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
#[cfg(test)]
mod tests;

use hashbrown::hash_set as base;

use crate::borrow::Borrow;
use crate::collections::TryReserveError;
use crate::fmt;
use crate::hash::{BuildHasher, Hash, RandomState};
use crate::iter::{Chain, FusedIterator};
use crate::ops::{BitAnd, BitOr, BitXor, Sub};

use super::map::map_try_reserve_error;

/// A [hash set] implemented as a `HashMap` where the value is `()`.
///
/// As with the [`HashMap`] type, a `HashSet` requires that the elements
/// implement the [`Eq`] and [`Hash`] traits. This can frequently be achieved by
/// using `#[derive(PartialEq, Eq, Hash)]`. If you implement these yourself,
/// it is important that the following property holds:
///
/// ```text
/// k1 == k2 -> hash(k1) == hash(k2)
/// ```
///
/// In other words, if two keys are equal, their hashes must be equal.
/// Violating this property is a logic error.
///
/// It is also a logic error for a key to be modified in such a way that the key's
/// hash, as determined by the [`Hash`] trait, or its equality, as determined by
/// the [`Eq`] trait, changes while it is in the map. This is normally only
/// possible through [`Cell`], [`RefCell`], global state, I/O, or unsafe code.
///
/// The behavior resulting from either logic error is not specified, but will
/// be encapsulated to the `HashSet` that observed the logic error and not
/// result in undefined behavior. This could include panics, incorrect results,
/// aborts, memory leaks, and non-termination.
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
/// // Type inference lets us omit an explicit type signature (which
/// // would be `HashSet<String>` in this example).
/// let mut books = HashSet::new();
///
/// // Add some books.
/// books.insert("A Dance With Dragons".to_string());
/// books.insert("To Kill a Mockingbird".to_string());
/// books.insert("The Odyssey".to_string());
/// books.insert("The Great Gatsby".to_string());
///
/// // Check for a specific one.
/// if !books.contains("The Winds of Winter") {
///     println!("We have {} books, but The Winds of Winter ain't one.",
///              books.len());
/// }
///
/// // Remove a book.
/// books.remove("The Odyssey");
///
/// // Iterate over everything.
/// for book in &books {
///     println!("{book}");
/// }
/// ```
///
/// The easiest way to use `HashSet` with a custom type is to derive
/// [`Eq`] and [`Hash`]. We must also derive [`PartialEq`],
/// which is required if [`Eq`] is derived.
///
/// ```
/// use std::collections::HashSet;
/// #[derive(Hash, Eq, PartialEq, Debug)]
/// struct Viking {
///     name: String,
///     power: usize,
/// }
///
/// let mut vikings = HashSet::new();
///
/// vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
/// vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
/// vikings.insert(Viking { name: "Olaf".to_string(), power: 4 });
/// vikings.insert(Viking { name: "Harald".to_string(), power: 8 });
///
/// // Use derived implementation to print the vikings.
/// for x in &vikings {
///     println!("{x:?}");
/// }
/// ```
///
/// A `HashSet` with a known list of items can be initialized from an array:
///
/// ```
/// use std::collections::HashSet;
///
/// let viking_names = HashSet::from(["Einar", "Olaf", "Harald"]);
/// ```
///
/// [hash set]: crate::collections#use-the-set-variant-of-any-of-these-maps-when
/// [`HashMap`]: crate::collections::HashMap
/// [`RefCell`]: crate::cell::RefCell
/// [`Cell`]: crate::cell::Cell
#[cfg_attr(not(test), rustc_diagnostic_item = "HashSet")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct HashSet<T, S = RandomState> {
    base: base::HashSet<T, S>,
}

impl<T> HashSet<T, RandomState> {
    /// Creates an empty `HashSet`.
    ///
    /// The hash set is initially created with a capacity of 0, so it will not allocate until it
    /// is first inserted into.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let set: HashSet<i32> = HashSet::new();
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> HashSet<T, RandomState> {
        Default::default()
    }

    /// Creates an empty `HashSet` with at least the specified capacity.
    ///
    /// The hash set will be able to hold at least `capacity` elements without
    /// reallocating. This method is allowed to allocate for more elements than
    /// `capacity`. If `capacity` is 0, the hash set will not allocate.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let set: HashSet<i32> = HashSet::with_capacity(10);
    /// assert!(set.capacity() >= 10);
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn with_capacity(capacity: usize) -> HashSet<T, RandomState> {
        HashSet::with_capacity_and_hasher(capacity, Default::default())
    }
}

impl<T, S> HashSet<T, S> {
    /// Returns the number of elements the set can hold without reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let set: HashSet<i32> = HashSet::with_capacity(100);
    /// assert!(set.capacity() >= 100);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize {
        self.base.capacity()
    }

