src/librustc_parse/parser/ty.rs

   1 use super::item::ParamCfg;
   2 use super::{Parser, PathStyle, PrevTokenKind, TokenType};
   3
   4 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
   5
   6 use rustc_errors::{pluralize, struct_span_err, Applicability, PResult};
   7 use rustc_span::source_map::Span;
   8 use rustc_span::symbol::{kw, sym};
   9 use syntax::ast::{
  10     self, BareFnTy, FunctionRetTy, GenericParam, Ident, Lifetime, MutTy, Ty, TyKind,
  11 };
  12 use syntax::ast::{
  13     GenericBound, GenericBounds, PolyTraitRef, TraitBoundModifier, TraitObjectSyntax,
  14 };
  15 use syntax::ast::{Mac, Mutability};
  16 use syntax::ptr::P;
  17 use syntax::token::{self, Token};
  18
  19 /// Any `?` or `?const` modifiers that appear at the start of a bound.
  20 struct BoundModifiers {
  21     /// `?Trait`.
  22     maybe: Option<Span>,
  23
  24     /// `?const Trait`.
  25     maybe_const: Option<Span>,
  26 }
  27
  28 impl BoundModifiers {
  29     fn to_trait_bound_modifier(&self) -> TraitBoundModifier {
  30         match (self.maybe, self.maybe_const) {
  31             (None, None) => TraitBoundModifier::None,
  32             (Some(_), None) => TraitBoundModifier::Maybe,
  33             (None, Some(_)) => TraitBoundModifier::MaybeConst,
  34             (Some(_), Some(_)) => TraitBoundModifier::MaybeConstMaybe,
  35         }
  36     }
  37 }
  38
  39 /// Returns `true` if `IDENT t` can start a type -- `IDENT::a::b`, `IDENT<u8, u8>`,
  40 /// `IDENT<<u8 as Trait>::AssocTy>`.
  41 ///
  42 /// Types can also be of the form `IDENT(u8, u8) -> u8`, however this assumes
  43 /// that `IDENT` is not the ident of a fn trait.
  44 fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
  45     t == &token::ModSep || t == &token::Lt || t == &token::BinOp(token::Shl)
  46 }
  47
  48 impl<'a> Parser<'a> {
  49     /// Parses a type.
  50     pub fn parse_ty(&mut self) -> PResult<'a, P<Ty>> {
  51         self.parse_ty_common(true, true, false)
  52     }
  53
  54     /// Parse a type suitable for a function or function pointer parameter.
  55     /// The difference from `parse_ty` is that this version allows `...`
  56     /// (`CVarArgs`) at the top level of the the type.
  57     pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
  58         self.parse_ty_common(true, true, true)
  59     }
  60
  61     /// Parses a type in restricted contexts where `+` is not permitted.
  62     ///
  63     /// Example 1: `&'a TYPE`
  64     ///     `+` is prohibited to maintain operator priority (P(+) < P(&)).
  65     /// Example 2: `value1 as TYPE + value2`
  66     ///     `+` is prohibited to avoid interactions with expression grammar.
  67     pub(super) fn parse_ty_no_plus(&mut self) -> PResult<'a, P<Ty>> {
  68         self.parse_ty_common(false, true, false)
  69     }
  70
  71     /// Parses an optional return type `[ -> TY ]` in a function declaration.
  72     pub(super) fn parse_ret_ty(
  73         &mut self,
  74         allow_plus: bool,
  75         allow_qpath_recovery: bool,
  76     ) -> PResult<'a, FunctionRetTy> {
  77         Ok(if self.eat(&token::RArrow) {
  78             // FIXME(Centril): Can we unconditionally `allow_plus`?
  79             FunctionRetTy::Ty(self.parse_ty_common(allow_plus, allow_qpath_recovery, false)?)
  80         } else {
  81             FunctionRetTy::Default(self.token.span.shrink_to_lo())
  82         })
  83     }
  84
  85     fn parse_ty_common(
  86         &mut self,
  87         allow_plus: bool,
  88         allow_qpath_recovery: bool,
  89         // Is `...` (`CVarArgs`) legal in the immediate top level call?
