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
//! This is an NFA-based parser, which calls out to the main Rust parser for named non-terminals
//! (which it commits to fully when it hits one in a grammar). There's a set of current NFA threads
//! and a set of next ones. Instead of NTs, we have a special case for Kleene star. The big-O, in
//! pathological cases, is worse than traditional use of NFA or Earley parsing, but it's an easier
//! fit for Macro-by-Example-style rules.
//!
//! (In order to prevent the pathological case, we'd need to lazily construct the resulting
//! `NamedMatch`es at the very end. It'd be a pain, and require more memory to keep around old
//! matcher positions, but it would also save overhead)
//!
//! We don't say this parser uses the Earley algorithm, because it's unnecessarily inaccurate.
//! The macro parser restricts itself to the features of finite state automata. Earley parsers
//! can be described as an extension of NFAs with completion rules, prediction rules, and recursion.
//!
//! Quick intro to how the parser works:
//!
//! A "matcher position" (a.k.a. "position" or "mp") is a dot in the middle of a matcher, usually
//! written as a `·`. For example `· a $( a )* a b` is one, as is `a $( · a )* a b`.
//!
//! The parser walks through the input a token at a time, maintaining a list
//! of threads consistent with the current position in the input string: `cur_mps`.
//!
//! As it processes them, it fills up `eof_mps` with threads that would be valid if
//! the macro invocation is now over, `bb_mps` with threads that are waiting on
//! a Rust non-terminal like `$e:expr`, and `next_mps` with threads that are waiting
//! on a particular token. Most of the logic concerns moving the · through the
//! repetitions indicated by Kleene stars. The rules for moving the · without
//! consuming any input are called epsilon transitions. It only advances or calls
//! out to the real Rust parser when no `cur_mps` threads remain.
//!
//! Example:
//!
//! ```text, ignore
//! Start parsing a a a a b against [· a $( a )* a b].
//!
//! Remaining input: a a a a b
//! next: [· a $( a )* a b]
//!
//! - - - Advance over an a. - - -
//!
//! Remaining input: a a a b
//! cur: [a · $( a )* a b]
//! Descend/Skip (first position).
//! next: [a $( · a )* a b]  [a $( a )* · a b].
//!
//! - - - Advance over an a. - - -
//!
//! Remaining input: a a b
//! cur: [a $( a · )* a b]  [a $( a )* a · b]
//! Follow epsilon transition: Finish/Repeat (first position)
//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
//!
//! - - - Advance over an a. - - - (this looks exactly like the last step)
//!
//! Remaining input: a b
//! cur: [a $( a · )* a b]  [a $( a )* a · b]
//! Follow epsilon transition: Finish/Repeat (first position)
//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
//!
//! - - - Advance over an a. - - - (this looks exactly like the last step)
//!
//! Remaining input: b
//! cur: [a $( a · )* a b]  [a $( a )* a · b]
//! Follow epsilon transition: Finish/Repeat (first position)
//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
//!
//! - - - Advance over a b. - - -
//!
//! Remaining input: ''
//! eof: [a $( a )* a b ·]
//! ```

pub(crate) use NamedMatch::*;
pub(crate) use ParseResult::*;

use crate::mbe::{KleeneOp, TokenTree};

use rustc_ast::token::{self, DocComment, Nonterminal, NonterminalKind, Token};
use rustc_lint_defs::pluralize;
use rustc_parse::parser::{NtOrTt, Parser};
use rustc_span::symbol::MacroRulesNormalizedIdent;
use rustc_span::Span;

use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::Lrc;
use rustc_span::symbol::Ident;
use std::borrow::Cow;
use std::collections::hash_map::Entry::{Occupied, Vacant};

/// A unit within a matcher that a `MatcherPos` can refer to. Similar to (and derived from)
/// `mbe::TokenTree`, but designed specifically for fast and easy traversal during matching.
/// Notable differences to `mbe::TokenTree`:
/// - It is non-recursive, i.e. there is no nesting.
/// - The end pieces of each sequence (the separator, if present, and the Kleene op) are
///   represented explicitly, as is the very end of the matcher.
///
/// This means a matcher can be represented by `&[MatcherLoc]`, and traversal mostly involves
/// simply incrementing the current matcher position index by one.
pub(super) enum MatcherLoc {
    Token {
        token: Token,
    },
    Delimited,
    Sequence {
        op: KleeneOp,
        num_metavar_decls: usize,
        idx_first_after: usize,
        next_metavar: usize,
        seq_depth: usize,
    },
    SequenceKleeneOpNoSep {
        op: KleeneOp,
        idx_first: usize,
    },
    SequenceSep {
        separator: Token,
    },
    SequenceKleeneOpAfterSep {
        idx_first: usize,
    },
    MetaVarDecl {
        span: Span,
        bind: Ident,
        kind: Option<NonterminalKind>,
        next_metavar: usize,
        seq_depth: usize,
    },
    Eof,
}

