rustc_parse/parser/
generics.rs

1use rustc_ast::{
2    self as ast, AttrVec, DUMMY_NODE_ID, GenericBounds, GenericParam, GenericParamKind, TyKind,
3    WhereClause, token,
4};
5use rustc_errors::{Applicability, Diag, PResult};
6use rustc_span::{Ident, Span, kw, sym};
7use thin_vec::ThinVec;
8
9use super::{ForceCollect, Parser, Trailing, UsePreAttrPos};
10use crate::diagnostics::{
11    self, MultipleWhereClauses, UnexpectedDefaultValueForLifetimeInGenericParameters,
12    UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
13    WhereClauseBeforeTupleStructBodySugg,
14};
15use crate::exp;
16
17enum PredicateKindOrStructBody {
18    PredicateKind(ast::WherePredicateKind),
19    StructBody(ThinVec<ast::FieldDef>),
20}
21
22impl<'a> Parser<'a> {
23    /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
24    ///
25    /// ```text
26    /// BOUND = LT_BOUND (e.g., `'a`)
27    /// ```
28    fn parse_lt_param_bounds(&mut self) -> GenericBounds {
29        let mut lifetimes = ThinVec::new();
30        while self.check_lifetime() {
31            lifetimes.push(ast::GenericBound::Outlives(self.expect_lifetime()));
32
33            if !self.eat_plus() {
34                break;
35            }
36        }
37        lifetimes
38    }
39
40    /// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
41    fn parse_ty_param(&mut self, preceding_attrs: AttrVec) -> PResult<'a, GenericParam> {
42        let ident = self.parse_ident()?;
43
44        // We might have a typo'd `Const` that was parsed as a type parameter.
45        if self.may_recover()
46            && ident.name.as_str().to_ascii_lowercase() == kw::Const.as_str()
47            && self.check_ident()
48        // `Const` followed by IDENT
49        {
50            return self.recover_const_param_with_mistyped_const(preceding_attrs, ident);
51        }
52
53        // Parse optional colon and param bounds.
54        let mut colon_span = None;
55        let bounds = if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon)) {
56            colon_span = Some(self.prev_token.span);
57            // recover from `impl Trait` in type param bound
58            if self.token.is_keyword(kw::Impl) {
59                let impl_span = self.token.span;
60                let snapshot = self.create_snapshot_for_diagnostic();
61                match self.parse_ty() {
62                    Ok(p) => {
63                        if let TyKind::ImplTrait(_, bounds) = &p.kind {
64                            let span = impl_span.to(self.token.span.shrink_to_lo());
65                            let mut err = self.dcx().struct_span_err(
66                                span,
67                                "expected trait bound, found `impl Trait` type",
68                            );
69                            err.span_label(span, "not a trait");
70                            if let [bound, ..] = &bounds[..] {
71                                err.span_suggestion_verbose(
72                                    impl_span.until(bound.span()),
73                                    "use the trait bounds directly",
74                                    String::new(),
75                                    Applicability::MachineApplicable,
76                                );
77                            }
78                            return Err(err);
79                        }
80                    }
81                    Err(err) => {
82                        err.cancel();
83                    }
84                }
85                self.restore_snapshot(snapshot);
86            }
87            self.parse_generic_bounds()?
88        } else {
89            ThinVec::new()
90        };
91
92        let default = if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Eq,
    token_type: crate::parser::token_type::TokenType::Eq,
}exp!(Eq)) { Some(self.parse_ty()?) } else { None };
93        Ok(GenericParam {
94            ident,
95            id: ast::DUMMY_NODE_ID,
96            attrs: preceding_attrs,
97            bounds,
98            kind: GenericParamKind::Type { default },
99            is_placeholder: false,
100            colon_span,
101        })
102    }
103
104    pub(crate) fn parse_const_param(
105        &mut self,
106        preceding_attrs: AttrVec,
107    ) -> PResult<'a, GenericParam> {
108        let const_span = self.token.span;
109
110        self.expect_keyword(crate::parser::token_type::ExpKeywordPair {
    kw: rustc_span::symbol::kw::Const,
    token_type: crate::parser::token_type::TokenType::KwConst,
}exp!(Const))?;
111        let ident = self.parse_ident()?;
112        if let Err(mut err) = self.expect(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon)) {
113            return if self.token.kind == token::Comma || self.token.kind == token::Gt {
114                // Recover parse from `<const N>` where the type is missing.
