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, PResult};
6use rustc_span::{Ident, Span, kw, sym};
7use thin_vec::ThinVec;
8
9use super::{ForceCollect, Parser, Trailing, UsePreAttrPos};
10use crate::errors::{
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 = Vec::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(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            Vec::new()
90        };
91
92        let default = if self.eat(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(exp!(Const))?;
111        let ident = self.parse_ident()?;
112        self.expect(exp!(Colon))?;
113        let ty = self.parse_ty()?;
114
115        // Parse optional const generics default value.
116        let default = if self.eat(exp!(Eq)) { Some(self.parse_const_arg()?) } else { None };
117        let span = if let Some(ref default) = default {
118            const_span.to(default.value.span)
119        } else {
120            const_span.to(ty.span)
121        };
122
123        Ok(GenericParam {
124            ident,
125            id: ast::DUMMY_NODE_ID,
126            attrs: preceding_attrs,
127            bounds: Vec::new(),
128            kind: GenericParamKind::Const { ty, span, default },
129            is_placeholder: false,
130            colon_span: None,
131        })
132    }
133
134    pub(crate) fn recover_const_param_with_mistyped_const(
135        &mut self,
136        preceding_attrs: AttrVec,
137        mistyped_const_ident: Ident,
138    ) -> PResult<'a, GenericParam> {
139        let ident = self.parse_ident()?;
140        self.expect(exp!(Colon))?;
141        let ty = self.parse_ty()?;
142
143        // Parse optional const generics default value.
144        let default = if self.eat(exp!(Eq)) { Some(self.parse_const_arg()?) } else { None };
145        let span = if let Some(ref default) = default {
146            mistyped_const_ident.span.to(default.value.span)
147        } else {
148            mistyped_const_ident.span.to(ty.span)
149        };
150
151        self.dcx()
152            .struct_span_err(
153                mistyped_const_ident.span,
154                format!("`const` keyword was mistyped as `{}`", mistyped_const_ident.as_str()),
155            )
156            .with_span_suggestion_verbose(
157                mistyped_const_ident.span,
158                "use the `const` keyword",
159                kw::Const,
160                Applicability::MachineApplicable,
161            )
162            .emit();
163
164        Ok(GenericParam {
165            ident,
166            id: ast::DUMMY_NODE_ID,
167            attrs: preceding_attrs,
168            bounds: Vec::new(),
169            kind: GenericParamKind::Const { ty, span, default },
170            is_placeholder: false,
171            colon_span: None,
172        })
173    }
174
175    /// Parse a (possibly empty) list of generic (lifetime, type, const) parameters.
176    ///
177    /// ```ebnf
178    /// GenericParams = (GenericParam ("," GenericParam)* ","?)?
179    /// ```
180    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {
181        let mut params = ThinVec::new();
182        let mut done = false;
183        while !done {
184            let attrs = self.parse_outer_attributes()?;
185            let param = self.collect_tokens(None, attrs, ForceCollect::No, |this, attrs| {
186                if this.eat_keyword_noexpect(kw::SelfUpper) {
187                    // `Self` as a generic param is invalid. Here we emit the diagnostic and continue parsing
188                    // as if `Self` never existed.
189                    this.dcx()
190                        .emit_err(UnexpectedSelfInGenericParameters { span: this.prev_token.span });
191
192                    // Eat a trailing comma, if it exists.
193                    let _ = this.eat(exp!(Comma));
194                }
195
196                let param = if this.check_lifetime() {
197                    let lifetime = this.expect_lifetime();
198                    // Parse lifetime parameter.
199                    let (colon_span, bounds) = if this.eat(exp!(Colon)) {
200                        (Some(this.prev_token.span), this.parse_lt_param_bounds())
201                    } else {
202                        (None, Vec::new())
203                    };
204
205                    if this.check_noexpect(&token::Eq) && this.look_ahead(1, |t| t.is_lifetime()) {
206                        let lo = this.token.span;
207                        // Parse `= 'lifetime`.
