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
//! Some code that abstracts away much of the boilerplate of writing
//! `derive` instances for traits. Among other things it manages getting
//! access to the fields of the 4 different sorts of structs and enum
//! variants, as well as creating the method and impl ast instances.
//!
//! Supported features (fairly exhaustive):
//!
//! - Methods taking any number of parameters of any type, and returning
//!   any type, other than vectors, bottom and closures.
//! - Generating `impl`s for types with type parameters and lifetimes
//!   (e.g., `Option<T>`), the parameters are automatically given the
//!   current trait as a bound. (This includes separate type parameters
//!   and lifetimes for methods.)
//! - Additional bounds on the type parameters (`TraitDef.additional_bounds`)
//!
//! The most important thing for implementors is the `Substructure` and
//! `SubstructureFields` objects. The latter groups 5 possibilities of the
//! arguments:
//!
//! - `Struct`, when `Self` is a struct (including tuple structs, e.g
//!   `struct T(i32, char)`).
//! - `EnumMatching`, when `Self` is an enum and all the arguments are the
//!   same variant of the enum (e.g., `Some(1)`, `Some(3)` and `Some(4)`)
//! - `EnumTag` when `Self` is an enum, for comparing the enum tags.
//! - `StaticEnum` and `StaticStruct` for static methods, where the type
//!   being derived upon is either an enum or struct respectively. (Any
//!   argument with type Self is just grouped among the non-self
//!   arguments.)
//!
//! In the first two cases, the values from the corresponding fields in
//! all the arguments are grouped together.
//!
//! The non-static cases have `Option<ident>` in several places associated
//! with field `expr`s. This represents the name of the field it is
//! associated with. It is only not `None` when the associated field has
//! an identifier in the source code. For example, the `x`s in the
//! following snippet
//!
//! ```rust
//! # #![allow(dead_code)]
//! struct A { x : i32 }
//!
//! struct B(i32);
//!
//! enum C {
//!     C0(i32),
//!     C1 { x: i32 }
//! }
//! ```
//!
//! The `i32`s in `B` and `C0` don't have an identifier, so the
//! `Option<ident>`s would be `None` for them.
//!
//! In the static cases, the structure is summarized, either into the just
//! spans of the fields or a list of spans and the field idents (for tuple
//! structs and record structs, respectively), or a list of these, for
//! enums (one for each variant). For empty struct and empty enum
//! variants, it is represented as a count of 0.
//!
//! # "`cs`" functions
//!
//! The `cs_...` functions ("combine substructure") are designed to
//! make life easier by providing some pre-made recipes for common
//! threads; mostly calling the function being derived on all the
//! arguments and then combining them back together in some way (or
//! letting the user chose that). They are not meant to be the only
//! way to handle the structures that this code creates.
//!
//! # Examples
//!
//! The following simplified `PartialEq` is used for in-code examples:
//!
//! ```rust
//! trait PartialEq {
//!     fn eq(&self, other: &Self) -> bool;
//! }
//! impl PartialEq for i32 {
//!     fn eq(&self, other: &i32) -> bool {
//!         *self == *other
//!     }
//! }
//! ```
//!
//! Some examples of the values of `SubstructureFields` follow, using the
//! above `PartialEq`, `A`, `B` and `C`.
//!
//! ## Structs
//!
//! When generating the `expr` for the `A` impl, the `SubstructureFields` is
//!
//! ```{.text}
//! Struct(vec![FieldInfo {
//!            span: <span of x>
//!            name: Some(<ident of x>),
//!            self_: <expr for &self.x>,
//!            other: vec![<expr for &other.x]
//!          }])
//! ```
//!
//! For the `B` impl, called with `B(a)` and `B(b)`,
//!
//! ```{.text}
//! Struct(vec![FieldInfo {
//!           span: <span of `i32`>,
//!           name: None,
//!           self_: <expr for &a>
//!           other: vec![<expr for &b>]
//!          }])
//! ```
//!
//! ## Enums
//!
//! When generating the `expr` for a call with `self == C0(a)` and `other
//! == C0(b)`, the SubstructureFields is
//!
//! ```{.text}
//! EnumMatching(0, <ast::Variant for C0>,
//!              vec![FieldInfo {
//!                 span: <span of i32>
//!                 name: None,
//!                 self_: <expr for &a>,
//!                 other: vec![<expr for &b>]
//!               }])
//! ```
//!
//! For `C1 {x}` and `C1 {x}`,
//!
//! ```{.text}
//! EnumMatching(1, <ast::Variant for C1>,
//!              vec![FieldInfo {
//!                 span: <span of x>
//!                 name: Some(<ident of x>),
//!                 self_: <expr for &self.x>,
//!                 other: vec![<expr for &other.x>]
//!                }])
//! ```
//!
//! For the tags,
//!
//! ```{.text}
//! EnumTag(
//!     &[<ident of self tag>, <ident of other tag>], <expr to combine with>)
//! ```
//! Note that this setup doesn't allow for the brute-force "match every variant
//! against every other variant" approach, which is bad because it produces a
//! quadratic amount of code (see #15375).
//!
//! ## Static
//!
//! A static method on the types above would result in,
//!
//! ```{.text}
//! StaticStruct(<ast::VariantData of A>, Named(vec![(<ident of x>, <span of x>)]))
//!
//! StaticStruct(<ast::VariantData of B>, Unnamed(vec![<span of x>]))
//!
//! StaticEnum(<ast::EnumDef of C>,
//!            vec![(<ident of C0>, <span of C0>, Unnamed(vec![<span of i32>])),
//!                 (<ident of C1>, <span of C1>, Named(vec![(<ident of x>, <span of x>)]))])
//! ```

pub use StaticFields::*;
pub use SubstructureFields::*;

use crate::deriving;
use rustc_ast::ptr::P;
use rustc_ast::{
    self as ast, BindingAnnotation, ByRef, EnumDef, Expr, Generics, Mutability, PatKind,
};
use rustc_ast::{GenericArg, GenericParamKind, VariantData};
use rustc_attr as attr;
use rustc_expand::base::{Annotatable, ExtCtxt};
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::Span;
use std::cell::RefCell;
use std::iter;
use std::vec;
use thin_vec::thin_vec;
use ty::{Bounds, Path, Ref, Self_, Ty};

pub mod ty;

pub struct TraitDef<'a> {
    /// The span for the current #[derive(Foo)] header.
    pub span: Span,

    /// Path of the trait, including any type parameters
    pub path: Path,

    /// Additional bounds required of any type parameters of the type,
    /// other than the current trait
    pub additional_bounds: Vec<Ty>,

    /// Any extra lifetimes and/or bounds, e.g., `D: serialize::Decoder`
    pub generics: Bounds,

