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
//! Generic data structure deserialization framework.
//!
//! The two most important traits in this module are [`Deserialize`] and
//! [`Deserializer`].
//!
//!  - **A type that implements `Deserialize` is a data structure** that can be
//!    deserialized from any data format supported by Serde, and conversely
//!  - **A type that implements `Deserializer` is a data format** that can
//!    deserialize any data structure supported by Serde.
//!
//! # The Deserialize trait
//!
//! Serde provides [`Deserialize`] implementations for many Rust primitive and
//! standard library types. The complete list is below. All of these can be
//! deserialized using Serde out of the box.
//!
//! Additionally, Serde provides a procedural macro called [`serde_derive`] to
//! automatically generate [`Deserialize`] implementations for structs and enums
//! in your program. See the [derive section of the manual] for how to use this.
//!
//! In rare cases it may be necessary to implement [`Deserialize`] manually for
//! some type in your program. See the [Implementing `Deserialize`] section of
//! the manual for more about this.
//!
//! Third-party crates may provide [`Deserialize`] implementations for types
//! that they expose. For example the [`linked-hash-map`] crate provides a
//! [`LinkedHashMap<K, V>`] type that is deserializable by Serde because the
//! crate provides an implementation of [`Deserialize`] for it.
//!
//! # The Deserializer trait
//!
//! [`Deserializer`] implementations are provided by third-party crates, for
//! example [`serde_json`], [`serde_yaml`] and [`bincode`].
//!
//! A partial list of well-maintained formats is given on the [Serde
//! website][data formats].
//!
//! # Implementations of Deserialize provided by Serde
//!
//! This is a slightly different set of types than what is supported for
//! serialization. Some types can be serialized by Serde but not deserialized.
//! One example is `OsStr`.
//!
//!  - **Primitive types**:
//!    - bool
//!    - i8, i16, i32, i64, i128, isize
//!    - u8, u16, u32, u64, u128, usize
//!    - f32, f64
//!    - char
//!  - **Compound types**:
//!    - \[T; 0\] through \[T; 32\]
//!    - tuples up to size 16
//!  - **Common standard library types**:
//!    - String
//!    - Option\<T\>
//!    - Result\<T, E\>
//!    - PhantomData\<T\>
//!  - **Wrapper types**:
//!    - Box\<T\>
//!    - Box\<\[T\]\>
//!    - Box\<str\>
//!    - Cow\<'a, T\>
//!    - Cell\<T\>
//!    - RefCell\<T\>
//!    - Mutex\<T\>
//!    - RwLock\<T\>
//!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
//!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
//!  - **Collection types**:
//!    - BTreeMap\<K, V\>
//!    - BTreeSet\<T\>
//!    - BinaryHeap\<T\>
//!    - HashMap\<K, V, H\>
//!    - HashSet\<T, H\>
//!    - LinkedList\<T\>
//!    - VecDeque\<T\>
//!    - Vec\<T\>
//!  - **Zero-copy types**:
//!    - &str
//!    - &\[u8\]
//!  - **FFI types**:
//!    - CString
//!    - Box\<CStr\>
//!    - OsString
//!  - **Miscellaneous standard library types**:
//!    - Duration
//!    - SystemTime
//!    - Path
//!    - PathBuf
//!    - Range\<T\>
//!    - RangeInclusive\<T\>
//!    - Bound\<T\>
//!    - num::NonZero*
//!    - `!` *(unstable)*
//!  - **Net types**:
//!    - IpAddr
//!    - Ipv4Addr
//!    - Ipv6Addr
//!    - SocketAddr
//!    - SocketAddrV4
//!    - SocketAddrV6
//!
//! [Implementing `Deserialize`]: https://serde.rs/impl-deserialize.html
//! [`Deserialize`]: ../trait.Deserialize.html
//! [`Deserializer`]: ../trait.Deserializer.html
//! [`LinkedHashMap<K, V>`]: https://docs.rs/linked-hash-map/*/linked_hash_map/struct.LinkedHashMap.html
//! [`bincode`]: https://github.com/servo/bincode
//! [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
//! [`serde_derive`]: https://crates.io/crates/serde_derive
//! [`serde_json`]: https://github.com/serde-rs/json
//! [`serde_yaml`]: https://github.com/dtolnay/serde-yaml
//! [derive section of the manual]: https://serde.rs/derive.html
//! [data formats]: https://serde.rs/#data-formats

use lib::*;

////////////////////////////////////////////////////////////////////////////////

pub mod value;

#[cfg(not(no_integer128))]
mod format;
mod ignored_any;
mod impls;
mod utf8;

pub use self::ignored_any::IgnoredAny;

#[cfg(feature = "std")]
#[doc(no_inline)]
pub use std::error::Error as StdError;
#[cfg(not(feature = "std"))]
#[doc(no_inline)]
pub use std_error::Error as StdError;

////////////////////////////////////////////////////////////////////////////////

macro_rules! declare_error_trait {
    (Error: Sized $(+ $($supertrait:ident)::+)*) => {
        /// The `Error` trait allows `Deserialize` implementations to create descriptive
        /// error messages belonging to the `Deserializer` against which they are
        /// currently running.
        ///
        /// Every `Deserializer` declares an `Error` type that encompasses both
        /// general-purpose deserialization errors as well as errors specific to the
        /// particular deserialization format. For example the `Error` type of
        /// `serde_json` can represent errors like an invalid JSON escape sequence or an
        /// unterminated string literal, in addition to the error cases that are part of
        /// this trait.
        ///
        /// Most deserializers should only need to provide the `Error::custom` method
        /// and inherit the default behavior for the other methods.
        ///
        /// # Example implementation
        ///
        /// The [example data format] presented on the website shows an error
        /// type appropriate for a basic JSON data format.
        ///
        /// [example data format]: https://serde.rs/data-format.html
        pub trait Error: Sized $(+ $($supertrait)::+)* {
            /// Raised when there is general error when deserializing a type.
            ///
            /// The message should not be capitalized and should not end with a period.
            ///
            /// ```edition2018
            /// # use std::str::FromStr;
            /// #
            /// # struct IpAddr;
            /// #
            /// # impl FromStr for IpAddr {
            /// #     type Err = String;
            /// #
            /// #     fn from_str(_: &str) -> Result<Self, String> {
            /// #         unimplemented!()
            /// #     }
            /// # }
            /// #
            /// use serde::de::{self, Deserialize, Deserializer};
            ///
            /// impl<'de> Deserialize<'de> for IpAddr {
            ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
            ///     where
            ///         D: Deserializer<'de>,
            ///     {
            ///         let s = String::deserialize(deserializer)?;
            ///         s.parse().map_err(de::Error::custom)
            ///     }
            /// }
            /// ```
            fn custom<T>(msg: T) -> Self
            where
                T: Display;

            /// Raised when a `Deserialize` receives a type different from what it was
            /// expecting.
            ///
            /// The `unexp` argument provides information about what type was received.
            /// This is the type that was present in the input file or other source data
            /// of the Deserializer.
            ///
            /// The `exp` argument provides information about what type was being
            /// expected. This is the type that is written in the program.
            ///
            /// For example if we try to deserialize a String out of a JSON file
            /// containing an integer, the unexpected type is the integer and the
            /// expected type is the string.
            #[cold]
            fn invalid_type(unexp: Unexpected, exp: &Expected) -> Self {
                Error::custom(format_args!("invalid type: {}, expected {}", unexp, exp))
            }

