref
//! [`as_ref`]: Option::as_ref
//!
//! ## Extracting the contained value
//!
//! These methods extract the contained value in an [`Option<T>`] when it
//! is the [`Some`] variant. If the [`Option`] is [`None`]:
//!
//! * [`expect`] panics with a provided custom message
//! * [`unwrap`] panics with a generic message
//! * [`unwrap_or`] returns the provided default value
//! * [`unwrap_or_default`] returns the default value of the type `T`
//!   (which must implement the [`Default`] trait)
//! * [`unwrap_or_else`] returns the result of evaluating the provided
//!   function
//!
//! [`expect`]: Option::expect
//! [`unwrap`]: Option::unwrap
//! [`unwrap_or`]: Option::unwrap_or
//! [`unwrap_or_default`]: Option::unwrap_or_default
//! [`unwrap_or_else`]: Option::unwrap_or_else
//!
//! ## Transforming contained values
//!
//! These methods transform [`Option`] to [`Result`]:
//!
//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
//!   [`Err(err)`] using the provided default `err` value
//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
//!   a value of [`Err`] using the provided function
//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
//!   [`Result`] of an [`Option`]
//!
//! [`Err(err)`]: Err
//! [`Ok(v)`]: Ok
//! [`Some(v)`]: Some
//! [`ok_or`]: Option::ok_or
//! [`ok_or_else`]: Option::ok_or_else
//! [`transpose`]: Option::transpose
//!
//! These methods transform the [`Some`] variant:
//!
//! * [`filter`] calls the provided predicate function on the contained
//!   value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
//!   if the function returns `true`; otherwise, returns [`None`]
//! * [`flatten`] removes one level of nesting from an
//!   [`Option<Option<T>>`]
//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
//!   provided function to the contained value of [`Some`] and leaving
//!   [`None`] values unchanged
//!
//! [`Some(t)`]: Some
//! [`filter`]: Option::filter
//! [`flatten`]: Option::flatten
//! [`map`]: Option::map
//!
//! These methods transform [`Option<T>`] to a value of a possibly
//! different type `U`:
//!
//! * [`map_or`] applies the provided function to the contained value of
//!   [`Some`], or returns the provided default value if the [`Option`] is
//!   [`None`]
//! * [`map_or_else`] applies the provided function to the contained value
//!   of [`Some`], or returns the result of evaluating the provided
//!   fallback function if the [`Option`] is [`None`]
//!
//! [`map_or`]: Option::map_or
//! [`map_or_else`]: Option::map_or_else
//!
//! These methods combine the [`Some`] variants of two [`Option`] values:
//!
//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
//!   provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
//! * [`zip_with`] calls the provided function `f` and returns
//!   [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
//!   [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
//!
//! [`Some(f(s, o))`]: Some
//! [`Some(o)`]: Some
//! [`Some(s)`]: Some
//! [`Some((s, o))`]: Some
//! [`zip`]: Option::zip
//! [`zip_with`]: Option::zip_with
//!
//! ## Boolean operators
//!
//! These methods treat the [`Option`] as a boolean value, where [`Some`]
//! acts like [`true`] and [`None`] acts like [`false`]. There are two
//! categories of these methods: ones that take an [`Option`] as input, and
//! ones that take a function as input (to be lazily evaluated).
//!
//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
//! input, and produce an [`Option`] as output. Only the [`and`] method can
//! produce an [`Option<U>`] value having a different inner type `U` than
//! [`Option<T>`].
//!
//! | method  | self      | input     | output    |
//! |---------|-----------|-----------|-----------|
//! | [`and`] | `None`    | (ignored) | `None`    |
//! | [`and`] | `Some(x)` | `None`    | `None`    |
//! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` |
//! | [`or`]  | `None`    | `None`    | `None`    |
//! | [`or`]  | `None`    | `Some(y)` | `Some(y)` |
//! | [`or`]  | `Some(x)` | (ignored) | `Some(x)` |
//! | [`xor`] | `None`    | `None`    | `None`    |
//! | [`xor`] | `None`    | `Some(y)` | `Some(y)` |
//! | [`xor`] | `Some(x)` | `None`    | `Some(x)` |
//! | [`xor`] | `Some(x)` | `Some(y)` | `None`    |
//!
//! [`and`]: Option::and
//! [`or`]: Option::or
//! [`xor`]: Option::xor
//!
//! The [`and_then`] and [`or_else`] methods take a function as input, and
//! only evaluate the function when they need to produce a new value. Only
//! the [`and_then`] method can produce an [`Option<U>`] value having a
//! different inner type `U` than [`Option<T>`].
//!
//! | method       | self      | function input | function result | output    |
//! |--------------|-----------|----------------|-----------------|-----------|
//! | [`and_then`] | `None`    | (not provided) | (not evaluated) | `None`    |
//! | [`and_then`] | `Some(x)` | `x`            | `None`          | `None`    |
//! | [`and_then`] | `Some(x)` | `x`            | `Some(y)`       | `Some(y)` |
//! | [`or_else`]  | `None`    | (not provided) | `None`          | `None`    |
//! | [`or_else`]  | `None`    | (not provided) | `Some(y)`       | `Some(y)` |
//! | [`or_else`]  | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` |
//!
//! [`and_then`]: Option::and_then
//! [`or_else`]: Option::or_else
//!
//! This is an example of using methods like [`and_then`] and [`or`] in a
//! pipeline of method calls. Early stages of the pipeline pass failure
//! values ([`None`]) through unchanged, and continue processing on
//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
//! message if it receives [`None`].
//!
//! ```
//! # use std::collections::BTreeMap;
//! let mut bt = BTreeMap::new();
//! bt.insert(20u8, "foo");
//! bt.insert(42u8, "bar");
//! let res = [0u8, 1, 11, 200, 22]
//!     .into_iter()
//!     .map(|x| {
//!         // `checked_sub()` returns `None` on error
//!         x.checked_sub(1)
//!             // same with `checked_mul()`
//!             .and_then(|x| x.checked_mul(2))
//!             // `BTreeMap::get` returns `None` on error
//!             .and_then(|x| bt.get(&x))
//!             // Substitute an error message if we have `None` so far
//!             .or(Some(&"error!"))
//!             .copied()
//!             // Won't panic because we unconditionally used `Some` above
//!             .unwrap()
//!     })
//!     .collect::<Vec<_>>();
//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
//! ```
//!
//! ## Comparison operators
//!
//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
//! [`PartialOrd`] implementation.  With this order, [`None`] compares as
//! less than any [`Some`], and two [`Some`] compare the same way as their
//! contained values would in `T`.  If `T` also implements
//! [`Ord`], then so does [`Option<T>`].
//!
//! ```
//! assert!(None < Some(0));
//! assert!(Some(0) < Some(1));
//! ```
//!
//! ## Iterating over `Option`
//!
