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
//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing. This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference. This is
//! useful for freezing mut things (that is, when the expected type is &T
//! but you have &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T). See
//! the various `src/test/ui/coerce/*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When inferring the generic arguments of functions, the argument
//! order is relevant, which can lead to the following edge case:
//!
//! ```ignore (illustrative)
//! fn foo<T>(a: T, b: T) {
//!     // ...
//! }
//!
//! foo(&7i32, &mut 7i32);
//! // This compiles, as we first infer `T` to be `&i32`,
//! // and then coerce `&mut 7i32` to `&7i32`.
//!
//! foo(&mut 7i32, &7i32);
//! // This does not compile, as we first infer `T` to be `&mut i32`
//! // and are then unable to coerce `&7i32` to `&mut i32`.
//! ```

use crate::FnCtxt;
use rustc_errors::{
    struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::Expr;
use rustc_hir_analysis::astconv::AstConv;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{Coercion, InferOk, InferResult};
use rustc_infer::traits::Obligation;
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::adjustment::{
    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::relate::RelateResult;
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::visit::TypeVisitable;
use rustc_middle::ty::{self, Ty, TypeAndMut};
use rustc_session::parse::feature_err;
use rustc_span::symbol::sym;
use rustc_span::{self, BytePos, DesugaringKind, Span};
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::infer::InferCtxtExt as _;
use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
use rustc_trait_selection::traits::{
    self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
};

use smallvec::{smallvec, SmallVec};
use std::ops::Deref;

struct Coerce<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    cause: ObligationCause<'tcx>,
    use_lub: bool,
    /// Determines whether or not allow_two_phase_borrow is set on any
    /// autoref adjustments we create while coercing. We don't want to
    /// allow deref coercions to create two-phase borrows, at least initially,
    /// but we do need two-phase borrows for function argument reborrows.
    /// See #47489 and #48598
    /// See docs on the "AllowTwoPhase" type for a more detailed discussion
    allow_two_phase: AllowTwoPhase,
}

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

type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;

struct CollectRetsVisitor<'tcx> {
    ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
}

impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
        if let hir::ExprKind::Ret(_) = expr.kind {
            self.ret_exprs.push(expr);
        }
        intravisit::walk_expr(self, expr);
    }
}

/// Coercing a mutable reference to an immutable works, while
/// coercing `&T` to `&mut T` should be forbidden.
fn coerce_mutbls<'tcx>(
    from_mutbl: hir::Mutability,
    to_mutbl: hir::Mutability,
) -> RelateResult<'tcx, ()> {
    if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
}

/// Do not require any adjustments, i.e. coerce `x -> x`.
fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
    vec![]
}

fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
    move |target| vec![Adjustment { kind, target }]
}

/// This always returns `Ok(...)`.
fn success<'tcx>(
    adj: Vec<Adjustment<'tcx>>,
    target: Ty<'tcx>,
    obligations: traits::PredicateObligations<'tcx>,
) -> CoerceResult<'tcx> {
    Ok(InferOk { value: (adj, target), obligations })
}

impl<'f, 'tcx> Coerce<'f, 'tcx> {
    fn new(
        fcx: &'f FnCtxt<'f, 'tcx>,
        cause: ObligationCause<'tcx>,
        allow_two_phase: AllowTwoPhase,
    ) -> Self {
        Coerce { fcx, cause, allow_two_phase, use_lub: false }
    }

    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
        debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
        self.commit_if_ok(|_| {
            if self.use_lub {
                self.at(&self.cause, self.fcx.param_env).lub(b, a)
            } else {
                self.at(&self.cause, self.fcx.param_env)
                    .sup(b, a)
                    .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
            }
        })
    }

    /// Unify two types (using sub or lub) and produce a specific coercion.
    fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
    where
        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
    {
        self.unify(a, b)
            .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
    }

    #[instrument(skip(self))]
    fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        // First, remove any resolved type variables (at the top level, at least):
        let a = self.shallow_resolve(a);
        let b = self.shallow_resolve(b);
        debug!("Coerce.tys({:?} => {:?})", a, b);

        // Just ignore error types.
        if a.references_error() || b.references_error() {
            return success(vec![], self.fcx.tcx.ty_error(), vec![]);
        }

        // Coercing from `!` to any type is allowed:
        if a.is_never() {
            return success(simple(Adjust::NeverToAny)(b), b, vec![]);
        }

        // Coercing *from* an unresolved inference variable means that
        // we have no information about the source type. This will always
        // ultimately fall back to some form of subtyping.
        if a.is_ty_var() {
            return self.coerce_from_inference_variable(a, b, identity);
        }

        // Consider coercing the subtype to a DST
        //
        // NOTE: this is wrapped in a `commit_if_ok` because it creates
        // a "spurious" type variable, and we don't want to have that
        // type variable in memory if the coercion fails.
        let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
        match unsize {
            Ok(_) => {
                debug!("coerce: unsize successful");
                return unsize;
            }
            Err(error) => {
                debug!(?error, "coerce: unsize failed");
            }
        }

        // Examine the supertype and consider auto-borrowing.
        match *b.kind() {
            ty::RawPtr(mt_b) => {
                return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
            }
            ty::Ref(r_b, _, mutbl_b) => {
                return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
            }
            ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
                return self.coerce_dyn_star(a, b, predicates, region);
            }
            _ => {}
        }

        match *a.kind() {
            ty::FnDef(..) => {
                // Function items are coercible to any closure
                // type; function pointers are not (that would
                // require double indirection).
                // Additionally, we permit coercion of function
                // items to drop the unsafe qualifier.
                self.coerce_from_fn_item(a, b)
            }
            ty::FnPtr(a_f) => {
                // We permit coercion of fn pointers to drop the
                // unsafe qualifier.
                self.coerce_from_fn_pointer(a, a_f, b)
            }
            ty::Closure(closure_def_id_a, substs_a) => {
                // Non-capturing closures are coercible to
                // function pointers or unsafe function pointers.
                // It cannot convert closures that require unsafe.
                self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
            }
            _ => {
                // Otherwise, just use unification rules.
                self.unify_and(a, b, identity)
            }
        }
    }

    /// Coercing *from* an inference variable. In this case, we have no information
    /// about the source type, so we can't really do a true coercion and we always
    /// fall back to subtyping (`unify_and`).
    fn coerce_from_inference_variable(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
    ) -> CoerceResult<'tcx> {
        debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
        assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
        assert!(self.shallow_resolve(b) == b);

        if b.is_ty_var() {
            // Two unresolved type variables: create a `Coerce` predicate.
            let target_ty = if self.use_lub {
                self.next_ty_var(TypeVariableOrigin {
                    kind: TypeVariableOriginKind::LatticeVariable,
                    span: self.cause.span,
                })
            } else {
                b
            };

            let mut obligations = Vec::with_capacity(2);
            for &source_ty in &[a, b] {
                if source_ty != target_ty {
                    obligations.push(Obligation::new(
                        self.tcx(),
                        self.cause.clone(),
                        self.param_env,
                        ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
                            a: source_ty,
                            b: target_ty,
                        })),
                    ));
                }
            }

            debug!(
                "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
                target_ty, obligations
            );
            let adjustments = make_adjustments(target_ty);
            InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
        } else {
            // One unresolved type variable: just apply subtyping, we may be able
            // to do something useful.
            self.unify_and(a, b, make_adjustments)
        }
    }

