compiler/rustc_parse/src/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 /// Signals whether parsing a type should recover `->`.
  47 ///
  48 /// More specifically, when parsing a function like:
  49 /// ```rust
  50 /// fn foo() => u8 { 0 }
  51 /// fn bar(): u8 { 0 }
  52 /// ```
  53 /// The compiler will try to recover interpreting `foo() => u8` as `foo() -> u8` when calling
  54 /// `parse_ty` with anything except `RecoverReturnSign::No`, and it will try to recover `bar(): u8`
  55 /// as `bar() -> u8` when passing `RecoverReturnSign::Yes` to `parse_ty`
  56 #[derive(Copy, Clone, PartialEq)]
  57 pub(super) enum RecoverReturnSign {
  58     Yes,
  59     OnlyFatArrow,
  60     No,
  61 }
  62
  63 impl RecoverReturnSign {
  64     /// [RecoverReturnSign::Yes] allows for recovering `fn foo() => u8` and `fn foo(): u8`,
  65     /// [RecoverReturnSign::OnlyFatArrow] allows for recovering only `fn foo() => u8` (recovering
  66     /// colons can cause problems when parsing where clauses), and
  67     /// [RecoverReturnSign::No] doesn't allow for any recovery of the return type arrow
  68     fn can_recover(self, token: &TokenKind) -> bool {
  69         match self {
  70             Self::Yes => matches!(token, token::FatArrow | token::Colon),
  71             Self::OnlyFatArrow => matches!(token, token::FatArrow),
  72             Self::No => false,
  73         }
  74     }
  75 }
  76
  77 // Is `...` (`CVarArgs`) legal at this level of type parsing?
  78 #[derive(PartialEq)]
  79 enum AllowCVariadic {
  80     Yes,
  81     No,
  82 }
  83
  84 /// Returns `true` if `IDENT t` can start a type -- `IDENT::a::b`, `IDENT<u8, u8>`,
  85 /// `IDENT<<u8 as Trait>::AssocTy>`.
  86 ///
  87 /// Types can also be of the form `IDENT(u8, u8) -> u8`, however this assumes
  88 /// that `IDENT` is not the ident of a fn trait.
  89 fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
  90     t == &token::ModSep || t == &token::Lt || t == &token::BinOp(token::Shl)
  91 }
  92
  93 impl<'a> Parser<'a> {
  94     /// Parses a type.
  95     pub fn parse_ty(&mut self) -> PResult<'a, P<Ty>> {
  96         self.parse_ty_common(
  97             AllowPlus::Yes,
  98             AllowCVariadic::No,
  99             RecoverQPath::Yes,
 100             RecoverReturnSign::Yes,
 101         )
 102     }
 103
 104     /// Parse a type suitable for a function or function pointer parameter.
 105     /// The difference from `parse_ty` is that this version allows `...`
 106     /// (`CVarArgs`) at the top level of the type.
 107     pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
 108         self.parse_ty_common(
 109             AllowPlus::Yes,
 110             AllowCVariadic::Yes,
 111             RecoverQPath::Yes,
 112             RecoverReturnSign::Yes,
 113         )
 114     }
 115
 116     /// Parses a type in restricted contexts where `+` is not permitted.
 117     ///
 118     /// Example 1: `&'a TYPE`
 119     ///     `+` is prohibited to maintain operator priority (P(+) < P(&)).
 120     /// Example 2: `value1 as TYPE + value2`
 121     ///     `+` is prohibited to avoid interactions with expression grammar.
 122     pub(super) fn parse_ty_no_plus(&mut self) -> PResult<'a, P<Ty>> {
 123         self.parse_ty_common(
 124             AllowPlus::No,
 125             AllowCVariadic::No,
 126             RecoverQPath::Yes,
 127             RecoverReturnSign::Yes,
 128         )
 129     }
 130
 131     /// Parse a type without recovering `:` as `->` to avoid breaking code such as `where fn() : for<'a>`
 132     pub(super) fn parse_ty_for_where_clause(&mut self) -> PResult<'a, P<Ty>> {
 133         self.parse_ty_common(
 134             AllowPlus::Yes,
 135             AllowCVariadic::Yes,
 136             RecoverQPath::Yes,
 137             RecoverReturnSign::OnlyFatArrow,
 138         )
 139     }
 140
 141     /// Parses an optional return type `[ -> TY ]` in a function declaration.