    /// An iterator visiting all elements in arbitrary order.
    /// The iterator element type is `&'a T`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a");
    /// set.insert("b");
    ///
    /// // Will print in an arbitrary order.
    /// for x in set.iter() {
    ///     println!("{x}");
    /// }
    /// ```
    ///
    /// # Performance
    ///
    /// In the current implementation, iterating over set takes O(capacity) time
    /// instead of O(len) because it internally visits empty buckets too.
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { base: self.base.iter() }
    }

    /// Returns the number of elements in the set.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert_eq!(v.len(), 0);
    /// v.insert(1);
    /// assert_eq!(v.len(), 1);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize {
        self.base.len()
    }

    /// Returns `true` if the set contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert!(v.is_empty());
    /// v.insert(1);
    /// assert!(!v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.base.is_empty()
    }

    /// Clears the set, returning all elements as an iterator. Keeps the
    /// allocated memory for reuse.
    ///
    /// If the returned iterator is dropped before being fully consumed, it
    /// drops the remaining elements. The returned iterator keeps a mutable
    /// borrow on the set to optimize its implementation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::from([1, 2, 3]);
    /// assert!(!set.is_empty());
    ///
    /// // print 1, 2, 3 in an arbitrary order
    /// for i in set.drain() {
    ///     println!("{i}");
    /// }
    ///
    /// assert!(set.is_empty());
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "drain", since = "1.6.0")]
    pub fn drain(&mut self) -> Drain<'_, T> {
        Drain { base: self.base.drain() }
    }

    /// Creates an iterator which uses a closure to determine if a value should be removed.
    ///
    /// If the closure returns true, then the value is removed and yielded.
    /// If the closure returns false, the value will remain in the list and will not be yielded
    /// by the iterator.
    ///
    /// If the returned `ExtractIf` is not exhausted, e.g. because it is dropped without iterating
    /// or the iteration short-circuits, then the remaining elements will be retained.
    /// Use [`retain`] with a negated predicate if you do not need the returned iterator.
    ///
    /// [`retain`]: HashSet::retain
    ///
    /// # Examples
    ///
    /// Splitting a set into even and odd values, reusing the original set:
    ///
    /// ```
    /// #![feature(hash_extract_if)]
    /// use std::collections::HashSet;
    ///
    /// let mut set: HashSet<i32> = (0..8).collect();
    /// let extracted: HashSet<i32> = set.extract_if(|v| v % 2 == 0).collect();
    ///
    /// let mut evens = extracted.into_iter().collect::<Vec<_>>();
    /// let mut odds = set.into_iter().collect::<Vec<_>>();
    /// evens.sort();
    /// odds.sort();
    ///
    /// assert_eq!(evens, vec![0, 2, 4, 6]);
    /// assert_eq!(odds, vec![1, 3, 5, 7]);
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[unstable(feature = "hash_extract_if", issue = "59618")]
    pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        ExtractIf { base: self.base.extract_if(pred) }
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
    /// The elements are visited in unsorted (and unspecified) order.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::from([1, 2, 3, 4, 5, 6]);
    /// set.retain(|&k| k % 2 == 0);
    /// assert_eq!(set, HashSet::from([2, 4, 6]));
    /// ```
    ///
    /// # Performance
    ///
    /// In the current implementation, this operation takes O(capacity) time
    /// instead of O(len) because it internally visits empty buckets too.
    #[rustc_lint_query_instability]
    #[stable(feature = "retain_hash_collection", since = "1.18.0")]
    pub fn retain<F>(&mut self, f: F)
    where
        F: FnMut(&T) -> bool,
    {
        self.base.retain(f)
    }

    /// Clears the set, removing all values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// v.insert(1);
    /// v.clear();
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        self.base.clear()
    }