//! An [`Option`] can be iterated over. This can be helpful if you need an
//! iterator that is conditionally empty. The iterator will either produce
//! a single value (when the [`Option`] is [`Some`]), or produce no values
//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
//! the [`Option`] is [`None`].
//!
//! [`Some(v)`]: Some
//! [`empty()`]: crate::iter::empty
//! [`once(v)`]: crate::iter::once
//!
//! Iterators over [`Option<T>`] come in three types:
//!
//! * [`into_iter`] consumes the [`Option`] and produces the contained
//!   value
//! * [`iter`] produces an immutable reference of type `&T` to the
//!   contained value
//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
//!   contained value
//!
//! [`into_iter`]: Option::into_iter
//! [`iter`]: Option::iter
//! [`iter_mut`]: Option::iter_mut
//!
//! An iterator over [`Option`] can be useful when chaining iterators, for
//! example, to conditionally insert items. (It's not always necessary to
//! explicitly call an iterator constructor: many [`Iterator`] methods that
//! accept other iterators will also accept iterable types that implement
//! [`IntoIterator`], which includes [`Option`].)
//!
//! ```
//! let yep = Some(42);
//! let nope = None;
//! // chain() already calls into_iter(), so we don't have to do so
//! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
//! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
//! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
//! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);
//! ```
//!
//! One reason to chain iterators in this way is that a function returning
//! `impl Iterator` must have all possible return values be of the same
//! concrete type. Chaining an iterated [`Option`] can help with that.
//!
//! ```
//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
//!     // Explicit returns to illustrate return types matching
//!     match do_insert {
//!         true => return (0..4).chain(Some(42)).chain(4..8),
//!         false => return (0..4).chain(None).chain(4..8),
//!     }
//! }
//! println!("{:?}", make_iter(true).collect::<Vec<_>>());
//! println!("{:?}", make_iter(false).collect::<Vec<_>>());
//! ```
//!
//! If we try to do the same thing, but using [`once()`] and [`empty()`],
//! we can't return `impl Iterator` anymore because the concrete types of
//! the return values differ.
//!
//! [`empty()`]: crate::iter::empty
//! [`once()`]: crate::iter::once
//!
//! ```compile_fail,E0308
//! # use std::iter::{empty, once};
//! // This won't compile because all possible returns from the function
//! // must have the same concrete type.
//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
//!     // Explicit returns to illustrate return types not matching
//!     match do_insert {
//!         true => return (0..4).chain(once(42)).chain(4..8),
//!         false => return (0..4).chain(empty()).chain(4..8),
//!     }
//! }
//! ```
//!
//! ## Collecting into `Option`
//!
//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
//! which allows an iterator over [`Option`] values to be collected into an
//! [`Option`] of a collection of each contained value of the original
//! [`Option`] values, or [`None`] if any of the elements was [`None`].
//!
//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E
//!
//! ```
//! let v = [Some(2), Some(4), None, Some(8)];
//! let res: Option<Vec<_>> = v.into_iter().collect();
//! assert_eq!(res, None);
//! let v = [Some(2), Some(4), Some(8)];
//! let res: Option<Vec<_>> = v.into_iter().collect();
//! assert_eq!(res, Some(vec![2, 4, 8]));
//! ```
//!
//! [`Option`] also implements the [`Product`][impl-Product] and
//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
//! to provide the [`product`][Iterator::product] and
//! [`sum`][Iterator::sum] methods.
//!
//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E
//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E
//!
//! ```
//! let v = [None, Some(1), Some(2), Some(3)];
//! let res: Option<i32> = v.into_iter().sum();
//! assert_eq!(res, None);
//! let v = [Some(1), Some(2), Some(21)];
//! let res: Option<i32> = v.into_iter().product();
//! assert_eq!(res, Some(42));
//! ```
//!
//! ## Modifying an [`Option`] in-place
//!
//! These methods return a mutable reference to the contained value of an
//! [`Option<T>`]:
//!
//! * [`insert`] inserts a value, dropping any old contents
//! * [`get_or_insert`] gets the current value, inserting a provided
//!   default value if it is [`None`]
//! * [`get_or_insert_default`] gets the current value, inserting the
//!   default value of type `T` (which must implement [`Default`]) if it is
//!   [`None`]
//! * [`get_or_insert_with`] gets the current value, inserting a default
//!   computed by the provided function if it is [`None`]
//!
//! [`get_or_insert`]: Option::get_or_insert
//! [`get_or_insert_default`]: Option::get_or_insert_default
//! [`get_or_insert_with`]: Option::get_or_insert_with
//! [`insert`]: Option::insert
//!
//! These methods transfer ownership of the contained value of an
//! [`Option`]:
//!
//! * [`take`] takes ownership of the contained value of an [`Option`], if
//!   any, replacing the [`Option`] with [`None`]
//! * [`replace`] takes ownership of the contained value of an [`Option`],
//!   if any, replacing the [`Option`] with a [`Some`] containing the
//!   provided value
//!
//! [`replace`]: Option::replace
//! [`take`]: Option::take
//!
//! # Examples
//!
//! Basic pattern matching on [`Option`]:
//!
//! ```
//! let msg = Some("howdy");
//!
//! // Take a reference to the contained string
//! if let Some(m) = &msg {
//!     println!("{}", *m);
//! }
//!
//! // Remove the contained string, destroying the Option
//! let unwrapped_msg = msg.unwrap_or("default message");
//! ```
//!
//! Initialize a result to [`None`] before a loop:
//!
//! ```
//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
//!
//! // A list of data to search through.
//! let all_the_big_things = [
//!     Kingdom::Plant(250, "redwood"),
//!     Kingdom::Plant(230, "noble fir"),
//!     Kingdom::Plant(229, "sugar pine"),
//!     Kingdom::Animal(25, "blue whale"),
//!     Kingdom::Animal(19, "fin whale"),
//!     Kingdom::Animal(15, "north pacific right whale"),
//! ];
//!
//! // We're going to search for the name of the biggest animal,
//! // but to start with we've just got `None`.
//! let mut name_of_biggest_animal = None;
//! let mut size_of_biggest_animal = 0;
//! for big_thing in &all_the_big_things {
//!     match *big_thing {
//!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
//!             // Now we've found the name of some big animal
//!             size_of_biggest_animal = size;
//!             name_of_biggest_animal = Some(name);
//!         }
//!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
//!     }
//! }
//!
//! match name_of_biggest_animal {
//!     Some(name) => println!("the biggest animal is {name}"),
//!     None => println!("there are no animals :("),
//! }
//! ```

