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
use super::suggest;
use super::CandidateSource;
use super::MethodError;
use super::NoMatchData;

use crate::check::FnCtxt;
use crate::errors::MethodCallOnUnknownType;
use crate::hir::def::DefKind;
use crate::hir::def_id::DefId;

use rustc_data_structures::fx::FxHashSet;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::def::Namespace;
use rustc_infer::infer::canonical::OriginalQueryValues;
use rustc_infer::infer::canonical::{Canonical, QueryResponse};
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
use rustc_middle::middle::stability;
use rustc_middle::ty::fast_reject::{simplify_type, TreatParams};
use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
use rustc_middle::ty::GenericParamDefKind;
use rustc_middle::ty::{self, ParamEnvAnd, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeVisitable};
use rustc_session::lint;
use rustc_span::def_id::LocalDefId;
use rustc_span::lev_distance::{
    find_best_match_for_name_with_substrings, lev_distance_with_substrings,
};
use rustc_span::symbol::sym;
use rustc_span::{symbol::Ident, Span, Symbol, DUMMY_SP};
use rustc_trait_selection::autoderef::{self, Autoderef};
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::query::method_autoderef::MethodAutoderefBadTy;
use rustc_trait_selection::traits::query::method_autoderef::{
    CandidateStep, MethodAutoderefStepsResult,
};
use rustc_trait_selection::traits::query::CanonicalTyGoal;
use rustc_trait_selection::traits::{self, ObligationCause};
use std::cmp::max;
use std::iter;
use std::mem;
use std::ops::Deref;

use smallvec::{smallvec, SmallVec};

use self::CandidateKind::*;
pub use self::PickKind::*;

/// Boolean flag used to indicate if this search is for a suggestion
/// or not. If true, we can allow ambiguity and so forth.
#[derive(Clone, Copy, Debug)]
pub struct IsSuggestion(pub bool);

struct ProbeContext<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    span: Span,
    mode: Mode,
    method_name: Option<Ident>,
    return_type: Option<Ty<'tcx>>,

    /// This is the OriginalQueryValues for the steps queries
    /// that are answered in steps.
    orig_steps_var_values: OriginalQueryValues<'tcx>,
    steps: &'tcx [CandidateStep<'tcx>],

    inherent_candidates: Vec<Candidate<'tcx>>,
    extension_candidates: Vec<Candidate<'tcx>>,
    impl_dups: FxHashSet<DefId>,

    /// Collects near misses when the candidate functions are missing a `self` keyword and is only
    /// used for error reporting
    static_candidates: Vec<CandidateSource>,

    /// When probing for names, include names that are close to the
    /// requested name (by Levensthein distance)
    allow_similar_names: bool,

    /// Some(candidate) if there is a private candidate
    private_candidate: Option<(DefKind, DefId)>,

    /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
    /// for error reporting
    unsatisfied_predicates:
        Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,

    is_suggestion: IsSuggestion,

    scope_expr_id: hir::HirId,
}

impl<'a, 'tcx> Deref for ProbeContext<'a, 'tcx> {
    type Target = FnCtxt<'a, 'tcx>;
    fn deref(&self) -> &Self::Target {
        self.fcx
    }
}

#[derive(Debug, Clone)]
struct Candidate<'tcx> {
    // Candidates are (I'm not quite sure, but they are mostly) basically
    // some metadata on top of a `ty::AssocItem` (without substs).
    //
    // However, method probing wants to be able to evaluate the predicates
    // for a function with the substs applied - for example, if a function
    // has `where Self: Sized`, we don't want to consider it unless `Self`
    // is actually `Sized`, and similarly, return-type suggestions want
    // to consider the "actual" return type.
    //
    // The way this is handled is through `xform_self_ty`. It contains
    // the receiver type of this candidate, but `xform_self_ty`,
    // `xform_ret_ty` and `kind` (which contains the predicates) have the
    // generic parameters of this candidate substituted with the *same set*
    // of inference variables, which acts as some weird sort of "query".
    //
    // When we check out a candidate, we require `xform_self_ty` to be
    // a subtype of the passed-in self-type, and this equates the type
    // variables in the rest of the fields.
    //
    // For example, if we have this candidate:
    // ```
    //    trait Foo {
    //        fn foo(&self) where Self: Sized;
    //    }
    // ```
    //
    // Then `xform_self_ty` will be `&'erased ?X` and `kind` will contain
    // the predicate `?X: Sized`, so if we are evaluating `Foo` for a
    // the receiver `&T`, we'll do the subtyping which will make `?X`
    // get the right value, then when we evaluate the predicate we'll check
    // if `T: Sized`.
    xform_self_ty: Ty<'tcx>,
    xform_ret_ty: Option<Ty<'tcx>>,
    item: ty::AssocItem,
    kind: CandidateKind<'tcx>,
    import_ids: SmallVec<[LocalDefId; 1]>,
}

#[derive(Debug, Clone)]
enum CandidateKind<'tcx> {
    InherentImplCandidate(
        SubstsRef<'tcx>,
        // Normalize obligations
        Vec<traits::PredicateObligation<'tcx>>,
    ),
    ObjectCandidate,
    TraitCandidate(ty::TraitRef<'tcx>),
    WhereClauseCandidate(
        // Trait
        ty::PolyTraitRef<'tcx>,
    ),
}

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum ProbeResult {
    NoMatch,
    BadReturnType,
    Match,
}

/// When adjusting a receiver we often want to do one of
///
/// - Add a `&` (or `&mut`), converting the receiver from `T` to `&T` (or `&mut T`)
/// - If the receiver has type `*mut T`, convert it to `*const T`
///
/// This type tells us which one to do.
///
/// Note that in principle we could do both at the same time. For example, when the receiver has
/// type `T`, we could autoref it to `&T`, then convert to `*const T`. Or, when it has type `*mut
/// T`, we could convert it to `*const T`, then autoref to `&*const T`. However, currently we do
/// (at most) one of these. Either the receiver has type `T` and we convert it to `&T` (or with
/// `mut`), or it has type `*mut T` and we convert it to `*const T`.
#[derive(Debug, PartialEq, Copy, Clone)]
pub enum AutorefOrPtrAdjustment {
    /// Receiver has type `T`, add `&` or `&mut` (it `T` is `mut`), and maybe also "unsize" it.
    /// Unsizing is used to convert a `[T; N]` to `[T]`, which only makes sense when autorefing.
    Autoref {
        mutbl: hir::Mutability,

        /// Indicates that the source expression should be "unsized" to a target type.
        /// This is special-cased for just arrays unsizing to slices.
        unsize: bool,
    },
    /// Receiver has type `*mut T`, convert to `*const T`
    ToConstPtr,
}

impl AutorefOrPtrAdjustment {
    fn get_unsize(&self) -> bool {
        match self {
            AutorefOrPtrAdjustment::Autoref { mutbl: _, unsize } => *unsize,
            AutorefOrPtrAdjustment::ToConstPtr => false,
        }
    }
}

#[derive(Debug, PartialEq, Clone)]
pub struct Pick<'tcx> {
    pub item: ty::AssocItem,
    pub kind: PickKind<'tcx>,
    pub import_ids: SmallVec<[LocalDefId; 1]>,

    /// Indicates that the source expression should be autoderef'd N times
    /// ```ignore (not-rust)
    /// A = expr | *expr | **expr | ...
    /// ```
    pub autoderefs: usize,

    /// Indicates that we want to add an autoref (and maybe also unsize it), or if the receiver is
    /// `*mut T`, convert it to `*const T`.
    pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment>,
    pub self_ty: Ty<'tcx>,
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub enum PickKind<'tcx> {
    InherentImplPick,
    ObjectPick,
    TraitPick,
    WhereClausePick(
        // Trait
        ty::PolyTraitRef<'tcx>,
    ),
}

pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;