    /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
    /// To match `A` with `B`, autoderef will be performed,
    /// calling `deref`/`deref_mut` where necessary.
    fn coerce_borrowed_pointer(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        r_b: ty::Region<'tcx>,
        mutbl_b: hir::Mutability,
    ) -> CoerceResult<'tcx> {
        debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);

        // If we have a parameter of type `&M T_a` and the value
        // provided is `expr`, we will be adding an implicit borrow,
        // meaning that we convert `f(expr)` to `f(&M *expr)`.  Therefore,
        // to type check, we will construct the type that `&M*expr` would
        // yield.

        let (r_a, mt_a) = match *a.kind() {
            ty::Ref(r_a, ty, mutbl) => {
                let mt_a = ty::TypeAndMut { ty, mutbl };
                coerce_mutbls(mt_a.mutbl, mutbl_b)?;
                (r_a, mt_a)
            }
            _ => return self.unify_and(a, b, identity),
        };

        let span = self.cause.span;

        let mut first_error = None;
        let mut r_borrow_var = None;
        let mut autoderef = self.autoderef(span, a);
        let mut found = None;

        for (referent_ty, autoderefs) in autoderef.by_ref() {
            if autoderefs == 0 {
                // Don't let this pass, otherwise it would cause
                // &T to autoref to &&T.
                continue;
            }

            // At this point, we have deref'd `a` to `referent_ty`.  So
            // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
            // In the autoderef loop for `&'a mut Vec<T>`, we would get
            // three callbacks:
            //
            // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
            // - `Vec<T>` -- 1 deref
            // - `[T]` -- 2 deref
            //
            // At each point after the first callback, we want to
            // check to see whether this would match out target type
            // (`&'b mut [T]`) if we autoref'd it. We can't just
            // compare the referent types, though, because we still
            // have to consider the mutability. E.g., in the case
            // we've been considering, we have an `&mut` reference, so
            // the `T` in `[T]` needs to be unified with equality.
            //
            // Therefore, we construct reference types reflecting what
            // the types will be after we do the final auto-ref and
            // compare those. Note that this means we use the target
            // mutability [1], since it may be that we are coercing
            // from `&mut T` to `&U`.
            //
            // One fine point concerns the region that we use. We
            // choose the region such that the region of the final
            // type that results from `unify` will be the region we
            // want for the autoref:
            //
            // - if in sub mode, that means we want to use `'b` (the
            //   region from the target reference) for both
            //   pointers [2]. This is because sub mode (somewhat
            //   arbitrarily) returns the subtype region.  In the case
            //   where we are coercing to a target type, we know we
            //   want to use that target type region (`'b`) because --
            //   for the program to type-check -- it must be the
            //   smaller of the two.
            //   - One fine point. It may be surprising that we can
            //     use `'b` without relating `'a` and `'b`. The reason
            //     that this is ok is that what we produce is
            //     effectively a `&'b *x` expression (if you could
            //     annotate the region of a borrow), and regionck has
            //     code that adds edges from the region of a borrow
            //     (`'b`, here) into the regions in the borrowed
            //     expression (`*x`, here).  (Search for "link".)
            // - if in lub mode, things can get fairly complicated. The
            //   easiest thing is just to make a fresh
            //   region variable [4], which effectively means we defer
            //   the decision to region inference (and regionck, which will add
            //   some more edges to this variable). However, this can wind up
            //   creating a crippling number of variables in some cases --
            //   e.g., #32278 -- so we optimize one particular case [3].
            //   Let me try to explain with some examples:
            //   - The "running example" above represents the simple case,
            //     where we have one `&` reference at the outer level and
            //     ownership all the rest of the way down. In this case,
            //     we want `LUB('a, 'b)` as the resulting region.
            //   - However, if there are nested borrows, that region is
            //     too strong. Consider a coercion from `&'a &'x Rc<T>` to
            //     `&'b T`. In this case, `'a` is actually irrelevant.
            //     The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
            //     we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
            //     (The errors actually show up in borrowck, typically, because
            //     this extra edge causes the region `'a` to be inferred to something
            //     too big, which then results in borrowck errors.)
            //   - We could track the innermost shared reference, but there is already
            //     code in regionck that has the job of creating links between
            //     the region of a borrow and the regions in the thing being
            //     borrowed (here, `'a` and `'x`), and it knows how to handle
            //     all the various cases. So instead we just make a region variable
            //     and let regionck figure it out.
            let r = if !self.use_lub {
                r_b // [2] above
            } else if autoderefs == 1 {
                r_a // [3] above
            } else {
                if r_borrow_var.is_none() {
                    // create var lazily, at most once
                    let coercion = Coercion(span);
                    let r = self.next_region_var(coercion);
                    r_borrow_var = Some(r); // [4] above
                }
                r_borrow_var.unwrap()
            };
            let derefd_ty_a = self.tcx.mk_ref(
                r,
                TypeAndMut {
                    ty: referent_ty,
                    mutbl: mutbl_b, // [1] above
                },
            );
            match self.unify(derefd_ty_a, b) {
                Ok(ok) => {
                    found = Some(ok);
                    break;
                }
                Err(err) => {
                    if first_error.is_none() {
                        first_error = Some(err);
                    }
                }
            }
        }

        // Extract type or return an error. We return the first error
        // we got, which should be from relating the "base" type
        // (e.g., in example above, the failure from relating `Vec<T>`
        // to the target type), since that should be the least
        // confusing.
        let Some(InferOk { value: ty, mut obligations }) = found else {
            let err = first_error.expect("coerce_borrowed_pointer had no error");
            debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
            return Err(err);
        };

        if ty == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
            // As a special case, if we would produce `&'a *x`, that's
            // a total no-op. We end up with the type `&'a T` just as
            // we started with.  In that case, just skip it
            // altogether. This is just an optimization.
            //
            // Note that for `&mut`, we DO want to reborrow --
            // otherwise, this would be a move, which might be an
            // error. For example `foo(self.x)` where `self` and
            // `self.x` both have `&mut `type would be a move of
            // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
            // which is a borrow.
            assert!(mutbl_b.is_not()); // can only coerce &T -> &U
            return success(vec![], ty, obligations);
        }

        let InferOk { value: mut adjustments, obligations: o } =
            self.adjust_steps_as_infer_ok(&autoderef);
        obligations.extend(o);
        obligations.extend(autoderef.into_obligations());

