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
use std::mem;

use rustc_ast::mut_visit::{self, MutVisitor};
use rustc_ast::token::{self, Delimiter, IdentIsRaw, Lit, LitKind, Nonterminal, Token, TokenKind};
use rustc_ast::tokenstream::{DelimSpacing, DelimSpan, Spacing, TokenStream, TokenTree};
use rustc_ast::ExprKind;
use rustc_data_structures::fx::FxHashMap;
use rustc_errors::{pluralize, Diag, DiagCtxtHandle, PResult};
use rustc_parse::lexer::nfc_normalize;
use rustc_parse::parser::ParseNtResult;
use rustc_session::parse::{ParseSess, SymbolGallery};
use rustc_span::hygiene::{LocalExpnId, Transparency};
use rustc_span::symbol::{sym, Ident, MacroRulesNormalizedIdent};
use rustc_span::{with_metavar_spans, Span, Symbol, SyntaxContext};
use smallvec::{smallvec, SmallVec};

use crate::errors::{
    CountRepetitionMisplaced, MetaVarExprUnrecognizedVar, MetaVarsDifSeqMatchers, MustRepeatOnce,
    NoSyntaxVarsExprRepeat, VarStillRepeating,
};
use crate::mbe::macro_parser::NamedMatch;
use crate::mbe::macro_parser::NamedMatch::*;
use crate::mbe::metavar_expr::{MetaVarExprConcatElem, RAW_IDENT_ERR};
use crate::mbe::{self, KleeneOp, MetaVarExpr};

// A Marker adds the given mark to the syntax context.
struct Marker(LocalExpnId, Transparency, FxHashMap<SyntaxContext, SyntaxContext>);

impl MutVisitor for Marker {
    const VISIT_TOKENS: bool = true;

    fn visit_span(&mut self, span: &mut Span) {
        // `apply_mark` is a relatively expensive operation, both due to taking hygiene lock, and
        // by itself. All tokens in a macro body typically have the same syntactic context, unless
        // it's some advanced case with macro-generated macros. So if we cache the marked version
        // of that context once, we'll typically have a 100% cache hit rate after that.
        let Marker(expn_id, transparency, ref mut cache) = *self;
        *span = span.map_ctxt(|ctxt| {
            *cache
                .entry(ctxt)
                .or_insert_with(|| ctxt.apply_mark(expn_id.to_expn_id(), transparency))
        });
    }
}

/// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
struct Frame<'a> {
    tts: &'a [mbe::TokenTree],
    idx: usize,
    kind: FrameKind,
}

enum FrameKind {
    Delimited { delim: Delimiter, span: DelimSpan, spacing: DelimSpacing },
    Sequence { sep: Option<Token>, kleene_op: KleeneOp },
}

impl<'a> Frame<'a> {
    fn new_delimited(src: &'a mbe::Delimited, span: DelimSpan, spacing: DelimSpacing) -> Frame<'a> {
        Frame {
            tts: &src.tts,
            idx: 0,
            kind: FrameKind::Delimited { delim: src.delim, span, spacing },
        }
    }

    fn new_sequence(
        src: &'a mbe::SequenceRepetition,
        sep: Option<Token>,
        kleene_op: KleeneOp,
    ) -> Frame<'a> {
        Frame { tts: &src.tts, idx: 0, kind: FrameKind::Sequence { sep, kleene_op } }
    }
}

impl<'a> Iterator for Frame<'a> {
    type Item = &'a mbe::TokenTree;

    fn next(&mut self) -> Option<&'a mbe::TokenTree> {
        let res = self.tts.get(self.idx);
        self.idx += 1;
        res
    }
}

