       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

    #[doc(keyword = "as")]
    //
    /// Cast between types, or rename an import.
    ///
    /// `as` is most commonly used to turn primitive types into other primitive types, but it has other
    /// uses that include turning pointers into addresses, addresses into pointers, and pointers into
    /// other pointers.
    ///
    /// ```rust
    /// let thing1: u8 = 89.0 as u8;
    /// assert_eq!('B' as u32, 66);
    /// assert_eq!(thing1 as char, 'Y');
    /// let thing2: f32 = thing1 as f32 + 10.5;
    /// assert_eq!(true as u8 + thing2 as u8, 100);
    /// ```
    ///
    /// In general, any cast that can be performed via ascribing the type can also be done using `as`,
    /// so instead of writing `let x: u32 = 123`, you can write `let x = 123 as u32` (note: `let x: u32
    /// = 123` would be best in that situation). The same is not true in the other direction, however;
    /// explicitly using `as` allows a few more coercions that aren't allowed implicitly, such as
    /// changing the type of a raw pointer or turning closures into raw pointers.
    ///
    /// `as` can be seen as the primitive for `From` and `Into`: `as` only works  with primitives
    /// (`u8`, `bool`, `str`, pointers, ...) whereas `From` and `Into`  also works with types like
    /// `String` or `Vec`.
    ///
    /// `as` can also be used with the `_` placeholder when the destination type can be inferred. Note
    /// that this can cause inference breakage and usually such code should use an explicit type for
    /// both clarity and stability. This is most useful when converting pointers using `as *const _` or
    /// `as *mut _` though the [`cast`][const-cast] method is recommended over `as *const _` and it is
    /// [the same][mut-cast] for `as *mut _`: those methods make the intent clearer.
    ///
    /// `as` is also used to rename imports in [`use`] and [`extern crate`][`crate`] statements:
    ///
    /// ```
    /// # #[allow(unused_imports)]
    /// use std::{mem as memory, net as network};
    /// // Now you can use the names `memory` and `network` to refer to `std::mem` and `std::net`.
    /// ```
    /// For more information on what `as` is capable of, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/operator-expr.html#type-cast-expressions
    /// [`crate`]: keyword.crate.html
    /// [`use`]: keyword.use.html
    /// [const-cast]: primitive.pointer.html#method.cast
    /// [mut-cast]: primitive.pointer.html#method.cast-1
    mod as_keyword {}

    #[doc(keyword = "break")]
    //
    /// Exit early from a loop.
    ///
    /// When `break` is encountered, execution of the associated loop body is
    /// immediately terminated.
    ///
    /// ```rust
    /// let mut last = 0;
    ///
    /// for x in 1..100 {
    ///     if x > 12 {
    ///         break;
    ///     }
    ///     last = x;
    /// }
    ///
    /// assert_eq!(last, 12);
    /// println!("{}", last);
    /// ```
    ///
    /// A break expression is normally associated with the innermost loop enclosing the
    /// `break` but a label can be used to specify which enclosing loop is affected.
    ///
    ///```rust
    /// 'outer: for i in 1..=5 {
    ///     println!("outer iteration (i): {}", i);
    ///
    ///     '_inner: for j in 1..=200 {
    ///         println!("    inner iteration (j): {}", j);
    ///         if j >= 3 {
    ///             // breaks from inner loop, let's outer loop continue.
    ///             break;
    ///         }
    ///         if i >= 2 {
    ///             // breaks from outer loop, and directly to "Bye".
    ///             break 'outer;
    ///         }
    ///     }
    /// }
    /// println!("Bye.");
    ///```
    ///
    /// When associated with `loop`, a break expression may be used to return a value from that loop.
    /// This is only valid with `loop` and not with any other type of loop.
    /// If no value is specified, `break;` returns `()`.
    /// Every `break` within a loop must return the same type.
    ///
    /// ```rust
    /// let (mut a, mut b) = (1, 1);
    /// let result = loop {
    ///     if b > 10 {
    ///         break b;
    ///     }
    ///     let c = a + b;
    ///     a = b;
    ///     b = c;
    /// };
    /// // first number in Fibonacci sequence over 10:
    /// assert_eq!(result, 13);
    /// println!("{}", result);
    /// ```
    ///
    /// For more details consult the [Reference on "break expression"] and the [Reference on "break and
    /// loop values"].
    ///
    /// [Reference on "break expression"]: ../reference/expressions/loop-expr.html#break-expressions
    /// [Reference on "break and loop values"]:
    /// ../reference/expressions/loop-expr.html#break-and-loop-values
    mod break_keyword {}

    #[doc(keyword = "const")]
    //
    /// Compile-time constants and compile-time evaluable functions.
    ///
    /// ## Compile-time constants
    ///
    /// Sometimes a certain value is used many times throughout a program, and it can become
    /// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
    /// make it a variable that gets carried around to each function that needs it. In these cases, the
    /// `const` keyword provides a convenient alternative to code duplication:
    ///
    /// ```rust
    /// const THING: u32 = 0xABAD1DEA;
    ///
    /// let foo = 123 + THING;
    /// ```
    ///
    /// Constants must be explicitly typed; unlike with `let`, you can't ignore their type and let the
    /// compiler figure it out. Any constant value can be defined in a `const`, which in practice happens
    /// to be most things that would be reasonable to have in a constant (barring `const fn`s). For
    /// example, you can't have a [`File`] as a `const`.
    ///
    /// [`File`]: crate::fs::File
    ///
    /// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses
    /// all others in a Rust program. For example, if you wanted to define a constant string, it would
    /// look like this:
    ///
    /// ```rust
    /// const WORDS: &'static str = "hello rust!";
    /// ```
    ///
    /// Thanks to static lifetime elision, you usually don't have to explicitly use `'static`:
    ///
    /// ```rust
    /// const WORDS: &str = "hello convenience!";
    /// ```
    ///
    /// `const` items looks remarkably similar to `static` items, which introduces some confusion as
    /// to which one should be used at which times. To put it simply, constants are inlined wherever
    /// they're used, making using them identical to simply replacing the name of the `const` with its
    /// value. Static variables, on the other hand, point to a single location in memory, which all
    /// accesses share. This means that, unlike with constants, they can't have destructors, and act as
    /// a single value across the entire codebase.
    ///
    /// Constants, like statics, should always be in `SCREAMING_SNAKE_CASE`.
    ///
    /// For more detail on `const`, see the [Rust Book] or the [Reference].
    ///
    /// ## Compile-time evaluable functions
    ///
    /// The other main use of the `const` keyword is in `const fn`. This marks a function as being
    /// callable in the body of a `const` or `static` item and in array initializers (commonly called
    /// "const contexts"). `const fn` are restricted in the set of operations they can perform, to
    /// ensure that they can be evaluated at compile-time. See the [Reference][const-eval] for more
    /// detail.
    ///
    /// Turning a `fn` into a `const fn` has no effect on run-time uses of that function.
    ///
    /// ## Other uses of `const`
    ///
    /// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
    /// T` and `*mut T`. More about `const` as used in raw pointers can be read at the Rust docs for the [pointer primitive].
    ///
    /// [pointer primitive]: primitive.pointer.html
    /// [Rust Book]:
    /// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
    /// [Reference]: ../reference/items/constant-items.html
    /// [const-eval]: ../reference/const_eval.html
    mod const_keyword {}

    #[doc(keyword = "continue")]
    //
    /// Skip to the next iteration of a loop.
    ///
    /// When `continue` is encountered, the current iteration is terminated, returning control to the
    /// loop head, typically continuing with the next iteration.
    ///
    ///```rust
    /// // Printing odd numbers by skipping even ones
    /// for number in 1..=10 {
    ///     if number % 2 == 0 {
    ///         continue;
    ///     }
    ///     println!("{}", number);
    /// }
    ///```
    ///
    /// Like `break`, `continue` is normally associated with the innermost enclosing loop, but labels
    /// may be used to specify the affected loop.
    ///
    ///```rust
    /// // Print Odd numbers under 30 with unit <= 5
    /// 'tens: for ten in 0..3 {
    ///     '_units: for unit in 0..=9 {
    ///         if unit % 2 == 0 {
    ///             continue;
    ///         }
    ///         if unit > 5 {
    ///             continue 'tens;
    ///         }
    ///         println!("{}", ten * 10 + unit);
    ///     }
    /// }
    ///```
    ///
    /// See [continue expressions] from the reference for more details.
    ///
    /// [continue expressions]: ../reference/expressions/loop-expr.html#continue-expressions
    mod continue_keyword {}

    #[doc(keyword = "crate")]
    //
    /// A Rust binary or library.
    ///
    /// The primary use of the `crate` keyword is as a part of `extern crate` declarations, which are
    /// used to specify a dependency on a crate external to the one it's declared in. Crates are the
    /// fundamental compilation unit of Rust code, and can be seen as libraries or projects. More can
    /// be read about crates in the [Reference].
    ///
    /// ```rust ignore
    /// extern crate rand;
    /// extern crate my_crate as thing;
    /// extern crate std; // implicitly added to the root of every Rust project
    /// ```
    ///
    /// The `as` keyword can be used to change what the crate is referred to as in your project. If a
    /// crate name includes a dash, it is implicitly imported with the dashes replaced by underscores.
    ///
    /// `crate` can also be used as in conjunction with `pub` to signify that the item it's attached to
    /// is public only to other members of the same crate it's in.
    ///
    /// ```rust
    /// # #[allow(unused_imports)]
    /// pub(crate) use std::io::Error as IoError;
    /// pub(crate) enum CoolMarkerType { }
    /// pub struct PublicThing {
    ///     pub(crate) semi_secret_thing: bool,
    /// }
    /// ```
    ///
    /// `crate` is also used to represent the absolute path of a module, where `crate` refers to the
    /// root of the current crate. For instance, `crate::foo::bar` refers to the name `bar` inside the
    /// module `foo`, from anywhere else in the same crate.
    ///
    /// [Reference]: ../reference/items/extern-crates.html
    mod crate_keyword {}