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

use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
use crate::marker::Destruct;
use crate::panicking::{panic, panic_str};
use crate::pin::Pin;
use crate::{
    convert, hint, mem,
    ops::{self, ControlFlow, Deref, DerefMut},
};

/// The `Option` type. See [the module level documentation](self) for more.
#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
#[rustc_diagnostic_item = "Option"]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum Option<T> {
    /// No value.
    #[lang = "None"]
    #[stable(feature = "rust1", since = "1.0.0")]
    None,
    /// Some value of type `T`.
    #[lang = "Some"]
    #[stable(feature = "rust1", since = "1.0.0")]
    Some(#[stable(feature = "rust1", since = "1.0.0")] T),
}

/////////////////////////////////////////////////////////////////////////////
// Type implementation
/////////////////////////////////////////////////////////////////////////////

impl<T> Option<T> {
    /////////////////////////////////////////////////////////////////////////
    // Querying the contained values
    /////////////////////////////////////////////////////////////////////////

    /// Returns `true` if the option is a [`Some`] value.
    ///
    /// # Examples
    ///
    /// ```
    /// let x: Option<u32> = Some(2);
    /// assert_eq!(x.is_some(), true);
    ///
    /// let x: Option<u32> = None;
    /// assert_eq!(x.is_some(), false);
    /// ```
    #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
    pub const fn is_some(&self) -> bool {
        matches!(*self, Some(_))
    }

    /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_some_with)]
    ///
    /// let x: Option<u32> = Some(2);
    /// assert_eq!(x.is_some_and(|&x| x > 1), true);
    ///
    /// let x: Option<u32> = Some(0);
    /// assert_eq!(x.is_some_and(|&x| x > 1), false);
    ///
    /// let x: Option<u32> = None;
    /// assert_eq!(x.is_some_and(|&x| x > 1), false);
    /// ```
    #[must_use]
    #[inline]
    #[unstable(feature = "is_some_with", issue = "93050")]
    pub fn is_some_and(&self, f: impl FnOnce(&T) -> bool) -> bool {
        matches!(self, Some(x) if f(x))
    }

    /// Returns `true` if the option is a [`None`] value.
    ///
    /// # Examples
    ///
    /// ```
    /// let x: Option<u32> = Some(2);
    /// assert_eq!(x.is_none(), false);
    ///
    /// let x: Option<u32> = None;
    /// assert_eq!(x.is_none(), true);
    /// ```
    #[must_use = "if you intended to assert that this doesn't have a value, consider \
                  `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
    pub const fn is_none(&self) -> bool {
        !self.is_some()
    }

    /////////////////////////////////////////////////////////////////////////
    // Adapter for working with references
    /////////////////////////////////////////////////////////////////////////

    /// Converts from `&Option<T>` to `Option<&T>`.
    ///
    /// # Examples
    ///
    /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, preserving
    /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
    /// so this technique uses `as_ref` to first take an `Option` to a reference
    /// to the value inside the original.
    ///
    /// [`map`]: Option::map
    /// [String]: ../../std/string/struct.String.html "String"
    ///
    /// ```
    /// let text: Option<String> = Some("Hello, world!".to_string());
    /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
    /// // then consume *that* with `map`, leaving `text` on the stack.
    /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
    /// println!("still can print text: {text:?}");
    /// ```
    #[inline]
    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const fn as_ref(&self) -> Option<&T> {
        match *self {
            Some(ref x) => Some(x),
            None => None,
        }
    }

    /// Converts from `&mut Option<T>` to `Option<&mut T>`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = Some(2);
    /// match x.as_mut() {
    ///     Some(v) => *v = 42,
    ///     None => {},
    /// }
    /// assert_eq!(x, Some(42));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn as_mut(&mut self) -> Option<&mut T> {
        match *self {
            Some(ref mut x) => Some(x),
            None => None,
        }
    }

    /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
    ///
    /// [&]: reference "shared reference"
    #[inline]
    #[must_use]
    #[stable(feature = "pin", since = "1.33.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
        match Pin::get_ref(self).as_ref() {
            // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
            // which is pinned.
            Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
            None => None,
        }
    }

    /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
    ///
    /// [&mut]: reference "mutable reference"
    #[inline]
    #[must_use]
    #[stable(feature = "pin", since = "1.33.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
        // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
        // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
        unsafe {
            match Pin::get_unchecked_mut(self).as_mut() {
                Some(x) => Some(Pin::new_unchecked(x)),
                None => None,
            }
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Getting to contained values
    /////////////////////////////////////////////////////////////////////////

    /// Returns the contained [`Some`] value, consuming the `self` value.
    ///
    /// # Panics
    ///
    /// Panics if the value is a [`None`] with a custom panic message provided by
    /// `msg`.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("value");
    /// assert_eq!(x.expect("fruits are healthy"), "value");
    /// ```
    ///
    /// ```should_panic
    /// let x: Option<&str> = None;
    /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
    /// ```
    #[inline]
    #[track_caller]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn expect(self, msg: &str) -> T {
        match self {
            Some(val) => val,
            None => expect_failed(msg),
        }
    }

    /// Returns the contained [`Some`] value, consuming the `self` value.
    ///
    /// Because this function may panic, its use is generally discouraged.
    /// Instead, prefer to use pattern matching and handle the [`None`]
    /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
    /// [`unwrap_or_default`].
    ///
    /// [`unwrap_or`]: Option::unwrap_or
    /// [`unwrap_or_else`]: Option::unwrap_or_else
    /// [`unwrap_or_default`]: Option::unwrap_or_default
    ///
    /// # Panics
    ///
    /// Panics if the self value equals [`None`].
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("air");
    /// assert_eq!(x.unwrap(), "air");
    /// ```
    ///
    /// ```should_panic
    /// let x: Option<&str> = None;
    /// assert_eq!(x.unwrap(), "air"); // fails
    /// ```
    #[inline]
    #[track_caller]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn unwrap(self) -> T {
        match self {
            Some(val) => val,
            None => panic("called `Option::unwrap()` on a `None` value"),
        }
    }

    /// Returns the contained [`Some`] value or a provided default.
    ///
    /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
    /// the result of a function call, it is recommended to use [`unwrap_or_else`],
    /// which is lazily evaluated.
    ///
    /// [`unwrap_or_else`]: Option::unwrap_or_else
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!(Some("car").unwrap_or("bike"), "car");
    /// assert_eq!(None.unwrap_or("bike"), "bike");
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn unwrap_or(self, default: T) -> T
    where
        T: ~const Destruct,
    {
        match self {
            Some(x) => x,
            None => default,
        }
    }