  90         allow_c_variadic: bool,
  91     ) -> PResult<'a, P<Ty>> {
  92         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
  93         maybe_whole!(self, NtTy, |x| x);
  94
  95         let lo = self.token.span;
  96         let mut impl_dyn_multi = false;
  97         let kind = if self.check(&token::OpenDelim(token::Paren)) {
  98             self.parse_ty_tuple_or_parens(lo, allow_plus)?
  99         } else if self.eat(&token::Not) {
 100             // Never type `!`
 101             TyKind::Never
 102         } else if self.eat(&token::BinOp(token::Star)) {
 103             self.parse_ty_ptr()?
 104         } else if self.eat(&token::OpenDelim(token::Bracket)) {
 105             self.parse_array_or_slice_ty()?
 106         } else if self.check(&token::BinOp(token::And)) || self.check(&token::AndAnd) {
 107             // Reference
 108             self.expect_and()?;
 109             self.parse_borrowed_pointee()?
 110         } else if self.eat_keyword_noexpect(kw::Typeof) {
 111             self.parse_typeof_ty()?
 112         } else if self.eat_keyword(kw::Underscore) {
 113             // A type to be inferred `_`
 114             TyKind::Infer
 115         } else if self.token_is_bare_fn_keyword() {
 116             // Function pointer type
 117             self.parse_ty_bare_fn(Vec::new())?
 118         } else if self.check_keyword(kw::For) {
 119             // Function pointer type or bound list (trait object type) starting with a poly-trait.
 120             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
 121             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
 122             let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 123             if self.token_is_bare_fn_keyword() {
 124                 self.parse_ty_bare_fn(lifetime_defs)?
 125             } else {
 126                 let path = self.parse_path(PathStyle::Type)?;
 127                 let parse_plus = allow_plus && self.check_plus();
 128                 self.parse_remaining_bounds(lifetime_defs, path, lo, parse_plus)?
 129             }
 130         } else if self.eat_keyword(kw::Impl) {
 131             self.parse_impl_ty(&mut impl_dyn_multi)?
 132         } else if self.is_explicit_dyn_type() {
 133             self.parse_dyn_ty(&mut impl_dyn_multi)?
 134         } else if self.check(&token::Question)
 135             || self.check_lifetime() && self.look_ahead(1, |t| t.is_like_plus())
 136         {
 137             // Bound list (trait object type)
 138             let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
 139             TyKind::TraitObject(bounds, TraitObjectSyntax::None)
 140         } else if self.eat_lt() {
 141             // Qualified path
 142             let (qself, path) = self.parse_qpath(PathStyle::Type)?;
 143             TyKind::Path(Some(qself), path)
 144         } else if self.token.is_path_start() {
 145             self.parse_path_start_ty(lo, allow_plus)?
 146         } else if self.eat(&token::DotDotDot) {
 147             if allow_c_variadic {
 148                 TyKind::CVarArgs
 149             } else {
 150                 // FIXME(Centril): Should we just allow `...` syntactically
 151                 // anywhere in a type and use semantic restrictions instead?
 152                 self.error_illegal_c_varadic_ty(lo);
 153                 TyKind::Err
 154             }
 155         } else {
 156             let msg = format!("expected type, found {}", super::token_descr(&self.token));
 157             let mut err = self.struct_span_err(self.token.span, &msg);
 158             err.span_label(self.token.span, "expected type");
 159             self.maybe_annotate_with_ascription(&mut err, true);
 160             return Err(err);
 161         };
 162
 163         let span = lo.to(self.prev_span);
 164         let ty = self.mk_ty(span, kind);
 165
 166         // Try to recover from use of `+` with incorrect priority.
 167         self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
 168         self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
 169         self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
 170     }
 171
 172     /// Parses either:
 173     /// - `(TYPE)`, a parenthesized type.
 174     /// - `(TYPE,)`, a tuple with a single field of type TYPE.