    #[doc(keyword = "else")]
    //
    /// What expression to evaluate when an [`if`] condition evaluates to [`false`].
    ///
    /// `else` expressions are optional. When no else expressions are supplied it is assumed to evaluate
    /// to the unit type `()`.
    ///
    /// The type that the `else` blocks evaluate to must be compatible with the type that the `if` block
    /// evaluates to.
    ///
    /// As can be seen below, `else` must be followed by either: `if`, `if let`, or a block `{}` and it
    /// will return the value of that expression.
    ///
    /// ```rust
    /// let result = if true == false {
    ///     "oh no"
    /// } else if "something" == "other thing" {
    ///     "oh dear"
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     "uh oh"
    /// } else {
    ///     println!("Sneaky side effect.");
    ///     "phew, nothing's broken"
    /// };
    /// ```
    ///
    /// Here's another example but here we do not try and return an expression:
    ///
    /// ```rust
    /// if true == false {
    ///     println!("oh no");
    /// } else if "something" == "other thing" {
    ///     println!("oh dear");
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     println!("uh oh");
    /// } else {
    ///     println!("phew, nothing's broken");
    /// }
    /// ```
    ///
    /// The above is _still_ an expression but it will always evaluate to `()`.
    ///
    /// There is possibly no limit to the number of `else` blocks that could follow an `if` expression
    /// however if you have several then a [`match`] expression might be preferable.
    ///
    /// Read more about control flow in the [Rust Book].
    ///
    /// [Rust Book]: ../book/ch03-05-control-flow.html#handling-multiple-conditions-with-else-if
    /// [`match`]: keyword.match.html
    /// [`false`]: keyword.false.html
    /// [`if`]: keyword.if.html
    mod else_keyword {}

    #[doc(keyword = "enum")]
    //
    /// A type that can be any one of several variants.
    ///
    /// Enums in Rust are similar to those of other compiled languages like C, but have important
    /// differences that make them considerably more powerful. What Rust calls enums are more commonly
    /// known as [Algebraic Data Types][ADT] if you're coming from a functional programming background.
    /// The important detail is that each enum variant can have data to go along with it.
    ///
    /// ```rust
    /// # struct Coord;
    /// enum SimpleEnum {
    ///     FirstVariant,
    ///     SecondVariant,
    ///     ThirdVariant,
    /// }
    ///
    /// enum Location {
    ///     Unknown,
    ///     Anonymous,
    ///     Known(Coord),
    /// }
    ///
    /// enum ComplexEnum {
    ///     Nothing,
    ///     Something(u32),
    ///     LotsOfThings {
    ///         usual_struct_stuff: bool,
    ///         blah: String,
    ///     }
    /// }
    ///
    /// enum EmptyEnum { }
    /// ```
    ///
    /// The first enum shown is the usual kind of enum you'd find in a C-style language. The second
    /// shows off a hypothetical example of something storing location data, with `Coord` being any
    /// other type that's needed, for example a struct. The third example demonstrates the kind of
    /// data a variant can store, ranging from nothing, to a tuple, to an anonymous struct.
    ///
    /// Instantiating enum variants involves explicitly using the enum's name as its namespace,
    /// followed by one of its variants. `SimpleEnum::SecondVariant` would be an example from above.
    /// When data follows along with a variant, such as with rust's built-in [`Option`] type, the data
    /// is added as the type describes, for example `Option::Some(123)`. The same follows with
    /// struct-like variants, with things looking like `ComplexEnum::LotsOfThings { usual_struct_stuff:
    /// true, blah: "hello!".to_string(), }`. Empty Enums are similar to [`!`] in that they cannot be
    /// instantiated at all, and are used mainly to mess with the type system in interesting ways.
    ///
    /// For more information, take a look at the [Rust Book] or the [Reference]
    ///
    /// [ADT]: https://en.wikipedia.org/wiki/Algebraic_data_type
    /// [Rust Book]: ../book/ch06-01-defining-an-enum.html
    /// [Reference]: ../reference/items/enumerations.html
    /// [`!`]: primitive.never.html
    mod enum_keyword {}

    #[doc(keyword = "extern")]
    //
    /// Link to or import external code.
    ///
    /// The `extern` keyword is used in two places in Rust. One is in conjunction with the [`crate`]
    /// keyword to make your Rust code aware of other Rust crates in your project, i.e., `extern crate
    /// lazy_static;`. The other use is in foreign function interfaces (FFI).
    ///
    /// `extern` is used in two different contexts within FFI. The first is in the form of external
    /// blocks, for declaring function interfaces that Rust code can call foreign code by.
    ///
    /// ```rust ignore
    /// #[link(name = "my_c_library")]
    /// extern "C" {
    ///     fn my_c_function(x: i32) -> bool;
    /// }
    /// ```
    ///
    /// This code would attempt to link with `libmy_c_library.so` on unix-like systems and
    /// `my_c_library.dll` on Windows at runtime, and panic if it can't find something to link to. Rust
    /// code could then use `my_c_function` as if it were any other unsafe Rust function. Working with
    /// non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.
    ///
    /// The mirror use case of FFI is also done via the `extern` keyword:
    ///
    /// ```rust
    /// #[no_mangle]
    /// pub extern fn callable_from_c(x: i32) -> bool {
    ///     x % 3 == 0
    /// }
    /// ```
    ///
    /// If compiled as a dylib, the resulting .so could then be linked to from a C library, and the
    /// function could be used as if it was from any other library.
    ///
    /// For more information on FFI, check the [Rust book] or the [Reference].
    ///
    /// [Rust book]:
    /// ../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
    /// [Reference]: ../reference/items/external-blocks.html
    /// [`crate`]: keyword.crate.html
    mod extern_keyword {}

    #[doc(keyword = "false")]
    //
    /// A value of type [`bool`] representing logical **false**.
    ///
    /// `false` is the logical opposite of [`true`].
    ///
    /// See the documentation for [`true`] for more information.
    ///
    /// [`true`]: keyword.true.html
    mod false_keyword {}

    #[doc(keyword = "fn")]
    //
    /// A function or function pointer.
    ///
    /// Functions are the primary way code is executed within Rust. Function blocks, usually just
    /// called functions, can be defined in a variety of different places and be assigned many
    /// different attributes and modifiers.
    ///
    /// Standalone functions that just sit within a module not attached to anything else are common,
    /// but most functions will end up being inside [`impl`] blocks, either on another type itself, or
    /// as a trait impl for that type.
    ///
    /// ```rust
    /// fn standalone_function() {
    ///     // code
    /// }
    ///
    /// pub fn public_thing(argument: bool) -> String {
    ///     // code
    ///     # "".to_string()
    /// }
    ///
    /// struct Thing {
    ///     foo: i32,
    /// }
    ///
    /// impl Thing {
    ///     pub fn new() -> Self {
    ///         Self {
    ///             foo: 42,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// In addition to presenting fixed types in the form of `fn name(arg: type, ..) -> return_type`,
    /// functions can also declare a list of type parameters along with trait bounds that they fall
    /// into.
    ///
    /// ```rust
    /// fn generic_function<T: Clone>(x: T) -> (T, T, T) {
    ///     (x.clone(), x.clone(), x.clone())
    /// }
    ///
    /// fn generic_where<T>(x: T) -> T
    ///     where T: std::ops::Add<Output = T> + Copy
    /// {
    ///     x + x + x
    /// }
    /// ```
    ///
    /// Declaring trait bounds in the angle brackets is functionally identical to using a `where`
    /// clause. It's up to the programmer to decide which works better in each situation, but `where`
    /// tends to be better when things get longer than one line.
    ///
    /// Along with being made public via `pub`, `fn` can also have an [`extern`] added for use in
    /// FFI.
    ///
    /// For more information on the various types of functions and how they're used, consult the [Rust
    /// book] or the [Reference].
    ///
    /// [`impl`]: keyword.impl.html
    /// [`extern`]: keyword.extern.html
    /// [Rust book]: ../book/ch03-03-how-functions-work.html
    /// [Reference]: ../reference/items/functions.html
    mod fn_keyword {}

    #[doc(keyword = "for")]
    //
    /// Iteration with [`in`], trait implementation with [`impl`], or [higher-ranked trait bounds]
    /// (`for<'a>`).
    ///
    /// The `for` keyword is used in many syntactic locations:
    ///
    /// * `for` is used in for-in-loops (see below).
    /// * `for` is used when implementing traits as in `impl Trait for Type` (see [`impl`] for more info
    ///   on that).
    /// * `for` is also used for [higher-ranked trait bounds] as in `for<'a> &'a T: PartialEq<i32>`.
    ///
    /// for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common
    /// practice within Rust, which is to loop over anything that implements [`IntoIterator`] until the
    /// iterator returned by `.into_iter()` returns `None` (or the loop body uses `break`).
    ///
    /// ```rust
    /// for i in 0..5 {
    ///     println!("{}", i * 2);
    /// }
    ///
    /// for i in std::iter::repeat(5) {
    ///     println!("turns out {} never stops being 5", i);
    ///     break; // would loop forever otherwise
    /// }
    ///
    /// 'outer: for x in 5..50 {
    ///     for y in 0..10 {
    ///         if x == y {
    ///             break 'outer;
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// As shown in the example above, `for` loops (along with all other loops) can be tagged, using
    /// similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the
    /// same tag to `break` breaks the tagged loop, which is useful for inner loops. It is definitely
    /// not a goto.
    ///
    /// A `for` loop expands as shown:
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// for loop_variable in iterator {
    ///     code()
    /// }
    /// ```
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// {
    ///     let mut _iter = std::iter::IntoIterator::into_iter(iterator);
    ///     loop {
    ///         match _iter.next() {
    ///             Some(loop_variable) => {
    ///                 code()
    ///             },
    ///             None => break,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// More details on the functionality shown can be seen at the [`IntoIterator`] docs.
    ///
    /// For more information on for-loops, see the [Rust book] or the [Reference].
    ///
    /// [`in`]: keyword.in.html
    /// [`impl`]: keyword.impl.html
    /// [higher-ranked trait bounds]: ../reference/trait-bounds.html#higher-ranked-trait-bounds
    /// [Rust book]:
    /// ../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    /// [Reference]: ../reference/expressions/loop-expr.html#iterator-loops
    mod for_keyword {}