115                let span = const_span.to(ident.span);
116                err.span_suggestion_verbose(
117                    ident.span.shrink_to_hi(),
118                    "you likely meant to write the type of the const parameter here",
119                    ": /* Type */".to_string(),
120                    Applicability::HasPlaceholders,
121                );
122                let kind = TyKind::Err(err.emit());
123                let ty = self.mk_ty(span, kind);
124                Ok(GenericParam {
125                    ident,
126                    id: ast::DUMMY_NODE_ID,
127                    attrs: preceding_attrs,
128                    bounds: ThinVec::new(),
129                    kind: GenericParamKind::Const { ty, span, default: None },
130                    is_placeholder: false,
131                    colon_span: None,
132                })
133            } else {
134                Err(err)
135            };
136        }
137        let ty = self.parse_ty()?;
138
139        // Parse optional const generics default value.
140        let default = if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Eq,
    token_type: crate::parser::token_type::TokenType::Eq,
}exp!(Eq)) { Some(self.parse_const_arg()?) } else { None };
141        let span = if let Some(ref default) = default {
142            const_span.to(default.value.span)
143        } else {
144            const_span.to(ty.span)
145        };
146
147        Ok(GenericParam {
148            ident,
149            id: ast::DUMMY_NODE_ID,
150            attrs: preceding_attrs,
151            bounds: ThinVec::new(),
152            kind: GenericParamKind::Const { ty, span, default },
153            is_placeholder: false,
154            colon_span: None,
155        })
156    }
157
158    pub(crate) fn recover_const_param_with_mistyped_const(
159        &mut self,
160        preceding_attrs: AttrVec,
161        mistyped_const_ident: Ident,
162    ) -> PResult<'a, GenericParam> {
163        let ident = self.parse_ident()?;
164        self.expect(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon))?;
165        let ty = self.parse_ty()?;
166
167        // Parse optional const generics default value.
168        let default = if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Eq,
    token_type: crate::parser::token_type::TokenType::Eq,
}exp!(Eq)) { Some(self.parse_const_arg()?) } else { None };
169        let span = if let Some(ref default) = default {
170            mistyped_const_ident.span.to(default.value.span)
171        } else {
172            mistyped_const_ident.span.to(ty.span)
173        };
174
175        self.dcx()
176            .struct_span_err(
177                mistyped_const_ident.span,
178                ::alloc::__export::must_use({
        ::alloc::fmt::format(format_args!("`const` keyword was mistyped as `{0}`",
                mistyped_const_ident.as_str()))
    })format!("`const` keyword was mistyped as `{}`", mistyped_const_ident.as_str()),
179            )
180            .with_span_suggestion_verbose(
181                mistyped_const_ident.span,
182                "use the `const` keyword",
183                kw::Const,
184                Applicability::MachineApplicable,
185            )
186            .emit();
187
188        Ok(GenericParam {
189            ident,
190            id: ast::DUMMY_NODE_ID,
191            attrs: preceding_attrs,
192            bounds: ThinVec::new(),
193            kind: GenericParamKind::Const { ty, span, default },
194            is_placeholder: false,
195            colon_span: None,
196        })
197    }
198
199    /// Parse a (possibly empty) list of generic (lifetime, type, const) parameters.
200    ///
201    /// ```ebnf
202    /// GenericParams = (GenericParam ("," GenericParam)* ","?)?
203    /// ```
204    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {
205        let mut params = ThinVec::new();
206        let mut done = false;
207        let prev = self.parsing_generics;
208        self.parsing_generics = true;
209        while !done {
210            let attrs = self.parse_outer_attributes()?;
211            let param = match self.collect_tokens(None, attrs, ForceCollect::No, |this, attrs| {
212                if this.eat_keyword_noexpect(kw::SelfUpper) {
213                    // `Self` as a generic param is invalid. Here we emit the diagnostic and continue parsing
214                    // as if `Self` never existed.
215                    this.dcx()
216                        .emit_err(UnexpectedSelfInGenericParameters { span: this.prev_token.span });
217
218                    // Eat a trailing comma, if it exists.
