1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
//! Error Reporting Code for the inference engine
//!
//! Because of the way inference, and in particular region inference,
//! works, it often happens that errors are not detected until far after
//! the relevant line of code has been type-checked. Therefore, there is
//! an elaborate system to track why a particular constraint in the
//! inference graph arose so that we can explain to the user what gave
//! rise to a particular error.
//!
//! The system is based around a set of "origin" types. An "origin" is the
//! reason that a constraint or inference variable arose. There are
//! different "origin" enums for different kinds of constraints/variables
//! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
//! a span, but also more information so that we can generate a meaningful
//! error message.
//!
//! Having a catalog of all the different reasons an error can arise is
//! also useful for other reasons, like cross-referencing FAQs etc, though
//! we are not really taking advantage of this yet.
//!
//! # Region Inference
//!
//! Region inference is particularly tricky because it always succeeds "in
//! the moment" and simply registers a constraint. Then, at the end, we
//! can compute the full graph and report errors, so we need to be able to
//! store and later report what gave rise to the conflicting constraints.
//!
//! # Subtype Trace
//!
//! Determining whether `T1 <: T2` often involves a number of subtypes and
//! subconstraints along the way. A "TypeTrace" is an extended version
//! of an origin that traces the types and other values that were being
//! compared. It is not necessarily comprehensive (in fact, at the time of
//! this writing it only tracks the root values being compared) but I'd
//! like to extend it to include significant "waypoints". For example, if
//! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
//! <: T4` fails, I'd like the trace to include enough information to say
//! "in the 2nd element of the tuple". Similarly, failures when comparing
//! arguments or return types in fn types should be able to cite the
//! specific position, etc.
//!
//! # Reality vs plan
//!
//! Of course, there is still a LOT of code in typeck that has yet to be
//! ported to this system, and which relies on string concatenation at the
//! time of error detection.

use super::lexical_region_resolve::RegionResolutionError;
use super::region_constraints::GenericKind;
use super::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TypeTrace, ValuePairs};

use crate::errors::{self, ObligationCauseFailureCode, TypeErrorAdditionalDiags};
use crate::infer;
use crate::infer::error_reporting::nice_region_error::find_anon_type::find_anon_type;
use crate::infer::ExpectedFound;
use crate::traits::{
    IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
    PredicateObligation,
};

use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_errors::{
    codes::*, pluralize, struct_span_code_err, Applicability, Diag, DiagCtxt, DiagStyledString,
    ErrorGuaranteed, IntoDiagArg,
};
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::intravisit::Visitor;
use rustc_hir::lang_items::LangItem;
use rustc_middle::dep_graph::DepContext;
use rustc_middle::ty::print::{with_forced_trimmed_paths, PrintError};
use rustc_middle::ty::relate::{self, RelateResult, TypeRelation};
use rustc_middle::ty::ToPredicate;
use rustc_middle::ty::{
    self, error::TypeError, IsSuggestable, List, Region, Ty, TyCtxt, TypeFoldable,
    TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
};
use rustc_span::{sym, symbol::kw, BytePos, DesugaringKind, Pos, Span};
use rustc_target::spec::abi;
use std::borrow::Cow;
use std::ops::{ControlFlow, Deref};
use std::path::PathBuf;
use std::{cmp, fmt, iter};

mod note;
mod note_and_explain;
mod suggest;

pub(crate) mod need_type_info;
pub mod sub_relations;
pub use need_type_info::TypeAnnotationNeeded;

pub mod nice_region_error;

/// Makes a valid string literal from a string by escaping special characters (" and \),
/// unless they are already escaped.
fn escape_literal(s: &str) -> String {
    let mut escaped = String::with_capacity(s.len());
    let mut chrs = s.chars().peekable();
    while let Some(first) = chrs.next() {
        match (first, chrs.peek()) {
            ('\\', Some(&delim @ '"') | Some(&delim @ '\'')) => {
                escaped.push('\\');
                escaped.push(delim);
                chrs.next();
            }
            ('"' | '\'', _) => {
                escaped.push('\\');
                escaped.push(first)
            }
            (c, _) => escaped.push(c),
        };
    }
    escaped
}

/// A helper for building type related errors. The `typeck_results`
/// field is only populated during an in-progress typeck.
/// Get an instance by calling `InferCtxt::err_ctxt` or `FnCtxt::err_ctxt`.
///
/// You must only create this if you intend to actually emit an error (or
/// perhaps a warning, though preferably not.) It provides a lot of utility
/// methods which should not be used during the happy path.
pub struct TypeErrCtxt<'a, 'tcx> {
    pub infcx: &'a InferCtxt<'tcx>,
    pub sub_relations: std::cell::RefCell<sub_relations::SubRelations>,

    pub typeck_results: Option<std::cell::Ref<'a, ty::TypeckResults<'tcx>>>,
    pub fallback_has_occurred: bool,

    pub normalize_fn_sig: Box<dyn Fn(ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx> + 'a>,

    pub autoderef_steps:
        Box<dyn Fn(Ty<'tcx>) -> Vec<(Ty<'tcx>, Vec<PredicateObligation<'tcx>>)> + 'a>,
}

impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
    pub fn dcx(&self) -> &'tcx DiagCtxt {
        self.infcx.tcx.dcx()
    }

    /// This is just to avoid a potential footgun of accidentally
    /// dropping `typeck_results` by calling `InferCtxt::err_ctxt`
    #[deprecated(note = "you already have a `TypeErrCtxt`")]
    #[allow(unused)]
    pub fn err_ctxt(&self) -> ! {
        bug!("called `err_ctxt` on `TypeErrCtxt`. Try removing the call");
    }
}

impl<'tcx> Deref for TypeErrCtxt<'_, 'tcx> {
    type Target = InferCtxt<'tcx>;
    fn deref(&self) -> &InferCtxt<'tcx> {
        self.infcx
    }
}

pub(super) fn note_and_explain_region<'tcx>(
    tcx: TyCtxt<'tcx>,
    err: &mut Diag<'_>,
    prefix: &str,
    region: ty::Region<'tcx>,
    suffix: &str,
    alt_span: Option<Span>,
) {
    let (description, span) = match *region {
        ty::ReEarlyParam(_) | ty::ReLateParam(_) | ty::RePlaceholder(_) | ty::ReStatic => {
            msg_span_from_named_region(tcx, region, alt_span)
        }

        ty::ReError(_) => return,

        // We shouldn't really be having unification failures with ReVar
        // and ReBound though.
        //
        // FIXME(@lcnr): Figure out whether this is reachable and if so, why.
        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => (format!("lifetime `{region}`"), alt_span),
    };

    emit_msg_span(err, prefix, description, span, suffix);
}

fn explain_free_region<'tcx>(
    tcx: TyCtxt<'tcx>,
    err: &mut Diag<'_>,
    prefix: &str,
    region: ty::Region<'tcx>,
    suffix: &str,
) {
    let (description, span) = msg_span_from_named_region(tcx, region, None);

    label_msg_span(err, prefix, description, span, suffix);
}

fn msg_span_from_named_region<'tcx>(
    tcx: TyCtxt<'tcx>,
    region: ty::Region<'tcx>,
    alt_span: Option<Span>,
) -> (String, Option<Span>) {
    match *region {
        ty::ReEarlyParam(ref br) => {
            let scope = region.free_region_binding_scope(tcx).expect_local();
            let span = if let Some(param) =
                tcx.hir().get_generics(scope).and_then(|generics| generics.get_named(br.name))
            {
                param.span
            } else {
                tcx.def_span(scope)
            };
            let text = if br.has_name() {
                format!("the lifetime `{}` as defined here", br.name)
            } else {
                "the anonymous lifetime as defined here".to_string()
            };
            (text, Some(span))
        }
        ty::ReLateParam(ref fr) => {
            if !fr.bound_region.is_named()
                && let Some((ty, _)) = find_anon_type(tcx, region, &fr.bound_region)
            {
                ("the anonymous lifetime defined here".to_string(), Some(ty.span))
            } else {
                let scope = region.free_region_binding_scope(tcx).expect_local();
                match fr.bound_region {
                    ty::BoundRegionKind::BrNamed(_, name) => {
                        let span = if let Some(param) = tcx
                            .hir()
                            .get_generics(scope)
                            .and_then(|generics| generics.get_named(name))
                        {
                            param.span
                        } else {
                            tcx.def_span(scope)
                        };
                        let text = if name == kw::UnderscoreLifetime {
                            "the anonymous lifetime as defined here".to_string()
                        } else {
                            format!("the lifetime `{name}` as defined here")
                        };
                        (text, Some(span))
                    }
                    ty::BrAnon => (
                        "the anonymous lifetime as defined here".to_string(),
                        Some(tcx.def_span(scope)),
                    ),
                    _ => (
                        format!("the lifetime `{region}` as defined here"),
                        Some(tcx.def_span(scope)),
                    ),
                }
            }
        }
        ty::ReStatic => ("the static lifetime".to_owned(), alt_span),
        ty::RePlaceholder(ty::PlaceholderRegion {
            bound: ty::BoundRegion { kind: ty::BoundRegionKind::BrNamed(def_id, name), .. },
            ..
        }) => (format!("the lifetime `{name}` as defined here"), Some(tcx.def_span(def_id))),
        ty::RePlaceholder(ty::PlaceholderRegion {
            bound: ty::BoundRegion { kind: ty::BoundRegionKind::BrAnon, .. },
            ..
        }) => ("an anonymous lifetime".to_owned(), None),
        _ => bug!("{:?}", region),
    }
}

fn emit_msg_span(
    err: &mut Diag<'_>,
    prefix: &str,
    description: String,
    span: Option<Span>,
    suffix: &str,
) {
    let message = format!("{prefix}{description}{suffix}");

    if let Some(span) = span {
        err.span_note(span, message);
    } else {
        err.note(message);
    }
}

fn label_msg_span(
    err: &mut Diag<'_>,
    prefix: &str,
    description: String,
    span: Option<Span>,
    suffix: &str,
) {
    let message = format!("{prefix}{description}{suffix}");

    if let Some(span) = span {
        err.span_label(span, message);
    } else {
        err.note(message);
    }
}

#[instrument(level = "trace", skip(tcx))]
pub fn unexpected_hidden_region_diagnostic<'tcx>(
    tcx: TyCtxt<'tcx>,
    span: Span,
    hidden_ty: Ty<'tcx>,
    hidden_region: ty::Region<'tcx>,
    opaque_ty_key: ty::OpaqueTypeKey<'tcx>,
) -> Diag<'tcx> {
    let mut err = tcx.dcx().create_err(errors::OpaqueCapturesLifetime {
        span,
        opaque_ty: Ty::new_opaque(tcx, opaque_ty_key.def_id.to_def_id(), opaque_ty_key.args),
        opaque_ty_span: tcx.def_span(opaque_ty_key.def_id),
    });

    // Explain the region we are capturing.
    match *hidden_region {
        ty::ReEarlyParam(_) | ty::ReLateParam(_) | ty::ReStatic => {
            // Assuming regionck succeeded (*), we ought to always be
            // capturing *some* region from the fn header, and hence it
            // ought to be free. So under normal circumstances, we will go
            // down this path which gives a decent human readable
            // explanation.
            //
            // (*) if not, the `tainted_by_errors` field would be set to
            // `Some(ErrorGuaranteed)` in any case, so we wouldn't be here at all.
            explain_free_region(
                tcx,
                &mut err,
                &format!("hidden type `{hidden_ty}` captures "),
                hidden_region,
                "",
            );
            if let Some(reg_info) = tcx.is_suitable_region(hidden_region) {
                let fn_returns = tcx.return_type_impl_or_dyn_traits(reg_info.def_id);
                nice_region_error::suggest_new_region_bound(
                    tcx,
                    &mut err,
                    fn_returns,
                    hidden_region.to_string(),
                    None,
                    format!("captures `{hidden_region}`"),
                    None,
                    Some(reg_info.def_id),
                )
            }
        }
        ty::RePlaceholder(_) => {
            explain_free_region(
                tcx,
                &mut err,
                &format!("hidden type `{}` captures ", hidden_ty),
                hidden_region,
                "",
            );
        }
        ty::ReError(_) => {
            err.downgrade_to_delayed_bug();
        }
        _ => {
            // Ugh. This is a painful case: the hidden region is not one
            // that we can easily summarize or explain. This can happen
            // in a case like
            // `tests/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
            //
            // ```
            // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
            //   if condition() { a } else { b }
            // }
            // ```
            //
            // Here the captured lifetime is the intersection of `'a` and
            // `'b`, which we can't quite express.

