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
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! See `doc.rs` for high-level documentation
#![allow(dead_code)] // FIXME -- just temporarily

pub use self::MethodMatchResult::*;
pub use self::MethodMatchedData::*;
use self::SelectionCandidate::*;
use self::BuiltinBoundConditions::*;
use self::EvaluationResult::*;

use super::{DerivedObligationCause};
use super::{PredicateObligation, Obligation, TraitObligation, ObligationCause};
use super::{ObligationCauseCode, BuiltinDerivedObligation};
use super::{SelectionError, Unimplemented, Overflow, OutputTypeParameterMismatch};
use super::{Selection};
use super::{SelectionResult};
use super::{VtableBuiltin, VtableImpl, VtableParam, VtableUnboxedClosure, VtableFnPointer};
use super::{VtableImplData, VtableBuiltinData};
use super::{util};

use middle::fast_reject;
use middle::mem_categorization::Typer;
use middle::subst::{Subst, Substs, VecPerParamSpace};
use middle::ty::{mod, AsPredicate, RegionEscape, ToPolyTraitRef, Ty};
use middle::infer;
use middle::infer::{InferCtxt, TypeFreshener};
use middle::ty_fold::TypeFoldable;
use std::cell::RefCell;
use std::collections::hash_map::HashMap;
use std::rc::Rc;
use syntax::{abi, ast};
use util::common::ErrorReported;
use util::ppaux::Repr;

pub struct SelectionContext<'cx, 'tcx:'cx> {
    infcx: &'cx InferCtxt<'cx, 'tcx>,
    param_env: &'cx ty::ParameterEnvironment<'tcx>,
    typer: &'cx (Typer<'tcx>+'cx),

    /// Freshener used specifically for skolemizing entries on the
    /// obligation stack. This ensures that all entries on the stack
    /// at one time will have the same set of skolemized entries,
    /// which is important for checking for trait bounds that
    /// recursively require themselves.
    freshener: TypeFreshener<'cx, 'tcx>,

    /// If true, indicates that the evaluation should be conservative
    /// and consider the possibility of types outside this crate.
    /// This comes up primarily when resolving ambiguity. Imagine
    /// there is some trait reference `$0 : Bar` where `$0` is an
    /// inference variable. If `intercrate` is true, then we can never
    /// say for sure that this reference is not implemented, even if
    /// there are *no impls at all for `Bar`*, because `$0` could be
    /// bound to some type that in a downstream crate that implements
    /// `Bar`. This is the suitable mode for coherence. Elsewhere,
    /// though, we set this to false, because we are only interested
    /// in types that the user could actually have written --- in
    /// other words, we consider `$0 : Bar` to be unimplemented if
    /// there is no type that the user could *actually name* that
    /// would satisfy it. This avoids crippling inference, basically.
    intercrate: bool,
}

// A stack that walks back up the stack frame.
struct TraitObligationStack<'prev, 'tcx: 'prev> {
    obligation: &'prev TraitObligation<'tcx>,

    /// Trait ref from `obligation` but skolemized with the
    /// selection-context's freshener. Used to check for recursion.
    fresh_trait_ref: ty::PolyTraitRef<'tcx>,

    previous: Option<&'prev TraitObligationStack<'prev, 'tcx>>
}

#[deriving(Clone)]
pub struct SelectionCache<'tcx> {
    hashmap: RefCell<HashMap<Rc<ty::TraitRef<'tcx>>,
                             SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
}

pub enum MethodMatchResult {
    MethodMatched(MethodMatchedData),
    MethodAmbiguous(/* list of impls that could apply */ Vec<ast::DefId>),
    MethodDidNotMatch,
}

#[deriving(Copy, Show)]
pub enum MethodMatchedData {
    // In the case of a precise match, we don't really need to store
    // how the match was found. So don't.
    PreciseMethodMatch,

    // In the case of a coercion, we need to know the precise impl so
    // that we can determine the type to which things were coerced.
    CoerciveMethodMatch(/* impl we matched */ ast::DefId)
}

/// The selection process begins by considering all impls, where
/// clauses, and so forth that might resolve an obligation.  Sometimes
/// we'll be able to say definitively that (e.g.) an impl does not
/// apply to the obligation: perhaps it is defined for `uint` but the
/// obligation is for `int`. In that case, we drop the impl out of the
/// list.  But the other cases are considered *candidates*.
///
/// Candidates can either be definitive or ambiguous. An ambiguous
/// candidate is one that might match or might not, depending on how
/// type variables wind up being resolved. This only occurs during inference.
///
/// For selection to succeed, there must be exactly one non-ambiguous
/// candidate.  Usually, it is not possible to have more than one
/// definitive candidate, due to the coherence rules. However, there is
/// one case where it could occur: if there is a blanket impl for a
/// trait (that is, an impl applied to all T), and a type parameter
/// with a where clause. In that case, we can have a candidate from the
/// where clause and a second candidate from the impl. This is not a
/// problem because coherence guarantees us that the impl which would
/// be used to satisfy the where clause is the same one that we see
/// now. To resolve this issue, therefore, we ignore impls if we find a
/// matching where clause. Part of the reason for this is that where
/// clauses can give additional information (like, the types of output
/// parameters) that would have to be inferred from the impl.
#[deriving(PartialEq,Eq,Show,Clone)]
enum SelectionCandidate<'tcx> {
    BuiltinCandidate(ty::BuiltinBound),
    ParamCandidate(ty::PolyTraitRef<'tcx>),
    ImplCandidate(ast::DefId),

    /// This is a trait matching with a projected type as `Self`, and
    /// we found an applicable bound in the trait definition.
    ProjectionCandidate,

    /// Implementation of a `Fn`-family trait by one of the
    /// anonymous types generated for a `||` expression.
    UnboxedClosureCandidate(/* closure */ ast::DefId, Substs<'tcx>),

    /// Implementation of a `Fn`-family trait by one of the anonymous
    /// types generated for a fn pointer type (e.g., `fn(int)->int`)
    FnPointerCandidate,

    ErrorCandidate,
}

struct SelectionCandidateSet<'tcx> {
    // a list of candidates that definitely apply to the current
    // obligation (meaning: types unify).
    vec: Vec<SelectionCandidate<'tcx>>,

    // if this is true, then there were candidates that might or might
    // not have applied, but we couldn't tell. This occurs when some
    // of the input types are type variables, in which case there are
    // various "builtin" rules that might or might not trigger.
    ambiguous: bool,
}

enum BuiltinBoundConditions<'tcx> {
    If(Vec<Ty<'tcx>>),
    ParameterBuiltin,
    AmbiguousBuiltin
}

#[deriving(Show)]
enum EvaluationResult<'tcx> {
    EvaluatedToOk,
    EvaluatedToAmbig,
    EvaluatedToErr(SelectionError<'tcx>),
}

impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
    pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>,
               param_env: &'cx ty::ParameterEnvironment<'tcx>,
               typer: &'cx Typer<'tcx>)
               -> SelectionContext<'cx, 'tcx> {
        SelectionContext {
            infcx: infcx,
            param_env: param_env,
            typer: typer,
            freshener: infcx.freshener(),
            intercrate: false,
        }
    }

    pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>,
                      param_env: &'cx ty::ParameterEnvironment<'tcx>,
                      typer: &'cx Typer<'tcx>)
                      -> SelectionContext<'cx, 'tcx> {
        SelectionContext {
            infcx: infcx,
            param_env: param_env,
            typer: typer,
            freshener: infcx.freshener(),
            intercrate: true,
        }
    }

    pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
        self.infcx
    }

    pub fn param_env(&self) -> &'cx ty::ParameterEnvironment<'tcx> {
        self.param_env
    }

    pub fn tcx(&self) -> &'cx ty::ctxt<'tcx> {
        self.infcx.tcx
    }

    ///////////////////////////////////////////////////////////////////////////
    // Selection
    //
    // The selection phase tries to identify *how* an obligation will
    // be resolved. For example, it will identify which impl or
    // parameter bound is to be used. The process can be inconclusive
    // if the self type in the obligation is not fully inferred. Selection
    // can result in an error in one of two ways:
    //
    // 1. If no applicable impl or parameter bound can be found.
    // 2. If the output type parameters in the obligation do not match
    //    those specified by the impl/bound. For example, if the obligation
    //    is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
    //    `impl<T> Iterable<T> for Vec<T>`, than an error would result.

    /// Evaluates whether the obligation can be satisfied. Returns an indication of whether the
    /// obligation can be satisfied and, if so, by what means. Never affects surrounding typing
    /// environment.
    pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
                  -> SelectionResult<'tcx, Selection<'tcx>> {
        debug!("select({})", obligation.repr(self.tcx()));
        assert!(!obligation.predicate.has_escaping_regions());

        let stack = self.push_stack(None, obligation);
        match try!(self.candidate_from_obligation(&stack)) {
            None => Ok(None),
            Some(candidate) => Ok(Some(try!(self.confirm_candidate(obligation, candidate)))),
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // EVALUATION
    //
    // Tests whether an obligation can be selected or whether an impl
    // can be applied to particular types. It skips the "confirmation"
    // step and hence completely ignores output type parameters.
    //
    // The result is "true" if the obligation *may* hold and "false" if
    // we can be sure it does not.