    /// Returns the contained [`Some`] value or computes it from a closure.
    ///
    /// # Examples
    ///
    /// ```
    /// let k = 10;
    /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
    /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn unwrap_or_else<F>(self, f: F) -> T
    where
        F: ~const FnOnce() -> T,
        F: ~const Destruct,
    {
        match self {
            Some(x) => x,
            None => f(),
        }
    }

    /// Returns the contained [`Some`] value or a default.
    ///
    /// Consumes the `self` argument then, if [`Some`], returns the contained
    /// value, otherwise if [`None`], returns the [default value] for that
    /// type.
    ///
    /// # Examples
    ///
    /// Converts a string to an integer, turning poorly-formed strings
    /// into 0 (the default value for integers). [`parse`] converts
    /// a string to any other type that implements [`FromStr`], returning
    /// [`None`] on error.
    ///
    /// ```
    /// let good_year_from_input = "1909";
    /// let bad_year_from_input = "190blarg";
    /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
    /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
    ///
    /// assert_eq!(1909, good_year);
    /// assert_eq!(0, bad_year);
    /// ```
    ///
    /// [default value]: Default::default
    /// [`parse`]: str::parse
    /// [`FromStr`]: crate::str::FromStr
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn unwrap_or_default(self) -> T
    where
        T: ~const Default,
    {
        match self {
            Some(x) => x,
            None => Default::default(),
        }
    }

    /// Returns the contained [`Some`] value, consuming the `self` value,
    /// without checking that the value is not [`None`].
    ///
    /// # Safety
    ///
    /// Calling this method on [`None`] is *[undefined behavior]*.
    ///
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("air");
    /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
    /// ```
    ///
    /// ```no_run
    /// let x: Option<&str> = None;
    /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
    /// ```
    #[inline]
    #[track_caller]
    #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const unsafe fn unwrap_unchecked(self) -> T {
        debug_assert!(self.is_some());
        match self {
            Some(val) => val,
            // SAFETY: the safety contract must be upheld by the caller.
            None => unsafe { hint::unreachable_unchecked() },
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Transforming contained values
    /////////////////////////////////////////////////////////////////////////

    /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
    ///
    /// # Examples
    ///
    /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming
    /// the original:
    ///
    /// [String]: ../../std/string/struct.String.html "String"
    /// ```
    /// let maybe_some_string = Some(String::from("Hello, World!"));
    /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
    /// let maybe_some_len = maybe_some_string.map(|s| s.len());
    ///
    /// assert_eq!(maybe_some_len, Some(13));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn map<U, F>(self, f: F) -> Option<U>
    where
        F: ~const FnOnce(T) -> U,
        F: ~const Destruct,
    {
        match self {
            Some(x) => Some(f(x)),
            None => None,
        }
    }

    /// Calls the provided closure with a reference to the contained value (if [`Some`]).
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(result_option_inspect)]
    ///
    /// let v = vec![1, 2, 3, 4, 5];
    ///
    /// // prints "got: 4"
    /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));
    ///
    /// // prints nothing
    /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));
    /// ```
    #[inline]
    #[unstable(feature = "result_option_inspect", issue = "91345")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn inspect<F>(self, f: F) -> Self
    where
        F: ~const FnOnce(&T),
        F: ~const Destruct,
    {
        if let Some(ref x) = self {
            f(x);
        }

        self
    }

    /// Returns the provided default result (if none),
    /// or applies a function to the contained value (if any).
    ///
    /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
    /// the result of a function call, it is recommended to use [`map_or_else`],
    /// which is lazily evaluated.
    ///
    /// [`map_or_else`]: Option::map_or_else
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("foo");
    /// assert_eq!(x.map_or(42, |v| v.len()), 3);
    ///
    /// let x: Option<&str> = None;
    /// assert_eq!(x.map_or(42, |v| v.len()), 42);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn map_or<U, F>(self, default: U, f: F) -> U
    where
        F: ~const FnOnce(T) -> U,
        F: ~const Destruct,
        U: ~const Destruct,
    {
        match self {
            Some(t) => f(t),
            None => default,
        }
    }

    /// Computes a default function result (if none), or
    /// applies a different function to the contained value (if any).
    ///
    /// # Examples
    ///
    /// ```
    /// let k = 21;
    ///
    /// let x = Some("foo");
    /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
    ///
    /// let x: Option<&str> = None;
    /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
    where
        D: ~const FnOnce() -> U,
        D: ~const Destruct,
        F: ~const FnOnce(T) -> U,
        F: ~const Destruct,
    {
        match self {
            Some(t) => f(t),
            None => default(),
        }
    }

    /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
    /// [`Ok(v)`] and [`None`] to [`Err(err)`].
    ///
    /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
    /// result of a function call, it is recommended to use [`ok_or_else`], which is
    /// lazily evaluated.
    ///
    /// [`Ok(v)`]: Ok
    /// [`Err(err)`]: Err
    /// [`Some(v)`]: Some
    /// [`ok_or_else`]: Option::ok_or_else
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("foo");
    /// assert_eq!(x.ok_or(0), Ok("foo"));
    ///
    /// let x: Option<&str> = None;
    /// assert_eq!(x.ok_or(0), Err(0));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn ok_or<E>(self, err: E) -> Result<T, E>
    where
        E: ~const Destruct,
    {
        match self {
            Some(v) => Ok(v),
            None => Err(err),
        }
    }

    /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
    /// [`Ok(v)`] and [`None`] to [`Err(err())`].
    ///
    /// [`Ok(v)`]: Ok
    /// [`Err(err())`]: Err
    /// [`Some(v)`]: Some
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("foo");
    /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
    ///
    /// let x: Option<&str> = None;
    /// assert_eq!(x.ok_or_else(|| 0), Err(0));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
    where
        F: ~const FnOnce() -> E,
        F: ~const Destruct,
    {
        match self {
            Some(v) => Ok(v),
            None => Err(err()),
        }
    }

    /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
    ///
    /// Leaves the original Option in-place, creating a new one with a reference
    /// to the original one, additionally coercing the contents via [`Deref`].
    ///
    /// # Examples
    ///
    /// ```
    /// let x: Option<String> = Some("hey".to_owned());
    /// assert_eq!(x.as_deref(), Some("hey"));
    ///
    /// let x: Option<String> = None;
    /// assert_eq!(x.as_deref(), None);
    /// ```
    #[stable(feature = "option_deref", since = "1.40.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn as_deref(&self) -> Option<&T::Target>
    where
        T: ~const Deref,
    {
        match self.as_ref() {
            Some(t) => Some(t.deref()),
            None => None,
        }
    }

