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
use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
use super::{Parser, Restrictions, TokenType};
use crate::maybe_whole;
use rustc_ast::ptr::P;
use rustc_ast::token::{self, Delimiter, Token, TokenKind};
use rustc_ast::{
    self as ast, AngleBracketedArg, AngleBracketedArgs, AnonConst, AssocConstraint,
    AssocConstraintKind, BlockCheckMode, GenericArg, GenericArgs, Generics, ParenthesizedArgs,
    Path, PathSegment, QSelf,
};
use rustc_errors::{pluralize, Applicability, PResult};
use rustc_span::source_map::{BytePos, Span};
use rustc_span::symbol::{kw, sym, Ident};

use std::mem;

/// Specifies how to parse a path.
#[derive(Copy, Clone, PartialEq)]
pub enum PathStyle {
    /// In some contexts, notably in expressions, paths with generic arguments are ambiguous
    /// with something else. For example, in expressions `segment < ....` can be interpreted
    /// as a comparison and `segment ( ....` can be interpreted as a function call.
    /// In all such contexts the non-path interpretation is preferred by default for practical
    /// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
    /// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
    Expr,
    /// In other contexts, notably in types, no ambiguity exists and paths can be written
    /// without the disambiguator, e.g., `x<y>` - unambiguously a path.
    /// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
    Type,
    /// A path with generic arguments disallowed, e.g., `foo::bar::Baz`, used in imports,
    /// visibilities or attributes.
    /// Technically, this variant is unnecessary and e.g., `Expr` can be used instead
    /// (paths in "mod" contexts have to be checked later for absence of generic arguments
    /// anyway, due to macros), but it is used to avoid weird suggestions about expected
    /// tokens when something goes wrong.
    Mod,
}

impl<'a> Parser<'a> {
    /// Parses a qualified path.
    /// Assumes that the leading `<` has been parsed already.
    ///
    /// `qualified_path = <type [as trait_ref]>::path`
    ///
    /// # Examples
    /// `<T>::default`
    /// `<T as U>::a`
    /// `<T as U>::F::a<S>` (without disambiguator)
    /// `<T as U>::F::a::<S>` (with disambiguator)
    pub(super) fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, Path)> {
        let lo = self.prev_token.span;
        let ty = self.parse_ty()?;

        // `path` will contain the prefix of the path up to the `>`,
        // if any (e.g., `U` in the `<T as U>::*` examples
        // above). `path_span` has the span of that path, or an empty
        // span in the case of something like `<T>::Bar`.
        let (mut path, path_span);
        if self.eat_keyword(kw::As) {
            let path_lo = self.token.span;
            path = self.parse_path(PathStyle::Type)?;
            path_span = path_lo.to(self.prev_token.span);
        } else {
            path_span = self.token.span.to(self.token.span);
            path = ast::Path { segments: Vec::new(), span: path_span, tokens: None };
        }

        // See doc comment for `unmatched_angle_bracket_count`.
        self.expect(&token::Gt)?;
        if self.unmatched_angle_bracket_count > 0 {
            self.unmatched_angle_bracket_count -= 1;
            debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
        }

        if !self.recover_colon_before_qpath_proj() {
            self.expect(&token::ModSep)?;
        }

        let qself = QSelf { ty, path_span, position: path.segments.len() };
        self.parse_path_segments(&mut path.segments, style, None)?;

        Ok((
            qself,
            Path { segments: path.segments, span: lo.to(self.prev_token.span), tokens: None },
        ))
    }

    /// Recover from an invalid single colon, when the user likely meant a qualified path.
    /// We avoid emitting this if not followed by an identifier, as our assumption that the user
    /// intended this to be a qualified path may not be correct.
    ///
    /// ```ignore (diagnostics)
    /// <Bar as Baz<T>>:Qux
    ///                ^ help: use double colon
    /// ```
    fn recover_colon_before_qpath_proj(&mut self) -> bool {
        if !self.check_noexpect(&TokenKind::Colon)
            || self.look_ahead(1, |t| !t.is_ident() || t.is_reserved_ident())
        {
            return false;
        }

        self.bump(); // colon

        self.diagnostic()
            .struct_span_err(
                self.prev_token.span,
                "found single colon before projection in qualified path",
            )
            .span_suggestion(
                self.prev_token.span,
                "use double colon",
                "::",
                Applicability::MachineApplicable,
            )
            .emit();

        true
    }

    pub(super) fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
        self.parse_path_inner(style, None)
    }

