src/librustc_parse/parser/ty.rs

   1 use super::{Parser, PathStyle, TokenType};
   2
   3 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
   4
   5 use rustc_ast::ptr::P;
   6 use rustc_ast::token::{self, Token, TokenKind};
   7 use rustc_ast::{self as ast, BareFnTy, FnRetTy, GenericParam, Lifetime, MutTy, Ty, TyKind};
   8 use rustc_ast::{GenericBound, GenericBounds, MacCall, Mutability};
   9 use rustc_ast::{PolyTraitRef, TraitBoundModifier, TraitObjectSyntax};
  10 use rustc_errors::{pluralize, struct_span_err, Applicability, PResult};
  11 use rustc_span::source_map::Span;
  12 use rustc_span::symbol::{kw, sym};
  13
  14 /// Any `?` or `?const` modifiers that appear at the start of a bound.
  15 struct BoundModifiers {
  16     /// `?Trait`.
  17     maybe: Option<Span>,
  18
  19     /// `?const Trait`.
  20     maybe_const: Option<Span>,
  21 }
  22
  23 impl BoundModifiers {
  24     fn to_trait_bound_modifier(&self) -> TraitBoundModifier {
  25         match (self.maybe, self.maybe_const) {
  26             (None, None) => TraitBoundModifier::None,
  27             (Some(_), None) => TraitBoundModifier::Maybe,
  28             (None, Some(_)) => TraitBoundModifier::MaybeConst,
  29             (Some(_), Some(_)) => TraitBoundModifier::MaybeConstMaybe,
  30         }
  31     }
  32 }
  33
  34 #[derive(Copy, Clone, PartialEq)]
  35 pub(super) enum AllowPlus {
  36     Yes,
  37     No,
  38 }
  39
  40 #[derive(PartialEq)]
  41 pub(super) enum RecoverQPath {
  42     Yes,
  43     No,
  44 }
  45
  46 // Is `...` (`CVarArgs`) legal at this level of type parsing?
  47 #[derive(PartialEq)]
  48 enum AllowCVariadic {
  49     Yes,
  50     No,
  51 }
  52
  53 /// Returns `true` if `IDENT t` can start a type -- `IDENT::a::b`, `IDENT<u8, u8>`,
  54 /// `IDENT<<u8 as Trait>::AssocTy>`.
  55 ///
  56 /// Types can also be of the form `IDENT(u8, u8) -> u8`, however this assumes
  57 /// that `IDENT` is not the ident of a fn trait.
  58 fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
  59     t == &token::ModSep || t == &token::Lt || t == &token::BinOp(token::Shl)
  60 }
  61
  62 impl<'a> Parser<'a> {
  63     /// Parses a type.
  64     pub fn parse_ty(&mut self) -> PResult<'a, P<Ty>> {
  65         self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::No)
  66     }
  67
  68     /// Parse a type suitable for a function or function pointer parameter.
  69     /// The difference from `parse_ty` is that this version allows `...`
  70     /// (`CVarArgs`) at the top level of the the type.
  71     pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
  72         self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::Yes)
  73     }
  74
  75     /// Parses a type in restricted contexts where `+` is not permitted.
  76     ///
  77     /// Example 1: `&'a TYPE`
  78     ///     `+` is prohibited to maintain operator priority (P(+) < P(&)).
  79     /// Example 2: `value1 as TYPE + value2`
  80     ///     `+` is prohibited to avoid interactions with expression grammar.
  81     pub(super) fn parse_ty_no_plus(&mut self) -> PResult<'a, P<Ty>> {
  82         self.parse_ty_common(AllowPlus::No, RecoverQPath::Yes, AllowCVariadic::No)
  83     }
  84
  85     /// Parses an optional return type `[ -> TY ]` in a function declaration.
  86     pub(super) fn parse_ret_ty(
  87         &mut self,
  88         allow_plus: AllowPlus,
  89         recover_qpath: RecoverQPath,
  90     ) -> PResult<'a, FnRetTy> {
  91         Ok(if self.eat(&token::RArrow) {
  92             // FIXME(Centril): Can we unconditionally `allow_plus`?