#[derive(PartialEq, Eq, Copy, Clone, Debug)]
pub enum Mode {
    // An expression of the form `receiver.method_name(...)`.
    // Autoderefs are performed on `receiver`, lookup is done based on the
    // `self` argument  of the method, and static methods aren't considered.
    MethodCall,
    // An expression of the form `Type::item` or `<T>::item`.
    // No autoderefs are performed, lookup is done based on the type each
    // implementation is for, and static methods are included.
    Path,
}

#[derive(PartialEq, Eq, Copy, Clone, Debug)]
pub enum ProbeScope {
    // Assemble candidates coming only from traits in scope.
    TraitsInScope,

    // Assemble candidates coming from all traits.
    AllTraits,
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    /// This is used to offer suggestions to users. It returns methods
    /// that could have been called which have the desired return
    /// type. Some effort is made to rule out methods that, if called,
    /// would result in an error (basically, the same criteria we
    /// would use to decide if a method is a plausible fit for
    /// ambiguity purposes).
    #[instrument(level = "debug", skip(self))]
    pub fn probe_for_return_type(
        &self,
        span: Span,
        mode: Mode,
        return_type: Ty<'tcx>,
        self_ty: Ty<'tcx>,
        scope_expr_id: hir::HirId,
    ) -> Vec<ty::AssocItem> {
        let method_names = self
            .probe_op(
                span,
                mode,
                None,
                Some(return_type),
                IsSuggestion(true),
                self_ty,
                scope_expr_id,
                ProbeScope::AllTraits,
                |probe_cx| Ok(probe_cx.candidate_method_names()),
            )
            .unwrap_or_default();
        method_names
            .iter()
            .flat_map(|&method_name| {
                self.probe_op(
                    span,
                    mode,
                    Some(method_name),
                    Some(return_type),
                    IsSuggestion(true),
                    self_ty,
                    scope_expr_id,
                    ProbeScope::AllTraits,
                    |probe_cx| probe_cx.pick(),
                )
                .ok()
                .map(|pick| pick.item)
            })
            .collect()
    }

    #[instrument(level = "debug", skip(self))]
    pub fn probe_for_name(
        &self,
        span: Span,
        mode: Mode,
        item_name: Ident,
        is_suggestion: IsSuggestion,
        self_ty: Ty<'tcx>,
        scope_expr_id: hir::HirId,
        scope: ProbeScope,
    ) -> PickResult<'tcx> {
        self.probe_op(
            span,
            mode,
            Some(item_name),
            None,
            is_suggestion,
            self_ty,
            scope_expr_id,
            scope,
            |probe_cx| probe_cx.pick(),
        )
    }

    fn probe_op<OP, R>(
        &'a self,
        span: Span,
        mode: Mode,
        method_name: Option<Ident>,
        return_type: Option<Ty<'tcx>>,
        is_suggestion: IsSuggestion,
        self_ty: Ty<'tcx>,
        scope_expr_id: hir::HirId,
        scope: ProbeScope,
        op: OP,
    ) -> Result<R, MethodError<'tcx>>
    where
        OP: FnOnce(ProbeContext<'a, 'tcx>) -> Result<R, MethodError<'tcx>>,
    {
        let mut orig_values = OriginalQueryValues::default();
        let param_env_and_self_ty = self.canonicalize_query(
            ParamEnvAnd { param_env: self.param_env, value: self_ty },
            &mut orig_values,
        );

        let steps = if mode == Mode::MethodCall {
            self.tcx.method_autoderef_steps(param_env_and_self_ty)
        } else {
            self.probe(|_| {
                // Mode::Path - the deref steps is "trivial". This turns
                // our CanonicalQuery into a "trivial" QueryResponse. This
                // is a bit inefficient, but I don't think that writing
                // special handling for this "trivial case" is a good idea.

                let infcx = &self.infcx;
                let (ParamEnvAnd { param_env: _, value: self_ty }, canonical_inference_vars) =
                    infcx.instantiate_canonical_with_fresh_inference_vars(
                        span,
                        &param_env_and_self_ty,
                    );
                debug!(
                    "probe_op: Mode::Path, param_env_and_self_ty={:?} self_ty={:?}",
                    param_env_and_self_ty, self_ty
                );
                MethodAutoderefStepsResult {
                    steps: infcx.tcx.arena.alloc_from_iter([CandidateStep {
                        self_ty: self.make_query_response_ignoring_pending_obligations(
                            canonical_inference_vars,
                            self_ty,
                        ),
                        autoderefs: 0,
                        from_unsafe_deref: false,
                        unsize: false,
                    }]),
                    opt_bad_ty: None,
                    reached_recursion_limit: false,
                }
            })
        };

        // If our autoderef loop had reached the recursion limit,
        // report an overflow error, but continue going on with
        // the truncated autoderef list.
        if steps.reached_recursion_limit {
            self.probe(|_| {
                let ty = &steps
                    .steps
                    .last()
                    .unwrap_or_else(|| span_bug!(span, "reached the recursion limit in 0 steps?"))
                    .self_ty;
                let ty = self
                    .probe_instantiate_query_response(span, &orig_values, ty)
                    .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
                autoderef::report_autoderef_recursion_limit_error(self.tcx, span, ty.value);
            });
        }

        // If we encountered an `_` type or an error type during autoderef, this is
        // ambiguous.
        if let Some(bad_ty) = &steps.opt_bad_ty {
            if is_suggestion.0 {
                // Ambiguity was encountered during a suggestion. Just keep going.
                debug!("ProbeContext: encountered ambiguity in suggestion");
            } else if bad_ty.reached_raw_pointer && !self.tcx.features().arbitrary_self_types {
                // this case used to be allowed by the compiler,
                // so we do a future-compat lint here for the 2015 edition
                // (see https://github.com/rust-lang/rust/issues/46906)
                if self.tcx.sess.rust_2018() {
                    self.tcx.sess.emit_err(MethodCallOnUnknownType { span });
                } else {
                    self.tcx.struct_span_lint_hir(
                        lint::builtin::TYVAR_BEHIND_RAW_POINTER,
                        scope_expr_id,
                        span,
                        |lint| {
                            lint.build("type annotations needed").emit();
                        },
                    );
                }
            } else {
                // Encountered a real ambiguity, so abort the lookup. If `ty` is not
                // an `Err`, report the right "type annotations needed" error pointing
                // to it.
                let ty = &bad_ty.ty;
                let ty = self
                    .probe_instantiate_query_response(span, &orig_values, ty)
                    .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
                let ty = self.structurally_resolved_type(span, ty.value);
                assert!(matches!(ty.kind(), ty::Error(_)));
                return Err(MethodError::NoMatch(NoMatchData {
                    static_candidates: Vec::new(),
                    unsatisfied_predicates: Vec::new(),
                    out_of_scope_traits: Vec::new(),
                    lev_candidate: None,
                    mode,
                }));
            }
        }

        debug!("ProbeContext: steps for self_ty={:?} are {:?}", self_ty, steps);