208                        this.bump(); // `=`
209                        this.bump(); // `'lifetime`
210                        let span = lo.to(this.prev_token.span);
211                        this.dcx().emit_err(UnexpectedDefaultValueForLifetimeInGenericParameters {
212                            span,
213                        });
214                    }
215
216                    Some(ast::GenericParam {
217                        ident: lifetime.ident,
218                        id: lifetime.id,
219                        attrs,
220                        bounds,
221                        kind: ast::GenericParamKind::Lifetime,
222                        is_placeholder: false,
223                        colon_span,
224                    })
225                } else if this.check_keyword(exp!(Const)) {
226                    // Parse const parameter.
227                    Some(this.parse_const_param(attrs)?)
228                } else if this.check_ident() {
229                    // Parse type parameter.
230                    Some(this.parse_ty_param(attrs)?)
231                } else if this.token.can_begin_type() {
232                    // Trying to write an associated type bound? (#26271)
233                    let snapshot = this.create_snapshot_for_diagnostic();
234                    let lo = this.token.span;
235                    match this.parse_ty_where_predicate_kind() {
236                        Ok(_) => {
237                            this.dcx().emit_err(errors::BadAssocTypeBounds {
238                                span: lo.to(this.prev_token.span),
239                            });
240                            // FIXME - try to continue parsing other generics?
241                        }
242                        Err(err) => {
243                            err.cancel();
244                            // FIXME - maybe we should overwrite 'self' outside of `collect_tokens`?
245                            this.restore_snapshot(snapshot);
246                        }
247                    }
248                    return Ok((None, Trailing::No, UsePreAttrPos::No));
249                } else {
250                    // Check for trailing attributes and stop parsing.
251                    if !attrs.is_empty() {
252                        if !params.is_empty() {
253                            this.dcx().emit_err(errors::AttrAfterGeneric { span: attrs[0].span });
254                        } else {
255                            this.dcx()
256                                .emit_err(errors::AttrWithoutGenerics { span: attrs[0].span });
257                        }
258                    }
259                    return Ok((None, Trailing::No, UsePreAttrPos::No));
260                };
261
262                if !this.eat(exp!(Comma)) {
263                    done = true;
264                }
265                // We just ate the comma, so no need to capture the trailing token.
266                Ok((param, Trailing::No, UsePreAttrPos::No))
267            })?;
268
269            if let Some(param) = param {
270                params.push(param);
271            } else {
272                break;
273            }
274        }
275        Ok(params)
276    }
277
278    /// Parses a set of optional generic type parameter declarations. Where
279    /// clauses are not parsed here, and must be added later via
280    /// `parse_where_clause()`.
281    ///
282    /// matches generics = ( ) | ( < > ) | ( < typaramseq ( , )? > ) | ( < lifetimes ( , )? > )
283    ///                  | ( < lifetimes , typaramseq ( , )? > )
284    /// where   typaramseq = ( typaram ) | ( typaram , typaramseq )
285    pub(super) fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
286        // invalid path separator `::` in function definition
287        // for example `fn invalid_path_separator::<T>() {}`
288        if self.eat_noexpect(&token::PathSep) {
289            self.dcx()
290                .emit_err(errors::InvalidPathSepInFnDefinition { span: self.prev_token.span });
291        }
292
293        let span_lo = self.token.span;
294        let (params, span) = if self.eat_lt() {
295            let params = self.parse_generic_params()?;
296            self.expect_gt_or_maybe_suggest_closing_generics(&params)?;
297            (params, span_lo.to(self.prev_token.span))
298        } else {
299            (ThinVec::new(), self.prev_token.span.shrink_to_hi())