  93             let ty = self.parse_ty_common(allow_plus, recover_qpath, AllowCVariadic::No)?;
  94             FnRetTy::Ty(ty)
  95         } else {
  96             FnRetTy::Default(self.token.span.shrink_to_lo())
  97         })
  98     }
  99
 100     fn parse_ty_common(
 101         &mut self,
 102         allow_plus: AllowPlus,
 103         recover_qpath: RecoverQPath,
 104         allow_c_variadic: AllowCVariadic,
 105     ) -> PResult<'a, P<Ty>> {
 106         let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
 107         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
 108         maybe_whole!(self, NtTy, |x| x);
 109
 110         let lo = self.token.span;
 111         let mut impl_dyn_multi = false;
 112         let kind = if self.check(&token::OpenDelim(token::Paren)) {
 113             self.parse_ty_tuple_or_parens(lo, allow_plus)?
 114         } else if self.eat(&token::Not) {
 115             // Never type `!`
 116             TyKind::Never
 117         } else if self.eat(&token::BinOp(token::Star)) {
 118             self.parse_ty_ptr()?
 119         } else if self.eat(&token::OpenDelim(token::Bracket)) {
 120             self.parse_array_or_slice_ty()?
 121         } else if self.check(&token::BinOp(token::And)) || self.check(&token::AndAnd) {
 122             // Reference
 123             self.expect_and()?;
 124             self.parse_borrowed_pointee()?
 125         } else if self.eat_keyword_noexpect(kw::Typeof) {
 126             self.parse_typeof_ty()?
 127         } else if self.eat_keyword(kw::Underscore) {
 128             // A type to be inferred `_`
 129             TyKind::Infer
 130         } else if self.check_fn_front_matter() {
 131             // Function pointer type
 132             self.parse_ty_bare_fn(lo, Vec::new())?
 133         } else if self.check_keyword(kw::For) {
 134             // Function pointer type or bound list (trait object type) starting with a poly-trait.
 135             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
 136             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
 137             let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 138             if self.check_fn_front_matter() {
 139                 self.parse_ty_bare_fn(lo, lifetime_defs)?
 140             } else {
 141                 let path = self.parse_path(PathStyle::Type)?;
 142                 let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
 143                 self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?
 144             }
 145         } else if self.eat_keyword(kw::Impl) {
 146             self.parse_impl_ty(&mut impl_dyn_multi)?
 147         } else if self.is_explicit_dyn_type() {
 148             self.parse_dyn_ty(&mut impl_dyn_multi)?
 149         } else if self.eat_lt() {
 150             // Qualified path
 151             let (qself, path) = self.parse_qpath(PathStyle::Type)?;
 152             TyKind::Path(Some(qself), path)
 153         } else if self.check_path() {
 154             self.parse_path_start_ty(lo, allow_plus)?
 155         } else if self.can_begin_bound() {
 156             self.parse_bare_trait_object(lo, allow_plus)?
 157         } else if self.eat(&token::DotDotDot) {
 158             if allow_c_variadic == AllowCVariadic::Yes {
 159                 TyKind::CVarArgs
 160             } else {
 161                 // FIXME(Centril): Should we just allow `...` syntactically
 162                 // anywhere in a type and use semantic restrictions instead?
 163                 self.error_illegal_c_varadic_ty(lo);
 164                 TyKind::Err
 165             }
 166         } else {
 167             let msg = format!("expected type, found {}", super::token_descr(&self.token));
 168             let mut err = self.struct_span_err(self.token.span, &msg);
 169             err.span_label(self.token.span, "expected type");
 170             self.maybe_annotate_with_ascription(&mut err, true);
 171             return Err(err);
 172         };
 173
 174         let span = lo.to(self.prev_token.span);
 175         let ty = self.mk_ty(span, kind);
 176
 177         // Try to recover from use of `+` with incorrect priority.
 178         self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
 179         self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
 180         self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
 181     }
 182
 183     /// Parses either:
 184     /// - `(TYPE)`, a parenthesized type.
 185     /// - `(TYPE,)`, a tuple with a single field of type TYPE.