            // We can at least report a really cryptic error for now.
            note_and_explain_region(
                tcx,
                &mut err,
                &format!("hidden type `{hidden_ty}` captures "),
                hidden_region,
                "",
                None,
            );
        }
    }

    err
}

impl<'tcx> InferCtxt<'tcx> {
    pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
        let (def_id, args) = match *ty.kind() {
            ty::Alias(_, ty::AliasTy { def_id, args, .. })
                if matches!(self.tcx.def_kind(def_id), DefKind::OpaqueTy) =>
            {
                (def_id, args)
            }
            ty::Alias(_, ty::AliasTy { def_id, args, .. })
                if self.tcx.is_impl_trait_in_trait(def_id) =>
            {
                (def_id, args)
            }
            _ => return None,
        };

        let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
        let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];

        self.tcx.explicit_item_bounds(def_id).iter_instantiated_copied(self.tcx, args).find_map(
            |(predicate, _)| {
                predicate
                    .kind()
                    .map_bound(|kind| match kind {
                        ty::ClauseKind::Projection(projection_predicate)
                            if projection_predicate.projection_ty.def_id == item_def_id =>
                        {
                            projection_predicate.term.ty()
                        }
                        _ => None,
                    })
                    .no_bound_vars()
                    .flatten()
            },
        )
    }
}

impl<'tcx> TypeErrCtxt<'_, 'tcx> {
    pub fn report_region_errors(
        &self,
        generic_param_scope: LocalDefId,
        errors: &[RegionResolutionError<'tcx>],
    ) -> ErrorGuaranteed {
        assert!(!errors.is_empty());

        if let Some(guaranteed) = self.infcx.tainted_by_errors() {
            return guaranteed;
        }

        debug!("report_region_errors(): {} errors to start", errors.len());

        // try to pre-process the errors, which will group some of them
        // together into a `ProcessedErrors` group:
        let errors = self.process_errors(errors);

        debug!("report_region_errors: {} errors after preprocessing", errors.len());

        let mut guar = None;
        for error in errors {
            debug!("report_region_errors: error = {:?}", error);

            guar = Some(if let Some(guar) = self.try_report_nice_region_error(&error) {
                guar
            } else {
                match error.clone() {
                    // These errors could indicate all manner of different
                    // problems with many different solutions. Rather
                    // than generate a "one size fits all" error, what we
                    // attempt to do is go through a number of specific
                    // scenarios and try to find the best way to present
                    // the error. If all of these fails, we fall back to a rather
                    // general bit of code that displays the error information
                    RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
                        if sub.is_placeholder() || sup.is_placeholder() {
                            self.report_placeholder_failure(origin, sub, sup).emit()
                        } else {
                            self.report_concrete_failure(origin, sub, sup).emit()
                        }
                    }

                    RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => self
                        .report_generic_bound_failure(
                            generic_param_scope,
                            origin.span(),
                            Some(origin),
                            param_ty,
                            sub,
                        ),

                    RegionResolutionError::SubSupConflict(
                        _,
                        var_origin,
                        sub_origin,
                        sub_r,
                        sup_origin,
                        sup_r,
                        _,
                    ) => {
                        if sub_r.is_placeholder() {
                            self.report_placeholder_failure(sub_origin, sub_r, sup_r).emit()
                        } else if sup_r.is_placeholder() {
                            self.report_placeholder_failure(sup_origin, sub_r, sup_r).emit()
                        } else {
                            self.report_sub_sup_conflict(
                                var_origin, sub_origin, sub_r, sup_origin, sup_r,
                            )
                        }
                    }

                    RegionResolutionError::UpperBoundUniverseConflict(
                        _,
                        _,
                        _,
                        sup_origin,
                        sup_r,
                    ) => {
                        assert!(sup_r.is_placeholder());

                        // Make a dummy value for the "sub region" --
                        // this is the initial value of the
                        // placeholder. In practice, we expect more
                        // tailored errors that don't really use this
                        // value.
                        let sub_r = self.tcx.lifetimes.re_erased;

                        self.report_placeholder_failure(sup_origin, sub_r, sup_r).emit()
                    }

                    RegionResolutionError::CannotNormalize(clause, origin) => {
                        let clause: ty::Clause<'tcx> =
                            clause.map_bound(ty::ClauseKind::TypeOutlives).to_predicate(self.tcx);
                        self.tcx
                            .dcx()
                            .struct_span_err(origin.span(), format!("cannot normalize `{clause}`"))
                            .emit()
                    }
                }
            })
        }

        guar.unwrap()
    }

    // This method goes through all the errors and try to group certain types
    // of error together, for the purpose of suggesting explicit lifetime
    // parameters to the user. This is done so that we can have a more
    // complete view of what lifetimes should be the same.
    // If the return value is an empty vector, it means that processing
    // failed (so the return value of this method should not be used).
    //
    // The method also attempts to weed out messages that seem like
    // duplicates that will be unhelpful to the end-user. But
    // obviously it never weeds out ALL errors.
    fn process_errors(
        &self,
        errors: &[RegionResolutionError<'tcx>],
    ) -> Vec<RegionResolutionError<'tcx>> {
        debug!("process_errors()");

        // We want to avoid reporting generic-bound failures if we can
        // avoid it: these have a very high rate of being unhelpful in
        // practice. This is because they are basically secondary
        // checks that test the state of the region graph after the
        // rest of inference is done, and the other kinds of errors
        // indicate that the region constraint graph is internally
        // inconsistent, so these test results are likely to be
        // meaningless.
        //
        // Therefore, we filter them out of the list unless they are
        // the only thing in the list.

        let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
            RegionResolutionError::GenericBoundFailure(..) => true,
            RegionResolutionError::ConcreteFailure(..)
            | RegionResolutionError::SubSupConflict(..)
            | RegionResolutionError::UpperBoundUniverseConflict(..)
            | RegionResolutionError::CannotNormalize(..) => false,
        };

        let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
            errors.to_owned()
        } else {
            errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
        };

        // sort the errors by span, for better error message stability.
        errors.sort_by_key(|u| match *u {
            RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
            RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
            RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _, _) => rvo.span(),
            RegionResolutionError::UpperBoundUniverseConflict(_, ref rvo, _, _, _) => rvo.span(),
            RegionResolutionError::CannotNormalize(_, ref sro) => sro.span(),
        });
        errors
    }

    /// Adds a note if the types come from similarly named crates
    fn check_and_note_conflicting_crates(&self, err: &mut Diag<'_>, terr: TypeError<'tcx>) {
        use hir::def_id::CrateNum;
        use rustc_hir::definitions::DisambiguatedDefPathData;
        use ty::print::Printer;
        use ty::GenericArg;

        struct AbsolutePathPrinter<'tcx> {
            tcx: TyCtxt<'tcx>,
            segments: Vec<String>,
        }

        impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
            fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
                self.tcx
            }

            fn print_region(&mut self, _region: ty::Region<'_>) -> Result<(), PrintError> {
                Err(fmt::Error)
            }

            fn print_type(&mut self, _ty: Ty<'tcx>) -> Result<(), PrintError> {
                Err(fmt::Error)
            }

            fn print_dyn_existential(
                &mut self,
                _predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
            ) -> Result<(), PrintError> {
                Err(fmt::Error)
            }

            fn print_const(&mut self, _ct: ty::Const<'tcx>) -> Result<(), PrintError> {
                Err(fmt::Error)
            }

            fn path_crate(&mut self, cnum: CrateNum) -> Result<(), PrintError> {
                self.segments = vec![self.tcx.crate_name(cnum).to_string()];
                Ok(())
            }
            fn path_qualified(
                &mut self,
                _self_ty: Ty<'tcx>,
                _trait_ref: Option<ty::TraitRef<'tcx>>,
            ) -> Result<(), PrintError> {
                Err(fmt::Error)
            }

            fn path_append_impl(
                &mut self,
                _print_prefix: impl FnOnce(&mut Self) -> Result<(), PrintError>,
                _disambiguated_data: &DisambiguatedDefPathData,
                _self_ty: Ty<'tcx>,
                _trait_ref: Option<ty::TraitRef<'tcx>>,
            ) -> Result<(), PrintError> {
                Err(fmt::Error)
            }
            fn path_append(
                &mut self,
                print_prefix: impl FnOnce(&mut Self) -> Result<(), PrintError>,
                disambiguated_data: &DisambiguatedDefPathData,
            ) -> Result<(), PrintError> {
                print_prefix(self)?;
                self.segments.push(disambiguated_data.to_string());
                Ok(())
            }
            fn path_generic_args(
                &mut self,
                print_prefix: impl FnOnce(&mut Self) -> Result<(), PrintError>,
                _args: &[GenericArg<'tcx>],
            ) -> Result<(), PrintError> {
                print_prefix(self)
            }
        }

        let report_path_match = |err: &mut Diag<'_>, did1: DefId, did2: DefId| {
            // Only report definitions from different crates. If both definitions
            // are from a local module we could have false positives, e.g.
            // let _ = [{struct Foo; Foo}, {struct Foo; Foo}];
            if did1.krate != did2.krate {
                let abs_path = |def_id| {
                    let mut printer = AbsolutePathPrinter { tcx: self.tcx, segments: vec![] };
                    printer.print_def_path(def_id, &[]).map(|_| printer.segments)
                };

                // We compare strings because DefPath can be different
                // for imported and non-imported crates
                let same_path = || -> Result<_, PrintError> {
                    Ok(self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2)
                        || abs_path(did1)? == abs_path(did2)?)
                };
                if same_path().unwrap_or(false) {
                    let crate_name = self.tcx.crate_name(did1.krate);
                    let msg = if did1.is_local() || did2.is_local() {
                        format!(
                            "the crate `{crate_name}` is compiled multiple times, possibly with different configurations"
                        )
                    } else {
                        format!(
                            "perhaps two different versions of crate `{crate_name}` are being used?"
                        )
                    };
                    err.note(msg);
                }
            }
        };
        match terr {
            TypeError::Sorts(ref exp_found) => {
                // if they are both "path types", there's a chance of ambiguity
                // due to different versions of the same crate
                if let (&ty::Adt(exp_adt, _), &ty::Adt(found_adt, _)) =
                    (exp_found.expected.kind(), exp_found.found.kind())
                {
                    report_path_match(err, exp_adt.did(), found_adt.did());
                }
            }
            TypeError::Traits(ref exp_found) => {
                report_path_match(err, exp_found.expected, exp_found.found);
            }
            _ => (), // FIXME(#22750) handle traits and stuff
        }
    }

    fn note_error_origin(
        &self,
        err: &mut Diag<'_>,
        cause: &ObligationCause<'tcx>,
        exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
        terr: TypeError<'tcx>,
    ) {
        match *cause.code() {
            ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
                let ty = self.resolve_vars_if_possible(root_ty);
                if !matches!(ty.kind(), ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)))
                {
                    // don't show type `_`
                    if span.desugaring_kind() == Some(DesugaringKind::ForLoop)
                        && let ty::Adt(def, args) = ty.kind()
                        && Some(def.did()) == self.tcx.get_diagnostic_item(sym::Option)
                    {
                        err.span_label(
                            span,
                            format!("this is an iterator with items of type `{}`", args.type_at(0)),
                        );
                    } else {
                        err.span_label(span, format!("this expression has type `{ty}`"));
                    }
                }
                if let Some(ty::error::ExpectedFound { found, .. }) = exp_found
                    && ty.is_box()
                    && ty.boxed_ty() == found
                    && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
                {
                    err.span_suggestion(
                        span,
                        "consider dereferencing the boxed value",
                        format!("*{snippet}"),
                        Applicability::MachineApplicable,
                    );
                }
            }
            ObligationCauseCode::Pattern { origin_expr: false, span: Some(span), .. } => {
                err.span_label(span, "expected due to this");
            }
            ObligationCauseCode::BlockTailExpression(
                _,
                hir::MatchSource::TryDesugar(scrut_hir_id),
            ) => {
                if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
                    let scrut_expr = self.tcx.hir().expect_expr(scrut_hir_id);
                    let scrut_ty = if let hir::ExprKind::Call(_, args) = &scrut_expr.kind {
                        let arg_expr = args.first().expect("try desugaring call w/out arg");
                        self.typeck_results
                            .as_ref()
                            .and_then(|typeck_results| typeck_results.expr_ty_opt(arg_expr))
                    } else {
                        bug!("try desugaring w/out call expr as scrutinee");
                    };

                    match scrut_ty {
                        Some(ty) if expected == ty => {
                            let source_map = self.tcx.sess.source_map();
                            err.span_suggestion(
                                source_map.end_point(cause.span()),
                                "try removing this `?`",
                                "",
                                Applicability::MachineApplicable,
                            );
                        }
                        _ => {}
                    }
                }
            }
            ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                arm_block_id,
                arm_span,
                arm_ty,
                prior_arm_block_id,
                prior_arm_span,
                prior_arm_ty,
                source,
                ref prior_non_diverging_arms,
                opt_suggest_box_span,
                scrut_span,
                ..
            }) => match source {
                hir::MatchSource::TryDesugar(scrut_hir_id) => {
                    if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
                        let scrut_expr = self.tcx.hir().expect_expr(scrut_hir_id);
                        let scrut_ty = if let hir::ExprKind::Call(_, args) = &scrut_expr.kind {
                            let arg_expr = args.first().expect("try desugaring call w/out arg");
                            self.typeck_results
                                .as_ref()
                                .and_then(|typeck_results| typeck_results.expr_ty_opt(arg_expr))
                        } else {
                            bug!("try desugaring w/out call expr as scrutinee");
                        };