    #[doc(keyword = "if")]
    //
    /// Evaluate a block if a condition holds.
    ///
    /// `if` is a familiar construct to most programmers, and is the main way you'll often do logic in
    /// your code. However, unlike in most languages, `if` blocks can also act as expressions.
    ///
    /// ```rust
    /// # let rude = true;
    /// if 1 == 2 {
    ///     println!("whoops, mathematics broke");
    /// } else {
    ///     println!("everything's fine!");
    /// }
    ///
    /// let greeting = if rude {
    ///     "sup nerd."
    /// } else {
    ///     "hello, friend!"
    /// };
    ///
    /// if let Ok(x) = "123".parse::<i32>() {
    ///     println!("{} double that and you get {}!", greeting, x * 2);
    /// }
    /// ```
    ///
    /// Shown above are the three typical forms an `if` block comes in. First is the usual kind of
    /// thing you'd see in many languages, with an optional `else` block. Second uses `if` as an
    /// expression, which is only possible if all branches return the same type. An `if` expression can
    /// be used everywhere you'd expect. The third kind of `if` block is an `if let` block, which
    /// behaves similarly to using a `match` expression:
    ///
    /// ```rust
    /// if let Some(x) = Some(123) {
    ///     // code
    ///     # let _ = x;
    /// } else {
    ///     // something else
    /// }
    ///
    /// match Some(123) {
    ///     Some(x) => {
    ///         // code
    ///         # let _ = x;
    ///     },
    ///     _ => {
    ///         // something else
    ///     },
    /// }
    /// ```
    ///
    /// Each kind of `if` expression can be mixed and matched as needed.
    ///
    /// ```rust
    /// if true == false {
    ///     println!("oh no");
    /// } else if "something" == "other thing" {
    ///     println!("oh dear");
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     println!("uh oh");
    /// } else {
    ///     println!("phew, nothing's broken");
    /// }
    /// ```
    ///
    /// The `if` keyword is used in one other place in Rust, namely as a part of pattern matching
    /// itself, allowing patterns such as `Some(x) if x > 200` to be used.
    ///
    /// For more information on `if` expressions, see the [Rust book] or the [Reference].
    ///
    /// [Rust book]: ../book/ch03-05-control-flow.html#if-expressions
    /// [Reference]: ../reference/expressions/if-expr.html
    mod if_keyword {}

    #[doc(keyword = "impl")]
    //
    /// Implement some functionality for a type.
    ///
    /// The `impl` keyword is primarily used to define implementations on types. Inherent
    /// implementations are standalone, while trait implementations are used to implement traits for
    /// types, or other traits.
    ///
    /// Functions and consts can both be defined in an implementation. A function defined in an
    /// `impl` block can be standalone, meaning it would be called like `Foo::bar()`. If the function
    /// takes `self`, `&self`, or `&mut self` as its first argument, it can also be called using
    /// method-call syntax, a familiar feature to any object oriented programmer, like `foo.bar()`.
    ///
    /// ```rust
    /// struct Example {
    ///     number: i32,
    /// }
    ///
    /// impl Example {
    ///     fn boo() {
    ///         println!("boo! Example::boo() was called!");
    ///     }
    ///
    ///     fn answer(&mut self) {
    ///         self.number += 42;
    ///     }
    ///
    ///     fn get_number(&self) -> i32 {
    ///         self.number
    ///     }
    /// }
    ///
    /// trait Thingy {
    ///     fn do_thingy(&self);
    /// }
    ///
    /// impl Thingy for Example {
    ///     fn do_thingy(&self) {
    ///         println!("doing a thing! also, number is {}!", self.number);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on implementations, see the [Rust book][book1] or the [Reference].
    ///
    /// The other use of the `impl` keyword is in `impl Trait` syntax, which can be seen as a shorthand
    /// for "a concrete type that implements this trait". Its primary use is working with closures,
    /// which have type definitions generated at compile time that can't be simply typed out.
    ///
    /// ```rust
    /// fn thing_returning_closure() -> impl Fn(i32) -> bool {
    ///     println!("here's a closure for you!");
    ///     |x: i32| x % 3 == 0
    /// }
    /// ```
    ///
    /// For more information on `impl Trait` syntax, see the [Rust book][book2].
    ///
    /// [book1]: ../book/ch05-03-method-syntax.html
    /// [Reference]: ../reference/items/implementations.html
    /// [book2]: ../book/ch10-02-traits.html#returning-types-that-implement-traits
    mod impl_keyword {}

    #[doc(keyword = "in")]
    //
    /// Iterate over a series of values with [`for`].
    ///
    /// The expression immediately following `in` must implement the [`IntoIterator`] trait.
    ///
    /// ## Literal Examples:
    ///
    ///    * `for _ in 1..3 {}` - Iterate over an exclusive range up to but excluding 3.
    ///    * `for _ in 1..=3 {}` - Iterate over an inclusive range up to and including 3.
    ///
    /// (Read more about [range patterns])
    ///
    /// [`IntoIterator`]: ../book/ch13-04-performance.html
    /// [range patterns]: ../reference/patterns.html?highlight=range#range-patterns
    /// [`for`]: keyword.for.html
    mod in_keyword {}

    #[doc(keyword = "let")]
    //
    /// Bind a value to a variable.
    ///
    /// The primary use for the `let` keyword is in `let` statements, which are used to introduce a new
    /// set of variables into the current scope, as given by a pattern.
    ///
    /// ```rust
    /// # #![allow(unused_assignments)]
    /// let thing1: i32 = 100;
    /// let thing2 = 200 + thing1;
    ///
    /// let mut changing_thing = true;
    /// changing_thing = false;
    ///
    /// let (part1, part2) = ("first", "second");
    ///
    /// struct Example {
    ///     a: bool,
    ///     b: u64,
    /// }
    ///
    /// let Example { a, b: _ } = Example {
    ///     a: true,
    ///     b: 10004,
    /// };
    /// assert!(a);
    /// ```
    ///
    /// The pattern is most commonly a single variable, which means no pattern matching is done and
    /// the expression given is bound to the variable. Apart from that, patterns used in `let` bindings
    /// can be as complicated as needed, given that the pattern is exhaustive. See the [Rust
    /// book][book1] for more information on pattern matching. The type of the pattern is optionally
    /// given afterwards, but if left blank is automatically inferred by the compiler if possible.
    ///
    /// Variables in Rust are immutable by default, and require the `mut` keyword to be made mutable.
    ///
    /// Multiple variables can be defined with the same name, known as shadowing. This doesn't affect
    /// the original variable in any way beyond being unable to directly access it beyond the point of
    /// shadowing. It continues to remain in scope, getting dropped only when it falls out of scope.
    /// Shadowed variables don't need to have the same type as the variables shadowing them.
    ///
    /// ```rust
    /// let shadowing_example = true;
    /// let shadowing_example = 123.4;
    /// let shadowing_example = shadowing_example as u32;
    /// let mut shadowing_example = format!("cool! {}", shadowing_example);
    /// shadowing_example += " something else!"; // not shadowing
    /// ```
    ///
    /// Other places the `let` keyword is used include along with [`if`], in the form of `if let`
    /// expressions. They're useful if the pattern being matched isn't exhaustive, such as with
    /// enumerations. `while let` also exists, which runs a loop with a pattern matched value until
    /// that pattern can't be matched.
    ///
    /// For more information on the `let` keyword, see the [Rust book][book2] or the [Reference]
    ///
    /// [book1]: ../book/ch06-02-match.html
    /// [`if`]: keyword.if.html
    /// [book2]: ../book/ch18-01-all-the-places-for-patterns.html#let-statements
    /// [Reference]: ../reference/statements.html#let-statements
    mod let_keyword {}

    #[doc(keyword = "while")]
    //
    /// Loop while a condition is upheld.
    ///
    /// A `while` expression is used for predicate loops. The `while` expression runs the conditional
    /// expression before running the loop body, then runs the loop body if the conditional
    /// expression evaluates to `true`, or exits the loop otherwise.
    ///
    /// ```rust
    /// let mut counter = 0;
    ///
    /// while counter < 10 {
    ///     println!("{}", counter);
    ///     counter += 1;
    /// }
    /// ```
    ///
    /// Like the [`for`] expression, we can use `break` and `continue`. A `while` expression
    /// cannot break with a value and always evaluates to `()` unlike [`loop`].
    ///
    /// ```rust
    /// let mut i = 1;
    ///
    /// while i < 100 {
    ///     i *= 2;
    ///     if i == 64 {
    ///         break; // Exit when `i` is 64.
    ///     }
    /// }
    /// ```
    ///
    /// As `if` expressions have their pattern matching variant in `if let`, so too do `while`
    /// expressions with `while let`. The `while let` expression matches the pattern against the
    /// expression, then runs the loop body if pattern matching succeeds, or exits the loop otherwise.
    /// We can use `break` and `continue` in `while let` expressions just like in `while`.
    ///
    /// ```rust
    /// let mut counter = Some(0);
    ///
    /// while let Some(i) = counter {
    ///     if i == 10 {
    ///         counter = None;
    ///     } else {
    ///         println!("{}", i);
    ///         counter = Some (i + 1);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on `while` and loops in general, see the [reference].
    ///
    /// [`for`]: keyword.for.html
    /// [`loop`]: keyword.loop.html
    /// [reference]: ../reference/expressions/loop-expr.html#predicate-loops
    mod while_keyword {}