    /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
    pub fn evaluate_obligation(&mut self,
                               obligation: &PredicateObligation<'tcx>)
                               -> bool
    {
        debug!("evaluate_obligation({})",
               obligation.repr(self.tcx()));

        self.evaluate_predicate_recursively(None, obligation).may_apply()
    }

    fn evaluate_builtin_bound_recursively<'o>(&mut self,
                                              bound: ty::BuiltinBound,
                                              previous_stack: &TraitObligationStack<'o, 'tcx>,
                                              ty: Ty<'tcx>)
                                              -> EvaluationResult<'tcx>
    {
        let obligation =
            util::predicate_for_builtin_bound(
                self.tcx(),
                previous_stack.obligation.cause.clone(),
                bound,
                previous_stack.obligation.recursion_depth + 1,
                ty);

        match obligation {
            Ok(obligation) => {
                self.evaluate_predicate_recursively(Some(previous_stack), &obligation)
            }
            Err(ErrorReported) => {
                EvaluatedToOk
            }
        }
    }

    fn evaluate_predicate_recursively<'o>(&mut self,
                                          previous_stack: Option<&TraitObligationStack<'o, 'tcx>>,
                                          obligation: &PredicateObligation<'tcx>)
                                           -> EvaluationResult<'tcx>
    {
        debug!("evaluate_predicate_recursively({})",
               obligation.repr(self.tcx()));

        match obligation.predicate {
            ty::Predicate::Trait(ref t) => {
                assert!(!t.has_escaping_regions());
                let obligation = obligation.with(t.clone());
                self.evaluate_obligation_recursively(previous_stack, &obligation)
            }

            ty::Predicate::Equate(ref p) => {
                let result = self.infcx.probe(|_| {
                    self.infcx.equality_predicate(obligation.cause.span, p)
                });
                match result {
                    Ok(()) => EvaluatedToOk,
                    Err(_) => EvaluatedToErr(Unimplemented),
                }
            }

            ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
                // we do not consider region relationships when
                // evaluating trait matches
                EvaluatedToOk
            }

            ty::Predicate::Projection(..) => {
                // FIXME(#20296) -- we should be able to give a more precise answer here
                EvaluatedToAmbig
            }
        }
    }

    fn evaluate_obligation_recursively<'o>(&mut self,
                                           previous_stack: Option<&TraitObligationStack<'o, 'tcx>>,
                                           obligation: &TraitObligation<'tcx>)
                                           -> EvaluationResult<'tcx>
    {
        debug!("evaluate_obligation_recursively({})",
               obligation.repr(self.tcx()));

        let stack = self.push_stack(previous_stack.map(|x| x), obligation);

        let result = self.evaluate_stack(&stack);

        debug!("result: {}", result);
        result
    }

    fn evaluate_stack<'o>(&mut self,
                          stack: &TraitObligationStack<'o, 'tcx>)
                          -> EvaluationResult<'tcx>
    {
        // In intercrate mode, whenever any of the types are unbound,
        // there can always be an impl. Even if there are no impls in
        // this crate, perhaps the type would be unified with
        // something from another crate that does provide an impl.
        //
        // In intracrate mode, we must still be conservative. The reason is
        // that we want to avoid cycles. Imagine an impl like:
        //
        //     impl<T:Eq> Eq for Vec<T>
        //
        // and a trait reference like `$0 : Eq` where `$0` is an
        // unbound variable. When we evaluate this trait-reference, we
        // will unify `$0` with `Vec<$1>` (for some fresh variable
        // `$1`), on the condition that `$1 : Eq`. We will then wind
        // up with many candidates (since that are other `Eq` impls
        // that apply) and try to winnow things down. This results in
        // a recurssive evaluation that `$1 : Eq` -- as you can
        // imagine, this is just where we started. To avoid that, we
        // check for unbound variables and return an ambiguous (hence possible)
        // match if we've seen this trait before.
        //
        // This suffices to allow chains like `FnMut` implemented in
        // terms of `Fn` etc, but we could probably make this more
        // precise still.
        let input_types = stack.fresh_trait_ref.0.input_types();
        let unbound_input_types = input_types.iter().any(|&t| ty::type_is_fresh(t));
        if
            unbound_input_types &&
             (self.intercrate ||
              stack.iter().skip(1).any(
                  |prev| stack.fresh_trait_ref.def_id() == prev.fresh_trait_ref.def_id()))
        {
            debug!("evaluate_stack({}) --> unbound argument, recursion -->  ambiguous",
                   stack.fresh_trait_ref.repr(self.tcx()));
            return EvaluatedToAmbig;
        }

        // If there is any previous entry on the stack that precisely
        // matches this obligation, then we can assume that the
        // obligation is satisfied for now (still all other conditions
        // must be met of course). One obvious case this comes up is
        // marker traits like `Send`. Think of a linked list:
        //
        //    struct List<T> { data: T, next: Option<Box<List<T>>> {
        //
        // `Box<List<T>>` will be `Send` if `T` is `Send` and
        // `Option<Box<List<T>>>` is `Send`, and in turn
        // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
        // `Send`.
        //
        // Note that we do this comparison using the `fresh_trait_ref`
        // fields. Because these have all been skolemized using
        // `self.freshener`, we can be sure that (a) this will not
        // affect the inferencer state and (b) that if we see two
        // skolemized types with the same index, they refer to the
        // same unbound type variable.
        if
            stack.iter()
            .skip(1) // skip top-most frame
            .any(|prev| stack.fresh_trait_ref == prev.fresh_trait_ref)
        {
            debug!("evaluate_stack({}) --> recursive",
                   stack.fresh_trait_ref.repr(self.tcx()));
            return EvaluatedToOk;
        }

        match self.candidate_from_obligation(stack) {
            Ok(Some(c)) => self.winnow_candidate(stack, &c),
            Ok(None) => EvaluatedToAmbig,
            Err(e) => EvaluatedToErr(e),
        }
    }

    /// Evaluates whether the impl with id `impl_def_id` could be applied to the self type
    /// `obligation_self_ty`. This can be used either for trait or inherent impls.
    pub fn evaluate_impl(&mut self,
                         impl_def_id: ast::DefId,
                         obligation: &TraitObligation<'tcx>)
                         -> bool
    {
        debug!("evaluate_impl(impl_def_id={}, obligation={})",
               impl_def_id.repr(self.tcx()),
               obligation.repr(self.tcx()));

        self.infcx.probe(|snapshot| {
            let (skol_obligation_trait_ref, skol_map) =
                self.infcx().skolemize_late_bound_regions(&obligation.predicate, snapshot);
            match self.match_impl(impl_def_id, obligation, snapshot,
                                  &skol_map, skol_obligation_trait_ref.trait_ref.clone()) {
                Ok(substs) => {
                    let vtable_impl = self.vtable_impl(impl_def_id,
                                                       substs,
                                                       obligation.cause.clone(),
                                                       obligation.recursion_depth + 1,
                                                       skol_map,
                                                       snapshot);
                    self.winnow_selection(None, VtableImpl(vtable_impl)).may_apply()
                }
                Err(()) => {
                    false
                }
            }
        })
    }

    ///////////////////////////////////////////////////////////////////////////
    // CANDIDATE ASSEMBLY
    //
    // The selection process begins by examining all in-scope impls,
    // caller obligations, and so forth and assembling a list of
    // candidates. See `doc.rs` and the `Candidate` type for more details.

    fn candidate_from_obligation<'o>(&mut self,
                                     stack: &TraitObligationStack<'o, 'tcx>)
                                     -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
    {
        // Watch out for overflow. This intentionally bypasses (and does
        // not update) the cache.
        let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
        if stack.obligation.recursion_depth >= recursion_limit {
            debug!("{} --> overflow (limit={})",
                   stack.obligation.repr(self.tcx()),
                   recursion_limit);
            return Err(Overflow)
        }

        // Check the cache. Note that we skolemize the trait-ref
        // separately rather than using `stack.fresh_trait_ref` -- this
        // is because we want the unbound variables to be replaced
        // with fresh skolemized types starting from index 0.
        let cache_fresh_trait_pred =
            self.infcx.freshen(stack.obligation.predicate.clone());
        debug!("candidate_from_obligation(cache_fresh_trait_pred={}, obligation={})",
               cache_fresh_trait_pred.repr(self.tcx()),
               stack.repr(self.tcx()));
        assert!(!stack.obligation.predicate.has_escaping_regions());

        match self.check_candidate_cache(&cache_fresh_trait_pred) {
            Some(c) => {
                debug!("CACHE HIT: cache_fresh_trait_pred={}, candidate={}",
                       cache_fresh_trait_pred.repr(self.tcx()),
                       c.repr(self.tcx()));
                return c;
            }
            None => { }
        }

        // If no match, compute result and insert into cache.
        let candidate = self.candidate_from_obligation_no_cache(stack);
        debug!("CACHE MISS: cache_fresh_trait_pred={}, candidate={}",
               cache_fresh_trait_pred.repr(self.tcx()), candidate.repr(self.tcx()));
        self.insert_candidate_cache(cache_fresh_trait_pred, candidate.clone());
        candidate
    }

    fn candidate_from_obligation_no_cache<'o>(&mut self,
                                              stack: &TraitObligationStack<'o, 'tcx>)
                                              -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
    {
        if ty::type_is_error(stack.obligation.predicate.0.self_ty()) {
            return Ok(Some(ErrorCandidate));
        }

        let candidate_set = try!(self.assemble_candidates(stack));

        if candidate_set.ambiguous {
            debug!("candidate set contains ambig");
            return Ok(None);
        }

        let mut candidates = candidate_set.vec;

        debug!("assembled {} candidates for {}: {}",
               candidates.len(),
               stack.repr(self.tcx()),
               candidates.repr(self.tcx()));

        // At this point, we know that each of the entries in the
        // candidate set is *individually* applicable. Now we have to
        // figure out if they contain mutual incompatibilities. This
        // frequently arises if we have an unconstrained input type --
        // for example, we are looking for $0:Eq where $0 is some
        // unconstrained type variable. In that case, we'll get a
        // candidate which assumes $0 == int, one that assumes $0 ==
        // uint, etc. This spells an ambiguity.

