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
use crate::base::{DummyResult, ExtCtxt, MacResult, TTMacroExpander};
use crate::base::{SyntaxExtension, SyntaxExtensionKind};
use crate::expand::{ensure_complete_parse, parse_ast_fragment, AstFragment, AstFragmentKind};
use crate::mbe;
use crate::mbe::macro_check;
use crate::mbe::macro_parser::{Error, ErrorReported, Failure, Success, TtParser};
use crate::mbe::macro_parser::{MatchedSeq, MatchedTokenTree, MatcherLoc};
use crate::mbe::transcribe::transcribe;

use rustc_ast as ast;
use rustc_ast::token::{self, Delimiter, NonterminalKind, Token, TokenKind, TokenKind::*};
use rustc_ast::tokenstream::{DelimSpan, TokenStream};
use rustc_ast::{NodeId, DUMMY_NODE_ID};
use rustc_ast_pretty::pprust;
use rustc_attr::{self as attr, TransparencyError};
use rustc_data_structures::fx::FxHashMap;
use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
use rustc_feature::Features;
use rustc_lint_defs::builtin::{
    RUST_2021_INCOMPATIBLE_OR_PATTERNS, SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
};
use rustc_lint_defs::BuiltinLintDiagnostics;
use rustc_parse::parser::Parser;
use rustc_session::parse::ParseSess;
use rustc_session::Session;
use rustc_span::edition::Edition;
use rustc_span::hygiene::Transparency;
use rustc_span::symbol::{kw, sym, Ident, MacroRulesNormalizedIdent};
use rustc_span::Span;

use std::borrow::Cow;
use std::collections::hash_map::Entry;
use std::{mem, slice};
use tracing::debug;

pub(crate) struct ParserAnyMacro<'a> {
    parser: Parser<'a>,

    /// Span of the expansion site of the macro this parser is for
    site_span: Span,
    /// The ident of the macro we're parsing
    macro_ident: Ident,
    lint_node_id: NodeId,
    is_trailing_mac: bool,
    arm_span: Span,
    /// Whether or not this macro is defined in the current crate
    is_local: bool,
}

pub(crate) fn annotate_err_with_kind(err: &mut Diagnostic, kind: AstFragmentKind, span: Span) {
    match kind {
        AstFragmentKind::Ty => {
            err.span_label(span, "this macro call doesn't expand to a type");
        }
        AstFragmentKind::Pat => {
            err.span_label(span, "this macro call doesn't expand to a pattern");
        }
        _ => {}
    };
}

fn emit_frag_parse_err(
    mut e: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed>,
    parser: &Parser<'_>,
    orig_parser: &mut Parser<'_>,
    site_span: Span,
    arm_span: Span,
    kind: AstFragmentKind,
) {
    // FIXME(davidtwco): avoid depending on the error message text
    if parser.token == token::Eof && e.message[0].0.expect_str().ends_with(", found `<eof>`") {
        if !e.span.is_dummy() {
            // early end of macro arm (#52866)
            e.replace_span_with(parser.sess.source_map().next_point(parser.token.span));
        }
        let msg = &e.message[0];
        e.message[0] = (
            rustc_errors::DiagnosticMessage::Str(format!(
                "macro expansion ends with an incomplete expression: {}",
                msg.0.expect_str().replace(", found `<eof>`", ""),
            )),
            msg.1,
        );
    }
    if e.span.is_dummy() {
        // Get around lack of span in error (#30128)
        e.replace_span_with(site_span);
        if !parser.sess.source_map().is_imported(arm_span) {
            e.span_label(arm_span, "in this macro arm");
        }
    } else if parser.sess.source_map().is_imported(parser.token.span) {
        e.span_label(site_span, "in this macro invocation");
    }
    match kind {
        // Try a statement if an expression is wanted but failed and suggest adding `;` to call.
        AstFragmentKind::Expr => match parse_ast_fragment(orig_parser, AstFragmentKind::Stmts) {
            Err(err) => err.cancel(),
            Ok(_) => {
                e.note(
                    "the macro call doesn't expand to an expression, but it can expand to a statement",
                );
                e.span_suggestion_verbose(
                    site_span.shrink_to_hi(),
                    "add `;` to interpret the expansion as a statement",
                    ";",
                    Applicability::MaybeIncorrect,
                );
            }
        },
        _ => annotate_err_with_kind(&mut e, kind, site_span),
    };
    e.emit();
}

impl<'a> ParserAnyMacro<'a> {
    pub(crate) fn make(mut self: Box<ParserAnyMacro<'a>>, kind: AstFragmentKind) -> AstFragment {
        let ParserAnyMacro {
            site_span,
            macro_ident,
            ref mut parser,
            lint_node_id,
            arm_span,
            is_trailing_mac,
            is_local,
        } = *self;
        let snapshot = &mut parser.create_snapshot_for_diagnostic();
        let fragment = match parse_ast_fragment(parser, kind) {
            Ok(f) => f,
            Err(err) => {
                emit_frag_parse_err(err, parser, snapshot, site_span, arm_span, kind);
                return kind.dummy(site_span);
            }
        };

        // We allow semicolons at the end of expressions -- e.g., the semicolon in
        // `macro_rules! m { () => { panic!(); } }` isn't parsed by `.parse_expr()`,
        // but `m!()` is allowed in expression positions (cf. issue #34706).
        if kind == AstFragmentKind::Expr && parser.token == token::Semi {
            if is_local {
                parser.sess.buffer_lint_with_diagnostic(
                    SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
                    parser.token.span,
                    lint_node_id,
                    "trailing semicolon in macro used in expression position",
                    BuiltinLintDiagnostics::TrailingMacro(is_trailing_mac, macro_ident),
                );
            }
            parser.bump();
        }