/// This can do Macro-By-Example transcription.
/// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
///   invocation. We are assuming we already know there is a match.
/// - `src` is the RHS of the MBE, that is, the "example" we are filling in.
///
/// For example,
///
/// ```rust
/// macro_rules! foo {
///     ($id:ident) => { println!("{}", stringify!($id)); }
/// }
///
/// foo!(bar);
/// ```
///
/// `interp` would contain `$id => bar` and `src` would contain `println!("{}", stringify!($id));`.
///
/// `transcribe` would return a `TokenStream` containing `println!("{}", stringify!(bar));`.
///
/// Along the way, we do some additional error checking.
pub(super) fn transcribe<'a>(
    psess: &'a ParseSess,
    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    src: &mbe::Delimited,
    src_span: DelimSpan,
    transparency: Transparency,
    expand_id: LocalExpnId,
) -> PResult<'a, TokenStream> {
    // Nothing for us to transcribe...
    if src.tts.is_empty() {
        return Ok(TokenStream::default());
    }

    // We descend into the RHS (`src`), expanding things as we go. This stack contains the things
    // we have yet to expand/are still expanding. We start the stack off with the whole RHS. The
    // choice of spacing values doesn't matter.
    let mut stack: SmallVec<[Frame<'_>; 1]> = smallvec![Frame::new_delimited(
        src,
        src_span,
        DelimSpacing::new(Spacing::Alone, Spacing::Alone)
    )];

    // As we descend in the RHS, we will need to be able to match nested sequences of matchers.
    // `repeats` keeps track of where we are in matching at each level, with the last element being
    // the most deeply nested sequence. This is used as a stack.
    let mut repeats: Vec<(usize, usize)> = Vec::new();

    // `result` contains resulting token stream from the TokenTree we just finished processing. At
    // the end, this will contain the full result of transcription, but at arbitrary points during
    // `transcribe`, `result` will contain subsets of the final result.
    //
    // Specifically, as we descend into each TokenTree, we will push the existing results onto the
    // `result_stack` and clear `results`. We will then produce the results of transcribing the
    // TokenTree into `results`. Then, as we unwind back out of the `TokenTree`, we will pop the
    // `result_stack` and append `results` too it to produce the new `results` up to that point.
    //
    // Thus, if we try to pop the `result_stack` and it is empty, we have reached the top-level
    // again, and we are done transcribing.
    let mut result: Vec<TokenTree> = Vec::new();
    let mut result_stack = Vec::new();
    let mut marker = Marker(expand_id, transparency, Default::default());

    let dcx = psess.dcx();
    loop {
        // Look at the last frame on the stack.
        // If it still has a TokenTree we have not looked at yet, use that tree.
        let Some(tree) = stack.last_mut().unwrap().next() else {
            // This else-case never produces a value for `tree` (it `continue`s or `return`s).

            // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
            // go back to the beginning of the sequence.
            let frame = stack.last_mut().unwrap();
            if let FrameKind::Sequence { sep, .. } = &frame.kind {
                let (repeat_idx, repeat_len) = repeats.last_mut().unwrap();
                *repeat_idx += 1;
                if repeat_idx < repeat_len {
                    frame.idx = 0;
                    if let Some(sep) = sep {
                        result.push(TokenTree::Token(sep.clone(), Spacing::Alone));
                    }
                    continue;
                }
            }

            // We are done with the top of the stack. Pop it. Depending on what it was, we do
            // different things. Note that the outermost item must be the delimited, wrapped RHS
            // that was passed in originally to `transcribe`.
            match stack.pop().unwrap().kind {
                // Done with a sequence. Pop from repeats.
                FrameKind::Sequence { .. } => {
                    repeats.pop();
                }

                // We are done processing a Delimited. If this is the top-level delimited, we are
                // done. Otherwise, we unwind the result_stack to append what we have produced to
                // any previous results.
                FrameKind::Delimited { delim, span, mut spacing, .. } => {
                    // Hack to force-insert a space after `]` in certain case.
                    // See discussion of the `hex-literal` crate in #114571.
                    if delim == Delimiter::Bracket {
                        spacing.close = Spacing::Alone;
                    }
                    if result_stack.is_empty() {
                        // No results left to compute! We are back at the top-level.
                        return Ok(TokenStream::new(result));
                    }

                    // Step back into the parent Delimited.
                    let tree = TokenTree::Delimited(span, spacing, delim, TokenStream::new(result));
                    result = result_stack.pop().unwrap();
                    result.push(tree);
                }
            }
            continue;
        };