        // If there is more than one candidate, first winnow them down
        // by considering extra conditions (nested obligations and so
        // forth). We don't winnow if there is exactly one
        // candidate. This is a relatively minor distinction but it
        // can lead to better inference and error-reporting. An
        // example would be if there was an impl:
        //
        //     impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
        //
        // and we were to see some code `foo.push_clone()` where `boo`
        // is a `Vec<Bar>` and `Bar` does not implement `Clone`.  If
        // we were to winnow, we'd wind up with zero candidates.
        // Instead, we select the right impl now but report `Bar does
        // not implement Clone`.
        if candidates.len() > 1 {
            candidates.retain(|c| self.winnow_candidate(stack, c).may_apply())
        }

        // If there are STILL multiple candidate, we can further reduce
        // the list by dropping duplicates.
        if candidates.len() > 1 {
            let mut i = 0;
            while i < candidates.len() {
                let is_dup =
                    range(0, candidates.len())
                    .filter(|&j| i != j)
                    .any(|j| self.candidate_should_be_dropped_in_favor_of(stack,
                                                                          &candidates[i],
                                                                          &candidates[j]));
                if is_dup {
                    debug!("Dropping candidate #{}/{}: {}",
                           i, candidates.len(), candidates[i].repr(self.tcx()));
                    candidates.swap_remove(i);
                } else {
                    debug!("Retaining candidate #{}/{}: {}",
                           i, candidates.len(), candidates[i].repr(self.tcx()));
                    i += 1;
                }
            }
        }

        // If there are *STILL* multiple candidates, give up and
        // report ambiguiuty.
        if candidates.len() > 1 {
            debug!("multiple matches, ambig");
            return Ok(None);
        }

        // If there are *NO* candidates, that there are no impls --
        // that we know of, anyway. Note that in the case where there
        // are unbound type variables within the obligation, it might
        // be the case that you could still satisfy the obligation
        // from another crate by instantiating the type variables with
        // a type from another crate that does have an impl. This case
        // is checked for in `evaluate_stack` (and hence users
        // who might care about this case, like coherence, should use
        // that function).
        if candidates.len() == 0 {
            return Err(Unimplemented);
        }

        // Just one candidate left.
        let candidate = candidates.pop().unwrap();
        Ok(Some(candidate))
    }

    fn pick_candidate_cache(&self,
                            cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
                            -> &SelectionCache<'tcx>
    {
        // High-level idea: we have to decide whether to consult the
        // cache that is specific to this scope, or to consult the
        // global cache. We want the cache that is specific to this
        // scope whenever where clauses might affect the result.

        // Avoid using the master cache during coherence and just rely
        // on the local cache. This effectively disables caching
        // during coherence. It is really just a simplification to
        // avoid us having to fear that coherence results "pollute"
        // the master cache. Since coherence executes pretty quickly,
        // it's not worth going to more trouble to increase the
        // hit-rate I don't think.
        if self.intercrate {
            return &self.param_env.selection_cache;
        }

        // If the trait refers to any parameters in scope, then use
        // the cache of the param-environment.
        if
            cache_fresh_trait_pred.0.input_types().iter().any(
                |&t| ty::type_has_self(t) || ty::type_has_params(t))
        {
            return &self.param_env.selection_cache;
        }

        // If the trait refers to unbound type variables, and there
        // are where clauses in scope, then use the local environment.
        // If there are no where clauses in scope, which is a very
        // common case, then we can use the global environment.
        // See the discussion in doc.rs for more details.
        if
            !self.param_env.caller_bounds.is_empty() &&
            cache_fresh_trait_pred.0.input_types().iter().any(
                |&t| ty::type_has_ty_infer(t))
        {
            return &self.param_env.selection_cache;
        }

        // Otherwise, we can use the global cache.
        &self.tcx().selection_cache
    }

    fn check_candidate_cache(&mut self,
                             cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
                             -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
    {
        let cache = self.pick_candidate_cache(cache_fresh_trait_pred);
        let hashmap = cache.hashmap.borrow();
        hashmap.get(&cache_fresh_trait_pred.0.trait_ref).map(|c| (*c).clone())
    }

    fn insert_candidate_cache(&mut self,
                              cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
                              candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
    {
        let cache = self.pick_candidate_cache(&cache_fresh_trait_pred);
        let mut hashmap = cache.hashmap.borrow_mut();
        hashmap.insert(cache_fresh_trait_pred.0.trait_ref.clone(), candidate);
    }

    fn assemble_candidates<'o>(&mut self,
                               stack: &TraitObligationStack<'o, 'tcx>)
                               -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
    {
        // Check for overflow.

        let TraitObligationStack { obligation, .. } = *stack;

        let mut candidates = SelectionCandidateSet {
            vec: Vec::new(),
            ambiguous: false
        };

        // Other bounds. Consider both in-scope bounds from fn decl
        // and applicable impls. There is a certain set of precedence rules here.

        match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
            Some(ty::BoundCopy) => {
                debug!("obligation self ty is {}",
                       obligation.predicate.0.self_ty().repr(self.tcx()));

                // If the user has asked for the older, compatibility
                // behavior, ignore user-defined impls here. This will
                // go away by the time 1.0 is released.
                if !self.tcx().sess.features.borrow().opt_out_copy {
                    try!(self.assemble_candidates_from_impls(obligation, &mut candidates.vec));
                }

                try!(self.assemble_builtin_bound_candidates(ty::BoundCopy,
                                                            stack,
                                                            &mut candidates));
            }
            Some(bound @ ty::BoundSend) |
            Some(bound @ ty::BoundSync) => {
                try!(self.assemble_candidates_from_impls(obligation, &mut candidates.vec));

                // No explicit impls were declared for this type, consider the fallback rules.
                if candidates.vec.is_empty() {
                    try!(self.assemble_builtin_bound_candidates(bound, stack, &mut candidates));
                }
            }

            Some(bound @ ty::BoundSized) => {
                // Sized and Copy are always automatically computed.
                try!(self.assemble_builtin_bound_candidates(bound, stack, &mut candidates));
            }

            None => {
                // For the time being, we ignore user-defined impls for builtin-bounds, other than
                // `Copy`.
                // (And unboxed candidates only apply to the Fn/FnMut/etc traits.)
                try!(self.assemble_unboxed_closure_candidates(obligation, &mut candidates));
                try!(self.assemble_fn_pointer_candidates(obligation, &mut candidates));
                try!(self.assemble_candidates_from_impls(obligation, &mut candidates.vec));
            }
        }

        self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
        try!(self.assemble_candidates_from_caller_bounds(obligation, &mut candidates));
        debug!("candidate list size: {}", candidates.vec.len());
        Ok(candidates)
    }

    fn assemble_candidates_from_projected_tys(&mut self,
                                              obligation: &TraitObligation<'tcx>,
                                              candidates: &mut SelectionCandidateSet<'tcx>)
    {
        let poly_trait_predicate =
            self.infcx().resolve_type_vars_if_possible(&obligation.predicate);

        debug!("assemble_candidates_for_projected_tys({},{})",
               obligation.repr(self.tcx()),
               poly_trait_predicate.repr(self.tcx()));

        // FIXME(#20297) -- just examining the self-type is very simplistic

        // before we go into the whole skolemization thing, just
        // quickly check if the self-type is a projection at all.
        let trait_def_id = match poly_trait_predicate.0.trait_ref.self_ty().sty {
            ty::ty_projection(ref data) => data.trait_ref.def_id,
            ty::ty_infer(ty::TyVar(_)) => {
                // If the self-type is an inference variable, then it MAY wind up
                // being a projected type, so induce an ambiguity.
                //
                // FIXME(#20297) -- being strict about this can cause
                // inference failures with BorrowFrom, which is
                // unfortunate. Can we do better here?
                candidates.ambiguous = true;
                return;
            }
            _ => { return; }
        };

        debug!("assemble_candidates_for_projected_tys: trait_def_id={}",
               trait_def_id.repr(self.tcx()));

        let result = self.infcx.probe(|snapshot| {
            self.match_projection_obligation_against_bounds_from_trait(obligation,
                                                                       snapshot)
        });

        if result {
            candidates.vec.push(ProjectionCandidate);
        }
    }

    fn match_projection_obligation_against_bounds_from_trait(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        snapshot: &infer::CombinedSnapshot)
        -> bool
    {
        let poly_trait_predicate =
            self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
        let (skol_trait_predicate, skol_map) =
            self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
        debug!("match_projection_obligation_against_bounds_from_trait: \
                skol_trait_predicate={} skol_map={}",
               skol_trait_predicate.repr(self.tcx()),
               skol_map.repr(self.tcx()));

        let projection_trait_ref = match skol_trait_predicate.trait_ref.self_ty().sty {
            ty::ty_projection(ref data) => &data.trait_ref,
            _ => {
                self.tcx().sess.span_bug(
                    obligation.cause.span,
                    format!("match_projection_obligation_against_bounds_from_trait() called \
                             but self-ty not a projection: {}",
                            skol_trait_predicate.trait_ref.self_ty().repr(self.tcx())).as_slice());
            }
        };
        debug!("match_projection_obligation_against_bounds_from_trait: \
                projection_trait_ref={}",
               projection_trait_ref.repr(self.tcx()));

        let trait_def = ty::lookup_trait_def(self.tcx(), projection_trait_ref.def_id);
        let bounds = trait_def.generics.to_bounds(self.tcx(), projection_trait_ref.substs);
        debug!("match_projection_obligation_against_bounds_from_trait: \
                bounds={}",
               bounds.repr(self.tcx()));

        let matching_bound =
            util::elaborate_predicates(self.tcx(), bounds.predicates.to_vec())
            .filter_to_traits()
            .find(
                |bound| self.infcx.probe(
                    |_| self.match_projection(obligation,
                                              bound.clone(),
                                              skol_trait_predicate.trait_ref.clone(),
                                              &skol_map,
                                              snapshot)));