    #[doc(keyword = "loop")]
    //
    /// Loop indefinitely.
    ///
    /// `loop` is used to define the simplest kind of loop supported in Rust. It runs the code inside
    /// it until the code uses `break` or the program exits.
    ///
    /// ```rust
    /// loop {
    ///     println!("hello world forever!");
    ///     # break;
    /// }
    ///
    /// let mut i = 1;
    /// loop {
    ///     println!("i is {}", i);
    ///     if i > 100 {
    ///         break;
    ///     }
    ///     i *= 2;
    /// }
    /// assert_eq!(i, 128);
    /// ```
    ///
    /// Unlike the other kinds of loops in Rust (`while`, `while let`, and `for`), loops can be used as
    /// expressions that return values via `break`.
    ///
    /// ```rust
    /// let mut i = 1;
    /// let something = loop {
    ///     i *= 2;
    ///     if i > 100 {
    ///         break i;
    ///     }
    /// };
    /// assert_eq!(something, 128);
    /// ```
    ///
    /// Every `break` in a loop has to have the same type. When it's not explicitly giving something,
    /// `break;` returns `()`.
    ///
    /// For more information on `loop` and loops in general, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/loop-expr.html
    mod loop_keyword {}

    #[doc(keyword = "match")]
    //
    /// Control flow based on pattern matching.
    ///
    /// `match` can be used to run code conditionally. Every pattern must
    /// be handled exhaustively either explicitly or by using wildcards like
    /// `_` in the `match`. Since `match` is an expression, values can also be
    /// returned.
    ///
    /// ```rust
    /// let opt = Option::None::<usize>;
    /// let x = match opt {
    ///     Some(int) => int,
    ///     None => 10,
    /// };
    /// assert_eq!(x, 10);
    ///
    /// let a_number = Option::Some(10);
    /// match a_number {
    ///     Some(x) if x <= 5 => println!("0 to 5 num = {}", x),
    ///     Some(x @ 6..=10) => println!("6 to 10 num = {}", x),
    ///     None => panic!(),
    ///     // all other numbers
    ///     _ => panic!(),
    /// }
    /// ```
    ///
    /// `match` can be used to gain access to the inner members of an enum
    /// and use them directly.
    ///
    /// ```rust
    /// enum Outer {
    ///     Double(Option<u8>, Option<String>),
    ///     Single(Option<u8>),
    ///     Empty
    /// }
    ///
    /// let get_inner = Outer::Double(None, Some(String::new()));
    /// match get_inner {
    ///     Outer::Double(None, Some(st)) => println!("{}", st),
    ///     Outer::Single(opt) => println!("{:?}", opt),
    ///     _ => panic!(),
    /// }
    /// ```
    ///
    /// For more information on `match` and matching in general, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/match-expr.html
    mod match_keyword {}

    #[doc(keyword = "mod")]
    //
    /// Organize code into [modules].
    ///
    /// Use `mod` to create new [modules] to encapsulate code, including other
    /// modules:
    ///
    /// ```
    /// mod foo {
    ///     mod bar {
    ///         type MyType = (u8, u8);
    ///         fn baz() {}
    ///     }
    /// }
    /// ```
    ///
    /// Like [`struct`]s and [`enum`]s, a module and its content are private by
    /// default, unaccessible to code outside of the module.
    ///
    /// To learn more about allowing access, see the documentation for the [`pub`]
    /// keyword.
    ///
    /// [`enum`]: keyword.enum.html
    /// [`pub`]: keyword.pub.html
    /// [`struct`]: keyword.struct.html
    /// [modules]: ../reference/items/modules.html
    mod mod_keyword {}

    #[doc(keyword = "move")]
    //
    /// Capture a [closure]'s environment by value.
    ///
    /// `move` converts any variables captured by reference or mutable reference
    /// to owned by value variables.
    ///
    /// ```rust
    /// let capture = "hello";
    /// let closure = move || {
    ///     println!("rust says {}", capture);
    /// };
    /// ```
    ///
    /// Note: `move` closures may still implement [`Fn`] or [`FnMut`], even though
    /// they capture variables by `move`. This is because the traits implemented by
    /// a closure type are determined by *what* the closure does with captured
    /// values, not *how* it captures them:
    ///
    /// ```rust
    /// fn create_fn() -> impl Fn() {
    ///     let text = "Fn".to_owned();
    ///
    ///     move || println!("This is a: {}", text)
    /// }
    ///
    ///     let fn_plain = create_fn();
    ///
    ///     fn_plain();
    /// ```
    ///
    /// `move` is often used when [threads] are involved.
    ///
    /// ```rust
    /// let x = 5;
    ///
    /// std::thread::spawn(move || {
    ///     println!("captured {} by value", x)
    /// }).join().unwrap();
    ///
    /// // x is no longer available
    /// ```
    ///
    /// `move` is also valid before an async block.
    ///
    /// ```rust
    /// let capture = "hello";
    /// let block = async move {
    ///     println!("rust says {} from async block", capture);
    /// };
    /// ```
    ///
    /// For more information on the `move` keyword, see the [closure]'s section
    /// of the Rust book or the [threads] section
    ///
    /// [closure]: ../book/ch13-01-closures.html
    /// [threads]: ../book/ch16-01-threads.html#using-move-closures-with-threads
    mod move_keyword {}

    #[doc(keyword = "mut")]
    //
    /// A mutable variable, reference, or pointer.
    ///
    /// `mut` can be used in several situations. The first is mutable variables,
    /// which can be used anywhere you can bind a value to a variable name. Some
    /// examples:
    ///
    /// ```rust
    /// // A mutable variable in the parameter list of a function.
    /// fn foo(mut x: u8, y: u8) -> u8 {
    ///     x += y;
    ///     x
    /// }
    ///
    /// // Modifying a mutable variable.
    /// # #[allow(unused_assignments)]
    /// let mut a = 5;
    /// a = 6;
    ///
    /// assert_eq!(foo(3, 4), 7);
    /// assert_eq!(a, 6);
    /// ```
    ///
    /// The second is mutable references. They can be created from `mut` variables
    /// and must be unique: no other variables can have a mutable reference, nor a
    /// shared reference.
    ///
    /// ```rust
    /// // Taking a mutable reference.
    /// fn push_two(v: &mut Vec<u8>) {
    ///     v.push(2);
    /// }
    ///
    /// // A mutable reference cannot be taken to a non-mutable variable.
    /// let mut v = vec![0, 1];
    /// // Passing a mutable reference.
    /// push_two(&mut v);
    ///
    /// assert_eq!(v, vec![0, 1, 2]);
    /// ```
    ///
    /// ```rust,compile_fail,E0502
    /// let mut v = vec![0, 1];
    /// let mut_ref_v = &mut v;
    /// ##[allow(unused)]
    /// let ref_v = &v;
    /// mut_ref_v.push(2);
    /// ```
    ///
    /// Mutable raw pointers work much like mutable references, with the added
    /// possibility of not pointing to a valid object. The syntax is `*mut Type`.
    ///
    /// More information on mutable references and pointers can be found in```
    /// [Reference].
    ///
    /// [Reference]: ../reference/types/pointer.html#mutable-references-mut
    mod mut_keyword {}

    #[doc(keyword = "pub")]
    //
    /// Make an item visible to others.
    ///
    /// The keyword `pub` makes any module, function, or data structure accessible from inside
    /// of external modules. The `pub` keyword may also be used in a `use` declaration to re-export
    /// an identifier from a namespace.
    ///
    /// For more information on the `pub` keyword, please see the visibility section
    /// of the [reference] and for some examples, see [Rust by Example].
    ///
    /// [reference]:../reference/visibility-and-privacy.html?highlight=pub#visibility-and-privacy
    /// [Rust by Example]:../rust-by-example/mod/visibility.html
    mod pub_keyword {}

    #[doc(keyword = "ref")]
    //
    /// Bind by reference during pattern matching.
    ///
    /// `ref` annotates pattern bindings to make them borrow rather than move.
    /// It is **not** a part of the pattern as far as matching is concerned: it does
    /// not affect *whether* a value is matched, only *how* it is matched.
    ///
    /// By default, [`match`] statements consume all they can, which can sometimes
    /// be a problem, when you don't really need the value to be moved and owned:
    ///
    /// ```compile_fail,E0382
    /// let maybe_name = Some(String::from("Alice"));
    /// // The variable 'maybe_name' is consumed here ...
    /// match maybe_name {
    ///     Some(n) => println!("Hello, {}", n),
    ///     _ => println!("Hello, world"),
    /// }
    /// // ... and is now unavailable.
    /// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
    /// ```
    ///
    /// Using the `ref` keyword, the value is only borrowed, not moved, making it
    /// available for use after the [`match`] statement:
    ///
    /// ```
    /// let maybe_name = Some(String::from("Alice"));
    /// // Using `ref`, the value is borrowed, not moved ...
    /// match maybe_name {
    ///     Some(ref n) => println!("Hello, {}", n),
    ///     _ => println!("Hello, world"),
    /// }
    /// // ... so it's available here!
    /// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
    /// ```
    ///
    /// # `&` vs `ref`
    ///
    /// - `&` denotes that your pattern expects a reference to an object. Hence `&`
    /// is a part of said pattern: `&Foo` matches different objects than `Foo` does.
    ///
    /// - `ref` indicates that you want a reference to an unpacked value. It is not
    /// matched against: `Foo(ref foo)` matches the same objects as `Foo(foo)`.
    ///
    /// See also the [Reference] for more information.
    ///
    /// [`match`]: keyword.match.html
    /// [Reference]: ../reference/patterns.html#identifier-patterns
    mod ref_keyword {}