 142     pub(super) fn parse_ret_ty(
 143         &mut self,
 144         allow_plus: AllowPlus,
 145         recover_qpath: RecoverQPath,
 146         recover_return_sign: RecoverReturnSign,
 147     ) -> PResult<'a, FnRetTy> {
 148         Ok(if self.eat(&token::RArrow) {
 149             // FIXME(Centril): Can we unconditionally `allow_plus`?
 150             let ty = self.parse_ty_common(
 151                 allow_plus,
 152                 AllowCVariadic::No,
 153                 recover_qpath,
 154                 recover_return_sign,
 155             )?;
 156             FnRetTy::Ty(ty)
 157         } else if recover_return_sign.can_recover(&self.token.kind) {
 158             // Don't `eat` to prevent `=>` from being added as an expected token which isn't
 159             // actually expected and could only confuse users
 160             self.bump();
 161             self.struct_span_err(self.prev_token.span, "return types are denoted using `->`")
 162                 .span_suggestion_short(
 163                     self.prev_token.span,
 164                     "use `->` instead",
 165                     "->".to_string(),
 166                     Applicability::MachineApplicable,
 167                 )
 168                 .emit();
 169             let ty = self.parse_ty_common(
 170                 allow_plus,
 171                 AllowCVariadic::No,
 172                 recover_qpath,
 173                 recover_return_sign,
 174             )?;
 175             FnRetTy::Ty(ty)
 176         } else {
 177             FnRetTy::Default(self.token.span.shrink_to_lo())
 178         })
 179     }
 180
 181     fn parse_ty_common(
 182         &mut self,
 183         allow_plus: AllowPlus,
 184         allow_c_variadic: AllowCVariadic,
 185         recover_qpath: RecoverQPath,
 186         recover_return_sign: RecoverReturnSign,
 187     ) -> PResult<'a, P<Ty>> {
 188         let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
 189         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
 190         maybe_whole!(self, NtTy, |x| x);
 191
 192         let lo = self.token.span;
 193         let mut impl_dyn_multi = false;
 194         let kind = if self.check(&token::OpenDelim(token::Paren)) {
 195             self.parse_ty_tuple_or_parens(lo, allow_plus)?
 196         } else if self.eat(&token::Not) {
 197             // Never type `!`
 198             TyKind::Never
 199         } else if self.eat(&token::BinOp(token::Star)) {
 200             self.parse_ty_ptr()?
 201         } else if self.eat(&token::OpenDelim(token::Bracket)) {
 202             self.parse_array_or_slice_ty()?
 203         } else if self.check(&token::BinOp(token::And)) || self.check(&token::AndAnd) {
 204             // Reference
 205             self.expect_and()?;
 206             self.parse_borrowed_pointee()?
 207         } else if self.eat_keyword_noexpect(kw::Typeof) {
 208             self.parse_typeof_ty()?
 209         } else if self.eat_keyword(kw::Underscore) {
 210             // A type to be inferred `_`
 211             TyKind::Infer
 212         } else if self.check_fn_front_matter() {
 213             // Function pointer type
 214             self.parse_ty_bare_fn(lo, Vec::new(), recover_return_sign)?
 215         } else if self.check_keyword(kw::For) {
 216             // Function pointer type or bound list (trait object type) starting with a poly-trait.
 217             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
 218             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
 219             let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 220             if self.check_fn_front_matter() {
 221                 self.parse_ty_bare_fn(lo, lifetime_defs, recover_return_sign)?
 222             } else {
 223                 let path = self.parse_path(PathStyle::Type)?;
 224                 let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
 225                 self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?
 226             }
 227         } else if self.eat_keyword(kw::Impl) {
 228             self.parse_impl_ty(&mut impl_dyn_multi)?
 229         } else if self.is_explicit_dyn_type() {
 230             self.parse_dyn_ty(&mut impl_dyn_multi)?