            /// Raised when a `Deserialize` receives a value of the right type but that
            /// is wrong for some other reason.
            ///
            /// The `unexp` argument provides information about what value was received.
            /// This is the value that was present in the input file or other source
            /// data of the Deserializer.
            ///
            /// The `exp` argument provides information about what value was being
            /// expected. This is the type that is written in the program.
            ///
            /// For example if we try to deserialize a String out of some binary data
            /// that is not valid UTF-8, the unexpected value is the bytes and the
            /// expected value is a string.
            #[cold]
            fn invalid_value(unexp: Unexpected, exp: &Expected) -> Self {
                Error::custom(format_args!("invalid value: {}, expected {}", unexp, exp))
            }

            /// Raised when deserializing a sequence or map and the input data contains
            /// too many or too few elements.
            ///
            /// The `len` argument is the number of elements encountered. The sequence
            /// or map may have expected more arguments or fewer arguments.
            ///
            /// The `exp` argument provides information about what data was being
            /// expected. For example `exp` might say that a tuple of size 6 was
            /// expected.
            #[cold]
            fn invalid_length(len: usize, exp: &Expected) -> Self {
                Error::custom(format_args!("invalid length {}, expected {}", len, exp))
            }

            /// Raised when a `Deserialize` enum type received a variant with an
            /// unrecognized name.
            #[cold]
            fn unknown_variant(variant: &str, expected: &'static [&'static str]) -> Self {
                if expected.is_empty() {
                    Error::custom(format_args!(
                        "unknown variant `{}`, there are no variants",
                        variant
                    ))
                } else {
                    Error::custom(format_args!(
                        "unknown variant `{}`, expected {}",
                        variant,
                        OneOf { names: expected }
                    ))
                }
            }

            /// Raised when a `Deserialize` struct type received a field with an
            /// unrecognized name.
            #[cold]
            fn unknown_field(field: &str, expected: &'static [&'static str]) -> Self {
                if expected.is_empty() {
                    Error::custom(format_args!(
                        "unknown field `{}`, there are no fields",
                        field
                    ))
                } else {
                    Error::custom(format_args!(
                        "unknown field `{}`, expected {}",
                        field,
                        OneOf { names: expected }
                    ))
                }
            }

            /// Raised when a `Deserialize` struct type expected to receive a required
            /// field with a particular name but that field was not present in the
            /// input.
            #[cold]
            fn missing_field(field: &'static str) -> Self {
                Error::custom(format_args!("missing field `{}`", field))
            }

            /// Raised when a `Deserialize` struct type received more than one of the
            /// same field.
            #[cold]
            fn duplicate_field(field: &'static str) -> Self {
                Error::custom(format_args!("duplicate field `{}`", field))
            }
        }
    }
}

#[cfg(feature = "std")]
declare_error_trait!(Error: Sized + StdError);

#[cfg(not(feature = "std"))]
declare_error_trait!(Error: Sized + Debug + Display);

/// `Unexpected` represents an unexpected invocation of any one of the `Visitor`
/// trait methods.
///
/// This is used as an argument to the `invalid_type`, `invalid_value`, and
/// `invalid_length` methods of the `Error` trait to build error messages.
///
/// ```edition2018
/// # use std::fmt;
/// #
/// # use serde::de::{self, Unexpected, Visitor};
/// #
/// # struct Example;
/// #
/// # impl<'de> Visitor<'de> for Example {
/// #     type Value = ();
/// #
/// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
/// #         write!(formatter, "definitely not a boolean")
/// #     }
/// #
/// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
/// where
///     E: de::Error,
/// {
///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
/// }
/// # }
/// ```
#[derive(Copy, Clone, PartialEq, Debug)]
pub enum Unexpected<'a> {
    /// The input contained a boolean value that was not expected.
    Bool(bool),

    /// The input contained an unsigned integer `u8`, `u16`, `u32` or `u64` that
    /// was not expected.
    Unsigned(u64),

    /// The input contained a signed integer `i8`, `i16`, `i32` or `i64` that
    /// was not expected.
    Signed(i64),

    /// The input contained a floating point `f32` or `f64` that was not
    /// expected.
    Float(f64),

    /// The input contained a `char` that was not expected.
    Char(char),

    /// The input contained a `&str` or `String` that was not expected.
    Str(&'a str),

    /// The input contained a `&[u8]` or `Vec<u8>` that was not expected.
    Bytes(&'a [u8]),

    /// The input contained a unit `()` that was not expected.
    Unit,

    /// The input contained an `Option<T>` that was not expected.
    Option,

    /// The input contained a newtype struct that was not expected.
    NewtypeStruct,

    /// The input contained a sequence that was not expected.
    Seq,

    /// The input contained a map that was not expected.
    Map,

    /// The input contained an enum that was not expected.
    Enum,

    /// The input contained a unit variant that was not expected.
    UnitVariant,

    /// The input contained a newtype variant that was not expected.
    NewtypeVariant,

    /// The input contained a tuple variant that was not expected.
    TupleVariant,

    /// The input contained a struct variant that was not expected.
    StructVariant,

    /// A message stating what uncategorized thing the input contained that was
    /// not expected.
    ///
    /// The message should be a noun or noun phrase, not capitalized and without
    /// a period. An example message is "unoriginal superhero".
    Other(&'a str),
}

impl<'a> fmt::Display for Unexpected<'a> {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        use self::Unexpected::*;
        match *self {
            Bool(b) => write!(formatter, "boolean `{}`", b),
            Unsigned(i) => write!(formatter, "integer `{}`", i),
            Signed(i) => write!(formatter, "integer `{}`", i),
            Float(f) => write!(formatter, "floating point `{}`", f),
            Char(c) => write!(formatter, "character `{}`", c),
            Str(s) => write!(formatter, "string {:?}", s),
            Bytes(_) => write!(formatter, "byte array"),
            Unit => write!(formatter, "unit value"),
            Option => write!(formatter, "Option value"),
            NewtypeStruct => write!(formatter, "newtype struct"),
            Seq => write!(formatter, "sequence"),
            Map => write!(formatter, "map"),
            Enum => write!(formatter, "enum"),
            UnitVariant => write!(formatter, "unit variant"),
            NewtypeVariant => write!(formatter, "newtype variant"),
            TupleVariant => write!(formatter, "tuple variant"),
            StructVariant => write!(formatter, "struct variant"),
            Other(other) => formatter.write_str(other),
        }
    }
}

/// `Expected` represents an explanation of what data a `Visitor` was expecting
/// to receive.
///
/// This is used as an argument to the `invalid_type`, `invalid_value`, and
/// `invalid_length` methods of the `Error` trait to build error messages. The
/// message should be a noun or noun phrase that completes the sentence "This
/// Visitor expects to receive ...", for example the message could be "an
/// integer between 0 and 64". The message should not be capitalized and should
/// not end with a period.
///
/// Within the context of a `Visitor` implementation, the `Visitor` itself
/// (`&self`) is an implementation of this trait.
///
/// ```edition2018
/// # use std::fmt;
/// #
/// # use serde::de::{self, Unexpected, Visitor};
/// #
/// # struct Example;
/// #
/// # impl<'de> Visitor<'de> for Example {
/// #     type Value = ();
/// #
/// #     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
/// #         write!(formatter, "definitely not a boolean")
/// #     }
/// #
/// fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
/// where
///     E: de::Error,
/// {
///     Err(de::Error::invalid_type(Unexpected::Bool(v), &self))
/// }
/// # }
/// ```
///
/// Outside of a `Visitor`, `&"..."` can be used.
///
/// ```edition2018
/// # use serde::de::{self, Unexpected};
/// #
/// # fn example<E>() -> Result<(), E>
/// # where
/// #     E: de::Error,
/// # {
/// #     let v = true;
/// return Err(de::Error::invalid_type(Unexpected::Bool(v), &"a negative integer"));
/// # }
/// ```
pub trait Expected {
    /// Format an explanation of what data was being expected. Same signature as
    /// the `Display` and `Debug` traits.
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result;
}

impl<'de, T> Expected for T
where
    T: Visitor<'de>,
{
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        self.expecting(formatter)
    }
}

impl<'a> Expected for &'a str {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str(self)
    }
}

impl<'a> Display for Expected + 'a {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        Expected::fmt(self, formatter)
    }
}