    #[doc(keyword = "return")]
    //
    /// Return a value from a function.
    ///
    /// A `return` marks the end of an execution path in a function:
    ///
    /// ```
    /// fn foo() -> i32 {
    ///     return 3;
    /// }
    /// assert_eq!(foo(), 3);
    /// ```
    ///
    /// `return` is not needed when the returned value is the last expression in the
    /// function. In this case the `;` is omitted:
    ///
    /// ```
    /// fn foo() -> i32 {
    ///     3
    /// }
    /// assert_eq!(foo(), 3);
    /// ```
    ///
    /// `return` returns from the function immediately (an "early return"):
    ///
    /// ```no_run
    /// use std::fs::File;
    /// use std::io::{Error, ErrorKind, Read, Result};
    ///
    /// fn main() -> Result<()> {
    ///     let mut file = match File::open("foo.txt") {
    ///         Ok(f) => f,
    ///         Err(e) => return Err(e),
    ///     };
    ///
    ///     let mut contents = String::new();
    ///     let size = match file.read_to_string(&mut contents) {
    ///         Ok(s) => s,
    ///         Err(e) => return Err(e),
    ///     };
    ///
    ///     if contents.contains("impossible!") {
    ///         return Err(Error::new(ErrorKind::Other, "oh no!"));
    ///     }
    ///
    ///     if size > 9000 {
    ///         return Err(Error::new(ErrorKind::Other, "over 9000!"));
    ///     }
    ///
    ///     assert_eq!(contents, "Hello, world!");
    ///     Ok(())
    /// }
    /// ```
    mod return_keyword {}

    #[doc(keyword = "self")]
    //
    /// The receiver of a method, or the current module.
    ///
    /// `self` is used in two situations: referencing the current module and marking
    /// the receiver of a method.
    ///
    /// In paths, `self` can be used to refer to the current module, either in a
    /// [`use`] statement or in a path to access an element:
    ///
    /// ```
    /// # #![allow(unused_imports)]
    /// use std::io::{self, Read};
    /// ```
    ///
    /// Is functionally the same as:
    ///
    /// ```
    /// # #![allow(unused_imports)]
    /// use std::io;
    /// use std::io::Read;
    /// ```
    ///
    /// Using `self` to access an element in the current module:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// # fn main() {}
    /// fn foo() {}
    /// fn bar() {
    ///     self::foo()
    /// }
    /// ```
    ///
    /// `self` as the current receiver for a method allows to omit the parameter
    /// type most of the time. With the exception of this particularity, `self` is
    /// used much like any other parameter:
    ///
    /// ```
    /// struct Foo(i32);
    ///
    /// impl Foo {
    ///     // No `self`.
    ///     fn new() -> Self {
    ///         Self(0)
    ///     }
    ///
    ///     // Consuming `self`.
    ///     fn consume(self) -> Self {
    ///         Self(self.0 + 1)
    ///     }
    ///
    ///     // Borrowing `self`.
    ///     fn borrow(&self) -> &i32 {
    ///         &self.0
    ///     }
    ///
    ///     // Borrowing `self` mutably.
    ///     fn borrow_mut(&mut self) -> &mut i32 {
    ///         &mut self.0
    ///     }
    /// }
    ///
    /// // This method must be called with a `Type::` prefix.
    /// let foo = Foo::new();
    /// assert_eq!(foo.0, 0);
    ///
    /// // Those two calls produces the same result.
    /// let foo = Foo::consume(foo);
    /// assert_eq!(foo.0, 1);
    /// let foo = foo.consume();
    /// assert_eq!(foo.0, 2);
    ///
    /// // Borrowing is handled automatically with the second syntax.
    /// let borrow_1 = Foo::borrow(&foo);
    /// let borrow_2 = foo.borrow();
    /// assert_eq!(borrow_1, borrow_2);
    ///
    /// // Borrowing mutably is handled automatically too with the second syntax.
    /// let mut foo = Foo::new();
    /// *Foo::borrow_mut(&mut foo) += 1;
    /// assert_eq!(foo.0, 1);
    /// *foo.borrow_mut() += 1;
    /// assert_eq!(foo.0, 2);
    /// ```
    ///
    /// Note that this automatic conversion when calling `foo.method()` is not
    /// limited to the examples above. See the [Reference] for more information.
    ///
    /// [`use`]: keyword.use.html
    /// [Reference]: ../reference/items/associated-items.html#methods
    mod self_keyword {}

    #[doc(keyword = "Self")]
    //
    /// The implementing type within a [`trait`] or [`impl`] block, or the current type within a type
    /// definition.
    ///
    /// Within a type definition:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// struct Node {
    ///     elem: i32,
    ///     // `Self` is a `Node` here.
    ///     next: Option<Box<Self>>,
    /// }
    /// ```
    ///
    /// In an [`impl`] block:
    ///
    /// ```
    /// struct Foo(i32);
    ///
    /// impl Foo {
    ///     fn new() -> Self {
    ///         Self(0)
    ///     }
    /// }
    ///
    /// assert_eq!(Foo::new().0, Foo(0).0);
    /// ```
    ///
    /// Generic parameters are implicit with `Self`:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// struct Wrap<T> {
    ///     elem: T,
    /// }
    ///
    /// impl<T> Wrap<T> {
    ///     fn new(elem: T) -> Self {
    ///         Self { elem }
    ///     }
    /// }
    /// ```
    ///
    /// In a [`trait`] definition and related [`impl`] block:
    ///
    /// ```
    /// trait Example {
    ///     fn example() -> Self;
    /// }
    ///
    /// struct Foo(i32);
    ///
    /// impl Example for Foo {
    ///     fn example() -> Self {
    ///         Self(42)
    ///     }
    /// }
    ///
    /// assert_eq!(Foo::example().0, Foo(42).0);
    /// ```
    ///
    /// [`impl`]: keyword.impl.html
    /// [`trait`]: keyword.trait.html
    mod self_upper_keyword {}

    #[doc(keyword = "static")]
    //
    /// A static item is a value which is valid for the entire duration of your
    /// program (a `'static` lifetime).
    ///
    /// On the surface, `static` items seem very similar to [`const`]s: both contain
    /// a value, both require type annotations and both can only be initialized with
    /// constant functions and values. However, `static`s are notably different in
    /// that they represent a location in memory. That means that you can have
    /// references to `static` items and potentially even modify them, making them
    /// essentially global variables.
    ///
    /// Static items do not call [`drop`] at the end of the program.
    ///
    /// There are two types of `static` items: those declared in association with
    /// the [`mut`] keyword and those without.
    ///
    /// Static items cannot be moved:
    ///
    /// ```rust,compile_fail,E0507
    /// static VEC: Vec<u32> = vec![];
    ///
    /// fn move_vec(v: Vec<u32>) -> Vec<u32> {
    ///     v
    /// }
    ///
    /// // This line causes an error
    /// move_vec(VEC);
    /// ```
    ///
    /// # Simple `static`s
    ///
    /// Accessing non-[`mut`] `static` items is considered safe, but some
    /// restrictions apply. Most notably, the type of a `static` value needs to
    /// implement the [`Sync`] trait, ruling out interior mutability containers
    /// like [`RefCell`]. See the [Reference] for more information.
    ///
    /// ```rust
    /// static FOO: [i32; 5] = [1, 2, 3, 4, 5];
    ///
    /// let r1 = &FOO as *const _;
    /// let r2 = &FOO as *const _;
    /// // With a strictly read-only static, references will have the same address
    /// assert_eq!(r1, r2);
    /// // A static item can be used just like a variable in many cases
    /// println!("{:?}", FOO);
    /// ```
    ///
    /// # Mutable `static`s
    ///
    /// If a `static` item is declared with the [`mut`] keyword, then it is allowed
    /// to be modified by the program. However, accessing mutable `static`s can
    /// cause undefined behavior in a number of ways, for example due to data races
    /// in a multithreaded context. As such, all accesses to mutable `static`s
    /// require an [`unsafe`] block.
    ///
    /// Despite their unsafety, mutable `static`s are necessary in many contexts:
    /// they can be used to represent global state shared by the whole program or in
    /// [`extern`] blocks to bind to variables from C libraries.
    ///
    /// In an [`extern`] block:
    ///
    /// ```rust,no_run
    /// # #![allow(dead_code)]
    /// extern "C" {
    ///     static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
    /// }
    /// ```
    ///
    /// Mutable `static`s, just like simple `static`s, have some restrictions that
    /// apply to them. See the [Reference] for more information.
    ///
    /// [`const`]: keyword.const.html
    /// [`extern`]: keyword.extern.html
    /// [`mut`]: keyword.mut.html
    /// [`unsafe`]: keyword.unsafe.html
    /// [`RefCell`]: cell::RefCell
    /// [Reference]: ../reference/items/static-items.html
    mod static_keyword {}