    /// Creates a new empty hash set which will use the given hasher to hash
    /// keys.
    ///
    /// The hash set is also created with the default initial capacity.
    ///
    /// Warning: `hasher` is normally randomly generated, and
    /// is designed to allow `HashSet`s to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the HashMap to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// use std::hash::RandomState;
    ///
    /// let s = RandomState::new();
    /// let mut set = HashSet::with_hasher(s);
    /// set.insert(2);
    /// ```
    #[inline]
    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
    #[rustc_const_unstable(feature = "const_collections_with_hasher", issue = "102575")]
    pub const fn with_hasher(hasher: S) -> HashSet<T, S> {
        HashSet { base: base::HashSet::with_hasher(hasher) }
    }

    /// Creates an empty `HashSet` with at least the specified capacity, using
    /// `hasher` to hash the keys.
    ///
    /// The hash set will be able to hold at least `capacity` elements without
    /// reallocating. This method is allowed to allocate for more elements than
    /// `capacity`. If `capacity` is 0, the hash set will not allocate.
    ///
    /// Warning: `hasher` is normally randomly generated, and
    /// is designed to allow `HashSet`s to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the HashMap to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// use std::hash::RandomState;
    ///
    /// let s = RandomState::new();
    /// let mut set = HashSet::with_capacity_and_hasher(10, s);
    /// set.insert(1);
    /// ```
    #[inline]
    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
    pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S> {
        HashSet { base: base::HashSet::with_capacity_and_hasher(capacity, hasher) }
    }

    /// Returns a reference to the set's [`BuildHasher`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// use std::hash::RandomState;
    ///
    /// let hasher = RandomState::new();
    /// let set: HashSet<i32> = HashSet::with_hasher(hasher);
    /// let hasher: &RandomState = set.hasher();
    /// ```
    #[inline]
    #[stable(feature = "hashmap_public_hasher", since = "1.9.0")]
    pub fn hasher(&self) -> &S {
        self.base.hasher()
    }
}

impl<T, S> HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    /// Reserves capacity for at least `additional` more elements to be inserted
    /// in the `HashSet`. The collection may reserve more space to speculatively
    /// avoid frequent reallocations. After calling `reserve`,
    /// capacity will be greater than or equal to `self.len() + additional`.
    /// Does nothing if capacity is already sufficient.
    ///
    /// # Panics
    ///
    /// Panics if the new allocation size overflows `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set: HashSet<i32> = HashSet::new();
    /// set.reserve(10);
    /// assert!(set.capacity() >= 10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        self.base.reserve(additional)
    }

    /// Tries to reserve capacity for at least `additional` more elements to be inserted
    /// in the `HashSet`. The collection may reserve more space to speculatively
    /// avoid frequent reallocations. After calling `try_reserve`,
    /// capacity will be greater than or equal to `self.len() + additional` if
    /// it returns `Ok(())`.
    /// Does nothing if capacity is already sufficient.
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set: HashSet<i32> = HashSet::new();
    /// set.try_reserve(10).expect("why is the test harness OOMing on a handful of bytes?");
    /// ```
    #[inline]
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.base.try_reserve(additional).map_err(map_try_reserve_error)
    }

    /// Shrinks the capacity of the set as much as possible. It will drop
    /// down as much as possible while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::with_capacity(100);
    /// set.insert(1);
    /// set.insert(2);
    /// assert!(set.capacity() >= 100);
    /// set.shrink_to_fit();
    /// assert!(set.capacity() >= 2);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn shrink_to_fit(&mut self) {
        self.base.shrink_to_fit()
    }

    /// Shrinks the capacity of the set with a lower limit. It will drop
    /// down no lower than the supplied limit while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// If the current capacity is less than the lower limit, this is a no-op.
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::with_capacity(100);
    /// set.insert(1);
    /// set.insert(2);
    /// assert!(set.capacity() >= 100);
    /// set.shrink_to(10);
    /// assert!(set.capacity() >= 10);
    /// set.shrink_to(0);
    /// assert!(set.capacity() >= 2);
    /// ```
    #[inline]
    #[stable(feature = "shrink_to", since = "1.56.0")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.base.shrink_to(min_capacity)
    }

    /// Visits the values representing the difference,
    /// i.e., the values that are in `self` but not in `other`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([4, 2, 3, 4]);
    ///
    /// // Can be seen as `a - b`.
    /// for x in a.difference(&b) {
    ///     println!("{x}"); // Print 1
    /// }
    ///
    /// let diff: HashSet<_> = a.difference(&b).collect();
    /// assert_eq!(diff, [1].iter().collect());
    ///
    /// // Note that difference is not symmetric,
    /// // and `b - a` means something else:
    /// let diff: HashSet<_> = b.difference(&a).collect();
    /// assert_eq!(diff, [4].iter().collect());
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn difference<'a>(&'a self, other: &'a HashSet<T, S>) -> Difference<'a, T, S> {
        Difference { iter: self.iter(), other }
    }