300        };
301        Ok(ast::Generics {
302            params,
303            where_clause: WhereClause {
304                has_where_token: false,
305                predicates: ThinVec::new(),
306                span: self.prev_token.span.shrink_to_hi(),
307            },
308            span,
309        })
310    }
311
312    /// Parses an experimental fn contract
313    /// (`contract_requires(WWW) contract_ensures(ZZZ)`)
314    pub(super) fn parse_contract(&mut self) -> PResult<'a, Option<Box<ast::FnContract>>> {
315        let (declarations, requires) = self.parse_contract_requires()?;
316        let ensures = self.parse_contract_ensures()?;
317
318        if requires.is_none() && ensures.is_none() {
319            Ok(None)
320        } else {
321            Ok(Some(Box::new(ast::FnContract { declarations, requires, ensures })))
322        }
323    }
324
325    fn parse_contract_requires(
326        &mut self,
327    ) -> PResult<'a, (ThinVec<rustc_ast::Stmt>, Option<Box<rustc_ast::Expr>>)> {
328        Ok(if self.eat_keyword_noexpect(exp!(ContractRequires).kw) {
329            self.psess.gated_spans.gate(sym::contracts_internals, self.prev_token.span);
330            let mut decls_and_precond = self.parse_block()?;
331
332            let precond = match decls_and_precond.stmts.pop() {
333                Some(precond) => match precond.kind {
334                    rustc_ast::StmtKind::Expr(expr) => expr,
335                    // Insert dummy node that will be rejected by typechecker to
336                    // avoid reinventing an error
337                    _ => self.mk_unit_expr(decls_and_precond.span),
338                },
339                None => self.mk_unit_expr(decls_and_precond.span),
340            };
341            let precond = self.mk_closure_expr(precond.span, precond);
342            let decls = decls_and_precond.stmts;
343            (decls, Some(precond))
344        } else {
345            (Default::default(), None)
346        })
347    }
348
349    fn parse_contract_ensures(&mut self) -> PResult<'a, Option<Box<rustc_ast::Expr>>> {
350        Ok(if self.eat_keyword_noexpect(exp!(ContractEnsures).kw) {
351            self.psess.gated_spans.gate(sym::contracts_internals, self.prev_token.span);
352            let postcond = self.parse_expr()?;
353            Some(postcond)
354        } else {
355            None
356        })
357    }
358
359    /// Parses an optional where-clause.
360    ///
361    /// ```ignore (only-for-syntax-highlight)
362    /// where T : Trait<U, V> + 'b, 'a : 'b
363    /// ```
364    pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
365        self.parse_where_clause_common(None).map(|(clause, _)| clause)
366    }
367
368    pub(super) fn parse_struct_where_clause(
369        &mut self,
370        struct_name: Ident,
371        body_insertion_point: Span,
372    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
373        self.parse_where_clause_common(Some((struct_name, body_insertion_point)))
374    }
375
376    fn parse_where_clause_common(
377        &mut self,
378        struct_: Option<(Ident, Span)>,
379    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
380        let mut where_clause = WhereClause {
381            has_where_token: false,
382            predicates: ThinVec::new(),
383            span: self.prev_token.span.shrink_to_hi(),
384        };
385        let mut tuple_struct_body = None;
386
387        if !self.eat_keyword(exp!(Where)) {
388            return Ok((where_clause, None));
389        }
390
391        if self.eat_noexpect(&token::Colon) {
392            let colon_span = self.prev_token.span;
393            self.dcx()
394                .struct_span_err(colon_span, "unexpected colon after `where`")
395                .with_span_suggestion_short(
396                    colon_span,
397                    "remove the colon",
398                    "",
399                    Applicability::MachineApplicable,
400                )
401                .emit();
402        }
403
404        where_clause.has_where_token = true;
405        let where_lo = self.prev_token.span;
406
407        // We are considering adding generics to the `where` keyword as an alternative higher-rank
408        // parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
409        // change we parse those generics now, but report an error.