 175     fn parse_ty_tuple_or_parens(&mut self, lo: Span, allow_plus: bool) -> PResult<'a, TyKind> {
 176         let mut trailing_plus = false;
 177         let (ts, trailing) = self.parse_paren_comma_seq(|p| {
 178             let ty = p.parse_ty()?;
 179             trailing_plus = p.prev_token_kind == PrevTokenKind::Plus;
 180             Ok(ty)
 181         })?;
 182
 183         if ts.len() == 1 && !trailing {
 184             let ty = ts.into_iter().nth(0).unwrap().into_inner();
 185             let maybe_bounds = allow_plus && self.token.is_like_plus();
 186             match ty.kind {
 187                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
 188                 TyKind::Path(None, path) if maybe_bounds => {
 189                     self.parse_remaining_bounds(Vec::new(), path, lo, true)
 190                 }
 191                 TyKind::TraitObject(mut bounds, TraitObjectSyntax::None)
 192                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
 193                 {
 194                     let path = match bounds.remove(0) {
 195                         GenericBound::Trait(pt, ..) => pt.trait_ref.path,
 196                         GenericBound::Outlives(..) => {
 197                             self.span_bug(ty.span, "unexpected lifetime bound")
 198                         }
 199                     };
 200                     self.parse_remaining_bounds(Vec::new(), path, lo, true)
 201                 }
 202                 // `(TYPE)`
 203                 _ => Ok(TyKind::Paren(P(ty))),
 204             }
 205         } else {
 206             Ok(TyKind::Tup(ts))
 207         }
 208     }
 209
 210     fn parse_remaining_bounds(
 211         &mut self,
 212         generic_params: Vec<GenericParam>,
 213         path: ast::Path,
 214         lo: Span,
 215         parse_plus: bool,
 216     ) -> PResult<'a, TyKind> {
 217         assert_ne!(self.token, token::Question);
 218
 219         let poly_trait_ref = PolyTraitRef::new(generic_params, path, lo.to(self.prev_span));
 220         let mut bounds = vec![GenericBound::Trait(poly_trait_ref, TraitBoundModifier::None)];
 221         if parse_plus {
 222             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
 223             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_span))?);
 224         }
 225         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 226     }
 227
 228     /// Parses a raw pointer type: `*[const | mut] $type`.
 229     fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
 230         let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
 231             let span = self.prev_span;
 232             let msg = "expected mut or const in raw pointer type";
 233             self.struct_span_err(span, msg)
 234                 .span_label(span, msg)
 235                 .help("use `*mut T` or `*const T` as appropriate")
 236                 .emit();
 237             Mutability::Not
 238         });
 239         let ty = self.parse_ty_no_plus()?;
 240         Ok(TyKind::Ptr(MutTy { ty, mutbl }))
 241     }
 242
 243     /// Parses an array (`[TYPE; EXPR]`) or slice (`[TYPE]`) type.
 244     /// The opening `[` bracket is already eaten.
 245     fn parse_array_or_slice_ty(&mut self) -> PResult<'a, TyKind> {
 246         let elt_ty = self.parse_ty()?;
 247         let ty = if self.eat(&token::Semi) {
 248             TyKind::Array(elt_ty, self.parse_anon_const_expr()?)
 249         } else {
 250             TyKind::Slice(elt_ty)
 251         };
 252         self.expect(&token::CloseDelim(token::Bracket))?;
 253         Ok(ty)
 254     }
 255
 256     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
 257         let opt_lifetime = if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
 258         let mutbl = self.parse_mutability();
 259         let ty = self.parse_ty_no_plus()?;
 260         Ok(TyKind::Rptr(opt_lifetime, MutTy { ty, mutbl }))
 261     }
 262
 263     // Parses the `typeof(EXPR)`.
 264     // To avoid ambiguity, the type is surrounded by parenthesis.
 265     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
 266         self.expect(&token::OpenDelim(token::Paren))?;
 267         let expr = self.parse_anon_const_expr()?;
 268         self.expect(&token::CloseDelim(token::Paren))?;
 269         Ok(TyKind::Typeof(expr))
 270     }
 271
 272     /// Is the current token one of the keywords that signals a bare function type?
 273     fn token_is_bare_fn_keyword(&mut self) -> bool {
 274         self.check_keyword(kw::Fn)
 275             || self.check_keyword(kw::Unsafe)
 276             || self.check_keyword(kw::Extern)
 277     }
 278
 279     /// Parses a function pointer type (`TyKind::BareFn`).