    /// Visits the values representing the symmetric difference,
    /// i.e., the values that are in `self` or in `other` but not in both.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([4, 2, 3, 4]);
    ///
    /// // Print 1, 4 in arbitrary order.
    /// for x in a.symmetric_difference(&b) {
    ///     println!("{x}");
    /// }
    ///
    /// let diff1: HashSet<_> = a.symmetric_difference(&b).collect();
    /// let diff2: HashSet<_> = b.symmetric_difference(&a).collect();
    ///
    /// assert_eq!(diff1, diff2);
    /// assert_eq!(diff1, [1, 4].iter().collect());
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn symmetric_difference<'a>(
        &'a self,
        other: &'a HashSet<T, S>,
    ) -> SymmetricDifference<'a, T, S> {
        SymmetricDifference { iter: self.difference(other).chain(other.difference(self)) }
    }

    /// Visits the values representing the intersection,
    /// i.e., the values that are both in `self` and `other`.
    ///
    /// When an equal element is present in `self` and `other`
    /// then the resulting `Intersection` may yield references to
    /// one or the other. This can be relevant if `T` contains fields which
    /// are not compared by its `Eq` implementation, and may hold different
    /// value between the two equal copies of `T` in the two sets.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([4, 2, 3, 4]);
    ///
    /// // Print 2, 3 in arbitrary order.
    /// for x in a.intersection(&b) {
    ///     println!("{x}");
    /// }
    ///
    /// let intersection: HashSet<_> = a.intersection(&b).collect();
    /// assert_eq!(intersection, [2, 3].iter().collect());
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S> {
        if self.len() <= other.len() {
            Intersection { iter: self.iter(), other }
        } else {
            Intersection { iter: other.iter(), other: self }
        }
    }

    /// Visits the values representing the union,
    /// i.e., all the values in `self` or `other`, without duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([4, 2, 3, 4]);
    ///
    /// // Print 1, 2, 3, 4 in arbitrary order.
    /// for x in a.union(&b) {
    ///     println!("{x}");
    /// }
    ///
    /// let union: HashSet<_> = a.union(&b).collect();
    /// assert_eq!(union, [1, 2, 3, 4].iter().collect());
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S> {
        if self.len() >= other.len() {
            Union { iter: self.iter().chain(other.difference(self)) }
        } else {
            Union { iter: other.iter().chain(self.difference(other)) }
        }
    }

    /// Returns `true` if the set contains a value.
    ///
    /// The value may be any borrowed form of the set's value type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the value type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let set = HashSet::from([1, 2, 3]);
    /// assert_eq!(set.contains(&1), true);
    /// assert_eq!(set.contains(&4), false);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.contains(value)
    }

    /// Returns a reference to the value in the set, if any, that is equal to the given value.
    ///
    /// The value may be any borrowed form of the set's value type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the value type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let set = HashSet::from([1, 2, 3]);
    /// assert_eq!(set.get(&2), Some(&2));
    /// assert_eq!(set.get(&4), None);
    /// ```
    #[inline]
    #[stable(feature = "set_recovery", since = "1.9.0")]
    pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T>
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.get(value)
    }

    /// Inserts the given `value` into the set if it is not present, then
    /// returns a reference to the value in the set.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::from([1, 2, 3]);
    /// assert_eq!(set.len(), 3);
    /// assert_eq!(set.get_or_insert(2), &2);
    /// assert_eq!(set.get_or_insert(100), &100);
    /// assert_eq!(set.len(), 4); // 100 was inserted
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    pub fn get_or_insert(&mut self, value: T) -> &T {
        // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
        // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
        self.base.get_or_insert(value)
    }

    /// Inserts an owned copy of the given `value` into the set if it is not
    /// present, then returns a reference to the value in the set.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::HashSet;
    ///
    /// let mut set: HashSet<String> = ["cat", "dog", "horse"]
    ///     .iter().map(|&pet| pet.to_owned()).collect();
    ///
    /// assert_eq!(set.len(), 3);
    /// for &pet in &["cat", "dog", "fish"] {
    ///     let value = set.get_or_insert_owned(pet);
    ///     assert_eq!(value, pet);
    /// }
    /// assert_eq!(set.len(), 4); // a new "fish" was inserted
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    pub fn get_or_insert_owned<Q: ?Sized>(&mut self, value: &Q) -> &T
    where
        T: Borrow<Q>,
        Q: Hash + Eq + ToOwned<Owned = T>,
    {
        // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
        // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
        self.base.get_or_insert_owned(value)
    }