        // this creates one big transaction so that all type variables etc
        // that we create during the probe process are removed later
        self.probe(|_| {
            let mut probe_cx = ProbeContext::new(
                self,
                span,
                mode,
                method_name,
                return_type,
                orig_values,
                steps.steps,
                is_suggestion,
                scope_expr_id,
            );

            probe_cx.assemble_inherent_candidates();
            match scope {
                ProbeScope::TraitsInScope => {
                    probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)
                }
                ProbeScope::AllTraits => probe_cx.assemble_extension_candidates_for_all_traits(),
            };
            op(probe_cx)
        })
    }
}

pub fn provide(providers: &mut ty::query::Providers) {
    providers.method_autoderef_steps = method_autoderef_steps;
}

fn method_autoderef_steps<'tcx>(
    tcx: TyCtxt<'tcx>,
    goal: CanonicalTyGoal<'tcx>,
) -> MethodAutoderefStepsResult<'tcx> {
    debug!("method_autoderef_steps({:?})", goal);

    tcx.infer_ctxt().enter_with_canonical(DUMMY_SP, &goal, |ref infcx, goal, inference_vars| {
        let ParamEnvAnd { param_env, value: self_ty } = goal;

        let mut autoderef =
            Autoderef::new(infcx, param_env, hir::CRATE_HIR_ID, DUMMY_SP, self_ty, DUMMY_SP)
                .include_raw_pointers()
                .silence_errors();
        let mut reached_raw_pointer = false;
        let mut steps: Vec<_> = autoderef
            .by_ref()
            .map(|(ty, d)| {
                let step = CandidateStep {
                    self_ty: infcx.make_query_response_ignoring_pending_obligations(
                        inference_vars.clone(),
                        ty,
                    ),
                    autoderefs: d,
                    from_unsafe_deref: reached_raw_pointer,
                    unsize: false,
                };
                if let ty::RawPtr(_) = ty.kind() {
                    // all the subsequent steps will be from_unsafe_deref
                    reached_raw_pointer = true;
                }
                step
            })
            .collect();

        let final_ty = autoderef.final_ty(true);
        let opt_bad_ty = match final_ty.kind() {
            ty::Infer(ty::TyVar(_)) | ty::Error(_) => Some(MethodAutoderefBadTy {
                reached_raw_pointer,
                ty: infcx
                    .make_query_response_ignoring_pending_obligations(inference_vars, final_ty),
            }),
            ty::Array(elem_ty, _) => {
                let dereferences = steps.len() - 1;

                steps.push(CandidateStep {
                    self_ty: infcx.make_query_response_ignoring_pending_obligations(
                        inference_vars,
                        infcx.tcx.mk_slice(*elem_ty),
                    ),
                    autoderefs: dereferences,
                    // this could be from an unsafe deref if we had
                    // a *mut/const [T; N]
                    from_unsafe_deref: reached_raw_pointer,
                    unsize: true,
                });

                None
            }
            _ => None,
        };

        debug!("method_autoderef_steps: steps={:?} opt_bad_ty={:?}", steps, opt_bad_ty);

        MethodAutoderefStepsResult {
            steps: tcx.arena.alloc_from_iter(steps),
            opt_bad_ty: opt_bad_ty.map(|ty| &*tcx.arena.alloc(ty)),
            reached_recursion_limit: autoderef.reached_recursion_limit(),
        }
    })
}

impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
    fn new(
        fcx: &'a FnCtxt<'a, 'tcx>,
        span: Span,
        mode: Mode,
        method_name: Option<Ident>,
        return_type: Option<Ty<'tcx>>,
        orig_steps_var_values: OriginalQueryValues<'tcx>,
        steps: &'tcx [CandidateStep<'tcx>],
        is_suggestion: IsSuggestion,
        scope_expr_id: hir::HirId,
    ) -> ProbeContext<'a, 'tcx> {
        ProbeContext {
            fcx,
            span,
            mode,
            method_name,
            return_type,
            inherent_candidates: Vec::new(),
            extension_candidates: Vec::new(),
            impl_dups: FxHashSet::default(),
            orig_steps_var_values,
            steps,
            static_candidates: Vec::new(),
            allow_similar_names: false,
            private_candidate: None,
            unsatisfied_predicates: Vec::new(),
            is_suggestion,
            scope_expr_id,
        }
    }

    fn reset(&mut self) {
        self.inherent_candidates.clear();
        self.extension_candidates.clear();
        self.impl_dups.clear();
        self.static_candidates.clear();
        self.private_candidate = None;
    }

    ///////////////////////////////////////////////////////////////////////////
    // CANDIDATE ASSEMBLY

    fn push_candidate(&mut self, candidate: Candidate<'tcx>, is_inherent: bool) {
        let is_accessible = if let Some(name) = self.method_name {
            let item = candidate.item;
            let def_scope = self
                .tcx
                .adjust_ident_and_get_scope(name, item.container_id(self.tcx), self.body_id)
                .1;
            item.visibility(self.tcx).is_accessible_from(def_scope, self.tcx)
        } else {
            true
        };
        if is_accessible {
            if is_inherent {
                self.inherent_candidates.push(candidate);
            } else {
                self.extension_candidates.push(candidate);
            }
        } else if self.private_candidate.is_none() {
            self.private_candidate =
                Some((candidate.item.kind.as_def_kind(), candidate.item.def_id));
        }
    }

    fn assemble_inherent_candidates(&mut self) {
        for step in self.steps.iter() {
            self.assemble_probe(&step.self_ty);
        }
    }

    fn assemble_probe(&mut self, self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>) {
        debug!("assemble_probe: self_ty={:?}", self_ty);
        let raw_self_ty = self_ty.value.value;
        match *raw_self_ty.kind() {
            ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
                // Subtle: we can't use `instantiate_query_response` here: using it will
                // commit to all of the type equalities assumed by inference going through
                // autoderef (see the `method-probe-no-guessing` test).
                //
                // However, in this code, it is OK if we end up with an object type that is
                // "more general" than the object type that we are evaluating. For *every*
                // object type `MY_OBJECT`, a function call that goes through a trait-ref
                // of the form `<MY_OBJECT as SuperTraitOf(MY_OBJECT)>::func` is a valid
                // `ObjectCandidate`, and it should be discoverable "exactly" through one
                // of the iterations in the autoderef loop, so there is no problem with it
                // being discoverable in another one of these iterations.
                //
                // Using `instantiate_canonical_with_fresh_inference_vars` on our
                // `Canonical<QueryResponse<Ty<'tcx>>>` and then *throwing away* the
                // `CanonicalVarValues` will exactly give us such a generalization - it
                // will still match the original object type, but it won't pollute our
                // type variables in any form, so just do that!
                let (QueryResponse { value: generalized_self_ty, .. }, _ignored_var_values) =
                    self.fcx
                        .instantiate_canonical_with_fresh_inference_vars(self.span, self_ty);

                self.assemble_inherent_candidates_from_object(generalized_self_ty);
                self.assemble_inherent_impl_candidates_for_type(p.def_id());
                if self.tcx.has_attr(p.def_id(), sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
                }
            }
            ty::Adt(def, _) => {
                let def_id = def.did();
                self.assemble_inherent_impl_candidates_for_type(def_id);
                if self.tcx.has_attr(def_id, sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
                }
            }
            ty::Foreign(did) => {
                self.assemble_inherent_impl_candidates_for_type(did);
                if self.tcx.has_attr(did, sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
                }
            }
            ty::Param(p) => {
                self.assemble_inherent_candidates_from_param(p);
            }
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Str
            | ty::Array(..)
            | ty::Slice(_)
            | ty::RawPtr(_)
            | ty::Ref(..)
            | ty::Never
            | ty::Tuple(..) => self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty),
            _ => {}
        }
    }

    fn assemble_inherent_candidates_for_incoherent_ty(&mut self, self_ty: Ty<'tcx>) {
        let Some(simp) = simplify_type(self.tcx, self_ty, TreatParams::AsInfer) else {
            bug!("unexpected incoherent type: {:?}", self_ty)
        };
        for &impl_def_id in self.tcx.incoherent_impls(simp) {
            self.assemble_inherent_impl_probe(impl_def_id);
        }
    }

    fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
        let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
        for &impl_def_id in impl_def_ids.iter() {
            self.assemble_inherent_impl_probe(impl_def_id);
        }
    }

    fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
        if !self.impl_dups.insert(impl_def_id) {
            return; // already visited
        }

        debug!("assemble_inherent_impl_probe {:?}", impl_def_id);

        for item in self.impl_or_trait_item(impl_def_id) {
            if !self.has_applicable_self(&item) {
                // No receiver declared. Not a candidate.
                self.record_static_candidate(CandidateSource::Impl(impl_def_id));
                continue;
            }

            let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
            let impl_ty = impl_ty.subst(self.tcx, impl_substs);

            debug!("impl_ty: {:?}", impl_ty);

            // Determine the receiver type that the method itself expects.
            let (xform_self_ty, xform_ret_ty) = self.xform_self_ty(&item, impl_ty, impl_substs);
            debug!("xform_self_ty: {:?}, xform_ret_ty: {:?}", xform_self_ty, xform_ret_ty);