        // Make sure we don't have any tokens left to parse so we don't silently drop anything.
        let path = ast::Path::from_ident(macro_ident.with_span_pos(site_span));
        ensure_complete_parse(parser, &path, kind.name(), site_span);
        fragment
    }
}

struct MacroRulesMacroExpander {
    node_id: NodeId,
    name: Ident,
    span: Span,
    transparency: Transparency,
    lhses: Vec<Vec<MatcherLoc>>,
    rhses: Vec<mbe::TokenTree>,
    valid: bool,
}

impl TTMacroExpander for MacroRulesMacroExpander {
    fn expand<'cx>(
        &self,
        cx: &'cx mut ExtCtxt<'_>,
        sp: Span,
        input: TokenStream,
    ) -> Box<dyn MacResult + 'cx> {
        if !self.valid {
            return DummyResult::any(sp);
        }
        expand_macro(
            cx,
            sp,
            self.span,
            self.node_id,
            self.name,
            self.transparency,
            input,
            &self.lhses,
            &self.rhses,
        )
    }
}

fn macro_rules_dummy_expander<'cx>(
    _: &'cx mut ExtCtxt<'_>,
    span: Span,
    _: TokenStream,
) -> Box<dyn MacResult + 'cx> {
    DummyResult::any(span)
}

fn trace_macros_note(cx_expansions: &mut FxHashMap<Span, Vec<String>>, sp: Span, message: String) {
    let sp = sp.macro_backtrace().last().map_or(sp, |trace| trace.call_site);
    cx_expansions.entry(sp).or_default().push(message);
}

/// Expands the rules based macro defined by `lhses` and `rhses` for a given
/// input `arg`.
fn expand_macro<'cx>(
    cx: &'cx mut ExtCtxt<'_>,
    sp: Span,
    def_span: Span,
    node_id: NodeId,
    name: Ident,
    transparency: Transparency,
    arg: TokenStream,
    lhses: &[Vec<MatcherLoc>],
    rhses: &[mbe::TokenTree],
) -> Box<dyn MacResult + 'cx> {
    let sess = &cx.sess.parse_sess;
    // Macros defined in the current crate have a real node id,
    // whereas macros from an external crate have a dummy id.
    let is_local = node_id != DUMMY_NODE_ID;

    if cx.trace_macros() {
        let msg = format!("expanding `{}! {{ {} }}`", name, pprust::tts_to_string(&arg));
        trace_macros_note(&mut cx.expansions, sp, msg);
    }

    // Which arm's failure should we report? (the one furthest along)
    let mut best_failure: Option<(Token, &str)> = None;

    // We create a base parser that can be used for the "black box" parts.
    // Every iteration needs a fresh copy of that parser. However, the parser
    // is not mutated on many of the iterations, particularly when dealing with
    // macros like this:
    //
    // macro_rules! foo {
    //     ("a") => (A);
    //     ("b") => (B);
    //     ("c") => (C);
    //     // ... etc. (maybe hundreds more)
    // }
    //
    // as seen in the `html5ever` benchmark. We use a `Cow` so that the base
    // parser is only cloned when necessary (upon mutation). Furthermore, we
    // reinitialize the `Cow` with the base parser at the start of every
    // iteration, so that any mutated parsers are not reused. This is all quite
    // hacky, but speeds up the `html5ever` benchmark significantly. (Issue
    // 68836 suggests a more comprehensive but more complex change to deal with
    // this situation.)
    let parser = parser_from_cx(sess, arg.clone());

    // Try each arm's matchers.
    let mut tt_parser = TtParser::new(name);
    for (i, lhs) in lhses.iter().enumerate() {
        // Take a snapshot of the state of pre-expansion gating at this point.
        // This is used so that if a matcher is not `Success(..)`ful,
        // then the spans which became gated when parsing the unsuccessful matcher
        // are not recorded. On the first `Success(..)`ful matcher, the spans are merged.
        let mut gated_spans_snapshot = mem::take(&mut *sess.gated_spans.spans.borrow_mut());

        match tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs) {
            Success(named_matches) => {
                // The matcher was `Success(..)`ful.
                // Merge the gated spans from parsing the matcher with the pre-existing ones.
                sess.gated_spans.merge(gated_spans_snapshot);

                let (rhs, rhs_span): (&mbe::Delimited, DelimSpan) = match &rhses[i] {
                    mbe::TokenTree::Delimited(span, delimited) => (&delimited, *span),
                    _ => cx.span_bug(sp, "malformed macro rhs"),
                };
                let arm_span = rhses[i].span();

                let rhs_spans = rhs.tts.iter().map(|t| t.span()).collect::<Vec<_>>();
                // rhs has holes ( `$id` and `$(...)` that need filled)
                let mut tts = match transcribe(cx, &named_matches, &rhs, rhs_span, transparency) {
                    Ok(tts) => tts,
                    Err(mut err) => {
                        err.emit();
                        return DummyResult::any(arm_span);
                    }
                };

                // Replace all the tokens for the corresponding positions in the macro, to maintain
                // proper positions in error reporting, while maintaining the macro_backtrace.
                if rhs_spans.len() == tts.len() {
                    tts = tts.map_enumerated(|i, tt| {
                        let mut tt = tt.clone();
                        let mut sp = rhs_spans[i];
                        sp = sp.with_ctxt(tt.span().ctxt());
                        tt.set_span(sp);
                        tt
                    });
                }

                if cx.trace_macros() {
                    let msg = format!("to `{}`", pprust::tts_to_string(&tts));
                    trace_macros_note(&mut cx.expansions, sp, msg);
                }

                let mut p = Parser::new(sess, tts, false, None);
                p.last_type_ascription = cx.current_expansion.prior_type_ascription;

                if is_local {
                    cx.resolver.record_macro_rule_usage(node_id, i);
                }

                // Let the context choose how to interpret the result.
                // Weird, but useful for X-macros.
                return Box::new(ParserAnyMacro {
                    parser: p,

                    // Pass along the original expansion site and the name of the macro
                    // so we can print a useful error message if the parse of the expanded
                    // macro leaves unparsed tokens.
                    site_span: sp,
                    macro_ident: name,
                    lint_node_id: cx.current_expansion.lint_node_id,
                    is_trailing_mac: cx.current_expansion.is_trailing_mac,
                    arm_span,
                    is_local,
                });
            }
            Failure(token, msg) => match best_failure {
                Some((ref best_token, _)) if best_token.span.lo() >= token.span.lo() => {}
                _ => best_failure = Some((token, msg)),
            },
            Error(err_sp, ref msg) => {
                let span = err_sp.substitute_dummy(sp);
                cx.struct_span_err(span, &msg).emit();
                return DummyResult::any(span);
            }
            ErrorReported => return DummyResult::any(sp),
        }