 280     /// ```
 281     /// [unsafe] [extern "ABI"] fn (S) -> T
 282     ///  ^~~~~^          ^~~~^     ^~^    ^
 283     ///    |               |        |     |
 284     ///    |               |        |   Return type
 285     /// Function Style    ABI  Parameter types
 286     /// ```
 287     fn parse_ty_bare_fn(&mut self, generic_params: Vec<GenericParam>) -> PResult<'a, TyKind> {
 288         let unsafety = self.parse_unsafety();
 289         let ext = self.parse_extern()?;
 290         self.expect_keyword(kw::Fn)?;
 291         let cfg = ParamCfg { is_self_allowed: false, is_name_required: |_| false };
 292         let decl = self.parse_fn_decl(cfg, false)?;
 293         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params, decl })))
 294     }
 295
 296     /// Parses an `impl B0 + ... + Bn` type.
 297     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 298         // Always parse bounds greedily for better error recovery.
 299         let bounds = self.parse_generic_bounds(None)?;
 300         *impl_dyn_multi = bounds.len() > 1 || self.prev_token_kind == PrevTokenKind::Plus;
 301         Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
 302     }
 303
 304     /// Is a `dyn B0 + ... + Bn` type allowed here?
 305     fn is_explicit_dyn_type(&mut self) -> bool {
 306         self.check_keyword(kw::Dyn)
 307             && (self.token.span.rust_2018()
 308                 || self.look_ahead(1, |t| {
 309                     t.can_begin_bound() && !can_continue_type_after_non_fn_ident(t)
 310                 }))
 311     }
 312
 313     /// Parses a `dyn B0 + ... + Bn` type.
 314     ///
 315     /// Note that this does *not* parse bare trait objects.
 316     fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 317         self.bump(); // `dyn`
 318         // Always parse bounds greedily for better error recovery.
 319         let bounds = self.parse_generic_bounds(None)?;
 320         *impl_dyn_multi = bounds.len() > 1 || self.prev_token_kind == PrevTokenKind::Plus;
 321         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn))
 322     }
 323
 324     /// Parses a type starting with a path.
 325     ///
 326     /// This can be:
 327     /// 1. a type macro, `mac!(...)`,
 328     /// 2. a bare trait object, `B0 + ... + Bn`,
 329     /// 3. or a path, `path::to::MyType`.
 330     fn parse_path_start_ty(&mut self, lo: Span, allow_plus: bool) -> PResult<'a, TyKind> {
 331         // Simple path
 332         let path = self.parse_path(PathStyle::Type)?;
 333         if self.eat(&token::Not) {
 334             // Macro invocation in type position
 335             Ok(TyKind::Mac(Mac {
 336                 path,
 337                 args: self.parse_mac_args()?,
 338                 prior_type_ascription: self.last_type_ascription,
 339             }))
 340         } else if allow_plus && self.check_plus() {
 341             // `Trait1 + Trait2 + 'a`
 342             self.parse_remaining_bounds(Vec::new(), path, lo, true)
 343         } else {
 344             // Just a type path.
 345             Ok(TyKind::Path(None, path))
 346         }
 347     }
 348
 349     fn error_illegal_c_varadic_ty(&self, lo: Span) {
 350         struct_span_err!(
 351             self.sess.span_diagnostic,
 352             lo.to(self.prev_span),
 353             E0743,
 354             "C-variadic type `...` may not be nested inside another type",
 355         )
 356         .emit();
 357     }
 358
 359     pub(super) fn parse_generic_bounds(
 360         &mut self,
 361         colon_span: Option<Span>,
 362     ) -> PResult<'a, GenericBounds> {
 363         self.parse_generic_bounds_common(true, colon_span)
 364     }
 365
 366     /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
 367     ///
 368     /// See `parse_generic_bound` for the `BOUND` grammar.
 369     fn parse_generic_bounds_common(
 370         &mut self,
 371         allow_plus: bool,
 372         colon_span: Option<Span>,
 373     ) -> PResult<'a, GenericBounds> {
 374         let mut bounds = Vec::new();
 375         let mut negative_bounds = Vec::new();
 376         while self.can_begin_bound() {
 377             match self.parse_generic_bound()? {
 378                 Ok(bound) => bounds.push(bound),
 379                 Err(neg_sp) => negative_bounds.push(neg_sp),
 380             }
 381             if !allow_plus || !self.eat_plus() {
 382                 break;
 383             }
 384         }
 385
 386         if !negative_bounds.is_empty() {
 387             self.error_negative_bounds(colon_span, &bounds, negative_bounds);
 388         }
 389
 390         Ok(bounds)
 391     }
 392
 393     /// Can the current token begin a bound?