219                    let _ = this.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Comma,
    token_type: crate::parser::token_type::TokenType::Comma,
}exp!(Comma));
220                }
221
222                let param = if this.check_lifetime() {
223                    let lifetime = this.expect_lifetime();
224                    // Parse lifetime parameter.
225                    let (colon_span, bounds) = if this.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon)) {
226                        (Some(this.prev_token.span), this.parse_lt_param_bounds())
227                    } else {
228                        (None, ThinVec::new())
229                    };
230
231                    if this.check_noexpect(&token::Eq) && this.look_ahead(1, |t| t.is_lifetime()) {
232                        let lo = this.token.span;
233                        // Parse `= 'lifetime`.
234                        this.bump(); // `=`
235                        this.bump(); // `'lifetime`
236                        let span = lo.to(this.prev_token.span);
237                        this.dcx().emit_err(UnexpectedDefaultValueForLifetimeInGenericParameters {
238                            span,
239                        });
240                    }
241
242                    Some(ast::GenericParam {
243                        ident: lifetime.ident,
244                        id: lifetime.id,
245                        attrs,
246                        bounds,
247                        kind: ast::GenericParamKind::Lifetime,
248                        is_placeholder: false,
249                        colon_span,
250                    })
251                } else if this.check_keyword(crate::parser::token_type::ExpKeywordPair {
    kw: rustc_span::symbol::kw::Const,
    token_type: crate::parser::token_type::TokenType::KwConst,
}exp!(Const)) {
252                    // Parse const parameter.
253                    Some(this.parse_const_param(attrs)?)
254                } else if this.check_ident() {
255                    // Parse type parameter.
256                    Some(this.parse_ty_param(attrs)?)
257                } else if this.token.can_begin_type() {
258                    // Trying to write an associated type bound? (#26271)
259                    let snapshot = this.create_snapshot_for_diagnostic();
260                    let lo = this.token.span;
261                    match this.parse_ty_where_predicate_kind() {
262                        Ok(_) => {
263                            this.dcx().emit_err(diagnostics::BadAssocTypeBounds {
264                                span: lo.to(this.prev_token.span),
265                            });
266                            // FIXME - try to continue parsing other generics?
267                        }
268                        Err(err) => {
269                            err.cancel();
270                            // FIXME - maybe we should overwrite 'self' outside of `collect_tokens`?
271                            this.restore_snapshot(snapshot);
272                        }
273                    }
274                    return Ok((None, Trailing::No, UsePreAttrPos::No));
275                } else {
276                    // Check for trailing attributes and stop parsing.
277                    if !attrs.is_empty() {
278                        if !params.is_empty() {
279                            this.dcx()
280                                .emit_err(diagnostics::AttrAfterGeneric { span: attrs[0].span });
281                        } else {
282                            this.dcx()
283                                .emit_err(diagnostics::AttrWithoutGenerics { span: attrs[0].span });
284                        }
285                    }
286                    return Ok((None, Trailing::No, UsePreAttrPos::No));
287                };
288
289                if !this.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Comma,
    token_type: crate::parser::token_type::TokenType::Comma,
}exp!(Comma)) {
290                    done = true;
291                }
292                // We just ate the comma, so no need to capture the trailing token.
293                Ok((param, Trailing::No, UsePreAttrPos::No))
294            }) {
295                Ok(param) => param,
296                Err(err) => {
297                    self.parsing_generics = prev;
298                    return Err(err);
299                }
300            };
301
302            if let Some(param) = param {
303                params.push(param);
304            } else {
305                break;
306            }
307        }
308        self.parsing_generics = prev;
309        Ok(params)
310    }
311
312    /// Parses a set of optional generic type parameter declarations. Where
313    /// clauses are not parsed here, and must be added later via
314    /// `parse_where_clause()`.
