compiler/rustc_parse/src/parser/generics.rs

   1 use super::{ForceCollect, Parser, TrailingToken};
   2
   3 use rustc_ast::token;
   4 use rustc_ast::{
   5     self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
   6 };
   7 use rustc_errors::{Applicability, PResult};
   8 use rustc_span::symbol::kw;
   9
  10 impl<'a> Parser<'a> {
  11     /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
  12     ///
  13     /// ```text
  14     /// BOUND = LT_BOUND (e.g., `'a`)
  15     /// ```
  16     fn parse_lt_param_bounds(&mut self) -> GenericBounds {
  17         let mut lifetimes = Vec::new();
  18         while self.check_lifetime() {
  19             lifetimes.push(ast::GenericBound::Outlives(self.expect_lifetime()));
  20
  21             if !self.eat_plus() {
  22                 break;
  23             }
  24         }
  25         lifetimes
  26     }
  27
  28     /// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
  29     fn parse_ty_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
  30         let ident = self.parse_ident()?;
  31
  32         // Parse optional colon and param bounds.
  33         let bounds = if self.eat(&token::Colon) {
  34             self.parse_generic_bounds(Some(self.prev_token.span))?
  35         } else {
  36             Vec::new()
  37         };
  38
  39         let default = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
  40
  41         Ok(GenericParam {
  42             ident,
  43             id: ast::DUMMY_NODE_ID,
  44             attrs: preceding_attrs.into(),
  45             bounds,
  46             kind: GenericParamKind::Type { default },
  47             is_placeholder: false,
  48         })
  49     }
  50
  51     crate fn parse_const_param(
  52         &mut self,
  53         preceding_attrs: Vec<Attribute>,
  54     ) -> PResult<'a, GenericParam> {
  55         let const_span = self.token.span;
  56
  57         self.expect_keyword(kw::Const)?;
  58         let ident = self.parse_ident()?;
  59         self.expect(&token::Colon)?;
  60         let ty = self.parse_ty()?;
  61
  62         // Parse optional const generics default value.
  63         let default = if self.eat(&token::Eq) { Some(self.parse_const_arg()?) } else { None };
  64
  65         Ok(GenericParam {
  66             ident,
  67             id: ast::DUMMY_NODE_ID,
  68             attrs: preceding_attrs.into(),
  69             bounds: Vec::new(),
  70             kind: GenericParamKind::Const { ty, kw_span: const_span, default },
  71             is_placeholder: false,
  72         })
  73     }
  74
  75     /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
  76     /// a trailing comma and erroneous trailing attributes.
  77     pub(super) fn parse_generic_params(&mut self) -> PResult<'a, Vec<ast::GenericParam>> {
  78         let mut params = Vec::new();
  79         let mut done = false;
  80         while !done {
  81             let attrs = self.parse_outer_attributes()?;
  82             let param =
  83                 self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
  84                     if this.eat_keyword_noexpect(kw::SelfUpper) {
  85                         // `Self` as a generic param is invalid. Here we emit the diagnostic and continue parsing
  86                         // as if `Self` never existed.
  87                         this.struct_span_err(
  88                             this.prev_token.span,
  89                             "unexpected keyword `Self` in generic parameters",
  90                         )
  91                         .note("you cannot use `Self` as a generic parameter because it is reserved for associated items")
  92                         .emit();
  93
  94                         this.eat(&token::Comma);
  95                     }
  96
  97                     let param = if this.check_lifetime() {
  98                         let lifetime = this.expect_lifetime();
  99                         // Parse lifetime parameter.
 100                         let bounds = if this.eat(&token::Colon) {
 101                             this.parse_lt_param_bounds()
 102                         } else {
 103                             Vec::new()
 104                         };
 105                         Some(ast::GenericParam {
 106                             ident: lifetime.ident,
 107                             id: lifetime.id,
 108                             attrs: attrs.into(),
 109                             bounds,
 110                             kind: ast::GenericParamKind::Lifetime,
 111                             is_placeholder: false,
 112                         })
 113                     } else if this.check_keyword(kw::Const) {
 114                         // Parse const parameter.
 115                         Some(this.parse_const_param(attrs)?)
 116                     } else if this.check_ident() {
 117                         // Parse type parameter.