        // At this point, we know we are in the middle of a TokenTree (the last one on `stack`).
        // `tree` contains the next `TokenTree` to be processed.
        match tree {
            // We are descending into a sequence. We first make sure that the matchers in the RHS
            // and the matches in `interp` have the same shape. Otherwise, either the caller or the
            // macro writer has made a mistake.
            seq @ mbe::TokenTree::Sequence(_, seq_rep) => {
                match lockstep_iter_size(seq, interp, &repeats) {
                    LockstepIterSize::Unconstrained => {
                        return Err(dcx.create_err(NoSyntaxVarsExprRepeat { span: seq.span() }));
                    }

                    LockstepIterSize::Contradiction(msg) => {
                        // FIXME: this really ought to be caught at macro definition time... It
                        // happens when two meta-variables are used in the same repetition in a
                        // sequence, but they come from different sequence matchers and repeat
                        // different amounts.
                        return Err(
                            dcx.create_err(MetaVarsDifSeqMatchers { span: seq.span(), msg })
                        );
                    }

                    LockstepIterSize::Constraint(len, _) => {
                        // We do this to avoid an extra clone above. We know that this is a
                        // sequence already.
                        let mbe::TokenTree::Sequence(sp, seq) = seq else { unreachable!() };

                        // Is the repetition empty?
                        if len == 0 {
                            if seq.kleene.op == KleeneOp::OneOrMore {
                                // FIXME: this really ought to be caught at macro definition
                                // time... It happens when the Kleene operator in the matcher and
                                // the body for the same meta-variable do not match.
                                return Err(dcx.create_err(MustRepeatOnce { span: sp.entire() }));
                            }
                        } else {
                            // 0 is the initial counter (we have done 0 repetitions so far). `len`
                            // is the total number of repetitions we should generate.
                            repeats.push((0, len));

                            // The first time we encounter the sequence we push it to the stack. It
                            // then gets reused (see the beginning of the loop) until we are done
                            // repeating.
                            stack.push(Frame::new_sequence(
                                seq_rep,
                                seq.separator.clone(),
                                seq.kleene.op,
                            ));
                        }
                    }
                }
            }

            // Replace the meta-var with the matched token tree from the invocation.
            mbe::TokenTree::MetaVar(mut sp, mut original_ident) => {
                // Find the matched nonterminal from the macro invocation, and use it to replace
                // the meta-var.
                //
                // We use `Spacing::Alone` everywhere here, because that's the conservative choice
                // and spacing of declarative macros is tricky. E.g. in this macro:
                // ```
                // macro_rules! idents {
                //     ($($a:ident,)*) => { stringify!($($a)*) }
                // }
                // ```
                // `$a` has no whitespace after it and will be marked `JointHidden`. If you then
                // call `idents!(x,y,z,)`, each of `x`, `y`, and `z` will be marked as `Joint`. So
                // if you choose to use `$x`'s spacing or the identifier's spacing, you'll end up
                // producing "xyz", which is bad because it effectively merges tokens.
                // `Spacing::Alone` is the safer option. Fortunately, `space_between` will avoid
                // some of the unnecessary whitespace.
                let ident = MacroRulesNormalizedIdent::new(original_ident);
                if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
                    let tt = match cur_matched {
                        MatchedSingle(ParseNtResult::Tt(tt)) => {
                            // `tt`s are emitted into the output stream directly as "raw tokens",
                            // without wrapping them into groups.
                            maybe_use_metavar_location(psess, &stack, sp, tt, &mut marker)
                        }
                        MatchedSingle(ParseNtResult::Ident(ident, is_raw)) => {
                            marker.visit_span(&mut sp);
                            let kind = token::NtIdent(*ident, *is_raw);
                            TokenTree::token_alone(kind, sp)
                        }
                        MatchedSingle(ParseNtResult::Lifetime(ident)) => {
                            marker.visit_span(&mut sp);
                            let kind = token::NtLifetime(*ident);
                            TokenTree::token_alone(kind, sp)
                        }
                        MatchedSingle(ParseNtResult::Nt(nt)) => {
                            // Other variables are emitted into the output stream as groups with
                            // `Delimiter::Invisible` to maintain parsing priorities.
                            // `Interpolated` is currently used for such groups in rustc parser.
                            marker.visit_span(&mut sp);
                            TokenTree::token_alone(token::Interpolated(nt.clone()), sp)
                        }
                        MatchedSeq(..) => {
                            // We were unable to descend far enough. This is an error.
                            return Err(dcx.create_err(VarStillRepeating { span: sp, ident }));
                        }
                    };
                    result.push(tt)
                } else {
                    // If we aren't able to match the meta-var, we push it back into the result but
                    // with modified syntax context. (I believe this supports nested macros).
                    marker.visit_span(&mut sp);
                    marker.visit_ident(&mut original_ident);
                    result.push(TokenTree::token_joint_hidden(token::Dollar, sp));
                    result.push(TokenTree::Token(
                        Token::from_ast_ident(original_ident),
                        Spacing::Alone,
                    ));
                }
            }