315    ///
316    /// matches generics = ( ) | ( < > ) | ( < typaramseq ( , )? > ) | ( < lifetimes ( , )? > )
317    ///                  | ( < lifetimes , typaramseq ( , )? > )
318    /// where   typaramseq = ( typaram ) | ( typaram , typaramseq )
319    pub(super) fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
320        // invalid path separator `::` in function definition
321        // for example `fn invalid_path_separator::<T>() {}`
322        if self.eat_noexpect(&token::PathSep) {
323            self.dcx()
324                .emit_err(diagnostics::InvalidPathSepInFnDefinition { span: self.prev_token.span });
325        }
326
327        let span_lo = self.token.span;
328        let (params, span) = if self.eat_lt() {
329            let params = self.parse_generic_params()?;
330            self.expect_gt_or_maybe_suggest_closing_generics(&params)?;
331            (params, span_lo.to(self.prev_token.span))
332        } else {
333            (ThinVec::new(), self.prev_token.span.shrink_to_hi())
334        };
335        Ok(ast::Generics {
336            params,
337            where_clause: WhereClause {
338                has_where_token: false,
339                predicates: ThinVec::new(),
340                span: self.prev_token.span.shrink_to_hi(),
341            },
342            span,
343        })
344    }
345
346    /// Parses an experimental fn contract
347    /// (`contract_requires(WWW) contract_ensures(ZZZ)`)
348    pub(super) fn parse_contract(&mut self) -> PResult<'a, Option<Box<ast::FnContract>>> {
349        let (declarations, requires) = self.parse_contract_requires()?;
350        let ensures = self.parse_contract_ensures()?;
351
352        if requires.is_none() && ensures.is_none() {
353            Ok(None)
354        } else {
355            Ok(Some(Box::new(ast::FnContract { declarations, requires, ensures })))
356        }
357    }
358
359    fn parse_contract_requires(
360        &mut self,
361    ) -> PResult<'a, (ThinVec<rustc_ast::Stmt>, Option<Box<rustc_ast::Expr>>)> {
362        Ok(if self.eat_keyword_noexpect(crate::parser::token_type::ExpKeywordPair {
    kw: rustc_span::symbol::kw::ContractRequires,
    token_type: crate::parser::token_type::TokenType::KwContractRequires,
}exp!(ContractRequires).kw) {
363            self.psess.gated_spans.gate(sym::contracts_internals, self.prev_token.span);
364            let mut decls_and_precond = self.parse_block()?;
365
366            let precond = match decls_and_precond.stmts.pop() {
367                Some(precond) => match precond.kind {
368                    rustc_ast::StmtKind::Expr(expr) => expr,
369                    // Insert dummy node that will be rejected by typechecker to
370                    // avoid reinventing an error
371                    _ => self.mk_unit_expr(decls_and_precond.span),
372                },
373                None => self.mk_unit_expr(decls_and_precond.span),
374            };
375            let precond = self.mk_closure_expr(precond.span, precond);
376            let decls = decls_and_precond.stmts;
377            (decls, Some(precond))
378        } else {
379            (Default::default(), None)
380        })
381    }
382
383    fn parse_contract_ensures(&mut self) -> PResult<'a, Option<Box<rustc_ast::Expr>>> {
384        Ok(if self.eat_keyword_noexpect(crate::parser::token_type::ExpKeywordPair {
    kw: rustc_span::symbol::kw::ContractEnsures,
    token_type: crate::parser::token_type::TokenType::KwContractEnsures,
}exp!(ContractEnsures).kw) {
385            self.psess.gated_spans.gate(sym::contracts_internals, self.prev_token.span);
386            let postcond = self.parse_expr()?;
387            Some(postcond)
388        } else {
389            None
390        })
391    }
392
393    /// Parses an optional where-clause.
394    ///
395    /// ```ignore (only-for-syntax-highlight)
396    /// where T : Trait<U, V> + 'b, 'a : 'b
397    /// ```
398    pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
399        self.parse_where_clause_common(None).map(|(clause, _)| clause)
400    }
401
402    pub(super) fn parse_struct_where_clause(
403        &mut self,
404        struct_name: Ident,
405        body_insertion_point: Span,
406    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
407        self.parse_where_clause_common(Some((struct_name, body_insertion_point)))
408    }
409
410    fn parse_where_clause_common(
411        &mut self,
412        struct_: Option<(Ident, Span)>,
413    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
414        let mut where_clause = WhereClause {
415            has_where_token: false,
416            predicates: ThinVec::new(),
417            span: self.prev_token.span.shrink_to_hi(),
418        };
419        let mut tuple_struct_body = None;
420
421        if !self.eat_keyword(crate::parser::token_type::ExpKeywordPair {
    kw: rustc_span::symbol::kw::Where,
    token_type: crate::parser::token_type::TokenType::KwWhere,
}exp!(Where)) {
422            return Ok((where_clause, None));
423        }
424
425        if self.eat_noexpect(&token::Colon) {
426            let colon_span = self.prev_token.span;
427            self.dcx()
428                .struct_span_err(colon_span, "unexpected colon after `where`")
429                .with_span_suggestion_short(
430                    colon_span,
431                    "remove the colon",
432                    "",
433                    Applicability::MachineApplicable,
434                )
435                .emit();
436        }
437
438        where_clause.has_where_token = true;
439        let where_lo = self.prev_token.span;
440
441        // We are considering adding generics to the `where` keyword as an alternative higher-rank
442        // parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
443        // change we parse those generics now, but report an error.