    /// Inserts a value computed from `f` into the set if the given `value` is
    /// not present, then returns a reference to the value in the set.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::HashSet;
    ///
    /// let mut set: HashSet<String> = ["cat", "dog", "horse"]
    ///     .iter().map(|&pet| pet.to_owned()).collect();
    ///
    /// assert_eq!(set.len(), 3);
    /// for &pet in &["cat", "dog", "fish"] {
    ///     let value = set.get_or_insert_with(pet, str::to_owned);
    ///     assert_eq!(value, pet);
    /// }
    /// assert_eq!(set.len(), 4); // a new "fish" was inserted
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    pub fn get_or_insert_with<Q: ?Sized, F>(&mut self, value: &Q, f: F) -> &T
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
        F: FnOnce(&Q) -> T,
    {
        // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
        // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
        self.base.get_or_insert_with(value, f)
    }

    /// Returns `true` if `self` has no elements in common with `other`.
    /// This is equivalent to checking for an empty intersection.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let a = HashSet::from([1, 2, 3]);
    /// let mut b = HashSet::new();
    ///
    /// assert_eq!(a.is_disjoint(&b), true);
    /// b.insert(4);
    /// assert_eq!(a.is_disjoint(&b), true);
    /// b.insert(1);
    /// assert_eq!(a.is_disjoint(&b), false);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_disjoint(&self, other: &HashSet<T, S>) -> bool {
        if self.len() <= other.len() {
            self.iter().all(|v| !other.contains(v))
        } else {
            other.iter().all(|v| !self.contains(v))
        }
    }

    /// Returns `true` if the set is a subset of another,
    /// i.e., `other` contains at least all the values in `self`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let sup = HashSet::from([1, 2, 3]);
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.is_subset(&sup), true);
    /// set.insert(2);
    /// assert_eq!(set.is_subset(&sup), true);
    /// set.insert(4);
    /// assert_eq!(set.is_subset(&sup), false);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_subset(&self, other: &HashSet<T, S>) -> bool {
        if self.len() <= other.len() { self.iter().all(|v| other.contains(v)) } else { false }
    }

    /// Returns `true` if the set is a superset of another,
    /// i.e., `self` contains at least all the values in `other`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let sub = HashSet::from([1, 2]);
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.is_superset(&sub), false);
    ///
    /// set.insert(0);
    /// set.insert(1);
    /// assert_eq!(set.is_superset(&sub), false);
    ///
    /// set.insert(2);
    /// assert_eq!(set.is_superset(&sub), true);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_superset(&self, other: &HashSet<T, S>) -> bool {
        other.is_subset(self)
    }

    /// Adds a value to the set.
    ///
    /// Returns whether the value was newly inserted. That is:
    ///
    /// - If the set did not previously contain this value, `true` is returned.
    /// - If the set already contained this value, `false` is returned,
    ///   and the set is not modified: original value is not replaced,
    ///   and the value passed as argument is dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::new();
    ///
    /// assert_eq!(set.insert(2), true);
    /// assert_eq!(set.insert(2), false);
    /// assert_eq!(set.len(), 1);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn insert(&mut self, value: T) -> bool {
        self.base.insert(value)
    }

    /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
    /// one. Returns the replaced value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::new();
    /// set.insert(Vec::<i32>::new());
    ///
    /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 0);
    /// set.replace(Vec::with_capacity(10));
    /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 10);
    /// ```
    #[inline]
    #[stable(feature = "set_recovery", since = "1.9.0")]
    pub fn replace(&mut self, value: T) -> Option<T> {
        self.base.replace(value)
    }

    /// Removes a value from the set. Returns whether the value was
    /// present in the set.
    ///
    /// The value may be any borrowed form of the set's value type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the value type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::new();
    ///
    /// set.insert(2);
    /// assert_eq!(set.remove(&2), true);
    /// assert_eq!(set.remove(&2), false);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.remove(value)
    }