        debug!("match_projection_obligation_against_bounds_from_trait: \
                matching_bound={}",
               matching_bound.repr(self.tcx()));
        match matching_bound {
            None => false,
            Some(bound) => {
                // Repeat the successful match, if any, this time outside of a probe.
                let result = self.match_projection(obligation,
                                                   bound,
                                                   skol_trait_predicate.trait_ref.clone(),
                                                   &skol_map,
                                                   snapshot);
                assert!(result);
                true
            }
        }
    }

    fn match_projection(&mut self,
                        obligation: &TraitObligation<'tcx>,
                        trait_bound: ty::PolyTraitRef<'tcx>,
                        skol_trait_ref: Rc<ty::TraitRef<'tcx>>,
                        skol_map: &infer::SkolemizationMap,
                        snapshot: &infer::CombinedSnapshot)
                        -> bool
    {
        assert!(!skol_trait_ref.has_escaping_regions());
        let origin = infer::RelateOutputImplTypes(obligation.cause.span);
        match self.infcx.sub_poly_trait_refs(false,
                                             origin,
                                             trait_bound.clone(),
                                             ty::Binder(skol_trait_ref.clone())) {
            Ok(()) => { }
            Err(_) => { return false; }
        }

        self.infcx.leak_check(skol_map, snapshot).is_ok()
    }

    /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
    /// supplied to find out whether it is listed among them.
    ///
    /// Never affects inference environment.
    fn assemble_candidates_from_caller_bounds(&mut self,
                                              obligation: &TraitObligation<'tcx>,
                                              candidates: &mut SelectionCandidateSet<'tcx>)
                                              -> Result<(),SelectionError<'tcx>>
    {
        debug!("assemble_candidates_from_caller_bounds({})",
               obligation.repr(self.tcx()));

        let caller_trait_refs: Vec<_> =
            self.param_env.caller_bounds.predicates.iter()
            .filter_map(|o| o.to_opt_poly_trait_ref())
            .collect();

        let all_bounds =
            util::transitive_bounds(
                self.tcx(), caller_trait_refs[]);

        let matching_bounds =
            all_bounds.filter(
                |bound| self.infcx.probe(
                    |_| self.match_where_clause(obligation, bound.clone())).is_ok());

        let param_candidates =
            matching_bounds.map(|bound| ParamCandidate(bound));

        candidates.vec.extend(param_candidates);

        Ok(())
    }

    /// Check for the artificial impl that the compiler will create for an obligation like `X :
    /// FnMut<..>` where `X` is an unboxed closure type.
    ///
    /// Note: the type parameters on an unboxed closure candidate are modeled as *output* type
    /// parameters and hence do not affect whether this trait is a match or not. They will be
    /// unified during the confirmation step.
    fn assemble_unboxed_closure_candidates(&mut self,
                                           obligation: &TraitObligation<'tcx>,
                                           candidates: &mut SelectionCandidateSet<'tcx>)
                                           -> Result<(),SelectionError<'tcx>>
    {
        let kind = match self.fn_family_trait_kind(obligation.predicate.0.def_id()) {
            Some(k) => k,
            None => { return Ok(()); }
        };

        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
        let (closure_def_id, substs) = match self_ty.sty {
            ty::ty_unboxed_closure(id, _, ref substs) => (id, substs.clone()),
            ty::ty_infer(ty::TyVar(_)) => {
                candidates.ambiguous = true;
                return Ok(());
            }
            _ => { return Ok(()); }
        };

        debug!("assemble_unboxed_candidates: self_ty={} kind={} obligation={}",
               self_ty.repr(self.tcx()),
               kind,
               obligation.repr(self.tcx()));

        let closure_kind = match self.typer.unboxed_closures().borrow().get(&closure_def_id) {
            Some(closure) => closure.kind,
            None => {
                self.tcx().sess.span_bug(
                    obligation.cause.span,
                    format!("No entry for unboxed closure: {}",
                            closure_def_id.repr(self.tcx()))[]);
            }
        };

        debug!("closure_kind = {}", closure_kind);

        if closure_kind == kind {
            candidates.vec.push(UnboxedClosureCandidate(closure_def_id, substs.clone()));
        }

        Ok(())
    }

    /// Implement one of the `Fn()` family for a fn pointer.
    fn assemble_fn_pointer_candidates(&mut self,
                                      obligation: &TraitObligation<'tcx>,
                                      candidates: &mut SelectionCandidateSet<'tcx>)
                                      -> Result<(),SelectionError<'tcx>>
    {
        // We provide a `Fn` impl for fn pointers. There is no need to provide
        // the other traits (e.g. `FnMut`) since those are provided by blanket
        // impls.
        if Some(obligation.predicate.def_id()) != self.tcx().lang_items.fn_trait() {
            return Ok(());
        }

        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
        match self_ty.sty {
            ty::ty_infer(..) => {
                candidates.ambiguous = true; // could wind up being a fn() type
            }

            // provide an impl, but only for suitable `fn` pointers
            ty::ty_bare_fn(_, &ty::BareFnTy {
                unsafety: ast::Unsafety::Normal,
                abi: abi::Rust,
                sig: ty::Binder(ty::FnSig {
                    inputs: _,
                    output: ty::FnConverging(_),
                    variadic: false
                })
            }) => {
                candidates.vec.push(FnPointerCandidate);
            }

            _ => { }
        }

        Ok(())
    }

    /// Search for impls that might apply to `obligation`.
    fn assemble_candidates_from_impls(&mut self,
                                      obligation: &TraitObligation<'tcx>,
                                      candidate_vec: &mut Vec<SelectionCandidate<'tcx>>)
                                      -> Result<(), SelectionError<'tcx>>
    {
        let all_impls = self.all_impls(obligation.predicate.def_id());
        for &impl_def_id in all_impls.iter() {
            self.infcx.probe(|snapshot| {
                let (skol_obligation_trait_pred, skol_map) =
                    self.infcx().skolemize_late_bound_regions(&obligation.predicate, snapshot);
                match self.match_impl(impl_def_id, obligation, snapshot,
                                      &skol_map, skol_obligation_trait_pred.trait_ref.clone()) {
                    Ok(_) => {
                        candidate_vec.push(ImplCandidate(impl_def_id));
                    }
                    Err(()) => { }
                }
            });
        }
        Ok(())
    }

    ///////////////////////////////////////////////////////////////////////////
    // WINNOW
    //
    // Winnowing is the process of attempting to resolve ambiguity by
    // probing further. During the winnowing process, we unify all
    // type variables (ignoring skolemization) and then we also
    // attempt to evaluate recursive bounds to see if they are
    // satisfied.

    /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
    /// obligations are met. Returns true if `candidate` remains viable after this further
    /// scrutiny.
    fn winnow_candidate<'o>(&mut self,
                            stack: &TraitObligationStack<'o, 'tcx>,
                            candidate: &SelectionCandidate<'tcx>)
                            -> EvaluationResult<'tcx>
    {
        debug!("winnow_candidate: candidate={}", candidate.repr(self.tcx()));
        let result = self.infcx.probe(|_| {
            let candidate = (*candidate).clone();
            match self.confirm_candidate(stack.obligation, candidate) {
                Ok(selection) => self.winnow_selection(Some(stack), selection),
                Err(error) => EvaluatedToErr(error),
            }
        });
        debug!("winnow_candidate depth={} result={}",
               stack.obligation.recursion_depth, result);
        result
    }

    fn winnow_selection<'o>(&mut self,
                            stack: Option<&TraitObligationStack<'o, 'tcx>>,
                            selection: Selection<'tcx>)
                            -> EvaluationResult<'tcx>
    {
        let mut result = EvaluatedToOk;
        for obligation in selection.iter_nested() {
            match self.evaluate_predicate_recursively(stack, obligation) {
                EvaluatedToErr(e) => { return EvaluatedToErr(e); }
                EvaluatedToAmbig => { result = EvaluatedToAmbig; }
                EvaluatedToOk => { }
            }
        }
        result
    }

    /// Returns true if `candidate_i` should be dropped in favor of `candidate_j`.
    ///
    /// This is generally true if either:
    /// - candidate i and candidate j are equivalent; or,
    /// - candidate i is a conrete impl and candidate j is a where clause bound,
    ///   and the concrete impl is applicable to the types in the where clause bound.
    ///
    /// The last case refers to cases where there are blanket impls (often conditional
    /// blanket impls) as well as a where clause. This can come down to one of two cases:
    ///
    /// - The impl is truly unconditional (it has no where clauses
    ///   of its own), in which case the where clause is
    ///   unnecessary, because coherence requires that we would
    ///   pick that particular impl anyhow (at least so long as we
    ///   don't have specialization).
    ///
    /// - The impl is conditional, in which case we may not have winnowed it out
    ///   because we don't know if the conditions apply, but the where clause is basically
    ///   telling us taht there is some impl, though not necessarily the one we see.
    ///
    /// In both cases we prefer to take the where clause, which is
    /// essentially harmless.  See issue #18453 for more details of
    /// a case where doing the opposite caused us harm.
    fn candidate_should_be_dropped_in_favor_of<'o>(&mut self,
                                                   stack: &TraitObligationStack<'o, 'tcx>,
                                                   candidate_i: &SelectionCandidate<'tcx>,
                                                   candidate_j: &SelectionCandidate<'tcx>)
                                                   -> bool
    {
        match (candidate_i, candidate_j) {
            (&ImplCandidate(impl_def_id), &ParamCandidate(ref bound)) => {
                debug!("Considering whether to drop param {} in favor of impl {}",
                       candidate_i.repr(self.tcx()),
                       candidate_j.repr(self.tcx()));