 394     fn can_begin_bound(&mut self) -> bool {
 395         // This needs to be synchronized with `TokenKind::can_begin_bound`.
 396         self.check_path()
 397         || self.check_lifetime()
 398         || self.check(&token::Not) // Used for error reporting only.
 399         || self.check(&token::Question)
 400         || self.check_keyword(kw::For)
 401         || self.check(&token::OpenDelim(token::Paren))
 402     }
 403
 404     fn error_negative_bounds(
 405         &self,
 406         colon_span: Option<Span>,
 407         bounds: &[GenericBound],
 408         negative_bounds: Vec<Span>,
 409     ) {
 410         let negative_bounds_len = negative_bounds.len();
 411         let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
 412         let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
 413         err.span_label(last_span, "negative bounds are not supported");
 414         if let Some(bound_list) = colon_span {
 415             let bound_list = bound_list.to(self.prev_span);
 416             let mut new_bound_list = String::new();
 417             if !bounds.is_empty() {
 418                 let mut snippets = bounds.iter().map(|bound| self.span_to_snippet(bound.span()));
 419                 while let Some(Ok(snippet)) = snippets.next() {
 420                     new_bound_list.push_str(" + ");
 421                     new_bound_list.push_str(&snippet);
 422                 }
 423                 new_bound_list = new_bound_list.replacen(" +", ":", 1);
 424             }
 425             err.tool_only_span_suggestion(
 426                 bound_list,
 427                 &format!("remove the bound{}", pluralize!(negative_bounds_len)),
 428                 new_bound_list,
 429                 Applicability::MachineApplicable,
 430             );
 431         }
 432         err.emit();
 433     }
 434
 435     /// Parses a bound according to the grammar:
 436     /// ```
 437     /// BOUND = TY_BOUND | LT_BOUND
 438     /// ```
 439     fn parse_generic_bound(&mut self) -> PResult<'a, Result<GenericBound, Span>> {
 440         let anchor_lo = self.prev_span;
 441         let lo = self.token.span;
 442         let has_parens = self.eat(&token::OpenDelim(token::Paren));
 443         let inner_lo = self.token.span;
 444         let is_negative = self.eat(&token::Not);
 445
 446         let modifiers = self.parse_ty_bound_modifiers();
 447         let bound = if self.token.is_lifetime() {
 448             self.error_lt_bound_with_modifiers(modifiers);
 449             self.parse_generic_lt_bound(lo, inner_lo, has_parens)?
 450         } else {
 451             self.parse_generic_ty_bound(lo, has_parens, modifiers)?
 452         };
 453
 454         Ok(if is_negative { Err(anchor_lo.to(self.prev_span)) } else { Ok(bound) })
 455     }
 456
 457     /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
 458     /// ```
 459     /// LT_BOUND = LIFETIME
 460     /// ```
 461     fn parse_generic_lt_bound(
 462         &mut self,
 463         lo: Span,
 464         inner_lo: Span,
 465         has_parens: bool,
 466     ) -> PResult<'a, GenericBound> {
 467         let bound = GenericBound::Outlives(self.expect_lifetime());
 468         if has_parens {
 469             // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
 470             // possibly introducing `GenericBound::Paren(P<GenericBound>)`?
 471             self.recover_paren_lifetime(lo, inner_lo)?;
 472         }
 473         Ok(bound)
 474     }
 475
 476     /// Emits an error if any trait bound modifiers were present.
 477     fn error_lt_bound_with_modifiers(&self, modifiers: BoundModifiers) {
 478         if let Some(span) = modifiers.maybe_const {
 479             self.struct_span_err(
 480                 span,
 481                 "`?const` may only modify trait bounds, not lifetime bounds",
 482             )
 483             .emit();
 484         }
 485
 486         if let Some(span) = modifiers.maybe {
 487             self.struct_span_err(span, "`?` may only modify trait bounds, not lifetime bounds")
 488                 .emit();
 489         }
 490     }
 491
 492     /// Recover on `('lifetime)` with `(` already eaten.