        // The matcher was not `Success(..)`ful.
        // Restore to the state before snapshotting and maybe try again.
        mem::swap(&mut gated_spans_snapshot, &mut sess.gated_spans.spans.borrow_mut());
    }
    drop(parser);

    let (token, label) = best_failure.expect("ran no matchers");
    let span = token.span.substitute_dummy(sp);
    let mut err = cx.struct_span_err(span, &parse_failure_msg(&token));
    err.span_label(span, label);
    if !def_span.is_dummy() && !cx.source_map().is_imported(def_span) {
        err.span_label(cx.source_map().guess_head_span(def_span), "when calling this macro");
    }

    // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
    if let Some((arg, comma_span)) = arg.add_comma() {
        for lhs in lhses {
            let parser = parser_from_cx(sess, arg.clone());
            if let Success(_) = tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs) {
                if comma_span.is_dummy() {
                    err.note("you might be missing a comma");
                } else {
                    err.span_suggestion_short(
                        comma_span,
                        "missing comma here",
                        ", ",
                        Applicability::MachineApplicable,
                    );
                }
            }
        }
    }
    err.emit();
    cx.trace_macros_diag();
    DummyResult::any(sp)
}

// Note that macro-by-example's input is also matched against a token tree:
//                   $( $lhs:tt => $rhs:tt );+
//
// Holy self-referential!

/// Converts a macro item into a syntax extension.
pub fn compile_declarative_macro(
    sess: &Session,
    features: &Features,
    def: &ast::Item,
    edition: Edition,
) -> (SyntaxExtension, Vec<(usize, Span)>) {
    debug!("compile_declarative_macro: {:?}", def);
    let mk_syn_ext = |expander| {
        SyntaxExtension::new(
            sess,
            SyntaxExtensionKind::LegacyBang(expander),
            def.span,
            Vec::new(),
            edition,
            def.ident.name,
            &def.attrs,
        )
    };
    let dummy_syn_ext = || (mk_syn_ext(Box::new(macro_rules_dummy_expander)), Vec::new());

    let diag = &sess.parse_sess.span_diagnostic;
    let lhs_nm = Ident::new(sym::lhs, def.span);
    let rhs_nm = Ident::new(sym::rhs, def.span);
    let tt_spec = Some(NonterminalKind::TT);

    // Parse the macro_rules! invocation
    let (macro_rules, body) = match &def.kind {
        ast::ItemKind::MacroDef(def) => (def.macro_rules, def.body.inner_tokens()),
        _ => unreachable!(),
    };

    // The pattern that macro_rules matches.
    // The grammar for macro_rules! is:
    // $( $lhs:tt => $rhs:tt );+
    // ...quasiquoting this would be nice.
    // These spans won't matter, anyways
    let argument_gram = vec![
        mbe::TokenTree::Sequence(
            DelimSpan::dummy(),
            mbe::SequenceRepetition {
                tts: vec![
                    mbe::TokenTree::MetaVarDecl(def.span, lhs_nm, tt_spec),
                    mbe::TokenTree::token(token::FatArrow, def.span),
                    mbe::TokenTree::MetaVarDecl(def.span, rhs_nm, tt_spec),
                ],
                separator: Some(Token::new(
                    if macro_rules { token::Semi } else { token::Comma },
                    def.span,
                )),
                kleene: mbe::KleeneToken::new(mbe::KleeneOp::OneOrMore, def.span),
                num_captures: 2,
            },
        ),
        // to phase into semicolon-termination instead of semicolon-separation
        mbe::TokenTree::Sequence(
            DelimSpan::dummy(),
            mbe::SequenceRepetition {
                tts: vec![mbe::TokenTree::token(
                    if macro_rules { token::Semi } else { token::Comma },
                    def.span,
                )],
                separator: None,
                kleene: mbe::KleeneToken::new(mbe::KleeneOp::ZeroOrMore, def.span),
                num_captures: 0,
            },
        ),
    ];
    // Convert it into `MatcherLoc` form.
    let argument_gram = mbe::macro_parser::compute_locs(&argument_gram);

    let parser = Parser::new(&sess.parse_sess, body, true, rustc_parse::MACRO_ARGUMENTS);
    let mut tt_parser =
        TtParser::new(Ident::with_dummy_span(if macro_rules { kw::MacroRules } else { kw::Macro }));
    let argument_map = match tt_parser.parse_tt(&mut Cow::Borrowed(&parser), &argument_gram) {
        Success(m) => m,
        Failure(token, msg) => {
            let s = parse_failure_msg(&token);
            let sp = token.span.substitute_dummy(def.span);
            sess.parse_sess.span_diagnostic.struct_span_err(sp, &s).span_label(sp, msg).emit();
            return dummy_syn_ext();
        }
        Error(sp, msg) => {
            sess.parse_sess
                .span_diagnostic
                .struct_span_err(sp.substitute_dummy(def.span), &msg)
                .emit();
            return dummy_syn_ext();
        }
        ErrorReported => {
            return dummy_syn_ext();
        }
    };

    let mut valid = true;

    // Extract the arguments:
    let lhses = match argument_map[&MacroRulesNormalizedIdent::new(lhs_nm)] {
        MatchedSeq(ref s) => s
            .iter()
            .map(|m| {
                if let MatchedTokenTree(ref tt) = *m {
                    let tt = mbe::quoted::parse(
                        tt.clone().into(),
                        true,
                        &sess.parse_sess,
                        def.id,
                        features,
                        edition,
                    )
                    .pop()
                    .unwrap();
                    valid &= check_lhs_nt_follows(&sess.parse_sess, &def, &tt);
                    return tt;
                }
                sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
            })
            .collect::<Vec<mbe::TokenTree>>(),
        _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs"),
    };

    let rhses = match argument_map[&MacroRulesNormalizedIdent::new(rhs_nm)] {
        MatchedSeq(ref s) => s
            .iter()
            .map(|m| {
                if let MatchedTokenTree(ref tt) = *m {
                    return mbe::quoted::parse(
                        tt.clone().into(),
                        false,
                        &sess.parse_sess,
                        def.id,
                        features,
                        edition,
                    )
                    .pop()
                    .unwrap();
                }
                sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
            })
            .collect::<Vec<mbe::TokenTree>>(),
        _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured rhs"),
    };