        // Now apply the autoref. We have to extract the region out of
        // the final ref type we got.
        let ty::Ref(r_borrow, _, _) = ty.kind() else {
            span_bug!(span, "expected a ref type, got {:?}", ty);
        };
        let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
        adjustments.push(Adjustment {
            kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
            target: ty,
        });

        debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);

        success(adjustments, ty, obligations)
    }

    // &[T; n] or &mut [T; n] -> &[T]
    // or &mut [T; n] -> &mut [T]
    // or &Concrete -> &Trait, etc.
    #[instrument(skip(self), level = "debug")]
    fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
        source = self.shallow_resolve(source);
        target = self.shallow_resolve(target);
        debug!(?source, ?target);

        // We don't apply any coercions incase either the source or target
        // aren't sufficiently well known but tend to instead just equate
        // them both.
        if source.is_ty_var() {
            debug!("coerce_unsized: source is a TyVar, bailing out");
            return Err(TypeError::Mismatch);
        }
        if target.is_ty_var() {
            debug!("coerce_unsized: target is a TyVar, bailing out");
            return Err(TypeError::Mismatch);
        }

        let traits =
            (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
        let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
            debug!("missing Unsize or CoerceUnsized traits");
            return Err(TypeError::Mismatch);
        };

        // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
        // a DST unless we have to. This currently comes out in the wash since
        // we can't unify [T] with U. But to properly support DST, we need to allow
        // that, at which point we will need extra checks on the target here.

        // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
        let reborrow = match (source.kind(), target.kind()) {
            (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
                coerce_mutbls(mutbl_a, mutbl_b)?;

                let coercion = Coercion(self.cause.span);
                let r_borrow = self.next_region_var(coercion);

                // We don't allow two-phase borrows here, at least for initial
                // implementation. If it happens that this coercion is a function argument,
                // the reborrow in coerce_borrowed_ptr will pick it up.
                let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);

                Some((
                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
                    Adjustment {
                        kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
                        target: self
                            .tcx
                            .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
                    },
                ))
            }
            (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
                coerce_mutbls(mt_a, mt_b)?;

                Some((
                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
                    Adjustment {
                        kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
                        target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
                    },
                ))
            }
            _ => None,
        };
        let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);

        // Setup either a subtyping or a LUB relationship between
        // the `CoerceUnsized` target type and the expected type.
        // We only have the latter, so we use an inference variable
        // for the former and let type inference do the rest.
        let origin = TypeVariableOrigin {
            kind: TypeVariableOriginKind::MiscVariable,
            span: self.cause.span,
        };
        let coerce_target = self.next_ty_var(origin);
        let mut coercion = self.unify_and(coerce_target, target, |target| {
            let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
            match reborrow {
                None => vec![unsize],
                Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
            }
        })?;

        let mut selcx = traits::SelectionContext::new(self);

        // Create an obligation for `Source: CoerceUnsized<Target>`.
        let cause = ObligationCause::new(
            self.cause.span,
            self.body_id,
            ObligationCauseCode::Coercion { source, target },
        );

        // Use a FIFO queue for this custom fulfillment procedure.
        //
        // A Vec (or SmallVec) is not a natural choice for a queue. However,
        // this code path is hot, and this queue usually has a max length of 1
        // and almost never more than 3. By using a SmallVec we avoid an
        // allocation, at the (very small) cost of (occasionally) having to
        // shift subsequent elements down when removing the front element.
        let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
            self.tcx,
            self.fcx.param_env,
            cause,
            coerce_unsized_did,
            0,
            [coerce_source, coerce_target]
        )];

        let mut has_unsized_tuple_coercion = false;
        let mut has_trait_upcasting_coercion = None;

        // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
        // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
        // inference might unify those two inner type variables later.
        let traits = [coerce_unsized_did, unsize_did];
        while !queue.is_empty() {
            let obligation = queue.remove(0);
            debug!("coerce_unsized resolve step: {:?}", obligation);
            let bound_predicate = obligation.predicate.kind();
            let trait_pred = match bound_predicate.skip_binder() {
                ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred))
                    if traits.contains(&trait_pred.def_id()) =>
                {
                    if unsize_did == trait_pred.def_id() {
                        let self_ty = trait_pred.self_ty();
                        let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
                        if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
                            (self_ty.kind(), unsize_ty.kind())
                            && data_a.principal_def_id() != data_b.principal_def_id()
                        {
                            debug!("coerce_unsized: found trait upcasting coercion");
                            has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
                        }
                        if let ty::Tuple(..) = unsize_ty.kind() {
                            debug!("coerce_unsized: found unsized tuple coercion");
                            has_unsized_tuple_coercion = true;
                        }
                    }
                    bound_predicate.rebind(trait_pred)
                }
                _ => {
                    coercion.obligations.push(obligation);
                    continue;
                }
            };
            match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
                // Uncertain or unimplemented.
                Ok(None) => {
                    if trait_pred.def_id() == unsize_did {
                        let trait_pred = self.resolve_vars_if_possible(trait_pred);
                        let self_ty = trait_pred.skip_binder().self_ty();
                        let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
                        debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
                        match (&self_ty.kind(), &unsize_ty.kind()) {
                            (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
                                if self.type_var_is_sized(*v) =>
                            {
                                debug!("coerce_unsized: have sized infer {:?}", v);
                                coercion.obligations.push(obligation);
                                // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
                                // for unsizing.
                            }
                            _ => {
                                // Some other case for `$0: Unsize<Something>`. Note that we
                                // hit this case even if `Something` is a sized type, so just
                                // don't do the coercion.
                                debug!("coerce_unsized: ambiguous unsize");
                                return Err(TypeError::Mismatch);
                            }
                        }
                    } else {
                        debug!("coerce_unsized: early return - ambiguous");
                        return Err(TypeError::Mismatch);
                    }
                }
                Err(traits::Unimplemented) => {
                    debug!("coerce_unsized: early return - can't prove obligation");
                    return Err(TypeError::Mismatch);
                }

                // Object safety violations or miscellaneous.
                Err(err) => {
                    self.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err);
                    // Treat this like an obligation and follow through
                    // with the unsizing - the lack of a coercion should
                    // be silent, as it causes a type mismatch later.
                }

                Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
            }
        }

        if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
            feature_err(
                &self.tcx.sess.parse_sess,
                sym::unsized_tuple_coercion,
                self.cause.span,
                "unsized tuple coercion is not stable enough for use and is subject to change",
            )
            .emit();
        }

        if let Some((sub, sup)) = has_trait_upcasting_coercion
            && !self.tcx().features().trait_upcasting
        {
            // Renders better when we erase regions, since they're not really the point here.
            let (sub, sup) = self.tcx.erase_regions((sub, sup));
            let mut err = feature_err(
                &self.tcx.sess.parse_sess,
                sym::trait_upcasting,
                self.cause.span,
                &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
            );
            err.note(&format!("required when coercing `{source}` into `{target}`"));
            err.emit();
        }