pub(super) fn compute_locs(matcher: &[TokenTree]) -> Vec<MatcherLoc> {
    fn inner(
        tts: &[TokenTree],
        locs: &mut Vec<MatcherLoc>,
        next_metavar: &mut usize,
        seq_depth: usize,
    ) {
        for tt in tts {
            match tt {
                TokenTree::Token(token) => {
                    locs.push(MatcherLoc::Token { token: token.clone() });
                }
                TokenTree::Delimited(span, delimited) => {
                    let open_token = Token::new(token::OpenDelim(delimited.delim), span.open);
                    let close_token = Token::new(token::CloseDelim(delimited.delim), span.close);

                    locs.push(MatcherLoc::Delimited);
                    locs.push(MatcherLoc::Token { token: open_token });
                    inner(&delimited.tts, locs, next_metavar, seq_depth);
                    locs.push(MatcherLoc::Token { token: close_token });
                }
                TokenTree::Sequence(_, seq) => {
                    // We can't determine `idx_first_after` and construct the final
                    // `MatcherLoc::Sequence` until after `inner()` is called and the sequence end
                    // pieces are processed. So we push a dummy value (`Eof` is cheapest to
                    // construct) now, and overwrite it with the proper value below.
                    let dummy = MatcherLoc::Eof;
                    locs.push(dummy);

                    let next_metavar_orig = *next_metavar;
                    let op = seq.kleene.op;
                    let idx_first = locs.len();
                    let idx_seq = idx_first - 1;
                    inner(&seq.tts, locs, next_metavar, seq_depth + 1);

                    if let Some(separator) = &seq.separator {
                        locs.push(MatcherLoc::SequenceSep { separator: separator.clone() });
                        locs.push(MatcherLoc::SequenceKleeneOpAfterSep { idx_first });
                    } else {
                        locs.push(MatcherLoc::SequenceKleeneOpNoSep { op, idx_first });
                    }

                    // Overwrite the dummy value pushed above with the proper value.
                    locs[idx_seq] = MatcherLoc::Sequence {
                        op,
                        num_metavar_decls: seq.num_captures,
                        idx_first_after: locs.len(),
                        next_metavar: next_metavar_orig,
                        seq_depth,
                    };
                }
                &TokenTree::MetaVarDecl(span, bind, kind) => {
                    locs.push(MatcherLoc::MetaVarDecl {
                        span,
                        bind,
                        kind,
                        next_metavar: *next_metavar,
                        seq_depth,
                    });
                    *next_metavar += 1;
                }
                TokenTree::MetaVar(..) | TokenTree::MetaVarExpr(..) => unreachable!(),
            }
        }
    }

    let mut locs = vec![];
    let mut next_metavar = 0;
    inner(matcher, &mut locs, &mut next_metavar, /* seq_depth */ 0);

    // A final entry is needed for eof.
    locs.push(MatcherLoc::Eof);

    locs
}

/// A single matcher position, representing the state of matching.
struct MatcherPos {
    /// The index into `TtParser::locs`, which represents the "dot".
    idx: usize,

    /// The matches made against metavar decls so far. On a successful match, this vector ends up
    /// with one element per metavar decl in the matcher. Each element records token trees matched
    /// against the relevant metavar by the black box parser. An element will be a `MatchedSeq` if
    /// the corresponding metavar decl is within a sequence.
    ///
    /// It is critical to performance that this is an `Lrc`, because it gets cloned frequently when
    /// processing sequences. Mostly for sequence-ending possibilities that must be tried but end
    /// up failing.
    matches: Lrc<Vec<NamedMatch>>,
}