444        if self.choose_generics_over_qpath(0) {
445            let generics = self.parse_generics()?;
446            self.dcx().emit_err(diagnostics::WhereOnGenerics { span: generics.span });
447        }
448
449        loop {
450            let where_sp = where_lo.to(self.prev_token.span);
451            let attrs = self.parse_outer_attributes()?;
452            let pred_lo = self.token.span;
453            let predicate = self.collect_tokens(None, attrs, ForceCollect::No, |this, attrs| {
454                for attr in &attrs {
455                    self.psess.gated_spans.gate(sym::where_clause_attrs, attr.span);
456                }
457                let kind = if this.check_lifetime() && this.look_ahead(1, |t| !t.is_like_plus()) {
458                    let lifetime = this.expect_lifetime();
459                    // Bounds starting with a colon are mandatory, but possibly empty.
460                    this.expect(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon))?;
461                    let bounds = this.parse_lt_param_bounds();
462                    Some(ast::WherePredicateKind::RegionPredicate(ast::WhereRegionPredicate {
463                        lifetime,
464                        bounds,
465                    }))
466                } else if this.check_type() {
467                    match this.parse_ty_where_predicate_kind_or_recover_tuple_struct_body(
468                        struct_, pred_lo, where_sp,
469                    )? {
470                        PredicateKindOrStructBody::PredicateKind(kind) => Some(kind),
471                        PredicateKindOrStructBody::StructBody(body) => {
472                            tuple_struct_body = Some(body);
473                            None
474                        }
475                    }
476                } else {
477                    if let [.., last] = &attrs[..] {
478                        if last.is_doc_comment() {
479                            this.dcx().emit_err(diagnostics::DocCommentDoesNotDocumentAnything {
480                                span: last.span,
481                                missing_comma: None,
482                            });
483                        } else {
484                            this.dcx().emit_err(diagnostics::AttrWithoutWherePredicates {
485                                span: last.span,
486                            });
487                        }
488                    }
489                    None
490                };
491                let predicate = kind.map(|kind| ast::WherePredicate {
492                    attrs,
493                    kind,
494                    id: DUMMY_NODE_ID,
495                    span: pred_lo.to(this.prev_token.span),
496                    is_placeholder: false,
497                });
498                Ok((predicate, Trailing::No, UsePreAttrPos::No))
499            })?;
500            match predicate {
501                Some(predicate) => where_clause.predicates.push(predicate),
502                None => break,
503            }
504
505            let prev_token = self.prev_token.span;
506            let ate_comma = self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Comma,
    token_type: crate::parser::token_type::TokenType::Comma,
}exp!(Comma));
507
508            if self.eat_keyword_noexpect(kw::Where) {
509                self.dcx().emit_err(MultipleWhereClauses {
510                    span: self.token.span,
511                    previous: pred_lo,
512                    between: prev_token.shrink_to_hi().to(self.prev_token.span),
513                });
514            } else if !ate_comma {
515                break;
516            }
517        }
518
519        where_clause.span = where_lo.to(self.prev_token.span);
520        Ok((where_clause, tuple_struct_body))
521    }
522
523    fn parse_ty_where_predicate_kind_or_recover_tuple_struct_body(
524        &mut self,
525        struct_: Option<(Ident, Span)>,
526        pred_lo: Span,
527        where_sp: Span,
528    ) -> PResult<'a, PredicateKindOrStructBody> {
529        let mut snapshot = None;
530
531        if let Some(struct_) = struct_
532            && self.may_recover()
533            && self.token == token::OpenParen
534        {
535            snapshot = Some((struct_, self.create_snapshot_for_diagnostic()));
536        };
537
538        match self.parse_ty_where_predicate_kind() {
539            Ok(pred) => Ok(PredicateKindOrStructBody::PredicateKind(pred)),
540            Err(type_err) => {
541                let Some(((struct_name, body_insertion_point), mut snapshot)) = snapshot else {
542                    return Err(type_err);
543                };
544
545                // Check if we might have encountered an out of place tuple struct body.