                        match scrut_ty {
                            Some(ty) if expected == ty => {
                                let source_map = self.tcx.sess.source_map();
                                err.span_suggestion(
                                    source_map.end_point(cause.span()),
                                    "try removing this `?`",
                                    "",
                                    Applicability::MachineApplicable,
                                );
                            }
                            _ => {}
                        }
                    }
                }
                _ => {
                    // `prior_arm_ty` can be `!`, `expected` will have better info when present.
                    let t = self.resolve_vars_if_possible(match exp_found {
                        Some(ty::error::ExpectedFound { expected, .. }) => expected,
                        _ => prior_arm_ty,
                    });
                    let source_map = self.tcx.sess.source_map();
                    let mut any_multiline_arm = source_map.is_multiline(arm_span);
                    if prior_non_diverging_arms.len() <= 4 {
                        for sp in prior_non_diverging_arms {
                            any_multiline_arm |= source_map.is_multiline(*sp);
                            err.span_label(*sp, format!("this is found to be of type `{t}`"));
                        }
                    } else if let Some(sp) = prior_non_diverging_arms.last() {
                        any_multiline_arm |= source_map.is_multiline(*sp);
                        err.span_label(
                            *sp,
                            format!("this and all prior arms are found to be of type `{t}`"),
                        );
                    }
                    let outer = if any_multiline_arm || !source_map.is_multiline(cause.span) {
                        // Cover just `match` and the scrutinee expression, not
                        // the entire match body, to reduce diagram noise.
                        cause.span.shrink_to_lo().to(scrut_span)
                    } else {
                        cause.span
                    };
                    let msg = "`match` arms have incompatible types";
                    err.span_label(outer, msg);
                    if let Some(subdiag) = self.suggest_remove_semi_or_return_binding(
                        prior_arm_block_id,
                        prior_arm_ty,
                        prior_arm_span,
                        arm_block_id,
                        arm_ty,
                        arm_span,
                    ) {
                        err.subdiagnostic(self.dcx(), subdiag);
                    }
                    if let Some(ret_sp) = opt_suggest_box_span {
                        // Get return type span and point to it.
                        self.suggest_boxing_for_return_impl_trait(
                            err,
                            ret_sp,
                            prior_non_diverging_arms
                                .iter()
                                .chain(std::iter::once(&arm_span))
                                .copied(),
                        );
                    }
                }
            },
            ObligationCauseCode::IfExpression(box IfExpressionCause {
                then_id,
                else_id,
                then_ty,
                else_ty,
                outer_span,
                opt_suggest_box_span,
            }) => {
                let then_span = self.find_block_span_from_hir_id(then_id);
                let else_span = self.find_block_span_from_hir_id(else_id);
                err.span_label(then_span, "expected because of this");
                if let Some(sp) = outer_span {
                    err.span_label(sp, "`if` and `else` have incompatible types");
                }
                if let Some(subdiag) = self.suggest_remove_semi_or_return_binding(
                    Some(then_id),
                    then_ty,
                    then_span,
                    Some(else_id),
                    else_ty,
                    else_span,
                ) {
                    err.subdiagnostic(self.dcx(), subdiag);
                }
                // don't suggest wrapping either blocks in `if .. {} else {}`
                let is_empty_arm = |id| {
                    let hir::Node::Block(blk) = self.tcx.hir_node(id) else {
                        return false;
                    };
                    if blk.expr.is_some() || !blk.stmts.is_empty() {
                        return false;
                    }
                    let Some((_, hir::Node::Expr(expr))) = self.tcx.hir().parent_iter(id).nth(1)
                    else {
                        return false;
                    };
                    matches!(expr.kind, hir::ExprKind::If(..))
                };
                if let Some(ret_sp) = opt_suggest_box_span
                    && !is_empty_arm(then_id)
                    && !is_empty_arm(else_id)
                {
                    self.suggest_boxing_for_return_impl_trait(
                        err,
                        ret_sp,
                        [then_span, else_span].into_iter(),
                    );
                }
            }
            ObligationCauseCode::LetElse => {
                err.help("try adding a diverging expression, such as `return` or `panic!(..)`");
                err.help("...or use `match` instead of `let...else`");
            }
            _ => {
                if let ObligationCauseCode::BindingObligation(_, span)
                | ObligationCauseCode::ExprBindingObligation(_, span, ..) =
                    cause.code().peel_derives()
                    && let TypeError::RegionsPlaceholderMismatch = terr
                {
                    err.span_note(*span, "the lifetime requirement is introduced here");
                }
            }
        }
    }

    /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
    /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
    /// populate `other_value` with `other_ty`.
    ///
    /// ```text
    /// Foo<Bar<Qux>>
    /// ^^^^--------^ this is highlighted
    /// |   |
    /// |   this type argument is exactly the same as the other type, not highlighted
    /// this is highlighted
    /// Bar<Qux>
    /// -------- this type is the same as a type argument in the other type, not highlighted
    /// ```
    fn highlight_outer(
        &self,
        value: &mut DiagStyledString,
        other_value: &mut DiagStyledString,
        name: String,
        sub: ty::GenericArgsRef<'tcx>,
        pos: usize,
        other_ty: Ty<'tcx>,
    ) {
        // `value` and `other_value` hold two incomplete type representation for display.
        // `name` is the path of both types being compared. `sub`
        value.push_highlighted(name);
        let len = sub.len();
        if len > 0 {
            value.push_highlighted("<");
        }

        // Output the lifetimes for the first type
        let lifetimes = sub
            .regions()
            .map(|lifetime| {
                let s = lifetime.to_string();
                if s.is_empty() { "'_".to_string() } else { s }
            })
            .collect::<Vec<_>>()
            .join(", ");
        if !lifetimes.is_empty() {
            if sub.regions().count() < len {
                value.push_normal(lifetimes + ", ");
            } else {
                value.push_normal(lifetimes);
            }
        }

        // Highlight all the type arguments that aren't at `pos` and compare the type argument at
        // `pos` and `other_ty`.
        for (i, type_arg) in sub.types().enumerate() {
            if i == pos {
                let values = self.cmp(type_arg, other_ty);
                value.0.extend((values.0).0);
                other_value.0.extend((values.1).0);
            } else {
                value.push_highlighted(type_arg.to_string());
            }

            if len > 0 && i != len - 1 {
                value.push_normal(", ");
            }
        }
        if len > 0 {
            value.push_highlighted(">");
        }
    }

    /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
    /// as that is the difference to the other type.
    ///
    /// For the following code:
    ///
    /// ```ignore (illustrative)
    /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
    /// ```
    ///
    /// The type error output will behave in the following way:
    ///
    /// ```text
    /// Foo<Bar<Qux>>
    /// ^^^^--------^ this is highlighted
    /// |   |
    /// |   this type argument is exactly the same as the other type, not highlighted
    /// this is highlighted
    /// Bar<Qux>
    /// -------- this type is the same as a type argument in the other type, not highlighted
    /// ```
    fn cmp_type_arg(
        &self,
        t1_out: &mut DiagStyledString,
        t2_out: &mut DiagStyledString,
        path: String,
        sub: &'tcx [ty::GenericArg<'tcx>],
        other_path: String,
        other_ty: Ty<'tcx>,
    ) -> Option<()> {
        // FIXME/HACK: Go back to `GenericArgsRef` to use its inherent methods,
        // ideally that shouldn't be necessary.
        let sub = self.tcx.mk_args(sub);
        for (i, ta) in sub.types().enumerate() {
            if ta == other_ty {
                self.highlight_outer(t1_out, t2_out, path, sub, i, other_ty);
                return Some(());
            }
            if let ty::Adt(def, _) = ta.kind() {
                let path_ = self.tcx.def_path_str(def.did());
                if path_ == other_path {
                    self.highlight_outer(t1_out, t2_out, path, sub, i, other_ty);
                    return Some(());
                }
            }
        }
        None
    }

    /// Adds a `,` to the type representation only if it is appropriate.
    fn push_comma(
        &self,
        value: &mut DiagStyledString,
        other_value: &mut DiagStyledString,
        len: usize,
        pos: usize,
    ) {
        if len > 0 && pos != len - 1 {
            value.push_normal(", ");
            other_value.push_normal(", ");
        }
    }

    /// Given two `fn` signatures highlight only sub-parts that are different.
    fn cmp_fn_sig(
        &self,
        sig1: &ty::PolyFnSig<'tcx>,
        sig2: &ty::PolyFnSig<'tcx>,
    ) -> (DiagStyledString, DiagStyledString) {
        let sig1 = &(self.normalize_fn_sig)(*sig1);
        let sig2 = &(self.normalize_fn_sig)(*sig2);

        let get_lifetimes = |sig| {
            use rustc_hir::def::Namespace;
            let (sig, reg) = ty::print::FmtPrinter::new(self.tcx, Namespace::TypeNS)
                .name_all_regions(sig)
                .unwrap();
            let lts: Vec<String> = reg.into_values().map(|kind| kind.to_string()).collect();
            (if lts.is_empty() { String::new() } else { format!("for<{}> ", lts.join(", ")) }, sig)
        };

        let (lt1, sig1) = get_lifetimes(sig1);
        let (lt2, sig2) = get_lifetimes(sig2);

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        let mut values =
            (DiagStyledString::normal("".to_string()), DiagStyledString::normal("".to_string()));

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        // ^^^^^^
        values.0.push(sig1.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
        values.1.push(sig2.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //        ^^^^^^^^^^
        if sig1.abi != abi::Abi::Rust {
            values.0.push(format!("extern {} ", sig1.abi), sig1.abi != sig2.abi);
        }
        if sig2.abi != abi::Abi::Rust {
            values.1.push(format!("extern {} ", sig2.abi), sig1.abi != sig2.abi);
        }

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //                   ^^^^^^^^
        let lifetime_diff = lt1 != lt2;
        values.0.push(lt1, lifetime_diff);
        values.1.push(lt2, lifetime_diff);

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //                           ^^^
        values.0.push_normal("fn(");
        values.1.push_normal("fn(");

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //                              ^^^^^
        let len1 = sig1.inputs().len();
        let len2 = sig2.inputs().len();
        if len1 == len2 {
            for (i, (l, r)) in iter::zip(sig1.inputs(), sig2.inputs()).enumerate() {
                let (x1, x2) = self.cmp(*l, *r);
                (values.0).0.extend(x1.0);
                (values.1).0.extend(x2.0);
                self.push_comma(&mut values.0, &mut values.1, len1, i);
            }
        } else {
            for (i, l) in sig1.inputs().iter().enumerate() {
                values.0.push_highlighted(l.to_string());
                if i != len1 - 1 {
                    values.0.push_highlighted(", ");
                }
            }
            for (i, r) in sig2.inputs().iter().enumerate() {
                values.1.push_highlighted(r.to_string());
                if i != len2 - 1 {
                    values.1.push_highlighted(", ");
                }
            }
        }

        if sig1.c_variadic {
            if len1 > 0 {
                values.0.push_normal(", ");
            }
            values.0.push("...", !sig2.c_variadic);
        }
        if sig2.c_variadic {
            if len2 > 0 {
                values.1.push_normal(", ");
            }
            values.1.push("...", !sig1.c_variadic);
        }

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //                                   ^
        values.0.push_normal(")");
        values.1.push_normal(")");

        // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
        //                                     ^^^^^^^^
        let output1 = sig1.output();
        let output2 = sig2.output();
        let (x1, x2) = self.cmp(output1, output2);
        if !output1.is_unit() {
            values.0.push_normal(" -> ");
            (values.0).0.extend(x1.0);
        }
        if !output2.is_unit() {
            values.1.push_normal(" -> ");
            (values.1).0.extend(x2.0);
        }
        values
    }

    /// Compares two given types, eliding parts that are the same between them and highlighting
    /// relevant differences, and return two representation of those types for highlighted printing.
    pub fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagStyledString, DiagStyledString) {
        debug!("cmp(t1={}, t1.kind={:?}, t2={}, t2.kind={:?})", t1, t1.kind(), t2, t2.kind());

        // helper functions
        let recurse = |t1, t2, values: &mut (DiagStyledString, DiagStyledString)| {
            let (x1, x2) = self.cmp(t1, t2);
            (values.0).0.extend(x1.0);
            (values.1).0.extend(x2.0);
        };

        fn fmt_region<'tcx>(region: ty::Region<'tcx>) -> String {
            let mut r = region.to_string();
            if r == "'_" {
                r.clear();
            } else {
                r.push(' ');
            }
            format!("&{r}")
        }

        fn push_ref<'tcx>(
            region: ty::Region<'tcx>,
            mutbl: hir::Mutability,
            s: &mut DiagStyledString,
        ) {
            s.push_highlighted(fmt_region(region));
            s.push_highlighted(mutbl.prefix_str());
        }

        fn maybe_highlight<T: Eq + ToString>(
            t1: T,
            t2: T,
            (buf1, buf2): &mut (DiagStyledString, DiagStyledString),
            tcx: TyCtxt<'_>,
        ) {
            let highlight = t1 != t2;
            let (t1, t2) = if highlight || tcx.sess.opts.verbose {
                (t1.to_string(), t2.to_string())
            } else {
                // The two types are the same, elide and don't highlight.
                ("_".into(), "_".into())
            };
            buf1.push(t1, highlight);
            buf2.push(t2, highlight);
        }

        fn cmp_ty_refs<'tcx>(
            r1: ty::Region<'tcx>,
            mut1: hir::Mutability,
            r2: ty::Region<'tcx>,
            mut2: hir::Mutability,
            ss: &mut (DiagStyledString, DiagStyledString),
        ) {
            let (r1, r2) = (fmt_region(r1), fmt_region(r2));
            if r1 != r2 {
                ss.0.push_highlighted(r1);
                ss.1.push_highlighted(r2);
            } else {
                ss.0.push_normal(r1);
                ss.1.push_normal(r2);
            }

            if mut1 != mut2 {
                ss.0.push_highlighted(mut1.prefix_str());
                ss.1.push_highlighted(mut2.prefix_str());
            } else {
                ss.0.push_normal(mut1.prefix_str());
                ss.1.push_normal(mut2.prefix_str());
            }
        }