    /// Can this trait be derived for unions?
    pub supports_unions: bool,

    pub methods: Vec<MethodDef<'a>>,

    pub associated_types: Vec<(Ident, Ty)>,
}

pub struct MethodDef<'a> {
    /// name of the method
    pub name: Symbol,
    /// List of generics, e.g., `R: rand::Rng`
    pub generics: Bounds,

    /// Is there is a `&self` argument? If not, it is a static function.
    pub explicit_self: bool,

    /// Arguments other than the self argument.
    pub nonself_args: Vec<(Ty, Symbol)>,

    /// Returns type
    pub ret_ty: Ty,

    pub attributes: ast::AttrVec,

    /// Can we combine fieldless variants for enums into a single match arm?
    /// If true, indicates that the trait operation uses the enum tag in some
    /// way.
    pub unify_fieldless_variants: bool,

    pub combine_substructure: RefCell<CombineSubstructureFunc<'a>>,
}

/// All the data about the data structure/method being derived upon.
pub struct Substructure<'a> {
    /// ident of self
    pub type_ident: Ident,
    /// Verbatim access to any non-selflike arguments, i.e. arguments that
    /// don't have type `&Self`.
    pub nonselflike_args: &'a [P<Expr>],
    pub fields: &'a SubstructureFields<'a>,
}

/// Summary of the relevant parts of a struct/enum field.
pub struct FieldInfo {
    pub span: Span,
    /// None for tuple structs/normal enum variants, Some for normal
    /// structs/struct enum variants.
    pub name: Option<Ident>,
    /// The expression corresponding to this field of `self`
    /// (specifically, a reference to it).
    pub self_expr: P<Expr>,
    /// The expressions corresponding to references to this field in
    /// the other selflike arguments.
    pub other_selflike_exprs: Vec<P<Expr>>,
}

/// Fields for a static method
pub enum StaticFields {
    /// Tuple and unit structs/enum variants like this.
    Unnamed(Vec<Span>, bool /*is tuple*/),
    /// Normal structs/struct variants.
    Named(Vec<(Ident, Span)>),
}

/// A summary of the possible sets of fields.
pub enum SubstructureFields<'a> {
    /// A non-static method with `Self` is a struct.
    Struct(&'a ast::VariantData, Vec<FieldInfo>),

    /// Matching variants of the enum: variant index, variant count, ast::Variant,
    /// fields: the field name is only non-`None` in the case of a struct
    /// variant.
    EnumMatching(usize, usize, &'a ast::Variant, Vec<FieldInfo>),

    /// The tag of an enum. The first field is a `FieldInfo` for the tags, as
    /// if they were fields. The second field is the expression to combine the
    /// tag expression with; it will be `None` if no match is necessary.
    EnumTag(FieldInfo, Option<P<Expr>>),

    /// A static method where `Self` is a struct.
    StaticStruct(&'a ast::VariantData, StaticFields),

    /// A static method where `Self` is an enum.
    StaticEnum(&'a ast::EnumDef, Vec<(Ident, Span, StaticFields)>),
}

/// Combine the values of all the fields together. The last argument is
/// all the fields of all the structures.
pub type CombineSubstructureFunc<'a> =
    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;

pub fn combine_substructure(
    f: CombineSubstructureFunc<'_>,
) -> RefCell<CombineSubstructureFunc<'_>> {
    RefCell::new(f)
}

struct TypeParameter {
    bound_generic_params: Vec<ast::GenericParam>,
    ty: P<ast::Ty>,
}

// The code snippets built up for derived code are sometimes used as blocks
// (e.g. in a function body) and sometimes used as expressions (e.g. in a match
// arm). This structure avoids committing to either form until necessary,
// avoiding the insertion of any unnecessary blocks.
//
// The statements come before the expression.
pub struct BlockOrExpr(Vec<ast::Stmt>, Option<P<Expr>>);

impl BlockOrExpr {
    pub fn new_stmts(stmts: Vec<ast::Stmt>) -> BlockOrExpr {
        BlockOrExpr(stmts, None)
    }

    pub fn new_expr(expr: P<Expr>) -> BlockOrExpr {
        BlockOrExpr(vec![], Some(expr))
    }

    pub fn new_mixed(stmts: Vec<ast::Stmt>, expr: Option<P<Expr>>) -> BlockOrExpr {
        BlockOrExpr(stmts, expr)
    }

    // Converts it into a block.
    fn into_block(mut self, cx: &ExtCtxt<'_>, span: Span) -> P<ast::Block> {
        if let Some(expr) = self.1 {
            self.0.push(cx.stmt_expr(expr));
        }
        cx.block(span, self.0)
    }

    // Converts it into an expression.
    fn into_expr(self, cx: &ExtCtxt<'_>, span: Span) -> P<Expr> {
        if self.0.is_empty() {
            match self.1 {
                None => cx.expr_block(cx.block(span, vec![])),
                Some(expr) => expr,
            }
        } else if self.0.len() == 1
            && let ast::StmtKind::Expr(expr) = &self.0[0].kind
            && self.1.is_none()
        {
            // There's only a single statement expression. Pull it out.
            expr.clone()
        } else {
            // Multiple statements and/or expressions.
            cx.expr_block(self.into_block(cx, span))
        }
    }
}

/// This method helps to extract all the type parameters referenced from a
/// type. For a type parameter `<T>`, it looks for either a `TyPath` that
/// is not global and starts with `T`, or a `TyQPath`.
/// Also include bound generic params from the input type.
fn find_type_parameters(
    ty: &ast::Ty,
    ty_param_names: &[Symbol],
    cx: &ExtCtxt<'_>,
) -> Vec<TypeParameter> {
    use rustc_ast::visit;

    struct Visitor<'a, 'b> {
        cx: &'a ExtCtxt<'b>,
        ty_param_names: &'a [Symbol],
        bound_generic_params_stack: Vec<ast::GenericParam>,
        type_params: Vec<TypeParameter>,
    }

    impl<'a, 'b> visit::Visitor<'a> for Visitor<'a, 'b> {
        fn visit_ty(&mut self, ty: &'a ast::Ty) {
            if let ast::TyKind::Path(_, ref path) = ty.kind {
                if let Some(segment) = path.segments.first() {
                    if self.ty_param_names.contains(&segment.ident.name) {
                        self.type_params.push(TypeParameter {
                            bound_generic_params: self.bound_generic_params_stack.clone(),
                            ty: P(ty.clone()),
                        });
                    }
                }
            }