                self.infcx.probe(|snapshot| {
                    let (skol_obligation_trait_ref, skol_map) =
                        self.infcx().skolemize_late_bound_regions(
                            &stack.obligation.predicate, snapshot);
                    let impl_substs =
                        self.rematch_impl(impl_def_id, stack.obligation, snapshot,
                                          &skol_map, skol_obligation_trait_ref.trait_ref.clone());
                    let impl_trait_ref =
                        ty::impl_trait_ref(self.tcx(), impl_def_id).unwrap();
                    let impl_trait_ref =
                        impl_trait_ref.subst(self.tcx(), &impl_substs);
                    let poly_impl_trait_ref =
                        ty::Binder(impl_trait_ref);
                    let origin =
                        infer::RelateOutputImplTypes(stack.obligation.cause.span);
                    self.infcx
                        .sub_poly_trait_refs(false, origin, poly_impl_trait_ref, bound.clone())
                        .is_ok()
                })
            }
            (&ProjectionCandidate, &ParamCandidate(_)) => {
                // FIXME(#20297) -- this gives where clauses precedent
                // over projections. Really these are just two means
                // of deducing information (one based on the where
                // clauses on the trait definition; one based on those
                // on the enclosing scope), and it'd be better to
                // integrate them more intelligently. But for now this
                // seems ok. If we DON'T give where clauses
                // precedence, we run into trouble in default methods,
                // where both the projection bounds for `Self::A` and
                // the where clauses are in scope.
                true
            }
            _ => {
                *candidate_i == *candidate_j
            }
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // BUILTIN BOUNDS
    //
    // These cover the traits that are built-in to the language
    // itself.  This includes `Copy` and `Sized` for sure. For the
    // moment, it also includes `Send` / `Sync` and a few others, but
    // those will hopefully change to library-defined traits in the
    // future.

    fn assemble_builtin_bound_candidates<'o>(&mut self,
                                             bound: ty::BuiltinBound,
                                             stack: &TraitObligationStack<'o, 'tcx>,
                                             candidates: &mut SelectionCandidateSet<'tcx>)
                                             -> Result<(),SelectionError<'tcx>>
    {
        match self.builtin_bound(bound, stack.obligation) {
            Ok(If(..)) => {
                debug!("builtin_bound: bound={}",
                       bound.repr(self.tcx()));
                candidates.vec.push(BuiltinCandidate(bound));
                Ok(())
            }
            Ok(ParameterBuiltin) => { Ok(()) }
            Ok(AmbiguousBuiltin) => { Ok(candidates.ambiguous = true) }
            Err(e) => { Err(e) }
        }
    }

    fn builtin_bound(&mut self,
                     bound: ty::BuiltinBound,
                     obligation: &TraitObligation<'tcx>)
                     -> Result<BuiltinBoundConditions<'tcx>,SelectionError<'tcx>>
    {
        // Note: these tests operate on types that may contain bound
        // regions. To be proper, we ought to skolemize here, but we
        // forego the skolemization and defer it until the
        // confirmation step.

        let self_ty = self.infcx.shallow_resolve(obligation.predicate.0.self_ty());
        return match self_ty.sty {
            ty::ty_infer(ty::IntVar(_)) |
            ty::ty_infer(ty::FloatVar(_)) |
            ty::ty_uint(_) |
            ty::ty_int(_) |
            ty::ty_bool |
            ty::ty_float(_) |
            ty::ty_bare_fn(..) |
            ty::ty_char => {
                // safe for everything
                Ok(If(Vec::new()))
            }

            ty::ty_uniq(referent_ty) => {  // Box<T>
                match bound {
                    ty::BoundCopy => {
                        Err(Unimplemented)
                    }

                    ty::BoundSized => {
                        Ok(If(Vec::new()))
                    }

                    ty::BoundSync |
                    ty::BoundSend => {
                        Ok(If(vec![referent_ty]))
                    }
                }
            }

            ty::ty_ptr(..) => {     // *const T, *mut T
                match bound {
                    ty::BoundCopy |
                    ty::BoundSized => {
                        Ok(If(Vec::new()))
                    }

                    ty::BoundSync |
                    ty::BoundSend => {
                        // sync and send are not implemented for *const, *mut
                        Err(Unimplemented)
                    }
                }
            }

            ty::ty_closure(ref c) => {
                match c.store {
                    ty::UniqTraitStore => {
                        // proc: Equivalent to `Box<FnOnce>`
                        match bound {
                            ty::BoundCopy => {
                                Err(Unimplemented)
                            }

                            ty::BoundSized => {
                                Ok(If(Vec::new()))
                            }

                            ty::BoundSync |
                            ty::BoundSend => {
                                if c.bounds.builtin_bounds.contains(&bound) {
                                    Ok(If(Vec::new()))
                                } else {
                                    Err(Unimplemented)
                                }
                            }
                        }
                    }
                    ty::RegionTraitStore(_, mutbl) => {
                        // ||: Equivalent to `&FnMut` or `&mut FnMut` or something like that.
                        match bound {
                            ty::BoundCopy => {
                                match mutbl {
                                    ast::MutMutable => {
                                        // &mut T is affine
                                        Err(Unimplemented)
                                    }
                                    ast::MutImmutable => {
                                        // &T is copyable, no matter what T is
                                        Ok(If(Vec::new()))
                                    }
                                }
                            }

                            ty::BoundSized => {
                                Ok(If(Vec::new()))
                            }

                            ty::BoundSync |
                            ty::BoundSend => {
                                if c.bounds.builtin_bounds.contains(&bound) {
                                    Ok(If(Vec::new()))
                                } else {
                                    Err(Unimplemented)
                                }
                            }
                        }
                    }
                }
            }

            ty::ty_trait(ref data) => {
                match bound {
                    ty::BoundSized => {
                        Err(Unimplemented)
                    }
                    ty::BoundCopy | ty::BoundSync | ty::BoundSend => {
                        if data.bounds.builtin_bounds.contains(&bound) {
                            Ok(If(Vec::new()))
                        } else {
                            // Recursively check all supertraits to find out if any further
                            // bounds are required and thus we must fulfill.
                            let principal =
                                data.principal_trait_ref_with_self_ty(self.tcx(),
                                                                      self.tcx().types.err);
                            for tr in util::supertraits(self.tcx(), principal) {
                                let td = ty::lookup_trait_def(self.tcx(), tr.def_id());
                                if td.bounds.builtin_bounds.contains(&bound) {
                                    return Ok(If(Vec::new()))
                                }
                            }

                            Err(Unimplemented)
                        }
                    }
                }
            }

            ty::ty_rptr(_, ty::mt { ty: referent_ty, mutbl }) => {
                // &mut T or &T
                match bound {
                    ty::BoundCopy => {
                        match mutbl {
                            // &mut T is affine and hence never `Copy`
                            ast::MutMutable => {
                                Err(Unimplemented)
                            }

                            // &T is always copyable
                            ast::MutImmutable => {
                                Ok(If(Vec::new()))
                            }
                        }
                    }

                    ty::BoundSized => {
                        Ok(If(Vec::new()))
                    }

                    ty::BoundSync |
                    ty::BoundSend => {
                        // Note: technically, a region pointer is only
                        // sendable if it has lifetime
                        // `'static`. However, we don't take regions
                        // into account when doing trait matching:
                        // instead, when we decide that `T : Send`, we
                        // will register a separate constraint with
                        // the region inferencer that `T : 'static`
                        // holds as well (because the trait `Send`
                        // requires it). This will ensure that there
                        // is no borrowed data in `T` (or else report
                        // an inference error). The reason we do it
                        // this way is that we do not yet *know* what
                        // lifetime the borrowed reference has, since
                        // we haven't finished running inference -- in
                        // other words, there's a kind of
                        // chicken-and-egg problem.
                        Ok(If(vec![referent_ty]))
                    }
                }
            }

            ty::ty_vec(element_ty, ref len) => {
                // [T, ..n] and [T]
                match bound {
                    ty::BoundCopy => {
                        match *len {
                            Some(_) => {
                                // [T, ..n] is copy iff T is copy
                                Ok(If(vec![element_ty]))
                            }
                            None => {
                                // [T] is unsized and hence affine
                                Err(Unimplemented)
                            }
                        }
                    }

                    ty::BoundSized => {
                        if len.is_some() {
                            Ok(If(Vec::new()))
                        } else {
                            Err(Unimplemented)
                        }
                    }

                    ty::BoundSync |
                    ty::BoundSend => {
                        Ok(If(vec![element_ty]))
                    }
                }
            }

            ty::ty_str => {
                // Equivalent to [u8]
                match bound {
                    ty::BoundSync |
                    ty::BoundSend => {
                        Ok(If(Vec::new()))
                    }

                    ty::BoundCopy |
                    ty::BoundSized => {
                        Err(Unimplemented)
                    }
                }
            }

            ty::ty_tup(ref tys) => {
                // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
                Ok(If(tys.clone()))
            }

            ty::ty_unboxed_closure(def_id, _, substs) => {
                // FIXME -- This case is tricky. In the case of by-ref
                // closures particularly, we need the results of
                // inference to decide how to reflect the type of each
                // upvar (the upvar may have type `T`, but the runtime
                // type could be `&mut`, `&`, or just `T`). For now,
                // though, we'll do this unsoundly and assume that all
                // captures are by value. Really what we ought to do
                // is reserve judgement and then intertwine this
                // analysis with closure inference.
                assert_eq!(def_id.krate, ast::LOCAL_CRATE);

                // Unboxed closures shouldn't be
                // implicitly copyable
                if bound == ty::BoundCopy {
                    return Ok(ParameterBuiltin);
                }

                match self.tcx().freevars.borrow().get(&def_id.node) {
                    None => {
                        // No upvars.
                        Ok(If(Vec::new()))
                    }