    #[doc(keyword = "struct")]
    //
    /// A type that is composed of other types.
    ///
    /// Structs in Rust come in three flavors: Structs with named fields, tuple structs, and unit
    /// structs.
    ///
    /// ```rust
    /// struct Regular {
    ///     field1: f32,
    ///     field2: String,
    ///     pub field3: bool
    /// }
    ///
    /// struct Tuple(u32, String);
    ///
    /// struct Unit;
    /// ```
    ///
    /// Regular structs are the most commonly used. Each field defined within them has a name and a
    /// type, and once defined can be accessed using `example_struct.field` syntax. The fields of a
    /// struct share its mutability, so `foo.bar = 2;` would only be valid if `foo` was mutable. Adding
    /// `pub` to a field makes it visible to code in other modules, as well as allowing it to be
    /// directly accessed and modified.
    ///
    /// Tuple structs are similar to regular structs, but its fields have no names. They are used like
    /// tuples, with deconstruction possible via `let TupleStruct(x, y) = foo;` syntax. For accessing
    /// individual variables, the same syntax is used as with regular tuples, namely `foo.0`, `foo.1`,
    /// etc, starting at zero.
    ///
    /// Unit structs are most commonly used as marker. They have a size of zero bytes, but unlike empty
    /// enums they can be instantiated, making them isomorphic to the unit type `()`. Unit structs are
    /// useful when you need to implement a trait on something, but don't need to store any data inside
    /// it.
    ///
    /// # Instantiation
    ///
    /// Structs can be instantiated in different ways, all of which can be mixed and
    /// matched as needed. The most common way to make a new struct is via a constructor method such as
    /// `new()`, but when that isn't available (or you're writing the constructor itself), struct
    /// literal syntax is used:
    ///
    /// ```rust
    /// # struct Foo { field1: f32, field2: String, etc: bool }
    /// let example = Foo {
    ///     field1: 42.0,
    ///     field2: "blah".to_string(),
    ///     etc: true,
    /// };
    /// ```
    ///
    /// It's only possible to directly instantiate a struct using struct literal syntax when all of its
    /// fields are visible to you.
    ///
    /// There are a handful of shortcuts provided to make writing constructors more convenient, most
    /// common of which is the Field Init shorthand. When there is a variable and a field of the same
    /// name, the assignment can be simplified from `field: field` into simply `field`. The following
    /// example of a hypothetical constructor demonstrates this:
    ///
    /// ```rust
    /// struct User {
    ///     name: String,
    ///     admin: bool,
    /// }
    ///
    /// impl User {
    ///     pub fn new(name: String) -> Self {
    ///         Self {
    ///             name,
    ///             admin: false,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// Another shortcut for struct instantiation is available, used when you need to make a new
    /// struct that has the same values as most of a previous struct of the same type, called struct
    /// update syntax:
    ///
    /// ```rust
    /// # struct Foo { field1: String, field2: () }
    /// # let thing = Foo { field1: "".to_string(), field2: () };
    /// let updated_thing = Foo {
    ///     field1: "a new value".to_string(),
    ///     ..thing
    /// };
    /// ```
    ///
    /// Tuple structs are instantiated in the same way as tuples themselves, except with the struct's
    /// name as a prefix: `Foo(123, false, 0.1)`.
    ///
    /// Empty structs are instantiated with just their name, and don't need anything else. `let thing =
    /// EmptyStruct;`
    ///
    /// # Style conventions
    ///
    /// Structs are always written in CamelCase, with few exceptions. While the trailing comma on a
    /// struct's list of fields can be omitted, it's usually kept for convenience in adding and
    /// removing fields down the line.
    ///
    /// For more information on structs, take a look at the [Rust Book][book] or the
    /// [Reference][reference].
    ///
    /// [`PhantomData`]: marker::PhantomData
    /// [book]: ../book/ch05-01-defining-structs.html
    /// [reference]: ../reference/items/structs.html
    mod struct_keyword {}

    #[doc(keyword = "super")]
    //
    /// The parent of the current [module].
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// # fn main() {}
    /// mod a {
    ///     pub fn foo() {}
    /// }
    /// mod b {
    ///     pub fn foo() {
    ///         super::a::foo(); // call a's foo function
    ///     }
    /// }
    /// ```
    ///
    /// It is also possible to use `super` multiple times: `super::super::foo`,
    /// going up the ancestor chain.
    ///
    /// See the [Reference] for more information.
    ///
    /// [module]: ../reference/items/modules.html
    /// [Reference]: ../reference/paths.html#super
    mod super_keyword {}

    #[doc(keyword = "trait")]
    //
    /// A common interface for a group of types.
    ///
    /// A `trait` is like an interface that data types can implement. When a type
    /// implements a trait it can be treated abstractly as that trait using generics
    /// or trait objects.
    ///
    /// Traits can be made up of three varieties of associated items:
    ///
    /// - functions and methods
    /// - types
    /// - constants
    ///
    /// Traits may also contain additional type parameters. Those type parameters
    /// or the trait itself can be constrained by other traits.
    ///
    /// Traits can serve as markers or carry other logical semantics that
    /// aren't expressed through their items. When a type implements that
    /// trait it is promising to uphold its contract. [`Send`] and [`Sync`] are two
    /// such marker traits present in the standard library.
    ///
    /// See the [Reference][Ref-Traits] for a lot more information on traits.
    ///
    /// # Examples
    ///
    /// Traits are declared using the `trait` keyword. Types can implement them
    /// using [`impl`] `Trait` [`for`] `Type`:
    ///
    /// ```rust
    /// trait Zero {
    ///     const ZERO: Self;
    ///     fn is_zero(&self) -> bool;
    /// }
    ///
    /// impl Zero for i32 {
    ///     const ZERO: Self = 0;
    ///
    ///     fn is_zero(&self) -> bool {
    ///         *self == Self::ZERO
    ///     }
    /// }
    ///
    /// assert_eq!(i32::ZERO, 0);
    /// assert!(i32::ZERO.is_zero());
    /// assert!(!4.is_zero());
    /// ```
    ///
    /// With an associated type:
    ///
    /// ```rust
    /// trait Builder {
    ///     type Built;
    ///
    ///     fn build(&self) -> Self::Built;
    /// }
    /// ```
    ///
    /// Traits can be generic, with constraints or without:
    ///
    /// ```rust
    /// trait MaybeFrom<T> {
    ///     fn maybe_from(value: T) -> Option<Self>
    ///     where
    ///         Self: Sized;
    /// }
    /// ```
    ///
    /// Traits can build upon the requirements of other traits. In the example
    /// below `Iterator` is a **supertrait** and `ThreeIterator` is a **subtrait**:
    ///
    /// ```rust
    /// trait ThreeIterator: std::iter::Iterator {
    ///     fn next_three(&mut self) -> Option<[Self::Item; 3]>;
    /// }
    /// ```
    ///
    /// Traits can be used in functions, as parameters:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// fn debug_iter<I: Iterator>(it: I) where I::Item: std::fmt::Debug {
    ///     for elem in it {
    ///         println!("{:#?}", elem);
    ///     }
    /// }
    ///
    /// // u8_len_1, u8_len_2 and u8_len_3 are equivalent
    ///
    /// fn u8_len_1(val: impl Into<Vec<u8>>) -> usize {
    ///     val.into().len()
    /// }
    ///
    /// fn u8_len_2<T: Into<Vec<u8>>>(val: T) -> usize {
    ///     val.into().len()
    /// }
    ///
    /// fn u8_len_3<T>(val: T) -> usize
    /// where
    ///     T: Into<Vec<u8>>,
    /// {
    ///     val.into().len()
    /// }
    /// ```
    ///
    /// Or as return types:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// fn from_zero_to(v: u8) -> impl Iterator<Item = u8> {
    ///     (0..v).into_iter()
    /// }
    /// ```
    ///
    /// The use of the [`impl`] keyword in this position allows the function writer
    /// to hide the concrete type as an implementation detail which can change
    /// without breaking user's code.
    ///
    /// # Trait objects
    ///
    /// A *trait object* is an opaque value of another type that implements a set of
    /// traits. A trait object implements all specified traits as well as their
    /// supertraits (if any).
    ///
    /// The syntax is the following: `dyn BaseTrait + AutoTrait1 + ... AutoTraitN`.
    /// Only one `BaseTrait` can be used so this will not compile:
    ///
    /// ```rust,compile_fail,E0225
    /// trait A {}
    /// trait B {}
    ///
    /// let _: Box<dyn A + B>;
    /// ```
    ///
    /// Neither will this, which is a syntax error:
    ///
    /// ```rust,compile_fail
    /// trait A {}
    /// trait B {}
    ///
    /// let _: Box<dyn A + dyn B>;
    /// ```
    ///
    /// On the other hand, this is correct:
    ///
    /// ```rust
    /// trait A {}
    ///
    /// let _: Box<dyn A + Send + Sync>;
    /// ```
    ///
    /// The [Reference][Ref-Trait-Objects] has more information about trait objects,
    /// their limitations and the differences between editions.
    ///
    /// # Unsafe traits
    ///
    /// Some traits may be unsafe to implement. Using the [`unsafe`] keyword in
    /// front of the trait's declaration is used to mark this:
    ///
    /// ```rust
    /// unsafe trait UnsafeTrait {}
    ///
    /// unsafe impl UnsafeTrait for i32 {}
    /// ```
    ///
    /// # Differences between the 2015 and 2018 editions
    ///
    /// In the 2015 edition the parameters pattern was not needed for traits:
    ///
    /// ```rust,edition2015
    /// trait Tr {
    ///     fn f(i32);
    /// }
    /// ```
    ///
    /// This behavior is no longer valid in edition 2018.
    ///
    /// [`for`]: keyword.for.html
    /// [`impl`]: keyword.impl.html
    /// [`unsafe`]: keyword.unsafe.html
    /// [Ref-Traits]: ../reference/items/traits.html
    /// [Ref-Trait-Objects]: ../reference/types/trait-object.html
    mod trait_keyword {}

    #[doc(keyword = "true")]
    //
    /// A value of type [`bool`] representing logical **true**.
    ///
    /// Logically `true` is not equal to [`false`].
    ///
    /// ## Control structures that check for **true**
    ///
    /// Several of Rust's control structures will check for a `bool` condition evaluating to **true**.
    ///
    ///   * The condition in an [`if`] expression must be of type `bool`.
    ///     Whenever that condition evaluates to **true**, the `if` expression takes
    ///     on the value of the first block. If however, the condition evaluates
    ///     to `false`, the expression takes on value of the `else` block if there is one.
    ///
    ///   * [`while`] is another control flow construct expecting a `bool`-typed condition.
    ///     As long as the condition evaluates to **true**, the `while` loop will continually
    ///     evaluate its associated block.
    ///
    ///   * [`match`] arms can have guard clauses on them.
    ///
    /// [`if`]: keyword.if.html
    /// [`while`]: keyword.while.html
    /// [`match`]: ../reference/expressions/match-expr.html#match-guards
    /// [`false`]: keyword.false.html
    mod true_keyword {}