// This type is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
rustc_data_structures::static_assert_size!(MatcherPos, 16);

impl MatcherPos {
    /// Adds `m` as a named match for the `metavar_idx`-th metavar. There are only two call sites,
    /// and both are hot enough to be always worth inlining.
    #[inline(always)]
    fn push_match(&mut self, metavar_idx: usize, seq_depth: usize, m: NamedMatch) {
        let matches = Lrc::make_mut(&mut self.matches);
        match seq_depth {
            0 => {
                // We are not within a sequence. Just append `m`.
                assert_eq!(metavar_idx, matches.len());
                matches.push(m);
            }
            _ => {
                // We are within a sequence. Find the final `MatchedSeq` at the appropriate depth
                // and append `m` to its vector.
                let mut curr = &mut matches[metavar_idx];
                for _ in 0..seq_depth - 1 {
                    match curr {
                        MatchedSeq(seq) => curr = seq.last_mut().unwrap(),
                        _ => unreachable!(),
                    }
                }
                match curr {
                    MatchedSeq(seq) => seq.push(m),
                    _ => unreachable!(),
                }
            }
        }
    }
}

enum EofMatcherPositions {
    None,
    One(MatcherPos),
    Multiple,
}

/// Represents the possible results of an attempted parse.
pub(crate) enum ParseResult<T> {
    /// Parsed successfully.
    Success(T),
    /// Arm failed to match. If the second parameter is `token::Eof`, it indicates an unexpected
    /// end of macro invocation. Otherwise, it indicates that no rules expected the given token.
    Failure(Token, &'static str),
    /// Fatal error (malformed macro?). Abort compilation.
    Error(rustc_span::Span, String),
    ErrorReported,
}

/// A `ParseResult` where the `Success` variant contains a mapping of
/// `MacroRulesNormalizedIdent`s to `NamedMatch`es. This represents the mapping
/// of metavars to the token trees they bind to.
pub(crate) type NamedParseResult = ParseResult<FxHashMap<MacroRulesNormalizedIdent, NamedMatch>>;

/// Count how many metavars declarations are in `matcher`.
pub(super) fn count_metavar_decls(matcher: &[TokenTree]) -> usize {
    matcher
        .iter()
        .map(|tt| match tt {
            TokenTree::MetaVarDecl(..) => 1,
            TokenTree::Sequence(_, seq) => seq.num_captures,
            TokenTree::Delimited(_, delim) => count_metavar_decls(&delim.tts),
            TokenTree::Token(..) => 0,
            TokenTree::MetaVar(..) | TokenTree::MetaVarExpr(..) => unreachable!(),
        })
        .sum()
}

/// `NamedMatch` is a pattern-match result for a single metavar. All
/// `MatchedNonterminal`s in the `NamedMatch` have the same non-terminal type
/// (expr, item, etc).
///
/// The in-memory structure of a particular `NamedMatch` represents the match
/// that occurred when a particular subset of a matcher was applied to a
/// particular token tree.
///
/// The width of each `MatchedSeq` in the `NamedMatch`, and the identity of
/// the `MatchedNtNonTts`s, will depend on the token tree it was applied
/// to: each `MatchedSeq` corresponds to a single repetition in the originating
/// token tree. The depth of the `NamedMatch` structure will therefore depend
/// only on the nesting depth of repetitions in the originating token tree it
/// was derived from.
///
/// In layperson's terms: `NamedMatch` will form a tree representing nested matches of a particular
/// meta variable. For example, if we are matching the following macro against the following
/// invocation...
///
/// ```rust
/// macro_rules! foo {
///   ($($($x:ident),+);+) => {}
/// }
///
/// foo!(a, b, c, d; a, b, c, d, e);
/// ```
///
/// Then, the tree will have the following shape:
///
/// ```ignore (private-internal)
/// # use NamedMatch::*;
/// MatchedSeq([
///   MatchedSeq([
///     MatchedNonterminal(a),
///     MatchedNonterminal(b),
///     MatchedNonterminal(c),
///     MatchedNonterminal(d),
///   ]),
///   MatchedSeq([
///     MatchedNonterminal(a),
///     MatchedNonterminal(b),
///     MatchedNonterminal(c),
///     MatchedNonterminal(d),
///     MatchedNonterminal(e),
///   ])
/// ])
/// ```
#[derive(Debug, Clone)]
pub(crate) enum NamedMatch {
    MatchedSeq(Vec<NamedMatch>),

    // A metavar match of type `tt`.
    MatchedTokenTree(rustc_ast::tokenstream::TokenTree),