 231         } else if self.eat_lt() {
 232             // Qualified path
 233             let (qself, path) = self.parse_qpath(PathStyle::Type)?;
 234             TyKind::Path(Some(qself), path)
 235         } else if self.check_path() {
 236             self.parse_path_start_ty(lo, allow_plus)?
 237         } else if self.can_begin_bound() {
 238             self.parse_bare_trait_object(lo, allow_plus)?
 239         } else if self.eat(&token::DotDotDot) {
 240             if allow_c_variadic == AllowCVariadic::Yes {
 241                 TyKind::CVarArgs
 242             } else {
 243                 // FIXME(Centril): Should we just allow `...` syntactically
 244                 // anywhere in a type and use semantic restrictions instead?
 245                 self.error_illegal_c_varadic_ty(lo);
 246                 TyKind::Err
 247             }
 248         } else {
 249             let msg = format!("expected type, found {}", super::token_descr(&self.token));
 250             let mut err = self.struct_span_err(self.token.span, &msg);
 251             err.span_label(self.token.span, "expected type");
 252             self.maybe_annotate_with_ascription(&mut err, true);
 253             return Err(err);
 254         };
 255
 256         let span = lo.to(self.prev_token.span);
 257         let ty = self.mk_ty(span, kind);
 258
 259         // Try to recover from use of `+` with incorrect priority.
 260         self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
 261         self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
 262         self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
 263     }
 264
 265     /// Parses either:
 266     /// - `(TYPE)`, a parenthesized type.
 267     /// - `(TYPE,)`, a tuple with a single field of type TYPE.
 268     fn parse_ty_tuple_or_parens(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 269         let mut trailing_plus = false;
 270         let (ts, trailing) = self.parse_paren_comma_seq(|p| {
 271             let ty = p.parse_ty()?;
 272             trailing_plus = p.prev_token.kind == TokenKind::BinOp(token::Plus);
 273             Ok(ty)
 274         })?;
 275
 276         if ts.len() == 1 && !trailing {
 277             let ty = ts.into_iter().next().unwrap().into_inner();
 278             let maybe_bounds = allow_plus == AllowPlus::Yes && self.token.is_like_plus();
 279             match ty.kind {
 280                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
 281                 TyKind::Path(None, path) if maybe_bounds => {
 282                     self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 283                 }
 284                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
 285                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
 286                 {
 287                     self.parse_remaining_bounds(bounds, true)
 288                 }
 289                 // `(TYPE)`
 290                 _ => Ok(TyKind::Paren(P(ty))),
 291             }
 292         } else {
 293             Ok(TyKind::Tup(ts))
 294         }
 295     }
 296
 297     fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 298         let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
 299         let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
 300         if lt_no_plus {
 301             self.struct_span_err(lo, "lifetime in trait object type must be followed by `+`").emit()
 302         }
 303         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 304     }
 305
 306     fn parse_remaining_bounds_path(
 307         &mut self,
 308         generic_params: Vec<GenericParam>,
 309         path: ast::Path,
 310         lo: Span,
 311         parse_plus: bool,
 312     ) -> PResult<'a, TyKind> {
 313         let poly_trait_ref = PolyTraitRef::new(generic_params, path, lo.to(self.prev_token.span));
 314         let bounds = vec![GenericBound::Trait(poly_trait_ref, TraitBoundModifier::None)];
 315         self.parse_remaining_bounds(bounds, parse_plus)
 316     }
 317
 318     /// Parse the remainder of a bare trait object type given an already parsed list.
 319     fn parse_remaining_bounds(
 320         &mut self,
 321         mut bounds: GenericBounds,
 322         plus: bool,
 323     ) -> PResult<'a, TyKind> {
 324         assert_ne!(self.token, token::Question);
 325         if plus {
 326             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
 327             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);
 328         }
 329         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 330     }
 331
 332     /// Parses a raw pointer type: `*[const | mut] $type`.