    for rhs in &rhses {
        valid &= check_rhs(&sess.parse_sess, rhs);
    }

    // don't abort iteration early, so that errors for multiple lhses can be reported
    for lhs in &lhses {
        valid &= check_lhs_no_empty_seq(&sess.parse_sess, slice::from_ref(lhs));
    }

    valid &= macro_check::check_meta_variables(&sess.parse_sess, def.id, def.span, &lhses, &rhses);

    let (transparency, transparency_error) = attr::find_transparency(&def.attrs, macro_rules);
    match transparency_error {
        Some(TransparencyError::UnknownTransparency(value, span)) => {
            diag.span_err(span, &format!("unknown macro transparency: `{}`", value));
        }
        Some(TransparencyError::MultipleTransparencyAttrs(old_span, new_span)) => {
            diag.span_err(vec![old_span, new_span], "multiple macro transparency attributes");
        }
        None => {}
    }

    // Compute the spans of the macro rules for unused rule linting.
    // To avoid warning noise, only consider the rules of this
    // macro for the lint, if all rules are valid.
    // Also, we are only interested in non-foreign macros.
    let rule_spans = if valid && def.id != DUMMY_NODE_ID {
        lhses
            .iter()
            .zip(rhses.iter())
            .enumerate()
            // If the rhs contains an invocation like compile_error!,
            // don't consider the rule for the unused rule lint.
            .filter(|(_idx, (_lhs, rhs))| !has_compile_error_macro(rhs))
            // We only take the span of the lhs here,
            // so that the spans of created warnings are smaller.
            .map(|(idx, (lhs, _rhs))| (idx, lhs.span()))
            .collect::<Vec<_>>()
    } else {
        Vec::new()
    };

    // Convert the lhses into `MatcherLoc` form, which is better for doing the
    // actual matching. Unless the matcher is invalid.
    let lhses = if valid {
        lhses
            .iter()
            .map(|lhs| {
                // Ignore the delimiters around the matcher.
                match lhs {
                    mbe::TokenTree::Delimited(_, delimited) => {
                        mbe::macro_parser::compute_locs(&delimited.tts)
                    }
                    _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "malformed macro lhs"),
                }
            })
            .collect()
    } else {
        vec![]
    };

    let expander = Box::new(MacroRulesMacroExpander {
        name: def.ident,
        span: def.span,
        node_id: def.id,
        transparency,
        lhses,
        rhses,
        valid,
    });
    (mk_syn_ext(expander), rule_spans)
}

fn check_lhs_nt_follows(sess: &ParseSess, def: &ast::Item, lhs: &mbe::TokenTree) -> bool {
    // lhs is going to be like TokenTree::Delimited(...), where the
    // entire lhs is those tts. Or, it can be a "bare sequence", not wrapped in parens.
    if let mbe::TokenTree::Delimited(_, delimited) = lhs {
        check_matcher(sess, def, &delimited.tts)
    } else {
        let msg = "invalid macro matcher; matchers must be contained in balanced delimiters";
        sess.span_diagnostic.span_err(lhs.span(), msg);
        false
    }
    // we don't abort on errors on rejection, the driver will do that for us
    // after parsing/expansion. we can report every error in every macro this way.
}

/// Checks that the lhs contains no repetition which could match an empty token
/// tree, because then the matcher would hang indefinitely.
fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[mbe::TokenTree]) -> bool {
    use mbe::TokenTree;
    for tt in tts {
        match *tt {
            TokenTree::Token(..)
            | TokenTree::MetaVar(..)
            | TokenTree::MetaVarDecl(..)
            | TokenTree::MetaVarExpr(..) => (),
            TokenTree::Delimited(_, ref del) => {
                if !check_lhs_no_empty_seq(sess, &del.tts) {
                    return false;
                }
            }
            TokenTree::Sequence(span, ref seq) => {
                if seq.separator.is_none()
                    && seq.tts.iter().all(|seq_tt| match *seq_tt {
                        TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Vis)) => true,
                        TokenTree::Sequence(_, ref sub_seq) => {
                            sub_seq.kleene.op == mbe::KleeneOp::ZeroOrMore
                                || sub_seq.kleene.op == mbe::KleeneOp::ZeroOrOne
                        }
                        _ => false,
                    })
                {
                    let sp = span.entire();
                    sess.span_diagnostic.span_err(sp, "repetition matches empty token tree");
                    return false;
                }
                if !check_lhs_no_empty_seq(sess, &seq.tts) {
                    return false;
                }
            }
        }
    }

    true
}

fn check_rhs(sess: &ParseSess, rhs: &mbe::TokenTree) -> bool {
    match *rhs {
        mbe::TokenTree::Delimited(..) => return true,
        _ => {
            sess.span_diagnostic.span_err(rhs.span(), "macro rhs must be delimited");
        }
    }
    false
}

fn check_matcher(sess: &ParseSess, def: &ast::Item, matcher: &[mbe::TokenTree]) -> bool {
    let first_sets = FirstSets::new(matcher);
    let empty_suffix = TokenSet::empty();
    let err = sess.span_diagnostic.err_count();
    check_matcher_core(sess, def, &first_sets, matcher, &empty_suffix);
    err == sess.span_diagnostic.err_count()
}

fn has_compile_error_macro(rhs: &mbe::TokenTree) -> bool {
    match rhs {
        mbe::TokenTree::Delimited(_sp, d) => {
            let has_compile_error = d.tts.array_windows::<3>().any(|[ident, bang, args]| {
                if let mbe::TokenTree::Token(ident) = ident &&
                        let TokenKind::Ident(ident, _) = ident.kind &&
                        ident == sym::compile_error &&
                        let mbe::TokenTree::Token(bang) = bang &&
                        let TokenKind::Not = bang.kind &&
                        let mbe::TokenTree::Delimited(_, del) = args &&
                        del.delim != Delimiter::Invisible
                    {
                        true
                    } else {
                        false
                    }
            });
            if has_compile_error { true } else { d.tts.iter().any(has_compile_error_macro) }
        }
        _ => false,
    }
}