    /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
    ///
    /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
    /// the inner type's [`Deref::Target`] type.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x: Option<String> = Some("hey".to_owned());
    /// assert_eq!(x.as_deref_mut().map(|x| {
    ///     x.make_ascii_uppercase();
    ///     x
    /// }), Some("HEY".to_owned().as_mut_str()));
    /// ```
    #[stable(feature = "option_deref", since = "1.40.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target>
    where
        T: ~const DerefMut,
    {
        match self.as_mut() {
            Some(t) => Some(t.deref_mut()),
            None => None,
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Iterator constructors
    /////////////////////////////////////////////////////////////////////////

    /// Returns an iterator over the possibly contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some(4);
    /// assert_eq!(x.iter().next(), Some(&4));
    ///
    /// let x: Option<u32> = None;
    /// assert_eq!(x.iter().next(), None);
    /// ```
    #[inline]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const fn iter(&self) -> Iter<'_, T> {
        Iter { inner: Item { opt: self.as_ref() } }
    }

    /// Returns a mutable iterator over the possibly contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = Some(4);
    /// match x.iter_mut().next() {
    ///     Some(v) => *v = 42,
    ///     None => {},
    /// }
    /// assert_eq!(x, Some(42));
    ///
    /// let mut x: Option<u32> = None;
    /// assert_eq!(x.iter_mut().next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter_mut(&mut self) -> IterMut<'_, T> {
        IterMut { inner: Item { opt: self.as_mut() } }
    }

    /////////////////////////////////////////////////////////////////////////
    // Boolean operations on the values, eager and lazy
    /////////////////////////////////////////////////////////////////////////

    /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some(2);
    /// let y: Option<&str> = None;
    /// assert_eq!(x.and(y), None);
    ///
    /// let x: Option<u32> = None;
    /// let y = Some("foo");
    /// assert_eq!(x.and(y), None);
    ///
    /// let x = Some(2);
    /// let y = Some("foo");
    /// assert_eq!(x.and(y), Some("foo"));
    ///
    /// let x: Option<u32> = None;
    /// let y: Option<&str> = None;
    /// assert_eq!(x.and(y), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn and<U>(self, optb: Option<U>) -> Option<U>
    where
        T: ~const Destruct,
        U: ~const Destruct,
    {
        match self {
            Some(_) => optb,
            None => None,
        }
    }

    /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
    /// wrapped value and returns the result.
    ///
    /// Some languages call this operation flatmap.
    ///
    /// # Examples
    ///
    /// ```
    /// fn sq_then_to_string(x: u32) -> Option<String> {
    ///     x.checked_mul(x).map(|sq| sq.to_string())
    /// }
    ///
    /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
    /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
    /// assert_eq!(None.and_then(sq_then_to_string), None);
    /// ```
    ///
    /// Often used to chain fallible operations that may return [`None`].
    ///
    /// ```
    /// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
    ///
    /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
    /// assert_eq!(item_0_1, Some(&"A1"));
    ///
    /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
    /// assert_eq!(item_2_0, None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn and_then<U, F>(self, f: F) -> Option<U>
    where
        F: ~const FnOnce(T) -> Option<U>,
        F: ~const Destruct,
    {
        match self {
            Some(x) => f(x),
            None => None,
        }
    }

    /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
    /// with the wrapped value and returns:
    ///
    /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
    ///   value), and
    /// - [`None`] if `predicate` returns `false`.
    ///
    /// This function works similar to [`Iterator::filter()`]. You can imagine
    /// the `Option<T>` being an iterator over one or zero elements. `filter()`
    /// lets you decide which elements to keep.
    ///
    /// # Examples
    ///
    /// ```rust
    /// fn is_even(n: &i32) -> bool {
    ///     n % 2 == 0
    /// }
    ///
    /// assert_eq!(None.filter(is_even), None);
    /// assert_eq!(Some(3).filter(is_even), None);
    /// assert_eq!(Some(4).filter(is_even), Some(4));
    /// ```
    ///
    /// [`Some(t)`]: Some
    #[inline]
    #[stable(feature = "option_filter", since = "1.27.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn filter<P>(self, predicate: P) -> Self
    where
        T: ~const Destruct,
        P: ~const FnOnce(&T) -> bool,
        P: ~const Destruct,
    {
        if let Some(x) = self {
            if predicate(&x) {
                return Some(x);
            }
        }
        None
    }

    /// Returns the option if it contains a value, otherwise returns `optb`.
    ///
    /// Arguments passed to `or` are eagerly evaluated; if you are passing the
    /// result of a function call, it is recommended to use [`or_else`], which is
    /// lazily evaluated.
    ///
    /// [`or_else`]: Option::or_else
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some(2);
    /// let y = None;
    /// assert_eq!(x.or(y), Some(2));
    ///
    /// let x = None;
    /// let y = Some(100);
    /// assert_eq!(x.or(y), Some(100));
    ///
    /// let x = Some(2);
    /// let y = Some(100);
    /// assert_eq!(x.or(y), Some(2));
    ///
    /// let x: Option<u32> = None;
    /// let y = None;
    /// assert_eq!(x.or(y), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn or(self, optb: Option<T>) -> Option<T>
    where
        T: ~const Destruct,
    {
        match self {
            Some(x) => Some(x),
            None => optb,
        }
    }

    /// Returns the option if it contains a value, otherwise calls `f` and
    /// returns the result.
    ///
    /// # Examples
    ///
    /// ```
    /// fn nobody() -> Option<&'static str> { None }
    /// fn vikings() -> Option<&'static str> { Some("vikings") }
    ///
    /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
    /// assert_eq!(None.or_else(vikings), Some("vikings"));
    /// assert_eq!(None.or_else(nobody), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn or_else<F>(self, f: F) -> Option<T>
    where
        F: ~const FnOnce() -> Option<T>,
        F: ~const Destruct,
    {
        match self {
            Some(x) => Some(x),
            None => f(),
        }
    }

    /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some(2);
    /// let y: Option<u32> = None;
    /// assert_eq!(x.xor(y), Some(2));
    ///
    /// let x: Option<u32> = None;
    /// let y = Some(2);
    /// assert_eq!(x.xor(y), Some(2));
    ///
    /// let x = Some(2);
    /// let y = Some(2);
    /// assert_eq!(x.xor(y), None);
    ///
    /// let x: Option<u32> = None;
    /// let y: Option<u32> = None;
    /// assert_eq!(x.xor(y), None);
    /// ```
    #[inline]
    #[stable(feature = "option_xor", since = "1.37.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn xor(self, optb: Option<T>) -> Option<T>
    where
        T: ~const Destruct,
    {
        match (self, optb) {
            (Some(a), None) => Some(a),
            (None, Some(b)) => Some(b),
            _ => None,
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Entry-like operations to insert a value and return a reference
    /////////////////////////////////////////////////////////////////////////