 333     fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
 334         let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
 335             let span = self.prev_token.span;
 336             let msg = "expected mut or const in raw pointer type";
 337             self.struct_span_err(span, msg)
 338                 .span_label(span, msg)
 339                 .help("use `*mut T` or `*const T` as appropriate")
 340                 .emit();
 341             Mutability::Not
 342         });
 343         let ty = self.parse_ty_no_plus()?;
 344         Ok(TyKind::Ptr(MutTy { ty, mutbl }))
 345     }
 346
 347     /// Parses an array (`[TYPE; EXPR]`) or slice (`[TYPE]`) type.
 348     /// The opening `[` bracket is already eaten.
 349     fn parse_array_or_slice_ty(&mut self) -> PResult<'a, TyKind> {
 350         let elt_ty = match self.parse_ty() {
 351             Ok(ty) => ty,
 352             Err(mut err)
 353                 if self.look_ahead(1, |t| t.kind == token::CloseDelim(token::Bracket))
 354                     | self.look_ahead(1, |t| t.kind == token::Semi) =>
 355             {
 356                 // Recover from `[LIT; EXPR]` and `[LIT]`
 357                 self.bump();
 358                 err.emit();
 359                 self.mk_ty(self.prev_token.span, TyKind::Err)
 360             }
 361             Err(err) => return Err(err),
 362         };
 363         let ty = if self.eat(&token::Semi) {
 364             TyKind::Array(elt_ty, self.parse_anon_const_expr()?)
 365         } else {
 366             TyKind::Slice(elt_ty)
 367         };
 368         self.expect(&token::CloseDelim(token::Bracket))?;
 369         Ok(ty)
 370     }
 371
 372     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
 373         let and_span = self.prev_token.span;
 374         let mut opt_lifetime =
 375             if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
 376         let mutbl = self.parse_mutability();
 377         if self.token.is_lifetime() && mutbl == Mutability::Mut && opt_lifetime.is_none() {
 378             // A lifetime is invalid here: it would be part of a bare trait bound, which requires
 379             // it to be followed by a plus, but we disallow plus in the pointee type.
 380             // So we can handle this case as an error here, and suggest `'a mut`.
 381             // If there *is* a plus next though, handling the error later provides better suggestions
 382             // (like adding parentheses)
 383             if !self.look_ahead(1, |t| t.is_like_plus()) {
 384                 let lifetime_span = self.token.span;
 385                 let span = and_span.to(lifetime_span);
 386
 387                 let mut err = self.struct_span_err(span, "lifetime must precede `mut`");
 388                 if let Ok(lifetime_src) = self.span_to_snippet(lifetime_span) {
 389                     err.span_suggestion(
 390                         span,
 391                         "place the lifetime before `mut`",
 392                         format!("&{} mut", lifetime_src),
 393                         Applicability::MaybeIncorrect,
 394                     );
 395                 }
 396                 err.emit();
 397
 398                 opt_lifetime = Some(self.expect_lifetime());
 399             }
 400         }
 401         let ty = self.parse_ty_no_plus()?;
 402         Ok(TyKind::Rptr(opt_lifetime, MutTy { ty, mutbl }))
 403     }
 404
 405     // Parses the `typeof(EXPR)`.
 406     // To avoid ambiguity, the type is surrounded by parenthesis.
 407     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
 408         self.expect(&token::OpenDelim(token::Paren))?;
 409         let expr = self.parse_anon_const_expr()?;
 410         self.expect(&token::CloseDelim(token::Paren))?;
 411         Ok(TyKind::Typeof(expr))
 412     }
 413
 414     /// Parses a function pointer type (`TyKind::BareFn`).
 415     /// ```
 416     /// [unsafe] [extern "ABI"] fn (S) -> T
 417     ///  ^~~~~^          ^~~~^     ^~^    ^
 418     ///    |               |        |     |
 419     ///    |               |        |   Return type
 420     /// Function Style    ABI  Parameter types
 421     /// ```
 422     /// We actually parse `FnHeader FnDecl`, but we error on `const` and `async` qualifiers.