            // We can't use normalize_associated_types_in as it will pollute the
            // fcx's fulfillment context after this probe is over.
            // Note: we only normalize `xform_self_ty` here since the normalization
            // of the return type can lead to inference results that prohibit
            // valid candidates from being found, see issue #85671
            // FIXME Postponing the normalization of the return type likely only hides a deeper bug,
            // which might be caused by the `param_env` itself. The clauses of the `param_env`
            // maybe shouldn't include `Param`s, but rather fresh variables or be canonicalized,
            // see issue #89650
            let cause = traits::ObligationCause::misc(self.span, self.body_id);
            let selcx = &mut traits::SelectionContext::new(self.fcx);
            let traits::Normalized { value: xform_self_ty, obligations } =
                traits::normalize(selcx, self.param_env, cause, xform_self_ty);
            debug!(
                "assemble_inherent_impl_probe after normalization: xform_self_ty = {:?}/{:?}",
                xform_self_ty, xform_ret_ty
            );

            self.push_candidate(
                Candidate {
                    xform_self_ty,
                    xform_ret_ty,
                    item,
                    kind: InherentImplCandidate(impl_substs, obligations),
                    import_ids: smallvec![],
                },
                true,
            );
        }
    }

    fn assemble_inherent_candidates_from_object(&mut self, self_ty: Ty<'tcx>) {
        debug!("assemble_inherent_candidates_from_object(self_ty={:?})", self_ty);

        let principal = match self_ty.kind() {
            ty::Dynamic(ref data, ..) => Some(data),
            _ => None,
        }
        .and_then(|data| data.principal())
        .unwrap_or_else(|| {
            span_bug!(
                self.span,
                "non-object {:?} in assemble_inherent_candidates_from_object",
                self_ty
            )
        });

        // It is illegal to invoke a method on a trait instance that refers to
        // the `Self` type. An [`ObjectSafetyViolation::SupertraitSelf`] error
        // will be reported by `object_safety.rs` if the method refers to the
        // `Self` type anywhere other than the receiver. Here, we use a
        // substitution that replaces `Self` with the object type itself. Hence,
        // a `&self` method will wind up with an argument type like `&dyn Trait`.
        let trait_ref = principal.with_self_ty(self.tcx, self_ty);
        self.elaborate_bounds(iter::once(trait_ref), |this, new_trait_ref, item| {
            let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);

            let (xform_self_ty, xform_ret_ty) =
                this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
            this.push_candidate(
                Candidate {
                    xform_self_ty,
                    xform_ret_ty,
                    item,
                    kind: ObjectCandidate,
                    import_ids: smallvec![],
                },
                true,
            );
        });
    }

    fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
        // FIXME: do we want to commit to this behavior for param bounds?
        debug!("assemble_inherent_candidates_from_param(param_ty={:?})", param_ty);

        let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
            let bound_predicate = predicate.kind();
            match bound_predicate.skip_binder() {
                ty::PredicateKind::Trait(trait_predicate) => {
                    match *trait_predicate.trait_ref.self_ty().kind() {
                        ty::Param(p) if p == param_ty => {
                            Some(bound_predicate.rebind(trait_predicate.trait_ref))
                        }
                        _ => None,
                    }
                }
                ty::PredicateKind::Subtype(..)
                | ty::PredicateKind::Coerce(..)
                | ty::PredicateKind::Projection(..)
                | ty::PredicateKind::RegionOutlives(..)
                | ty::PredicateKind::WellFormed(..)
                | ty::PredicateKind::ObjectSafe(..)
                | ty::PredicateKind::ClosureKind(..)
                | ty::PredicateKind::TypeOutlives(..)
                | ty::PredicateKind::ConstEvaluatable(..)
                | ty::PredicateKind::ConstEquate(..)
                | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
            }
        });

        self.elaborate_bounds(bounds, |this, poly_trait_ref, item| {
            let trait_ref = this.erase_late_bound_regions(poly_trait_ref);

            let (xform_self_ty, xform_ret_ty) =
                this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);

            // Because this trait derives from a where-clause, it
            // should not contain any inference variables or other
            // artifacts. This means it is safe to put into the
            // `WhereClauseCandidate` and (eventually) into the
            // `WhereClausePick`.
            assert!(!trait_ref.substs.needs_infer());

            this.push_candidate(
                Candidate {
                    xform_self_ty,
                    xform_ret_ty,
                    item,
                    kind: WhereClauseCandidate(poly_trait_ref),
                    import_ids: smallvec![],
                },
                true,
            );
        });
    }

    // Do a search through a list of bounds, using a callback to actually
    // create the candidates.
    fn elaborate_bounds<F>(
        &mut self,
        bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
        mut mk_cand: F,
    ) where
        F: for<'b> FnMut(&mut ProbeContext<'b, 'tcx>, ty::PolyTraitRef<'tcx>, ty::AssocItem),
    {
        let tcx = self.tcx;
        for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
            debug!("elaborate_bounds(bound_trait_ref={:?})", bound_trait_ref);
            for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
                if !self.has_applicable_self(&item) {
                    self.record_static_candidate(CandidateSource::Trait(bound_trait_ref.def_id()));
                } else {
                    mk_cand(self, bound_trait_ref, item);
                }
            }
        }
    }

    fn assemble_extension_candidates_for_traits_in_scope(&mut self, expr_hir_id: hir::HirId) {
        let mut duplicates = FxHashSet::default();
        let opt_applicable_traits = self.tcx.in_scope_traits(expr_hir_id);
        if let Some(applicable_traits) = opt_applicable_traits {
            for trait_candidate in applicable_traits.iter() {
                let trait_did = trait_candidate.def_id;
                if duplicates.insert(trait_did) {
                    self.assemble_extension_candidates_for_trait(
                        &trait_candidate.import_ids,
                        trait_did,
                    );
                }
            }
        }
    }

    fn assemble_extension_candidates_for_all_traits(&mut self) {
        let mut duplicates = FxHashSet::default();
        for trait_info in suggest::all_traits(self.tcx) {
            if duplicates.insert(trait_info.def_id) {
                self.assemble_extension_candidates_for_trait(&smallvec![], trait_info.def_id);
            }
        }
    }

    pub fn matches_return_type(
        &self,
        method: &ty::AssocItem,
        self_ty: Option<Ty<'tcx>>,
        expected: Ty<'tcx>,
    ) -> bool {
        match method.kind {
            ty::AssocKind::Fn => {
                let fty = self.tcx.bound_fn_sig(method.def_id);
                self.probe(|_| {
                    let substs = self.fresh_substs_for_item(self.span, method.def_id);
                    let fty = fty.subst(self.tcx, substs);
                    let fty =
                        self.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, fty);

                    if let Some(self_ty) = self_ty {
                        if self
                            .at(&ObligationCause::dummy(), self.param_env)
                            .sup(fty.inputs()[0], self_ty)
                            .is_err()
                        {
                            return false;
                        }
                    }
                    self.can_sub(self.param_env, fty.output(), expected).is_ok()
                })
            }
            _ => false,
        }
    }

    fn assemble_extension_candidates_for_trait(
        &mut self,
        import_ids: &SmallVec<[LocalDefId; 1]>,
        trait_def_id: DefId,
    ) {
        debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})", trait_def_id);
        let trait_substs = self.fresh_item_substs(trait_def_id);
        let trait_ref = ty::TraitRef::new(trait_def_id, trait_substs);

        if self.tcx.is_trait_alias(trait_def_id) {
            // For trait aliases, assume all supertraits are relevant.
            let bounds = iter::once(ty::Binder::dummy(trait_ref));
            self.elaborate_bounds(bounds, |this, new_trait_ref, item| {
                let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);