        // process starts here
        match (t1.kind(), t2.kind()) {
            (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
                let did1 = def1.did();
                let did2 = def2.did();

                let generics1 = self.tcx.generics_of(did1);
                let generics2 = self.tcx.generics_of(did2);

                let non_default_after_default = generics1
                    .check_concrete_type_after_default(self.tcx, sub1)
                    || generics2.check_concrete_type_after_default(self.tcx, sub2);
                let sub_no_defaults_1 = if non_default_after_default {
                    generics1.own_args(sub1)
                } else {
                    generics1.own_args_no_defaults(self.tcx, sub1)
                };
                let sub_no_defaults_2 = if non_default_after_default {
                    generics2.own_args(sub2)
                } else {
                    generics2.own_args_no_defaults(self.tcx, sub2)
                };
                let mut values = (DiagStyledString::new(), DiagStyledString::new());
                let path1 = self.tcx.def_path_str(did1);
                let path2 = self.tcx.def_path_str(did2);
                if did1 == did2 {
                    // Easy case. Replace same types with `_` to shorten the output and highlight
                    // the differing ones.
                    //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
                    //     Foo<Bar, _>
                    //     Foo<Quz, _>
                    //         ---  ^ type argument elided
                    //         |
                    //         highlighted in output
                    values.0.push_normal(path1);
                    values.1.push_normal(path2);

                    // Avoid printing out default generic parameters that are common to both
                    // types.
                    let len1 = sub_no_defaults_1.len();
                    let len2 = sub_no_defaults_2.len();
                    let common_len = cmp::min(len1, len2);
                    let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
                    let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
                    let common_default_params =
                        iter::zip(remainder1.iter().rev(), remainder2.iter().rev())
                            .filter(|(a, b)| a == b)
                            .count();
                    let len = sub1.len() - common_default_params;
                    let consts_offset = len - sub1.consts().count();

                    // Only draw `<...>` if there are lifetime/type arguments.
                    if len > 0 {
                        values.0.push_normal("<");
                        values.1.push_normal("<");
                    }

                    fn lifetime_display(lifetime: Region<'_>) -> String {
                        let s = lifetime.to_string();
                        if s.is_empty() { "'_".to_string() } else { s }
                    }
                    // At one point we'd like to elide all lifetimes here, they are irrelevant for
                    // all diagnostics that use this output
                    //
                    //     Foo<'x, '_, Bar>
                    //     Foo<'y, '_, Qux>
                    //         ^^  ^^  --- type arguments are not elided
                    //         |   |
                    //         |   elided as they were the same
                    //         not elided, they were different, but irrelevant
                    //
                    // For bound lifetimes, keep the names of the lifetimes,
                    // even if they are the same so that it's clear what's happening
                    // if we have something like
                    //
                    // for<'r, 's> fn(Inv<'r>, Inv<'s>)
                    // for<'r> fn(Inv<'r>, Inv<'r>)
                    let lifetimes = sub1.regions().zip(sub2.regions());
                    for (i, lifetimes) in lifetimes.enumerate() {
                        let l1 = lifetime_display(lifetimes.0);
                        let l2 = lifetime_display(lifetimes.1);
                        if lifetimes.0 != lifetimes.1 {
                            values.0.push_highlighted(l1);
                            values.1.push_highlighted(l2);
                        } else if lifetimes.0.is_bound() || self.tcx.sess.opts.verbose {
                            values.0.push_normal(l1);
                            values.1.push_normal(l2);
                        } else {
                            values.0.push_normal("'_");
                            values.1.push_normal("'_");
                        }
                        self.push_comma(&mut values.0, &mut values.1, len, i);
                    }

                    // We're comparing two types with the same path, so we compare the type
                    // arguments for both. If they are the same, do not highlight and elide from the
                    // output.
                    //     Foo<_, Bar>
                    //     Foo<_, Qux>
                    //         ^ elided type as this type argument was the same in both sides
                    let type_arguments = sub1.types().zip(sub2.types());
                    let regions_len = sub1.regions().count();
                    let num_display_types = consts_offset - regions_len;
                    for (i, (ta1, ta2)) in type_arguments.take(num_display_types).enumerate() {
                        let i = i + regions_len;
                        if ta1 == ta2 && !self.tcx.sess.opts.verbose {
                            values.0.push_normal("_");
                            values.1.push_normal("_");
                        } else {
                            recurse(ta1, ta2, &mut values);
                        }
                        self.push_comma(&mut values.0, &mut values.1, len, i);
                    }

                    // Do the same for const arguments, if they are equal, do not highlight and
                    // elide them from the output.
                    let const_arguments = sub1.consts().zip(sub2.consts());
                    for (i, (ca1, ca2)) in const_arguments.enumerate() {
                        let i = i + consts_offset;
                        maybe_highlight(ca1, ca2, &mut values, self.tcx);
                        self.push_comma(&mut values.0, &mut values.1, len, i);
                    }

                    // Close the type argument bracket.
                    // Only draw `<...>` if there are lifetime/type arguments.
                    if len > 0 {
                        values.0.push_normal(">");
                        values.1.push_normal(">");
                    }
                    values
                } else {
                    // Check for case:
                    //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
                    //     Foo<Bar<Qux>
                    //         ------- this type argument is exactly the same as the other type
                    //     Bar<Qux>
                    if self
                        .cmp_type_arg(
                            &mut values.0,
                            &mut values.1,
                            path1.clone(),
                            sub_no_defaults_1,
                            path2.clone(),
                            t2,
                        )
                        .is_some()
                    {
                        return values;
                    }
                    // Check for case:
                    //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
                    //     Bar<Qux>
                    //     Foo<Bar<Qux>>
                    //         ------- this type argument is exactly the same as the other type
                    if self
                        .cmp_type_arg(
                            &mut values.1,
                            &mut values.0,
                            path2,
                            sub_no_defaults_2,
                            path1,
                            t1,
                        )
                        .is_some()
                    {
                        return values;
                    }

                    // We can't find anything in common, highlight relevant part of type path.
                    //     let x: foo::bar::Baz<Qux> = y:<foo::bar::Bar<Zar>>();
                    //     foo::bar::Baz<Qux>
                    //     foo::bar::Bar<Zar>
                    //               -------- this part of the path is different

                    let t1_str = t1.to_string();
                    let t2_str = t2.to_string();
                    let min_len = t1_str.len().min(t2_str.len());

                    const SEPARATOR: &str = "::";
                    let separator_len = SEPARATOR.len();
                    let split_idx: usize =
                        iter::zip(t1_str.split(SEPARATOR), t2_str.split(SEPARATOR))
                            .take_while(|(mod1_str, mod2_str)| mod1_str == mod2_str)
                            .map(|(mod_str, _)| mod_str.len() + separator_len)
                            .sum();

                    debug!(?separator_len, ?split_idx, ?min_len, "cmp");

                    if split_idx >= min_len {
                        // paths are identical, highlight everything
                        (
                            DiagStyledString::highlighted(t1_str),
                            DiagStyledString::highlighted(t2_str),
                        )
                    } else {
                        let (common, uniq1) = t1_str.split_at(split_idx);
                        let (_, uniq2) = t2_str.split_at(split_idx);
                        debug!(?common, ?uniq1, ?uniq2, "cmp");

                        values.0.push_normal(common);
                        values.0.push_highlighted(uniq1);
                        values.1.push_normal(common);
                        values.1.push_highlighted(uniq2);

                        values
                    }
                }
            }

            // When finding `&T != &T`, compare the references, then recurse into pointee type
            (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2)) => {
                let mut values = (DiagStyledString::new(), DiagStyledString::new());
                cmp_ty_refs(r1, mutbl1, r2, mutbl2, &mut values);
                recurse(ref_ty1, ref_ty2, &mut values);
                values
            }
            // When finding T != &T, highlight the borrow
            (&ty::Ref(r1, ref_ty1, mutbl1), _) => {
                let mut values = (DiagStyledString::new(), DiagStyledString::new());
                push_ref(r1, mutbl1, &mut values.0);
                recurse(ref_ty1, t2, &mut values);
                values
            }
            (_, &ty::Ref(r2, ref_ty2, mutbl2)) => {
                let mut values = (DiagStyledString::new(), DiagStyledString::new());
                push_ref(r2, mutbl2, &mut values.1);
                recurse(t1, ref_ty2, &mut values);
                values
            }

            // When encountering tuples of the same size, highlight only the differing types
            (&ty::Tuple(args1), &ty::Tuple(args2)) if args1.len() == args2.len() => {
                let mut values = (DiagStyledString::normal("("), DiagStyledString::normal("("));
                let len = args1.len();
                for (i, (left, right)) in args1.iter().zip(args2).enumerate() {
                    recurse(left, right, &mut values);
                    self.push_comma(&mut values.0, &mut values.1, len, i);
                }
                if len == 1 {
                    // Keep the output for single element tuples as `(ty,)`.
                    values.0.push_normal(",");
                    values.1.push_normal(",");
                }
                values.0.push_normal(")");
                values.1.push_normal(")");
                values
            }

            (ty::FnDef(did1, args1), ty::FnDef(did2, args2)) => {
                let sig1 = self.tcx.fn_sig(*did1).instantiate(self.tcx, args1);
                let sig2 = self.tcx.fn_sig(*did2).instantiate(self.tcx, args2);
                let mut values = self.cmp_fn_sig(&sig1, &sig2);
                let path1 = format!(" {{{}}}", self.tcx.def_path_str_with_args(*did1, args1));
                let path2 = format!(" {{{}}}", self.tcx.def_path_str_with_args(*did2, args2));
                let same_path = path1 == path2;
                values.0.push(path1, !same_path);
                values.1.push(path2, !same_path);
                values
            }

            (ty::FnDef(did1, args1), ty::FnPtr(sig2)) => {
                let sig1 = self.tcx.fn_sig(*did1).instantiate(self.tcx, args1);
                let mut values = self.cmp_fn_sig(&sig1, sig2);
                values.0.push_highlighted(format!(
                    " {{{}}}",
                    self.tcx.def_path_str_with_args(*did1, args1)
                ));
                values
            }

            (ty::FnPtr(sig1), ty::FnDef(did2, args2)) => {
                let sig2 = self.tcx.fn_sig(*did2).instantiate(self.tcx, args2);
                let mut values = self.cmp_fn_sig(sig1, &sig2);
                values
                    .1
                    .push_normal(format!(" {{{}}}", self.tcx.def_path_str_with_args(*did2, args2)));
                values
            }

            (ty::FnPtr(sig1), ty::FnPtr(sig2)) => self.cmp_fn_sig(sig1, sig2),

            _ => {
                let mut strs = (DiagStyledString::new(), DiagStyledString::new());
                maybe_highlight(t1, t2, &mut strs, self.tcx);
                strs
            }
        }
    }

    /// Extend a type error with extra labels pointing at "non-trivial" types, like closures and
    /// the return type of `async fn`s.
    ///
    /// `secondary_span` gives the caller the opportunity to expand `diag` with a `span_label`.
    ///
    /// `swap_secondary_and_primary` is used to make projection errors in particular nicer by using
    /// the message in `secondary_span` as the primary label, and apply the message that would
    /// otherwise be used for the primary label on the `secondary_span` `Span`. This applies on
    /// E0271, like `tests/ui/issues/issue-39970.stderr`.
    #[instrument(
        level = "debug",
        skip(self, diag, secondary_span, swap_secondary_and_primary, prefer_label)
    )]
    pub fn note_type_err(
        &self,
        diag: &mut Diag<'_>,
        cause: &ObligationCause<'tcx>,
        secondary_span: Option<(Span, Cow<'static, str>)>,
        mut values: Option<ValuePairs<'tcx>>,
        terr: TypeError<'tcx>,
        swap_secondary_and_primary: bool,
        prefer_label: bool,
    ) {
        let span = cause.span();