////////////////////////////////////////////////////////////////////////////////

/// A **data structure** that can be deserialized from any data format supported
/// by Serde.
///
/// Serde provides `Deserialize` implementations for many Rust primitive and
/// standard library types. The complete list is [here][de]. All of these can
/// be deserialized using Serde out of the box.
///
/// Additionally, Serde provides a procedural macro called `serde_derive` to
/// automatically generate `Deserialize` implementations for structs and enums
/// in your program. See the [derive section of the manual][derive] for how to
/// use this.
///
/// In rare cases it may be necessary to implement `Deserialize` manually for
/// some type in your program. See the [Implementing
/// `Deserialize`][impl-deserialize] section of the manual for more about this.
///
/// Third-party crates may provide `Deserialize` implementations for types that
/// they expose. For example the `linked-hash-map` crate provides a
/// `LinkedHashMap<K, V>` type that is deserializable by Serde because the crate
/// provides an implementation of `Deserialize` for it.
///
/// [de]: https://docs.serde.rs/serde/de/index.html
/// [derive]: https://serde.rs/derive.html
/// [impl-deserialize]: https://serde.rs/impl-deserialize.html
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by `Self` when deserialized. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
pub trait Deserialize<'de>: Sized {
    /// Deserialize this value from the given Serde deserializer.
    ///
    /// See the [Implementing `Deserialize`][impl-deserialize] section of the
    /// manual for more information about how to implement this method.
    ///
    /// [impl-deserialize]: https://serde.rs/impl-deserialize.html
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>;

    /// Deserializes a value into `self` from the given Deserializer.
    ///
    /// The purpose of this method is to allow the deserializer to reuse
    /// resources and avoid copies. As such, if this method returns an error,
    /// `self` will be in an indeterminate state where some parts of the struct
    /// have been overwritten. Although whatever state that is will be
    /// memory-safe.
    ///
    /// This is generally useful when repeatedly deserializing values that
    /// are processed one at a time, where the value of `self` doesn't matter
    /// when the next deserialization occurs.
    ///
    /// If you manually implement this, your recursive deserializations should
    /// use `deserialize_in_place`.
    ///
    /// This method is stable and an official public API, but hidden from the
    /// documentation because it is almost never what newbies are looking for.
    /// Showing it in rustdoc would cause it to be featured more prominently
    /// than it deserves.
    #[doc(hidden)]
    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
    where
        D: Deserializer<'de>,
    {
        // Default implementation just delegates to `deserialize` impl.
        *place = Deserialize::deserialize(deserializer)?;
        Ok(())
    }
}

/// A data structure that can be deserialized without borrowing any data from
/// the deserializer.
///
/// This is primarily useful for trait bounds on functions. For example a
/// `from_str` function may be able to deserialize a data structure that borrows
/// from the input string, but a `from_reader` function may only deserialize
/// owned data.
///
/// ```edition2018
/// # use serde::de::{Deserialize, DeserializeOwned};
/// # use std::io::{Read, Result};
/// #
/// # trait Ignore {
/// fn from_str<'a, T>(s: &'a str) -> Result<T>
/// where
///     T: Deserialize<'a>;
///
/// fn from_reader<R, T>(rdr: R) -> Result<T>
/// where
///     R: Read,
///     T: DeserializeOwned;
/// # }
/// ```
///
/// # Lifetime
///
/// The relationship between `Deserialize` and `DeserializeOwned` in trait
/// bounds is explained in more detail on the page [Understanding deserializer
/// lifetimes].
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
pub trait DeserializeOwned: for<'de> Deserialize<'de> {}
impl<T> DeserializeOwned for T where T: for<'de> Deserialize<'de> {}

/// `DeserializeSeed` is the stateful form of the `Deserialize` trait. If you
/// ever find yourself looking for a way to pass data into a `Deserialize` impl,
/// this trait is the way to do it.
///
/// As one example of stateful deserialization consider deserializing a JSON
/// array into an existing buffer. Using the `Deserialize` trait we could
/// deserialize a JSON array into a `Vec<T>` but it would be a freshly allocated
/// `Vec<T>`; there is no way for `Deserialize` to reuse a previously allocated
/// buffer. Using `DeserializeSeed` instead makes this possible as in the
/// example code below.
///
/// The canonical API for stateless deserialization looks like this:
///
/// ```edition2018
/// # use serde::Deserialize;
/// #
/// # enum Error {}
/// #
/// fn func<'de, T: Deserialize<'de>>() -> Result<T, Error>
/// # {
/// #     unimplemented!()
/// # }
/// ```
///
/// Adjusting an API like this to support stateful deserialization is a matter
/// of accepting a seed as input:
///
/// ```edition2018
/// # use serde::de::DeserializeSeed;
/// #
/// # enum Error {}
/// #
/// fn func_seed<'de, T: DeserializeSeed<'de>>(seed: T) -> Result<T::Value, Error>
/// # {
/// #     let _ = seed;
/// #     unimplemented!()
/// # }
/// ```
///
/// In practice the majority of deserialization is stateless. An API expecting a
/// seed can be appeased by passing `std::marker::PhantomData` as a seed in the
/// case of stateless deserialization.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by `Self::Value` when deserialized. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example
///
/// Suppose we have JSON that looks like `[[1, 2], [3, 4, 5], [6]]` and we need
/// to deserialize it into a flat representation like `vec![1, 2, 3, 4, 5, 6]`.
/// Allocating a brand new `Vec<T>` for each subarray would be slow. Instead we
/// would like to allocate a single `Vec<T>` and then deserialize each subarray
/// into it. This requires stateful deserialization using the `DeserializeSeed`
/// trait.
///
/// ```edition2018
/// use std::fmt;
/// use std::marker::PhantomData;
///
/// use serde::de::{Deserialize, DeserializeSeed, Deserializer, SeqAccess, Visitor};
///
/// // A DeserializeSeed implementation that uses stateful deserialization to
/// // append array elements onto the end of an existing vector. The preexisting
/// // state ("seed") in this case is the Vec<T>. The `deserialize` method of
/// // `ExtendVec` will be traversing the inner arrays of the JSON input and
/// // appending each integer into the existing Vec.
/// struct ExtendVec<'a, T: 'a>(&'a mut Vec<T>);
///
/// impl<'de, 'a, T> DeserializeSeed<'de> for ExtendVec<'a, T>
/// where
///     T: Deserialize<'de>,
/// {
///     // The return type of the `deserialize` method. This implementation
///     // appends onto an existing vector but does not create any new data
///     // structure, so the return type is ().
///     type Value = ();
///
///     fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
///     where
///         D: Deserializer<'de>,
///     {
///         // Visitor implementation that will walk an inner array of the JSON
///         // input.
///         struct ExtendVecVisitor<'a, T: 'a>(&'a mut Vec<T>);
///
///         impl<'de, 'a, T> Visitor<'de> for ExtendVecVisitor<'a, T>
///         where
///             T: Deserialize<'de>,
///         {
///             type Value = ();
///
///             fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
///                 write!(formatter, "an array of integers")
///             }
///
///             fn visit_seq<A>(self, mut seq: A) -> Result<(), A::Error>
///             where
///                 A: SeqAccess<'de>,
///             {
///                 // Visit each element in the inner array and push it onto
///                 // the existing vector.
///                 while let Some(elem) = seq.next_element()? {
///                     self.0.push(elem);
///                 }
///                 Ok(())
///             }
///         }
///
///         deserializer.deserialize_seq(ExtendVecVisitor(self.0))
///     }
/// }
///
/// // Visitor implementation that will walk the outer array of the JSON input.
/// struct FlattenedVecVisitor<T>(PhantomData<T>);
///
/// impl<'de, T> Visitor<'de> for FlattenedVecVisitor<T>
/// where
///     T: Deserialize<'de>,
/// {
///     // This Visitor constructs a single Vec<T> to hold the flattened
///     // contents of the inner arrays.
///     type Value = Vec<T>;
///
///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
///         write!(formatter, "an array of arrays")
///     }
///
///     fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
///     where
///         A: SeqAccess<'de>,
///     {
///         // Create a single Vec to hold the flattened contents.
///         let mut vec = Vec::new();
///
///         // Each iteration through this loop is one inner array.
///         while let Some(()) = seq.next_element_seed(ExtendVec(&mut vec))? {
///             // Nothing to do; inner array has been appended into `vec`.
///         }
///
///         // Return the finished vec.
///         Ok(vec)
///     }
/// }
///
/// # fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
/// # where
/// #     D: Deserializer<'de>,
/// # {
/// let visitor = FlattenedVecVisitor(PhantomData);
/// let flattened: Vec<u64> = deserializer.deserialize_seq(visitor)?;
/// #     Ok(())
/// # }
/// ```
pub trait DeserializeSeed<'de>: Sized {
    /// The type produced by using this seed.
    type Value;