    /// Removes and returns the value in the set, if any, that is equal to the given one.
    ///
    /// The value may be any borrowed form of the set's value type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the value type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set = HashSet::from([1, 2, 3]);
    /// assert_eq!(set.take(&2), Some(2));
    /// assert_eq!(set.take(&2), None);
    /// ```
    #[inline]
    #[stable(feature = "set_recovery", since = "1.9.0")]
    pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T>
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.take(value)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for HashSet<T, S>
where
    T: Clone,
    S: Clone,
{
    #[inline]
    fn clone(&self) -> Self {
        Self { base: self.base.clone() }
    }

    #[inline]
    fn clone_from(&mut self, other: &Self) {
        self.base.clone_from(&other.base);
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> PartialEq for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    fn eq(&self, other: &HashSet<T, S>) -> bool {
        if self.len() != other.len() {
            return false;
        }

        self.iter().all(|key| other.contains(key))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Eq for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> fmt::Debug for HashSet<T, S>
where
    T: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_set().entries(self.iter()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> FromIterator<T> for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher + Default,
{
    #[inline]
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> HashSet<T, S> {
        let mut set = HashSet::with_hasher(Default::default());
        set.extend(iter);
        set
    }
}

#[stable(feature = "std_collections_from_array", since = "1.56.0")]
// Note: as what is currently the most convenient built-in way to construct
// a HashSet, a simple usage of this function must not *require* the user
// to provide a type annotation in order to infer the third type parameter
// (the hasher parameter, conventionally "S").
// To that end, this impl is defined using RandomState as the concrete
// type of S, rather than being generic over `S: BuildHasher + Default`.
// It is expected that users who want to specify a hasher will manually use
// `with_capacity_and_hasher`.
// If type parameter defaults worked on impls, and if type parameter
// defaults could be mixed with const generics, then perhaps
// this could be generalized.
// See also the equivalent impl on HashMap.
impl<T, const N: usize> From<[T; N]> for HashSet<T, RandomState>
where
    T: Eq + Hash,
{
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let set1 = HashSet::from([1, 2, 3, 4]);
    /// let set2: HashSet<_> = [1, 2, 3, 4].into();
    /// assert_eq!(set1, set2);
    /// ```
    fn from(arr: [T; N]) -> Self {
        Self::from_iter(arr)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Extend<T> for HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    #[inline]
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        self.base.extend(iter);
    }

    #[inline]
    fn extend_one(&mut self, item: T) {
        self.base.insert(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.base.extend_reserve(additional);
    }
}

#[stable(feature = "hash_extend_copy", since = "1.4.0")]
impl<'a, T, S> Extend<&'a T> for HashSet<T, S>
where
    T: 'a + Eq + Hash + Copy,
    S: BuildHasher,
{
    #[inline]
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }

    #[inline]
    fn extend_one(&mut self, &item: &'a T) {
        self.base.insert(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        Extend::<T>::extend_reserve(self, additional)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Default for HashSet<T, S>
where
    S: Default,
{
    /// Creates an empty `HashSet<T, S>` with the `Default` value for the hasher.
    #[inline]
    fn default() -> HashSet<T, S> {
        HashSet { base: Default::default() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> BitOr<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the union of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([3, 4, 5]);
    ///
    /// let set = &a | &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2, 3, 4, 5];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.union(rhs).cloned().collect()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> BitAnd<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the intersection of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([2, 3, 4]);
    ///
    /// let set = &a & &b;
    ///
    /// let mut i = 0;
    /// let expected = [2, 3];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitand(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.intersection(rhs).cloned().collect()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> BitXor<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the symmetric difference of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([3, 4, 5]);
    ///
    /// let set = &a ^ &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2, 4, 5];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn bitxor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.symmetric_difference(rhs).cloned().collect()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Sub<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// Returns the difference of `self` and `rhs` as a new `HashSet<T, S>`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let a = HashSet::from([1, 2, 3]);
    /// let b = HashSet::from([3, 4, 5]);
    ///
    /// let set = &a - &b;
    ///
    /// let mut i = 0;
    /// let expected = [1, 2];
    /// for x in &set {
    ///     assert!(expected.contains(x));
    ///     i += 1;
    /// }
    /// assert_eq!(i, expected.len());
    /// ```
    fn sub(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
        self.difference(rhs).cloned().collect()
    }
}

/// An iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`iter`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`iter`]: HashSet::iter
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
///
/// let mut iter = a.iter();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, K: 'a> {
    base: base::Iter<'a, K>,
}