        // For some types of errors, expected-found does not make
        // sense, so just ignore the values we were given.
        if let TypeError::CyclicTy(_) = terr {
            values = None;
        }
        struct OpaqueTypesVisitor<'tcx> {
            types: FxIndexMap<TyCategory, FxIndexSet<Span>>,
            expected: FxIndexMap<TyCategory, FxIndexSet<Span>>,
            found: FxIndexMap<TyCategory, FxIndexSet<Span>>,
            ignore_span: Span,
            tcx: TyCtxt<'tcx>,
        }

        impl<'tcx> OpaqueTypesVisitor<'tcx> {
            fn visit_expected_found(
                tcx: TyCtxt<'tcx>,
                expected: impl TypeVisitable<TyCtxt<'tcx>>,
                found: impl TypeVisitable<TyCtxt<'tcx>>,
                ignore_span: Span,
            ) -> Self {
                let mut types_visitor = OpaqueTypesVisitor {
                    types: Default::default(),
                    expected: Default::default(),
                    found: Default::default(),
                    ignore_span,
                    tcx,
                };
                // The visitor puts all the relevant encountered types in `self.types`, but in
                // here we want to visit two separate types with no relation to each other, so we
                // move the results from `types` to `expected` or `found` as appropriate.
                expected.visit_with(&mut types_visitor);
                std::mem::swap(&mut types_visitor.expected, &mut types_visitor.types);
                found.visit_with(&mut types_visitor);
                std::mem::swap(&mut types_visitor.found, &mut types_visitor.types);
                types_visitor
            }

            fn report(&self, err: &mut Diag<'_>) {
                self.add_labels_for_types(err, "expected", &self.expected);
                self.add_labels_for_types(err, "found", &self.found);
            }

            fn add_labels_for_types(
                &self,
                err: &mut Diag<'_>,
                target: &str,
                types: &FxIndexMap<TyCategory, FxIndexSet<Span>>,
            ) {
                for (kind, values) in types.iter() {
                    let count = values.len();
                    for &sp in values {
                        err.span_label(
                            sp,
                            format!(
                                "{}{} {:#}{}",
                                if count == 1 { "the " } else { "one of the " },
                                target,
                                kind,
                                pluralize!(count),
                            ),
                        );
                    }
                }
            }
        }

        impl<'tcx> ty::visit::TypeVisitor<TyCtxt<'tcx>> for OpaqueTypesVisitor<'tcx> {
            fn visit_ty(&mut self, t: Ty<'tcx>) {
                if let Some((kind, def_id)) = TyCategory::from_ty(self.tcx, t) {
                    let span = self.tcx.def_span(def_id);
                    // Avoid cluttering the output when the "found" and error span overlap:
                    //
                    // error[E0308]: mismatched types
                    //   --> $DIR/issue-20862.rs:2:5
                    //    |
                    // LL |     |y| x + y
                    //    |     ^^^^^^^^^
                    //    |     |
                    //    |     the found closure
                    //    |     expected `()`, found closure
                    //    |
                    //    = note: expected unit type `()`
                    //                 found closure `{closure@$DIR/issue-20862.rs:2:5: 2:14 x:_}`
                    //
                    // Also ignore opaque `Future`s that come from async fns.
                    if !self.ignore_span.overlaps(span)
                        && !span.is_desugaring(DesugaringKind::Async)
                    {
                        self.types.entry(kind).or_default().insert(span);
                    }
                }
                t.super_visit_with(self)
            }
        }

        debug!("note_type_err(diag={:?})", diag);
        enum Mismatch<'a> {
            Variable(ty::error::ExpectedFound<Ty<'a>>),
            Fixed(&'static str),
        }
        let (expected_found, exp_found, is_simple_error, values) = match values {
            None => (None, Mismatch::Fixed("type"), false, None),
            Some(values) => {
                let values = self.resolve_vars_if_possible(values);
                let (is_simple_error, exp_found) = match values {
                    ValuePairs::Terms(infer::ExpectedFound { expected, found }) => {
                        match (expected.unpack(), found.unpack()) {
                            (ty::TermKind::Ty(expected), ty::TermKind::Ty(found)) => {
                                let is_simple_err = expected.is_simple_text(self.tcx)
                                    && found.is_simple_text(self.tcx);
                                OpaqueTypesVisitor::visit_expected_found(
                                    self.tcx, expected, found, span,
                                )
                                .report(diag);

                                (
                                    is_simple_err,
                                    Mismatch::Variable(infer::ExpectedFound { expected, found }),
                                )
                            }
                            (ty::TermKind::Const(_), ty::TermKind::Const(_)) => {
                                (false, Mismatch::Fixed("constant"))
                            }
                            _ => (false, Mismatch::Fixed("type")),
                        }
                    }
                    ValuePairs::PolySigs(infer::ExpectedFound { expected, found }) => {
                        OpaqueTypesVisitor::visit_expected_found(self.tcx, expected, found, span)
                            .report(diag);
                        (false, Mismatch::Fixed("signature"))
                    }
                    ValuePairs::PolyTraitRefs(_) => (false, Mismatch::Fixed("trait")),
                    ValuePairs::Aliases(infer::ExpectedFound { expected, .. }) => {
                        (false, Mismatch::Fixed(self.tcx.def_descr(expected.def_id)))
                    }
                    ValuePairs::Regions(_) => (false, Mismatch::Fixed("lifetime")),
                    ValuePairs::ExistentialTraitRef(_) => {
                        (false, Mismatch::Fixed("existential trait ref"))
                    }
                    ValuePairs::ExistentialProjection(_) => {
                        (false, Mismatch::Fixed("existential projection"))
                    }
                };
                let Some(vals) = self.values_str(values) else {
                    // Derived error. Cancel the emitter.
                    // NOTE(eddyb) this was `.cancel()`, but `diag`
                    // is borrowed, so we can't fully defuse it.
                    diag.downgrade_to_delayed_bug();
                    return;
                };
                (Some(vals), exp_found, is_simple_error, Some(values))
            }
        };

        let mut label_or_note = |span: Span, msg: Cow<'static, str>| {
            if (prefer_label && is_simple_error) || &[span] == diag.span.primary_spans() {
                diag.span_label(span, msg);
            } else {
                diag.span_note(span, msg);
            }
        };
        if let Some((sp, msg)) = secondary_span {
            if swap_secondary_and_primary {
                let terr = if let Some(infer::ValuePairs::Terms(infer::ExpectedFound {
                    expected,
                    ..
                })) = values
                {
                    Cow::from(format!("expected this to be `{expected}`"))
                } else {
                    terr.to_string(self.tcx)
                };
                label_or_note(sp, terr);
                label_or_note(span, msg);
            } else {
                label_or_note(span, terr.to_string(self.tcx));
                label_or_note(sp, msg);
            }
        } else {
            if let Some(values) = values
                && let Some((e, f)) = values.ty()
                && let TypeError::ArgumentSorts(..) | TypeError::Sorts(_) = terr
            {
                let e = self.tcx.erase_regions(e);
                let f = self.tcx.erase_regions(f);
                let expected = with_forced_trimmed_paths!(e.sort_string(self.tcx));
                let found = with_forced_trimmed_paths!(f.sort_string(self.tcx));
                if expected == found {
                    label_or_note(span, terr.to_string(self.tcx));
                } else {
                    label_or_note(span, Cow::from(format!("expected {expected}, found {found}")));
                }
            } else {
                label_or_note(span, terr.to_string(self.tcx));
            }
        }

        if let Some((expected, found, path)) = expected_found {
            let (expected_label, found_label, exp_found) = match exp_found {
                Mismatch::Variable(ef) => (
                    ef.expected.prefix_string(self.tcx),
                    ef.found.prefix_string(self.tcx),
                    Some(ef),
                ),
                Mismatch::Fixed(s) => (s.into(), s.into(), None),
            };

            enum Similar<'tcx> {
                Adts { expected: ty::AdtDef<'tcx>, found: ty::AdtDef<'tcx> },
                PrimitiveFound { expected: ty::AdtDef<'tcx>, found: Ty<'tcx> },
                PrimitiveExpected { expected: Ty<'tcx>, found: ty::AdtDef<'tcx> },
            }

            let similarity = |ExpectedFound { expected, found }: ExpectedFound<Ty<'tcx>>| {
                if let ty::Adt(expected, _) = expected.kind()
                    && let Some(primitive) = found.primitive_symbol()
                {
                    let path = self.tcx.def_path(expected.did()).data;
                    let name = path.last().unwrap().data.get_opt_name();
                    if name == Some(primitive) {
                        return Some(Similar::PrimitiveFound { expected: *expected, found });
                    }
                } else if let Some(primitive) = expected.primitive_symbol()
                    && let ty::Adt(found, _) = found.kind()
                {
                    let path = self.tcx.def_path(found.did()).data;
                    let name = path.last().unwrap().data.get_opt_name();
                    if name == Some(primitive) {
                        return Some(Similar::PrimitiveExpected { expected, found: *found });
                    }
                } else if let ty::Adt(expected, _) = expected.kind()
                    && let ty::Adt(found, _) = found.kind()
                {
                    if !expected.did().is_local() && expected.did().krate == found.did().krate {
                        // Most likely types from different versions of the same crate
                        // are in play, in which case this message isn't so helpful.
                        // A "perhaps two different versions..." error is already emitted for that.
                        return None;
                    }
                    let f_path = self.tcx.def_path(found.did()).data;
                    let e_path = self.tcx.def_path(expected.did()).data;

                    if let (Some(e_last), Some(f_last)) = (e_path.last(), f_path.last())
                        && e_last == f_last
                    {
                        return Some(Similar::Adts { expected: *expected, found: *found });
                    }
                }
                None
            };

            match terr {
                // If two types mismatch but have similar names, mention that specifically.
                TypeError::Sorts(values) if let Some(s) = similarity(values) => {
                    let diagnose_primitive =
                        |prim: Ty<'tcx>, shadow: Ty<'tcx>, defid: DefId, diag: &mut Diag<'_>| {
                            let name = shadow.sort_string(self.tcx);
                            diag.note(format!(
                                "{prim} and {name} have similar names, but are actually distinct types"
                            ));
                            diag.note(format!("{prim} is a primitive defined by the language"));
                            let def_span = self.tcx.def_span(defid);
                            let msg = if defid.is_local() {
                                format!("{name} is defined in the current crate")
                            } else {
                                let crate_name = self.tcx.crate_name(defid.krate);
                                format!("{name} is defined in crate `{crate_name}`")
                            };
                            diag.span_note(def_span, msg);
                        };

                    let diagnose_adts =
                        |expected_adt: ty::AdtDef<'tcx>,
                         found_adt: ty::AdtDef<'tcx>,
                         diag: &mut Diag<'_>| {
                            let found_name = values.found.sort_string(self.tcx);
                            let expected_name = values.expected.sort_string(self.tcx);

                            let found_defid = found_adt.did();
                            let expected_defid = expected_adt.did();

                            diag.note(format!("{found_name} and {expected_name} have similar names, but are actually distinct types"));
                            for (defid, name) in
                                [(found_defid, found_name), (expected_defid, expected_name)]
                            {
                                let def_span = self.tcx.def_span(defid);

                                let msg = if found_defid.is_local() && expected_defid.is_local() {
                                    let module = self
                                        .tcx
                                        .parent_module_from_def_id(defid.expect_local())
                                        .to_def_id();
                                    let module_name =
                                        self.tcx.def_path(module).to_string_no_crate_verbose();
                                    format!(
                                        "{name} is defined in module `crate{module_name}` of the current crate"
                                    )
                                } else if defid.is_local() {
                                    format!("{name} is defined in the current crate")
                                } else {
                                    let crate_name = self.tcx.crate_name(defid.krate);
                                    format!("{name} is defined in crate `{crate_name}`")
                                };
                                diag.span_note(def_span, msg);
                            }
                        };