    /// Inserts `value` into the option, then returns a mutable reference to it.
    ///
    /// If the option already contains a value, the old value is dropped.
    ///
    /// See also [`Option::get_or_insert`], which doesn't update the value if
    /// the option already contains [`Some`].
    ///
    /// # Example
    ///
    /// ```
    /// let mut opt = None;
    /// let val = opt.insert(1);
    /// assert_eq!(*val, 1);
    /// assert_eq!(opt.unwrap(), 1);
    /// let val = opt.insert(2);
    /// assert_eq!(*val, 2);
    /// *val = 3;
    /// assert_eq!(opt.unwrap(), 3);
    /// ```
    #[must_use = "if you intended to set a value, consider assignment instead"]
    #[inline]
    #[stable(feature = "option_insert", since = "1.53.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn insert(&mut self, value: T) -> &mut T
    where
        T: ~const Destruct,
    {
        *self = Some(value);

        // SAFETY: the code above just filled the option
        unsafe { self.as_mut().unwrap_unchecked() }
    }

    /// Inserts `value` into the option if it is [`None`], then
    /// returns a mutable reference to the contained value.
    ///
    /// See also [`Option::insert`], which updates the value even if
    /// the option already contains [`Some`].
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = None;
    ///
    /// {
    ///     let y: &mut u32 = x.get_or_insert(5);
    ///     assert_eq!(y, &5);
    ///
    ///     *y = 7;
    /// }
    ///
    /// assert_eq!(x, Some(7));
    /// ```
    #[inline]
    #[stable(feature = "option_entry", since = "1.20.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn get_or_insert(&mut self, value: T) -> &mut T
    where
        T: ~const Destruct,
    {
        if let None = *self {
            *self = Some(value);
        }

        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
        // variant in the code above.
        unsafe { self.as_mut().unwrap_unchecked() }
    }

    /// Inserts the default value into the option if it is [`None`], then
    /// returns a mutable reference to the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(option_get_or_insert_default)]
    ///
    /// let mut x = None;
    ///
    /// {
    ///     let y: &mut u32 = x.get_or_insert_default();
    ///     assert_eq!(y, &0);
    ///
    ///     *y = 7;
    /// }
    ///
    /// assert_eq!(x, Some(7));
    /// ```
    #[inline]
    #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn get_or_insert_default(&mut self) -> &mut T
    where
        T: ~const Default,
    {
        #[rustc_allow_const_fn_unstable(const_fn_trait_bound)]
        const fn default<T: ~const Default>() -> T {
            T::default()
        }

        self.get_or_insert_with(default)
    }

    /// Inserts a value computed from `f` into the option if it is [`None`],
    /// then returns a mutable reference to the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = None;
    ///
    /// {
    ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
    ///     assert_eq!(y, &5);
    ///
    ///     *y = 7;
    /// }
    ///
    /// assert_eq!(x, Some(7));
    /// ```
    #[inline]
    #[stable(feature = "option_entry", since = "1.20.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
    where
        F: ~const FnOnce() -> T,
        F: ~const Destruct,
    {
        if let None = *self {
            // the compiler isn't smart enough to know that we are not dropping a `T`
            // here and wants us to ensure `T` can be dropped at compile time.
            mem::forget(mem::replace(self, Some(f())))
        }

        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
        // variant in the code above.
        unsafe { self.as_mut().unwrap_unchecked() }
    }

    /////////////////////////////////////////////////////////////////////////
    // Misc
    /////////////////////////////////////////////////////////////////////////

    /// Takes the value out of the option, leaving a [`None`] in its place.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = Some(2);
    /// let y = x.take();
    /// assert_eq!(x, None);
    /// assert_eq!(y, Some(2));
    ///
    /// let mut x: Option<u32> = None;
    /// let y = x.take();
    /// assert_eq!(x, None);
    /// assert_eq!(y, None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn take(&mut self) -> Option<T> {
        // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
        mem::replace(self, None)
    }

    /// Replaces the actual value in the option by the value given in parameter,
    /// returning the old value if present,
    /// leaving a [`Some`] in its place without deinitializing either one.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = Some(2);
    /// let old = x.replace(5);
    /// assert_eq!(x, Some(5));
    /// assert_eq!(old, Some(2));
    ///
    /// let mut x = None;
    /// let old = x.replace(3);
    /// assert_eq!(x, Some(3));
    /// assert_eq!(old, None);
    /// ```
    #[inline]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    #[stable(feature = "option_replace", since = "1.31.0")]
    pub const fn replace(&mut self, value: T) -> Option<T> {
        mem::replace(self, Some(value))
    }

    /// Returns `true` if the option is a [`Some`] value containing the given value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(option_result_contains)]
    ///
    /// let x: Option<u32> = Some(2);
    /// assert_eq!(x.contains(&2), true);
    ///
    /// let x: Option<u32> = Some(3);
    /// assert_eq!(x.contains(&2), false);
    ///
    /// let x: Option<u32> = None;
    /// assert_eq!(x.contains(&2), false);
    /// ```
    #[must_use]
    #[inline]
    #[unstable(feature = "option_result_contains", issue = "62358")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn contains<U>(&self, x: &U) -> bool
    where
        U: ~const PartialEq<T>,
    {
        match self {
            Some(y) => x.eq(y),
            None => false,
        }
    }

    /// Zips `self` with another `Option`.
    ///
    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
    /// Otherwise, `None` is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some(1);
    /// let y = Some("hi");
    /// let z = None::<u8>;
    ///
    /// assert_eq!(x.zip(y), Some((1, "hi")));
    /// assert_eq!(x.zip(z), None);
    /// ```
    #[stable(feature = "option_zip_option", since = "1.46.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
    where
        T: ~const Destruct,
        U: ~const Destruct,
    {
        match (self, other) {
            (Some(a), Some(b)) => Some((a, b)),
            _ => None,
        }
    }