410        if self.choose_generics_over_qpath(0) {
411            let generics = self.parse_generics()?;
412            self.dcx().emit_err(errors::WhereOnGenerics { span: generics.span });
413        }
414
415        loop {
416            let where_sp = where_lo.to(self.prev_token.span);
417            let attrs = self.parse_outer_attributes()?;
418            let pred_lo = self.token.span;
419            let predicate = self.collect_tokens(None, attrs, ForceCollect::No, |this, attrs| {
420                for attr in &attrs {
421                    self.psess.gated_spans.gate(sym::where_clause_attrs, attr.span);
422                }
423                let kind = if this.check_lifetime() && this.look_ahead(1, |t| !t.is_like_plus()) {
424                    let lifetime = this.expect_lifetime();
425                    // Bounds starting with a colon are mandatory, but possibly empty.
426                    this.expect(exp!(Colon))?;
427                    let bounds = this.parse_lt_param_bounds();
428                    Some(ast::WherePredicateKind::RegionPredicate(ast::WhereRegionPredicate {
429                        lifetime,
430                        bounds,
431                    }))
432                } else if this.check_type() {
433                    match this.parse_ty_where_predicate_kind_or_recover_tuple_struct_body(
434                        struct_, pred_lo, where_sp,
435                    )? {
436                        PredicateKindOrStructBody::PredicateKind(kind) => Some(kind),
437                        PredicateKindOrStructBody::StructBody(body) => {
438                            tuple_struct_body = Some(body);
439                            None
440                        }
441                    }
442                } else {
443                    None
444                };
445                let predicate = kind.map(|kind| ast::WherePredicate {
446                    attrs,
447                    kind,
448                    id: DUMMY_NODE_ID,
449                    span: pred_lo.to(this.prev_token.span),
450                    is_placeholder: false,
451                });
452                Ok((predicate, Trailing::No, UsePreAttrPos::No))
453            })?;
454            match predicate {
455                Some(predicate) => where_clause.predicates.push(predicate),
456                None => break,
457            }
458
459            let prev_token = self.prev_token.span;
460            let ate_comma = self.eat(exp!(Comma));
461
462            if self.eat_keyword_noexpect(kw::Where) {
463                self.dcx().emit_err(MultipleWhereClauses {
464                    span: self.token.span,
465                    previous: pred_lo,
466                    between: prev_token.shrink_to_hi().to(self.prev_token.span),
467                });
468            } else if !ate_comma {
469                break;
470            }
471        }
472
473        where_clause.span = where_lo.to(self.prev_token.span);
474        Ok((where_clause, tuple_struct_body))
475    }
476
477    fn parse_ty_where_predicate_kind_or_recover_tuple_struct_body(
478        &mut self,
479        struct_: Option<(Ident, Span)>,
480        pred_lo: Span,
481        where_sp: Span,
482    ) -> PResult<'a, PredicateKindOrStructBody> {
483        let mut snapshot = None;
484
485        if let Some(struct_) = struct_
486            && self.may_recover()
487            && self.token == token::OpenParen
488        {
489            snapshot = Some((struct_, self.create_snapshot_for_diagnostic()));
490        };
491
492        match self.parse_ty_where_predicate_kind() {
493            Ok(pred) => Ok(PredicateKindOrStructBody::PredicateKind(pred)),
494            Err(type_err) => {
495                let Some(((struct_name, body_insertion_point), mut snapshot)) = snapshot else {
496                    return Err(type_err);
497                };
498
499                // Check if we might have encountered an out of place tuple struct body.
500                match snapshot.parse_tuple_struct_body() {
501                    // Since we don't know the exact reason why we failed to parse the
502                    // predicate (we might have stumbled upon something bogus like `(T): ?`),
503                    // employ a simple heuristic to weed out some pathological cases:
504                    // Look for a semicolon (strong indicator) or anything that might mark
505                    // the end of the item (weak indicator) following the body.