/// An owning iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`into_iter`] method on [`HashSet`]
/// (provided by the [`IntoIterator`] trait). See its documentation for more.
///
/// [`into_iter`]: IntoIterator::into_iter
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
///
/// let mut iter = a.into_iter();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<K> {
    base: base::IntoIter<K>,
}

/// A draining iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`drain`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`drain`]: HashSet::drain
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let mut a = HashSet::from([1, 2, 3]);
///
/// let mut drain = a.drain();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Drain<'a, K: 'a> {
    base: base::Drain<'a, K>,
}

/// A draining, filtering iterator over the items of a `HashSet`.
///
/// This `struct` is created by the [`extract_if`] method on [`HashSet`].
///
/// [`extract_if`]: HashSet::extract_if
///
/// # Examples
///
/// ```
/// #![feature(hash_extract_if)]
///
/// use std::collections::HashSet;
///
/// let mut a = HashSet::from([1, 2, 3]);
///
/// let mut extract_ifed = a.extract_if(|v| v % 2 == 0);
/// ```
#[unstable(feature = "hash_extract_if", issue = "59618")]
pub struct ExtractIf<'a, K, F>
where
    F: FnMut(&K) -> bool,
{
    base: base::ExtractIf<'a, K, F>,
}

/// A lazy iterator producing elements in the intersection of `HashSet`s.
///
/// This `struct` is created by the [`intersection`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`intersection`]: HashSet::intersection
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
/// let b = HashSet::from([4, 2, 3, 4]);
///
/// let mut intersection = a.intersection(&b);
/// ```
#[must_use = "this returns the intersection as an iterator, \
              without modifying either input set"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Intersection<'a, T: 'a, S: 'a> {
    // iterator of the first set
    iter: Iter<'a, T>,
    // the second set
    other: &'a HashSet<T, S>,
}

/// A lazy iterator producing elements in the difference of `HashSet`s.
///
/// This `struct` is created by the [`difference`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`difference`]: HashSet::difference
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
/// let b = HashSet::from([4, 2, 3, 4]);
///
/// let mut difference = a.difference(&b);
/// ```
#[must_use = "this returns the difference as an iterator, \
              without modifying either input set"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Difference<'a, T: 'a, S: 'a> {
    // iterator of the first set
    iter: Iter<'a, T>,
    // the second set
    other: &'a HashSet<T, S>,
}

/// A lazy iterator producing elements in the symmetric difference of `HashSet`s.
///
/// This `struct` is created by the [`symmetric_difference`] method on
/// [`HashSet`]. See its documentation for more.
///
/// [`symmetric_difference`]: HashSet::symmetric_difference
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
/// let b = HashSet::from([4, 2, 3, 4]);
///
/// let mut intersection = a.symmetric_difference(&b);
/// ```
#[must_use = "this returns the difference as an iterator, \
              without modifying either input set"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SymmetricDifference<'a, T: 'a, S: 'a> {
    iter: Chain<Difference<'a, T, S>, Difference<'a, T, S>>,
}

/// A lazy iterator producing elements in the union of `HashSet`s.
///
/// This `struct` is created by the [`union`] method on [`HashSet`].
/// See its documentation for more.
///
/// [`union`]: HashSet::union
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a = HashSet::from([1, 2, 3]);
/// let b = HashSet::from([4, 2, 3, 4]);
///
/// let mut union_iter = a.union(&b);
/// ```
#[must_use = "this returns the union as an iterator, \
              without modifying either input set"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Union<'a, T: 'a, S: 'a> {
    iter: Chain<Iter<'a, T>, Difference<'a, T, S>>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, S> IntoIterator for &'a HashSet<T, S> {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    #[inline]
    #[rustc_lint_query_instability]
    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

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

    /// Creates a consuming iterator, that is, one that moves each value out
    /// of the set in arbitrary order. The set cannot be used after calling
    /// this.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a".to_string());
    /// set.insert("b".to_string());
    ///
    /// // Not possible to collect to a Vec<String> with a regular `.iter()`.
    /// let v: Vec<String> = set.into_iter().collect();
    ///
    /// // Will print in an arbitrary order.
    /// for x in &v {
    ///     println!("{x}");
    /// }
    /// ```
    #[inline]
    #[rustc_lint_query_instability]
    fn into_iter(self) -> IntoIter<T> {
        IntoIter { base: self.base.into_iter() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K> Clone for Iter<'_, K> {
    #[inline]
    fn clone(&self) -> Self {
        Iter { base: self.base.clone() }
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K> Iterator for Iter<'a, K> {
    type Item = &'a K;