        Ok(coercion)
    }

    fn coerce_dyn_star(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
        b_region: ty::Region<'tcx>,
    ) -> CoerceResult<'tcx> {
        if !self.tcx.features().dyn_star {
            return Err(TypeError::Mismatch);
        }

        if let ty::Dynamic(a_data, _, _) = a.kind()
            && let ty::Dynamic(b_data, _, _) = b.kind()
            && a_data.principal_def_id() == b_data.principal_def_id()
        {
            return self.unify_and(a, b, |_| vec![]);
        }

        // Check the obligations of the cast -- for example, when casting
        // `usize` to `dyn* Clone + 'static`:
        let mut obligations: Vec<_> = predicates
            .iter()
            .map(|predicate| {
                // For each existential predicate (e.g., `?Self: Clone`) substitute
                // the type of the expression (e.g., `usize` in our example above)
                // and then require that the resulting predicate (e.g., `usize: Clone`)
                // holds (it does).
                let predicate = predicate.with_self_ty(self.tcx, a);
                Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
            })
            .chain([
                // Enforce the region bound (e.g., `usize: 'static`, in our example).
                Obligation::new(
                    self.tcx,
                    self.cause.clone(),
                    self.param_env,
                    ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
                        ty::OutlivesPredicate(a, b_region),
                    ))),
                ),
            ])
            .collect();

        // Enforce that the type is `usize`/pointer-sized.
        obligations.push(Obligation::new(
            self.tcx,
            self.cause.clone(),
            self.param_env,
            ty::Binder::dummy(
                self.tcx.at(self.cause.span).mk_trait_ref(hir::LangItem::PointerSized, [a]),
            ),
        ));

        Ok(InferOk {
            value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
            obligations,
        })
    }

    fn coerce_from_safe_fn<F, G>(
        &self,
        a: Ty<'tcx>,
        fn_ty_a: ty::PolyFnSig<'tcx>,
        b: Ty<'tcx>,
        to_unsafe: F,
        normal: G,
    ) -> CoerceResult<'tcx>
    where
        F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
        G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
    {
        self.commit_if_ok(|snapshot| {
            let result = if let ty::FnPtr(fn_ty_b) = b.kind()
                && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
                    (fn_ty_a.unsafety(), fn_ty_b.unsafety())
            {
                let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
                self.unify_and(unsafe_a, b, to_unsafe)
            } else {
                self.unify_and(a, b, normal)
            };

            // FIXME(#73154): This is a hack. Currently LUB can generate
            // unsolvable constraints. Additionally, it returns `a`
            // unconditionally, even when the "LUB" is `b`. In the future, we
            // want the coerced type to be the actual supertype of these two,
            // but for now, we want to just error to ensure we don't lock
            // ourselves into a specific behavior with NLL.
            self.leak_check(false, snapshot)?;

            result
        })
    }

    fn coerce_from_fn_pointer(
        &self,
        a: Ty<'tcx>,
        fn_ty_a: ty::PolyFnSig<'tcx>,
        b: Ty<'tcx>,
    ) -> CoerceResult<'tcx> {
        //! Attempts to coerce from the type of a Rust function item
        //! into a closure or a `proc`.
        //!

        let b = self.shallow_resolve(b);
        debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);

        self.coerce_from_safe_fn(
            a,
            fn_ty_a,
            b,
            simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
            identity,
        )
    }

    fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        //! Attempts to coerce from the type of a Rust function item
        //! into a closure or a `proc`.

        let b = self.shallow_resolve(b);
        let InferOk { value: b, mut obligations } =
            self.at(&self.cause, self.param_env).normalize(b);
        debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);

        match b.kind() {
            ty::FnPtr(b_sig) => {
                let a_sig = a.fn_sig(self.tcx);
                if let ty::FnDef(def_id, _) = *a.kind() {
                    // Intrinsics are not coercible to function pointers
                    if self.tcx.is_intrinsic(def_id) {
                        return Err(TypeError::IntrinsicCast);
                    }

                    // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).

                    if b_sig.unsafety() == hir::Unsafety::Normal
                        && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
                    {
                        return Err(TypeError::TargetFeatureCast(def_id));
                    }
                }

                let InferOk { value: a_sig, obligations: o1 } =
                    self.at(&self.cause, self.param_env).normalize(a_sig);
                obligations.extend(o1);

                let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
                let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
                    a_fn_pointer,
                    a_sig,
                    b,
                    |unsafe_ty| {
                        vec![
                            Adjustment {
                                kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
                                target: a_fn_pointer,
                            },
                            Adjustment {
                                kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
                                target: unsafe_ty,
                            },
                        ]
                    },
                    simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
                )?;

                obligations.extend(o2);
                Ok(InferOk { value, obligations })
            }
            _ => self.unify_and(a, b, identity),
        }
    }

    fn coerce_closure_to_fn(
        &self,
        a: Ty<'tcx>,
        closure_def_id_a: DefId,
        substs_a: SubstsRef<'tcx>,
        b: Ty<'tcx>,
    ) -> CoerceResult<'tcx> {
        //! Attempts to coerce from the type of a non-capturing closure
        //! into a function pointer.
        //!

        let b = self.shallow_resolve(b);

        match b.kind() {
            // At this point we haven't done capture analysis, which means
            // that the ClosureSubsts just contains an inference variable instead
            // of tuple of captured types.
            //
            // All we care here is if any variable is being captured and not the exact paths,
            // so we check `upvars_mentioned` for root variables being captured.
            ty::FnPtr(fn_ty)
                if self
                    .tcx
                    .upvars_mentioned(closure_def_id_a.expect_local())
                    .map_or(true, |u| u.is_empty()) =>
            {
                // We coerce the closure, which has fn type
                //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
                // to
                //     `fn(arg0,arg1,...) -> _`
                // or
                //     `unsafe fn(arg0,arg1,...) -> _`
                let closure_sig = substs_a.as_closure().sig();
                let unsafety = fn_ty.unsafety();
                let pointer_ty =
                    self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
                debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
                self.unify_and(
                    pointer_ty,
                    b,
                    simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
                )
            }
            _ => self.unify_and(a, b, identity),
        }
    }

    fn coerce_unsafe_ptr(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        mutbl_b: hir::Mutability,
    ) -> CoerceResult<'tcx> {
        debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);

        let (is_ref, mt_a) = match *a.kind() {
            ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
            ty::RawPtr(mt) => (false, mt),
            _ => return self.unify_and(a, b, identity),
        };
        coerce_mutbls(mt_a.mutbl, mutbl_b)?;