// `The FirstSets` for a matcher is a mapping from subsequences in the
// matcher to the FIRST set for that subsequence.
//
// This mapping is partially precomputed via a backwards scan over the
// token trees of the matcher, which provides a mapping from each
// repetition sequence to its *first* set.
//
// (Hypothetically, sequences should be uniquely identifiable via their
// spans, though perhaps that is false, e.g., for macro-generated macros
// that do not try to inject artificial span information. My plan is
// to try to catch such cases ahead of time and not include them in
// the precomputed mapping.)
struct FirstSets<'tt> {
    // this maps each TokenTree::Sequence `$(tt ...) SEP OP` that is uniquely identified by its
    // span in the original matcher to the First set for the inner sequence `tt ...`.
    //
    // If two sequences have the same span in a matcher, then map that
    // span to None (invalidating the mapping here and forcing the code to
    // use a slow path).
    first: FxHashMap<Span, Option<TokenSet<'tt>>>,
}

impl<'tt> FirstSets<'tt> {
    fn new(tts: &'tt [mbe::TokenTree]) -> FirstSets<'tt> {
        use mbe::TokenTree;

        let mut sets = FirstSets { first: FxHashMap::default() };
        build_recur(&mut sets, tts);
        return sets;

        // walks backward over `tts`, returning the FIRST for `tts`
        // and updating `sets` at the same time for all sequence
        // substructure we find within `tts`.
        fn build_recur<'tt>(sets: &mut FirstSets<'tt>, tts: &'tt [TokenTree]) -> TokenSet<'tt> {
            let mut first = TokenSet::empty();
            for tt in tts.iter().rev() {
                match *tt {
                    TokenTree::Token(..)
                    | TokenTree::MetaVar(..)
                    | TokenTree::MetaVarDecl(..)
                    | TokenTree::MetaVarExpr(..) => {
                        first.replace_with(TtHandle::TtRef(tt));
                    }
                    TokenTree::Delimited(span, ref delimited) => {
                        build_recur(sets, &delimited.tts);
                        first.replace_with(TtHandle::from_token_kind(
                            token::OpenDelim(delimited.delim),
                            span.open,
                        ));
                    }
                    TokenTree::Sequence(sp, ref seq_rep) => {
                        let subfirst = build_recur(sets, &seq_rep.tts);

                        match sets.first.entry(sp.entire()) {
                            Entry::Vacant(vac) => {
                                vac.insert(Some(subfirst.clone()));
                            }
                            Entry::Occupied(mut occ) => {
                                // if there is already an entry, then a span must have collided.
                                // This should not happen with typical macro_rules macros,
                                // but syntax extensions need not maintain distinct spans,
                                // so distinct syntax trees can be assigned the same span.
                                // In such a case, the map cannot be trusted; so mark this
                                // entry as unusable.
                                occ.insert(None);
                            }
                        }

                        // If the sequence contents can be empty, then the first
                        // token could be the separator token itself.

                        if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
                            first.add_one_maybe(TtHandle::from_token(sep.clone()));
                        }

                        // Reverse scan: Sequence comes before `first`.
                        if subfirst.maybe_empty
                            || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrMore
                            || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrOne
                        {
                            // If sequence is potentially empty, then
                            // union them (preserving first emptiness).
                            first.add_all(&TokenSet { maybe_empty: true, ..subfirst });
                        } else {
                            // Otherwise, sequence guaranteed
                            // non-empty; replace first.
                            first = subfirst;
                        }
                    }
                }
            }

            first
        }
    }

    // walks forward over `tts` until all potential FIRST tokens are
    // identified.
    fn first(&self, tts: &'tt [mbe::TokenTree]) -> TokenSet<'tt> {
        use mbe::TokenTree;

        let mut first = TokenSet::empty();
        for tt in tts.iter() {
            assert!(first.maybe_empty);
            match *tt {
                TokenTree::Token(..)
                | TokenTree::MetaVar(..)
                | TokenTree::MetaVarDecl(..)
                | TokenTree::MetaVarExpr(..) => {
                    first.add_one(TtHandle::TtRef(tt));
                    return first;
                }
                TokenTree::Delimited(span, ref delimited) => {
                    first.add_one(TtHandle::from_token_kind(
                        token::OpenDelim(delimited.delim),
                        span.open,
                    ));
                    return first;
                }
                TokenTree::Sequence(sp, ref seq_rep) => {
                    let subfirst_owned;
                    let subfirst = match self.first.get(&sp.entire()) {
                        Some(&Some(ref subfirst)) => subfirst,
                        Some(&None) => {
                            subfirst_owned = self.first(&seq_rep.tts);
                            &subfirst_owned
                        }
                        None => {
                            panic!("We missed a sequence during FirstSets construction");
                        }
                    };

                    // If the sequence contents can be empty, then the first
                    // token could be the separator token itself.
                    if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
                        first.add_one_maybe(TtHandle::from_token(sep.clone()));
                    }

                    assert!(first.maybe_empty);
                    first.add_all(subfirst);
                    if subfirst.maybe_empty
                        || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrMore
                        || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrOne
                    {
                        // Continue scanning for more first
                        // tokens, but also make sure we
                        // restore empty-tracking state.
                        first.maybe_empty = true;
                        continue;
                    } else {
                        return first;
                    }
                }
            }
        }

        // we only exit the loop if `tts` was empty or if every
        // element of `tts` matches the empty sequence.
        assert!(first.maybe_empty);
        first
    }
}

// Most `mbe::TokenTree`s are pre-existing in the matcher, but some are defined
// implicitly, such as opening/closing delimiters and sequence repetition ops.
// This type encapsulates both kinds. It implements `Clone` while avoiding the
// need for `mbe::TokenTree` to implement `Clone`.
#[derive(Debug)]
enum TtHandle<'tt> {
    /// This is used in most cases.
    TtRef(&'tt mbe::TokenTree),