    /// Parses simple paths.
    ///
    /// `path = [::] segment+`
    /// `segment = ident | ident[::]<args> | ident[::](args) [-> type]`
    ///
    /// # Examples
    /// `a::b::C<D>` (without disambiguator)
    /// `a::b::C::<D>` (with disambiguator)
    /// `Fn(Args)` (without disambiguator)
    /// `Fn::(Args)` (with disambiguator)
    pub(super) fn parse_path_inner(
        &mut self,
        style: PathStyle,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, Path> {
        let reject_generics_if_mod_style = |parser: &Parser<'_>, path: &Path| {
            // Ensure generic arguments don't end up in attribute paths, such as:
            //
            //     macro_rules! m {
            //         ($p:path) => { #[$p] struct S; }
            //     }
            //
            //     m!(inline<u8>); //~ ERROR: unexpected generic arguments in path
            //
            if style == PathStyle::Mod && path.segments.iter().any(|segment| segment.args.is_some())
            {
                parser
                    .struct_span_err(
                        path.segments
                            .iter()
                            .filter_map(|segment| segment.args.as_ref())
                            .map(|arg| arg.span())
                            .collect::<Vec<_>>(),
                        "unexpected generic arguments in path",
                    )
                    .emit();
            }
        };

        maybe_whole!(self, NtPath, |path| {
            reject_generics_if_mod_style(self, &path);
            path.into_inner()
        });

        if let token::Interpolated(nt) = &self.token.kind {
            if let token::NtTy(ty) = &**nt {
                if let ast::TyKind::Path(None, path) = &ty.kind {
                    let path = path.clone();
                    self.bump();
                    reject_generics_if_mod_style(self, &path);
                    return Ok(path);
                }
            }
        }

        let lo = self.token.span;
        let mut segments = Vec::new();
        let mod_sep_ctxt = self.token.span.ctxt();
        if self.eat(&token::ModSep) {
            segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
        }
        self.parse_path_segments(&mut segments, style, ty_generics)?;

        Ok(Path { segments, span: lo.to(self.prev_token.span), tokens: None })
    }

    pub(super) fn parse_path_segments(
        &mut self,
        segments: &mut Vec<PathSegment>,
        style: PathStyle,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, ()> {
        loop {
            let segment = self.parse_path_segment(style, ty_generics)?;
            if style == PathStyle::Expr {
                // In order to check for trailing angle brackets, we must have finished
                // recursing (`parse_path_segment` can indirectly call this function),
                // that is, the next token must be the highlighted part of the below example:
                //
                // `Foo::<Bar as Baz<T>>::Qux`
                //                      ^ here
                //
                // As opposed to the below highlight (if we had only finished the first
                // recursion):
                //
                // `Foo::<Bar as Baz<T>>::Qux`
                //                     ^ here
                //
                // `PathStyle::Expr` is only provided at the root invocation and never in
                // `parse_path_segment` to recurse and therefore can be checked to maintain
                // this invariant.
                self.check_trailing_angle_brackets(&segment, &[&token::ModSep]);
            }
            segments.push(segment);

            if self.is_import_coupler() || !self.eat(&token::ModSep) {
                return Ok(());
            }
        }
    }

    pub(super) fn parse_path_segment(
        &mut self,
        style: PathStyle,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, PathSegment> {
        let ident = self.parse_path_segment_ident()?;
        let is_args_start = |token: &Token| {
            matches!(
                token.kind,
                token::Lt
                    | token::BinOp(token::Shl)
                    | token::OpenDelim(Delimiter::Parenthesis)
                    | token::LArrow
            )
        };
        let check_args_start = |this: &mut Self| {
            this.expected_tokens.extend_from_slice(&[
                TokenType::Token(token::Lt),
                TokenType::Token(token::OpenDelim(Delimiter::Parenthesis)),
            ]);
            is_args_start(&this.token)
        };

        Ok(
            if style == PathStyle::Type && check_args_start(self)
                || style != PathStyle::Mod
                    && self.check(&token::ModSep)
                    && self.look_ahead(1, |t| is_args_start(t))
            {
                // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
                // it isn't, then we reset the unmatched angle bracket count as we're about to start
                // parsing a new path.
                if style == PathStyle::Expr {
                    self.unmatched_angle_bracket_count = 0;
                    self.max_angle_bracket_count = 0;
                }