    // A metavar match of any type other than `tt`.
    MatchedNonterminal(Lrc<Nonterminal>),
}

/// Performs a token equality check, ignoring syntax context (that is, an unhygienic comparison)
fn token_name_eq(t1: &Token, t2: &Token) -> bool {
    if let (Some((ident1, is_raw1)), Some((ident2, is_raw2))) = (t1.ident(), t2.ident()) {
        ident1.name == ident2.name && is_raw1 == is_raw2
    } else if let (Some(ident1), Some(ident2)) = (t1.lifetime(), t2.lifetime()) {
        ident1.name == ident2.name
    } else {
        t1.kind == t2.kind
    }
}

// Note: the vectors could be created and dropped within `parse_tt`, but to avoid excess
// allocations we have a single vector for each kind that is cleared and reused repeatedly.
pub struct TtParser {
    macro_name: Ident,

    /// The set of current mps to be processed. This should be empty by the end of a successful
    /// execution of `parse_tt_inner`.
    cur_mps: Vec<MatcherPos>,

    /// The set of newly generated mps. These are used to replenish `cur_mps` in the function
    /// `parse_tt`.
    next_mps: Vec<MatcherPos>,

    /// The set of mps that are waiting for the black-box parser.
    bb_mps: Vec<MatcherPos>,

    /// Pre-allocate an empty match array, so it can be cloned cheaply for macros with many rules
    /// that have no metavars.
    empty_matches: Lrc<Vec<NamedMatch>>,
}

impl TtParser {
    pub(super) fn new(macro_name: Ident) -> TtParser {
        TtParser {
            macro_name,
            cur_mps: vec![],
            next_mps: vec![],
            bb_mps: vec![],
            empty_matches: Lrc::new(vec![]),
        }
    }

    /// Process the matcher positions of `cur_mps` until it is empty. In the process, this will
    /// produce more mps in `next_mps` and `bb_mps`.
    ///
    /// # Returns
    ///
    /// `Some(result)` if everything is finished, `None` otherwise. Note that matches are kept
    /// track of through the mps generated.
    fn parse_tt_inner(
        &mut self,
        matcher: &[MatcherLoc],
        token: &Token,
    ) -> Option<NamedParseResult> {
        // Matcher positions that would be valid if the macro invocation was over now. Only
        // modified if `token == Eof`.
        let mut eof_mps = EofMatcherPositions::None;

        while let Some(mut mp) = self.cur_mps.pop() {
            match &matcher[mp.idx] {
                MatcherLoc::Token { token: t } => {
                    // If it's a doc comment, we just ignore it and move on to the next tt in the
                    // matcher. This is a bug, but #95267 showed that existing programs rely on
                    // this behaviour, and changing it would require some care and a transition
                    // period.
                    //
                    // If the token matches, we can just advance the parser.
                    //
                    // Otherwise, this match has failed, there is nothing to do, and hopefully
                    // another mp in `cur_mps` will match.
                    if matches!(t, Token { kind: DocComment(..), .. }) {
                        mp.idx += 1;
                        self.cur_mps.push(mp);
                    } else if token_name_eq(&t, token) {
                        mp.idx += 1;
                        self.next_mps.push(mp);
                    }
                }
                MatcherLoc::Delimited => {
                    // Entering the delimiter is trivial.
                    mp.idx += 1;
                    self.cur_mps.push(mp);
                }
                &MatcherLoc::Sequence {
                    op,
                    num_metavar_decls,
                    idx_first_after,
                    next_metavar,
                    seq_depth,
                } => {
                    // Install an empty vec for each metavar within the sequence.
                    for metavar_idx in next_metavar..next_metavar + num_metavar_decls {
                        mp.push_match(metavar_idx, seq_depth, MatchedSeq(vec![]));
                    }

                    if op == KleeneOp::ZeroOrMore || op == KleeneOp::ZeroOrOne {
                        // Try zero matches of this sequence, by skipping over it.
                        self.cur_mps.push(MatcherPos {
                            idx: idx_first_after,
                            matches: mp.matches.clone(), // a cheap clone
                        });
                    }

