1 use super::{ForceCollect, Parser, TrailingToken};
5 self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
7 use rustc_errors::PResult;
8 use rustc_span::symbol::kw;
11 /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
14 /// BOUND = LT_BOUND (e.g., `'a`)
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()));
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()?;
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))?
39 let default = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
43 id: ast::DUMMY_NODE_ID,
44 attrs: preceding_attrs.into(),
46 kind: GenericParamKind::Type { default },
47 is_placeholder: false,
51 crate fn parse_const_param(
53 preceding_attrs: Vec<Attribute>,
54 ) -> PResult<'a, GenericParam> {
55 let const_span = self.token.span;
57 self.expect_keyword(kw::Const)?;
58 let ident = self.parse_ident()?;
59 self.expect(&token::Colon)?;
60 let ty = self.parse_ty()?;
62 // Parse optional const generics default value.
63 let default = if self.eat(&token::Eq) { Some(self.parse_const_arg()?) } else { None };
67 id: ast::DUMMY_NODE_ID,
68 attrs: preceding_attrs.into(),
70 kind: GenericParamKind::Const { ty, kw_span: const_span, default },
71 is_placeholder: false,
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();
81 let attrs = self.parse_outer_attributes()?;
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.
89 "unexpected keyword `Self` in generic parameters",
91 .note("you cannot use `Self` as a generic parameter because it is reserved for associated items")
94 this.eat(&token::Comma);
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()
105 Some(ast::GenericParam {
106 ident: lifetime.ident,
110 kind: ast::GenericParamKind::Lifetime,
111 is_placeholder: false,
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.clone();
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",
128 .span_label(where_predicate.span(), "belongs in `where` clause")
130 // FIXME - try to continue parsing other generics?
131 return Ok((None, TrailingToken::None));
135 // FIXME - maybe we should overwrite 'self' outside of `collect_tokens`?
137 return Ok((None, TrailingToken::None));
141 // Check for trailing attributes and stop parsing.
142 if !attrs.is_empty() {
143 if !params.is_empty() {
144 this.struct_span_err(
146 "trailing attribute after generic parameter",
148 .span_label(attrs[0].span, "attributes must go before parameters")
151 this.struct_span_err(
153 "attribute without generic parameters",
157 "attributes are only permitted when preceding parameters",
162 return Ok((None, TrailingToken::None));
165 if !this.eat(&token::Comma) {
168 // We just ate the comma, so no need to use `TrailingToken`
169 Ok((param, TrailingToken::None))
172 if let Some(param) = param {
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()`.
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()?;
193 (params, span_lo.to(self.prev_token.span))
195 (vec![], self.prev_token.span.shrink_to_hi())
199 where_clause: WhereClause {
200 has_where_token: false,
201 predicates: Vec::new(),
202 span: self.prev_token.span.shrink_to_hi(),
208 /// Parses an optional where-clause and places it in `generics`.
210 /// ```ignore (only-for-syntax-highlight)
211 /// where T : Trait<U, V> + 'b, 'a : 'b
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(),
220 if !self.eat_keyword(kw::Where) {
221 return Ok(where_clause);
223 where_clause.has_where_token = true;
224 let lo = self.prev_token.span;
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(
233 "generic parameters on `where` clauses are reserved for future use",
235 .span_label(generics.span, "currently unsupported")
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),
253 } else if self.check_type() {
254 where_clause.predicates.push(self.parse_ty_where_predicate()?);
259 if !self.eat(&token::Comma) {
264 where_clause.span = lo.to(self.prev_token.span);
268 fn parse_ty_where_predicate(&mut self) -> PResult<'a, ast::WherePredicate> {
269 let lo = self.token.span;
270 // Parse optional `for<'a, 'b>`.
271 // This `for` is parsed greedily and applies to the whole predicate,
272 // the bounded type can have its own `for` applying only to it.
274 // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
275 // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
276 // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
277 let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
279 // Parse type with mandatory colon and (possibly empty) bounds,
280 // or with mandatory equality sign and the second type.
281 let ty = self.parse_ty_for_where_clause()?;
282 if self.eat(&token::Colon) {
283 let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
284 Ok(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
285 span: lo.to(self.prev_token.span),
286 bound_generic_params: lifetime_defs,
290 // FIXME: Decide what should be used here, `=` or `==`.
291 // FIXME: We are just dropping the binders in lifetime_defs on the floor here.
292 } else if self.eat(&token::Eq) || self.eat(&token::EqEq) {
293 let rhs_ty = self.parse_ty()?;
294 Ok(ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
295 span: lo.to(self.prev_token.span),
298 id: ast::DUMMY_NODE_ID,
305 pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
306 // There's an ambiguity between generic parameters and qualified paths in impls.
307 // If we see `<` it may start both, so we have to inspect some following tokens.
308 // The following combinations can only start generics,
309 // but not qualified paths (with one exception):
310 // `<` `>` - empty generic parameters
311 // `<` `#` - generic parameters with attributes
312 // `<` (LIFETIME|IDENT) `>` - single generic parameter
313 // `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
314 // `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
315 // `<` (LIFETIME|IDENT) `=` - generic parameter with a default
316 // `<` const - generic const parameter
317 // The only truly ambiguous case is
318 // `<` IDENT `>` `::` IDENT ...
319 // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
320 // because this is what almost always expected in practice, qualified paths in impls
321 // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
322 self.look_ahead(start, |t| t == &token::Lt)
323 && (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
324 || self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
325 && self.look_ahead(start + 2, |t| {
326 matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
328 || self.is_keyword_ahead(start + 1, &[kw::Const]))