 118                         Some(this.parse_ty_param(attrs)?)
 119                     } else if this.token.can_begin_type() {
 120                         // Trying to write an associated type bound? (#26271)
 121                         let snapshot = this.create_snapshot_for_diagnostic();
 122                         match this.parse_ty_where_predicate() {
 123                             Ok(where_predicate) => {
 124                                 this.struct_span_err(
 125                                     where_predicate.span(),
 126                                     "bounds on associated types do not belong here",
 127                                 )
 128                                 .span_label(where_predicate.span(), "belongs in `where` clause")
 129                                 .emit();
 130                                 // FIXME - try to continue parsing other generics?
 131                                 return Ok((None, TrailingToken::None));
 132                             }
 133                             Err(err) => {
 134                                 err.cancel();
 135                                 // FIXME - maybe we should overwrite 'self' outside of `collect_tokens`?
 136                                 this.restore_snapshot(snapshot);
 137                                 return Ok((None, TrailingToken::None));
 138                             }
 139                         }
 140                     } else {
 141                         // Check for trailing attributes and stop parsing.
 142                         if !attrs.is_empty() {
 143                             if !params.is_empty() {
 144                                 this.struct_span_err(
 145                                     attrs[0].span,
 146                                     "trailing attribute after generic parameter",
 147                                 )
 148                                 .span_label(attrs[0].span, "attributes must go before parameters")
 149                                 .emit();
 150                             } else {
 151                                 this.struct_span_err(
 152                                     attrs[0].span,
 153                                     "attribute without generic parameters",
 154                                 )
 155                                 .span_label(
 156                                     attrs[0].span,
 157                                     "attributes are only permitted when preceding parameters",
 158                                 )
 159                                 .emit();
 160                             }
 161                         }
 162                         return Ok((None, TrailingToken::None));
 163                     };
 164
 165                     if !this.eat(&token::Comma) {
 166                         done = true;
 167                     }
 168                     // We just ate the comma, so no need to use `TrailingToken`
 169                     Ok((param, TrailingToken::None))
 170                 })?;
 171
 172             if let Some(param) = param {
 173                 params.push(param);
 174             } else {
 175                 break;
 176             }
 177         }
 178         Ok(params)
 179     }
 180
 181     /// Parses a set of optional generic type parameter declarations. Where
 182     /// clauses are not parsed here, and must be added later via
 183     /// `parse_where_clause()`.
 184     ///
 185     /// matches generics = ( ) | ( < > ) | ( < typaramseq ( , )? > ) | ( < lifetimes ( , )? > )
 186     ///                  | ( < lifetimes , typaramseq ( , )? > )
 187     /// where   typaramseq = ( typaram ) | ( typaram , typaramseq )
 188     pub(super) fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
 189         let span_lo = self.token.span;
 190         let (params, span) = if self.eat_lt() {
 191             let params = self.parse_generic_params()?;
 192             self.expect_gt()?;
 193             (params, span_lo.to(self.prev_token.span))
 194         } else {
 195             (vec![], self.prev_token.span.shrink_to_hi())
 196         };
 197         Ok(ast::Generics {
 198             params,
 199             where_clause: WhereClause {
 200                 has_where_token: false,
 201                 predicates: Vec::new(),
 202                 span: self.prev_token.span.shrink_to_hi(),
 203             },
 204             span,
 205         })
 206     }
 207
 208     /// Parses an optional where-clause and places it in `generics`.
 209     ///
 210     /// ```ignore (only-for-syntax-highlight)
 211     /// where T : Trait<U, V> + 'b, 'a : 'b
 212     /// ```
 213     pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
 214         let mut where_clause = WhereClause {
 215             has_where_token: false,
 216             predicates: Vec::new(),
 217             span: self.prev_token.span.shrink_to_hi(),
 218         };
 219
 220         if !self.eat_keyword(kw::Where) {
 221             return Ok(where_clause);
 222         }
 223         where_clause.has_where_token = true;
 224         let lo = self.prev_token.span;
 225
 226         // We are considering adding generics to the `where` keyword as an alternative higher-rank
 227         // parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
 228         // change we parse those generics now, but report an error.