                    // Try one or more matches of this sequence, by entering it.
                    mp.idx += 1;
                    self.cur_mps.push(mp);
                }
                &MatcherLoc::SequenceKleeneOpNoSep { op, idx_first } => {
                    // We are past the end of a sequence with no separator. Try ending the
                    // sequence. If that's not possible, `ending_mp` will fail quietly when it is
                    // processed next time around the loop.
                    let ending_mp = MatcherPos {
                        idx: mp.idx + 1,             // +1 skips the Kleene op
                        matches: mp.matches.clone(), // a cheap clone
                    };
                    self.cur_mps.push(ending_mp);

                    if op != KleeneOp::ZeroOrOne {
                        // Try another repetition.
                        mp.idx = idx_first;
                        self.cur_mps.push(mp);
                    }
                }
                MatcherLoc::SequenceSep { separator } => {
                    // We are past the end of a sequence with a separator but we haven't seen the
                    // separator yet. Try ending the sequence. If that's not possible, `ending_mp`
                    // will fail quietly when it is processed next time around the loop.
                    let ending_mp = MatcherPos {
                        idx: mp.idx + 2,             // +2 skips the separator and the Kleene op
                        matches: mp.matches.clone(), // a cheap clone
                    };
                    self.cur_mps.push(ending_mp);

                    if token_name_eq(token, separator) {
                        // The separator matches the current token. Advance past it.
                        mp.idx += 1;
                        self.next_mps.push(mp);
                    }
                }
                &MatcherLoc::SequenceKleeneOpAfterSep { idx_first } => {
                    // We are past the sequence separator. This can't be a `?` Kleene op, because
                    // they don't permit separators. Try another repetition.
                    mp.idx = idx_first;
                    self.cur_mps.push(mp);
                }
                &MatcherLoc::MetaVarDecl { span, kind, .. } => {
                    // Built-in nonterminals never start with these tokens, so we can eliminate
                    // them from consideration. We use the span of the metavariable declaration
                    // to determine any edition-specific matching behavior for non-terminals.
                    if let Some(kind) = kind {
                        if Parser::nonterminal_may_begin_with(kind, token) {
                            self.bb_mps.push(mp);
                        }
                    } else {
                        // E.g. `$e` instead of `$e:expr`, reported as a hard error if actually used.
                        // Both this check and the one in `nameize` are necessary, surprisingly.
                        return Some(Error(span, "missing fragment specifier".to_string()));
                    }
                }
                MatcherLoc::Eof => {
                    // We are past the matcher's end, and not in a sequence. Try to end things.
                    debug_assert_eq!(mp.idx, matcher.len() - 1);
                    if *token == token::Eof {
                        eof_mps = match eof_mps {
                            EofMatcherPositions::None => EofMatcherPositions::One(mp),
                            EofMatcherPositions::One(_) | EofMatcherPositions::Multiple => {
                                EofMatcherPositions::Multiple
                            }
                        }
                    }
                }
            }
        }

        // If we reached the end of input, check that there is EXACTLY ONE possible matcher.
        // Otherwise, either the parse is ambiguous (which is an error) or there is a syntax error.
        if *token == token::Eof {
            Some(match eof_mps {
                EofMatcherPositions::One(mut eof_mp) => {
                    // Need to take ownership of the matches from within the `Lrc`.
                    Lrc::make_mut(&mut eof_mp.matches);
                    let matches = Lrc::try_unwrap(eof_mp.matches).unwrap().into_iter();
                    self.nameize(matcher, matches)
                }
                EofMatcherPositions::Multiple => {
                    Error(token.span, "ambiguity: multiple successful parses".to_string())
                }
                EofMatcherPositions::None => Failure(
                    Token::new(
                        token::Eof,
                        if token.span.is_dummy() { token.span } else { token.span.shrink_to_hi() },
                    ),
                    "missing tokens in macro arguments",
                ),
            })
        } else {
            None
        }
    }

    /// Match the token stream from `parser` against `matcher`.
    pub(super) fn parse_tt(
        &mut self,
        parser: &mut Cow<'_, Parser<'_>>,
        matcher: &[MatcherLoc],
    ) -> NamedParseResult {
        // A queue of possible matcher positions. We initialize it with the matcher position in
        // which the "dot" is before the first token of the first token tree in `matcher`.
        // `parse_tt_inner` then processes all of these possible matcher positions and produces
        // possible next positions into `next_mps`. After some post-processing, the contents of
        // `next_mps` replenish `cur_mps` and we start over again.
        self.cur_mps.clear();
        self.cur_mps.push(MatcherPos { idx: 0, matches: self.empty_matches.clone() });

        loop {
            self.next_mps.clear();
            self.bb_mps.clear();