    /// Equivalent to the more common `Deserialize::deserialize` method, except
    /// with some initial piece of data (the seed) passed in.
    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: Deserializer<'de>;
}

impl<'de, T> DeserializeSeed<'de> for PhantomData<T>
where
    T: Deserialize<'de>,
{
    type Value = T;

    #[inline]
    fn deserialize<D>(self, deserializer: D) -> Result<T, D::Error>
    where
        D: Deserializer<'de>,
    {
        T::deserialize(deserializer)
    }
}

////////////////////////////////////////////////////////////////////////////////

/// A **data format** that can deserialize any data structure supported by
/// Serde.
///
/// The role of this trait is to define the deserialization half of the [Serde
/// data model], which is a way to categorize every Rust data type into one of
/// 29 possible types. Each method of the `Deserializer` trait corresponds to one
/// of the types of the data model.
///
/// Implementations of `Deserialize` map themselves into this data model by
/// passing to the `Deserializer` a `Visitor` implementation that can receive
/// these various types.
///
/// The types that make up the Serde data model are:
///
///  - **14 primitive types**
///    - bool
///    - i8, i16, i32, i64, i128
///    - u8, u16, u32, u64, u128
///    - f32, f64
///    - char
///  - **string**
///    - UTF-8 bytes with a length and no null terminator.
///    - When serializing, all strings are handled equally. When deserializing,
///      there are three flavors of strings: transient, owned, and borrowed.
///  - **byte array** - \[u8\]
///    - Similar to strings, during deserialization byte arrays can be
///      transient, owned, or borrowed.
///  - **option**
///    - Either none or some value.
///  - **unit**
///    - The type of `()` in Rust. It represents an anonymous value containing
///      no data.
///  - **unit_struct**
///    - For example `struct Unit` or `PhantomData<T>`. It represents a named
///      value containing no data.
///  - **unit_variant**
///    - For example the `E::A` and `E::B` in `enum E { A, B }`.
///  - **newtype_struct**
///    - For example `struct Millimeters(u8)`.
///  - **newtype_variant**
///    - For example the `E::N` in `enum E { N(u8) }`.
///  - **seq**
///    - A variably sized heterogeneous sequence of values, for example `Vec<T>`
///      or `HashSet<T>`. When serializing, the length may or may not be known
///      before iterating through all the data. When deserializing, the length
///      is determined by looking at the serialized data.
///  - **tuple**
///    - A statically sized heterogeneous sequence of values for which the
///      length will be known at deserialization time without looking at the
///      serialized data, for example `(u8,)` or `(String, u64, Vec<T>)` or
///      `[u64; 10]`.
///  - **tuple_struct**
///    - A named tuple, for example `struct Rgb(u8, u8, u8)`.
///  - **tuple_variant**
///    - For example the `E::T` in `enum E { T(u8, u8) }`.
///  - **map**
///    - A heterogeneous key-value pairing, for example `BTreeMap<K, V>`.
///  - **struct**
///    - A heterogeneous key-value pairing in which the keys are strings and
///      will be known at deserialization time without looking at the serialized
///      data, for example `struct S { r: u8, g: u8, b: u8 }`.
///  - **struct_variant**
///    - For example the `E::S` in `enum E { S { r: u8, g: u8, b: u8 } }`.
///
/// The `Deserializer` trait supports two entry point styles which enables
/// different kinds of deserialization.
///
/// 1. The `deserialize` method. Self-describing data formats like JSON are able
///    to look at the serialized data and tell what it represents. For example
///    the JSON deserializer may see an opening curly brace (`{`) and know that
///    it is seeing a map. If the data format supports
///    `Deserializer::deserialize_any`, it will drive the Visitor using whatever
///    type it sees in the input. JSON uses this approach when deserializing
///    `serde_json::Value` which is an enum that can represent any JSON
///    document. Without knowing what is in a JSON document, we can deserialize
///    it to `serde_json::Value` by going through
///    `Deserializer::deserialize_any`.
///
/// 2. The various `deserialize_*` methods. Non-self-describing formats like
///    Bincode need to be told what is in the input in order to deserialize it.
///    The `deserialize_*` methods are hints to the deserializer for how to
///    interpret the next piece of input. Non-self-describing formats are not
///    able to deserialize something like `serde_json::Value` which relies on
///    `Deserializer::deserialize_any`.
///
/// When implementing `Deserialize`, you should avoid relying on
/// `Deserializer::deserialize_any` unless you need to be told by the
/// Deserializer what type is in the input. Know that relying on
/// `Deserializer::deserialize_any` means your data type will be able to
/// deserialize from self-describing formats only, ruling out Bincode and many
/// others.
///
/// [Serde data model]: https://serde.rs/data-model.html
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed from the input when deserializing. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example implementation
///
/// The [example data format] presented on the website contains example code for
/// a basic JSON `Deserializer`.
///
/// [example data format]: https://serde.rs/data-format.html
pub trait Deserializer<'de>: Sized {
    /// The error type that can be returned if some error occurs during
    /// deserialization.
    type Error: Error;

    /// Require the `Deserializer` to figure out how to drive the visitor based
    /// on what data type is in the input.
    ///
    /// When implementing `Deserialize`, you should avoid relying on
    /// `Deserializer::deserialize_any` unless you need to be told by the
    /// Deserializer what type is in the input. Know that relying on
    /// `Deserializer::deserialize_any` means your data type will be able to
    /// deserialize from self-describing formats only, ruling out Bincode and
    /// many others.
    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `bool` value.
    fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an `i8` value.
    fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an `i16` value.
    fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an `i32` value.
    fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an `i64` value.
    fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    serde_if_integer128! {
        /// Hint that the `Deserialize` type is expecting an `i128` value.
        ///
        /// This method is available only on Rust compiler versions >=1.26. The
        /// default behavior unconditionally returns an error.
        fn deserialize_i128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>
        {
            let _ = visitor;
            Err(Error::custom("i128 is not supported"))
        }
    }

    /// Hint that the `Deserialize` type is expecting a `u8` value.
    fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `u16` value.
    fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `u32` value.
    fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `u64` value.
    fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    serde_if_integer128! {
        /// Hint that the `Deserialize` type is expecting an `u128` value.
        ///
        /// This method is available only on Rust compiler versions >=1.26. The
        /// default behavior unconditionally returns an error.
        fn deserialize_u128<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>
        {
            let _ = visitor;
            Err(Error::custom("u128 is not supported"))
        }
    }