        // Check that the types which they point at are compatible.
        let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
        // Although references and unsafe ptrs have the same
        // representation, we still register an Adjust::DerefRef so that
        // regionck knows that the region for `a` must be valid here.
        if is_ref {
            self.unify_and(a_unsafe, b, |target| {
                vec![
                    Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
                    Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
                ]
            })
        } else if mt_a.mutbl != mutbl_b {
            self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
        } else {
            self.unify_and(a_unsafe, b, identity)
        }
    }
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    /// Attempt to coerce an expression to a type, and return the
    /// adjusted type of the expression, if successful.
    /// Adjustments are only recorded if the coercion succeeded.
    /// The expressions *must not* have any pre-existing adjustments.
    pub fn try_coerce(
        &self,
        expr: &hir::Expr<'_>,
        expr_ty: Ty<'tcx>,
        target: Ty<'tcx>,
        allow_two_phase: AllowTwoPhase,
        cause: Option<ObligationCause<'tcx>>,
    ) -> RelateResult<'tcx, Ty<'tcx>> {
        let source = self.resolve_vars_with_obligations(expr_ty);
        debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);

        let cause =
            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
        let coerce = Coerce::new(self, cause, allow_two_phase);
        let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;

        let (adjustments, _) = self.register_infer_ok_obligations(ok);
        self.apply_adjustments(expr, adjustments);
        Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
    }

    /// Same as `try_coerce()`, but without side-effects.
    ///
    /// Returns false if the coercion creates any obligations that result in
    /// errors.
    pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
        let source = self.resolve_vars_with_obligations(expr_ty);
        debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);

        let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
        // We don't ever need two-phase here since we throw out the result of the coercion
        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
        self.probe(|_| {
            let Ok(ok) = coerce.coerce(source, target) else {
                return false;
            };
            let ocx = ObligationCtxt::new_in_snapshot(self);
            ocx.register_obligations(ok.obligations);
            ocx.select_where_possible().is_empty()
        })
    }

    /// Given a type and a target type, this function will calculate and return
    /// how many dereference steps needed to achieve `expr_ty <: target`. If
    /// it's not possible, return `None`.
    pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
        let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
        // We don't ever need two-phase here since we throw out the result of the coercion
        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
        coerce
            .autoderef(rustc_span::DUMMY_SP, expr_ty)
            .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
    }

    /// Given a type, this function will calculate and return the type given
    /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
    ///
    /// This function is for diagnostics only, since it does not register
    /// trait or region sub-obligations. (presumably we could, but it's not
    /// particularly important for diagnostics...)
    pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
        self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
            self.infcx
                .type_implements_trait(
                    self.tcx.lang_items().deref_mut_trait()?,
                    [expr_ty],
                    self.param_env,
                )
                .may_apply()
                .then(|| deref_ty)
        })
    }

    /// Given some expressions, their known unified type and another expression,
    /// tries to unify the types, potentially inserting coercions on any of the
    /// provided expressions and returns their LUB (aka "common supertype").
    ///
    /// This is really an internal helper. From outside the coercion
    /// module, you should instantiate a `CoerceMany` instance.
    fn try_find_coercion_lub<E>(
        &self,
        cause: &ObligationCause<'tcx>,
        exprs: &[E],
        prev_ty: Ty<'tcx>,
        new: &hir::Expr<'_>,
        new_ty: Ty<'tcx>,
    ) -> RelateResult<'tcx, Ty<'tcx>>
    where
        E: AsCoercionSite,
    {
        let prev_ty = self.resolve_vars_with_obligations(prev_ty);
        let new_ty = self.resolve_vars_with_obligations(new_ty);
        debug!(
            "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
            prev_ty,
            new_ty,
            exprs.len()
        );

        // The following check fixes #88097, where the compiler erroneously
        // attempted to coerce a closure type to itself via a function pointer.
        if prev_ty == new_ty {
            return Ok(prev_ty);
        }

        // Special-case that coercion alone cannot handle:
        // Function items or non-capturing closures of differing IDs or InternalSubsts.
        let (a_sig, b_sig) = {
            let is_capturing_closure = |ty: Ty<'tcx>| {
                if let &ty::Closure(closure_def_id, _substs) = ty.kind() {
                    self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
                } else {
                    false
                }
            };
            if is_capturing_closure(prev_ty) || is_capturing_closure(new_ty) {
                (None, None)
            } else {
                match (prev_ty.kind(), new_ty.kind()) {
                    (ty::FnDef(..), ty::FnDef(..)) => {
                        // Don't reify if the function types have a LUB, i.e., they
                        // are the same function and their parameters have a LUB.
                        match self
                            .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
                        {
                            // We have a LUB of prev_ty and new_ty, just return it.
                            Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
                            Err(_) => {
                                (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
                            }
                        }
                    }
                    (ty::Closure(_, substs), ty::FnDef(..)) => {
                        let b_sig = new_ty.fn_sig(self.tcx);
                        let a_sig = self
                            .tcx
                            .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
                        (Some(a_sig), Some(b_sig))
                    }
                    (ty::FnDef(..), ty::Closure(_, substs)) => {
                        let a_sig = prev_ty.fn_sig(self.tcx);
                        let b_sig = self
                            .tcx
                            .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
                        (Some(a_sig), Some(b_sig))
                    }
                    (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
                        Some(self.tcx.signature_unclosure(
                            substs_a.as_closure().sig(),
                            hir::Unsafety::Normal,
                        )),
                        Some(self.tcx.signature_unclosure(
                            substs_b.as_closure().sig(),
                            hir::Unsafety::Normal,
                        )),
                    ),
                    _ => (None, None),
                }
            }
        };
        if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
            // Intrinsics are not coercible to function pointers.
            if a_sig.abi() == Abi::RustIntrinsic
                || a_sig.abi() == Abi::PlatformIntrinsic
                || b_sig.abi() == Abi::RustIntrinsic
                || b_sig.abi() == Abi::PlatformIntrinsic
            {
                return Err(TypeError::IntrinsicCast);
            }
            // The signature must match.
            let (a_sig, b_sig) = self.normalize(new.span, (a_sig, b_sig));
            let sig = self
                .at(cause, self.param_env)
                .trace(prev_ty, new_ty)
                .lub(a_sig, b_sig)
                .map(|ok| self.register_infer_ok_obligations(ok))?;

            // Reify both sides and return the reified fn pointer type.
            let fn_ptr = self.tcx.mk_fn_ptr(sig);
            let prev_adjustment = match prev_ty.kind() {
                ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
                ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
                _ => unreachable!(),
            };
            let next_adjustment = match new_ty.kind() {
                ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
                ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
                _ => unreachable!(),
            };
            for expr in exprs.iter().map(|e| e.as_coercion_site()) {
                self.apply_adjustments(
                    expr,
                    vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
                );
            }
            self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
            return Ok(fn_ptr);
        }

        // Configure a Coerce instance to compute the LUB.
        // We don't allow two-phase borrows on any autorefs this creates since we
        // probably aren't processing function arguments here and even if we were,
        // they're going to get autorefed again anyway and we can apply 2-phase borrows
        // at that time.
        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
        coerce.use_lub = true;

        // First try to coerce the new expression to the type of the previous ones,
        // but only if the new expression has no coercion already applied to it.
        let mut first_error = None;
        if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
            let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
            match result {
                Ok(ok) => {
                    let (adjustments, target) = self.register_infer_ok_obligations(ok);
                    self.apply_adjustments(new, adjustments);
                    debug!(
                        "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
                        new_ty, prev_ty, target
                    );
                    return Ok(target);
                }
                Err(e) => first_error = Some(e),
            }
        }