506                    Ok(body)
507                        if matches!(snapshot.token.kind, token::Semi | token::Eof)
508                            || snapshot.token.can_begin_item() =>
509                    {
510                        type_err.cancel();
511
512                        let body_sp = pred_lo.to(snapshot.prev_token.span);
513                        let map = self.psess.source_map();
514
515                        self.dcx().emit_err(WhereClauseBeforeTupleStructBody {
516                            span: where_sp,
517                            name: struct_name.span,
518                            body: body_sp,
519                            sugg: map.span_to_snippet(body_sp).ok().map(|body| {
520                                WhereClauseBeforeTupleStructBodySugg {
521                                    left: body_insertion_point.shrink_to_hi(),
522                                    snippet: body,
523                                    right: map.end_point(where_sp).to(body_sp),
524                                }
525                            }),
526                        });
527
528                        self.restore_snapshot(snapshot);
529                        Ok(PredicateKindOrStructBody::StructBody(body))
530                    }
531                    Ok(_) => Err(type_err),
532                    Err(body_err) => {
533                        body_err.cancel();
534                        Err(type_err)
535                    }
536                }
537            }
538        }
539    }
540
541    fn parse_ty_where_predicate_kind(&mut self) -> PResult<'a, ast::WherePredicateKind> {
542        // Parse optional `for<'a, 'b>`.
543        // This `for` is parsed greedily and applies to the whole predicate,
544        // the bounded type can have its own `for` applying only to it.
545        // Examples:
546        // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
547        // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
548        // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
549        let (bound_vars, _) = self.parse_higher_ranked_binder()?;
550
551        // Parse type with mandatory colon and (possibly empty) bounds,
552        // or with mandatory equality sign and the second type.
553        let ty = self.parse_ty_for_where_clause()?;
554        if self.eat(exp!(Colon)) {
555            let bounds = self.parse_generic_bounds()?;
556            Ok(ast::WherePredicateKind::BoundPredicate(ast::WhereBoundPredicate {
557                bound_generic_params: bound_vars,
558                bounded_ty: ty,
559                bounds,
560            }))
561        // FIXME: Decide what should be used here, `=` or `==`.
562        // FIXME: We are just dropping the binders in lifetime_defs on the floor here.
563        } else if self.eat(exp!(Eq)) || self.eat(exp!(EqEq)) {
564            let rhs_ty = self.parse_ty()?;
565            Ok(ast::WherePredicateKind::EqPredicate(ast::WhereEqPredicate { lhs_ty: ty, rhs_ty }))
566        } else {
567            self.maybe_recover_bounds_doubled_colon(&ty)?;
568            self.unexpected_any()
569        }
570    }
571
572    pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
573        // There's an ambiguity between generic parameters and qualified paths in impls.
574        // If we see `<` it may start both, so we have to inspect some following tokens.
575        // The following combinations can only start generics,
576        // but not qualified paths (with one exception):
577        //     `<` `>` - empty generic parameters
578        //     `<` `#` - generic parameters with attributes
579        //     `<` (LIFETIME|IDENT) `>` - single generic parameter
580        //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
581        //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
582        //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
583        //     `<` const                - generic const parameter
584        //     `<` IDENT `?`            - RECOVERY for `impl<T ?Bound` missing a `:`, meant to
585        //                                avoid the `T?` to `Option<T>` recovery for types.
586        // The only truly ambiguous case is
587        //     `<` IDENT `>` `::` IDENT ...
588        // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
589        // because this is what almost always expected in practice, qualified paths in impls
590        // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
591        self.look_ahead(start, |t| t == &token::Lt)
592            && (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
593                || self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
594                    && self.look_ahead(start + 2, |t| {
595                        matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
596                        // Recovery-only branch -- this could be removed,
597                        // since it only affects diagnostics currently.
598                            || t.kind == token::Question
599                    })
600                || self.is_keyword_ahead(start + 1, &[kw::Const]))
601    }
602}
rustc_parse/parser/generics.rs

rustc_parse/parser/
generics.rs