 493     fn recover_paren_lifetime(&mut self, lo: Span, inner_lo: Span) -> PResult<'a, ()> {
 494         let inner_span = inner_lo.to(self.prev_span);
 495         self.expect(&token::CloseDelim(token::Paren))?;
 496         let mut err = self.struct_span_err(
 497             lo.to(self.prev_span),
 498             "parenthesized lifetime bounds are not supported",
 499         );
 500         if let Ok(snippet) = self.span_to_snippet(inner_span) {
 501             err.span_suggestion_short(
 502                 lo.to(self.prev_span),
 503                 "remove the parentheses",
 504                 snippet,
 505                 Applicability::MachineApplicable,
 506             );
 507         }
 508         err.emit();
 509         Ok(())
 510     }
 511
 512     /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `?const Trait`.
 513     ///
 514     /// If no modifiers are present, this does not consume any tokens.
 515     ///
 516     /// ```
 517     /// TY_BOUND_MODIFIERS = "?" ["const" ["?"]]
 518     /// ```
 519     fn parse_ty_bound_modifiers(&mut self) -> BoundModifiers {
 520         if !self.eat(&token::Question) {
 521             return BoundModifiers { maybe: None, maybe_const: None };
 522         }
 523
 524         // `? ...`
 525         let first_question = self.prev_span;
 526         if !self.eat_keyword(kw::Const) {
 527             return BoundModifiers { maybe: Some(first_question), maybe_const: None };
 528         }
 529
 530         // `?const ...`
 531         let maybe_const = first_question.to(self.prev_span);
 532         self.sess.gated_spans.gate(sym::const_trait_bound_opt_out, maybe_const);
 533         if !self.eat(&token::Question) {
 534             return BoundModifiers { maybe: None, maybe_const: Some(maybe_const) };
 535         }
 536
 537         // `?const ? ...`
 538         let second_question = self.prev_span;
 539         BoundModifiers { maybe: Some(second_question), maybe_const: Some(maybe_const) }
 540     }
 541
 542     /// Parses a type bound according to:
 543     /// ```
 544     /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
 545     /// TY_BOUND_NOPAREN = [TY_BOUND_MODIFIERS] [for<LT_PARAM_DEFS>] SIMPLE_PATH
 546     /// ```
 547     ///
 548     /// For example, this grammar accepts `?const ?for<'a: 'b> m::Trait<'a>`.
 549     fn parse_generic_ty_bound(
 550         &mut self,
 551         lo: Span,
 552         has_parens: bool,
 553         modifiers: BoundModifiers,
 554     ) -> PResult<'a, GenericBound> {
 555         let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 556         let path = self.parse_path(PathStyle::Type)?;
 557         if has_parens {
 558             self.expect(&token::CloseDelim(token::Paren))?;
 559         }
 560
 561         let modifier = modifiers.to_trait_bound_modifier();
 562         let poly_trait = PolyTraitRef::new(lifetime_defs, path, lo.to(self.prev_span));
 563         Ok(GenericBound::Trait(poly_trait, modifier))
 564     }
 565
 566     /// Optionally parses `for<$generic_params>`.
 567     pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
 568         if self.eat_keyword(kw::For) {
 569             self.expect_lt()?;
 570             let params = self.parse_generic_params()?;
 571             self.expect_gt()?;
 572             // We rely on AST validation to rule out invalid cases: There must not be type
 573             // parameters, and the lifetime parameters must not have bounds.
 574             Ok(params)
 575         } else {
 576             Ok(Vec::new())
 577         }
 578     }
 579
 580     pub fn check_lifetime(&mut self) -> bool {
 581         self.expected_tokens.push(TokenType::Lifetime);
 582         self.token.is_lifetime()
 583     }
 584
 585     /// Parses a single lifetime `'a` or panics.
 586     pub fn expect_lifetime(&mut self) -> Lifetime {
 587         if let Some(ident) = self.token.lifetime() {
 588             let span = self.token.span;
 589             self.bump();
 590             Lifetime { ident: Ident::new(ident.name, span), id: ast::DUMMY_NODE_ID }
 591         } else {
 592             self.span_bug(self.token.span, "not a lifetime")
 593         }
 594     }
 595
 596     pub(super) fn mk_ty(&self, span: Span, kind: TyKind) -> P<Ty> {
 597         P(Ty { kind, span, id: ast::DUMMY_NODE_ID })
 598     }
 599 }