    /// Zips `self` and another `Option` with function `f`.
    ///
    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
    /// Otherwise, `None` is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(option_zip)]
    ///
    /// #[derive(Debug, PartialEq)]
    /// struct Point {
    ///     x: f64,
    ///     y: f64,
    /// }
    ///
    /// impl Point {
    ///     fn new(x: f64, y: f64) -> Self {
    ///         Self { x, y }
    ///     }
    /// }
    ///
    /// let x = Some(17.5);
    /// let y = Some(42.7);
    ///
    /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
    /// assert_eq!(x.zip_with(None, Point::new), None);
    /// ```
    #[unstable(feature = "option_zip", issue = "70086")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
    where
        F: ~const FnOnce(T, U) -> R,
        F: ~const Destruct,
        T: ~const Destruct,
        U: ~const Destruct,
    {
        match (self, other) {
            (Some(a), Some(b)) => Some(f(a, b)),
            _ => None,
        }
    }
}

impl<T, U> Option<(T, U)> {
    /// Unzips an option containing a tuple of two options.
    ///
    /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
    /// Otherwise, `(None, None)` is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(unzip_option)]
    ///
    /// let x = Some((1, "hi"));
    /// let y = None::<(u8, u32)>;
    ///
    /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
    /// assert_eq!(y.unzip(), (None, None));
    /// ```
    #[inline]
    #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")]
    pub const fn unzip(self) -> (Option<T>, Option<U>) {
        match self {
            Some((a, b)) => (Some(a), Some(b)),
            None => (None, None),
        }
    }
}

impl<T> Option<&T> {
    /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
    /// option.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = 12;
    /// let opt_x = Some(&x);
    /// assert_eq!(opt_x, Some(&12));
    /// let copied = opt_x.copied();
    /// assert_eq!(copied, Some(12));
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "copied", since = "1.35.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn copied(self) -> Option<T>
    where
        T: Copy,
    {
        // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
        // ready yet, should be reverted when possible to avoid code repetition
        match self {
            Some(&v) => Some(v),
            None => None,
        }
    }

    /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
    /// option.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = 12;
    /// let opt_x = Some(&x);
    /// assert_eq!(opt_x, Some(&12));
    /// let cloned = opt_x.cloned();
    /// assert_eq!(cloned, Some(12));
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
    pub const fn cloned(self) -> Option<T>
    where
        T: ~const Clone,
    {
        match self {
            Some(t) => Some(t.clone()),
            None => None,
        }
    }
}

impl<T> Option<&mut T> {
    /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
    /// option.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = 12;
    /// let opt_x = Some(&mut x);
    /// assert_eq!(opt_x, Some(&mut 12));
    /// let copied = opt_x.copied();
    /// assert_eq!(copied, Some(12));
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "copied", since = "1.35.0")]
    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
    pub const fn copied(self) -> Option<T>
    where
        T: Copy,
    {
        match self {
            Some(&mut t) => Some(t),
            None => None,
        }
    }

    /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
    /// option.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut x = 12;
    /// let opt_x = Some(&mut x);
    /// assert_eq!(opt_x, Some(&mut 12));
    /// let cloned = opt_x.cloned();
    /// assert_eq!(cloned, Some(12));
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
    #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
    pub const fn cloned(self) -> Option<T>
    where
        T: ~const Clone,
    {
        match self {
            Some(t) => Some(t.clone()),
            None => None,
        }
    }
}

impl<T, E> Option<Result<T, E>> {
    /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
    ///
    /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
    /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
    /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
    ///
    /// # Examples
    ///
    /// ```
    /// #[derive(Debug, Eq, PartialEq)]
    /// struct SomeErr;
    ///
    /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
    /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
    /// assert_eq!(x, y.transpose());
    /// ```
    #[inline]
    #[stable(feature = "transpose_result", since = "1.33.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn transpose(self) -> Result<Option<T>, E> {
        match self {
            Some(Ok(x)) => Ok(Some(x)),
            Some(Err(e)) => Err(e),
            None => Ok(None),
        }
    }
}

// This is a separate function to reduce the code size of .expect() itself.
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[cfg_attr(feature = "panic_immediate_abort", inline)]
#[cold]
#[track_caller]
#[rustc_const_unstable(feature = "const_option", issue = "67441")]
const fn expect_failed(msg: &str) -> ! {
    panic_str(msg)
}

/////////////////////////////////////////////////////////////////////////////
// Trait implementations
/////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T> const Clone for Option<T>
where
    T: ~const Clone + ~const Destruct,
{
    #[inline]
    fn clone(&self) -> Self {
        match self {
            Some(x) => Some(x.clone()),
            None => None,
        }
    }

    #[inline]
    fn clone_from(&mut self, source: &Self) {
        match (self, source) {
            (Some(to), Some(from)) => to.clone_from(from),
            (to, from) => *to = from.clone(),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for Option<T> {
    /// Returns [`None`][Option::None].
    ///
    /// # Examples
    ///
    /// ```
    /// let opt: Option<u32> = Option::default();
    /// assert!(opt.is_none());
    /// ```
    #[inline]
    fn default() -> Option<T> {
        None
    }
}

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

    /// Returns a consuming iterator over the possibly contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Some("string");
    /// let v: Vec<&str> = x.into_iter().collect();
    /// assert_eq!(v, ["string"]);
    ///
    /// let x = None;
    /// let v: Vec<&str> = x.into_iter().collect();
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    fn into_iter(self) -> IntoIter<T> {
        IntoIter { inner: Item { opt: self } }
    }
}

#[stable(since = "1.4.0", feature = "option_iter")]
impl<'a, T> IntoIterator for &'a Option<T> {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

#[stable(since = "1.4.0", feature = "option_iter")]
impl<'a, T> IntoIterator for &'a mut Option<T> {
    type Item = &'a mut T;
    type IntoIter = IterMut<'a, T>;

    fn into_iter(self) -> IterMut<'a, T> {
        self.iter_mut()
    }
}

#[stable(since = "1.12.0", feature = "option_from")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const From<T> for Option<T> {
    /// Moves `val` into a new [`Some`].
    ///
    /// # Examples
    ///
    /// ```
    /// let o: Option<u8> = Option::from(67);
    ///
    /// assert_eq!(Some(67), o);
    /// ```
    fn from(val: T) -> Option<T> {
        Some(val)
    }
}

#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
    /// Converts from `&Option<T>` to `Option<&T>`.
    ///
    /// # Examples
    ///
    /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
    /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
    /// so this technique uses `from` to first take an [`Option`] to a reference
    /// to the value inside the original.
    ///
    /// [`map`]: Option::map
    /// [String]: ../../std/string/struct.String.html "String"
    ///
    /// ```
    /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
    /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
    ///
    /// println!("Can still print s: {s:?}");
    ///
    /// assert_eq!(o, Some(18));
    /// ```
    fn from(o: &'a Option<T>) -> Option<&'a T> {
        o.as_ref()
    }
}