                let (xform_self_ty, xform_ret_ty) =
                    this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
                this.push_candidate(
                    Candidate {
                        xform_self_ty,
                        xform_ret_ty,
                        item,
                        import_ids: import_ids.clone(),
                        kind: TraitCandidate(new_trait_ref),
                    },
                    false,
                );
            });
        } else {
            debug_assert!(self.tcx.is_trait(trait_def_id));
            for item in self.impl_or_trait_item(trait_def_id) {
                // Check whether `trait_def_id` defines a method with suitable name.
                if !self.has_applicable_self(&item) {
                    debug!("method has inapplicable self");
                    self.record_static_candidate(CandidateSource::Trait(trait_def_id));
                    continue;
                }

                let (xform_self_ty, xform_ret_ty) =
                    self.xform_self_ty(&item, trait_ref.self_ty(), trait_substs);
                self.push_candidate(
                    Candidate {
                        xform_self_ty,
                        xform_ret_ty,
                        item,
                        import_ids: import_ids.clone(),
                        kind: TraitCandidate(trait_ref),
                    },
                    false,
                );
            }
        }
    }

    fn candidate_method_names(&self) -> Vec<Ident> {
        let mut set = FxHashSet::default();
        let mut names: Vec<_> = self
            .inherent_candidates
            .iter()
            .chain(&self.extension_candidates)
            .filter(|candidate| {
                if let Some(return_ty) = self.return_type {
                    self.matches_return_type(&candidate.item, None, return_ty)
                } else {
                    true
                }
            })
            .map(|candidate| candidate.item.ident(self.tcx))
            .filter(|&name| set.insert(name))
            .collect();

        // Sort them by the name so we have a stable result.
        names.sort_by(|a, b| a.as_str().partial_cmp(b.as_str()).unwrap());
        names
    }

    ///////////////////////////////////////////////////////////////////////////
    // THE ACTUAL SEARCH

    fn pick(mut self) -> PickResult<'tcx> {
        assert!(self.method_name.is_some());

        if let Some(r) = self.pick_core() {
            return r;
        }

        debug!("pick: actual search failed, assemble diagnostics");

        let static_candidates = mem::take(&mut self.static_candidates);
        let private_candidate = self.private_candidate.take();
        let unsatisfied_predicates = mem::take(&mut self.unsatisfied_predicates);

        // things failed, so lets look at all traits, for diagnostic purposes now:
        self.reset();

        let span = self.span;
        let tcx = self.tcx;

        self.assemble_extension_candidates_for_all_traits();

        let out_of_scope_traits = match self.pick_core() {
            Some(Ok(p)) => vec![p.item.container_id(self.tcx)],
            //Some(Ok(p)) => p.iter().map(|p| p.item.container().id()).collect(),
            Some(Err(MethodError::Ambiguity(v))) => v
                .into_iter()
                .map(|source| match source {
                    CandidateSource::Trait(id) => id,
                    CandidateSource::Impl(impl_id) => match tcx.trait_id_of_impl(impl_id) {
                        Some(id) => id,
                        None => span_bug!(span, "found inherent method when looking at traits"),
                    },
                })
                .collect(),
            Some(Err(MethodError::NoMatch(NoMatchData {
                out_of_scope_traits: others, ..
            }))) => {
                assert!(others.is_empty());
                vec![]
            }
            _ => vec![],
        };

        if let Some((kind, def_id)) = private_candidate {
            return Err(MethodError::PrivateMatch(kind, def_id, out_of_scope_traits));
        }
        let lev_candidate = self.probe_for_lev_candidate()?;

        Err(MethodError::NoMatch(NoMatchData {
            static_candidates,
            unsatisfied_predicates,
            out_of_scope_traits,
            lev_candidate,
            mode: self.mode,
        }))
    }

    fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
        let mut unstable_candidates = Vec::new();
        let pick = self.pick_all_method(Some(&mut unstable_candidates));

        // In this case unstable picking is done by `pick_method`.
        if !self.tcx.sess.opts.unstable_opts.pick_stable_methods_before_any_unstable {
            return pick;
        }

        match pick {
            // Emit a lint if there are unstable candidates alongside the stable ones.
            //
            // We suppress warning if we're picking the method only because it is a
            // suggestion.
            Some(Ok(ref p)) if !self.is_suggestion.0 && !unstable_candidates.is_empty() => {
                self.emit_unstable_name_collision_hint(p, &unstable_candidates);
                pick
            }
            Some(_) => pick,
            None => self.pick_all_method(None),
        }
    }

    fn pick_all_method(
        &mut self,
        mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>> {
        let steps = self.steps.clone();
        steps
            .iter()
            .filter(|step| {
                debug!("pick_all_method: step={:?}", step);
                // skip types that are from a type error or that would require dereferencing
                // a raw pointer
                !step.self_ty.references_error() && !step.from_unsafe_deref
            })
            .flat_map(|step| {
                let InferOk { value: self_ty, obligations: _ } = self
                    .fcx
                    .probe_instantiate_query_response(
                        self.span,
                        &self.orig_steps_var_values,
                        &step.self_ty,
                    )
                    .unwrap_or_else(|_| {
                        span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
                    });
                self.pick_by_value_method(step, self_ty, unstable_candidates.as_deref_mut())
                    .or_else(|| {
                        self.pick_autorefd_method(
                            step,
                            self_ty,
                            hir::Mutability::Not,
                            unstable_candidates.as_deref_mut(),
                        )
                        .or_else(|| {
                            self.pick_autorefd_method(
                                step,
                                self_ty,
                                hir::Mutability::Mut,
                                unstable_candidates.as_deref_mut(),
                            )
                        })
                        .or_else(|| {
                            self.pick_const_ptr_method(
                                step,
                                self_ty,
                                unstable_candidates.as_deref_mut(),
                            )
                        })
                    })
            })
            .next()
    }

    /// For each type `T` in the step list, this attempts to find a method where
    /// the (transformed) self type is exactly `T`. We do however do one
    /// transformation on the adjustment: if we are passing a region pointer in,
    /// we will potentially *reborrow* it to a shorter lifetime. This allows us
    /// to transparently pass `&mut` pointers, in particular, without consuming
    /// them for their entire lifetime.
    fn pick_by_value_method(
        &mut self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>> {
        if step.unsize {
            return None;
        }

        self.pick_method(self_ty, unstable_candidates).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;

                // Insert a `&*` or `&mut *` if this is a reference type:
                if let ty::Ref(_, _, mutbl) = *step.self_ty.value.value.kind() {
                    pick.autoderefs += 1;
                    pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
                        mutbl,
                        unsize: pick.autoref_or_ptr_adjustment.map_or(false, |a| a.get_unsize()),
                    })
                }

                pick
            })
        })
    }

    fn pick_autorefd_method(
        &mut self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        mutbl: hir::Mutability,
        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>> {
        let tcx = self.tcx;

        // In general, during probing we erase regions.
        let region = tcx.lifetimes.re_erased;

        let autoref_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: self_ty, mutbl });
        self.pick_method(autoref_ty, unstable_candidates).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;
                pick.autoref_or_ptr_adjustment =
                    Some(AutorefOrPtrAdjustment::Autoref { mutbl, unsize: step.unsize });
                pick
            })
        })
    }