        // Then try to coerce the previous expressions to the type of the new one.
        // This requires ensuring there are no coercions applied to *any* of the
        // previous expressions, other than noop reborrows (ignoring lifetimes).
        for expr in exprs {
            let expr = expr.as_coercion_site();
            let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
                &[
                    Adjustment { kind: Adjust::Deref(_), .. },
                    Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
                ] => {
                    match *self.node_ty(expr.hir_id).kind() {
                        ty::Ref(_, _, mt_orig) => {
                            let mutbl_adj: hir::Mutability = mutbl_adj.into();
                            // Reborrow that we can safely ignore, because
                            // the next adjustment can only be a Deref
                            // which will be merged into it.
                            mutbl_adj == mt_orig
                        }
                        _ => false,
                    }
                }
                &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
                _ => false,
            };

            if !noop {
                debug!(
                    "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
                    expr,
                );

                return self
                    .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
                    .map(|ok| self.register_infer_ok_obligations(ok));
            }
        }

        match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
            Err(_) => {
                // Avoid giving strange errors on failed attempts.
                if let Some(e) = first_error {
                    Err(e)
                } else {
                    self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
                        .map(|ok| self.register_infer_ok_obligations(ok))
                }
            }
            Ok(ok) => {
                let (adjustments, target) = self.register_infer_ok_obligations(ok);
                for expr in exprs {
                    let expr = expr.as_coercion_site();
                    self.apply_adjustments(expr, adjustments.clone());
                }
                debug!(
                    "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
                    prev_ty, new_ty, target
                );
                Ok(target)
            }
        }
    }
}

/// CoerceMany encapsulates the pattern you should use when you have
/// many expressions that are all getting coerced to a common
/// type. This arises, for example, when you have a match (the result
/// of each arm is coerced to a common type). It also arises in less
/// obvious places, such as when you have many `break foo` expressions
/// that target the same loop, or the various `return` expressions in
/// a function.
///
/// The basic protocol is as follows:
///
/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
///   This will also serve as the "starting LUB". The expectation is
///   that this type is something which all of the expressions *must*
///   be coercible to. Use a fresh type variable if needed.
/// - For each expression whose result is to be coerced, invoke `coerce()` with.
///   - In some cases we wish to coerce "non-expressions" whose types are implicitly
///     unit. This happens for example if you have a `break` with no expression,
///     or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
///   - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
///     from you so that you don't have to worry your pretty head about it.
///     But if an error is reported, the final type will be `err`.
///   - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
///     previously coerced expressions.
/// - When all done, invoke `complete()`. This will return the LUB of
///   all your expressions.
///   - WARNING: I don't believe this final type is guaranteed to be
///     related to your initial `expected_ty` in any particular way,
///     although it will typically be a subtype, so you should check it.
///   - Invoking `complete()` may cause us to go and adjust the "adjustments" on
///     previously coerced expressions.
///
/// Example:
///
/// ```ignore (illustrative)
/// let mut coerce = CoerceMany::new(expected_ty);
/// for expr in exprs {
///     let expr_ty = fcx.check_expr_with_expectation(expr, expected);
///     coerce.coerce(fcx, &cause, expr, expr_ty);
/// }
/// let final_ty = coerce.complete(fcx);
/// ```
pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
    expected_ty: Ty<'tcx>,
    final_ty: Option<Ty<'tcx>>,
    expressions: Expressions<'tcx, 'exprs, E>,
    pushed: usize,
}

/// The type of a `CoerceMany` that is storing up the expressions into
/// a buffer. We use this in `check/mod.rs` for things like `break`.
pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;

enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
    Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
    UpFront(&'exprs [E]),
}

impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
    /// The usual case; collect the set of expressions dynamically.
    /// If the full set of coercion sites is known before hand,
    /// consider `with_coercion_sites()` instead to avoid allocation.
    pub fn new(expected_ty: Ty<'tcx>) -> Self {
        Self::make(expected_ty, Expressions::Dynamic(vec![]))
    }

    /// As an optimization, you can create a `CoerceMany` with a
    /// pre-existing slice of expressions. In this case, you are
    /// expected to pass each element in the slice to `coerce(...)` in
    /// order. This is used with arrays in particular to avoid
    /// needlessly cloning the slice.
    pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
        Self::make(expected_ty, Expressions::UpFront(coercion_sites))
    }

    fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
        CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
    }

    /// Returns the "expected type" with which this coercion was
    /// constructed. This represents the "downward propagated" type
    /// that was given to us at the start of typing whatever construct
    /// we are typing (e.g., the match expression).
    ///
    /// Typically, this is used as the expected type when
    /// type-checking each of the alternative expressions whose types
    /// we are trying to merge.
    pub fn expected_ty(&self) -> Ty<'tcx> {
        self.expected_ty
    }

    /// Returns the current "merged type", representing our best-guess
    /// at the LUB of the expressions we've seen so far (if any). This
    /// isn't *final* until you call `self.complete()`, which will return
    /// the merged type.
    pub fn merged_ty(&self) -> Ty<'tcx> {
        self.final_ty.unwrap_or(self.expected_ty)
    }

    /// Indicates that the value generated by `expression`, which is
    /// of type `expression_ty`, is one of the possibilities that we
    /// could coerce from. This will record `expression`, and later
    /// calls to `coerce` may come back and add adjustments and things
    /// if necessary.
    pub fn coerce<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        expression: &'tcx hir::Expr<'tcx>,
        expression_ty: Ty<'tcx>,
    ) {
        self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
    }

    /// Indicates that one of the inputs is a "forced unit". This
    /// occurs in a case like `if foo { ... };`, where the missing else
    /// generates a "forced unit". Another example is a `loop { break;
    /// }`, where the `break` has no argument expression. We treat
    /// these cases slightly differently for error-reporting
    /// purposes. Note that these tend to correspond to cases where
    /// the `()` expression is implicit in the source, and hence we do
    /// not take an expression argument.
    ///
    /// The `augment_error` gives you a chance to extend the error
    /// message, in case any results (e.g., we use this to suggest
    /// removing a `;`).
    pub fn coerce_forced_unit<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        augment_error: &mut dyn FnMut(&mut Diagnostic),
        label_unit_as_expected: bool,
    ) {
        self.coerce_inner(
            fcx,
            cause,
            None,
            fcx.tcx.mk_unit(),
            Some(augment_error),
            label_unit_as_expected,
        )
    }

    /// The inner coercion "engine". If `expression` is `None`, this
    /// is a forced-unit case, and hence `expression_ty` must be
    /// `Nil`.
    #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
    pub(crate) fn coerce_inner<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        expression: Option<&'tcx hir::Expr<'tcx>>,
        mut expression_ty: Ty<'tcx>,
        augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
        label_expression_as_expected: bool,
    ) {
        // Incorporate whatever type inference information we have
        // until now; in principle we might also want to process
        // pending obligations, but doing so should only improve
        // compatibility (hopefully that is true) by helping us
        // uncover never types better.
        if expression_ty.is_ty_var() {
            expression_ty = fcx.infcx.shallow_resolve(expression_ty);
        }