                    match s {
                        Similar::Adts { expected, found } => diagnose_adts(expected, found, diag),
                        Similar::PrimitiveFound { expected, found: prim } => {
                            diagnose_primitive(prim, values.expected, expected.did(), diag)
                        }
                        Similar::PrimitiveExpected { expected: prim, found } => {
                            diagnose_primitive(prim, values.found, found.did(), diag)
                        }
                    }
                }
                TypeError::Sorts(values) => {
                    let extra = expected == found;
                    let sort_string = |ty: Ty<'tcx>| match (extra, ty.kind()) {
                        (true, ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. })) => {
                            let sm = self.tcx.sess.source_map();
                            let pos = sm.lookup_char_pos(self.tcx.def_span(*def_id).lo());
                            format!(
                                " (opaque type at <{}:{}:{}>)",
                                sm.filename_for_diagnostics(&pos.file.name),
                                pos.line,
                                pos.col.to_usize() + 1,
                            )
                        }
                        (true, ty::Alias(ty::Projection, proj))
                            if self.tcx.is_impl_trait_in_trait(proj.def_id) =>
                        {
                            let sm = self.tcx.sess.source_map();
                            let pos = sm.lookup_char_pos(self.tcx.def_span(proj.def_id).lo());
                            format!(
                                " (trait associated opaque type at <{}:{}:{}>)",
                                sm.filename_for_diagnostics(&pos.file.name),
                                pos.line,
                                pos.col.to_usize() + 1,
                            )
                        }
                        (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
                        (false, _) => "".to_string(),
                    };
                    if !(values.expected.is_simple_text(self.tcx)
                        && values.found.is_simple_text(self.tcx))
                        || (exp_found.is_some_and(|ef| {
                            // This happens when the type error is a subset of the expectation,
                            // like when you have two references but one is `usize` and the other
                            // is `f32`. In those cases we still want to show the `note`. If the
                            // value from `ef` is `Infer(_)`, then we ignore it.
                            if !ef.expected.is_ty_or_numeric_infer() {
                                ef.expected != values.expected
                            } else if !ef.found.is_ty_or_numeric_infer() {
                                ef.found != values.found
                            } else {
                                false
                            }
                        }))
                    {
                        if let Some(ExpectedFound { found: found_ty, .. }) = exp_found {
                            // `Future` is a special opaque type that the compiler
                            // will try to hide in some case such as `async fn`, so
                            // to make an error more use friendly we will
                            // avoid to suggest a mismatch type with a
                            // type that the user usually are not using
                            // directly such as `impl Future<Output = u8>`.
                            if !self.tcx.ty_is_opaque_future(found_ty) {
                                diag.note_expected_found_extra(
                                    &expected_label,
                                    expected,
                                    &found_label,
                                    found,
                                    &sort_string(values.expected),
                                    &sort_string(values.found),
                                );
                                if let Some(path) = path {
                                    diag.note(format!(
                                        "the full type name has been written to '{}'",
                                        path.display(),
                                    ));
                                    diag.note("consider using `--verbose` to print the full type name to the console");
                                }
                            }
                        }
                    }
                }
                _ => {
                    debug!(
                        "note_type_err: exp_found={:?}, expected={:?} found={:?}",
                        exp_found, expected, found
                    );
                    if !is_simple_error || terr.must_include_note() {
                        diag.note_expected_found(&expected_label, expected, &found_label, found);
                    }
                }
            }
        }
        let exp_found = match exp_found {
            Mismatch::Variable(exp_found) => Some(exp_found),
            Mismatch::Fixed(_) => None,
        };
        let exp_found = match terr {
            // `terr` has more accurate type information than `exp_found` in match expressions.
            ty::error::TypeError::Sorts(terr)
                if exp_found.is_some_and(|ef| terr.found == ef.found) =>
            {
                Some(terr)
            }
            _ => exp_found,
        };
        debug!("exp_found {:?} terr {:?} cause.code {:?}", exp_found, terr, cause.code());
        if let Some(exp_found) = exp_found {
            let should_suggest_fixes =
                if let ObligationCauseCode::Pattern { root_ty, .. } = cause.code() {
                    // Skip if the root_ty of the pattern is not the same as the expected_ty.
                    // If these types aren't equal then we've probably peeled off a layer of arrays.
                    self.same_type_modulo_infer(*root_ty, exp_found.expected)
                } else {
                    true
                };

            // FIXME(#73154): For now, we do leak check when coercing function
            // pointers in typeck, instead of only during borrowck. This can lead
            // to these `RegionsInsufficientlyPolymorphic` errors that aren't helpful.
            if should_suggest_fixes
                && !matches!(terr, TypeError::RegionsInsufficientlyPolymorphic(..))
            {
                self.suggest_tuple_pattern(cause, &exp_found, diag);
                self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
                self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
                self.suggest_function_pointers(cause, span, &exp_found, diag);
                self.suggest_turning_stmt_into_expr(cause, &exp_found, diag);
            }
        }

        self.check_and_note_conflicting_crates(diag, terr);

        self.note_and_explain_type_err(diag, terr, cause, span, cause.body_id.to_def_id());
        if let Some(exp_found) = exp_found
            && let exp_found = TypeError::Sorts(exp_found)
            && exp_found != terr
        {
            self.note_and_explain_type_err(diag, exp_found, cause, span, cause.body_id.to_def_id());
        }

        if let Some(ValuePairs::PolyTraitRefs(exp_found)) = values
            && let ty::Closure(def_id, _) = exp_found.expected.skip_binder().self_ty().kind()
            && let Some(def_id) = def_id.as_local()
            && terr.involves_regions()
        {
            let span = self.tcx.def_span(def_id);
            diag.span_note(span, "this closure does not fulfill the lifetime requirements");
            self.suggest_for_all_lifetime_closure(
                span,
                self.tcx.hir_node_by_def_id(def_id),
                &exp_found,
                diag,
            );
        }

        // It reads better to have the error origin as the final
        // thing.
        self.note_error_origin(diag, cause, exp_found, terr);

        debug!(?diag);
    }

    pub fn type_error_additional_suggestions(
        &self,
        trace: &TypeTrace<'tcx>,
        terr: TypeError<'tcx>,
    ) -> Vec<TypeErrorAdditionalDiags> {
        use crate::traits::ObligationCauseCode::{BlockTailExpression, MatchExpressionArm};
        let mut suggestions = Vec::new();
        let span = trace.cause.span();
        let values = self.resolve_vars_if_possible(trace.values);
        if let Some((expected, found)) = values.ty() {
            match (expected.kind(), found.kind()) {
                (ty::Tuple(_), ty::Tuple(_)) => {}
                // If a tuple of length one was expected and the found expression has
                // parentheses around it, perhaps the user meant to write `(expr,)` to
                // build a tuple (issue #86100)
                (ty::Tuple(fields), _) => {
                    suggestions.extend(self.suggest_wrap_to_build_a_tuple(span, found, fields))
                }
                // If a byte was expected and the found expression is a char literal
                // containing a single ASCII character, perhaps the user meant to write `b'c'` to
                // specify a byte literal
                (ty::Uint(ty::UintTy::U8), ty::Char) => {
                    if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
                        && let Some(code) =
                            code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
                        // forbid all Unicode escapes
                        && !code.starts_with("\\u")
                        // forbids literal Unicode characters beyond ASCII
                        && code.chars().next().is_some_and(|c| c.is_ascii())
                    {
                        suggestions.push(TypeErrorAdditionalDiags::MeantByteLiteral {
                            span,
                            code: escape_literal(code),
                        })
                    }
                }
                // If a character was expected and the found expression is a string literal
                // containing a single character, perhaps the user meant to write `'c'` to
                // specify a character literal (issue #92479)
                (ty::Char, ty::Ref(_, r, _)) if r.is_str() => {
                    if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
                        && let Some(code) = code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
                        && code.chars().count() == 1
                    {
                        suggestions.push(TypeErrorAdditionalDiags::MeantCharLiteral {
                            span,
                            code: escape_literal(code),
                        })
                    }
                }
                // If a string was expected and the found expression is a character literal,
                // perhaps the user meant to write `"s"` to specify a string literal.
                (ty::Ref(_, r, _), ty::Char) if r.is_str() => {
                    if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
                        if let Some(code) =
                            code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
                        {
                            suggestions.push(TypeErrorAdditionalDiags::MeantStrLiteral {
                                span,
                                code: escape_literal(code),
                            })
                        }
                    }
                }
                // For code `if Some(..) = expr `, the type mismatch may be expected `bool` but found `()`,
                // we try to suggest to add the missing `let` for `if let Some(..) = expr`
                (ty::Bool, ty::Tuple(list)) => {
                    if list.len() == 0 {
                        suggestions.extend(self.suggest_let_for_letchains(&trace.cause, span));
                    }
                }
                (ty::Array(_, _), ty::Array(_, _)) => {
                    suggestions.extend(self.suggest_specify_actual_length(terr, trace, span))
                }
                _ => {}
            }
        }
        let code = trace.cause.code();
        if let &(MatchExpressionArm(box MatchExpressionArmCause { source, .. })
        | BlockTailExpression(.., source)) = code
            && let hir::MatchSource::TryDesugar(_) = source
            && let Some((expected_ty, found_ty, _)) = self.values_str(trace.values)
        {
            suggestions.push(TypeErrorAdditionalDiags::TryCannotConvert {
                found: found_ty.content(),
                expected: expected_ty.content(),
            });
        }
        suggestions
    }

    fn suggest_specify_actual_length(
        &self,
        terr: TypeError<'_>,
        trace: &TypeTrace<'_>,
        span: Span,
    ) -> Option<TypeErrorAdditionalDiags> {
        let hir = self.tcx.hir();
        let TypeError::FixedArraySize(sz) = terr else {
            return None;
        };
        let tykind = match self.tcx.hir_node_by_def_id(trace.cause.body_id) {
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. }) => {
                let body = hir.body(*body_id);
                struct LetVisitor {
                    span: Span,
                }
                impl<'v> Visitor<'v> for LetVisitor {
                    type Result = ControlFlow<&'v hir::TyKind<'v>>;
                    fn visit_stmt(&mut self, s: &'v hir::Stmt<'v>) -> Self::Result {
                        // Find a local statement where the initializer has
                        // the same span as the error and the type is specified.
                        if let hir::Stmt {
                            kind:
                                hir::StmtKind::Let(hir::Local {
                                    init: Some(hir::Expr { span: init_span, .. }),
                                    ty: Some(array_ty),
                                    ..
                                }),
                            ..
                        } = s
                            && init_span == &self.span
                        {
                            ControlFlow::Break(&array_ty.peel_refs().kind)
                        } else {
                            ControlFlow::Continue(())
                        }
                    }
                }
                LetVisitor { span }.visit_body(body).break_value()
            }
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(ty, _, _), .. }) => {
                Some(&ty.peel_refs().kind)
            }
            _ => None,
        };
        if let Some(tykind) = tykind
            && let hir::TyKind::Array(_, length) = tykind
            && let hir::ArrayLen::Body(hir::AnonConst { hir_id, .. }) = length
        {
            let span = self.tcx.hir().span(*hir_id);
            Some(TypeErrorAdditionalDiags::ConsiderSpecifyingLength { span, length: sz.found })
        } else {
            None
        }
    }

    pub fn report_and_explain_type_error(
        &self,
        trace: TypeTrace<'tcx>,
        terr: TypeError<'tcx>,
    ) -> Diag<'tcx> {
        debug!("report_and_explain_type_error(trace={:?}, terr={:?})", trace, terr);

        let span = trace.cause.span();
        let failure_code = trace.cause.as_failure_code_diag(
            terr,
            span,
            self.type_error_additional_suggestions(&trace, terr),
        );
        let mut diag = self.tcx.dcx().create_err(failure_code);
        self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr, false, false);
        diag
    }

    fn suggest_wrap_to_build_a_tuple(
        &self,
        span: Span,
        found: Ty<'tcx>,
        expected_fields: &List<Ty<'tcx>>,
    ) -> Option<TypeErrorAdditionalDiags> {
        let [expected_tup_elem] = expected_fields[..] else { return None };

        if !self.same_type_modulo_infer(expected_tup_elem, found) {
            return None;
        }

        let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) else { return None };

        let sugg = if code.starts_with('(') && code.ends_with(')') {
            let before_close = span.hi() - BytePos::from_u32(1);
            TypeErrorAdditionalDiags::TupleOnlyComma {
                span: span.with_hi(before_close).shrink_to_hi(),
            }
        } else {
            TypeErrorAdditionalDiags::TupleAlsoParentheses {
                span_low: span.shrink_to_lo(),
                span_high: span.shrink_to_hi(),
            }
        };
        Some(sugg)
    }

    fn values_str(
        &self,
        values: ValuePairs<'tcx>,
    ) -> Option<(DiagStyledString, DiagStyledString, Option<PathBuf>)> {
        match values {
            infer::Regions(exp_found) => self.expected_found_str(exp_found),
            infer::Terms(exp_found) => self.expected_found_str_term(exp_found),
            infer::Aliases(exp_found) => self.expected_found_str(exp_found),
            infer::ExistentialTraitRef(exp_found) => self.expected_found_str(exp_found),
            infer::ExistentialProjection(exp_found) => self.expected_found_str(exp_found),
            infer::PolyTraitRefs(exp_found) => {
                let pretty_exp_found = ty::error::ExpectedFound {
                    expected: exp_found.expected.print_trait_sugared(),
                    found: exp_found.found.print_trait_sugared(),
                };
                match self.expected_found_str(pretty_exp_found) {
                    Some((expected, found, _)) if expected == found => {
                        self.expected_found_str(exp_found)
                    }
                    ret => ret,
                }
            }
            infer::PolySigs(exp_found) => {
                let exp_found = self.resolve_vars_if_possible(exp_found);
                if exp_found.references_error() {
                    return None;
                }
                let (exp, fnd) = self.cmp_fn_sig(&exp_found.expected, &exp_found.found);
                Some((exp, fnd, None))
            }
        }
    }

    fn expected_found_str_term(
        &self,
        exp_found: ty::error::ExpectedFound<ty::Term<'tcx>>,
    ) -> Option<(DiagStyledString, DiagStyledString, Option<PathBuf>)> {
        let exp_found = self.resolve_vars_if_possible(exp_found);
        if exp_found.references_error() {
            return None;
        }