            // Replace meta-variable expressions with the result of their expansion.
            mbe::TokenTree::MetaVarExpr(sp, expr) => {
                transcribe_metavar_expr(
                    dcx,
                    expr,
                    interp,
                    &mut marker,
                    &repeats,
                    &mut result,
                    sp,
                    &psess.symbol_gallery,
                )?;
            }

            // If we are entering a new delimiter, we push its contents to the `stack` to be
            // processed, and we push all of the currently produced results to the `result_stack`.
            // We will produce all of the results of the inside of the `Delimited` and then we will
            // jump back out of the Delimited, pop the result_stack and add the new results back to
            // the previous results (from outside the Delimited).
            mbe::TokenTree::Delimited(mut span, spacing, delimited) => {
                mut_visit::visit_delim_span(&mut marker, &mut span);
                stack.push(Frame::new_delimited(delimited, span, *spacing));
                result_stack.push(mem::take(&mut result));
            }

            // Nothing much to do here. Just push the token to the result, being careful to
            // preserve syntax context.
            mbe::TokenTree::Token(token) => {
                let mut token = token.clone();
                mut_visit::visit_token(&mut marker, &mut token);
                let tt = TokenTree::Token(token, Spacing::Alone);
                result.push(tt);
            }

            // There should be no meta-var declarations in the invocation of a macro.
            mbe::TokenTree::MetaVarDecl(..) => panic!("unexpected `TokenTree::MetaVarDecl`"),
        }
    }
}

/// Store the metavariable span for this original span into a side table.
/// FIXME: Try to put the metavariable span into `SpanData` instead of a side table (#118517).
/// An optimal encoding for inlined spans will need to be selected to minimize regressions.
/// The side table approach is relatively good, but not perfect due to collisions.
/// In particular, collisions happen when token is passed as an argument through several macro
/// calls, like in recursive macros.
/// The old heuristic below is used to improve spans in case of collisions, but diagnostics are
/// still degraded sometimes in those cases.
///
/// The old heuristic:
///
/// Usually metavariables `$var` produce interpolated tokens, which have an additional place for
/// keeping both the original span and the metavariable span. For `tt` metavariables that's not the
/// case however, and there's no place for keeping a second span. So we try to give the single
/// produced span a location that would be most useful in practice (the hygiene part of the span
/// must not be changed).
///
/// Different locations are useful for different purposes:
/// - The original location is useful when we need to report a diagnostic for the original token in
///   isolation, without combining it with any surrounding tokens. This case occurs, but it is not
///   very common in practice.
/// - The metavariable location is useful when we need to somehow combine the token span with spans
///   of its surrounding tokens. This is the most common way to use token spans.
///
/// So this function replaces the original location with the metavariable location in all cases
/// except these two:
/// - The metavariable is an element of undelimited sequence `$($tt)*`.
///   These are typically used for passing larger amounts of code, and tokens in that code usually
///   combine with each other and not with tokens outside of the sequence.
/// - The metavariable span comes from a different crate, then we prefer the more local span.
fn maybe_use_metavar_location(
    psess: &ParseSess,
    stack: &[Frame<'_>],
    mut metavar_span: Span,
    orig_tt: &TokenTree,
    marker: &mut Marker,
) -> TokenTree {
    let undelimited_seq = matches!(
        stack.last(),
        Some(Frame {
            tts: [_],
            kind: FrameKind::Sequence {
                sep: None,
                kleene_op: KleeneOp::ZeroOrMore | KleeneOp::OneOrMore,
                ..
            },
            ..
        })
    );
    if undelimited_seq {
        // Do not record metavar spans for tokens from undelimited sequences, for perf reasons.
        return orig_tt.clone();
    }