    /// Hint that the `Deserialize` type is expecting a `f32` value.
    fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `f64` value.
    fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a `char` value.
    fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a string value and does
    /// not benefit from taking ownership of buffered data owned by the
    /// `Deserializer`.
    ///
    /// If the `Visitor` would benefit from taking ownership of `String` data,
    /// indicate this to the `Deserializer` by using `deserialize_string`
    /// instead.
    fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a string value and would
    /// benefit from taking ownership of buffered data owned by the
    /// `Deserializer`.
    ///
    /// If the `Visitor` would not benefit from taking ownership of `String`
    /// data, indicate that to the `Deserializer` by using `deserialize_str`
    /// instead.
    fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a byte array and does not
    /// benefit from taking ownership of buffered data owned by the
    /// `Deserializer`.
    ///
    /// If the `Visitor` would benefit from taking ownership of `Vec<u8>` data,
    /// indicate this to the `Deserializer` by using `deserialize_byte_buf`
    /// instead.
    fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a byte array and would
    /// benefit from taking ownership of buffered data owned by the
    /// `Deserializer`.
    ///
    /// If the `Visitor` would not benefit from taking ownership of `Vec<u8>`
    /// data, indicate that to the `Deserializer` by using `deserialize_bytes`
    /// instead.
    fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an optional value.
    ///
    /// This allows deserializers that encode an optional value as a nullable
    /// value to convert the null value into `None` and a regular value into
    /// `Some(value)`.
    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a unit value.
    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a unit struct with a
    /// particular name.
    fn deserialize_unit_struct<V>(
        self,
        name: &'static str,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a newtype struct with a
    /// particular name.
    fn deserialize_newtype_struct<V>(
        self,
        name: &'static str,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a sequence of values.
    fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a sequence of values and
    /// knows how many values there are without looking at the serialized data.
    fn deserialize_tuple<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a tuple struct with a
    /// particular name and number of fields.
    fn deserialize_tuple_struct<V>(
        self,
        name: &'static str,
        len: usize,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a map of key-value pairs.
    fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting a struct with a particular
    /// name and fields.
    fn deserialize_struct<V>(
        self,
        name: &'static str,
        fields: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting an enum value with a
    /// particular name and possible variants.
    fn deserialize_enum<V>(
        self,
        name: &'static str,
        variants: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type is expecting the name of a struct
    /// field or the discriminant of an enum variant.
    fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Hint that the `Deserialize` type needs to deserialize a value whose type
    /// doesn't matter because it is ignored.
    ///
    /// Deserializers for non-self-describing formats may not support this mode.
    fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Determine whether `Deserialize` implementations should expect to
    /// deserialize their human-readable form.
    ///
    /// Some types have a human-readable form that may be somewhat expensive to
    /// construct, as well as a binary form that is compact and efficient.
    /// Generally text-based formats like JSON and YAML will prefer to use the
    /// human-readable one and binary formats like Bincode will prefer the
    /// compact one.
    ///
    /// ```edition2018
    /// # use std::ops::Add;
    /// # use std::str::FromStr;
    /// #
    /// # struct Timestamp;
    /// #
    /// # impl Timestamp {
    /// #     const EPOCH: Timestamp = Timestamp;
    /// # }
    /// #
    /// # impl FromStr for Timestamp {
    /// #     type Err = String;
    /// #     fn from_str(_: &str) -> Result<Self, Self::Err> {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// #
    /// # struct Duration;
    /// #
    /// # impl Duration {
    /// #     fn seconds(_: u64) -> Self { unimplemented!() }
    /// # }
    /// #
    /// # impl Add<Duration> for Timestamp {
    /// #     type Output = Timestamp;
    /// #     fn add(self, _: Duration) -> Self::Output {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// #
    /// use serde::de::{self, Deserialize, Deserializer};
    ///
    /// impl<'de> Deserialize<'de> for Timestamp {
    ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    ///     where
    ///         D: Deserializer<'de>,
    ///     {
    ///         if deserializer.is_human_readable() {
    ///             // Deserialize from a human-readable string like "2015-05-15T17:01:00Z".
    ///             let s = String::deserialize(deserializer)?;
    ///             Timestamp::from_str(&s).map_err(de::Error::custom)
    ///         } else {
    ///             // Deserialize from a compact binary representation, seconds since
    ///             // the Unix epoch.
    ///             let n = u64::deserialize(deserializer)?;
    ///             Ok(Timestamp::EPOCH + Duration::seconds(n))
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// The default implementation of this method returns `true`. Data formats
    /// may override this to `false` to request a compact form for types that
    /// support one. Note that modifying this method to change a format from
    /// human-readable to compact or vice versa should be regarded as a breaking
    /// change, as a value serialized in human-readable mode is not required to
    /// deserialize from the same data in compact mode.
    #[inline]
    fn is_human_readable(&self) -> bool {
        true
    }

    // Not public API.
    #[cfg(all(not(no_serde_derive), any(feature = "std", feature = "alloc")))]
    #[doc(hidden)]
    fn __deserialize_content<V>(
        self,
        _: ::actually_private::T,
        visitor: V,
    ) -> Result<::private::de::Content<'de>, Self::Error>
    where
        V: Visitor<'de, Value = ::private::de::Content<'de>>,
    {
        self.deserialize_any(visitor)
    }
}

////////////////////////////////////////////////////////////////////////////////

/// This trait represents a visitor that walks through a deserializer.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the requirement for lifetime of data
/// that may be borrowed by `Self::Value`. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example
///
/// ```edition2018
/// # use std::fmt;
/// #
/// # use serde::de::{self, Unexpected, Visitor};
/// #
/// /// A visitor that deserializes a long string - a string containing at least
/// /// some minimum number of bytes.
/// struct LongString {
///     min: usize,
/// }
///
/// impl<'de> Visitor<'de> for LongString {
///     type Value = String;
///
///     fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
///         write!(formatter, "a string containing at least {} bytes", self.min)
///     }
///
///     fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
///     where
///         E: de::Error,
///     {
///         if s.len() >= self.min {
///             Ok(s.to_owned())
///         } else {
///             Err(de::Error::invalid_value(Unexpected::Str(s), &self))
///         }
///     }
/// }
/// ```
pub trait Visitor<'de>: Sized {
    /// The value produced by this visitor.
    type Value;

    /// Format a message stating what data this Visitor expects to receive.
    ///
    /// This is used in error messages. The message should complete the sentence
    /// "This Visitor expects to receive ...", for example the message could be
    /// "an integer between 0 and 64". The message should not be capitalized and
    /// should not end with a period.
    ///
    /// ```edition2018
    /// # use std::fmt;
    /// #
    /// # struct S {
    /// #     max: usize,
    /// # }
    /// #
    /// # impl<'de> serde::de::Visitor<'de> for S {
    /// #     type Value = ();
    /// #
    /// fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    ///     write!(formatter, "an integer between 0 and {}", self.max)
    /// }
    /// # }
    /// ```
    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result;

    /// The input contains a boolean.
    ///
    /// The default implementation fails with a type error.
    fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Bool(v), &self))
    }

    /// The input contains an `i8`.
    ///
    /// The default implementation forwards to [`visit_i64`].
    ///
    /// [`visit_i64`]: #method.visit_i64
    fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_i64(v as i64)
    }

    /// The input contains an `i16`.
    ///
    /// The default implementation forwards to [`visit_i64`].
    ///
    /// [`visit_i64`]: #method.visit_i64
    fn visit_i16<E>(self, v: i16) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_i64(v as i64)
    }

    /// The input contains an `i32`.
    ///
    /// The default implementation forwards to [`visit_i64`].
    ///
    /// [`visit_i64`]: #method.visit_i64
    fn visit_i32<E>(self, v: i32) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_i64(v as i64)
    }