                // Generic arguments are found - `<`, `(`, `::<` or `::(`.
                self.eat(&token::ModSep);
                let lo = self.token.span;
                let args = if self.eat_lt() {
                    // `<'a, T, A = U>`
                    let args = self.parse_angle_args_with_leading_angle_bracket_recovery(
                        style,
                        lo,
                        ty_generics,
                    )?;
                    self.expect_gt().map_err(|mut err| {
                        // Attempt to find places where a missing `>` might belong.
                        if let Some(arg) = args
                            .iter()
                            .rev()
                            .skip_while(|arg| matches!(arg, AngleBracketedArg::Constraint(_)))
                            .next()
                        {
                            err.span_suggestion_verbose(
                                arg.span().shrink_to_hi(),
                                "you might have meant to end the type parameters here",
                                ">",
                                Applicability::MaybeIncorrect,
                            );
                        }
                        err
                    })?;
                    let span = lo.to(self.prev_token.span);
                    AngleBracketedArgs { args, span }.into()
                } else {
                    // `(T, U) -> R`
                    let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
                    let inputs_span = lo.to(self.prev_token.span);
                    let output =
                        self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
                    let span = ident.span.to(self.prev_token.span);
                    ParenthesizedArgs { span, inputs, inputs_span, output }.into()
                };

                PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
            } else {
                // Generic arguments are not found.
                PathSegment::from_ident(ident)
            },
        )
    }

    pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
        match self.token.ident() {
            Some((ident, false)) if ident.is_path_segment_keyword() => {
                self.bump();
                Ok(ident)
            }
            _ => self.parse_ident(),
        }
    }

    /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
    /// For the purposes of understanding the parsing logic of generic arguments, this function
    /// can be thought of being the same as just calling `self.parse_angle_args()` if the source
    /// had the correct amount of leading angle brackets.
    ///
    /// ```ignore (diagnostics)
    /// bar::<<<<T as Foo>::Output>();
    ///      ^^ help: remove extra angle brackets
    /// ```
    fn parse_angle_args_with_leading_angle_bracket_recovery(
        &mut self,
        style: PathStyle,
        lo: Span,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, Vec<AngleBracketedArg>> {
        // We need to detect whether there are extra leading left angle brackets and produce an
        // appropriate error and suggestion. This cannot be implemented by looking ahead at
        // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
        // then there won't be matching `>` tokens to find.
        //
        // To explain how this detection works, consider the following example:
        //
        // ```ignore (diagnostics)
        // bar::<<<<T as Foo>::Output>();
        //      ^^ help: remove extra angle brackets
        // ```
        //
        // Parsing of the left angle brackets starts in this function. We start by parsing the
        // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
        // `eat_lt`):
        //
        // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
        // *Unmatched count:* 1
        // *`parse_path_segment` calls deep:* 0
        //
        // This has the effect of recursing as this function is called if a `<` character
        // is found within the expected generic arguments:
        //
        // *Upcoming tokens:* `<<<T as Foo>::Output>;`
        // *Unmatched count:* 2
        // *`parse_path_segment` calls deep:* 1
        //
        // Eventually we will have recursed until having consumed all of the `<` tokens and
        // this will be reflected in the count:
        //
        // *Upcoming tokens:* `T as Foo>::Output>;`
        // *Unmatched count:* 4
        // `parse_path_segment` calls deep:* 3
        //
        // The parser will continue until reaching the first `>` - this will decrement the
        // unmatched angle bracket count and return to the parent invocation of this function
        // having succeeded in parsing:
        //
        // *Upcoming tokens:* `::Output>;`
        // *Unmatched count:* 3
        // *`parse_path_segment` calls deep:* 2
        //
        // This will continue until the next `>` character which will also return successfully
        // to the parent invocation of this function and decrement the count:
        //
        // *Upcoming tokens:* `;`
        // *Unmatched count:* 2
        // *`parse_path_segment` calls deep:* 1
        //
        // At this point, this function will expect to find another matching `>` character but
        // won't be able to and will return an error. This will continue all the way up the
        // call stack until the first invocation:
        //
        // *Upcoming tokens:* `;`
        // *Unmatched count:* 2
        // *`parse_path_segment` calls deep:* 0
        //
        // In doing this, we have managed to work out how many unmatched leading left angle
        // brackets there are, but we cannot recover as the unmatched angle brackets have
        // already been consumed. To remedy this, we keep a snapshot of the parser state
        // before we do the above. We can then inspect whether we ended up with a parsing error
        // and unmatched left angle brackets and if so, restore the parser state before we
        // consumed any `<` characters to emit an error and consume the erroneous tokens to
        // recover by attempting to parse again.
        //
        // In practice, the recursion of this function is indirect and there will be other
        // locations that consume some `<` characters - as long as we update the count when
        // this happens, it isn't an issue.