546                match snapshot.parse_tuple_struct_body() {
547                    // Since we don't know the exact reason why we failed to parse the
548                    // predicate (we might have stumbled upon something bogus like `(T): ?`),
549                    // employ a simple heuristic to weed out some pathological cases:
550                    // Look for a semicolon (strong indicator) or anything that might mark
551                    // the end of the item (weak indicator) following the body.
552                    Ok(body)
553                        if #[allow(non_exhaustive_omitted_patterns)] match snapshot.token.kind {
    token::Semi | token::Eof => true,
    _ => false,
}matches!(snapshot.token.kind, token::Semi | token::Eof)
554                            || snapshot.token.can_begin_item() =>
555                    {
556                        type_err.cancel();
557
558                        let body_sp = pred_lo.to(snapshot.prev_token.span);
559                        let map = self.psess.source_map();
560
561                        self.dcx().emit_err(WhereClauseBeforeTupleStructBody {
562                            span: where_sp,
563                            name: struct_name.span,
564                            body: body_sp,
565                            sugg: map.span_to_snippet(body_sp).ok().map(|body| {
566                                WhereClauseBeforeTupleStructBodySugg {
567                                    left: body_insertion_point.shrink_to_hi(),
568                                    snippet: body,
569                                    right: map.end_point(where_sp).to(body_sp),
570                                }
571                            }),
572                        });
573
574                        self.restore_snapshot(snapshot);
575                        Ok(PredicateKindOrStructBody::StructBody(body))
576                    }
577                    Ok(_) => Err(type_err),
578                    Err(body_err) => {
579                        body_err.cancel();
580                        Err(type_err)
581                    }
582                }
583            }
584        }
585    }
586
587    fn parse_ty_where_predicate_kind(&mut self) -> PResult<'a, ast::WherePredicateKind> {
588        // Parse optional `for<'a, 'b>`.
589        // This `for` is parsed greedily and applies to the whole predicate,
590        // the bounded type can have its own `for` applying only to it.
591        // Examples:
592        // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
593        // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
594        // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
595        let (bound_vars, _) = self.parse_higher_ranked_binder()?;
596
597        let ty = self.parse_ty_for_where_clause()?;
598
599        if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Colon,
    token_type: crate::parser::token_type::TokenType::Colon,
}exp!(Colon)) {
600            // The bounds may be empty; we intentionally accept predicates like  `Ty:`.
601            let bounds = self.parse_generic_bounds()?;
602
603            return Ok(ast::WherePredicateKind::BoundPredicate(ast::WhereBoundPredicate {
604                bound_generic_params: bound_vars,
605                bounded_ty: ty,
606                bounds,
607            }));
608        }
609
610        // NOTE: If we ever end up impl'ing and stabilizing equality predicates (#20041),
611        //       we need to pick between `=` and `==`, both is not an option!
612        if self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::Eq,
    token_type: crate::parser::token_type::TokenType::Eq,
}exp!(Eq)) || self.eat(crate::parser::token_type::ExpTokenPair {
    tok: rustc_ast::token::EqEq,
    token_type: crate::parser::token_type::TokenType::EqEq,
}exp!(EqEq)) {
613            let lhs_ty = ty;
614            let rhs_ty = self.parse_ty()?;
615
616            // NOTE: If we ever end up impl'ing equality predicates,
617            //       we ought to track the binder in the AST node!