    /// The input contains an `i64`.
    ///
    /// The default implementation fails with a type error.
    fn visit_i64<E>(self, v: i64) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Signed(v), &self))
    }

    serde_if_integer128! {
        /// The input contains a `i128`.
        ///
        /// This method is available only on Rust compiler versions >=1.26. The
        /// default implementation fails with a type error.
        fn visit_i128<E>(self, v: i128) -> Result<Self::Value, E>
        where
            E: Error,
        {
            let mut buf = [0u8; 58];
            let mut writer = format::Buf::new(&mut buf);
            fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as i128", v)).unwrap();
            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &self))
        }
    }

    /// The input contains a `u8`.
    ///
    /// The default implementation forwards to [`visit_u64`].
    ///
    /// [`visit_u64`]: #method.visit_u64
    fn visit_u8<E>(self, v: u8) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_u64(v as u64)
    }

    /// The input contains a `u16`.
    ///
    /// The default implementation forwards to [`visit_u64`].
    ///
    /// [`visit_u64`]: #method.visit_u64
    fn visit_u16<E>(self, v: u16) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_u64(v as u64)
    }

    /// The input contains a `u32`.
    ///
    /// The default implementation forwards to [`visit_u64`].
    ///
    /// [`visit_u64`]: #method.visit_u64
    fn visit_u32<E>(self, v: u32) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_u64(v as u64)
    }

    /// The input contains a `u64`.
    ///
    /// The default implementation fails with a type error.
    fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Unsigned(v), &self))
    }

    serde_if_integer128! {
        /// The input contains a `u128`.
        ///
        /// This method is available only on Rust compiler versions >=1.26. The
        /// default implementation fails with a type error.
        fn visit_u128<E>(self, v: u128) -> Result<Self::Value, E>
        where
            E: Error,
        {
            let mut buf = [0u8; 57];
            let mut writer = format::Buf::new(&mut buf);
            fmt::Write::write_fmt(&mut writer, format_args!("integer `{}` as u128", v)).unwrap();
            Err(Error::invalid_type(Unexpected::Other(writer.as_str()), &self))
        }
    }

    /// The input contains an `f32`.
    ///
    /// The default implementation forwards to [`visit_f64`].
    ///
    /// [`visit_f64`]: #method.visit_f64
    fn visit_f32<E>(self, v: f32) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_f64(v as f64)
    }

    /// The input contains an `f64`.
    ///
    /// The default implementation fails with a type error.
    fn visit_f64<E>(self, v: f64) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Float(v), &self))
    }

    /// The input contains a `char`.
    ///
    /// The default implementation forwards to [`visit_str`] as a one-character
    /// string.
    ///
    /// [`visit_str`]: #method.visit_str
    #[inline]
    fn visit_char<E>(self, v: char) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_str(utf8::encode(v).as_str())
    }

    /// The input contains a string. The lifetime of the string is ephemeral and
    /// it may be destroyed after this method returns.
    ///
    /// This method allows the `Deserializer` to avoid a copy by retaining
    /// ownership of any buffered data. `Deserialize` implementations that do
    /// not benefit from taking ownership of `String` data should indicate that
    /// to the deserializer by using `Deserializer::deserialize_str` rather than
    /// `Deserializer::deserialize_string`.
    ///
    /// It is never correct to implement `visit_string` without implementing
    /// `visit_str`. Implement neither, both, or just `visit_str`.
    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Str(v), &self))
    }

    /// The input contains a string that lives at least as long as the
    /// `Deserializer`.
    ///
    /// This enables zero-copy deserialization of strings in some formats. For
    /// example JSON input containing the JSON string `"borrowed"` can be
    /// deserialized with zero copying into a `&'a str` as long as the input
    /// data outlives `'a`.
    ///
    /// The default implementation forwards to `visit_str`.
    #[inline]
    fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_str(v)
    }

    /// The input contains a string and ownership of the string is being given
    /// to the `Visitor`.
    ///
    /// This method allows the `Visitor` to avoid a copy by taking ownership of
    /// a string created by the `Deserializer`. `Deserialize` implementations
    /// that benefit from taking ownership of `String` data should indicate that
    /// to the deserializer by using `Deserializer::deserialize_string` rather
    /// than `Deserializer::deserialize_str`, although not every deserializer
    /// will honor such a request.
    ///
    /// It is never correct to implement `visit_string` without implementing
    /// `visit_str`. Implement neither, both, or just `visit_str`.
    ///
    /// The default implementation forwards to `visit_str` and then drops the
    /// `String`.
    #[inline]
    #[cfg(any(feature = "std", feature = "alloc"))]
    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_str(&v)
    }

    /// The input contains a byte array. The lifetime of the byte array is
    /// ephemeral and it may be destroyed after this method returns.
    ///
    /// This method allows the `Deserializer` to avoid a copy by retaining
    /// ownership of any buffered data. `Deserialize` implementations that do
    /// not benefit from taking ownership of `Vec<u8>` data should indicate that
    /// to the deserializer by using `Deserializer::deserialize_bytes` rather
    /// than `Deserializer::deserialize_byte_buf`.
    ///
    /// It is never correct to implement `visit_byte_buf` without implementing
    /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
    where
        E: Error,
    {
        let _ = v;
        Err(Error::invalid_type(Unexpected::Bytes(v), &self))
    }

    /// The input contains a byte array that lives at least as long as the
    /// `Deserializer`.
    ///
    /// This enables zero-copy deserialization of bytes in some formats. For
    /// example Bincode data containing bytes can be deserialized with zero
    /// copying into a `&'a [u8]` as long as the input data outlives `'a`.
    ///
    /// The default implementation forwards to `visit_bytes`.
    #[inline]
    fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_bytes(v)
    }

    /// The input contains a byte array and ownership of the byte array is being
    /// given to the `Visitor`.
    ///
    /// This method allows the `Visitor` to avoid a copy by taking ownership of
    /// a byte buffer created by the `Deserializer`. `Deserialize`
    /// implementations that benefit from taking ownership of `Vec<u8>` data
    /// should indicate that to the deserializer by using
    /// `Deserializer::deserialize_byte_buf` rather than
    /// `Deserializer::deserialize_bytes`, although not every deserializer will
    /// honor such a request.
    ///
    /// It is never correct to implement `visit_byte_buf` without implementing
    /// `visit_bytes`. Implement neither, both, or just `visit_bytes`.
    ///
    /// The default implementation forwards to `visit_bytes` and then drops the
    /// `Vec<u8>`.
    #[cfg(any(feature = "std", feature = "alloc"))]
    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
    where
        E: Error,
    {
        self.visit_bytes(&v)
    }

    /// The input contains an optional that is absent.
    ///
    /// The default implementation fails with a type error.
    fn visit_none<E>(self) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Option, &self))
    }

    /// The input contains an optional that is present.
    ///
    /// The default implementation fails with a type error.
    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: Deserializer<'de>,
    {
        let _ = deserializer;
        Err(Error::invalid_type(Unexpected::Option, &self))
    }

    /// The input contains a unit `()`.
    ///
    /// The default implementation fails with a type error.
    fn visit_unit<E>(self) -> Result<Self::Value, E>
    where
        E: Error,
    {
        Err(Error::invalid_type(Unexpected::Unit, &self))
    }

    /// The input contains a newtype struct.
    ///
    /// The content of the newtype struct may be read from the given
    /// `Deserializer`.
    ///
    /// The default implementation fails with a type error.
    fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: Deserializer<'de>,
    {
        let _ = deserializer;
        Err(Error::invalid_type(Unexpected::NewtypeStruct, &self))
    }