 423     fn parse_ty_bare_fn(
 424         &mut self,
 425         lo: Span,
 426         params: Vec<GenericParam>,
 427         recover_return_sign: RecoverReturnSign,
 428     ) -> PResult<'a, TyKind> {
 429         let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
 430         let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
 431         let whole_span = lo.to(self.prev_token.span);
 432         if let ast::Const::Yes(span) = constness {
 433             self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
 434         }
 435         if let ast::Async::Yes { span, .. } = asyncness {
 436             self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
 437         }
 438         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
 439     }
 440
 441     /// Emit an error for the given bad function pointer qualifier.
 442     fn error_fn_ptr_bad_qualifier(&self, span: Span, qual_span: Span, qual: &str) {
 443         self.struct_span_err(span, &format!("an `fn` pointer type cannot be `{}`", qual))
 444             .span_label(qual_span, format!("`{}` because of this", qual))
 445             .span_suggestion_short(
 446                 qual_span,
 447                 &format!("remove the `{}` qualifier", qual),
 448                 String::new(),
 449                 Applicability::MaybeIncorrect,
 450             )
 451             .emit();
 452     }
 453
 454     /// Parses an `impl B0 + ... + Bn` type.
 455     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 456         // Always parse bounds greedily for better error recovery.
 457         let bounds = self.parse_generic_bounds(None)?;
 458         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 459         Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
 460     }
 461
 462     /// Is a `dyn B0 + ... + Bn` type allowed here?
 463     fn is_explicit_dyn_type(&mut self) -> bool {
 464         self.check_keyword(kw::Dyn)
 465             && (self.token.uninterpolated_span().rust_2018()
 466                 || self.look_ahead(1, |t| {
 467                     t.can_begin_bound() && !can_continue_type_after_non_fn_ident(t)
 468                 }))
 469     }
 470
 471     /// Parses a `dyn B0 + ... + Bn` type.
 472     ///
 473     /// Note that this does *not* parse bare trait objects.
 474     fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 475         self.bump(); // `dyn`
 476         // Always parse bounds greedily for better error recovery.
 477         let bounds = self.parse_generic_bounds(None)?;
 478         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 479         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn))
 480     }
 481
 482     /// Parses a type starting with a path.
 483     ///
 484     /// This can be:
 485     /// 1. a type macro, `mac!(...)`,
 486     /// 2. a bare trait object, `B0 + ... + Bn`,
 487     /// 3. or a path, `path::to::MyType`.
 488     fn parse_path_start_ty(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 489         // Simple path
 490         let path = self.parse_path(PathStyle::Type)?;
 491         if self.eat(&token::Not) {
 492             // Macro invocation in type position
 493             Ok(TyKind::MacCall(MacCall {
 494                 path,
 495                 args: self.parse_mac_args()?,
 496                 prior_type_ascription: self.last_type_ascription,
 497             }))
 498         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
 499             // `Trait1 + Trait2 + 'a`
 500             self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 501         } else {
 502             // Just a type path.
 503             Ok(TyKind::Path(None, path))
 504         }
 505     }
 506
 507     fn error_illegal_c_varadic_ty(&self, lo: Span) {
 508         struct_span_err!(
 509             self.sess.span_diagnostic,
 510             lo.to(self.prev_token.span),
 511             E0743,
 512             "C-variadic type `...` may not be nested inside another type",
 513         )
 514         .emit();
 515     }
 516
 517     pub(super) fn parse_generic_bounds(
 518         &mut self,
 519         colon_span: Option<Span>,
 520     ) -> PResult<'a, GenericBounds> {
 521         self.parse_generic_bounds_common(AllowPlus::Yes, colon_span)
 522     }
 523
 524     /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
 525     ///
 526     /// See `parse_generic_bound` for the `BOUND` grammar.
 527     fn parse_generic_bounds_common(
 528         &mut self,
 529         allow_plus: AllowPlus,
 530         colon_span: Option<Span>,
 531     ) -> PResult<'a, GenericBounds> {
 532         let mut bounds = Vec::new();
 533         let mut negative_bounds = Vec::new();
 534         while self.can_begin_bound() {
 535             match self.parse_generic_bound()? {
 536                 Ok(bound) => bounds.push(bound),
 537                 Err(neg_sp) => negative_bounds.push(neg_sp),
 538             }
 539             if allow_plus == AllowPlus::No || !self.eat_plus() {
 540                 break;
 541             }
 542         }
 543
 544         if !negative_bounds.is_empty() {
 545             self.error_negative_bounds(colon_span, &bounds, negative_bounds);
 546         }
 547
 548         Ok(bounds)
 549     }
 550
 551     /// Can the current token begin a bound?