        let is_first_invocation = style == PathStyle::Expr;
        // Take a snapshot before attempting to parse - we can restore this later.
        let snapshot = if is_first_invocation { Some(self.clone()) } else { None };

        debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
        match self.parse_angle_args(ty_generics) {
            Ok(args) => Ok(args),
            Err(e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
                // Swap `self` with our backup of the parser state before attempting to parse
                // generic arguments.
                let snapshot = mem::replace(self, snapshot.unwrap());

                // Eat the unmatched angle brackets.
                let all_angle_brackets = (0..snapshot.unmatched_angle_bracket_count)
                    .fold(true, |a, _| a && self.eat_lt());

                if !all_angle_brackets {
                    // If there are other tokens in between the extraneous `<`s, we cannot simply
                    // suggest to remove them. This check also prevents us from accidentally ending
                    // up in the middle of a multibyte character (issue #84104).
                    let _ = mem::replace(self, snapshot);
                    Err(e)
                } else {
                    // Cancel error from being unable to find `>`. We know the error
                    // must have been this due to a non-zero unmatched angle bracket
                    // count.
                    e.cancel();

                    debug!(
                        "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
                         snapshot.count={:?}",
                        snapshot.unmatched_angle_bracket_count,
                    );

                    // Make a span over ${unmatched angle bracket count} characters.
                    // This is safe because `all_angle_brackets` ensures that there are only `<`s,
                    // i.e. no multibyte characters, in this range.
                    let span =
                        lo.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count));
                    self.struct_span_err(
                        span,
                        &format!(
                            "unmatched angle bracket{}",
                            pluralize!(snapshot.unmatched_angle_bracket_count)
                        ),
                    )
                    .span_suggestion(
                        span,
                        &format!(
                            "remove extra angle bracket{}",
                            pluralize!(snapshot.unmatched_angle_bracket_count)
                        ),
                        "",
                        Applicability::MachineApplicable,
                    )
                    .emit();

                    // Try again without unmatched angle bracket characters.
                    self.parse_angle_args(ty_generics)
                }
            }
            Err(e) => Err(e),
        }
    }

    /// Parses (possibly empty) list of generic arguments / associated item constraints,
    /// possibly including trailing comma.
    pub(super) fn parse_angle_args(
        &mut self,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, Vec<AngleBracketedArg>> {
        let mut args = Vec::new();
        while let Some(arg) = self.parse_angle_arg(ty_generics)? {
            args.push(arg);
            if !self.eat(&token::Comma) {
                if self.check_noexpect(&TokenKind::Semi)
                    && self.look_ahead(1, |t| t.is_ident() || t.is_lifetime())
                {
                    // Add `>` to the list of expected tokens.
                    self.check(&token::Gt);
                    // Handle `,` to `;` substitution
                    let mut err = self.unexpected::<()>().unwrap_err();
                    self.bump();
                    err.span_suggestion_verbose(
                        self.prev_token.span.until(self.token.span),
                        "use a comma to separate type parameters",
                        ", ",
                        Applicability::MachineApplicable,
                    );
                    err.emit();
                    continue;
                }
                if !self.token.kind.should_end_const_arg() {
                    if self.handle_ambiguous_unbraced_const_arg(&mut args)? {
                        // We've managed to (partially) recover, so continue trying to parse
                        // arguments.
                        continue;
                    }
                }
                break;
            }
        }
        Ok(args)
    }