    /// The input contains a sequence of elements.
    ///
    /// The default implementation fails with a type error.
    fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
    where
        A: SeqAccess<'de>,
    {
        let _ = seq;
        Err(Error::invalid_type(Unexpected::Seq, &self))
    }

    /// The input contains a key-value map.
    ///
    /// The default implementation fails with a type error.
    fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>
    where
        A: MapAccess<'de>,
    {
        let _ = map;
        Err(Error::invalid_type(Unexpected::Map, &self))
    }

    /// The input contains an enum.
    ///
    /// The default implementation fails with a type error.
    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
    where
        A: EnumAccess<'de>,
    {
        let _ = data;
        Err(Error::invalid_type(Unexpected::Enum, &self))
    }

    // Used when deserializing a flattened Option field. Not public API.
    #[doc(hidden)]
    fn __private_visit_untagged_option<D>(self, _: D) -> Result<Self::Value, ()>
    where
        D: Deserializer<'de>,
    {
        Err(())
    }
}

////////////////////////////////////////////////////////////////////////////////

/// Provides a `Visitor` access to each element of a sequence in the input.
///
/// This is a trait that a `Deserializer` passes to a `Visitor` implementation,
/// which deserializes each item in a sequence.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by deserialized sequence elements. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example implementation
///
/// The [example data format] presented on the website demonstrates an
/// implementation of `SeqAccess` for a basic JSON data format.
///
/// [example data format]: https://serde.rs/data-format.html
pub trait SeqAccess<'de> {
    /// The error type that can be returned if some error occurs during
    /// deserialization.
    type Error: Error;

    /// This returns `Ok(Some(value))` for the next value in the sequence, or
    /// `Ok(None)` if there are no more remaining items.
    ///
    /// `Deserialize` implementations should typically use
    /// `SeqAccess::next_element` instead.
    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
    where
        T: DeserializeSeed<'de>;

    /// This returns `Ok(Some(value))` for the next value in the sequence, or
    /// `Ok(None)` if there are no more remaining items.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `SeqAccess` implementations should not override the default behavior.
    #[inline]
    fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
    where
        T: Deserialize<'de>,
    {
        self.next_element_seed(PhantomData)
    }

    /// Returns the number of elements remaining in the sequence, if known.
    #[inline]
    fn size_hint(&self) -> Option<usize> {
        None
    }
}

impl<'de, 'a, A: ?Sized> SeqAccess<'de> for &'a mut A
where
    A: SeqAccess<'de>,
{
    type Error = A::Error;

    #[inline]
    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
    where
        T: DeserializeSeed<'de>,
    {
        (**self).next_element_seed(seed)
    }

    #[inline]
    fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
    where
        T: Deserialize<'de>,
    {
        (**self).next_element()
    }

    #[inline]
    fn size_hint(&self) -> Option<usize> {
        (**self).size_hint()
    }
}

////////////////////////////////////////////////////////////////////////////////

/// Provides a `Visitor` access to each entry of a map in the input.
///
/// This is a trait that a `Deserializer` passes to a `Visitor` implementation.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by deserialized map entries. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example implementation
///
/// The [example data format] presented on the website demonstrates an
/// implementation of `MapAccess` for a basic JSON data format.
///
/// [example data format]: https://serde.rs/data-format.html
pub trait MapAccess<'de> {
    /// The error type that can be returned if some error occurs during
    /// deserialization.
    type Error: Error;

    /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
    /// if there are no more remaining entries.
    ///
    /// `Deserialize` implementations should typically use
    /// `MapAccess::next_key` or `MapAccess::next_entry` instead.
    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
    where
        K: DeserializeSeed<'de>;

    /// This returns a `Ok(value)` for the next value in the map.
    ///
    /// `Deserialize` implementations should typically use
    /// `MapAccess::next_value` instead.
    ///
    /// # Panics
    ///
    /// Calling `next_value_seed` before `next_key_seed` is incorrect and is
    /// allowed to panic or return bogus results.
    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
    where
        V: DeserializeSeed<'de>;

    /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
    /// the map, or `Ok(None)` if there are no more remaining items.
    ///
    /// `MapAccess` implementations should override the default behavior if a
    /// more efficient implementation is possible.
    ///
    /// `Deserialize` implementations should typically use
    /// `MapAccess::next_entry` instead.
    #[inline]
    fn next_entry_seed<K, V>(
        &mut self,
        kseed: K,
        vseed: V,
    ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
    where
        K: DeserializeSeed<'de>,
        V: DeserializeSeed<'de>,
    {
        match try!(self.next_key_seed(kseed)) {
            Some(key) => {
                let value = try!(self.next_value_seed(vseed));
                Ok(Some((key, value)))
            }
            None => Ok(None),
        }
    }

    /// This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)`
    /// if there are no more remaining entries.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `MapAccess` implementations should not override the default behavior.
    #[inline]
    fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
    where
        K: Deserialize<'de>,
    {
        self.next_key_seed(PhantomData)
    }

    /// This returns a `Ok(value)` for the next value in the map.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `MapAccess` implementations should not override the default behavior.
    ///
    /// # Panics
    ///
    /// Calling `next_value` before `next_key` is incorrect and is allowed to
    /// panic or return bogus results.
    #[inline]
    fn next_value<V>(&mut self) -> Result<V, Self::Error>
    where
        V: Deserialize<'de>,
    {
        self.next_value_seed(PhantomData)
    }

    /// This returns `Ok(Some((key, value)))` for the next (key-value) pair in
    /// the map, or `Ok(None)` if there are no more remaining items.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `MapAccess` implementations should not override the default behavior.
    #[inline]
    fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
    where
        K: Deserialize<'de>,
        V: Deserialize<'de>,
    {
        self.next_entry_seed(PhantomData, PhantomData)
    }

    /// Returns the number of entries remaining in the map, if known.
    #[inline]
    fn size_hint(&self) -> Option<usize> {
        None
    }
}

impl<'de, 'a, A: ?Sized> MapAccess<'de> for &'a mut A
where
    A: MapAccess<'de>,
{
    type Error = A::Error;

    #[inline]
    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
    where
        K: DeserializeSeed<'de>,
    {
        (**self).next_key_seed(seed)
    }

    #[inline]
    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
    where
        V: DeserializeSeed<'de>,
    {
        (**self).next_value_seed(seed)
    }

    #[inline]
    fn next_entry_seed<K, V>(
        &mut self,
        kseed: K,
        vseed: V,
    ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
    where
        K: DeserializeSeed<'de>,
        V: DeserializeSeed<'de>,
    {
        (**self).next_entry_seed(kseed, vseed)
    }

    #[inline]
    fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
    where
        K: Deserialize<'de>,
        V: Deserialize<'de>,
    {
        (**self).next_entry()
    }

    #[inline]
    fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
    where
        K: Deserialize<'de>,
    {
        (**self).next_key()
    }

    #[inline]
    fn next_value<V>(&mut self) -> Result<V, Self::Error>
    where
        V: Deserialize<'de>,
    {
        (**self).next_value()
    }

    #[inline]
    fn size_hint(&self) -> Option<usize> {
        (**self).size_hint()
    }
}

////////////////////////////////////////////////////////////////////////////////

/// Provides a `Visitor` access to the data of an enum in the input.
///
/// `EnumAccess` is created by the `Deserializer` and passed to the
/// `Visitor` in order to identify which variant of an enum to deserialize.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by the deserialized enum variant. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example implementation
///
/// The [example data format] presented on the website demonstrates an
/// implementation of `EnumAccess` for a basic JSON data format.
///
/// [example data format]: https://serde.rs/data-format.html
pub trait EnumAccess<'de>: Sized {
    /// The error type that can be returned if some error occurs during
    /// deserialization.
    type Error: Error;
    /// The `Visitor` that will be used to deserialize the content of the enum
    /// variant.
    type Variant: VariantAccess<'de, Error = Self::Error>;