            visit::walk_ty(self, ty)
        }

        // Place bound generic params on a stack, to extract them when a type is encountered.
        fn visit_poly_trait_ref(&mut self, trait_ref: &'a ast::PolyTraitRef) {
            let stack_len = self.bound_generic_params_stack.len();
            self.bound_generic_params_stack.extend(trait_ref.bound_generic_params.iter().cloned());

            visit::walk_poly_trait_ref(self, trait_ref);

            self.bound_generic_params_stack.truncate(stack_len);
        }

        fn visit_mac_call(&mut self, mac: &ast::MacCall) {
            self.cx.span_err(mac.span(), "`derive` cannot be used on items with type macros");
        }
    }

    let mut visitor = Visitor {
        cx,
        ty_param_names,
        bound_generic_params_stack: Vec::new(),
        type_params: Vec::new(),
    };
    visit::Visitor::visit_ty(&mut visitor, ty);

    visitor.type_params
}

impl<'a> TraitDef<'a> {
    pub fn expand(
        self,
        cx: &mut ExtCtxt<'_>,
        mitem: &ast::MetaItem,
        item: &'a Annotatable,
        push: &mut dyn FnMut(Annotatable),
    ) {
        self.expand_ext(cx, mitem, item, push, false);
    }

    pub fn expand_ext(
        self,
        cx: &mut ExtCtxt<'_>,
        mitem: &ast::MetaItem,
        item: &'a Annotatable,
        push: &mut dyn FnMut(Annotatable),
        from_scratch: bool,
    ) {
        match *item {
            Annotatable::Item(ref item) => {
                let is_packed = item.attrs.iter().any(|attr| {
                    for r in attr::find_repr_attrs(&cx.sess, attr) {
                        if let attr::ReprPacked(_) = r {
                            return true;
                        }
                    }
                    false
                });
                let has_no_type_params = match item.kind {
                    ast::ItemKind::Struct(_, ref generics)
                    | ast::ItemKind::Enum(_, ref generics)
                    | ast::ItemKind::Union(_, ref generics) => !generics
                        .params
                        .iter()
                        .any(|param| matches!(param.kind, ast::GenericParamKind::Type { .. })),
                    _ => unreachable!(),
                };
                let container_id = cx.current_expansion.id.expn_data().parent.expect_local();
                let always_copy = has_no_type_params && cx.resolver.has_derive_copy(container_id);

                let newitem = match item.kind {
                    ast::ItemKind::Struct(ref struct_def, ref generics) => self.expand_struct_def(
                        cx,
                        &struct_def,
                        item.ident,
                        generics,
                        from_scratch,
                        is_packed,
                        always_copy,
                    ),
                    ast::ItemKind::Enum(ref enum_def, ref generics) => {
                        // We ignore `is_packed`/`always_copy` here, because
                        // `repr(packed)` enums cause an error later on.
                        //
                        // This can only cause further compilation errors
                        // downstream in blatantly illegal code, so it
                        // is fine.
                        self.expand_enum_def(cx, enum_def, item.ident, generics, from_scratch)
                    }
                    ast::ItemKind::Union(ref struct_def, ref generics) => {
                        if self.supports_unions {
                            self.expand_struct_def(
                                cx,
                                &struct_def,
                                item.ident,
                                generics,
                                from_scratch,
                                is_packed,
                                always_copy,
                            )
                        } else {
                            cx.span_err(mitem.span, "this trait cannot be derived for unions");
                            return;
                        }
                    }
                    _ => unreachable!(),
                };
                // Keep the lint attributes of the previous item to control how the
                // generated implementations are linted
                let mut attrs = newitem.attrs.clone();
                attrs.extend(
                    item.attrs
                        .iter()
                        .filter(|a| {
                            [
                                sym::allow,
                                sym::warn,
                                sym::deny,
                                sym::forbid,
                                sym::stable,
                                sym::unstable,
                            ]
                            .contains(&a.name_or_empty())
                        })
                        .cloned(),
                );
                push(Annotatable::Item(P(ast::Item { attrs, ..(*newitem).clone() })))
            }
            _ => unreachable!(),
        }
    }

    /// Given that we are deriving a trait `DerivedTrait` for a type like:
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// struct Struct<'a, ..., 'z, A, B: DeclaredTrait, C, ..., Z> where C: WhereTrait {
    ///     a: A,
    ///     b: B::Item,
    ///     b1: <B as DeclaredTrait>::Item,
    ///     c1: <C as WhereTrait>::Item,
    ///     c2: Option<<C as WhereTrait>::Item>,
    ///     ...
    /// }
    /// ```
    ///
    /// create an impl like:
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// impl<'a, ..., 'z, A, B: DeclaredTrait, C, ... Z> where
    ///     C:                       WhereTrait,
    ///     A: DerivedTrait + B1 + ... + BN,
    ///     B: DerivedTrait + B1 + ... + BN,
    ///     C: DerivedTrait + B1 + ... + BN,
    ///     B::Item:                 DerivedTrait + B1 + ... + BN,
    ///     <C as WhereTrait>::Item: DerivedTrait + B1 + ... + BN,
    ///     ...
    /// {
    ///     ...
    /// }
    /// ```
    ///
    /// where B1, ..., BN are the bounds given by `bounds_paths`.'. Z is a phantom type, and
    /// therefore does not get bound by the derived trait.
    fn create_derived_impl(
        &self,
        cx: &mut ExtCtxt<'_>,
        type_ident: Ident,
        generics: &Generics,
        field_tys: Vec<P<ast::Ty>>,
        methods: Vec<P<ast::AssocItem>>,
    ) -> P<ast::Item> {
        let trait_path = self.path.to_path(cx, self.span, type_ident, generics);

        // Transform associated types from `deriving::ty::Ty` into `ast::AssocItem`
        let associated_types = self.associated_types.iter().map(|&(ident, ref type_def)| {
            P(ast::AssocItem {
                id: ast::DUMMY_NODE_ID,
                span: self.span,
                ident,
                vis: ast::Visibility {
                    span: self.span.shrink_to_lo(),
                    kind: ast::VisibilityKind::Inherited,
                    tokens: None,
                },
                attrs: ast::AttrVec::new(),
                kind: ast::AssocItemKind::TyAlias(Box::new(ast::TyAlias {
                    defaultness: ast::Defaultness::Final,
                    generics: Generics::default(),
                    where_clauses: (
                        ast::TyAliasWhereClause::default(),
                        ast::TyAliasWhereClause::default(),
                    ),
                    where_predicates_split: 0,
                    bounds: Vec::new(),
                    ty: Some(type_def.to_ty(cx, self.span, type_ident, generics)),
                })),
                tokens: None,
            })
        });

        let Generics { mut params, mut where_clause, .. } =
            self.generics.to_generics(cx, self.span, type_ident, generics);
        where_clause.span = generics.where_clause.span;
        let ctxt = self.span.ctxt();
        let span = generics.span.with_ctxt(ctxt);