 186     fn parse_ty_tuple_or_parens(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 187         let mut trailing_plus = false;
 188         let (ts, trailing) = self.parse_paren_comma_seq(|p| {
 189             let ty = p.parse_ty()?;
 190             trailing_plus = p.prev_token.kind == TokenKind::BinOp(token::Plus);
 191             Ok(ty)
 192         })?;
 193
 194         if ts.len() == 1 && !trailing {
 195             let ty = ts.into_iter().next().unwrap().into_inner();
 196             let maybe_bounds = allow_plus == AllowPlus::Yes && self.token.is_like_plus();
 197             match ty.kind {
 198                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
 199                 TyKind::Path(None, path) if maybe_bounds => {
 200                     self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 201                 }
 202                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
 203                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
 204                 {
 205                     self.parse_remaining_bounds(bounds, true)
 206                 }
 207                 // `(TYPE)`
 208                 _ => Ok(TyKind::Paren(P(ty))),
 209             }
 210         } else {
 211             Ok(TyKind::Tup(ts))
 212         }
 213     }
 214
 215     fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 216         let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
 217         let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
 218         if lt_no_plus {
 219             self.struct_span_err(lo, "lifetime in trait object type must be followed by `+`").emit()
 220         }
 221         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 222     }
 223
 224     fn parse_remaining_bounds_path(
 225         &mut self,
 226         generic_params: Vec<GenericParam>,
 227         path: ast::Path,
 228         lo: Span,
 229         parse_plus: bool,
 230     ) -> PResult<'a, TyKind> {
 231         let poly_trait_ref = PolyTraitRef::new(generic_params, path, lo.to(self.prev_token.span));
 232         let bounds = vec![GenericBound::Trait(poly_trait_ref, TraitBoundModifier::None)];
 233         self.parse_remaining_bounds(bounds, parse_plus)
 234     }
 235
 236     /// Parse the remainder of a bare trait object type given an already parsed list.
 237     fn parse_remaining_bounds(
 238         &mut self,
 239         mut bounds: GenericBounds,
 240         plus: bool,
 241     ) -> PResult<'a, TyKind> {
 242         assert_ne!(self.token, token::Question);
 243         if plus {
 244             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
 245             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);
 246         }
 247         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 248     }
 249
 250     /// Parses a raw pointer type: `*[const | mut] $type`.
 251     fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
 252         let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
 253             let span = self.prev_token.span;
 254             let msg = "expected mut or const in raw pointer type";
 255             self.struct_span_err(span, msg)
 256                 .span_label(span, msg)
 257                 .help("use `*mut T` or `*const T` as appropriate")
 258                 .emit();
 259             Mutability::Not
 260         });
 261         let ty = self.parse_ty_no_plus()?;
 262         Ok(TyKind::Ptr(MutTy { ty, mutbl }))
 263     }
 264
 265     /// Parses an array (`[TYPE; EXPR]`) or slice (`[TYPE]`) type.
 266     /// The opening `[` bracket is already eaten.
 267     fn parse_array_or_slice_ty(&mut self) -> PResult<'a, TyKind> {
 268         let elt_ty = self.parse_ty()?;
 269         let ty = if self.eat(&token::Semi) {
 270             TyKind::Array(elt_ty, self.parse_anon_const_expr()?)
 271         } else {
 272             TyKind::Slice(elt_ty)
 273         };
 274         self.expect(&token::CloseDelim(token::Bracket))?;
 275         Ok(ty)
 276     }
 277
 278     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
 279         let opt_lifetime = if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
 280         let mutbl = self.parse_mutability();
 281         let ty = self.parse_ty_no_plus()?;
 282         Ok(TyKind::Rptr(opt_lifetime, MutTy { ty, mutbl }))
 283     }
 284
 285     // Parses the `typeof(EXPR)`.
 286     // To avoid ambiguity, the type is surrounded by parenthesis.
 287     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
 288         self.expect(&token::OpenDelim(token::Paren))?;
 289         let expr = self.parse_anon_const_expr()?;
 290         self.expect(&token::CloseDelim(token::Paren))?;
 291         Ok(TyKind::Typeof(expr))
 292     }
 293
 294     /// Parses a function pointer type (`TyKind::BareFn`).
 295     /// ```
 296     /// [unsafe] [extern "ABI"] fn (S) -> T
 297     ///  ^~~~~^          ^~~~^     ^~^    ^
 298     ///    |               |        |     |
 299     ///    |               |        |   Return type
 300     /// Function Style    ABI  Parameter types
 301     /// ```
 302     /// We actually parse `FnHeader FnDecl`, but we error on `const` and `async` qualifiers.