    let insert = |mspans: &mut FxHashMap<_, _>, s, ms| match mspans.try_insert(s, ms) {
        Ok(_) => true,
        Err(err) => *err.entry.get() == ms, // Tried to insert the same span, still success
    };
    marker.visit_span(&mut metavar_span);
    let no_collision = match orig_tt {
        TokenTree::Token(token, ..) => {
            with_metavar_spans(|mspans| insert(mspans, token.span, metavar_span))
        }
        TokenTree::Delimited(dspan, ..) => with_metavar_spans(|mspans| {
            insert(mspans, dspan.open, metavar_span)
                && insert(mspans, dspan.close, metavar_span)
                && insert(mspans, dspan.entire(), metavar_span)
        }),
    };
    if no_collision || psess.source_map().is_imported(metavar_span) {
        return orig_tt.clone();
    }

    // Setting metavar spans for the heuristic spans gives better opportunities for combining them
    // with neighboring spans even despite their different syntactic contexts.
    match orig_tt {
        TokenTree::Token(Token { kind, span }, spacing) => {
            let span = metavar_span.with_ctxt(span.ctxt());
            with_metavar_spans(|mspans| insert(mspans, span, metavar_span));
            TokenTree::Token(Token { kind: kind.clone(), span }, *spacing)
        }
        TokenTree::Delimited(dspan, dspacing, delimiter, tts) => {
            let open = metavar_span.with_ctxt(dspan.open.ctxt());
            let close = metavar_span.with_ctxt(dspan.close.ctxt());
            with_metavar_spans(|mspans| {
                insert(mspans, open, metavar_span) && insert(mspans, close, metavar_span)
            });
            let dspan = DelimSpan::from_pair(open, close);
            TokenTree::Delimited(dspan, *dspacing, *delimiter, tts.clone())
        }
    }
}

/// Lookup the meta-var named `ident` and return the matched token tree from the invocation using
/// the set of matches `interpolations`.
///
/// See the definition of `repeats` in the `transcribe` function. `repeats` is used to descend
/// into the right place in nested matchers. If we attempt to descend too far, the macro writer has
/// made a mistake, and we return `None`.
fn lookup_cur_matched<'a>(
    ident: MacroRulesNormalizedIdent,
    interpolations: &'a FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    repeats: &[(usize, usize)],
) -> Option<&'a NamedMatch> {
    interpolations.get(&ident).map(|mut matched| {
        for &(idx, _) in repeats {
            match matched {
                MatchedSingle(_) => break,
                MatchedSeq(ads) => matched = ads.get(idx).unwrap(),
            }
        }

        matched
    })
}

/// An accumulator over a TokenTree to be used with `fold`. During transcription, we need to make
/// sure that the size of each sequence and all of its nested sequences are the same as the sizes
/// of all the matched (nested) sequences in the macro invocation. If they don't match, somebody
/// has made a mistake (either the macro writer or caller).
#[derive(Clone)]
enum LockstepIterSize {
    /// No constraints on length of matcher. This is true for any TokenTree variants except a
    /// `MetaVar` with an actual `MatchedSeq` (as opposed to a `MatchedNonterminal`).
    Unconstrained,

    /// A `MetaVar` with an actual `MatchedSeq`. The length of the match and the name of the
    /// meta-var are returned.
    Constraint(usize, MacroRulesNormalizedIdent),