    /// This is only used for implicit token trees. The `mbe::TokenTree` *must*
    /// be `mbe::TokenTree::Token`. No other variants are allowed. We store an
    /// `mbe::TokenTree` rather than a `Token` so that `get()` can return a
    /// `&mbe::TokenTree`.
    Token(mbe::TokenTree),
}

impl<'tt> TtHandle<'tt> {
    fn from_token(tok: Token) -> Self {
        TtHandle::Token(mbe::TokenTree::Token(tok))
    }

    fn from_token_kind(kind: TokenKind, span: Span) -> Self {
        TtHandle::from_token(Token::new(kind, span))
    }

    // Get a reference to a token tree.
    fn get(&'tt self) -> &'tt mbe::TokenTree {
        match self {
            TtHandle::TtRef(tt) => tt,
            TtHandle::Token(token_tt) => &token_tt,
        }
    }
}

impl<'tt> PartialEq for TtHandle<'tt> {
    fn eq(&self, other: &TtHandle<'tt>) -> bool {
        self.get() == other.get()
    }
}

impl<'tt> Clone for TtHandle<'tt> {
    fn clone(&self) -> Self {
        match self {
            TtHandle::TtRef(tt) => TtHandle::TtRef(tt),

            // This variant *must* contain a `mbe::TokenTree::Token`, and not
            // any other variant of `mbe::TokenTree`.
            TtHandle::Token(mbe::TokenTree::Token(tok)) => {
                TtHandle::Token(mbe::TokenTree::Token(tok.clone()))
            }

            _ => unreachable!(),
        }
    }
}

// A set of `mbe::TokenTree`s, which may include `TokenTree::Match`s
// (for macro-by-example syntactic variables). It also carries the
// `maybe_empty` flag; that is true if and only if the matcher can
// match an empty token sequence.
//
// The First set is computed on submatchers like `$($a:expr b),* $(c)* d`,
// which has corresponding FIRST = {$a:expr, c, d}.
// Likewise, `$($a:expr b),* $(c)+ d` has FIRST = {$a:expr, c}.
//
// (Notably, we must allow for *-op to occur zero times.)
#[derive(Clone, Debug)]
struct TokenSet<'tt> {
    tokens: Vec<TtHandle<'tt>>,
    maybe_empty: bool,
}

impl<'tt> TokenSet<'tt> {
    // Returns a set for the empty sequence.
    fn empty() -> Self {
        TokenSet { tokens: Vec::new(), maybe_empty: true }
    }

    // Returns the set `{ tok }` for the single-token (and thus
    // non-empty) sequence [tok].
    fn singleton(tt: TtHandle<'tt>) -> Self {
        TokenSet { tokens: vec![tt], maybe_empty: false }
    }

    // Changes self to be the set `{ tok }`.
    // Since `tok` is always present, marks self as non-empty.
    fn replace_with(&mut self, tt: TtHandle<'tt>) {
        self.tokens.clear();
        self.tokens.push(tt);
        self.maybe_empty = false;
    }

    // Changes self to be the empty set `{}`; meant for use when
    // the particular token does not matter, but we want to
    // record that it occurs.
    fn replace_with_irrelevant(&mut self) {
        self.tokens.clear();
        self.maybe_empty = false;
    }

    // Adds `tok` to the set for `self`, marking sequence as non-empy.
    fn add_one(&mut self, tt: TtHandle<'tt>) {
        if !self.tokens.contains(&tt) {
            self.tokens.push(tt);
        }
        self.maybe_empty = false;
    }

    // Adds `tok` to the set for `self`. (Leaves `maybe_empty` flag alone.)
    fn add_one_maybe(&mut self, tt: TtHandle<'tt>) {
        if !self.tokens.contains(&tt) {
            self.tokens.push(tt);
        }
    }

    // Adds all elements of `other` to this.
    //
    // (Since this is a set, we filter out duplicates.)
    //
    // If `other` is potentially empty, then preserves the previous
    // setting of the empty flag of `self`. If `other` is guaranteed
    // non-empty, then `self` is marked non-empty.
    fn add_all(&mut self, other: &Self) {
        for tt in &other.tokens {
            if !self.tokens.contains(tt) {
                self.tokens.push(tt.clone());
            }
        }
        if !other.maybe_empty {
            self.maybe_empty = false;
        }
    }
}

// Checks that `matcher` is internally consistent and that it
// can legally be followed by a token `N`, for all `N` in `follow`.
// (If `follow` is empty, then it imposes no constraint on
// the `matcher`.)
//
// Returns the set of NT tokens that could possibly come last in
// `matcher`. (If `matcher` matches the empty sequence, then
// `maybe_empty` will be set to true.)
//
// Requires that `first_sets` is pre-computed for `matcher`;
// see `FirstSets::new`.
fn check_matcher_core<'tt>(
    sess: &ParseSess,
    def: &ast::Item,
    first_sets: &FirstSets<'tt>,
    matcher: &'tt [mbe::TokenTree],
    follow: &TokenSet<'tt>,
) -> TokenSet<'tt> {
    use mbe::TokenTree;

    let mut last = TokenSet::empty();

    // 2. For each token and suffix  [T, SUFFIX] in M:
    // ensure that T can be followed by SUFFIX, and if SUFFIX may be empty,
    // then ensure T can also be followed by any element of FOLLOW.
    'each_token: for i in 0..matcher.len() {
        let token = &matcher[i];
        let suffix = &matcher[i + 1..];

        let build_suffix_first = || {
            let mut s = first_sets.first(suffix);
            if s.maybe_empty {
                s.add_all(follow);
            }
            s
        };

        // (we build `suffix_first` on demand below; you can tell
        // which cases are supposed to fall through by looking for the
        // initialization of this variable.)
        let suffix_first;

        // First, update `last` so that it corresponds to the set
        // of NT tokens that might end the sequence `... token`.
        match *token {
            TokenTree::Token(..)
            | TokenTree::MetaVar(..)
            | TokenTree::MetaVarDecl(..)
            | TokenTree::MetaVarExpr(..) => {
                if token_can_be_followed_by_any(token) {
                    // don't need to track tokens that work with any,
                    last.replace_with_irrelevant();
                    // ... and don't need to check tokens that can be
                    // followed by anything against SUFFIX.
                    continue 'each_token;
                } else {
                    last.replace_with(TtHandle::TtRef(token));
                    suffix_first = build_suffix_first();
                }
            }
            TokenTree::Delimited(span, ref d) => {
                let my_suffix = TokenSet::singleton(TtHandle::from_token_kind(
                    token::CloseDelim(d.delim),
                    span.close,
                ));
                check_matcher_core(sess, def, first_sets, &d.tts, &my_suffix);
                // don't track non NT tokens
                last.replace_with_irrelevant();