 229         if self.choose_generics_over_qpath(0) {
 230             let generics = self.parse_generics()?;
 231             self.struct_span_err(
 232                 generics.span,
 233                 "generic parameters on `where` clauses are reserved for future use",
 234             )
 235             .span_label(generics.span, "currently unsupported")
 236             .emit();
 237         }
 238
 239         loop {
 240             let lo = self.token.span;
 241             if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
 242                 let lifetime = self.expect_lifetime();
 243                 // Bounds starting with a colon are mandatory, but possibly empty.
 244                 self.expect(&token::Colon)?;
 245                 let bounds = self.parse_lt_param_bounds();
 246                 where_clause.predicates.push(ast::WherePredicate::RegionPredicate(
 247                     ast::WhereRegionPredicate {
 248                         span: lo.to(self.prev_token.span),
 249                         lifetime,
 250                         bounds,
 251                     },
 252                 ));
 253             } else if self.check_type() {
 254                 where_clause.predicates.push(self.parse_ty_where_predicate()?);
 255             } else {
 256                 break;
 257             }
 258
 259             let prev_token = self.prev_token.span;
 260             let ate_comma = self.eat(&token::Comma);
 261
 262             if self.eat_keyword_noexpect(kw::Where) {
 263                 let msg = "cannot define duplicate `where` clauses on an item";
 264                 let mut err = self.struct_span_err(self.token.span, msg);
 265                 err.span_label(lo, "previous `where` clause starts here");
 266                 err.span_suggestion_verbose(
 267                     prev_token.shrink_to_hi().to(self.prev_token.span),
 268                     "consider joining the two `where` clauses into one",
 269                     ",".to_owned(),
 270                     Applicability::MaybeIncorrect,
 271                 );
 272                 err.emit();
 273             } else if !ate_comma {
 274                 break;
 275             }
 276         }
 277
 278         where_clause.span = lo.to(self.prev_token.span);
 279         Ok(where_clause)
 280     }
 281
 282     fn parse_ty_where_predicate(&mut self) -> PResult<'a, ast::WherePredicate> {
 283         let lo = self.token.span;
 284         // Parse optional `for<'a, 'b>`.
 285         // This `for` is parsed greedily and applies to the whole predicate,
 286         // the bounded type can have its own `for` applying only to it.
 287         // Examples:
 288         // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
 289         // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
 290         // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
 291         let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 292
 293         // Parse type with mandatory colon and (possibly empty) bounds,
 294         // or with mandatory equality sign and the second type.
 295         let ty = self.parse_ty_for_where_clause()?;
 296         if self.eat(&token::Colon) {
 297             let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
 298             Ok(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
 299                 span: lo.to(self.prev_token.span),
 300                 bound_generic_params: lifetime_defs,
 301                 bounded_ty: ty,
 302                 bounds,
 303             }))
 304         // FIXME: Decide what should be used here, `=` or `==`.
 305         // FIXME: We are just dropping the binders in lifetime_defs on the floor here.
 306         } else if self.eat(&token::Eq) || self.eat(&token::EqEq) {
 307             let rhs_ty = self.parse_ty()?;
 308             Ok(ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
 309                 span: lo.to(self.prev_token.span),
 310                 lhs_ty: ty,
 311                 rhs_ty,
 312                 id: ast::DUMMY_NODE_ID,
 313             }))
 314         } else {
 315             self.maybe_recover_bounds_doubled_colon(&ty)?;
 316             self.unexpected()
 317         }
 318     }
 319
 320     pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
 321         // There's an ambiguity between generic parameters and qualified paths in impls.
 322         // If we see `<` it may start both, so we have to inspect some following tokens.
 323         // The following combinations can only start generics,
 324         // but not qualified paths (with one exception):
 325         //     `<` `>` - empty generic parameters
 326         //     `<` `#` - generic parameters with attributes
 327         //     `<` (LIFETIME|IDENT) `>` - single generic parameter
 328         //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
 329         //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
 330         //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
 331         //     `<` const                - generic const parameter
 332         // The only truly ambiguous case is
 333         //     `<` IDENT `>` `::` IDENT ...
 334         // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
 335         // because this is what almost always expected in practice, qualified paths in impls
 336         // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
 337         self.look_ahead(start, |t| t == &token::Lt)
 338             && (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
 339                 || self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
 340                     && self.look_ahead(start + 2, |t| {
 341                         matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
 342                     })
 343                 || self.is_keyword_ahead(start + 1, &[kw::Const]))
 344     }
 345 }