    #[doc(keyword = "type")]
    //
    /// Define an alias for an existing type.
    ///
    /// The syntax is `type Name = ExistingType;`.
    ///
    /// # Examples
    ///
    /// `type` does **not** create a new type:
    ///
    /// ```rust
    /// type Meters = u32;
    /// type Kilograms = u32;
    ///
    /// let m: Meters = 3;
    /// let k: Kilograms = 3;
    ///
    /// assert_eq!(m, k);
    /// ```
    ///
    /// In traits, `type` is used to declare an [associated type]:
    ///
    /// ```rust
    /// trait Iterator {
    ///     // associated type declaration
    ///     type Item;
    ///     fn next(&mut self) -> Option<Self::Item>;
    /// }
    ///
    /// struct Once<T>(Option<T>);
    ///
    /// impl<T> Iterator for Once<T> {
    ///     // associated type definition
    ///     type Item = T;
    ///     fn next(&mut self) -> Option<Self::Item> {
    ///         self.0.take()
    ///     }
    /// }
    /// ```
    ///
    /// [`trait`]: keyword.trait.html
    /// [associated type]: ../reference/items/associated-items.html#associated-types
    mod type_keyword {}

    #[doc(keyword = "unsafe")]
    //
    /// Code or interfaces whose [memory safety] cannot be verified by the type
    /// system.
    ///
    /// The `unsafe` keyword has two uses: to declare the existence of contracts the
    /// compiler can't check (`unsafe fn` and `unsafe trait`), and to declare that a
    /// programmer has checked that these contracts have been upheld (`unsafe {}`
    /// and `unsafe impl`, but also `unsafe fn` -- see below). They are not mutually
    /// exclusive, as can be seen in `unsafe fn`.
    ///
    /// # Unsafe abilities
    ///
    /// **No matter what, Safe Rust can't cause Undefined Behavior**. This is
    /// referred to as [soundness]: a well-typed program actually has the desired
    /// properties. The [Nomicon][nomicon-soundness] has a more detailed explanation
    /// on the subject.
    ///
    /// To ensure soundness, Safe Rust is restricted enough that it can be
    /// automatically checked. Sometimes, however, it is necessary to write code
    /// that is correct for reasons which are too clever for the compiler to
    /// understand. In those cases, you need to use Unsafe Rust.
    ///
    /// Here are the abilities Unsafe Rust has in addition to Safe Rust:
    ///
    /// - Dereference [raw pointers]
    /// - Implement `unsafe` [`trait`]s
    /// - Call `unsafe` functions
    /// - Mutate [`static`]s (including [`extern`]al ones)
    /// - Access fields of [`union`]s
    ///
    /// However, this extra power comes with extra responsibilities: it is now up to
    /// you to ensure soundness. The `unsafe` keyword helps by clearly marking the
    /// pieces of code that need to worry about this.
    ///
    /// ## The different meanings of `unsafe`
    ///
    /// Not all uses of `unsafe` are equivalent: some are here to mark the existence
    /// of a contract the programmer must check, others are to say "I have checked
    /// the contract, go ahead and do this". The following
    /// [discussion on Rust Internals] has more in-depth explanations about this but
    /// here is a summary of the main points:
    ///
    /// - `unsafe fn`: calling this function means abiding by a contract the
    /// compiler cannot enforce.
    /// - `unsafe trait`: implementing the [`trait`] means abiding by a
    /// contract the compiler cannot enforce.
    /// - `unsafe {}`: the contract necessary to call the operations inside the
    /// block has been checked by the programmer and is guaranteed to be respected.
    /// - `unsafe impl`: the contract necessary to implement the trait has been
    /// checked by the programmer and is guaranteed to be respected.
    ///
    /// `unsafe fn` also acts like an `unsafe {}` block
    /// around the code inside the function. This means it is not just a signal to
    /// the caller, but also promises that the preconditions for the operations
    /// inside the function are upheld. Mixing these two meanings can be confusing
    /// and [proposal]s exist to use `unsafe {}` blocks inside such functions when
    /// making `unsafe` operations.
    ///
    /// See the [Rustnomicon] and the [Reference] for more informations.
    ///
    /// # Examples
    ///
    /// ## Marking elements as `unsafe`
    ///
    /// `unsafe` can be used on functions. Note that functions and statics declared
    /// in [`extern`] blocks are implicitly marked as `unsafe` (but not functions
    /// declared as `extern "something" fn ...`). Mutable statics are always unsafe,
    /// wherever they are declared. Methods can also be declared as `unsafe`:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// static mut FOO: &str = "hello";
    ///
    /// unsafe fn unsafe_fn() {}
    ///
    /// extern "C" {
    ///     fn unsafe_extern_fn();
    ///     static BAR: *mut u32;
    /// }
    ///
    /// trait SafeTraitWithUnsafeMethod {
    ///     unsafe fn unsafe_method(&self);
    /// }
    ///
    /// struct S;
    ///
    /// impl S {
    ///     unsafe fn unsafe_method_on_struct() {}
    /// }
    /// ```
    ///
    /// Traits can also be declared as `unsafe`:
    ///
    /// ```rust
    /// unsafe trait UnsafeTrait {}
    /// ```
    ///
    /// Since `unsafe fn` and `unsafe trait` indicate that there is a safety
    /// contract that the compiler cannot enforce, documenting it is important. The
    /// standard library has many examples of this, like the following which is an
    /// extract from [`Vec::set_len`]. The `# Safety` section explains the contract
    /// that must be fulfilled to safely call the function.
    ///
    /// ```rust,ignore (stub-to-show-doc-example)
    /// /// Forces the length of the vector to `new_len`.
    /// ///
    /// /// This is a low-level operation that maintains none of the normal
    /// /// invariants of the type. Normally changing the length of a vector
    /// /// is done using one of the safe operations instead, such as
    /// /// `truncate`, `resize`, `extend`, or `clear`.
    /// ///
    /// /// # Safety
    /// ///
    /// /// - `new_len` must be less than or equal to `capacity()`.
    /// /// - The elements at `old_len..new_len` must be initialized.
    /// pub unsafe fn set_len(&mut self, new_len: usize)
    /// ```
    ///
    /// ## Using `unsafe {}` blocks and `impl`s
    ///
    /// Performing `unsafe` operations requires an `unsafe {}` block:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// /// Dereference the given pointer.
    /// ///
    /// /// # Safety
    /// ///
    /// /// `ptr` must be aligned and must not be dangling.
    /// unsafe fn deref_unchecked(ptr: *const i32) -> i32 {
    ///     *ptr
    /// }
    ///
    /// let a = 3;
    /// let b = &a as *const _;
    /// // SAFETY: `a` has not been dropped and references are always aligned,
    /// // so `b` is a valid address.
    /// unsafe { assert_eq!(*b, deref_unchecked(b)); };
    /// ```
    ///
    /// Traits marked as `unsafe` must be [`impl`]emented using `unsafe impl`. This
    /// makes a guarantee to other `unsafe` code that the implementation satisfies
    /// the trait's safety contract. The [Send] and [Sync] traits are examples of
    /// this behaviour in the standard library.
    ///
    /// ```rust
    /// /// Implementors of this trait must guarantee an element is always
    /// /// accessible with index 3.
    /// unsafe trait ThreeIndexable<T> {
    ///     /// Returns a reference to the element with index 3 in `&self`.
    ///     fn three(&self) -> &T;
    /// }
    ///
    /// // The implementation of `ThreeIndexable` for `[T; 4]` is `unsafe`
    /// // because the implementor must abide by a contract the compiler cannot
    /// // check but as a programmer we know there will always be a valid element
    /// // at index 3 to access.
    /// unsafe impl<T> ThreeIndexable<T> for [T; 4] {
    ///     fn three(&self) -> &T {
    ///         // SAFETY: implementing the trait means there always is an element
    ///         // with index 3 accessible.
    ///         unsafe { self.get_unchecked(3) }
    ///     }
    /// }
    ///
    /// let a = [1, 2, 4, 8];
    /// assert_eq!(a.three(), &8);
    /// ```
    ///
    /// [`extern`]: keyword.extern.html
    /// [`trait`]: keyword.trait.html
    /// [`static`]: keyword.static.html
    /// [`union`]: keyword.union.html
    /// [`impl`]: keyword.impl.html
    /// [raw pointers]: ../reference/types/pointer.html
    /// [memory safety]: ../book/ch19-01-unsafe-rust.html
    /// [Rustnomicon]: ../nomicon/index.html
    /// [nomicon-soundness]: ../nomicon/safe-unsafe-meaning.html
    /// [soundness]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library
    /// [Reference]: ../reference/unsafety.html
    /// [proposal]: https://github.com/rust-lang/rfcs/pull/2585
    /// [discussion on Rust Internals]: https://internals.rust-lang.org/t/what-does-unsafe-mean/6696
    mod unsafe_keyword {}