        // Create the generic parameters
        params.extend(generics.params.iter().map(|param| match &param.kind {
            GenericParamKind::Lifetime { .. } => param.clone(),
            GenericParamKind::Type { .. } => {
                // I don't think this can be moved out of the loop, since
                // a GenericBound requires an ast id
                let bounds: Vec<_> =
                    // extra restrictions on the generics parameters to the
                    // type being derived upon
                    self.additional_bounds.iter().map(|p| {
                        cx.trait_bound(p.to_path(cx, self.span, type_ident, generics))
                    }).chain(
                        // require the current trait
                        iter::once(cx.trait_bound(trait_path.clone()))
                    ).chain(
                        // also add in any bounds from the declaration
                        param.bounds.iter().cloned()
                    ).collect();

                cx.typaram(param.ident.span.with_ctxt(ctxt), param.ident, bounds, None)
            }
            GenericParamKind::Const { ty, kw_span, .. } => {
                let const_nodefault_kind = GenericParamKind::Const {
                    ty: ty.clone(),
                    kw_span: kw_span.with_ctxt(ctxt),

                    // We can't have default values inside impl block
                    default: None,
                };
                let mut param_clone = param.clone();
                param_clone.kind = const_nodefault_kind;
                param_clone
            }
        }));

        // and similarly for where clauses
        where_clause.predicates.extend(generics.where_clause.predicates.iter().map(|clause| {
            match clause {
                ast::WherePredicate::BoundPredicate(wb) => {
                    let span = wb.span.with_ctxt(ctxt);
                    ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
                        span,
                        ..wb.clone()
                    })
                }
                ast::WherePredicate::RegionPredicate(wr) => {
                    let span = wr.span.with_ctxt(ctxt);
                    ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate {
                        span,
                        ..wr.clone()
                    })
                }
                ast::WherePredicate::EqPredicate(we) => {
                    let span = we.span.with_ctxt(ctxt);
                    ast::WherePredicate::EqPredicate(ast::WhereEqPredicate { span, ..we.clone() })
                }
            }
        }));

        {
            // Extra scope required here so ty_params goes out of scope before params is moved

            let mut ty_params = params
                .iter()
                .filter(|param| matches!(param.kind, ast::GenericParamKind::Type { .. }))
                .peekable();

            if ty_params.peek().is_some() {
                let ty_param_names: Vec<Symbol> =
                    ty_params.map(|ty_param| ty_param.ident.name).collect();

                for field_ty in field_tys {
                    let field_ty_params = find_type_parameters(&field_ty, &ty_param_names, cx);

                    for field_ty_param in field_ty_params {
                        // if we have already handled this type, skip it
                        if let ast::TyKind::Path(_, ref p) = field_ty_param.ty.kind {
                            if p.segments.len() == 1
                                && ty_param_names.contains(&p.segments[0].ident.name)
                            {
                                continue;
                            };
                        }
                        let mut bounds: Vec<_> = self
                            .additional_bounds
                            .iter()
                            .map(|p| cx.trait_bound(p.to_path(cx, self.span, type_ident, generics)))
                            .collect();

                        // require the current trait
                        bounds.push(cx.trait_bound(trait_path.clone()));

                        let predicate = ast::WhereBoundPredicate {
                            span: self.span,
                            bound_generic_params: field_ty_param.bound_generic_params,
                            bounded_ty: field_ty_param.ty,
                            bounds,
                        };

                        let predicate = ast::WherePredicate::BoundPredicate(predicate);
                        where_clause.predicates.push(predicate);
                    }
                }
            }
        }

        let trait_generics = Generics { params, where_clause, span };

        // Create the reference to the trait.
        let trait_ref = cx.trait_ref(trait_path);

        let self_params: Vec<_> = generics
            .params
            .iter()
            .map(|param| match param.kind {
                GenericParamKind::Lifetime { .. } => {
                    GenericArg::Lifetime(cx.lifetime(param.ident.span.with_ctxt(ctxt), param.ident))
                }
                GenericParamKind::Type { .. } => {
                    GenericArg::Type(cx.ty_ident(param.ident.span.with_ctxt(ctxt), param.ident))
                }
                GenericParamKind::Const { .. } => {
                    GenericArg::Const(cx.const_ident(param.ident.span.with_ctxt(ctxt), param.ident))
                }
            })
            .collect();

        // Create the type of `self`.
        let path = cx.path_all(self.span, false, vec![type_ident], self_params);
        let self_type = cx.ty_path(path);

        let attr = cx.attribute(cx.meta_word(self.span, sym::automatically_derived));
        let attrs = thin_vec![attr];
        let opt_trait_ref = Some(trait_ref);

        cx.item(
            self.span,
            Ident::empty(),
            attrs,
            ast::ItemKind::Impl(Box::new(ast::Impl {
                unsafety: ast::Unsafe::No,
                polarity: ast::ImplPolarity::Positive,
                defaultness: ast::Defaultness::Final,
                constness: ast::Const::No,
                generics: trait_generics,
                of_trait: opt_trait_ref,
                self_ty: self_type,
                items: methods.into_iter().chain(associated_types).collect(),
            })),
        )
    }

    fn expand_struct_def(
        &self,
        cx: &mut ExtCtxt<'_>,
        struct_def: &'a VariantData,
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
        is_packed: bool,
        always_copy: bool,
    ) -> P<ast::Item> {
        let field_tys: Vec<P<ast::Ty>> =
            struct_def.fields().iter().map(|field| field.ty.clone()).collect();

        let methods = self
            .methods
            .iter()
            .map(|method_def| {
                let (explicit_self, selflike_args, nonselflike_args, nonself_arg_tys) =
                    method_def.extract_arg_details(cx, self, type_ident, generics);

                let body = if from_scratch || method_def.is_static() {
                    method_def.expand_static_struct_method_body(
                        cx,
                        self,
                        struct_def,
                        type_ident,
                        &nonselflike_args,
                    )
                } else {
                    method_def.expand_struct_method_body(
                        cx,
                        self,
                        struct_def,
                        type_ident,
                        &selflike_args,
                        &nonselflike_args,
                        is_packed,
                        always_copy,
                    )
                };

                method_def.create_method(
                    cx,
                    self,
                    type_ident,
                    generics,
                    explicit_self,
                    nonself_arg_tys,
                    body,
                )
            })
            .collect();

        self.create_derived_impl(cx, type_ident, generics, field_tys, methods)
    }

    fn expand_enum_def(
        &self,
        cx: &mut ExtCtxt<'_>,
        enum_def: &'a EnumDef,
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
    ) -> P<ast::Item> {
        let mut field_tys = Vec::new();