 303     fn parse_ty_bare_fn(&mut self, lo: Span, params: Vec<GenericParam>) -> PResult<'a, TyKind> {
 304         let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
 305         let decl = self.parse_fn_decl(|_| false, AllowPlus::No)?;
 306         let whole_span = lo.to(self.prev_token.span);
 307         if let ast::Const::Yes(span) = constness {
 308             self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
 309         }
 310         if let ast::Async::Yes { span, .. } = asyncness {
 311             self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
 312         }
 313         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
 314     }
 315
 316     /// Emit an error for the given bad function pointer qualifier.
 317     fn error_fn_ptr_bad_qualifier(&self, span: Span, qual_span: Span, qual: &str) {
 318         self.struct_span_err(span, &format!("an `fn` pointer type cannot be `{}`", qual))
 319             .span_label(qual_span, format!("`{}` because of this", qual))
 320             .span_suggestion_short(
 321                 qual_span,
 322                 &format!("remove the `{}` qualifier", qual),
 323                 String::new(),
 324                 Applicability::MaybeIncorrect,
 325             )
 326             .emit();
 327     }
 328
 329     /// Parses an `impl B0 + ... + Bn` type.
 330     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 331         // Always parse bounds greedily for better error recovery.
 332         let bounds = self.parse_generic_bounds(None)?;
 333         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 334         Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
 335     }
 336
 337     /// Is a `dyn B0 + ... + Bn` type allowed here?
 338     fn is_explicit_dyn_type(&mut self) -> bool {
 339         self.check_keyword(kw::Dyn)
 340             && (self.token.uninterpolated_span().rust_2018()
 341                 || self.look_ahead(1, |t| {
 342                     t.can_begin_bound() && !can_continue_type_after_non_fn_ident(t)
 343                 }))
 344     }
 345
 346     /// Parses a `dyn B0 + ... + Bn` type.
 347     ///
 348     /// Note that this does *not* parse bare trait objects.
 349     fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 350         self.bump(); // `dyn`
 351         // Always parse bounds greedily for better error recovery.
 352         let bounds = self.parse_generic_bounds(None)?;
 353         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 354         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn))
 355     }
 356
 357     /// Parses a type starting with a path.
 358     ///
 359     /// This can be:
 360     /// 1. a type macro, `mac!(...)`,
 361     /// 2. a bare trait object, `B0 + ... + Bn`,
 362     /// 3. or a path, `path::to::MyType`.
 363     fn parse_path_start_ty(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 364         // Simple path
 365         let path = self.parse_path(PathStyle::Type)?;
 366         if self.eat(&token::Not) {
 367             // Macro invocation in type position
 368             Ok(TyKind::MacCall(MacCall {
 369                 path,
 370                 args: self.parse_mac_args()?,
 371                 prior_type_ascription: self.last_type_ascription,
 372             }))
 373         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
 374             // `Trait1 + Trait2 + 'a`
 375             self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 376         } else {
 377             // Just a type path.
 378             Ok(TyKind::Path(None, path))
 379         }
 380     }
 381
 382     fn error_illegal_c_varadic_ty(&self, lo: Span) {
 383         struct_span_err!(
 384             self.sess.span_diagnostic,
 385             lo.to(self.prev_token.span),
 386             E0743,
 387             "C-variadic type `...` may not be nested inside another type",
 388         )
 389         .emit();
 390     }
 391
 392     pub(super) fn parse_generic_bounds(
 393         &mut self,
 394         colon_span: Option<Span>,
 395     ) -> PResult<'a, GenericBounds> {
 396         self.parse_generic_bounds_common(AllowPlus::Yes, colon_span)
 397     }
 398
 399     /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
 400     ///
 401     /// See `parse_generic_bound` for the `BOUND` grammar.
 402     fn parse_generic_bounds_common(
 403         &mut self,
 404         allow_plus: AllowPlus,
 405         colon_span: Option<Span>,
 406     ) -> PResult<'a, GenericBounds> {
 407         let mut bounds = Vec::new();
 408         let mut negative_bounds = Vec::new();
 409         while self.can_begin_bound() {
 410             match self.parse_generic_bound()? {
 411                 Ok(bound) => bounds.push(bound),
 412                 Err(neg_sp) => negative_bounds.push(neg_sp),
 413             }
 414             if allow_plus == AllowPlus::No || !self.eat_plus() {
 415                 break;
 416             }
 417         }
 418
 419         if !negative_bounds.is_empty() {
 420             self.error_negative_bounds(colon_span, &bounds, negative_bounds);
 421         }
 422
 423         Ok(bounds)
 424     }
 425
 426     /// Can the current token begin a bound?