    /// Parses a single argument in the angle arguments `<...>` of a path segment.
    fn parse_angle_arg(
        &mut self,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, Option<AngleBracketedArg>> {
        let lo = self.token.span;
        let arg = self.parse_generic_arg(ty_generics)?;
        match arg {
            Some(arg) => {
                // we are using noexpect here because we first want to find out if either `=` or `:`
                // is present and then use that info to push the other token onto the tokens list
                let separated =
                    self.check_noexpect(&token::Colon) || self.check_noexpect(&token::Eq);
                if separated && (self.check(&token::Colon) | self.check(&token::Eq)) {
                    let arg_span = arg.span();
                    let (binder, ident, gen_args) = match self.get_ident_from_generic_arg(&arg) {
                        Ok(ident_gen_args) => ident_gen_args,
                        Err(()) => return Ok(Some(AngleBracketedArg::Arg(arg))),
                    };
                    if binder {
                        // FIXME(compiler-errors): this could be improved by suggesting lifting
                        // this up to the trait, at least before this becomes real syntax.
                        // e.g. `Trait<for<'a> Assoc = Ty>` -> `for<'a> Trait<Assoc = Ty>`
                        return Err(self.struct_span_err(
                            arg_span,
                            "`for<...>` is not allowed on associated type bounds",
                        ));
                    }
                    let kind = if self.eat(&token::Colon) {
                        // Parse associated type constraint bound.

                        let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
                        AssocConstraintKind::Bound { bounds }
                    } else if self.eat(&token::Eq) {
                        self.parse_assoc_equality_term(ident, self.prev_token.span)?
                    } else {
                        unreachable!();
                    };

                    let span = lo.to(self.prev_token.span);

                    // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
                    if let AssocConstraintKind::Bound { .. } = kind {
                        self.sess.gated_spans.gate(sym::associated_type_bounds, span);
                    }
                    let constraint =
                        AssocConstraint { id: ast::DUMMY_NODE_ID, ident, gen_args, kind, span };
                    Ok(Some(AngleBracketedArg::Constraint(constraint)))
                } else {
                    // we only want to suggest `:` and `=` in contexts where the previous token
                    // is an ident and the current token or the next token is an ident
                    if self.prev_token.is_ident()
                        && (self.token.is_ident() || self.look_ahead(1, |token| token.is_ident()))
                    {
                        self.check(&token::Colon);
                        self.check(&token::Eq);
                    }
                    Ok(Some(AngleBracketedArg::Arg(arg)))
                }
            }
            _ => Ok(None),
        }
    }

    /// Parse the term to the right of an associated item equality constraint.
    /// That is, parse `<term>` in `Item = <term>`.
    /// Right now, this only admits types in `<term>`.
    fn parse_assoc_equality_term(
        &mut self,
        ident: Ident,
        eq: Span,
    ) -> PResult<'a, AssocConstraintKind> {
        let arg = self.parse_generic_arg(None)?;
        let span = ident.span.to(self.prev_token.span);
        let term = match arg {
            Some(GenericArg::Type(ty)) => ty.into(),
            Some(GenericArg::Const(c)) => {
                self.sess.gated_spans.gate(sym::associated_const_equality, span);
                c.into()
            }
            Some(GenericArg::Lifetime(lt)) => {
                self.struct_span_err(span, "associated lifetimes are not supported")
                    .span_label(lt.ident.span, "the lifetime is given here")
                    .help("if you meant to specify a trait object, write `dyn Trait + 'lifetime`")
                    .emit();
                self.mk_ty(span, ast::TyKind::Err).into()
            }
            None => {
                let after_eq = eq.shrink_to_hi();
                let before_next = self.token.span.shrink_to_lo();
                let mut err = self
                    .struct_span_err(after_eq.to(before_next), "missing type to the right of `=`");
                if matches!(self.token.kind, token::Comma | token::Gt) {
                    err.span_suggestion(
                        self.sess.source_map().next_point(eq).to(before_next),
                        "to constrain the associated type, add a type after `=`",
                        " TheType",
                        Applicability::HasPlaceholders,
                    );
                    err.span_suggestion(
                        eq.to(before_next),
                        &format!("remove the `=` if `{}` is a type", ident),
                        "",
                        Applicability::MaybeIncorrect,
                    )
                } else {
                    err.span_label(
                        self.token.span,
                        &format!("expected type, found {}", super::token_descr(&self.token)),
                    )
                };
                return Err(err);
            }
        };
        Ok(AssocConstraintKind::Equality { term })
    }

    /// We do not permit arbitrary expressions as const arguments. They must be one of:
    /// - An expression surrounded in `{}`.
    /// - A literal.
    /// - A numeric literal prefixed by `-`.
    /// - A single-segment path.
    pub(super) fn expr_is_valid_const_arg(&self, expr: &P<rustc_ast::Expr>) -> bool {
        match &expr.kind {
            ast::ExprKind::Block(_, _) | ast::ExprKind::Lit(_) => true,
            ast::ExprKind::Unary(ast::UnOp::Neg, expr) => {
                matches!(expr.kind, ast::ExprKind::Lit(_))
            }
            // We can only resolve single-segment paths at the moment, because multi-segment paths
            // require type-checking: see `visit_generic_arg` in `src/librustc_resolve/late.rs`.
            ast::ExprKind::Path(None, path)
                if path.segments.len() == 1 && path.segments[0].args.is_none() =>
            {
                true
            }
            _ => false,
        }
    }