#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
    /// Converts from `&mut Option<T>` to `Option<&mut T>`
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = Some(String::from("Hello"));
    /// let o: Option<&mut String> = Option::from(&mut s);
    ///
    /// match o {
    ///     Some(t) => *t = String::from("Hello, Rustaceans!"),
    ///     None => (),
    /// }
    ///
    /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
    /// ```
    fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
        o.as_mut()
    }
}

/////////////////////////////////////////////////////////////////////////////
// The Option Iterators
/////////////////////////////////////////////////////////////////////////////

#[derive(Clone, Debug)]
struct Item<A> {
    opt: Option<A>,
}

impl<A> Iterator for Item<A> {
    type Item = A;

    #[inline]
    fn next(&mut self) -> Option<A> {
        self.opt.take()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        match self.opt {
            Some(_) => (1, Some(1)),
            None => (0, Some(0)),
        }
    }
}

impl<A> DoubleEndedIterator for Item<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        self.opt.take()
    }
}

impl<A> ExactSizeIterator for Item<A> {}
impl<A> FusedIterator for Item<A> {}
unsafe impl<A> TrustedLen for Item<A> {}

/// An iterator over a reference to the [`Some`] variant of an [`Option`].
///
/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
///
/// This `struct` is created by the [`Option::iter`] function.
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Iter<'a, A: 'a> {
    inner: Item<&'a A>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> Iterator for Iter<'a, A> {
    type Item = &'a A;

    #[inline]
    fn next(&mut self) -> Option<&'a A> {
        self.inner.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a A> {
        self.inner.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> ExactSizeIterator for Iter<'_, A> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<A> FusedIterator for Iter<'_, A> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for Iter<'_, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Clone for Iter<'_, A> {
    #[inline]
    fn clone(&self) -> Self {
        Iter { inner: self.inner.clone() }
    }
}

/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
///
/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
///
/// This `struct` is created by the [`Option::iter_mut`] function.
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct IterMut<'a, A: 'a> {
    inner: Item<&'a mut A>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> Iterator for IterMut<'a, A> {
    type Item = &'a mut A;

    #[inline]
    fn next(&mut self) -> Option<&'a mut A> {
        self.inner.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut A> {
        self.inner.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> ExactSizeIterator for IterMut<'_, A> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<A> FusedIterator for IterMut<'_, A> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for IterMut<'_, A> {}

/// An iterator over the value in [`Some`] variant of an [`Option`].
///
/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
///
/// This `struct` is created by the [`Option::into_iter`] function.
#[derive(Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<A> {
    inner: Item<A>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Iterator for IntoIter<A> {
    type Item = A;

    #[inline]
    fn next(&mut self) -> Option<A> {
        self.inner.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> DoubleEndedIterator for IntoIter<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        self.inner.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> ExactSizeIterator for IntoIter<A> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<A> FusedIterator for IntoIter<A> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for IntoIter<A> {}

/////////////////////////////////////////////////////////////////////////////
// FromIterator
/////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
    /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
    /// no further elements are taken, and the [`None`][Option::None] is
    /// returned. Should no [`None`][Option::None] occur, a container of type
    /// `V` containing the values of each [`Option`] is returned.
    ///
    /// # Examples
    ///
    /// Here is an example which increments every integer in a vector.
    /// We use the checked variant of `add` that returns `None` when the
    /// calculation would result in an overflow.
    ///
    /// ```
    /// let items = vec![0_u16, 1, 2];
    ///
    /// let res: Option<Vec<u16>> = items
    ///     .iter()
    ///     .map(|x| x.checked_add(1))
    ///     .collect();
    ///
    /// assert_eq!(res, Some(vec![1, 2, 3]));
    /// ```
    ///
    /// As you can see, this will return the expected, valid items.
    ///
    /// Here is another example that tries to subtract one from another list
    /// of integers, this time checking for underflow:
    ///
    /// ```
    /// let items = vec![2_u16, 1, 0];
    ///
    /// let res: Option<Vec<u16>> = items
    ///     .iter()
    ///     .map(|x| x.checked_sub(1))
    ///     .collect();
    ///
    /// assert_eq!(res, None);
    /// ```
    ///
    /// Since the last element is zero, it would underflow. Thus, the resulting
    /// value is `None`.
    ///
    /// Here is a variation on the previous example, showing that no
    /// further elements are taken from `iter` after the first `None`.
    ///
    /// ```
    /// let items = vec![3_u16, 2, 1, 10];
    ///
    /// let mut shared = 0;
    ///
    /// let res: Option<Vec<u16>> = items
    ///     .iter()
    ///     .map(|x| { shared += x; x.checked_sub(2) })
    ///     .collect();
    ///
    /// assert_eq!(res, None);
    /// assert_eq!(shared, 6);
    /// ```
    ///
    /// Since the third element caused an underflow, no further elements were taken,
    /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
    #[inline]
    fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
        // FIXME(#11084): This could be replaced with Iterator::scan when this
        // performance bug is closed.

        iter::try_process(iter.into_iter(), |i| i.collect())
    }
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const ops::Try for Option<T> {
    type Output = T;
    type Residual = Option<convert::Infallible>;

    #[inline]
    fn from_output(output: Self::Output) -> Self {
        Some(output)
    }

    #[inline]
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
        match self {
            Some(v) => ControlFlow::Continue(v),
            None => ControlFlow::Break(None),
        }
    }
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T> const ops::FromResidual for Option<T> {
    #[inline]
    fn from_residual(residual: Option<convert::Infallible>) -> Self {
        match residual {
            None => None,
        }
    }
}

#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
impl<T> ops::Residual<T> for Option<convert::Infallible> {
    type TryType = Option<T>;
}

impl<T> Option<Option<T>> {
    /// Converts from `Option<Option<T>>` to `Option<T>`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let x: Option<Option<u32>> = Some(Some(6));
    /// assert_eq!(Some(6), x.flatten());
    ///
    /// let x: Option<Option<u32>> = Some(None);
    /// assert_eq!(None, x.flatten());
    ///
    /// let x: Option<Option<u32>> = None;
    /// assert_eq!(None, x.flatten());
    /// ```
    ///
    /// Flattening only removes one level of nesting at a time:
    ///
    /// ```
    /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
    /// assert_eq!(Some(Some(6)), x.flatten());
    /// assert_eq!(Some(6), x.flatten().flatten());
    /// ```
    #[inline]
    #[stable(feature = "option_flattening", since = "1.40.0")]
    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
    pub const fn flatten(self) -> Option<T> {
        match self {
            Some(inner) => inner,
            None => None,
        }
    }
}