 427     fn can_begin_bound(&mut self) -> bool {
 428         // This needs to be synchronized with `TokenKind::can_begin_bound`.
 429         self.check_path()
 430         || self.check_lifetime()
 431         || self.check(&token::Not) // Used for error reporting only.
 432         || self.check(&token::Question)
 433         || self.check_keyword(kw::For)
 434         || self.check(&token::OpenDelim(token::Paren))
 435     }
 436
 437     fn error_negative_bounds(
 438         &self,
 439         colon_span: Option<Span>,
 440         bounds: &[GenericBound],
 441         negative_bounds: Vec<Span>,
 442     ) {
 443         let negative_bounds_len = negative_bounds.len();
 444         let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
 445         let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
 446         err.span_label(last_span, "negative bounds are not supported");
 447         if let Some(bound_list) = colon_span {
 448             let bound_list = bound_list.to(self.prev_token.span);
 449             let mut new_bound_list = String::new();
 450             if !bounds.is_empty() {
 451                 let mut snippets = bounds.iter().map(|bound| self.span_to_snippet(bound.span()));
 452                 while let Some(Ok(snippet)) = snippets.next() {
 453                     new_bound_list.push_str(" + ");
 454                     new_bound_list.push_str(&snippet);
 455                 }
 456                 new_bound_list = new_bound_list.replacen(" +", ":", 1);
 457             }
 458             err.tool_only_span_suggestion(
 459                 bound_list,
 460                 &format!("remove the bound{}", pluralize!(negative_bounds_len)),
 461                 new_bound_list,
 462                 Applicability::MachineApplicable,
 463             );
 464         }
 465         err.emit();
 466     }
 467
 468     /// Parses a bound according to the grammar:
 469     /// ```
 470     /// BOUND = TY_BOUND | LT_BOUND
 471     /// ```
 472     fn parse_generic_bound(&mut self) -> PResult<'a, Result<GenericBound, Span>> {
 473         let anchor_lo = self.prev_token.span;
 474         let lo = self.token.span;
 475         let has_parens = self.eat(&token::OpenDelim(token::Paren));
 476         let inner_lo = self.token.span;
 477         let is_negative = self.eat(&token::Not);
 478
 479         let modifiers = self.parse_ty_bound_modifiers();
 480         let bound = if self.token.is_lifetime() {
 481             self.error_lt_bound_with_modifiers(modifiers);
 482             self.parse_generic_lt_bound(lo, inner_lo, has_parens)?
 483         } else {
 484             self.parse_generic_ty_bound(lo, has_parens, modifiers)?
 485         };
 486
 487         Ok(if is_negative { Err(anchor_lo.to(self.prev_token.span)) } else { Ok(bound) })
 488     }
 489
 490     /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
 491     /// ```
 492     /// LT_BOUND = LIFETIME
 493     /// ```
 494     fn parse_generic_lt_bound(
 495         &mut self,
 496         lo: Span,
 497         inner_lo: Span,
 498         has_parens: bool,
 499     ) -> PResult<'a, GenericBound> {
 500         let bound = GenericBound::Outlives(self.expect_lifetime());
 501         if has_parens {
 502             // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
 503             // possibly introducing `GenericBound::Paren(P<GenericBound>)`?
 504             self.recover_paren_lifetime(lo, inner_lo)?;
 505         }
 506         Ok(bound)
 507     }
 508
 509     /// Emits an error if any trait bound modifiers were present.
 510     fn error_lt_bound_with_modifiers(&self, modifiers: BoundModifiers) {
 511         if let Some(span) = modifiers.maybe_const {
 512             self.struct_span_err(
 513                 span,
 514                 "`?const` may only modify trait bounds, not lifetime bounds",
 515             )
 516             .emit();
 517         }
 518
 519         if let Some(span) = modifiers.maybe {
 520             self.struct_span_err(span, "`?` may only modify trait bounds, not lifetime bounds")
 521                 .emit();
 522         }
 523     }
 524
 525     /// Recover on `('lifetime)` with `(` already eaten.