    /// Two `Constraint`s on the same sequence had different lengths. This is an error.
    Contradiction(String),
}

impl LockstepIterSize {
    /// Find incompatibilities in matcher/invocation sizes.
    /// - `Unconstrained` is compatible with everything.
    /// - `Contradiction` is incompatible with everything.
    /// - `Constraint(len)` is only compatible with other constraints of the same length.
    fn with(self, other: LockstepIterSize) -> LockstepIterSize {
        match self {
            LockstepIterSize::Unconstrained => other,
            LockstepIterSize::Contradiction(_) => self,
            LockstepIterSize::Constraint(l_len, l_id) => match other {
                LockstepIterSize::Unconstrained => self,
                LockstepIterSize::Contradiction(_) => other,
                LockstepIterSize::Constraint(r_len, _) if l_len == r_len => self,
                LockstepIterSize::Constraint(r_len, r_id) => {
                    let msg = format!(
                        "meta-variable `{}` repeats {} time{}, but `{}` repeats {} time{}",
                        l_id,
                        l_len,
                        pluralize!(l_len),
                        r_id,
                        r_len,
                        pluralize!(r_len),
                    );
                    LockstepIterSize::Contradiction(msg)
                }
            },
        }
    }
}

/// Given a `tree`, make sure that all sequences have the same length as the matches for the
/// appropriate meta-vars in `interpolations`.
///
/// Note that if `repeats` does not match the exact correct depth of a meta-var,
/// `lookup_cur_matched` will return `None`, which is why this still works even in the presence of
/// multiple nested matcher sequences.
///
/// Example: `$($($x $y)+*);+` -- we need to make sure that `x` and `y` repeat the same amount as
/// each other at the given depth when the macro was invoked. If they don't it might mean they were
/// declared at depths which weren't equal or there was a compiler bug. For example, if we have 3 repetitions of
/// the outer sequence and 4 repetitions of the inner sequence for `x`, we should have the same for
/// `y`; otherwise, we can't transcribe them both at the given depth.
fn lockstep_iter_size(
    tree: &mbe::TokenTree,
    interpolations: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    repeats: &[(usize, usize)],
) -> LockstepIterSize {
    use mbe::TokenTree;
    match tree {
        TokenTree::Delimited(.., delimited) => {
            delimited.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
                size.with(lockstep_iter_size(tt, interpolations, repeats))
            })
        }
        TokenTree::Sequence(_, seq) => {
            seq.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
                size.with(lockstep_iter_size(tt, interpolations, repeats))
            })
        }
        TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) => {
            let name = MacroRulesNormalizedIdent::new(*name);
            match lookup_cur_matched(name, interpolations, repeats) {
                Some(matched) => match matched {
                    MatchedSingle(_) => LockstepIterSize::Unconstrained,
                    MatchedSeq(ads) => LockstepIterSize::Constraint(ads.len(), name),
                },
                _ => LockstepIterSize::Unconstrained,
            }
        }
        TokenTree::MetaVarExpr(_, expr) => {
            expr.for_each_metavar(LockstepIterSize::Unconstrained, |lis, ident| {
                lis.with(lockstep_iter_size(
                    &TokenTree::MetaVar(ident.span, *ident),
                    interpolations,
                    repeats,
                ))
            })
        }
        TokenTree::Token(..) => LockstepIterSize::Unconstrained,
    }
}

/// Used solely by the `count` meta-variable expression, counts the outer-most repetitions at a
/// given optional nested depth.
///
/// For example, a macro parameter of `$( { $( $foo:ident ),* } )*` called with `{ a, b } { c }`:
///
/// * `[ $( ${count(foo)} ),* ]` will return [2, 1] with a, b = 2 and c = 1
/// * `[ $( ${count(foo, 0)} ),* ]` will be the same as `[ $( ${count(foo)} ),* ]`
/// * `[ $( ${count(foo, 1)} ),* ]` will return an error because `${count(foo, 1)}` is
///   declared inside a single repetition and the index `1` implies two nested repetitions.
fn count_repetitions<'a>(
    dcx: DiagCtxtHandle<'a>,
    depth_user: usize,
    mut matched: &NamedMatch,
    repeats: &[(usize, usize)],
    sp: &DelimSpan,
) -> PResult<'a, usize> {
    // Recursively count the number of matches in `matched` at given depth
    // (or at the top-level of `matched` if no depth is given).
    fn count<'a>(depth_curr: usize, depth_max: usize, matched: &NamedMatch) -> PResult<'a, usize> {
        match matched {
            MatchedSingle(_) => Ok(1),
            MatchedSeq(named_matches) => {
                if depth_curr == depth_max {
                    Ok(named_matches.len())
                } else {
                    named_matches.iter().map(|elem| count(depth_curr + 1, depth_max, elem)).sum()
                }
            }
        }
    }