                    Some(freevars) => {
                        let tys: Vec<Ty> =
                            freevars
                            .iter()
                            .map(|freevar| {
                                let freevar_def_id = freevar.def.def_id();
                                self.typer.node_ty(freevar_def_id.node).subst(self.tcx(), substs)
                            })
                            .collect();
                        Ok(If(tys))
                    }
                }
            }

            ty::ty_struct(def_id, substs) => {
                let types: Vec<Ty> =
                    ty::struct_fields(self.tcx(), def_id, substs)
                    .iter()
                    .map(|f| f.mt.ty)
                    .collect();
                nominal(self, bound, def_id, types)
            }

            ty::ty_enum(def_id, substs) => {
                let types: Vec<Ty> =
                    ty::substd_enum_variants(self.tcx(), def_id, substs)
                    .iter()
                    .flat_map(|variant| variant.args.iter())
                    .map(|&ty| ty)
                    .collect();
                nominal(self, bound, def_id, types)
            }

            ty::ty_projection(_) |
            ty::ty_param(_) => {
                // Note: A type parameter is only considered to meet a
                // particular bound if there is a where clause telling
                // us that it does, and that case is handled by
                // `assemble_candidates_from_caller_bounds()`.
                Ok(ParameterBuiltin)
            }

            ty::ty_infer(ty::TyVar(_)) => {
                // Unbound type variable. Might or might not have
                // applicable impls and so forth, depending on what
                // those type variables wind up being bound to.
                Ok(AmbiguousBuiltin)
            }

            ty::ty_err => {
                Ok(If(Vec::new()))
            }

            ty::ty_open(_) |
            ty::ty_infer(ty::FreshTy(_)) |
            ty::ty_infer(ty::FreshIntTy(_)) => {
                self.tcx().sess.bug(
                    format!(
                        "asked to assemble builtin bounds of unexpected type: {}",
                        self_ty.repr(self.tcx()))[]);
            }
        };

        fn nominal<'cx, 'tcx>(this: &mut SelectionContext<'cx, 'tcx>,
                              bound: ty::BuiltinBound,
                              def_id: ast::DefId,
                              types: Vec<Ty<'tcx>>)
                              -> Result<BuiltinBoundConditions<'tcx>,SelectionError<'tcx>>
        {
            // First check for markers and other nonsense.
            let tcx = this.tcx();
            match bound {
                ty::BoundSend => {
                    if
                        Some(def_id) == tcx.lang_items.no_send_bound() ||
                        Some(def_id) == tcx.lang_items.managed_bound()
                    {
                        return Err(Unimplemented)
                    }
                }

                ty::BoundCopy => {
                    // This is an Opt-In Built-In Trait. So, unless
                    // the user is asking for the old behavior, we
                    // don't supply any form of builtin impl.
                    if !this.tcx().sess.features.borrow().opt_out_copy {
                        return Ok(ParameterBuiltin)
                    } else {
                        // Older, backwards compatibility behavior:
                        if
                            Some(def_id) == tcx.lang_items.no_copy_bound() ||
                            Some(def_id) == tcx.lang_items.managed_bound() ||
                            ty::has_dtor(tcx, def_id)
                        {
                            return Err(Unimplemented);
                        }
                    }
                }

                ty::BoundSync => {
                    if
                        Some(def_id) == tcx.lang_items.no_sync_bound() ||
                        Some(def_id) == tcx.lang_items.managed_bound() ||
                        Some(def_id) == tcx.lang_items.unsafe_type()
                    {
                        return Err(Unimplemented)
                    }
                }

                ty::BoundSized => { }
            }

            Ok(If(types))
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // CONFIRMATION
    //
    // Confirmation unifies the output type parameters of the trait
    // with the values found in the obligation, possibly yielding a
    // type error.  See `doc.rs` for more details.

    fn confirm_candidate(&mut self,
                         obligation: &TraitObligation<'tcx>,
                         candidate: SelectionCandidate<'tcx>)
                         -> Result<Selection<'tcx>,SelectionError<'tcx>>
    {
        debug!("confirm_candidate({}, {})",
               obligation.repr(self.tcx()),
               candidate.repr(self.tcx()));

        match candidate {
            BuiltinCandidate(builtin_bound) => {
                Ok(VtableBuiltin(
                    try!(self.confirm_builtin_candidate(obligation, builtin_bound))))
            }

            ErrorCandidate => {
                Ok(VtableBuiltin(VtableBuiltinData { nested: VecPerParamSpace::empty() }))
            }

            ParamCandidate(param) => {
                self.confirm_param_candidate(obligation, param);
                Ok(VtableParam)
            }

            ImplCandidate(impl_def_id) => {
                let vtable_impl =
                    try!(self.confirm_impl_candidate(obligation, impl_def_id));
                Ok(VtableImpl(vtable_impl))
            }

            UnboxedClosureCandidate(closure_def_id, substs) => {
                try!(self.confirm_unboxed_closure_candidate(obligation, closure_def_id, &substs));
                Ok(VtableUnboxedClosure(closure_def_id, substs))
            }

            FnPointerCandidate => {
                let fn_type =
                    try!(self.confirm_fn_pointer_candidate(obligation));
                Ok(VtableFnPointer(fn_type))
            }

            ProjectionCandidate => {
                self.confirm_projection_candidate(obligation);
                Ok(VtableParam)
            }
        }
    }

    fn confirm_projection_candidate(&mut self,
                                    obligation: &TraitObligation<'tcx>)
    {
        let _: Result<(),()> =
            self.infcx.try(|snapshot| {
                let result =
                    self.match_projection_obligation_against_bounds_from_trait(obligation,
                                                                               snapshot);
                assert!(result);
                Ok(())
            });
    }

    fn confirm_param_candidate(&mut self,
                               obligation: &TraitObligation<'tcx>,
                               param: ty::PolyTraitRef<'tcx>)
    {
        debug!("confirm_param_candidate({},{})",
               obligation.repr(self.tcx()),
               param.repr(self.tcx()));

        // During evaluation, we already checked that this
        // where-clause trait-ref could be unified with the obligation
        // trait-ref. Repeat that unification now without any
        // transactional boundary; it should not fail.
        match self.confirm_poly_trait_refs(obligation.cause.clone(),
                                           obligation.predicate.to_poly_trait_ref(),
                                           param.clone()) {
            Ok(()) => { }
            Err(_) => {
                self.tcx().sess.bug(
                    format!("Where clause `{}` was applicable to `{}` but now is not",
                            param.repr(self.tcx()),
                            obligation.repr(self.tcx())).as_slice());
            }
        }
    }

    fn confirm_builtin_candidate(&mut self,
                                 obligation: &TraitObligation<'tcx>,
                                 bound: ty::BuiltinBound)
                                 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>,
                                           SelectionError<'tcx>>
    {
        debug!("confirm_builtin_candidate({})",
               obligation.repr(self.tcx()));

        match try!(self.builtin_bound(bound, obligation)) {
            If(nested) => Ok(self.vtable_builtin_data(obligation, bound, nested)),
            AmbiguousBuiltin | ParameterBuiltin => {
                self.tcx().sess.span_bug(
                    obligation.cause.span,
                    format!("builtin bound for {} was ambig",
                            obligation.repr(self.tcx()))[]);
            }
        }
    }

    fn vtable_builtin_data(&mut self,
                           obligation: &TraitObligation<'tcx>,
                           bound: ty::BuiltinBound,
                           nested: Vec<Ty<'tcx>>)
                           -> VtableBuiltinData<PredicateObligation<'tcx>>
    {
        let derived_cause = self.derived_cause(obligation, BuiltinDerivedObligation);
        let obligations = nested.iter().map(|&bound_ty| {
            // the obligation might be higher-ranked, e.g. for<'a> &'a
            // int : Copy. In that case, we will wind up with
            // late-bound regions in the `nested` vector. So for each
            // one we instantiate to a skolemized region, do our work
            // to produce something like `&'0 int : Copy`, and then
            // re-bind it. This is a bit of busy-work but preserves
            // the invariant that we only manipulate free regions, not
            // bound ones.
            self.infcx.try(|snapshot| {
                let (skol_ty, skol_map) =
                    self.infcx().skolemize_late_bound_regions(&ty::Binder(bound_ty), snapshot);
                let skol_predicate =
                    util::predicate_for_builtin_bound(
                        self.tcx(),
                        derived_cause.clone(),
                        bound,
                        obligation.recursion_depth + 1,
                        skol_ty);
                match skol_predicate {
                    Ok(skol_predicate) => Ok(self.infcx().plug_leaks(skol_map, snapshot,
                                                                     &skol_predicate)),
                    Err(ErrorReported) => Err(ErrorReported)
                }
            })
        }).collect::<Result<_, _>>();
        let mut obligations = match obligations {
            Ok(o) => o,
            Err(ErrorReported) => Vec::new()
        };

        // as a special case, `Send` requires `'static`
        if bound == ty::BoundSend {
            obligations.push(Obligation {
                cause: obligation.cause.clone(),
                recursion_depth: obligation.recursion_depth+1,
                predicate: ty::Binder(ty::OutlivesPredicate(obligation.self_ty(),
                                                            ty::ReStatic)).as_predicate(),
            });
        }

        let obligations = VecPerParamSpace::new(obligations, Vec::new(), Vec::new());

        debug!("vtable_builtin_data: obligations={}",
               obligations.repr(self.tcx()));

        VtableBuiltinData { nested: obligations }
    }

    fn confirm_impl_candidate(&mut self,
                              obligation: &TraitObligation<'tcx>,
                              impl_def_id: ast::DefId)
                              -> Result<VtableImplData<'tcx, PredicateObligation<'tcx>>,
                                        SelectionError<'tcx>>
    {
        debug!("confirm_impl_candidate({},{})",
               obligation.repr(self.tcx()),
               impl_def_id.repr(self.tcx()));