        for variant in &enum_def.variants {
            field_tys.extend(variant.data.fields().iter().map(|field| field.ty.clone()));
        }

        let methods = self
            .methods
            .iter()
            .map(|method_def| {
                let (explicit_self, selflike_args, nonselflike_args, nonself_arg_tys) =
                    method_def.extract_arg_details(cx, self, type_ident, generics);

                let body = if from_scratch || method_def.is_static() {
                    method_def.expand_static_enum_method_body(
                        cx,
                        self,
                        enum_def,
                        type_ident,
                        &nonselflike_args,
                    )
                } else {
                    method_def.expand_enum_method_body(
                        cx,
                        self,
                        enum_def,
                        type_ident,
                        selflike_args,
                        &nonselflike_args,
                    )
                };

                method_def.create_method(
                    cx,
                    self,
                    type_ident,
                    generics,
                    explicit_self,
                    nonself_arg_tys,
                    body,
                )
            })
            .collect();

        self.create_derived_impl(cx, type_ident, generics, field_tys, methods)
    }
}

impl<'a> MethodDef<'a> {
    fn call_substructure_method(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        nonselflike_args: &[P<Expr>],
        fields: &SubstructureFields<'_>,
    ) -> BlockOrExpr {
        let span = trait_.span;
        let substructure = Substructure { type_ident, nonselflike_args, fields };
        let mut f = self.combine_substructure.borrow_mut();
        let f: &mut CombineSubstructureFunc<'_> = &mut *f;
        f(cx, span, &substructure)
    }

    fn get_ret_ty(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        generics: &Generics,
        type_ident: Ident,
    ) -> P<ast::Ty> {
        self.ret_ty.to_ty(cx, trait_.span, type_ident, generics)
    }

    fn is_static(&self) -> bool {
        !self.explicit_self
    }

    // The return value includes:
    // - explicit_self: The `&self` arg, if present.
    // - selflike_args: Expressions for `&self` (if present) and also any other
    //   args with the same type (e.g. the `other` arg in `PartialEq::eq`).
    // - nonselflike_args: Expressions for all the remaining args.
    // - nonself_arg_tys: Additional information about all the args other than
    //   `&self`.
    fn extract_arg_details(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        generics: &Generics,
    ) -> (Option<ast::ExplicitSelf>, Vec<P<Expr>>, Vec<P<Expr>>, Vec<(Ident, P<ast::Ty>)>) {
        let mut selflike_args = Vec::new();
        let mut nonselflike_args = Vec::new();
        let mut nonself_arg_tys = Vec::new();
        let span = trait_.span;

        let explicit_self = if self.explicit_self {
            let (self_expr, explicit_self) = ty::get_explicit_self(cx, span);
            selflike_args.push(self_expr);
            Some(explicit_self)
        } else {
            None
        };

        for (ty, name) in self.nonself_args.iter() {
            let ast_ty = ty.to_ty(cx, span, type_ident, generics);
            let ident = Ident::new(*name, span);
            nonself_arg_tys.push((ident, ast_ty));

            let arg_expr = cx.expr_ident(span, ident);

            match ty {
                // Selflike (`&Self`) arguments only occur in non-static methods.
                Ref(box Self_, _) if !self.is_static() => selflike_args.push(arg_expr),
                Self_ => cx.span_bug(span, "`Self` in non-return position"),
                _ => nonselflike_args.push(arg_expr),
            }
        }

        (explicit_self, selflike_args, nonselflike_args, nonself_arg_tys)
    }

    fn create_method(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        generics: &Generics,
        explicit_self: Option<ast::ExplicitSelf>,
        nonself_arg_tys: Vec<(Ident, P<ast::Ty>)>,
        body: BlockOrExpr,
    ) -> P<ast::AssocItem> {
        let span = trait_.span;
        // Create the generics that aren't for `Self`.
        let fn_generics = self.generics.to_generics(cx, span, type_ident, generics);

        let args = {
            let self_arg = explicit_self.map(|explicit_self| {
                let ident = Ident::with_dummy_span(kw::SelfLower).with_span_pos(span);
                ast::Param::from_self(ast::AttrVec::default(), explicit_self, ident)
            });
            let nonself_args =
                nonself_arg_tys.into_iter().map(|(name, ty)| cx.param(span, name, ty));
            self_arg.into_iter().chain(nonself_args).collect()
        };

        let ret_type = self.get_ret_ty(cx, trait_, generics, type_ident);

        let method_ident = Ident::new(self.name, span);
        let fn_decl = cx.fn_decl(args, ast::FnRetTy::Ty(ret_type));
        let body_block = body.into_block(cx, span);

        let trait_lo_sp = span.shrink_to_lo();

        let sig = ast::FnSig { header: ast::FnHeader::default(), decl: fn_decl, span };
        let defaultness = ast::Defaultness::Final;

        // Create the method.
        P(ast::AssocItem {
            id: ast::DUMMY_NODE_ID,
            attrs: self.attributes.clone(),
            span,
            vis: ast::Visibility {
                span: trait_lo_sp,
                kind: ast::VisibilityKind::Inherited,
                tokens: None,
            },
            ident: method_ident,
            kind: ast::AssocItemKind::Fn(Box::new(ast::Fn {
                defaultness,
                sig,
                generics: fn_generics,
                body: Some(body_block),
            })),
            tokens: None,
        })
    }

    /// The normal case uses field access.
    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// struct A { x: u8, y: u8 }
    ///
    /// // equivalent to:
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         self.x == other.x && self.y == other.y
    ///     }
    /// }
    /// ```
    /// But if the struct is `repr(packed)`, we can't use something like
    /// `&self.x` because that might cause an unaligned ref. So for any trait
    /// method that takes a reference, if the struct impls `Copy` then we use a
    /// local block to force a copy:
    /// ```
    /// # struct A { x: u8, y: u8 }
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         // Desugars to `{ self.x }.eq(&{ other.y }) && ...`
    ///         { self.x } == { other.y } && { self.y } == { other.y }
    ///     }
    /// }
    /// impl Hash for A {
    ///     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
    ///         ::core::hash::Hash::hash(&{ self.x }, state);
    ///         ::core::hash::Hash::hash(&{ self.y }, state)
    ///     }
    /// }
    /// ```
    /// If the struct doesn't impl `Copy`, we use let-destructuring with `ref`:
    /// ```
    /// # struct A { x: u8, y: u8 }
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         let Self { x: ref __self_0_0, y: ref __self_0_1 } = *self;
    ///         let Self { x: ref __self_1_0, y: ref __self_1_1 } = *other;
    ///         *__self_0_0 == *__self_1_0 && *__self_0_1 == *__self_1_1
    ///     }
    /// }
    /// ```
    /// This latter case only works if the fields match the alignment required
    /// by the `packed(N)` attribute. (We'll get errors later on if not.)
    fn expand_struct_method_body<'b>(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        struct_def: &'b VariantData,
        type_ident: Ident,
        selflike_args: &[P<Expr>],
        nonselflike_args: &[P<Expr>],
        is_packed: bool,
        always_copy: bool,
    ) -> BlockOrExpr {
        let span = trait_.span;
        assert!(selflike_args.len() == 1 || selflike_args.len() == 2);