        // If we see any error types, just propagate that error
        // upwards.
        if expression_ty.references_error() || self.merged_ty().references_error() {
            self.final_ty = Some(fcx.tcx.ty_error());
            return;
        }

        // Handle the actual type unification etc.
        let result = if let Some(expression) = expression {
            if self.pushed == 0 {
                // Special-case the first expression we are coercing.
                // To be honest, I'm not entirely sure why we do this.
                // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
                fcx.try_coerce(
                    expression,
                    expression_ty,
                    self.expected_ty,
                    AllowTwoPhase::No,
                    Some(cause.clone()),
                )
            } else {
                match self.expressions {
                    Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
                        cause,
                        exprs,
                        self.merged_ty(),
                        expression,
                        expression_ty,
                    ),
                    Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
                        cause,
                        &coercion_sites[0..self.pushed],
                        self.merged_ty(),
                        expression,
                        expression_ty,
                    ),
                }
            }
        } else {
            // this is a hack for cases where we default to `()` because
            // the expression etc has been omitted from the source. An
            // example is an `if let` without an else:
            //
            //     if let Some(x) = ... { }
            //
            // we wind up with a second match arm that is like `_ =>
            // ()`.  That is the case we are considering here. We take
            // a different path to get the right "expected, found"
            // message and so forth (and because we know that
            // `expression_ty` will be unit).
            //
            // Another example is `break` with no argument expression.
            assert!(expression_ty.is_unit(), "if let hack without unit type");
            fcx.at(cause, fcx.param_env)
                .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
                .map(|infer_ok| {
                    fcx.register_infer_ok_obligations(infer_ok);
                    expression_ty
                })
        };

        debug!(?result);
        match result {
            Ok(v) => {
                self.final_ty = Some(v);
                if let Some(e) = expression {
                    match self.expressions {
                        Expressions::Dynamic(ref mut buffer) => buffer.push(e),
                        Expressions::UpFront(coercion_sites) => {
                            // if the user gave us an array to validate, check that we got
                            // the next expression in the list, as expected
                            assert_eq!(
                                coercion_sites[self.pushed].as_coercion_site().hir_id,
                                e.hir_id
                            );
                        }
                    }
                    self.pushed += 1;
                }
            }
            Err(coercion_error) => {
                // Mark that we've failed to coerce the types here to suppress
                // any superfluous errors we might encounter while trying to
                // emit or provide suggestions on how to fix the initial error.
                fcx.set_tainted_by_errors(
                    fcx.tcx.sess.delay_span_bug(cause.span, "coercion error but no error emitted"),
                );
                let (expected, found) = if label_expression_as_expected {
                    // In the case where this is a "forced unit", like
                    // `break`, we want to call the `()` "expected"
                    // since it is implied by the syntax.
                    // (Note: not all force-units work this way.)"
                    (expression_ty, self.merged_ty())
                } else {
                    // Otherwise, the "expected" type for error
                    // reporting is the current unification type,
                    // which is basically the LUB of the expressions
                    // we've seen so far (combined with the expected
                    // type)
                    (self.merged_ty(), expression_ty)
                };
                let (expected, found) = fcx.resolve_vars_if_possible((expected, found));

                let mut err;
                let mut unsized_return = false;
                let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
                match *cause.code() {
                    ObligationCauseCode::ReturnNoExpression => {
                        err = struct_span_err!(
                            fcx.tcx.sess,
                            cause.span,
                            E0069,
                            "`return;` in a function whose return type is not `()`"
                        );
                        err.span_label(cause.span, "return type is not `()`");
                    }
                    ObligationCauseCode::BlockTailExpression(blk_id) => {
                        let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
                        err = self.report_return_mismatched_types(
                            cause,
                            expected,
                            found,
                            coercion_error.clone(),
                            fcx,
                            parent_id,
                            expression,
                            Some(blk_id),
                        );
                        if !fcx.tcx.features().unsized_locals {
                            unsized_return = self.is_return_ty_unsized(fcx, blk_id);
                        }
                        if let Some(expression) = expression
                            && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
                              intravisit::walk_block(& mut visitor, loop_blk);
                        }
                    }
                    ObligationCauseCode::ReturnValue(id) => {
                        err = self.report_return_mismatched_types(
                            cause,
                            expected,
                            found,
                            coercion_error.clone(),
                            fcx,
                            id,
                            expression,
                            None,
                        );
                        if !fcx.tcx.features().unsized_locals {
                            let id = fcx.tcx.hir().get_parent_node(id);
                            unsized_return = self.is_return_ty_unsized(fcx, id);
                        }
                    }
                    _ => {
                        err = fcx.err_ctxt().report_mismatched_types(
                            cause,
                            expected,
                            found,
                            coercion_error.clone(),
                        );
                    }
                }

                if let Some(augment_error) = augment_error {
                    augment_error(&mut err);
                }

                let is_insufficiently_polymorphic =
                    matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));

                if !is_insufficiently_polymorphic && let Some(expr) = expression {
                    fcx.emit_coerce_suggestions(
                        &mut err,
                        expr,
                        found,
                        expected,
                        None,
                        Some(coercion_error),
                    );
                }

                if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
                    self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
                }
                let reported = err.emit_unless(unsized_return);

                self.final_ty = Some(fcx.tcx.ty_error_with_guaranteed(reported));
            }
        }
    }
    fn note_unreachable_loop_return(
        &self,
        err: &mut Diagnostic,
        expr: &hir::Expr<'tcx>,
        ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
    ) {
        let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
        let mut span: MultiSpan = vec![loop_span].into();
        span.push_span_label(loop_span, "this might have zero elements to iterate on");
        const MAXITER: usize = 3;
        let iter = ret_exprs.iter().take(MAXITER);
        for ret_expr in iter {
            span.push_span_label(
                ret_expr.span,
                "if the loop doesn't execute, this value would never get returned",
            );
        }
        err.span_note(
            span,
            "the function expects a value to always be returned, but loops might run zero times",
        );
        if MAXITER < ret_exprs.len() {
            err.note(&format!(
                "if the loop doesn't execute, {} other values would never get returned",
                ret_exprs.len() - MAXITER
            ));
        }
        err.help(
            "return a value for the case when the loop has zero elements to iterate on, or \
           consider changing the return type to account for that possibility",
        );
    }

    fn report_return_mismatched_types<'a>(
        &self,
        cause: &ObligationCause<'tcx>,
        expected: Ty<'tcx>,
        found: Ty<'tcx>,
        ty_err: TypeError<'tcx>,
        fcx: &FnCtxt<'a, 'tcx>,
        id: hir::HirId,
        expression: Option<&'tcx hir::Expr<'tcx>>,
        blk_id: Option<hir::HirId>,
    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
        let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);

        let mut pointing_at_return_type = false;
        let mut fn_output = None;