                // also, we don't need to check delimited sequences
                // against SUFFIX
                continue 'each_token;
            }
            TokenTree::Sequence(_, ref seq_rep) => {
                suffix_first = build_suffix_first();
                // The trick here: when we check the interior, we want
                // to include the separator (if any) as a potential
                // (but not guaranteed) element of FOLLOW. So in that
                // case, we make a temp copy of suffix and stuff
                // delimiter in there.
                //
                // FIXME: Should I first scan suffix_first to see if
                // delimiter is already in it before I go through the
                // work of cloning it? But then again, this way I may
                // get a "tighter" span?
                let mut new;
                let my_suffix = if let Some(sep) = &seq_rep.separator {
                    new = suffix_first.clone();
                    new.add_one_maybe(TtHandle::from_token(sep.clone()));
                    &new
                } else {
                    &suffix_first
                };

                // At this point, `suffix_first` is built, and
                // `my_suffix` is some TokenSet that we can use
                // for checking the interior of `seq_rep`.
                let next = check_matcher_core(sess, def, first_sets, &seq_rep.tts, my_suffix);
                if next.maybe_empty {
                    last.add_all(&next);
                } else {
                    last = next;
                }

                // the recursive call to check_matcher_core already ran the 'each_last
                // check below, so we can just keep going forward here.
                continue 'each_token;
            }
        }

        // (`suffix_first` guaranteed initialized once reaching here.)

        // Now `last` holds the complete set of NT tokens that could
        // end the sequence before SUFFIX. Check that every one works with `suffix`.
        for tt in &last.tokens {
            if let &TokenTree::MetaVarDecl(span, name, Some(kind)) = tt.get() {
                for next_token in &suffix_first.tokens {
                    let next_token = next_token.get();

                    // Check if the old pat is used and the next token is `|`
                    // to warn about incompatibility with Rust 2021.
                    // We only emit this lint if we're parsing the original
                    // definition of this macro_rules, not while (re)parsing
                    // the macro when compiling another crate that is using the
                    // macro. (See #86567.)
                    // Macros defined in the current crate have a real node id,
                    // whereas macros from an external crate have a dummy id.
                    if def.id != DUMMY_NODE_ID
                        && matches!(kind, NonterminalKind::PatParam { inferred: true })
                        && matches!(next_token, TokenTree::Token(token) if token.kind == BinOp(token::BinOpToken::Or))
                    {
                        // It is suggestion to use pat_param, for example: $x:pat -> $x:pat_param.
                        let suggestion = quoted_tt_to_string(&TokenTree::MetaVarDecl(
                            span,
                            name,
                            Some(NonterminalKind::PatParam { inferred: false }),
                        ));
                        sess.buffer_lint_with_diagnostic(
                            &RUST_2021_INCOMPATIBLE_OR_PATTERNS,
                            span,
                            ast::CRATE_NODE_ID,
                            "the meaning of the `pat` fragment specifier is changing in Rust 2021, which may affect this macro",
                            BuiltinLintDiagnostics::OrPatternsBackCompat(span, suggestion),
                        );
                    }
                    match is_in_follow(next_token, kind) {
                        IsInFollow::Yes => {}
                        IsInFollow::No(possible) => {
                            let may_be = if last.tokens.len() == 1 && suffix_first.tokens.len() == 1
                            {
                                "is"
                            } else {
                                "may be"
                            };

                            let sp = next_token.span();
                            let mut err = sess.span_diagnostic.struct_span_err(
                                sp,
                                &format!(
                                    "`${name}:{frag}` {may_be} followed by `{next}`, which \
                                     is not allowed for `{frag}` fragments",
                                    name = name,
                                    frag = kind,
                                    next = quoted_tt_to_string(next_token),
                                    may_be = may_be
                                ),
                            );
                            err.span_label(sp, format!("not allowed after `{}` fragments", kind));

                            if kind == NonterminalKind::PatWithOr
                                && sess.edition.rust_2021()
                                && next_token.is_token(&BinOp(token::BinOpToken::Or))
                            {
                                let suggestion = quoted_tt_to_string(&TokenTree::MetaVarDecl(
                                    span,
                                    name,
                                    Some(NonterminalKind::PatParam { inferred: false }),
                                ));
                                err.span_suggestion(
                                    span,
                                    "try a `pat_param` fragment specifier instead",
                                    suggestion,
                                    Applicability::MaybeIncorrect,
                                );
                            }

                            let msg = "allowed there are: ";
                            match possible {
                                &[] => {}
                                &[t] => {
                                    err.note(&format!(
                                        "only {} is allowed after `{}` fragments",
                                        t, kind,
                                    ));
                                }
                                ts => {
                                    err.note(&format!(
                                        "{}{} or {}",
                                        msg,
                                        ts[..ts.len() - 1]
                                            .iter()
                                            .copied()
                                            .collect::<Vec<_>>()
                                            .join(", "),
                                        ts[ts.len() - 1],
                                    ));
                                }
                            }
                            err.emit();
                        }
                    }
                }
            }
        }
    }
    last
}

fn token_can_be_followed_by_any(tok: &mbe::TokenTree) -> bool {
    if let mbe::TokenTree::MetaVarDecl(_, _, Some(kind)) = *tok {
        frag_can_be_followed_by_any(kind)
    } else {
        // (Non NT's can always be followed by anything in matchers.)
        true
    }
}