        Some(match (exp_found.expected.unpack(), exp_found.found.unpack()) {
            (ty::TermKind::Ty(expected), ty::TermKind::Ty(found)) => {
                let (mut exp, mut fnd) = self.cmp(expected, found);
                // Use the terminal width as the basis to determine when to compress the printed
                // out type, but give ourselves some leeway to avoid ending up creating a file for
                // a type that is somewhat shorter than the path we'd write to.
                let len = self.tcx.sess().diagnostic_width() + 40;
                let exp_s = exp.content();
                let fnd_s = fnd.content();
                let mut path = None;
                if exp_s.len() > len {
                    let exp_s = self.tcx.short_ty_string(expected, &mut path);
                    exp = DiagStyledString::highlighted(exp_s);
                }
                if fnd_s.len() > len {
                    let fnd_s = self.tcx.short_ty_string(found, &mut path);
                    fnd = DiagStyledString::highlighted(fnd_s);
                }
                (exp, fnd, path)
            }
            _ => (
                DiagStyledString::highlighted(exp_found.expected.to_string()),
                DiagStyledString::highlighted(exp_found.found.to_string()),
                None,
            ),
        })
    }

    /// Returns a string of the form "expected `{}`, found `{}`".
    fn expected_found_str<T: fmt::Display + TypeFoldable<TyCtxt<'tcx>>>(
        &self,
        exp_found: ty::error::ExpectedFound<T>,
    ) -> Option<(DiagStyledString, DiagStyledString, Option<PathBuf>)> {
        let exp_found = self.resolve_vars_if_possible(exp_found);
        if exp_found.references_error() {
            return None;
        }

        Some((
            DiagStyledString::highlighted(exp_found.expected.to_string()),
            DiagStyledString::highlighted(exp_found.found.to_string()),
            None,
        ))
    }

    pub fn report_generic_bound_failure(
        &self,
        generic_param_scope: LocalDefId,
        span: Span,
        origin: Option<SubregionOrigin<'tcx>>,
        bound_kind: GenericKind<'tcx>,
        sub: Region<'tcx>,
    ) -> ErrorGuaranteed {
        self.construct_generic_bound_failure(generic_param_scope, span, origin, bound_kind, sub)
            .emit()
    }

    pub fn construct_generic_bound_failure(
        &self,
        generic_param_scope: LocalDefId,
        span: Span,
        origin: Option<SubregionOrigin<'tcx>>,
        bound_kind: GenericKind<'tcx>,
        sub: Region<'tcx>,
    ) -> Diag<'tcx> {
        if let Some(SubregionOrigin::CompareImplItemObligation {
            span,
            impl_item_def_id,
            trait_item_def_id,
        }) = origin
        {
            return self.report_extra_impl_obligation(
                span,
                impl_item_def_id,
                trait_item_def_id,
                &format!("`{bound_kind}: {sub}`"),
            );
        }

        let labeled_user_string = match bound_kind {
            GenericKind::Param(ref p) => format!("the parameter type `{p}`"),
            GenericKind::Placeholder(ref p) => format!("the placeholder type `{p:?}`"),
            GenericKind::Alias(ref p) => match p.kind(self.tcx) {
                ty::Projection | ty::Inherent => {
                    format!("the associated type `{p}`")
                }
                ty::Weak => format!("the type alias `{p}`"),
                ty::Opaque => format!("the opaque type `{p}`"),
            },
        };

        let mut err = self
            .tcx
            .dcx()
            .struct_span_err(span, format!("{labeled_user_string} may not live long enough"));
        err.code(match sub.kind() {
            ty::ReEarlyParam(_) | ty::ReLateParam(_) if sub.has_name() => E0309,
            ty::ReStatic => E0310,
            _ => E0311,
        });

        '_explain: {
            let (description, span) = match sub.kind() {
                ty::ReEarlyParam(_) | ty::ReLateParam(_) | ty::ReStatic => {
                    msg_span_from_named_region(self.tcx, sub, Some(span))
                }
                _ => (format!("lifetime `{sub}`"), Some(span)),
            };
            let prefix = format!("{labeled_user_string} must be valid for ");
            label_msg_span(&mut err, &prefix, description, span, "...");
            if let Some(origin) = origin {
                self.note_region_origin(&mut err, &origin);
            }
        }

        'suggestion: {
            let msg = "consider adding an explicit lifetime bound";

            if (bound_kind, sub).has_infer_regions()
                || (bound_kind, sub).has_placeholders()
                || !bound_kind.is_suggestable(self.tcx, false)
            {
                let lt_name = sub.get_name_or_anon().to_string();
                err.help(format!("{msg} `{bound_kind}: {lt_name}`..."));
                break 'suggestion;
            }

            let mut generic_param_scope = generic_param_scope;
            while self.tcx.def_kind(generic_param_scope) == DefKind::OpaqueTy {
                generic_param_scope = self.tcx.local_parent(generic_param_scope);
            }

            // type_param_sugg_span is (span, has_bounds)
            let (type_scope, type_param_sugg_span) = match bound_kind {
                GenericKind::Param(ref param) => {
                    let generics = self.tcx.generics_of(generic_param_scope);
                    let def_id = generics.type_param(param, self.tcx).def_id.expect_local();
                    let scope = self.tcx.local_def_id_to_hir_id(def_id).owner.def_id;
                    // Get the `hir::Param` to verify whether it already has any bounds.
                    // We do this to avoid suggesting code that ends up as `T: 'a'b`,
                    // instead we suggest `T: 'a + 'b` in that case.
                    let hir_generics = self.tcx.hir().get_generics(scope).unwrap();
                    let sugg_span = match hir_generics.bounds_span_for_suggestions(def_id) {
                        Some(span) => Some((span, true)),
                        // If `param` corresponds to `Self`, no usable suggestion span.
                        None if generics.has_self && param.index == 0 => None,
                        None => Some((self.tcx.def_span(def_id).shrink_to_hi(), false)),
                    };
                    (scope, sugg_span)
                }
                _ => (generic_param_scope, None),
            };
            let suggestion_scope = {
                let lifetime_scope = match sub.kind() {
                    ty::ReStatic => hir::def_id::CRATE_DEF_ID,
                    _ => match self.tcx.is_suitable_region(sub) {
                        Some(info) => info.def_id,
                        None => generic_param_scope,
                    },
                };
                match self.tcx.is_descendant_of(type_scope.into(), lifetime_scope.into()) {
                    true => type_scope,
                    false => lifetime_scope,
                }
            };

            let mut suggs = vec![];
            let lt_name = self.suggest_name_region(sub, &mut suggs);

            if let Some((sp, has_lifetimes)) = type_param_sugg_span
                && suggestion_scope == type_scope
            {
                let suggestion =
                    if has_lifetimes { format!(" + {lt_name}") } else { format!(": {lt_name}") };
                suggs.push((sp, suggestion))
            } else if let GenericKind::Alias(ref p) = bound_kind
                && let ty::Projection = p.kind(self.tcx)
                && let DefKind::AssocTy = self.tcx.def_kind(p.def_id)
                && let Some(ty::ImplTraitInTraitData::Trait { .. }) =
                    self.tcx.opt_rpitit_info(p.def_id)
            {
                // The lifetime found in the `impl` is longer than the one on the RPITIT.
                // Do not suggest `<Type as Trait>::{opaque}: 'static`.
            } else if let Some(generics) = self.tcx.hir().get_generics(suggestion_scope) {
                let pred = format!("{bound_kind}: {lt_name}");
                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred);
                suggs.push((generics.tail_span_for_predicate_suggestion(), suggestion))
            } else {
                let consider = format!("{msg} `{bound_kind}: {sub}`...");
                err.help(consider);
            }

            if !suggs.is_empty() {
                err.multipart_suggestion_verbose(
                    msg,
                    suggs,
                    Applicability::MaybeIncorrect, // Issue #41966
                );
            }
        }

        err
    }

    pub fn suggest_name_region(
        &self,
        lifetime: Region<'tcx>,
        add_lt_suggs: &mut Vec<(Span, String)>,
    ) -> String {
        struct LifetimeReplaceVisitor<'tcx, 'a> {
            tcx: TyCtxt<'tcx>,
            needle: hir::LifetimeName,
            new_lt: &'a str,
            add_lt_suggs: &'a mut Vec<(Span, String)>,
        }

        impl<'hir, 'tcx> hir::intravisit::Visitor<'hir> for LifetimeReplaceVisitor<'tcx, '_> {
            fn visit_lifetime(&mut self, lt: &'hir hir::Lifetime) {
                if lt.res == self.needle {
                    let (pos, span) = lt.suggestion_position();
                    let new_lt = &self.new_lt;
                    let sugg = match pos {
                        hir::LifetimeSuggestionPosition::Normal => format!("{new_lt}"),
                        hir::LifetimeSuggestionPosition::Ampersand => format!("{new_lt} "),
                        hir::LifetimeSuggestionPosition::ElidedPath => format!("<{new_lt}>"),
                        hir::LifetimeSuggestionPosition::ElidedPathArgument => {
                            format!("{new_lt}, ")
                        }
                        hir::LifetimeSuggestionPosition::ObjectDefault => format!("+ {new_lt}"),
                    };
                    self.add_lt_suggs.push((span, sugg));
                }
            }

            fn visit_ty(&mut self, ty: &'hir hir::Ty<'hir>) {
                let hir::TyKind::OpaqueDef(item_id, _, _) = ty.kind else {
                    return hir::intravisit::walk_ty(self, ty);
                };
                let opaque_ty = self.tcx.hir().item(item_id).expect_opaque_ty();
                if let Some(&(_, b)) =
                    opaque_ty.lifetime_mapping.iter().find(|&(a, _)| a.res == self.needle)
                {
                    let prev_needle =
                        std::mem::replace(&mut self.needle, hir::LifetimeName::Param(b));
                    for bound in opaque_ty.bounds {
                        self.visit_param_bound(bound);
                    }
                    self.needle = prev_needle;
                }
            }
        }

        let (lifetime_def_id, lifetime_scope) = match self.tcx.is_suitable_region(lifetime) {
            Some(info) if !lifetime.has_name() => {
                (info.bound_region.get_id().unwrap().expect_local(), info.def_id)
            }
            _ => return lifetime.get_name_or_anon().to_string(),
        };

        let new_lt = {
            let generics = self.tcx.generics_of(lifetime_scope);
            let mut used_names =
                iter::successors(Some(generics), |g| g.parent.map(|p| self.tcx.generics_of(p)))
                    .flat_map(|g| &g.params)
                    .filter(|p| matches!(p.kind, ty::GenericParamDefKind::Lifetime))
                    .map(|p| p.name)
                    .collect::<Vec<_>>();
            let hir_id = self.tcx.local_def_id_to_hir_id(lifetime_scope);
            // consider late-bound lifetimes ...
            used_names.extend(self.tcx.late_bound_vars(hir_id).into_iter().filter_map(
                |p| match p {
                    ty::BoundVariableKind::Region(lt) => lt.get_name(),
                    _ => None,
                },
            ));
            (b'a'..=b'z')
                .map(|c| format!("'{}", c as char))
                .find(|candidate| !used_names.iter().any(|e| e.as_str() == candidate))
                .unwrap_or("'lt".to_string())
        };

        let mut visitor = LifetimeReplaceVisitor {
            tcx: self.tcx,
            needle: hir::LifetimeName::Param(lifetime_def_id),
            add_lt_suggs,
            new_lt: &new_lt,
        };
        match self.tcx.expect_hir_owner_node(lifetime_scope) {
            hir::OwnerNode::Item(i) => visitor.visit_item(i),
            hir::OwnerNode::ForeignItem(i) => visitor.visit_foreign_item(i),
            hir::OwnerNode::ImplItem(i) => visitor.visit_impl_item(i),
            hir::OwnerNode::TraitItem(i) => visitor.visit_trait_item(i),
            hir::OwnerNode::Crate(_) => bug!("OwnerNode::Crate doesn't not have generics"),
            hir::OwnerNode::AssocOpaqueTy(..) => unreachable!(),
        }

        let ast_generics = self.tcx.hir().get_generics(lifetime_scope).unwrap();
        let sugg = ast_generics
            .span_for_lifetime_suggestion()
            .map(|span| (span, format!("{new_lt}, ")))
            .unwrap_or_else(|| (ast_generics.span, format!("<{new_lt}>")));
        add_lt_suggs.push(sugg);

        new_lt
    }

    fn report_sub_sup_conflict(
        &self,
        var_origin: RegionVariableOrigin,
        sub_origin: SubregionOrigin<'tcx>,
        sub_region: Region<'tcx>,
        sup_origin: SubregionOrigin<'tcx>,
        sup_region: Region<'tcx>,
    ) -> ErrorGuaranteed {
        let mut err = self.report_inference_failure(var_origin);

        note_and_explain_region(
            self.tcx,
            &mut err,
            "first, the lifetime cannot outlive ",
            sup_region,
            "...",
            None,
        );

        debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
        debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
        debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
        debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
        debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);

        if let infer::Subtype(ref sup_trace) = sup_origin
            && let infer::Subtype(ref sub_trace) = sub_origin
            && let Some((sup_expected, sup_found, _)) = self.values_str(sup_trace.values)
            && let Some((sub_expected, sub_found, _)) = self.values_str(sub_trace.values)
            && sub_expected == sup_expected
            && sub_found == sup_found
        {
            note_and_explain_region(
                self.tcx,
                &mut err,
                "...but the lifetime must also be valid for ",
                sub_region,
                "...",
                None,
            );
            err.span_note(
                sup_trace.cause.span,
                format!("...so that the {}", sup_trace.cause.as_requirement_str()),
            );

            err.note_expected_found(&"", sup_expected, &"", sup_found);
            return if sub_region.is_error() | sup_region.is_error() {
                err.delay_as_bug()
            } else {
                err.emit()
            };
        }

        self.note_region_origin(&mut err, &sup_origin);

        note_and_explain_region(
            self.tcx,
            &mut err,
            "but, the lifetime must be valid for ",
            sub_region,
            "...",
            None,
        );

        self.note_region_origin(&mut err, &sub_origin);
        if sub_region.is_error() | sup_region.is_error() { err.delay_as_bug() } else { err.emit() }
    }