    #[doc(keyword = "use")]
    //
    /// Import or rename items from other crates or modules.
    ///
    /// Usually a `use` keyword is used to shorten the path required to refer to a module item.
    /// The keyword may appear in modules, blocks and even functions, usually at the top.
    ///
    /// The most basic usage of the keyword is `use path::to::item;`,
    /// though a number of convenient shortcuts are supported:
    ///
    ///   * Simultaneously binding a list of paths with a common prefix,
    ///     using the glob-like brace syntax `use a::b::{c, d, e::f, g::h::i};`
    ///   * Simultaneously binding a list of paths with a common prefix and their common parent module,
    ///     using the [`self`] keyword, such as `use a::b::{self, c, d::e};`
    ///   * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`.
    ///     This can also be used with the last two features: `use a::b::{self as ab, c as abc}`.
    ///   * Binding all paths matching a given prefix,
    ///     using the asterisk wildcard syntax `use a::b::*;`.
    ///   * Nesting groups of the previous features multiple times,
    ///     such as `use a::b::{self as ab, c, d::{*, e::f}};`
    ///   * Reexporting with visibility modifiers such as `pub use a::b;`
    ///   * Importing with `_` to only import the methods of a trait without binding it to a name
    ///     (to avoid conflict for example): `use ::std::io::Read as _;`.
    ///
    /// Using path qualifiers like [`crate`], [`super`] or [`self`] is supported: `use crate::a::b;`.
    ///
    /// Note that when the wildcard `*` is used on a type, it does not import its methods (though
    /// for `enum`s it imports the variants, as shown in the example below).
    ///
    /// ```compile_fail,edition2018
    /// enum ExampleEnum {
    ///     VariantA,
    ///     VariantB,
    /// }
    ///
    /// impl ExampleEnum {
    ///     fn new() -> Self {
    ///         Self::VariantA
    ///     }
    /// }
    ///
    /// use ExampleEnum::*;
    ///
    /// // Compiles.
    /// let _ = VariantA;
    ///
    /// // Does not compile !
    /// let n = new();
    /// ```
    ///
    /// For more information on `use` and paths in general, see the [Reference].
    ///
    /// The differences about paths and the `use` keyword between the 2015 and 2018 editions
    /// can also be found in the [Reference].
    ///
    /// [`crate`]: keyword.crate.html
    /// [`self`]: keyword.self.html
    /// [`super`]: keyword.super.html
    /// [Reference]: ../reference/items/use-declarations.html
    mod use_keyword {}

    #[doc(keyword = "where")]
    //
    /// Add constraints that must be upheld to use an item.
    ///
    /// `where` allows specifying constraints on lifetime and generic parameters.
    /// The [RFC] introducing `where` contains detailed informations about the
    /// keyword.
    ///
    /// # Examples
    ///
    /// `where` can be used for constraints with traits:
    ///
    /// ```rust
    /// fn new<T: Default>() -> T {
    ///     T::default()
    /// }
    ///
    /// fn new_where<T>() -> T
    /// where
    ///     T: Default,
    /// {
    ///     T::default()
    /// }
    ///
    /// assert_eq!(0.0, new());
    /// assert_eq!(0.0, new_where());
    ///
    /// assert_eq!(0, new());
    /// assert_eq!(0, new_where());
    /// ```
    ///
    /// `where` can also be used for lifetimes.
    ///
    /// This compiles because `longer` outlives `shorter`, thus the constraint is
    /// respected:
    ///
    /// ```rust
    /// fn select<'short, 'long>(s1: &'short str, s2: &'long str, second: bool) -> &'short str
    /// where
    ///     'long: 'short,
    /// {
    ///     if second { s2 } else { s1 }
    /// }
    ///
    /// let outer = String::from("Long living ref");
    /// let longer = &outer;
    /// {
    ///     let inner = String::from("Short living ref");
    ///     let shorter = &inner;
    ///
    ///     assert_eq!(select(shorter, longer, false), shorter);
    ///     assert_eq!(select(shorter, longer, true), longer);
    /// }
    /// ```
    ///
    /// On the other hand, this will not compile because the `where 'b: 'a` clause
    /// is missing: the `'b` lifetime is not known to live at least as long as `'a`
    /// which means this function cannot ensure it always returns a valid reference:
    ///
    /// ```rust,compile_fail,E0623
    /// fn select<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
    /// {
    ///     if second { s2 } else { s1 }
    /// }
    /// ```
    ///
    /// `where` can also be used to express more complicated constraints that cannot
    /// be written with the `<T: Trait>` syntax:
    ///
    /// ```rust
    /// fn first_or_default<I>(mut i: I) -> I::Item
    /// where
    ///     I: Iterator,
    ///     I::Item: Default,
    /// {
    ///     i.next().unwrap_or_else(I::Item::default)
    /// }
    ///
    /// assert_eq!(first_or_default(vec![1, 2, 3].into_iter()), 1);
    /// assert_eq!(first_or_default(Vec::<i32>::new().into_iter()), 0);
    /// ```
    ///
    /// `where` is available anywhere generic and lifetime parameters are available,
    /// as can be seen with the [`Cow`](crate::borrow::Cow) type from the standard
    /// library:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// pub enum Cow<'a, B>
    /// where
    ///     B: 'a + ToOwned + ?Sized,
    ///  {
    ///     Borrowed(&'a B),
    ///     Owned(<B as ToOwned>::Owned),
    /// }
    /// ```
    ///
    /// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/0135-where.md
    mod where_keyword {}

    // 2018 Edition keywords

    #[doc(keyword = "async")]
    //
    /// Return a [`Future`] instead of blocking the current thread.
    ///
    /// Use `async` in front of `fn`, `closure`, or a `block` to turn the marked code into a `Future`.
    /// As such the code will not be run immediately, but will only be evaluated when the returned
    /// future is `.await`ed.
    ///
    /// We have written an [async book] detailing async/await and trade-offs compared to using threads.
    ///
    /// ## Editions
    ///
    /// `async` is a keyword from the 2018 edition onwards.
    ///
    /// It is available for use in stable rust from version 1.39 onwards.
    ///
    /// [`Future`]: future::Future
    /// [async book]: https://rust-lang.github.io/async-book/
    mod async_keyword {}

    #[doc(keyword = "await")]
    //
    /// Suspend execution until the result of a [`Future`] is ready.
    ///
    /// `.await`ing a future will suspend the current function's execution until the `executor`
    /// has run the future to completion.
    ///
    /// Read the [async book] for details on how async/await and executors work.
    ///
    /// ## Editions
    ///
    /// `await` is a keyword from the 2018 edition onwards.
    ///
    /// It is available for use in stable rust from version 1.39 onwards.
    ///
    /// [`Future`]: future::Future
    /// [async book]: https://rust-lang.github.io/async-book/
    mod await_keyword {}

    #[doc(keyword = "dyn")]
    //
    /// `dyn` is a prefix of a [trait object]'s type.
    ///
    /// The `dyn` keyword is used to highlight that calls to methods on the associated `Trait`
    /// are dynamically dispatched. To use the trait this way, it must be 'object safe'.
    ///
    /// Unlike generic parameters or `impl Trait`, the compiler does not know the concrete type that
    /// is being passed. That is, the type has been [erased].
    /// As such, a `dyn Trait` reference contains _two_ pointers.
    /// One pointer goes to the data (e.g., an instance of a struct).
    /// Another pointer goes to a map of method call names to function pointers
    /// (known as a virtual method table or vtable).
    ///
    /// At run-time, when a method needs to be called on the `dyn Trait`, the vtable is consulted to get
    /// the function pointer and then that function pointer is called.
    ///
    /// ## Trade-offs
    ///
    /// The above indirection is the additional runtime cost of calling a function on a `dyn Trait`.
    /// Methods called by dynamic dispatch generally cannot be inlined by the compiler.
    ///
    /// However, `dyn Trait` is likely to produce smaller code than `impl Trait` / generic parameters as
    /// the method won't be duplicated for each concrete type.
    ///
    /// Read more about `object safety` and [trait object]s.
    ///
    /// [trait object]: ../book/ch17-02-trait-objects.html
    /// [erased]: https://en.wikipedia.org/wiki/Type_erasure
    mod dyn_keyword {}

    #[doc(keyword = "union")]
    //
    /// The [Rust equivalent of a C-style union][union].
    ///
    /// A `union` looks like a [`struct`] in terms of declaration, but all of its
    /// fields exist in the same memory, superimposed over one another. For instance,
    /// if we wanted some bits in memory that we sometimes interpret as a `u32` and
    /// sometimes as an `f32`, we could write:
    ///
    /// ```rust
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let mut u = IntOrFloat { f: 1.0 };
    /// // Reading the fields of an union is always unsafe
    /// assert_eq!(unsafe { u.i }, 1065353216);
    /// // Updating through any of the field will modify all of them
    /// u.i = 1073741824;
    /// assert_eq!(unsafe { u.f }, 2.0);
    /// ```
    ///
    /// # Matching on unions
    ///
    /// It is possible to use pattern matching on `union`s. A single field name must
    /// be used and it must match the name of one of the `union`'s field.
    /// Like reading from a `union`, pattern matching on a `union` requires `unsafe`.
    ///
    /// ```rust
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let u = IntOrFloat { f: 1.0 };
    ///
    /// unsafe {
    ///     match u {
    ///         IntOrFloat { i: 10 } => println!("Found exactly ten!"),
    ///         // Matching the field `f` provides an `f32`.
    ///         IntOrFloat { f } => println!("Found f = {} !", f),
    ///     }
    /// }
    /// ```
    ///
    /// # References to union fields
    ///
    /// All fields in a `union` are all at the same place in memory which means
    /// borrowing one borrows the entire `union`, for the same lifetime:
    ///
    /// ```rust,compile_fail,E0502
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let mut u = IntOrFloat { f: 1.0 };
    ///
    /// let f = unsafe { &u.f };
    /// // This will not compile because the field has already been borrowed, even
    /// // if only immutably
    /// let i = unsafe { &mut u.i };
    ///
    /// *i = 10;
    /// println!("f = {} and i = {}", f, i);
    /// ```
    ///
    /// See the [Reference][union] for more informations on `union`s.
    ///
    /// [`struct`]: keyword.struct.html
    /// [union]: ../reference/items/unions.html
    mod union_keyword {}