618            let _ = bound_vars;
619
620            let mut diag = self.dcx().struct_span_err(
621                lhs_ty.span.to(rhs_ty.span),
622                "general type equality constraints are not supported",
623            );
624            diag.note(
625                "see issue #20041 <https://github.com/rust-lang/rust/issues/20041> \
626                 for more information",
627            );
628            diag.span(lhs_ty.span.to(rhs_ty.span));
629            diag.span_label(lhs_ty.span.to(rhs_ty.span), "not supported");
630
631            suggest_replacing_equality_pred_with_assoc_item_constraint(&mut diag, *lhs_ty, *rhs_ty);
632
633            return Err(diag);
634        }
635
636        self.maybe_recover_bounds_doubled_colon(&ty)?;
637        self.unexpected_any()
638    }
639
640    pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
641        // There's an ambiguity between generic parameters and qualified paths in impls.
642        // If we see `<` it may start both, so we have to inspect some following tokens.
643        // The following combinations can only start generics,
644        // but not qualified paths (with one exception):
645        //     `<` `>` - empty generic parameters
646        //     `<` `#` - generic parameters with attributes
647        //     `<` (LIFETIME|IDENT) `>` - single generic parameter
648        //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
649        //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
650        //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
651        //     `<` const                - generic const parameter
652        //     `<` IDENT `?`            - RECOVERY for `impl<T ?Bound` missing a `:`, meant to
653        //                                avoid the `T?` to `Option<T>` recovery for types.
654        // The only truly ambiguous case is
655        //     `<` IDENT `>` `::` IDENT ...
656        // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
657        // because this is what almost always expected in practice, qualified paths in impls
658        // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
659        self.look_ahead(start, |t| t == &token::Lt)
660            && (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
661                || self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
662                    && self.look_ahead(start + 2, |t| {
663                        #[allow(non_exhaustive_omitted_patterns)] match t.kind {
    token::Gt | token::Comma | token::Colon | token::Eq => true,
    _ => false,
}matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
664                        // Recovery-only branch -- this could be removed,
665                        // since it only affects diagnostics currently.
666                            || t.kind == token::Question
667                    })
668                || self.is_keyword_ahead(start + 1, &[kw::Const]))
669    }
670}
671
672fn suggest_replacing_equality_pred_with_assoc_item_constraint(
673    diag: &mut Diag<'_>,
674    lhs_ty: ast::Ty,
675    rhs_ty: ast::Ty,
676) {
677    let TyKind::Path(qself, ast::Path { segments, .. }) = lhs_ty.kind else { return };
678
679    let mut parts = Vec::new();
680    let applicability = match qself {
681        // We have something like `Ty::Item<i32> = Rhs`.
682        None if let [self_ty_seg, assoc_item_seg] = &segments[..]
683            && self_ty_seg.ident.name != kw::PathRoot =>
684        {
685            parts.push((
686                self_ty_seg.span().between(assoc_item_seg.span()),
687                ": /* Trait */</* ... */".into(),
688            ));
689            Applicability::HasPlaceholders
690        }
691        Some(qself) if let [assoc_item_seg] = &segments[qself.position..] => {
692            parts.push((lhs_ty.span.until(qself.ty.span), String::new()));
693
694            // We have something like `<Option<usize> as self::Trait<i32>>::Item = Rhs`.
695            if let trait_segs @ [.., final_trait_seg] = &segments[..qself.position] {
696                parts.push((qself.ty.span.between(trait_segs[0].span()), ": ".into()));
697                let (span, snippet) = match &final_trait_seg.args {
698                    Some(args) => {
699                        let ast::GenericArgs::AngleBracketed(args) = args else { return };
700                        let Some(args) = args.args.last() else { return };
701                        (args.span(), ", ")
702                    }
703                    None => (final_trait_seg.span(), "<"),
704                };
705                parts.push((span.between(assoc_item_seg.span()), snippet.into()));
706                Applicability::MaybeIncorrect
707            }
708            // We have something like `<[u8]>::Item == Rhs`.
709            else {
710                parts.push((
711                    qself.ty.span.between(assoc_item_seg.span()),
712                    ": /* Trait */</* ... */".into(),
713                ));
714                Applicability::HasPlaceholders
715            }
716        }
717        _ => return,
718    };
719
720    parts.push((lhs_ty.span.between(rhs_ty.span), " = ".into()));
721    parts.push((rhs_ty.span.shrink_to_hi(), ">".into()));
722
723    diag.multipart_suggestion(
724        "replace it with an associated item constraint if possible",
725        parts,
726        applicability,
727    );
728}
rustc_parse/parser/generics.rs

rustc_parse/parser/
generics.rs