/// Returns `true` if a fragment of type `frag` can be followed by any sort of
/// token. We use this (among other things) as a useful approximation
/// for when `frag` can be followed by a repetition like `$(...)*` or
/// `$(...)+`. In general, these can be a bit tricky to reason about,
/// so we adopt a conservative position that says that any fragment
/// specifier which consumes at most one token tree can be followed by
/// a fragment specifier (indeed, these fragments can be followed by
/// ANYTHING without fear of future compatibility hazards).
fn frag_can_be_followed_by_any(kind: NonterminalKind) -> bool {
    matches!(
        kind,
        NonterminalKind::Item           // always terminated by `}` or `;`
        | NonterminalKind::Block        // exactly one token tree
        | NonterminalKind::Ident        // exactly one token tree
        | NonterminalKind::Literal      // exactly one token tree
        | NonterminalKind::Meta         // exactly one token tree
        | NonterminalKind::Lifetime     // exactly one token tree
        | NonterminalKind::TT // exactly one token tree
    )
}

enum IsInFollow {
    Yes,
    No(&'static [&'static str]),
}

/// Returns `true` if `frag` can legally be followed by the token `tok`. For
/// fragments that can consume an unbounded number of tokens, `tok`
/// must be within a well-defined follow set. This is intended to
/// guarantee future compatibility: for example, without this rule, if
/// we expanded `expr` to include a new binary operator, we might
/// break macros that were relying on that binary operator as a
/// separator.
// when changing this do not forget to update doc/book/macros.md!
fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
    use mbe::TokenTree;

    if let TokenTree::Token(Token { kind: token::CloseDelim(_), .. }) = *tok {
        // closing a token tree can never be matched by any fragment;
        // iow, we always require that `(` and `)` match, etc.
        IsInFollow::Yes
    } else {
        match kind {
            NonterminalKind::Item => {
                // since items *must* be followed by either a `;` or a `}`, we can
                // accept anything after them
                IsInFollow::Yes
            }
            NonterminalKind::Block => {
                // anything can follow block, the braces provide an easy boundary to
                // maintain
                IsInFollow::Yes
            }
            NonterminalKind::Stmt | NonterminalKind::Expr => {
                const TOKENS: &[&str] = &["`=>`", "`,`", "`;`"];
                match tok {
                    TokenTree::Token(token) => match token.kind {
                        FatArrow | Comma | Semi => IsInFollow::Yes,
                        _ => IsInFollow::No(TOKENS),
                    },
                    _ => IsInFollow::No(TOKENS),
                }
            }
            NonterminalKind::PatParam { .. } => {
                const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`|`", "`if`", "`in`"];
                match tok {
                    TokenTree::Token(token) => match token.kind {
                        FatArrow | Comma | Eq | BinOp(token::Or) => IsInFollow::Yes,
                        Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
                        _ => IsInFollow::No(TOKENS),
                    },
                    _ => IsInFollow::No(TOKENS),
                }
            }
            NonterminalKind::PatWithOr { .. } => {
                const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`if`", "`in`"];
                match tok {
                    TokenTree::Token(token) => match token.kind {
                        FatArrow | Comma | Eq => IsInFollow::Yes,
                        Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
                        _ => IsInFollow::No(TOKENS),
                    },
                    _ => IsInFollow::No(TOKENS),
                }
            }
            NonterminalKind::Path | NonterminalKind::Ty => {
                const TOKENS: &[&str] = &[
                    "`{`", "`[`", "`=>`", "`,`", "`>`", "`=`", "`:`", "`;`", "`|`", "`as`",
                    "`where`",
                ];
                match tok {
                    TokenTree::Token(token) => match token.kind {
                        OpenDelim(Delimiter::Brace)
                        | OpenDelim(Delimiter::Bracket)
                        | Comma
                        | FatArrow
                        | Colon
                        | Eq
                        | Gt
                        | BinOp(token::Shr)
                        | Semi
                        | BinOp(token::Or) => IsInFollow::Yes,
                        Ident(name, false) if name == kw::As || name == kw::Where => {
                            IsInFollow::Yes
                        }
                        _ => IsInFollow::No(TOKENS),
                    },
                    TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Block)) => IsInFollow::Yes,
                    _ => IsInFollow::No(TOKENS),
                }
            }
            NonterminalKind::Ident | NonterminalKind::Lifetime => {
                // being a single token, idents and lifetimes are harmless
                IsInFollow::Yes
            }
            NonterminalKind::Literal => {
                // literals may be of a single token, or two tokens (negative numbers)
                IsInFollow::Yes
            }
            NonterminalKind::Meta | NonterminalKind::TT => {
                // being either a single token or a delimited sequence, tt is
                // harmless
                IsInFollow::Yes
            }
            NonterminalKind::Vis => {
                // Explicitly disallow `priv`, on the off chance it comes back.
                const TOKENS: &[&str] = &["`,`", "an ident", "a type"];
                match tok {
                    TokenTree::Token(token) => match token.kind {
                        Comma => IsInFollow::Yes,
                        Ident(name, is_raw) if is_raw || name != kw::Priv => IsInFollow::Yes,
                        _ => {
                            if token.can_begin_type() {
                                IsInFollow::Yes
                            } else {
                                IsInFollow::No(TOKENS)
                            }
                        }
                    },
                    TokenTree::MetaVarDecl(
                        _,
                        _,
                        Some(NonterminalKind::Ident | NonterminalKind::Ty | NonterminalKind::Path),
                    ) => IsInFollow::Yes,
                    _ => IsInFollow::No(TOKENS),
                }
            }
        }
    }
}

fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
    match *tt {
        mbe::TokenTree::Token(ref token) => pprust::token_to_string(&token).into(),
        mbe::TokenTree::MetaVar(_, name) => format!("${}", name),
        mbe::TokenTree::MetaVarDecl(_, name, Some(kind)) => format!("${}:{}", name, kind),
        mbe::TokenTree::MetaVarDecl(_, name, None) => format!("${}:", name),
        _ => panic!(
            "{}",
            "unexpected mbe::TokenTree::{Sequence or Delimited} \
             in follow set checker"
        ),
    }
}

fn parser_from_cx(sess: &ParseSess, tts: TokenStream) -> Parser<'_> {
    Parser::new(sess, tts, true, rustc_parse::MACRO_ARGUMENTS)
}

/// Generates an appropriate parsing failure message. For EOF, this is "unexpected end...". For
/// other tokens, this is "unexpected token...".
fn parse_failure_msg(tok: &Token) -> String {
    match tok.kind {
        token::Eof => "unexpected end of macro invocation".to_string(),
        _ => format!("no rules expected the token `{}`", pprust::token_to_string(tok),),
    }
}