            // Process `cur_mps` until either we have finished the input or we need to get some
            // parsing from the black-box parser done.
            if let Some(res) = self.parse_tt_inner(matcher, &parser.token) {
                return res;
            }

            // `parse_tt_inner` handled all of `cur_mps`, so it's empty.
            assert!(self.cur_mps.is_empty());

            // Error messages here could be improved with links to original rules.
            match (self.next_mps.len(), self.bb_mps.len()) {
                (0, 0) => {
                    // There are no possible next positions AND we aren't waiting for the black-box
                    // parser: syntax error.
                    return Failure(
                        parser.token.clone(),
                        "no rules expected this token in macro call",
                    );
                }

                (_, 0) => {
                    // Dump all possible `next_mps` into `cur_mps` for the next iteration. Then
                    // process the next token.
                    self.cur_mps.append(&mut self.next_mps);
                    parser.to_mut().bump();
                }

                (0, 1) => {
                    // We need to call the black-box parser to get some nonterminal.
                    let mut mp = self.bb_mps.pop().unwrap();
                    let loc = &matcher[mp.idx];
                    if let &MatcherLoc::MetaVarDecl {
                        span,
                        kind: Some(kind),
                        next_metavar,
                        seq_depth,
                        ..
                    } = loc
                    {
                        // We use the span of the metavariable declaration to determine any
                        // edition-specific matching behavior for non-terminals.
                        let nt = match parser.to_mut().parse_nonterminal(kind) {
                            Err(mut err) => {
                                err.span_label(
                                    span,
                                    format!(
                                        "while parsing argument for this `{kind}` macro fragment"
                                    ),
                                )
                                .emit();
                                return ErrorReported;
                            }
                            Ok(nt) => nt,
                        };
                        let m = match nt {
                            NtOrTt::Nt(nt) => MatchedNonterminal(Lrc::new(nt)),
                            NtOrTt::Tt(tt) => MatchedTokenTree(tt),
                        };
                        mp.push_match(next_metavar, seq_depth, m);
                        mp.idx += 1;
                    } else {
                        unreachable!()
                    }
                    self.cur_mps.push(mp);
                }

                (_, _) => {
                    // Too many possibilities!
                    return self.ambiguity_error(matcher, parser.token.span);
                }
            }

            assert!(!self.cur_mps.is_empty());
        }
    }

    fn ambiguity_error(
        &self,
        matcher: &[MatcherLoc],
        token_span: rustc_span::Span,
    ) -> NamedParseResult {
        let nts = self
            .bb_mps
            .iter()
            .map(|mp| match &matcher[mp.idx] {
                MatcherLoc::MetaVarDecl { bind, kind: Some(kind), .. } => {
                    format!("{} ('{}')", kind, bind)
                }
                _ => unreachable!(),
            })
            .collect::<Vec<String>>()
            .join(" or ");

        Error(
            token_span,
            format!(
                "local ambiguity when calling macro `{}`: multiple parsing options: {}",
                self.macro_name,
                match self.next_mps.len() {
                    0 => format!("built-in NTs {}.", nts),
                    n => format!("built-in NTs {} or {n} other option{s}.", nts, s = pluralize!(n)),
                }
            ),
        )
    }

    fn nameize<I: Iterator<Item = NamedMatch>>(
        &self,
        matcher: &[MatcherLoc],
        mut res: I,
    ) -> NamedParseResult {
        // Make that each metavar has _exactly one_ binding. If so, insert the binding into the
        // `NamedParseResult`. Otherwise, it's an error.
        let mut ret_val = FxHashMap::default();
        for loc in matcher {
            if let &MatcherLoc::MetaVarDecl { span, bind, kind, .. } = loc {
                if kind.is_some() {
                    match ret_val.entry(MacroRulesNormalizedIdent::new(bind)) {
                        Vacant(spot) => spot.insert(res.next().unwrap()),
                        Occupied(..) => {
                            return Error(span, format!("duplicated bind name: {}", bind));
                        }
                    };
                } else {
                    // E.g. `$e` instead of `$e:expr`, reported as a hard error if actually used.
                    // Both this check and the one in `parse_tt_inner` are necessary, surprisingly.
                    return Error(span, "missing fragment specifier".to_string());
                }
            }
        }
        Success(ret_val)
    }
}