    /// `variant` is called to identify which variant to deserialize.
    ///
    /// `Deserialize` implementations should typically use `EnumAccess::variant`
    /// instead.
    fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
    where
        V: DeserializeSeed<'de>;

    /// `variant` is called to identify which variant to deserialize.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `EnumAccess` implementations should not override the default behavior.
    #[inline]
    fn variant<V>(self) -> Result<(V, Self::Variant), Self::Error>
    where
        V: Deserialize<'de>,
    {
        self.variant_seed(PhantomData)
    }
}

/// `VariantAccess` is a visitor that is created by the `Deserializer` and
/// passed to the `Deserialize` to deserialize the content of a particular enum
/// variant.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed by the deserialized enum variant. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example implementation
///
/// The [example data format] presented on the website demonstrates an
/// implementation of `VariantAccess` for a basic JSON data format.
///
/// [example data format]: https://serde.rs/data-format.html
pub trait VariantAccess<'de>: Sized {
    /// The error type that can be returned if some error occurs during
    /// deserialization. Must match the error type of our `EnumAccess`.
    type Error: Error;

    /// Called when deserializing a variant with no values.
    ///
    /// If the data contains a different type of variant, the following
    /// `invalid_type` error should be constructed:
    ///
    /// ```edition2018
    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
    /// #
    /// # struct X;
    /// #
    /// # impl<'de> VariantAccess<'de> for X {
    /// #     type Error = value::Error;
    /// #
    /// fn unit_variant(self) -> Result<(), Self::Error> {
    ///     // What the data actually contained; suppose it is a tuple variant.
    ///     let unexp = Unexpected::TupleVariant;
    ///     Err(de::Error::invalid_type(unexp, &"unit variant"))
    /// }
    /// #
    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
    /// #     where
    /// #         T: DeserializeSeed<'de>,
    /// #     { unimplemented!() }
    /// #
    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// #
    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// # }
    /// ```
    fn unit_variant(self) -> Result<(), Self::Error>;

    /// Called when deserializing a variant with a single value.
    ///
    /// `Deserialize` implementations should typically use
    /// `VariantAccess::newtype_variant` instead.
    ///
    /// If the data contains a different type of variant, the following
    /// `invalid_type` error should be constructed:
    ///
    /// ```edition2018
    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
    /// #
    /// # struct X;
    /// #
    /// # impl<'de> VariantAccess<'de> for X {
    /// #     type Error = value::Error;
    /// #
    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
    /// #         unimplemented!()
    /// #     }
    /// #
    /// fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
    /// where
    ///     T: DeserializeSeed<'de>,
    /// {
    ///     // What the data actually contained; suppose it is a unit variant.
    ///     let unexp = Unexpected::UnitVariant;
    ///     Err(de::Error::invalid_type(unexp, &"newtype variant"))
    /// }
    /// #
    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// #
    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// # }
    /// ```
    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
    where
        T: DeserializeSeed<'de>;

    /// Called when deserializing a variant with a single value.
    ///
    /// This method exists as a convenience for `Deserialize` implementations.
    /// `VariantAccess` implementations should not override the default
    /// behavior.
    #[inline]
    fn newtype_variant<T>(self) -> Result<T, Self::Error>
    where
        T: Deserialize<'de>,
    {
        self.newtype_variant_seed(PhantomData)
    }

    /// Called when deserializing a tuple-like variant.
    ///
    /// The `len` is the number of fields expected in the tuple variant.
    ///
    /// If the data contains a different type of variant, the following
    /// `invalid_type` error should be constructed:
    ///
    /// ```edition2018
    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
    /// #
    /// # struct X;
    /// #
    /// # impl<'de> VariantAccess<'de> for X {
    /// #     type Error = value::Error;
    /// #
    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
    /// #         unimplemented!()
    /// #     }
    /// #
    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
    /// #     where
    /// #         T: DeserializeSeed<'de>,
    /// #     { unimplemented!() }
    /// #
    /// fn tuple_variant<V>(
    ///     self,
    ///     _len: usize,
    ///     _visitor: V,
    /// ) -> Result<V::Value, Self::Error>
    /// where
    ///     V: Visitor<'de>,
    /// {
    ///     // What the data actually contained; suppose it is a unit variant.
    ///     let unexp = Unexpected::UnitVariant;
    ///     Err(de::Error::invalid_type(unexp, &"tuple variant"))
    /// }
    /// #
    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// # }
    /// ```
    fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;

    /// Called when deserializing a struct-like variant.
    ///
    /// The `fields` are the names of the fields of the struct variant.
    ///
    /// If the data contains a different type of variant, the following
    /// `invalid_type` error should be constructed:
    ///
    /// ```edition2018
    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
    /// #
    /// # struct X;
    /// #
    /// # impl<'de> VariantAccess<'de> for X {
    /// #     type Error = value::Error;
    /// #
    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
    /// #         unimplemented!()
    /// #     }
    /// #
    /// #     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
    /// #     where
    /// #         T: DeserializeSeed<'de>,
    /// #     { unimplemented!() }
    /// #
    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
    /// #     where
    /// #         V: Visitor<'de>,
    /// #     { unimplemented!() }
    /// #
    /// fn struct_variant<V>(
    ///     self,
    ///     _fields: &'static [&'static str],
    ///     _visitor: V,
    /// ) -> Result<V::Value, Self::Error>
    /// where
    ///     V: Visitor<'de>,
    /// {
    ///     // What the data actually contained; suppose it is a unit variant.
    ///     let unexp = Unexpected::UnitVariant;
    ///     Err(de::Error::invalid_type(unexp, &"struct variant"))
    /// }
    /// # }
    /// ```
    fn struct_variant<V>(
        self,
        fields: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>;
}

////////////////////////////////////////////////////////////////////////////////

/// Converts an existing value into a `Deserializer` from which other values can
/// be deserialized.
///
/// # Lifetime
///
/// The `'de` lifetime of this trait is the lifetime of data that may be
/// borrowed from the resulting `Deserializer`. See the page [Understanding
/// deserializer lifetimes] for a more detailed explanation of these lifetimes.
///
/// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
///
/// # Example
///
/// ```edition2018
/// use std::str::FromStr;
/// use serde::Deserialize;
/// use serde::de::{value, IntoDeserializer};
///
/// #[derive(Deserialize)]
/// enum Setting {
///     On,
///     Off,
/// }
///
/// impl FromStr for Setting {
///     type Err = value::Error;
///
///     fn from_str(s: &str) -> Result<Self, Self::Err> {
///         Self::deserialize(s.into_deserializer())
///     }
/// }
/// ```
pub trait IntoDeserializer<'de, E: Error = value::Error> {
    /// The type of the deserializer being converted into.
    type Deserializer: Deserializer<'de, Error = E>;

    /// Convert this value into a deserializer.
    fn into_deserializer(self) -> Self::Deserializer;
}

////////////////////////////////////////////////////////////////////////////////

/// Used in error messages.
///
/// - expected `a`
/// - expected `a` or `b`
/// - expected one of `a`, `b`, `c`
///
/// The slice of names must not be empty.
struct OneOf {
    names: &'static [&'static str],
}

impl Display for OneOf {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        match self.names.len() {
            0 => panic!(), // special case elsewhere
            1 => write!(formatter, "`{}`", self.names[0]),
            2 => write!(formatter, "`{}` or `{}`", self.names[0], self.names[1]),
            _ => {
                try!(write!(formatter, "one of "));
                for (i, alt) in self.names.iter().enumerate() {
                    if i > 0 {
                        try!(write!(formatter, ", "));
                    }
                    try!(write!(formatter, "`{}`", alt));
                }
                Ok(())
            }
        }
    }
}