    /// Maximum depth
    fn depth(counter: usize, matched: &NamedMatch) -> usize {
        match matched {
            MatchedSingle(_) => counter,
            MatchedSeq(named_matches) => {
                let rslt = counter + 1;
                if let Some(elem) = named_matches.first() { depth(rslt, elem) } else { rslt }
            }
        }
    }

    let depth_max = depth(0, matched)
        .checked_sub(1)
        .and_then(|el| el.checked_sub(repeats.len()))
        .unwrap_or_default();
    if depth_user > depth_max {
        return Err(out_of_bounds_err(dcx, depth_max + 1, sp.entire(), "count"));
    }

    // `repeats` records all of the nested levels at which we are currently
    // matching meta-variables. The meta-var-expr `count($x)` only counts
    // matches that occur in this "subtree" of the `NamedMatch` where we
    // are currently transcribing, so we need to descend to that subtree
    // before we start counting. `matched` contains the various levels of the
    // tree as we descend, and its final value is the subtree we are currently at.
    for &(idx, _) in repeats {
        if let MatchedSeq(ads) = matched {
            matched = &ads[idx];
        }
    }

    if let MatchedSingle(_) = matched {
        return Err(dcx.create_err(CountRepetitionMisplaced { span: sp.entire() }));
    }

    count(depth_user, depth_max, matched)
}

/// Returns a `NamedMatch` item declared on the LHS given an arbitrary [Ident]
fn matched_from_ident<'ctx, 'interp, 'rslt>(
    dcx: DiagCtxtHandle<'ctx>,
    ident: Ident,
    interp: &'interp FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
) -> PResult<'ctx, &'rslt NamedMatch>
where
    'interp: 'rslt,
{
    let span = ident.span;
    let key = MacroRulesNormalizedIdent::new(ident);
    interp.get(&key).ok_or_else(|| dcx.create_err(MetaVarExprUnrecognizedVar { span, key }))
}

/// Used by meta-variable expressions when an user input is out of the actual declared bounds. For
/// example, index(999999) in an repetition of only three elements.
fn out_of_bounds_err<'a>(dcx: DiagCtxtHandle<'a>, max: usize, span: Span, ty: &str) -> Diag<'a> {
    let msg = if max == 0 {
        format!(
            "meta-variable expression `{ty}` with depth parameter \
             must be called inside of a macro repetition"
        )
    } else {
        format!(
            "depth parameter of meta-variable expression `{ty}` \
             must be less than {max}"
        )
    };
    dcx.struct_span_err(span, msg)
}