    /// If `self_ty` is `*mut T` then this picks `*const T` methods. The reason why we have a
    /// special case for this is because going from `*mut T` to `*const T` with autoderefs and
    /// autorefs would require dereferencing the pointer, which is not safe.
    fn pick_const_ptr_method(
        &mut self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>> {
        // Don't convert an unsized reference to ptr
        if step.unsize {
            return None;
        }

        let &ty::RawPtr(ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut }) = self_ty.kind() else {
            return None;
        };

        let const_self_ty = ty::TypeAndMut { ty, mutbl: hir::Mutability::Not };
        let const_ptr_ty = self.tcx.mk_ptr(const_self_ty);
        self.pick_method(const_ptr_ty, unstable_candidates).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;
                pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
                pick
            })
        })
    }

    fn pick_method_with_unstable(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
        debug!("pick_method_with_unstable(self_ty={})", self.ty_to_string(self_ty));

        let mut possibly_unsatisfied_predicates = Vec::new();
        let mut unstable_candidates = Vec::new();

        for (kind, candidates) in
            &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
        {
            debug!("searching {} candidates", kind);
            let res = self.consider_candidates(
                self_ty,
                candidates.iter(),
                &mut possibly_unsatisfied_predicates,
                Some(&mut unstable_candidates),
            );
            if let Some(pick) = res {
                if !self.is_suggestion.0 && !unstable_candidates.is_empty() {
                    if let Ok(p) = &pick {
                        // Emit a lint if there are unstable candidates alongside the stable ones.
                        //
                        // We suppress warning if we're picking the method only because it is a
                        // suggestion.
                        self.emit_unstable_name_collision_hint(p, &unstable_candidates);
                    }
                }
                return Some(pick);
            }
        }

        debug!("searching unstable candidates");
        let res = self.consider_candidates(
            self_ty,
            unstable_candidates.iter().map(|(c, _)| c),
            &mut possibly_unsatisfied_predicates,
            None,
        );
        if res.is_none() {
            self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
        }
        res
    }

    fn pick_method(
        &mut self,
        self_ty: Ty<'tcx>,
        mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>> {
        if !self.tcx.sess.opts.unstable_opts.pick_stable_methods_before_any_unstable {
            return self.pick_method_with_unstable(self_ty);
        }

        debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));

        let mut possibly_unsatisfied_predicates = Vec::new();

        for (kind, candidates) in
            &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
        {
            debug!("searching {} candidates", kind);
            let res = self.consider_candidates(
                self_ty,
                candidates.iter(),
                &mut possibly_unsatisfied_predicates,
                unstable_candidates.as_deref_mut(),
            );
            if let Some(pick) = res {
                return Some(pick);
            }
        }

        // `pick_method` may be called twice for the same self_ty if no stable methods
        // match. Only extend once.
        if unstable_candidates.is_some() {
            self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
        }
        None
    }

    fn consider_candidates<'b, ProbesIter>(
        &self,
        self_ty: Ty<'tcx>,
        probes: ProbesIter,
        possibly_unsatisfied_predicates: &mut Vec<(
            ty::Predicate<'tcx>,
            Option<ty::Predicate<'tcx>>,
            Option<ObligationCause<'tcx>>,
        )>,
        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
    ) -> Option<PickResult<'tcx>>
    where
        ProbesIter: Iterator<Item = &'b Candidate<'tcx>> + Clone,
        'tcx: 'b,
    {
        let mut applicable_candidates: Vec<_> = probes
            .clone()
            .map(|probe| {
                (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
            })
            .filter(|&(_, status)| status != ProbeResult::NoMatch)
            .collect();

        debug!("applicable_candidates: {:?}", applicable_candidates);

        if applicable_candidates.len() > 1 {
            if let Some(pick) =
                self.collapse_candidates_to_trait_pick(self_ty, &applicable_candidates)
            {
                return Some(Ok(pick));
            }
        }

        if let Some(uc) = unstable_candidates {
            applicable_candidates.retain(|&(p, _)| {
                if let stability::EvalResult::Deny { feature, .. } =
                    self.tcx.eval_stability(p.item.def_id, None, self.span, None)
                {
                    uc.push((p.clone(), feature));
                    return false;
                }
                true
            });
        }

        if applicable_candidates.len() > 1 {
            let sources = probes.map(|p| self.candidate_source(p, self_ty)).collect();
            return Some(Err(MethodError::Ambiguity(sources)));
        }

        applicable_candidates.pop().map(|(probe, status)| {
            if status == ProbeResult::Match {
                Ok(probe.to_unadjusted_pick(self_ty))
            } else {
                Err(MethodError::BadReturnType)
            }
        })
    }

    fn emit_unstable_name_collision_hint(
        &self,
        stable_pick: &Pick<'_>,
        unstable_candidates: &[(Candidate<'tcx>, Symbol)],
    ) {
        self.tcx.struct_span_lint_hir(
            lint::builtin::UNSTABLE_NAME_COLLISIONS,
            self.scope_expr_id,
            self.span,
            |lint| {
                let def_kind = stable_pick.item.kind.as_def_kind();
                let mut diag = lint.build(&format!(
                    "{} {} with this name may be added to the standard library in the future",
                    def_kind.article(),
                    def_kind.descr(stable_pick.item.def_id),
                ));
                match (stable_pick.item.kind, stable_pick.item.container) {
                    (ty::AssocKind::Fn, _) => {
                        // FIXME: This should be a `span_suggestion` instead of `help`
                        // However `self.span` only
                        // highlights the method name, so we can't use it. Also consider reusing
                        // the code from `report_method_error()`.
                        diag.help(&format!(
                            "call with fully qualified syntax `{}(...)` to keep using the current \
                             method",
                            self.tcx.def_path_str(stable_pick.item.def_id),
                        ));
                    }
                    (ty::AssocKind::Const, ty::AssocItemContainer::TraitContainer) => {
                        let def_id = stable_pick.item.container_id(self.tcx);
                        diag.span_suggestion(
                            self.span,
                            "use the fully qualified path to the associated const",
                            format!(
                                "<{} as {}>::{}",
                                stable_pick.self_ty,
                                self.tcx.def_path_str(def_id),
                                stable_pick.item.name
                            ),
                            Applicability::MachineApplicable,
                        );
                    }
                    _ => {}
                }
                if self.tcx.sess.is_nightly_build() {
                    for (candidate, feature) in unstable_candidates {
                        diag.help(&format!(
                            "add `#![feature({})]` to the crate attributes to enable `{}`",
                            feature,
                            self.tcx.def_path_str(candidate.item.def_id),
                        ));
                    }
                }

                diag.emit();
            },
        );
    }

    fn select_trait_candidate(
        &self,
        trait_ref: ty::TraitRef<'tcx>,
    ) -> traits::SelectionResult<'tcx, traits::Selection<'tcx>> {
        let cause = traits::ObligationCause::misc(self.span, self.body_id);
        let predicate = ty::Binder::dummy(trait_ref).to_poly_trait_predicate();
        let obligation = traits::Obligation::new(cause, self.param_env, predicate);
        traits::SelectionContext::new(self).select(&obligation)
    }

    fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
        match candidate.kind {
            InherentImplCandidate(..) => {
                CandidateSource::Impl(candidate.item.container_id(self.tcx))
            }
            ObjectCandidate | WhereClauseCandidate(_) => {
                CandidateSource::Trait(candidate.item.container_id(self.tcx))
            }
            TraitCandidate(trait_ref) => self.probe(|_| {
                let _ = self
                    .at(&ObligationCause::dummy(), self.param_env)
                    .define_opaque_types(false)
                    .sup(candidate.xform_self_ty, self_ty);
                match self.select_trait_candidate(trait_ref) {
                    Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
                        // If only a single impl matches, make the error message point
                        // to that impl.
                        CandidateSource::Impl(impl_data.impl_def_id)
                    }
                    _ => CandidateSource::Trait(candidate.item.container_id(self.tcx)),
                }
            }),
        }
    }

    fn consider_probe(
        &self,
        self_ty: Ty<'tcx>,
        probe: &Candidate<'tcx>,
        possibly_unsatisfied_predicates: &mut Vec<(
            ty::Predicate<'tcx>,
            Option<ty::Predicate<'tcx>>,
            Option<ObligationCause<'tcx>>,
        )>,
    ) -> ProbeResult {
        debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);

        self.probe(|_| {
            // First check that the self type can be related.
            let sub_obligations = match self
                .at(&ObligationCause::dummy(), self.param_env)
                .define_opaque_types(false)
                .sup(probe.xform_self_ty, self_ty)
            {
                Ok(InferOk { obligations, value: () }) => obligations,
                Err(err) => {
                    debug!("--> cannot relate self-types {:?}", err);
                    return ProbeResult::NoMatch;
                }
            };

            let mut result = ProbeResult::Match;
            let mut xform_ret_ty = probe.xform_ret_ty;
            debug!(?xform_ret_ty);

            let selcx = &mut traits::SelectionContext::new(self);
            let cause = traits::ObligationCause::misc(self.span, self.body_id);

            let mut parent_pred = None;