        let mk_body = |cx, selflike_fields| {
            self.call_substructure_method(
                cx,
                trait_,
                type_ident,
                nonselflike_args,
                &Struct(struct_def, selflike_fields),
            )
        };

        if !is_packed {
            let selflike_fields =
                trait_.create_struct_field_access_fields(cx, selflike_args, struct_def, false);
            mk_body(cx, selflike_fields)
        } else if always_copy {
            let selflike_fields =
                trait_.create_struct_field_access_fields(cx, selflike_args, struct_def, true);
            mk_body(cx, selflike_fields)
        } else {
            // Neither packed nor copy. Need to use ref patterns.
            let prefixes: Vec<_> =
                (0..selflike_args.len()).map(|i| format!("__self_{}", i)).collect();
            let addr_of = always_copy;
            let selflike_fields =
                trait_.create_struct_pattern_fields(cx, struct_def, &prefixes, addr_of);
            let mut body = mk_body(cx, selflike_fields);

            let struct_path = cx.path(span, vec![Ident::new(kw::SelfUpper, type_ident.span)]);
            let by_ref = ByRef::from(is_packed && !always_copy);
            let patterns =
                trait_.create_struct_patterns(cx, struct_path, struct_def, &prefixes, by_ref);

            // Do the let-destructuring.
            let mut stmts: Vec<_> = iter::zip(selflike_args, patterns)
                .map(|(selflike_arg_expr, pat)| {
                    let selflike_arg_expr = cx.expr_deref(span, selflike_arg_expr.clone());
                    cx.stmt_let_pat(span, pat, selflike_arg_expr)
                })
                .collect();
            stmts.extend(std::mem::take(&mut body.0));
            BlockOrExpr(stmts, body.1)
        }
    }

    fn expand_static_struct_method_body(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        struct_def: &VariantData,
        type_ident: Ident,
        nonselflike_args: &[P<Expr>],
    ) -> BlockOrExpr {
        let summary = trait_.summarise_struct(cx, struct_def);

        self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            nonselflike_args,
            &StaticStruct(struct_def, summary),
        )
    }

    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// enum A {
    ///     A1,
    ///     A2(i32)
    /// }
    /// ```
    /// is equivalent to:
    /// ```
    /// impl ::core::cmp::PartialEq for A {
    ///     #[inline]
    ///     fn eq(&self, other: &A) -> bool {
    ///         let __self_tag = ::core::intrinsics::discriminant_value(self);
    ///         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
    ///         __self_tag == __arg1_tag &&
    ///             match (self, other) {
    ///                 (A::A2(__self_0), A::A2(__arg1_0)) =>
    ///                     *__self_0 == *__arg1_0,
    ///                 _ => true,
    ///             }
    ///     }
    /// }
    /// ```
    /// Creates a tag check combined with a match for a tuple of all
    /// `selflike_args`, with an arm for each variant with fields, possibly an
    /// arm for each fieldless variant (if `!unify_fieldless_variants` is not
    /// true), and possibly a default arm.
    fn expand_enum_method_body<'b>(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        enum_def: &'b EnumDef,
        type_ident: Ident,
        selflike_args: Vec<P<Expr>>,
        nonselflike_args: &[P<Expr>],
    ) -> BlockOrExpr {
        let span = trait_.span;
        let variants = &enum_def.variants;

        // Traits that unify fieldless variants always use the tag(s).
        let uses_tags = self.unify_fieldless_variants;

        // There is no sensible code to be generated for *any* deriving on a
        // zero-variant enum. So we just generate a failing expression.
        if variants.is_empty() {
            return BlockOrExpr(vec![], Some(deriving::call_unreachable(cx, span)));
        }

        let prefixes = iter::once("__self".to_string())
            .chain(
                selflike_args
                    .iter()
                    .enumerate()
                    .skip(1)
                    .map(|(arg_count, _selflike_arg)| format!("__arg{}", arg_count)),
            )
            .collect::<Vec<String>>();

        // Build a series of let statements mapping each selflike_arg
        // to its discriminant value.
        //
        // e.g. for `PartialEq::eq` builds two statements:
        // ```
        // let __self_tag = ::core::intrinsics::discriminant_value(self);
        // let __arg1_tag = ::core::intrinsics::discriminant_value(other);
        // ```
        let get_tag_pieces = |cx: &ExtCtxt<'_>| {
            let tag_idents: Vec<_> = prefixes
                .iter()
                .map(|name| Ident::from_str_and_span(&format!("{}_tag", name), span))
                .collect();

            let mut tag_exprs: Vec<_> = tag_idents
                .iter()
                .map(|&ident| cx.expr_addr_of(span, cx.expr_ident(span, ident)))
                .collect();

            let self_expr = tag_exprs.remove(0);
            let other_selflike_exprs = tag_exprs;
            let tag_field = FieldInfo { span, name: None, self_expr, other_selflike_exprs };

            let tag_let_stmts: Vec<_> = iter::zip(&tag_idents, &selflike_args)
                .map(|(&ident, selflike_arg)| {
                    let variant_value = deriving::call_intrinsic(
                        cx,
                        span,
                        sym::discriminant_value,
                        vec![selflike_arg.clone()],
                    );
                    cx.stmt_let(span, false, ident, variant_value)
                })
                .collect();

            (tag_field, tag_let_stmts)
        };

        // There are some special cases involving fieldless enums where no
        // match is necessary.
        let all_fieldless = variants.iter().all(|v| v.data.fields().is_empty());
        if all_fieldless {
            if uses_tags && variants.len() > 1 {
                // If the type is fieldless and the trait uses the tag and
                // there are multiple variants, we need just an operation on
                // the tag(s).
                let (tag_field, mut tag_let_stmts) = get_tag_pieces(cx);
                let mut tag_check = self.call_substructure_method(
                    cx,
                    trait_,
                    type_ident,
                    nonselflike_args,
                    &EnumTag(tag_field, None),
                );
                tag_let_stmts.append(&mut tag_check.0);
                return BlockOrExpr(tag_let_stmts, tag_check.1);
            }

            if variants.len() == 1 {
                // If there is a single variant, we don't need an operation on
                // the tag(s). Just use the most degenerate result.
                return self.call_substructure_method(
                    cx,
                    trait_,
                    type_ident,
                    nonselflike_args,
                    &EnumMatching(0, 1, &variants[0], Vec::new()),
                );
            };
        }