 526     fn recover_paren_lifetime(&mut self, lo: Span, inner_lo: Span) -> PResult<'a, ()> {
 527         let inner_span = inner_lo.to(self.prev_token.span);
 528         self.expect(&token::CloseDelim(token::Paren))?;
 529         let mut err = self.struct_span_err(
 530             lo.to(self.prev_token.span),
 531             "parenthesized lifetime bounds are not supported",
 532         );
 533         if let Ok(snippet) = self.span_to_snippet(inner_span) {
 534             err.span_suggestion_short(
 535                 lo.to(self.prev_token.span),
 536                 "remove the parentheses",
 537                 snippet,
 538                 Applicability::MachineApplicable,
 539             );
 540         }
 541         err.emit();
 542         Ok(())
 543     }
 544
 545     /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `?const Trait`.
 546     ///
 547     /// If no modifiers are present, this does not consume any tokens.
 548     ///
 549     /// ```
 550     /// TY_BOUND_MODIFIERS = "?" ["const" ["?"]]
 551     /// ```
 552     fn parse_ty_bound_modifiers(&mut self) -> BoundModifiers {
 553         if !self.eat(&token::Question) {
 554             return BoundModifiers { maybe: None, maybe_const: None };
 555         }
 556
 557         // `? ...`
 558         let first_question = self.prev_token.span;
 559         if !self.eat_keyword(kw::Const) {
 560             return BoundModifiers { maybe: Some(first_question), maybe_const: None };
 561         }
 562
 563         // `?const ...`
 564         let maybe_const = first_question.to(self.prev_token.span);
 565         self.sess.gated_spans.gate(sym::const_trait_bound_opt_out, maybe_const);
 566         if !self.eat(&token::Question) {
 567             return BoundModifiers { maybe: None, maybe_const: Some(maybe_const) };
 568         }
 569
 570         // `?const ? ...`
 571         let second_question = self.prev_token.span;
 572         BoundModifiers { maybe: Some(second_question), maybe_const: Some(maybe_const) }
 573     }
 574
 575     /// Parses a type bound according to:
 576     /// ```
 577     /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
 578     /// TY_BOUND_NOPAREN = [TY_BOUND_MODIFIERS] [for<LT_PARAM_DEFS>] SIMPLE_PATH
 579     /// ```
 580     ///
 581     /// For example, this grammar accepts `?const ?for<'a: 'b> m::Trait<'a>`.
 582     fn parse_generic_ty_bound(
 583         &mut self,
 584         lo: Span,
 585         has_parens: bool,
 586         modifiers: BoundModifiers,
 587     ) -> PResult<'a, GenericBound> {
 588         let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 589         let path = self.parse_path(PathStyle::Type)?;
 590         if has_parens {
 591             self.expect(&token::CloseDelim(token::Paren))?;
 592         }
 593
 594         let modifier = modifiers.to_trait_bound_modifier();
 595         let poly_trait = PolyTraitRef::new(lifetime_defs, path, lo.to(self.prev_token.span));
 596         Ok(GenericBound::Trait(poly_trait, modifier))
 597     }
 598
 599     /// Optionally parses `for<$generic_params>`.
 600     pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
 601         if self.eat_keyword(kw::For) {
 602             self.expect_lt()?;
 603             let params = self.parse_generic_params()?;
 604             self.expect_gt()?;
 605             // We rely on AST validation to rule out invalid cases: There must not be type
 606             // parameters, and the lifetime parameters must not have bounds.
 607             Ok(params)
 608         } else {
 609             Ok(Vec::new())
 610         }
 611     }
 612
 613     pub(super) fn check_lifetime(&mut self) -> bool {
 614         self.expected_tokens.push(TokenType::Lifetime);
 615         self.token.is_lifetime()
 616     }
 617
 618     /// Parses a single lifetime `'a` or panics.
 619     pub(super) fn expect_lifetime(&mut self) -> Lifetime {
 620         if let Some(ident) = self.token.lifetime() {
 621             self.bump();
 622             Lifetime { ident, id: ast::DUMMY_NODE_ID }
 623         } else {
 624             self.span_bug(self.token.span, "not a lifetime")
 625         }
 626     }
 627
 628     pub(super) fn mk_ty(&self, span: Span, kind: TyKind) -> P<Ty> {
 629         P(Ty { kind, span, id: ast::DUMMY_NODE_ID })
 630     }
 631 }