            // If so, impls may carry other conditions (e.g., where
            // clauses) that must be considered. Make sure that those
            // match as well (or at least may match, sometimes we
            // don't have enough information to fully evaluate).
            match probe.kind {
                InherentImplCandidate(ref substs, ref ref_obligations) => {
                    // `xform_ret_ty` hasn't been normalized yet, only `xform_self_ty`,
                    // see the reasons mentioned in the comments in `assemble_inherent_impl_probe`
                    // for why this is necessary
                    let traits::Normalized {
                        value: normalized_xform_ret_ty,
                        obligations: normalization_obligations,
                    } = traits::normalize(selcx, self.param_env, cause.clone(), probe.xform_ret_ty);
                    xform_ret_ty = normalized_xform_ret_ty;
                    debug!("xform_ret_ty after normalization: {:?}", xform_ret_ty);

                    // Check whether the impl imposes obligations we have to worry about.
                    let impl_def_id = probe.item.container_id(self.tcx);
                    let impl_bounds = self.tcx.predicates_of(impl_def_id);
                    let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
                    let traits::Normalized { value: impl_bounds, obligations: norm_obligations } =
                        traits::normalize(selcx, self.param_env, cause.clone(), impl_bounds);

                    // Convert the bounds into obligations.
                    let impl_obligations = traits::predicates_for_generics(
                        move |_, _| cause.clone(),
                        self.param_env,
                        impl_bounds,
                    );

                    let candidate_obligations = impl_obligations
                        .chain(norm_obligations.into_iter())
                        .chain(ref_obligations.iter().cloned())
                        .chain(normalization_obligations.into_iter());

                    // Evaluate those obligations to see if they might possibly hold.
                    for o in candidate_obligations {
                        let o = self.resolve_vars_if_possible(o);
                        if !self.predicate_may_hold(&o) {
                            result = ProbeResult::NoMatch;
                            possibly_unsatisfied_predicates.push((
                                o.predicate,
                                None,
                                Some(o.cause),
                            ));
                        }
                    }
                }

                ObjectCandidate | WhereClauseCandidate(..) => {
                    // These have no additional conditions to check.
                }

                TraitCandidate(trait_ref) => {
                    if let Some(method_name) = self.method_name {
                        // Some trait methods are excluded for arrays before 2021.
                        // (`array.into_iter()` wants a slice iterator for compatibility.)
                        if self_ty.is_array() && !method_name.span.rust_2021() {
                            let trait_def = self.tcx.trait_def(trait_ref.def_id);
                            if trait_def.skip_array_during_method_dispatch {
                                return ProbeResult::NoMatch;
                            }
                        }
                    }
                    let predicate =
                        ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx);
                    parent_pred = Some(predicate);
                    let obligation = traits::Obligation::new(cause, self.param_env, predicate);
                    if !self.predicate_may_hold(&obligation) {
                        result = ProbeResult::NoMatch;
                        if self.probe(|_| {
                            match self.select_trait_candidate(trait_ref) {
                                Err(_) => return true,
                                Ok(Some(impl_source))
                                    if !impl_source.borrow_nested_obligations().is_empty() =>
                                {
                                    for obligation in impl_source.borrow_nested_obligations() {
                                        // Determine exactly which obligation wasn't met, so
                                        // that we can give more context in the error.
                                        if !self.predicate_may_hold(obligation) {
                                            let nested_predicate =
                                                self.resolve_vars_if_possible(obligation.predicate);
                                            let predicate =
                                                self.resolve_vars_if_possible(predicate);
                                            let p = if predicate == nested_predicate {
                                                // Avoid "`MyStruct: Foo` which is required by
                                                // `MyStruct: Foo`" in E0599.
                                                None
                                            } else {
                                                Some(predicate)
                                            };
                                            possibly_unsatisfied_predicates.push((
                                                nested_predicate,
                                                p,
                                                Some(obligation.cause.clone()),
                                            ));
                                        }
                                    }
                                }
                                _ => {
                                    // Some nested subobligation of this predicate
                                    // failed.
                                    let predicate = self.resolve_vars_if_possible(predicate);
                                    possibly_unsatisfied_predicates.push((predicate, None, None));
                                }
                            }
                            false
                        }) {
                            // This candidate's primary obligation doesn't even
                            // select - don't bother registering anything in
                            // `potentially_unsatisfied_predicates`.
                            return ProbeResult::NoMatch;
                        }
                    }
                }
            }

            // Evaluate those obligations to see if they might possibly hold.
            for o in sub_obligations {
                let o = self.resolve_vars_if_possible(o);
                if !self.predicate_may_hold(&o) {
                    result = ProbeResult::NoMatch;
                    possibly_unsatisfied_predicates.push((o.predicate, parent_pred, Some(o.cause)));
                }
            }

            if let ProbeResult::Match = result {
                if let (Some(return_ty), Some(xform_ret_ty)) = (self.return_type, xform_ret_ty) {
                    let xform_ret_ty = self.resolve_vars_if_possible(xform_ret_ty);
                    debug!(
                        "comparing return_ty {:?} with xform ret ty {:?}",
                        return_ty, probe.xform_ret_ty
                    );
                    if self
                        .at(&ObligationCause::dummy(), self.param_env)
                        .define_opaque_types(false)
                        .sup(return_ty, xform_ret_ty)
                        .is_err()
                    {
                        return ProbeResult::BadReturnType;
                    }
                }
            }

            result
        })
    }

    /// Sometimes we get in a situation where we have multiple probes that are all impls of the
    /// same trait, but we don't know which impl to use. In this case, since in all cases the
    /// external interface of the method can be determined from the trait, it's ok not to decide.
    /// We can basically just collapse all of the probes for various impls into one where-clause
    /// probe. This will result in a pending obligation so when more type-info is available we can
    /// make the final decision.
    ///
    /// Example (`src/test/ui/method-two-trait-defer-resolution-1.rs`):
    ///
    /// ```ignore (illustrative)
    /// trait Foo { ... }
    /// impl Foo for Vec<i32> { ... }
    /// impl Foo for Vec<usize> { ... }
    /// ```
    ///
    /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
    /// use, so it's ok to just commit to "using the method from the trait Foo".
    fn collapse_candidates_to_trait_pick(
        &self,
        self_ty: Ty<'tcx>,
        probes: &[(&Candidate<'tcx>, ProbeResult)],
    ) -> Option<Pick<'tcx>> {
        // Do all probes correspond to the same trait?
        let container = probes[0].0.item.trait_container(self.tcx)?;
        for (p, _) in &probes[1..] {
            let p_container = p.item.trait_container(self.tcx)?;
            if p_container != container {
                return None;
            }
        }

        // FIXME: check the return type here somehow.
        // If so, just use this trait and call it a day.
        Some(Pick {
            item: probes[0].0.item,
            kind: TraitPick,
            import_ids: probes[0].0.import_ids.clone(),
            autoderefs: 0,
            autoref_or_ptr_adjustment: None,
            self_ty,
        })
    }