    #[inline]
    fn next(&mut self) -> Option<&'a K> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
    #[inline]
    fn count(self) -> usize {
        self.base.len()
    }
    #[inline]
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.base.fold(init, f)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K> ExactSizeIterator for Iter<'_, K> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K> FusedIterator for Iter<'_, K> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K: fmt::Debug> fmt::Debug for Iter<'_, K> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<K> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
    #[inline]
    fn count(self) -> usize {
        self.base.len()
    }
    #[inline]
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.base.fold(init, f)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K> ExactSizeIterator for IntoIter<K> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K> FusedIterator for IntoIter<K> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K: fmt::Debug> fmt::Debug for IntoIter<K> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.base, f)
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<K> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
    #[inline]
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.base.fold(init, f)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K> ExactSizeIterator for Drain<'_, K> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K> FusedIterator for Drain<'_, K> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K: fmt::Debug> fmt::Debug for Drain<'_, K> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.base, f)
    }
}

#[unstable(feature = "hash_extract_if", issue = "59618")]
impl<K, F> Iterator for ExtractIf<'_, K, F>
where
    F: FnMut(&K) -> bool,
{
    type Item = K;

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

#[unstable(feature = "hash_extract_if", issue = "59618")]
impl<K, F> FusedIterator for ExtractIf<'_, K, F> where F: FnMut(&K) -> bool {}

#[unstable(feature = "hash_extract_if", issue = "59618")]
impl<'a, K, F> fmt::Debug for ExtractIf<'a, K, F>
where
    F: FnMut(&K) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ExtractIf").finish_non_exhaustive()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for Intersection<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        Intersection { iter: self.iter.clone(), ..*self }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, S> Iterator for Intersection<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    type Item = &'a T;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        loop {
            let elt = self.iter.next()?;
            if self.other.contains(elt) {
                return Some(elt);
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper)
    }

    #[inline]
    fn fold<B, F>(self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.iter.fold(init, |acc, elt| if self.other.contains(elt) { f(acc, elt) } else { acc })
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T, S> fmt::Debug for Intersection<'_, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Intersection<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for Difference<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        Difference { iter: self.iter.clone(), ..*self }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, S> Iterator for Difference<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    type Item = &'a T;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        loop {
            let elt = self.iter.next()?;
            if !self.other.contains(elt) {
                return Some(elt);
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper)
    }

    #[inline]
    fn fold<B, F>(self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.iter.fold(init, |acc, elt| if self.other.contains(elt) { acc } else { f(acc, elt) })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Difference<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T, S> fmt::Debug for Difference<'_, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for SymmetricDifference<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        SymmetricDifference { iter: self.iter.clone() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, S> Iterator for SymmetricDifference<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    type Item = &'a T;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        self.iter.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.iter.fold(init, f)
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for SymmetricDifference<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T, S> fmt::Debug for SymmetricDifference<'_, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for Union<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        Union { iter: self.iter.clone() }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Union<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<T, S> fmt::Debug for Union<'_, T, S>
where
    T: fmt::Debug + Eq + Hash,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, S> Iterator for Union<'a, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    type Item = &'a T;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        self.iter.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }
    #[inline]
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    {
        self.iter.fold(init, f)
    }
}

#[allow(dead_code)]
fn assert_covariance() {
    fn set<'new>(v: HashSet<&'static str>) -> HashSet<&'new str> {
        v
    }
    fn iter<'a, 'new>(v: Iter<'a, &'static str>) -> Iter<'a, &'new str> {
        v
    }
    fn into_iter<'new>(v: IntoIter<&'static str>) -> IntoIter<&'new str> {
        v
    }
    fn difference<'a, 'new>(
        v: Difference<'a, &'static str, RandomState>,
    ) -> Difference<'a, &'new str, RandomState> {
        v
    }
    fn symmetric_difference<'a, 'new>(
        v: SymmetricDifference<'a, &'static str, RandomState>,
    ) -> SymmetricDifference<'a, &'new str, RandomState> {
        v
    }
    fn intersection<'a, 'new>(
        v: Intersection<'a, &'static str, RandomState>,
    ) -> Intersection<'a, &'new str, RandomState> {
        v
    }
    fn union<'a, 'new>(
        v: Union<'a, &'static str, RandomState>,
    ) -> Union<'a, &'new str, RandomState> {
        v
    }
    fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> {
        d
    }
}