        // These arms are of the form:
        // (Variant1, Variant1, ...) => Body1
        // (Variant2, Variant2, ...) => Body2
        // ...
        // where each tuple has length = selflike_args.len()
        let mut match_arms: Vec<ast::Arm> = variants
            .iter()
            .enumerate()
            .filter(|&(_, v)| !(self.unify_fieldless_variants && v.data.fields().is_empty()))
            .map(|(index, variant)| {
                // A single arm has form (&VariantK, &VariantK, ...) => BodyK
                // (see "Final wrinkle" note below for why.)

                let addr_of = false; // because enums can't be repr(packed)
                let fields =
                    trait_.create_struct_pattern_fields(cx, &variant.data, &prefixes, addr_of);

                let sp = variant.span.with_ctxt(trait_.span.ctxt());
                let variant_path = cx.path(sp, vec![type_ident, variant.ident]);
                let by_ref = ByRef::No; // because enums can't be repr(packed)
                let mut subpats: Vec<_> = trait_.create_struct_patterns(
                    cx,
                    variant_path,
                    &variant.data,
                    &prefixes,
                    by_ref,
                );

                // `(VariantK, VariantK, ...)` or just `VariantK`.
                let single_pat = if subpats.len() == 1 {
                    subpats.pop().unwrap()
                } else {
                    cx.pat_tuple(span, subpats)
                };

                // For the BodyK, we need to delegate to our caller,
                // passing it an EnumMatching to indicate which case
                // we are in.
                //
                // Now, for some given VariantK, we have built up
                // expressions for referencing every field of every
                // Self arg, assuming all are instances of VariantK.
                // Build up code associated with such a case.
                let substructure = EnumMatching(index, variants.len(), variant, fields);
                let arm_expr = self
                    .call_substructure_method(
                        cx,
                        trait_,
                        type_ident,
                        nonselflike_args,
                        &substructure,
                    )
                    .into_expr(cx, span);

                cx.arm(span, single_pat, arm_expr)
            })
            .collect();

        // Add a default arm to the match, if necessary.
        let first_fieldless = variants.iter().find(|v| v.data.fields().is_empty());
        let default = match first_fieldless {
            Some(v) if self.unify_fieldless_variants => {
                // We need a default case that handles all the fieldless
                // variants. The index and actual variant aren't meaningful in
                // this case, so just use dummy values.
                Some(
                    self.call_substructure_method(
                        cx,
                        trait_,
                        type_ident,
                        nonselflike_args,
                        &EnumMatching(0, variants.len(), v, Vec::new()),
                    )
                    .into_expr(cx, span),
                )
            }
            _ if variants.len() > 1 && selflike_args.len() > 1 => {
                // Because we know that all the arguments will match if we reach
                // the match expression we add the unreachable intrinsics as the
                // result of the default which should help llvm in optimizing it.
                Some(deriving::call_unreachable(cx, span))
            }
            _ => None,
        };
        if let Some(arm) = default {
            match_arms.push(cx.arm(span, cx.pat_wild(span), arm));
        }

        // Create a match expression with one arm per discriminant plus
        // possibly a default arm, e.g.:
        //      match (self, other) {
        //          (Variant1, Variant1, ...) => Body1
        //          (Variant2, Variant2, ...) => Body2,
        //          ...
        //          _ => ::core::intrinsics::unreachable()
        //      }
        let get_match_expr = |mut selflike_args: Vec<P<Expr>>| {
            let match_arg = if selflike_args.len() == 1 {
                selflike_args.pop().unwrap()
            } else {
                cx.expr(span, ast::ExprKind::Tup(selflike_args))
            };
            cx.expr_match(span, match_arg, match_arms)
        };

        // If the trait uses the tag and there are multiple variants, we need
        // to add a tag check operation before the match. Otherwise, the match
        // is enough.
        if uses_tags && variants.len() > 1 {
            let (tag_field, mut tag_let_stmts) = get_tag_pieces(cx);

            // Combine a tag check with the match.
            let mut tag_check_plus_match = self.call_substructure_method(
                cx,
                trait_,
                type_ident,
                nonselflike_args,
                &EnumTag(tag_field, Some(get_match_expr(selflike_args))),
            );
            tag_let_stmts.append(&mut tag_check_plus_match.0);
            BlockOrExpr(tag_let_stmts, tag_check_plus_match.1)
        } else {
            BlockOrExpr(vec![], Some(get_match_expr(selflike_args)))
        }
    }

    fn expand_static_enum_method_body(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        enum_def: &EnumDef,
        type_ident: Ident,
        nonselflike_args: &[P<Expr>],
    ) -> BlockOrExpr {
        let summary = enum_def
            .variants
            .iter()
            .map(|v| {
                let sp = v.span.with_ctxt(trait_.span.ctxt());
                let summary = trait_.summarise_struct(cx, &v.data);
                (v.ident, sp, summary)
            })
            .collect();
        self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            nonselflike_args,
            &StaticEnum(enum_def, summary),
        )
    }
}

// general helper methods.
impl<'a> TraitDef<'a> {
    fn summarise_struct(&self, cx: &mut ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
        let mut named_idents = Vec::new();
        let mut just_spans = Vec::new();
        for field in struct_def.fields() {
            let sp = field.span.with_ctxt(self.span.ctxt());
            match field.ident {
                Some(ident) => named_idents.push((ident, sp)),
                _ => just_spans.push(sp),
            }
        }

        let is_tuple = matches!(struct_def, ast::VariantData::Tuple(..));
        match (just_spans.is_empty(), named_idents.is_empty()) {
            (false, false) => {
                cx.span_bug(self.span, "a struct with named and unnamed fields in generic `derive`")
            }
            // named fields
            (_, false) => Named(named_idents),
            // unnamed fields
            (false, _) => Unnamed(just_spans, is_tuple),
            // empty
            _ => Named(Vec::new()),
        }
    }

    fn create_struct_patterns(
        &self,
        cx: &mut ExtCtxt<'_>,
        struct_path: ast::Path,
        struct_def: &'a VariantData,
        prefixes: &[String],
        by_ref: ByRef,
    ) -> Vec<P<ast::Pat>> {
        prefixes
            .iter()
            .map(|prefix| {
                let pieces_iter =
                    struct_def.fields().iter().enumerate().map(|(i, struct_field)| {
                        let sp = struct_field.span.with_ctxt(self.span.ctxt());
                        let ident = self.mk_pattern_ident(prefix, i);
                        let path = ident.with_span_pos(sp);
                        (
                            sp,
                            struct_field.ident,
                            cx.pat(
                                path.span,
                                PatKind::Ident(
                                    BindingAnnotation(by_ref, Mutability::Not),
                                    path,
                                    None,
                                ),
                            ),
                        )
                    });