    /// Determine whether an error associated with the given span and definition
    /// should be treated as being caused by the implicit `From` conversion
    /// within `?` desugaring.
    pub fn is_try_conversion(&self, span: Span, trait_def_id: DefId) -> bool {
        span.is_desugaring(DesugaringKind::QuestionMark)
            && self.tcx.is_diagnostic_item(sym::From, trait_def_id)
    }

    /// Structurally compares two types, modulo any inference variables.
    ///
    /// Returns `true` if two types are equal, or if one type is an inference variable compatible
    /// with the other type. A TyVar inference type is compatible with any type, and an IntVar or
    /// FloatVar inference type are compatible with themselves or their concrete types (Int and
    /// Float types, respectively). When comparing two ADTs, these rules apply recursively.
    pub fn same_type_modulo_infer<T: relate::Relate<'tcx>>(&self, a: T, b: T) -> bool {
        let (a, b) = self.resolve_vars_if_possible((a, b));
        SameTypeModuloInfer(self).relate(a, b).is_ok()
    }
}

struct SameTypeModuloInfer<'a, 'tcx>(&'a InferCtxt<'tcx>);

impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.0.tcx
    }

    fn tag(&self) -> &'static str {
        "SameTypeModuloInfer"
    }

    fn relate_with_variance<T: relate::Relate<'tcx>>(
        &mut self,
        _variance: ty::Variance,
        _info: ty::VarianceDiagInfo<'tcx>,
        a: T,
        b: T,
    ) -> relate::RelateResult<'tcx, T> {
        self.relate(a, b)
    }

    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
        match (a.kind(), b.kind()) {
            (ty::Int(_) | ty::Uint(_), ty::Infer(ty::InferTy::IntVar(_)))
            | (
                ty::Infer(ty::InferTy::IntVar(_)),
                ty::Int(_) | ty::Uint(_) | ty::Infer(ty::InferTy::IntVar(_)),
            )
            | (ty::Float(_), ty::Infer(ty::InferTy::FloatVar(_)))
            | (
                ty::Infer(ty::InferTy::FloatVar(_)),
                ty::Float(_) | ty::Infer(ty::InferTy::FloatVar(_)),
            )
            | (ty::Infer(ty::InferTy::TyVar(_)), _)
            | (_, ty::Infer(ty::InferTy::TyVar(_))) => Ok(a),
            (ty::Infer(_), _) | (_, ty::Infer(_)) => Err(TypeError::Mismatch),
            _ => relate::structurally_relate_tys(self, a, b),
        }
    }

    fn regions(
        &mut self,
        a: ty::Region<'tcx>,
        b: ty::Region<'tcx>,
    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
        if (a.is_var() && b.is_free())
            || (b.is_var() && a.is_free())
            || (a.is_var() && b.is_var())
            || a == b
        {
            Ok(a)
        } else {
            Err(TypeError::Mismatch)
        }
    }

    fn binders<T>(
        &mut self,
        a: ty::Binder<'tcx, T>,
        b: ty::Binder<'tcx, T>,
    ) -> relate::RelateResult<'tcx, ty::Binder<'tcx, T>>
    where
        T: relate::Relate<'tcx>,
    {
        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
    }

    fn consts(
        &mut self,
        a: ty::Const<'tcx>,
        _b: ty::Const<'tcx>,
    ) -> relate::RelateResult<'tcx, ty::Const<'tcx>> {
        // FIXME(compiler-errors): This could at least do some first-order
        // relation
        Ok(a)
    }
}

impl<'tcx> InferCtxt<'tcx> {
    fn report_inference_failure(&self, var_origin: RegionVariableOrigin) -> Diag<'tcx> {
        let br_string = |br: ty::BoundRegionKind| {
            let mut s = match br {
                ty::BrNamed(_, name) => name.to_string(),
                _ => String::new(),
            };
            if !s.is_empty() {
                s.push(' ');
            }
            s
        };
        let var_description = match var_origin {
            infer::MiscVariable(_) => String::new(),
            infer::PatternRegion(_) => " for pattern".to_string(),
            infer::AddrOfRegion(_) => " for borrow expression".to_string(),
            infer::Autoref(_) => " for autoref".to_string(),
            infer::Coercion(_) => " for automatic coercion".to_string(),
            infer::BoundRegion(_, br, infer::FnCall) => {
                format!(" for lifetime parameter {}in function call", br_string(br))
            }
            infer::BoundRegion(_, br, infer::HigherRankedType) => {
                format!(" for lifetime parameter {}in generic type", br_string(br))
            }
            infer::BoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
                " for lifetime parameter {}in trait containing associated type `{}`",
                br_string(br),
                self.tcx.associated_item(def_id).name
            ),
            infer::RegionParameterDefinition(_, name) => {
                format!(" for lifetime parameter `{name}`")
            }
            infer::UpvarRegion(ref upvar_id, _) => {
                let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
                format!(" for capture of `{var_name}` by closure")
            }
            infer::Nll(..) => bug!("NLL variable found in lexical phase"),
        };

        struct_span_code_err!(
            self.tcx.dcx(),
            var_origin.span(),
            E0495,
            "cannot infer an appropriate lifetime{} due to conflicting requirements",
            var_description
        )
    }
}

pub enum FailureCode {
    Error0317,
    Error0580,
    Error0308,
    Error0644,
}

#[extension(pub trait ObligationCauseExt<'tcx>)]
impl<'tcx> ObligationCause<'tcx> {
    fn as_failure_code(&self, terr: TypeError<'tcx>) -> FailureCode {
        use self::FailureCode::*;
        use crate::traits::ObligationCauseCode::*;
        match self.code() {
            IfExpressionWithNoElse => Error0317,
            MainFunctionType => Error0580,
            CompareImplItemObligation { .. }
            | MatchExpressionArm(_)
            | IfExpression { .. }
            | LetElse
            | StartFunctionType
            | LangFunctionType(_)
            | IntrinsicType
            | MethodReceiver => Error0308,

            // In the case where we have no more specific thing to
            // say, also take a look at the error code, maybe we can
            // tailor to that.
            _ => match terr {
                TypeError::CyclicTy(ty)
                    if ty.is_closure() || ty.is_coroutine() || ty.is_coroutine_closure() =>
                {
                    Error0644
                }
                TypeError::IntrinsicCast => Error0308,
                _ => Error0308,
            },
        }
    }
    fn as_failure_code_diag(
        &self,
        terr: TypeError<'tcx>,
        span: Span,
        subdiags: Vec<TypeErrorAdditionalDiags>,
    ) -> ObligationCauseFailureCode {
        use crate::traits::ObligationCauseCode::*;
        match self.code() {
            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => {
                ObligationCauseFailureCode::MethodCompat { span, subdiags }
            }
            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => {
                ObligationCauseFailureCode::TypeCompat { span, subdiags }
            }
            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
                ObligationCauseFailureCode::ConstCompat { span, subdiags }
            }
            BlockTailExpression(.., hir::MatchSource::TryDesugar(_)) => {
                ObligationCauseFailureCode::TryCompat { span, subdiags }
            }
            MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
                hir::MatchSource::TryDesugar(_) => {
                    ObligationCauseFailureCode::TryCompat { span, subdiags }
                }
                _ => ObligationCauseFailureCode::MatchCompat { span, subdiags },
            },
            IfExpression { .. } => ObligationCauseFailureCode::IfElseDifferent { span, subdiags },
            IfExpressionWithNoElse => ObligationCauseFailureCode::NoElse { span },
            LetElse => ObligationCauseFailureCode::NoDiverge { span, subdiags },
            MainFunctionType => ObligationCauseFailureCode::FnMainCorrectType { span },
            StartFunctionType => ObligationCauseFailureCode::FnStartCorrectType { span, subdiags },
            &LangFunctionType(lang_item_name) => {
                ObligationCauseFailureCode::FnLangCorrectType { span, subdiags, lang_item_name }
            }
            IntrinsicType => ObligationCauseFailureCode::IntrinsicCorrectType { span, subdiags },
            MethodReceiver => ObligationCauseFailureCode::MethodCorrectType { span, subdiags },

            // In the case where we have no more specific thing to
            // say, also take a look at the error code, maybe we can
            // tailor to that.
            _ => match terr {
                TypeError::CyclicTy(ty)
                    if ty.is_closure() || ty.is_coroutine() || ty.is_coroutine_closure() =>
                {
                    ObligationCauseFailureCode::ClosureSelfref { span }
                }
                TypeError::IntrinsicCast => {
                    ObligationCauseFailureCode::CantCoerce { span, subdiags }
                }
                _ => ObligationCauseFailureCode::Generic { span, subdiags },
            },
        }
    }

    fn as_requirement_str(&self) -> &'static str {
        use crate::traits::ObligationCauseCode::*;
        match self.code() {
            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => {
                "method type is compatible with trait"
            }
            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => {
                "associated type is compatible with trait"
            }
            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
                "const is compatible with trait"
            }
            MainFunctionType => "`main` function has the correct type",
            StartFunctionType => "`#[start]` function has the correct type",
            LangFunctionType(_) => "lang item function has the correct type",
            IntrinsicType => "intrinsic has the correct type",
            MethodReceiver => "method receiver has the correct type",
            _ => "types are compatible",
        }
    }
}

/// Newtype to allow implementing IntoDiagArg
pub struct ObligationCauseAsDiagArg<'tcx>(pub ObligationCause<'tcx>);

impl IntoDiagArg for ObligationCauseAsDiagArg<'_> {
    fn into_diag_arg(self) -> rustc_errors::DiagArgValue {
        use crate::traits::ObligationCauseCode::*;
        let kind = match self.0.code() {
            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => "method_compat",
            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => "type_compat",
            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => "const_compat",
            MainFunctionType => "fn_main_correct_type",
            StartFunctionType => "fn_start_correct_type",
            LangFunctionType(_) => "fn_lang_correct_type",
            IntrinsicType => "intrinsic_correct_type",
            MethodReceiver => "method_correct_type",
            _ => "other",
        }
        .into();
        rustc_errors::DiagArgValue::Str(kind)
    }
}

/// This is a bare signal of what kind of type we're dealing with. `ty::TyKind` tracks
/// extra information about each type, but we only care about the category.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub enum TyCategory {
    Closure,
    Opaque,
    OpaqueFuture,
    Coroutine(hir::CoroutineKind),
    Foreign,
}

impl fmt::Display for TyCategory {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Closure => "closure".fmt(f),
            Self::Opaque => "opaque type".fmt(f),
            Self::OpaqueFuture => "future".fmt(f),
            Self::Coroutine(gk) => gk.fmt(f),
            Self::Foreign => "foreign type".fmt(f),
        }
    }
}

impl TyCategory {
    pub fn from_ty(tcx: TyCtxt<'_>, ty: Ty<'_>) -> Option<(Self, DefId)> {
        match *ty.kind() {
            ty::Closure(def_id, _) => Some((Self::Closure, def_id)),
            ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) => {
                let kind =
                    if tcx.ty_is_opaque_future(ty) { Self::OpaqueFuture } else { Self::Opaque };
                Some((kind, def_id))
            }
            ty::Coroutine(def_id, ..) => {
                Some((Self::Coroutine(tcx.coroutine_kind(def_id).unwrap()), def_id))
            }
            ty::Foreign(def_id) => Some((Self::Foreign, def_id)),
            _ => None,
        }
    }
}

impl<'tcx> InferCtxt<'tcx> {
    /// Given a [`hir::Block`], get the span of its last expression or
    /// statement, peeling off any inner blocks.
    pub fn find_block_span(&self, block: &'tcx hir::Block<'tcx>) -> Span {
        let block = block.innermost_block();
        if let Some(expr) = &block.expr {
            expr.span
        } else if let Some(stmt) = block.stmts.last() {
            // possibly incorrect trailing `;` in the else arm
            stmt.span
        } else {
            // empty block; point at its entirety
            block.span
        }
    }

    /// Given a [`hir::HirId`] for a block, get the span of its last expression
    /// or statement, peeling off any inner blocks.
    pub fn find_block_span_from_hir_id(&self, hir_id: hir::HirId) -> Span {
        match self.tcx.hir_node(hir_id) {
            hir::Node::Block(blk) => self.find_block_span(blk),
            // The parser was in a weird state if either of these happen, but
            // it's better not to panic.
            hir::Node::Expr(e) => e.span,
            _ => rustc_span::DUMMY_SP,
        }
    }
}