    /// Parse a const argument, e.g. `<3>`. It is assumed the angle brackets will be parsed by
    /// the caller.
    pub(super) fn parse_const_arg(&mut self) -> PResult<'a, AnonConst> {
        // Parse const argument.
        let value = if let token::OpenDelim(Delimiter::Brace) = self.token.kind {
            self.parse_block_expr(None, self.token.span, BlockCheckMode::Default)?
        } else {
            self.handle_unambiguous_unbraced_const_arg()?
        };
        Ok(AnonConst { id: ast::DUMMY_NODE_ID, value })
    }

    /// Parse a generic argument in a path segment.
    /// This does not include constraints, e.g., `Item = u8`, which is handled in `parse_angle_arg`.
    pub(super) fn parse_generic_arg(
        &mut self,
        ty_generics: Option<&Generics>,
    ) -> PResult<'a, Option<GenericArg>> {
        let start = self.token.span;
        let arg = if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
            // Parse lifetime argument.
            GenericArg::Lifetime(self.expect_lifetime())
        } else if self.check_const_arg() {
            // Parse const argument.
            GenericArg::Const(self.parse_const_arg()?)
        } else if self.check_type() {
            // Parse type argument.
            let is_const_fn =
                self.look_ahead(1, |t| t.kind == token::OpenDelim(Delimiter::Parenthesis));
            let mut snapshot = self.create_snapshot_for_diagnostic();
            match self.parse_ty() {
                Ok(ty) => GenericArg::Type(ty),
                Err(err) => {
                    if is_const_fn {
                        match (*snapshot).parse_expr_res(Restrictions::CONST_EXPR, None) {
                            Ok(expr) => {
                                self.restore_snapshot(snapshot);
                                return Ok(Some(self.dummy_const_arg_needs_braces(err, expr.span)));
                            }
                            Err(err) => {
                                err.cancel();
                            }
                        }
                    }
                    // Try to recover from possible `const` arg without braces.
                    return self.recover_const_arg(start, err).map(Some);
                }
            }
        } else if self.token.is_keyword(kw::Const) {
            return self.recover_const_param_declaration(ty_generics);
        } else {
            // Fall back by trying to parse a const-expr expression. If we successfully do so,
            // then we should report an error that it needs to be wrapped in braces.
            let snapshot = self.create_snapshot_for_diagnostic();
            match self.parse_expr_res(Restrictions::CONST_EXPR, None) {
                Ok(expr) => {
                    return Ok(Some(self.dummy_const_arg_needs_braces(
                        self.struct_span_err(expr.span, "invalid const generic expression"),
                        expr.span,
                    )));
                }
                Err(err) => {
                    self.restore_snapshot(snapshot);
                    err.cancel();
                    return Ok(None);
                }
            }
        };
        Ok(Some(arg))
    }

    /// Given a arg inside of generics, we try to destructure it as if it were the LHS in
    /// `LHS = ...`, i.e. an associated type binding.
    /// This returns a bool indicating if there are any `for<'a, 'b>` binder args, the
    /// identifier, and any GAT arguments.
    fn get_ident_from_generic_arg(
        &self,
        gen_arg: &GenericArg,
    ) -> Result<(bool, Ident, Option<GenericArgs>), ()> {
        if let GenericArg::Type(ty) = gen_arg {
            if let ast::TyKind::Path(qself, path) = &ty.kind
                && qself.is_none()
                && let [seg] = path.segments.as_slice()
            {
                return Ok((false, seg.ident, seg.args.as_deref().cloned()));
            } else if let ast::TyKind::TraitObject(bounds, ast::TraitObjectSyntax::None) = &ty.kind
                && let [ast::GenericBound::Trait(trait_ref, ast::TraitBoundModifier::None)] =
                    bounds.as_slice()
                && let [seg] = trait_ref.trait_ref.path.segments.as_slice()
            {
                return Ok((true, seg.ident, seg.args.as_deref().cloned()));
            }
        }
        Err(())
    }
}