        // First, create the substitutions by matching the impl again,
        // this time not in a probe.
        self.infcx.try(|snapshot| {
            let (skol_obligation_trait_ref, skol_map) =
                self.infcx().skolemize_late_bound_regions(&obligation.predicate, snapshot);
            let substs =
                self.rematch_impl(impl_def_id, obligation,
                                  snapshot, &skol_map, skol_obligation_trait_ref.trait_ref);
            debug!("confirm_impl_candidate substs={}", substs);
            Ok(self.vtable_impl(impl_def_id, substs, obligation.cause.clone(),
                                obligation.recursion_depth + 1, skol_map, snapshot))
        })
    }

    fn vtable_impl(&mut self,
                   impl_def_id: ast::DefId,
                   substs: Substs<'tcx>,
                   cause: ObligationCause<'tcx>,
                   recursion_depth: uint,
                   skol_map: infer::SkolemizationMap,
                   snapshot: &infer::CombinedSnapshot)
                   -> VtableImplData<'tcx, PredicateObligation<'tcx>>
    {
        debug!("vtable_impl(impl_def_id={}, substs={}, recursion_depth={}, skol_map={})",
               impl_def_id.repr(self.tcx()),
               substs.repr(self.tcx()),
               recursion_depth,
               skol_map.repr(self.tcx()));

        let impl_predicates =
            self.impl_predicates(cause,
                                 recursion_depth,
                                 impl_def_id,
                                 &substs,
                                 skol_map,
                                 snapshot);

        debug!("vtable_impl: impl_def_id={} impl_predicates={}",
               impl_def_id.repr(self.tcx()),
               impl_predicates.repr(self.tcx()));

        VtableImplData { impl_def_id: impl_def_id,
                         substs: substs,
                         nested: impl_predicates }
    }

    fn confirm_fn_pointer_candidate(&mut self,
                                    obligation: &TraitObligation<'tcx>)
                                    -> Result<ty::Ty<'tcx>,SelectionError<'tcx>>
    {
        debug!("confirm_fn_pointer_candidate({})",
               obligation.repr(self.tcx()));

        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
        let sig = match self_ty.sty {
            ty::ty_bare_fn(_, &ty::BareFnTy {
                unsafety: ast::Unsafety::Normal,
                abi: abi::Rust,
                ref sig
            }) => {
                sig
            }
            _ => {
                self.tcx().sess.span_bug(
                    obligation.cause.span,
                    format!("Fn pointer candidate for inappropriate self type: {}",
                            self_ty.repr(self.tcx()))[]);
            }
        };

        let arguments_tuple = ty::mk_tup(self.tcx(), sig.0.inputs.to_vec());
        let output_type = sig.0.output.unwrap();
        let substs =
            Substs::new_trait(
                vec![arguments_tuple, output_type],
                vec![],
                self_ty);
        let trait_ref = ty::Binder(Rc::new(ty::TraitRef {
            def_id: obligation.predicate.def_id(),
            substs: self.tcx().mk_substs(substs),
        }));

        try!(self.confirm_poly_trait_refs(obligation.cause.clone(),
                                          obligation.predicate.to_poly_trait_ref(),
                                          trait_ref));
        Ok(self_ty)
    }

    fn confirm_unboxed_closure_candidate(&mut self,
                                         obligation: &TraitObligation<'tcx>,
                                         closure_def_id: ast::DefId,
                                         substs: &Substs<'tcx>)
                                         -> Result<(),SelectionError<'tcx>>
    {
        debug!("confirm_unboxed_closure_candidate({},{},{})",
               obligation.repr(self.tcx()),
               closure_def_id.repr(self.tcx()),
               substs.repr(self.tcx()));

        let closure_type = match self.typer.unboxed_closures().borrow().get(&closure_def_id) {
            Some(closure) => closure.closure_type.clone(),
            None => {
                self.tcx().sess.span_bug(
                    obligation.cause.span,
                    format!("No entry for unboxed closure: {}",
                            closure_def_id.repr(self.tcx()))[]);
            }
        };

        let closure_sig = &closure_type.sig;
        let arguments_tuple = closure_sig.0.inputs[0];
        let substs =
            Substs::new_trait(
                vec![arguments_tuple.subst(self.tcx(), substs),
                     closure_sig.0.output.unwrap().subst(self.tcx(), substs)],
                vec![],
                obligation.self_ty());
        let trait_ref = ty::Binder(Rc::new(ty::TraitRef {
            def_id: obligation.predicate.def_id(),
            substs: self.tcx().mk_substs(substs),
        }));

        debug!("confirm_unboxed_closure_candidate(closure_def_id={}, trait_ref={})",
               closure_def_id.repr(self.tcx()),
               trait_ref.repr(self.tcx()));

        self.confirm_poly_trait_refs(obligation.cause.clone(),
                                     obligation.predicate.to_poly_trait_ref(),
                                     trait_ref)
    }

    /// In the case of unboxed closure types and fn pointers,
    /// we currently treat the input type parameters on the trait as
    /// outputs. This means that when we have a match we have only
    /// considered the self type, so we have to go back and make sure
    /// to relate the argument types too.  This is kind of wrong, but
    /// since we control the full set of impls, also not that wrong,
    /// and it DOES yield better error messages (since we don't report
    /// errors as if there is no applicable impl, but rather report
    /// errors are about mismatched argument types.
    ///
    /// Here is an example. Imagine we have an unboxed closure expression
    /// and we desugared it so that the type of the expression is
    /// `Closure`, and `Closure` expects an int as argument. Then it
    /// is "as if" the compiler generated this impl:
    ///
    ///     impl Fn(int) for Closure { ... }
    ///
    /// Now imagine our obligation is `Fn(uint) for Closure`. So far
    /// we have matched the self-type `Closure`. At this point we'll
    /// compare the `int` to `uint` and generate an error.
    ///
    /// Note that this checking occurs *after* the impl has selected,
    /// because these output type parameters should not affect the
    /// selection of the impl. Therefore, if there is a mismatch, we
    /// report an error to the user.
    fn confirm_poly_trait_refs(&mut self,
                               obligation_cause: ObligationCause,
                               obligation_trait_ref: ty::PolyTraitRef<'tcx>,
                               expected_trait_ref: ty::PolyTraitRef<'tcx>)
                               -> Result<(), SelectionError<'tcx>>
    {
        let origin = infer::RelateOutputImplTypes(obligation_cause.span);

        let obligation_trait_ref = obligation_trait_ref.clone();
        match self.infcx.sub_poly_trait_refs(false,
                                             origin,
                                             expected_trait_ref.clone(),
                                             obligation_trait_ref.clone()) {
            Ok(()) => Ok(()),
            Err(e) => Err(OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // Matching
    //
    // Matching is a common path used for both evaluation and
    // confirmation.  It basically unifies types that appear in impls
    // and traits. This does affect the surrounding environment;
    // therefore, when used during evaluation, match routines must be
    // run inside of a `probe()` so that their side-effects are
    // contained.

    fn rematch_impl(&mut self,
                    impl_def_id: ast::DefId,
                    obligation: &TraitObligation<'tcx>,
                    snapshot: &infer::CombinedSnapshot,
                    skol_map: &infer::SkolemizationMap,
                    skol_obligation_trait_ref: Rc<ty::TraitRef<'tcx>>)
                    -> Substs<'tcx>
    {
        match self.match_impl(impl_def_id, obligation, snapshot,
                              skol_map, skol_obligation_trait_ref) {
            Ok(substs) => {
                substs
            }
            Err(()) => {
                self.tcx().sess.bug(
                    format!("Impl {} was matchable against {} but now is not",
                            impl_def_id.repr(self.tcx()),
                            obligation.repr(self.tcx()))[]);
            }
        }
    }

    fn match_impl(&mut self,
                  impl_def_id: ast::DefId,
                  obligation: &TraitObligation<'tcx>,
                  snapshot: &infer::CombinedSnapshot,
                  skol_map: &infer::SkolemizationMap,
                  skol_obligation_trait_ref: Rc<ty::TraitRef<'tcx>>)
                  -> Result<Substs<'tcx>, ()>
    {
        let impl_trait_ref = ty::impl_trait_ref(self.tcx(), impl_def_id).unwrap();

        // Before we create the substitutions and everything, first
        // consider a "quick reject". This avoids creating more types
        // and so forth that we need to.
        if self.fast_reject_trait_refs(obligation, &*impl_trait_ref) {
            return Err(());
        }

        let impl_substs = util::fresh_substs_for_impl(self.infcx,
                                                      obligation.cause.span,
                                                      impl_def_id);

        let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
                                                  &impl_substs);

        debug!("match_impl(impl_def_id={}, obligation={}, \
               impl_trait_ref={}, skol_obligation_trait_ref={})",
               impl_def_id.repr(self.tcx()),
               obligation.repr(self.tcx()),
               impl_trait_ref.repr(self.tcx()),
               skol_obligation_trait_ref.repr(self.tcx()));

        let origin = infer::RelateOutputImplTypes(obligation.cause.span);
        match self.infcx.sub_trait_refs(false,
                                        origin,
                                        impl_trait_ref,
                                        skol_obligation_trait_ref) {
            Ok(()) => { }
            Err(e) => {
                debug!("match_impl: failed sub_trait_refs due to `{}`",
                       ty::type_err_to_str(self.tcx(), &e));
                return Err(());
            }
        }

        match self.infcx.leak_check(skol_map, snapshot) {
            Ok(()) => { }
            Err(e) => {
                debug!("match_impl: failed leak check due to `{}`",
                       ty::type_err_to_str(self.tcx(), &e));
                return Err(());
            }
        }

        debug!("match_impl: success impl_substs={}", impl_substs.repr(self.tcx()));
        Ok(impl_substs)
    }

    fn fast_reject_trait_refs(&mut self,
                              obligation: &TraitObligation,
                              impl_trait_ref: &ty::TraitRef)
                              -> bool
    {
        // We can avoid creating type variables and doing the full
        // substitution if we find that any of the input types, when
        // simplified, do not match.