    /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
    /// candidate method where the method name may have been misspelled. Similarly to other
    /// Levenshtein based suggestions, we provide at most one such suggestion.
    fn probe_for_lev_candidate(&mut self) -> Result<Option<ty::AssocItem>, MethodError<'tcx>> {
        debug!("probing for method names similar to {:?}", self.method_name);

        let steps = self.steps.clone();
        self.probe(|_| {
            let mut pcx = ProbeContext::new(
                self.fcx,
                self.span,
                self.mode,
                self.method_name,
                self.return_type,
                self.orig_steps_var_values.clone(),
                steps,
                IsSuggestion(true),
                self.scope_expr_id,
            );
            pcx.allow_similar_names = true;
            pcx.assemble_inherent_candidates();

            let method_names = pcx.candidate_method_names();
            pcx.allow_similar_names = false;
            let applicable_close_candidates: Vec<ty::AssocItem> = method_names
                .iter()
                .filter_map(|&method_name| {
                    pcx.reset();
                    pcx.method_name = Some(method_name);
                    pcx.assemble_inherent_candidates();
                    pcx.pick_core().and_then(|pick| pick.ok()).map(|pick| pick.item)
                })
                .collect();

            if applicable_close_candidates.is_empty() {
                Ok(None)
            } else {
                let best_name = {
                    let names = applicable_close_candidates
                        .iter()
                        .map(|cand| cand.name)
                        .collect::<Vec<Symbol>>();
                    find_best_match_for_name_with_substrings(
                        &names,
                        self.method_name.unwrap().name,
                        None,
                    )
                }
                .unwrap();
                Ok(applicable_close_candidates.into_iter().find(|method| method.name == best_name))
            }
        })
    }

    ///////////////////////////////////////////////////////////////////////////
    // MISCELLANY
    fn has_applicable_self(&self, item: &ty::AssocItem) -> bool {
        // "Fast track" -- check for usage of sugar when in method call
        // mode.
        //
        // In Path mode (i.e., resolving a value like `T::next`), consider any
        // associated value (i.e., methods, constants) but not types.
        match self.mode {
            Mode::MethodCall => item.fn_has_self_parameter,
            Mode::Path => match item.kind {
                ty::AssocKind::Type => false,
                ty::AssocKind::Fn | ty::AssocKind::Const => true,
            },
        }
        // FIXME -- check for types that deref to `Self`,
        // like `Rc<Self>` and so on.
        //
        // Note also that the current code will break if this type
        // includes any of the type parameters defined on the method
        // -- but this could be overcome.
    }

    fn record_static_candidate(&mut self, source: CandidateSource) {
        self.static_candidates.push(source);
    }

    #[instrument(level = "debug", skip(self))]
    fn xform_self_ty(
        &self,
        item: &ty::AssocItem,
        impl_ty: Ty<'tcx>,
        substs: SubstsRef<'tcx>,
    ) -> (Ty<'tcx>, Option<Ty<'tcx>>) {
        if item.kind == ty::AssocKind::Fn && self.mode == Mode::MethodCall {
            let sig = self.xform_method_sig(item.def_id, substs);
            (sig.inputs()[0], Some(sig.output()))
        } else {
            (impl_ty, None)
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn xform_method_sig(&self, method: DefId, substs: SubstsRef<'tcx>) -> ty::FnSig<'tcx> {
        let fn_sig = self.tcx.bound_fn_sig(method);
        debug!(?fn_sig);

        assert!(!substs.has_escaping_bound_vars());

        // It is possible for type parameters or early-bound lifetimes
        // to appear in the signature of `self`. The substitutions we
        // are given do not include type/lifetime parameters for the
        // method yet. So create fresh variables here for those too,
        // if there are any.
        let generics = self.tcx.generics_of(method);
        assert_eq!(substs.len(), generics.parent_count as usize);

        let xform_fn_sig = if generics.params.is_empty() {
            fn_sig.subst(self.tcx, substs)
        } else {
            let substs = InternalSubsts::for_item(self.tcx, method, |param, _| {
                let i = param.index as usize;
                if i < substs.len() {
                    substs[i]
                } else {
                    match param.kind {
                        GenericParamDefKind::Lifetime => {
                            // In general, during probe we erase regions.
                            self.tcx.lifetimes.re_erased.into()
                        }
                        GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
                            self.var_for_def(self.span, param)
                        }
                    }
                }
            });
            fn_sig.subst(self.tcx, substs)
        };

        self.erase_late_bound_regions(xform_fn_sig)
    }

    /// Gets the type of an impl and generate substitutions with inference vars.
    fn impl_ty_and_substs(
        &self,
        impl_def_id: DefId,
    ) -> (ty::EarlyBinder<Ty<'tcx>>, SubstsRef<'tcx>) {
        (self.tcx.bound_type_of(impl_def_id), self.fresh_item_substs(impl_def_id))
    }

    fn fresh_item_substs(&self, def_id: DefId) -> SubstsRef<'tcx> {
        InternalSubsts::for_item(self.tcx, def_id, |param, _| match param.kind {
            GenericParamDefKind::Lifetime => self.tcx.lifetimes.re_erased.into(),
            GenericParamDefKind::Type { .. } => self
                .next_ty_var(TypeVariableOrigin {
                    kind: TypeVariableOriginKind::SubstitutionPlaceholder,
                    span: self.tcx.def_span(def_id),
                })
                .into(),
            GenericParamDefKind::Const { .. } => {
                let span = self.tcx.def_span(def_id);
                let origin = ConstVariableOrigin {
                    kind: ConstVariableOriginKind::SubstitutionPlaceholder,
                    span,
                };
                self.next_const_var(self.tcx.type_of(param.def_id), origin).into()
            }
        })
    }

    /// Replaces late-bound-regions bound by `value` with `'static` using
    /// `ty::erase_late_bound_regions`.
    ///
    /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
    /// method matching. It is reasonable during the probe phase because we don't consider region
    /// relationships at all. Therefore, we can just replace all the region variables with 'static
    /// rather than creating fresh region variables. This is nice for two reasons:
    ///
    /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
    ///    particular method call, it winds up creating fewer types overall, which helps for memory
    ///    usage. (Admittedly, this is a rather small effect, though measurable.)
    ///
    /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
    ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
    ///    regions with actual region variables as is proper, we'd have to ensure that the same
    ///    region got replaced with the same variable, which requires a bit more coordination
    ///    and/or tracking the substitution and
    ///    so forth.
    fn erase_late_bound_regions<T>(&self, value: ty::Binder<'tcx, T>) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        self.tcx.erase_late_bound_regions(value)
    }

    /// Finds the method with the appropriate name (or return type, as the case may be). If
    /// `allow_similar_names` is set, find methods with close-matching names.
    // The length of the returned iterator is nearly always 0 or 1 and this
    // method is fairly hot.
    fn impl_or_trait_item(&self, def_id: DefId) -> SmallVec<[ty::AssocItem; 1]> {
        if let Some(name) = self.method_name {
            if self.allow_similar_names {
                let max_dist = max(name.as_str().len(), 3) / 3;
                self.tcx
                    .associated_items(def_id)
                    .in_definition_order()
                    .filter(|x| {
                        if x.kind.namespace() != Namespace::ValueNS {
                            return false;
                        }
                        match lev_distance_with_substrings(name.as_str(), x.name.as_str(), max_dist)
                        {
                            Some(d) => d > 0,
                            None => false,
                        }
                    })
                    .copied()
                    .collect()
            } else {
                self.fcx
                    .associated_value(def_id, name)
                    .map_or_else(SmallVec::new, |x| SmallVec::from_buf([x]))
            }
        } else {
            self.tcx.associated_items(def_id).in_definition_order().copied().collect()
        }
    }
}

impl<'tcx> Candidate<'tcx> {
    fn to_unadjusted_pick(&self, self_ty: Ty<'tcx>) -> Pick<'tcx> {
        Pick {
            item: self.item,
            kind: match self.kind {
                InherentImplCandidate(..) => InherentImplPick,
                ObjectCandidate => ObjectPick,
                TraitCandidate(_) => TraitPick,
                WhereClauseCandidate(ref trait_ref) => {
                    // Only trait derived from where-clauses should
                    // appear here, so they should not contain any
                    // inference variables or other artifacts. This
                    // means they are safe to put into the
                    // `WhereClausePick`.
                    assert!(
                        !trait_ref.skip_binder().substs.needs_infer()
                            && !trait_ref.skip_binder().substs.has_placeholders()
                    );

                    WhereClausePick(*trait_ref)
                }
            },
            import_ids: self.import_ids.clone(),
            autoderefs: 0,
            autoref_or_ptr_adjustment: None,
            self_ty,
        }
    }
}