fn transcribe_metavar_expr<'a>(
    dcx: DiagCtxtHandle<'a>,
    expr: &MetaVarExpr,
    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    marker: &mut Marker,
    repeats: &[(usize, usize)],
    result: &mut Vec<TokenTree>,
    sp: &DelimSpan,
    symbol_gallery: &SymbolGallery,
) -> PResult<'a, ()> {
    let mut visited_span = || {
        let mut span = sp.entire();
        marker.visit_span(&mut span);
        span
    };
    match *expr {
        MetaVarExpr::Concat(ref elements) => {
            let mut concatenated = String::new();
            for element in elements.into_iter() {
                let symbol = match element {
                    MetaVarExprConcatElem::Ident(elem) => elem.name,
                    MetaVarExprConcatElem::Literal(elem) => *elem,
                    MetaVarExprConcatElem::Var(ident) => {
                        match matched_from_ident(dcx, *ident, interp)? {
                            NamedMatch::MatchedSeq(named_matches) => {
                                let curr_idx = repeats.last().unwrap().0;
                                match &named_matches[curr_idx] {
                                    // FIXME(c410-f3r) Nested repetitions are unimplemented
                                    MatchedSeq(_) => unimplemented!(),
                                    MatchedSingle(pnr) => {
                                        extract_symbol_from_pnr(dcx, pnr, ident.span)?
                                    }
                                }
                            }
                            NamedMatch::MatchedSingle(pnr) => {
                                extract_symbol_from_pnr(dcx, pnr, ident.span)?
                            }
                        }
                    }
                };
                concatenated.push_str(symbol.as_str());
            }
            let symbol = nfc_normalize(&concatenated);
            let concatenated_span = visited_span();
            if !rustc_lexer::is_ident(symbol.as_str()) {
                return Err(dcx.struct_span_err(
                    concatenated_span,
                    "`${concat(..)}` is not generating a valid identifier",
                ));
            }
            symbol_gallery.insert(symbol, concatenated_span);
            // The current implementation marks the span as coming from the macro regardless of
            // contexts of the concatenated identifiers but this behavior may change in the
            // future.
            result.push(TokenTree::Token(
                Token::from_ast_ident(Ident::new(symbol, concatenated_span)),
                Spacing::Alone,
            ));
        }
        MetaVarExpr::Count(original_ident, depth) => {
            let matched = matched_from_ident(dcx, original_ident, interp)?;
            let count = count_repetitions(dcx, depth, matched, repeats, sp)?;
            let tt = TokenTree::token_alone(
                TokenKind::lit(token::Integer, sym::integer(count), None),
                visited_span(),
            );
            result.push(tt);
        }
        MetaVarExpr::Ignore(original_ident) => {
            // Used to ensure that `original_ident` is present in the LHS
            let _ = matched_from_ident(dcx, original_ident, interp)?;
        }
        MetaVarExpr::Index(depth) => match repeats.iter().nth_back(depth) {
            Some((index, _)) => {
                result.push(TokenTree::token_alone(
                    TokenKind::lit(token::Integer, sym::integer(*index), None),
                    visited_span(),
                ));
            }
            None => return Err(out_of_bounds_err(dcx, repeats.len(), sp.entire(), "index")),
        },
        MetaVarExpr::Len(depth) => match repeats.iter().nth_back(depth) {
            Some((_, length)) => {
                result.push(TokenTree::token_alone(
                    TokenKind::lit(token::Integer, sym::integer(*length), None),
                    visited_span(),
                ));
            }
            None => return Err(out_of_bounds_err(dcx, repeats.len(), sp.entire(), "len")),
        },
    }
    Ok(())
}

/// Extracts an metavariable symbol that can be an identifier, a token tree or a literal.
fn extract_symbol_from_pnr<'a>(
    dcx: DiagCtxtHandle<'a>,
    pnr: &ParseNtResult,
    span_err: Span,
) -> PResult<'a, Symbol> {
    match pnr {
        ParseNtResult::Ident(nt_ident, is_raw) => {
            if let IdentIsRaw::Yes = is_raw {
                return Err(dcx.struct_span_err(span_err, RAW_IDENT_ERR));
            }
            return Ok(nt_ident.name);
        }
        ParseNtResult::Tt(TokenTree::Token(
            Token { kind: TokenKind::Ident(symbol, is_raw), .. },
            _,
        )) => {
            if let IdentIsRaw::Yes = is_raw {
                return Err(dcx.struct_span_err(span_err, RAW_IDENT_ERR));
            }
            return Ok(*symbol);
        }
        ParseNtResult::Tt(TokenTree::Token(
            Token {
                kind: TokenKind::Literal(Lit { kind: LitKind::Str, symbol, suffix: None }),
                ..
            },
            _,
        )) => {
            return Ok(*symbol);
        }
        ParseNtResult::Nt(nt)
            if let Nonterminal::NtLiteral(expr) = &**nt
                && let ExprKind::Lit(Lit { kind: LitKind::Str, symbol, suffix: None }) =
                    &expr.kind =>
        {
            return Ok(*symbol);
        }
        _ => Err(dcx
            .struct_err(
                "metavariables of `${concat(..)}` must be of type `ident`, `literal` or `tt`",
            )
            .with_note("currently only string literals are supported")
            .with_span(span_err)),
    }
}