        obligation.predicate.0.input_types().iter()
            .zip(impl_trait_ref.input_types().iter())
            .any(|(&obligation_ty, &impl_ty)| {
                let simplified_obligation_ty =
                    fast_reject::simplify_type(self.tcx(), obligation_ty, true);
                let simplified_impl_ty =
                    fast_reject::simplify_type(self.tcx(), impl_ty, false);

                simplified_obligation_ty.is_some() &&
                    simplified_impl_ty.is_some() &&
                    simplified_obligation_ty != simplified_impl_ty
            })
    }

    fn match_where_clause(&mut self,
                          obligation: &TraitObligation<'tcx>,
                          where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
                        -> Result<(),()>
    {
        debug!("match_where_clause: obligation={} where_clause_trait_ref={}",
               obligation.repr(self.tcx()),
               where_clause_trait_ref.repr(self.tcx()));

        let origin = infer::RelateOutputImplTypes(obligation.cause.span);
        match self.infcx.sub_poly_trait_refs(false,
                                             origin,
                                             where_clause_trait_ref,
                                             obligation.predicate.to_poly_trait_ref()) {
            Ok(()) => Ok(()),
            Err(_) => Err(()),
        }
    }

    /// Determines whether the self type declared against
    /// `impl_def_id` matches `obligation_self_ty`. If successful,
    /// returns the substitutions used to make them match. See
    /// `match_impl()`. For example, if `impl_def_id` is declared
    /// as:
    ///
    ///    impl<T:Copy> Foo for ~T { ... }
    ///
    /// and `obligation_self_ty` is `int`, we'd back an `Err(_)`
    /// result. But if `obligation_self_ty` were `~int`, we'd get
    /// back `Ok(T=int)`.
    fn match_inherent_impl(&mut self,
                           impl_def_id: ast::DefId,
                           obligation_cause: &ObligationCause,
                           obligation_self_ty: Ty<'tcx>)
                           -> Result<Substs<'tcx>,()>
    {
        // Create fresh type variables for each type parameter declared
        // on the impl etc.
        let impl_substs = util::fresh_substs_for_impl(self.infcx,
                                                      obligation_cause.span,
                                                      impl_def_id);

        // Find the self type for the impl.
        let impl_self_ty = ty::lookup_item_type(self.tcx(), impl_def_id).ty;
        let impl_self_ty = impl_self_ty.subst(self.tcx(), &impl_substs);

        debug!("match_impl_self_types(obligation_self_ty={}, impl_self_ty={})",
               obligation_self_ty.repr(self.tcx()),
               impl_self_ty.repr(self.tcx()));

        match self.match_self_types(obligation_cause,
                                    impl_self_ty,
                                    obligation_self_ty) {
            Ok(()) => {
                debug!("Matched impl_substs={}", impl_substs.repr(self.tcx()));
                Ok(impl_substs)
            }
            Err(()) => {
                debug!("NoMatch");
                Err(())
            }
        }
    }

    fn match_self_types(&mut self,
                        cause: &ObligationCause,

                        // The self type provided by the impl/caller-obligation:
                        provided_self_ty: Ty<'tcx>,

                        // The self type the obligation is for:
                        required_self_ty: Ty<'tcx>)
                        -> Result<(),()>
    {
        // FIXME(#5781) -- equating the types is stronger than
        // necessary. Should consider variance of trait w/r/t Self.

        let origin = infer::RelateSelfType(cause.span);
        match self.infcx.eq_types(false,
                                  origin,
                                  provided_self_ty,
                                  required_self_ty) {
            Ok(()) => Ok(()),
            Err(_) => Err(()),
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // Miscellany

    fn push_stack<'o,'s:'o>(&mut self,
                            previous_stack: Option<&'s TraitObligationStack<'s, 'tcx>>,
                            obligation: &'o TraitObligation<'tcx>)
                            -> TraitObligationStack<'o, 'tcx>
    {
        let fresh_trait_ref =
            obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);

        TraitObligationStack {
            obligation: obligation,
            fresh_trait_ref: fresh_trait_ref,
            previous: previous_stack.map(|p| p), // FIXME variance
        }
    }

    /// Returns set of all impls for a given trait.
    fn all_impls(&self, trait_def_id: ast::DefId) -> Vec<ast::DefId> {
        ty::populate_implementations_for_trait_if_necessary(self.tcx(),
                                                            trait_def_id);
        match self.tcx().trait_impls.borrow().get(&trait_def_id) {
            None => Vec::new(),
            Some(impls) => impls.borrow().clone()
        }
    }

    fn impl_predicates(&self,
                       cause: ObligationCause<'tcx>,
                       recursion_depth: uint,
                       impl_def_id: ast::DefId,
                       impl_substs: &Substs<'tcx>,
                       skol_map: infer::SkolemizationMap,
                       snapshot: &infer::CombinedSnapshot)
                       -> VecPerParamSpace<PredicateObligation<'tcx>>
    {
        let impl_generics = ty::lookup_item_type(self.tcx(), impl_def_id).generics;
        let bounds = impl_generics.to_bounds(self.tcx(), impl_substs);
        let bounds = self.infcx().plug_leaks(skol_map, snapshot, &bounds);
        util::predicates_for_generics(self.tcx(), cause, recursion_depth, &bounds)
    }

    fn fn_family_trait_kind(&self,
                            trait_def_id: ast::DefId)
                            -> Option<ty::UnboxedClosureKind>
    {
        let tcx = self.tcx();
        if Some(trait_def_id) == tcx.lang_items.fn_trait() {
            Some(ty::FnUnboxedClosureKind)
        } else if Some(trait_def_id) == tcx.lang_items.fn_mut_trait() {
            Some(ty::FnMutUnboxedClosureKind)
        } else if Some(trait_def_id) == tcx.lang_items.fn_once_trait() {
            Some(ty::FnOnceUnboxedClosureKind)
        } else {
            None
        }
    }

    #[allow(unused_comparisons)]
    fn derived_cause(&self,
                     obligation: &TraitObligation<'tcx>,
                     variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
                     -> ObligationCause<'tcx>
    {
        /*!
         * Creates a cause for obligations that are derived from
         * `obligation` by a recursive search (e.g., for a builtin
         * bound, or eventually a `impl Foo for ..`). If `obligation`
         * is itself a derived obligation, this is just a clone, but
         * otherwise we create a "derived obligation" cause so as to
         * keep track of the original root obligation for error
         * reporting.
         */

        // NOTE(flaper87): As of now, it keeps track of the whole error
        // chain. Ideally, we should have a way to configure this either
        // by using -Z verbose or just a CLI argument.
        if obligation.recursion_depth >= 0 {
            let derived_cause = DerivedObligationCause {
                parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
                parent_code: Rc::new(obligation.cause.code.clone()),
            };
            ObligationCause::new(obligation.cause.span,
                                 obligation.cause.body_id,
                                 variant(derived_cause))
        } else {
            obligation.cause.clone()
        }
    }
}

impl<'tcx> Repr<'tcx> for SelectionCandidate<'tcx> {
    fn repr(&self, tcx: &ty::ctxt<'tcx>) -> String {
        match *self {
            ErrorCandidate => format!("ErrorCandidate"),
            BuiltinCandidate(b) => format!("BuiltinCandidate({})", b),
            ParamCandidate(ref a) => format!("ParamCandidate({})", a.repr(tcx)),
            ImplCandidate(a) => format!("ImplCandidate({})", a.repr(tcx)),
            ProjectionCandidate => format!("ProjectionCandidate"),
            FnPointerCandidate => format!("FnPointerCandidate"),
            UnboxedClosureCandidate(c, ref s) => {
                format!("UnboxedClosureCandidate({},{})", c, s.repr(tcx))
            }
        }
    }
}

impl<'tcx> SelectionCache<'tcx> {
    pub fn new() -> SelectionCache<'tcx> {
        SelectionCache {
            hashmap: RefCell::new(HashMap::new())
        }
    }
}

impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
    fn iter(&self) -> Option<&TraitObligationStack<'o, 'tcx>> {
        Some(self)
    }
}

impl<'o, 'tcx> Iterator<&'o TraitObligationStack<'o,'tcx>>
           for Option<&'o TraitObligationStack<'o, 'tcx>>
{
    fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
        match *self {
            Some(o) => {
                *self = o.previous;
                Some(o)
            }
            None => {
                None
            }
        }
    }
}

impl<'o, 'tcx> Repr<'tcx> for TraitObligationStack<'o, 'tcx> {
    fn repr(&self, tcx: &ty::ctxt<'tcx>) -> String {
        format!("TraitObligationStack({})",
                self.obligation.repr(tcx))
    }
}

impl<'tcx> EvaluationResult<'tcx> {
    fn may_apply(&self) -> bool {
        match *self {
            EvaluatedToOk |
            EvaluatedToAmbig |
            EvaluatedToErr(Overflow) |
            EvaluatedToErr(OutputTypeParameterMismatch(..)) => {
                true
            }
            EvaluatedToErr(Unimplemented) => {
                false
            }
        }
    }
}

impl MethodMatchResult {
    pub fn may_apply(&self) -> bool {
        match *self {
            MethodMatched(_) => true,
            MethodAmbiguous(_) => true,
            MethodDidNotMatch => false,
        }
    }
}