                let struct_path = struct_path.clone();
                match *struct_def {
                    VariantData::Struct(..) => {
                        let field_pats = pieces_iter
                            .map(|(sp, ident, pat)| {
                                if ident.is_none() {
                                    cx.span_bug(
                                        sp,
                                        "a braced struct with unnamed fields in `derive`",
                                    );
                                }
                                ast::PatField {
                                    ident: ident.unwrap(),
                                    is_shorthand: false,
                                    attrs: ast::AttrVec::new(),
                                    id: ast::DUMMY_NODE_ID,
                                    span: pat.span.with_ctxt(self.span.ctxt()),
                                    pat,
                                    is_placeholder: false,
                                }
                            })
                            .collect();
                        cx.pat_struct(self.span, struct_path, field_pats)
                    }
                    VariantData::Tuple(..) => {
                        let subpats = pieces_iter.map(|(_, _, subpat)| subpat).collect();
                        cx.pat_tuple_struct(self.span, struct_path, subpats)
                    }
                    VariantData::Unit(..) => cx.pat_path(self.span, struct_path),
                }
            })
            .collect()
    }

    fn create_fields<F>(&self, struct_def: &'a VariantData, mk_exprs: F) -> Vec<FieldInfo>
    where
        F: Fn(usize, &ast::FieldDef, Span) -> Vec<P<ast::Expr>>,
    {
        struct_def
            .fields()
            .iter()
            .enumerate()
            .map(|(i, struct_field)| {
                // For this field, get an expr for each selflike_arg. E.g. for
                // `PartialEq::eq`, one for each of `&self` and `other`.
                let sp = struct_field.span.with_ctxt(self.span.ctxt());
                let mut exprs: Vec<_> = mk_exprs(i, struct_field, sp);
                let self_expr = exprs.remove(0);
                let other_selflike_exprs = exprs;
                FieldInfo {
                    span: sp.with_ctxt(self.span.ctxt()),
                    name: struct_field.ident,
                    self_expr,
                    other_selflike_exprs,
                }
            })
            .collect()
    }

    fn mk_pattern_ident(&self, prefix: &str, i: usize) -> Ident {
        Ident::from_str_and_span(&format!("{}_{}", prefix, i), self.span)
    }

    fn create_struct_pattern_fields(
        &self,
        cx: &mut ExtCtxt<'_>,
        struct_def: &'a VariantData,
        prefixes: &[String],
        addr_of: bool,
    ) -> Vec<FieldInfo> {
        self.create_fields(struct_def, |i, _struct_field, sp| {
            prefixes
                .iter()
                .map(|prefix| {
                    let ident = self.mk_pattern_ident(prefix, i);
                    let expr = cx.expr_path(cx.path_ident(sp, ident));
                    if addr_of { cx.expr_addr_of(sp, expr) } else { expr }
                })
                .collect()
        })
    }

    fn create_struct_field_access_fields(
        &self,
        cx: &mut ExtCtxt<'_>,
        selflike_args: &[P<Expr>],
        struct_def: &'a VariantData,
        copy: bool,
    ) -> Vec<FieldInfo> {
        self.create_fields(struct_def, |i, struct_field, sp| {
            selflike_args
                .iter()
                .map(|selflike_arg| {
                    // Note: we must use `struct_field.span` rather than `sp` in the
                    // `unwrap_or_else` case otherwise the hygiene is wrong and we get
                    // "field `0` of struct `Point` is private" errors on tuple
                    // structs.
                    let mut field_expr = cx.expr(
                        sp,
                        ast::ExprKind::Field(
                            selflike_arg.clone(),
                            struct_field.ident.unwrap_or_else(|| {
                                Ident::from_str_and_span(&i.to_string(), struct_field.span)
                            }),
                        ),
                    );
                    if copy {
                        field_expr = cx.expr_block(
                            cx.block(struct_field.span, vec![cx.stmt_expr(field_expr)]),
                        );
                    }
                    cx.expr_addr_of(sp, field_expr)
                })
                .collect()
        })
    }
}

/// The function passed to `cs_fold` is called repeatedly with a value of this
/// type. It describes one part of the code generation. The result is always an
/// expression.
pub enum CsFold<'a> {
    /// The basic case: a field expression for one or more selflike args. E.g.
    /// for `PartialEq::eq` this is something like `self.x == other.x`.
    Single(&'a FieldInfo),

    /// The combination of two field expressions. E.g. for `PartialEq::eq` this
    /// is something like `<field1 equality> && <field2 equality>`.
    Combine(Span, P<Expr>, P<Expr>),

    // The fallback case for a struct or enum variant with no fields.
    Fieldless,
}

/// Folds over fields, combining the expressions for each field in a sequence.
/// Statics may not be folded over.
pub fn cs_fold<F>(
    use_foldl: bool,
    cx: &mut ExtCtxt<'_>,
    trait_span: Span,
    substructure: &Substructure<'_>,
    mut f: F,
) -> P<Expr>
where
    F: FnMut(&mut ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
{
    match substructure.fields {
        EnumMatching(.., all_fields) | Struct(_, all_fields) => {
            if all_fields.is_empty() {
                return f(cx, CsFold::Fieldless);
            }

            let (base_field, rest) = if use_foldl {
                all_fields.split_first().unwrap()
            } else {
                all_fields.split_last().unwrap()
            };

            let base_expr = f(cx, CsFold::Single(base_field));

            let op = |old, field: &FieldInfo| {
                let new = f(cx, CsFold::Single(field));
                f(cx, CsFold::Combine(field.span, old, new))
            };

            if use_foldl {
                rest.iter().fold(base_expr, op)
            } else {
                rest.iter().rfold(base_expr, op)
            }
        }
        EnumTag(tag_field, match_expr) => {
            let tag_check_expr = f(cx, CsFold::Single(tag_field));
            if let Some(match_expr) = match_expr {
                if use_foldl {
                    f(cx, CsFold::Combine(trait_span, tag_check_expr, match_expr.clone()))
                } else {
                    f(cx, CsFold::Combine(trait_span, match_expr.clone(), tag_check_expr))
                }
            } else {
                tag_check_expr
            }
        }
        StaticEnum(..) | StaticStruct(..) => cx.span_bug(trait_span, "static function in `derive`"),
    }
}