        let parent_id = fcx.tcx.hir().get_parent_node(id);
        let parent = fcx.tcx.hir().get(parent_id);
        if let Some(expr) = expression
            && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
            && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
        {
            fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
        }
        // Verify that this is a tail expression of a function, otherwise the
        // label pointing out the cause for the type coercion will be wrong
        // as prior return coercions would not be relevant (#57664).
        let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
            pointing_at_return_type =
                fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
            if let (Some(cond_expr), true, false) = (
                fcx.tcx.hir().get_if_cause(expr.hir_id),
                expected.is_unit(),
                pointing_at_return_type,
            )
                // If the block is from an external macro or try (`?`) desugaring, then
                // do not suggest adding a semicolon, because there's nowhere to put it.
                // See issues #81943 and #87051.
                && matches!(
                    cond_expr.span.desugaring_kind(),
                    None | Some(DesugaringKind::WhileLoop)
                ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
                    && !matches!(
                        cond_expr.kind,
                        hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
                    )
            {
                err.span_label(cond_expr.span, "expected this to be `()`");
                if expr.can_have_side_effects() {
                    fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
                }
            }
            fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
        } else {
            fcx.get_fn_decl(parent_id)
        };

        if let Some((fn_decl, can_suggest)) = fn_decl {
            if blk_id.is_none() {
                pointing_at_return_type |= fcx.suggest_missing_return_type(
                    &mut err,
                    &fn_decl,
                    expected,
                    found,
                    can_suggest,
                    fcx.tcx.hir().get_parent_item(id).into(),
                );
            }
            if !pointing_at_return_type {
                fn_output = Some(&fn_decl.output); // `impl Trait` return type
            }
        }

        let parent_id = fcx.tcx.hir().get_parent_item(id);
        let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);

        if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
            (expression, blk_id, fcx.get_node_fn_decl(parent_item))
        {
            fcx.suggest_missing_break_or_return_expr(
                &mut err,
                expr,
                fn_decl,
                expected,
                found,
                id,
                parent_id.into(),
            );
        }

        let ret_coercion_span = fcx.ret_coercion_span.get();

        if let Some(sp) = ret_coercion_span
            // If the closure has an explicit return type annotation, or if
            // the closure's return type has been inferred from outside
            // requirements (such as an Fn* trait bound), then a type error
            // may occur at the first return expression we see in the closure
            // (if it conflicts with the declared return type). Skip adding a
            // note in this case, since it would be incorrect.
            && let Some(fn_sig) = fcx.body_fn_sig()
            && fn_sig.output().is_ty_var()
        {
            err.span_note(
                sp,
                &format!(
                    "return type inferred to be `{}` here",
                    expected
                ),
            );
        }

        if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
            self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
        }

        err
    }

    fn add_impl_trait_explanation<'a>(
        &self,
        err: &mut Diagnostic,
        cause: &ObligationCause<'tcx>,
        fcx: &FnCtxt<'a, 'tcx>,
        expected: Ty<'tcx>,
        sp: Span,
        fn_output: &hir::FnRetTy<'_>,
    ) {
        let return_sp = fn_output.span();
        err.span_label(return_sp, "expected because this return type...");
        err.span_label(
            sp,
            format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
        );
        let impl_trait_msg = "for information on `impl Trait`, see \
                <https://doc.rust-lang.org/book/ch10-02-traits.html\
                #returning-types-that-implement-traits>";
        let trait_obj_msg = "for information on trait objects, see \
                <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
                #using-trait-objects-that-allow-for-values-of-different-types>";
        err.note("to return `impl Trait`, all returned values must be of the same type");
        err.note(impl_trait_msg);
        let snippet = fcx
            .tcx
            .sess
            .source_map()
            .span_to_snippet(return_sp)
            .unwrap_or_else(|_| "dyn Trait".to_string());
        let mut snippet_iter = snippet.split_whitespace();
        let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
        // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
        let mut is_object_safe = false;
        if let hir::FnRetTy::Return(ty) = fn_output
            // Get the return type.
            && let hir::TyKind::OpaqueDef(..) = ty.kind
        {
            let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
            // Get the `impl Trait`'s `DefId`.
            if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = ty.kind()
                // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
                // get the `Trait`'s `DefId`.
                && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
                    fcx.tcx.hir().expect_item(def_id.expect_local()).kind
            {
                // Are of this `impl Trait`'s traits object safe?
                is_object_safe = bounds.iter().all(|bound| {
                    bound
                        .trait_ref()
                        .and_then(|t| t.trait_def_id())
                        .map_or(false, |def_id| {
                            fcx.tcx.object_safety_violations(def_id).is_empty()
                        })
                })
            }
        };
        if has_impl {
            if is_object_safe {
                err.multipart_suggestion(
                    "you could change the return type to be a boxed trait object",
                    vec![
                        (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
                        (return_sp.shrink_to_hi(), ">".to_string()),
                    ],
                    Applicability::MachineApplicable,
                );
                let sugg = [sp, cause.span]
                    .into_iter()
                    .flat_map(|sp| {
                        [
                            (sp.shrink_to_lo(), "Box::new(".to_string()),
                            (sp.shrink_to_hi(), ")".to_string()),
                        ]
                        .into_iter()
                    })
                    .collect::<Vec<_>>();
                err.multipart_suggestion(
                    "if you change the return type to expect trait objects, box the returned \
                     expressions",
                    sugg,
                    Applicability::MaybeIncorrect,
                );
            } else {
                err.help(&format!(
                    "if the trait `{}` were object safe, you could return a boxed trait object",
                    &snippet[5..]
                ));
            }
            err.note(trait_obj_msg);
        }
        err.help("you could instead create a new `enum` with a variant for each returned type");
    }

    fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
        if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
            && let hir::FnRetTy::Return(ty) = fn_decl.output
            && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
            && let ty::Dynamic(..) = ty.kind()
        {
            return true;
        }
        false
    }

    pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
        if let Some(final_ty) = self.final_ty {
            final_ty
        } else {
            // If we only had inputs that were of type `!` (or no
            // inputs at all), then the final type is `!`.
            assert_eq!(self.pushed, 0);
            fcx.tcx.types.never
        }
    }
}

/// Something that can be converted into an expression to which we can
/// apply a coercion.
pub trait AsCoercionSite {
    fn as_coercion_site(&self) -> &hir::Expr<'_>;
}

impl AsCoercionSite for hir::Expr<'_> {
    fn as_coercion_site(&self) -> &hir::Expr<'_> {
        self
    }
}

impl<'a, T> AsCoercionSite for &'a T
where
    T: AsCoercionSite,
{
    fn as_coercion_site(&self) -> &hir::Expr<'_> {
        (**self).as_coercion_site()
    }
}

impl AsCoercionSite for ! {
    fn as_coercion_site(&self) -> &hir::Expr<'_> {
        unreachable!()
    }
}

impl AsCoercionSite for hir::Arm<'_> {
    fn as_coercion_site(&self) -> &hir::Expr<'_> {
        &self.body
    }
}