 552     fn can_begin_bound(&mut self) -> bool {
 553         // This needs to be synchronized with `TokenKind::can_begin_bound`.
 554         self.check_path()
 555         || self.check_lifetime()
 556         || self.check(&token::Not) // Used for error reporting only.
 557         || self.check(&token::Question)
 558         || self.check_keyword(kw::For)
 559         || self.check(&token::OpenDelim(token::Paren))
 560     }
 561
 562     fn error_negative_bounds(
 563         &self,
 564         colon_span: Option<Span>,
 565         bounds: &[GenericBound],
 566         negative_bounds: Vec<Span>,
 567     ) {
 568         let negative_bounds_len = negative_bounds.len();
 569         let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
 570         let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
 571         err.span_label(last_span, "negative bounds are not supported");
 572         if let Some(bound_list) = colon_span {
 573             let bound_list = bound_list.to(self.prev_token.span);
 574             let mut new_bound_list = String::new();
 575             if !bounds.is_empty() {
 576                 let mut snippets = bounds.iter().map(|bound| self.span_to_snippet(bound.span()));
 577                 while let Some(Ok(snippet)) = snippets.next() {
 578                     new_bound_list.push_str(" + ");
 579                     new_bound_list.push_str(&snippet);
 580                 }
 581                 new_bound_list = new_bound_list.replacen(" +", ":", 1);
 582             }
 583             err.tool_only_span_suggestion(
 584                 bound_list,
 585                 &format!("remove the bound{}", pluralize!(negative_bounds_len)),
 586                 new_bound_list,
 587                 Applicability::MachineApplicable,
 588             );
 589         }
 590         err.emit();
 591     }
 592
 593     /// Parses a bound according to the grammar:
 594     /// ```
 595     /// BOUND = TY_BOUND | LT_BOUND
 596     /// ```
 597     fn parse_generic_bound(&mut self) -> PResult<'a, Result<GenericBound, Span>> {
 598         let anchor_lo = self.prev_token.span;
 599         let lo = self.token.span;
 600         let has_parens = self.eat(&token::OpenDelim(token::Paren));
 601         let inner_lo = self.token.span;
 602         let is_negative = self.eat(&token::Not);
 603
 604         let modifiers = self.parse_ty_bound_modifiers();
 605         let bound = if self.token.is_lifetime() {
 606             self.error_lt_bound_with_modifiers(modifiers);
 607             self.parse_generic_lt_bound(lo, inner_lo, has_parens)?
 608         } else {
 609             self.parse_generic_ty_bound(lo, has_parens, modifiers)?
 610         };
 611
 612         Ok(if is_negative { Err(anchor_lo.to(self.prev_token.span)) } else { Ok(bound) })
 613     }
 614
 615     /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
 616     /// ```
 617     /// LT_BOUND = LIFETIME
 618     /// ```
 619     fn parse_generic_lt_bound(
 620         &mut self,
 621         lo: Span,
 622         inner_lo: Span,
 623         has_parens: bool,
 624     ) -> PResult<'a, GenericBound> {
 625         let bound = GenericBound::Outlives(self.expect_lifetime());
 626         if has_parens {
 627             // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
 628             // possibly introducing `GenericBound::Paren(P<GenericBound>)`?
 629             self.recover_paren_lifetime(lo, inner_lo)?;
 630         }
 631         Ok(bound)
 632     }
 633
 634     /// Emits an error if any trait bound modifiers were present.
 635     fn error_lt_bound_with_modifiers(&self, modifiers: BoundModifiers) {
 636         if let Some(span) = modifiers.maybe_const {
 637             self.struct_span_err(
 638                 span,
 639                 "`?const` may only modify trait bounds, not lifetime bounds",
 640             )
 641             .emit();
 642         }
 643
 644         if let Some(span) = modifiers.maybe {
 645             self.struct_span_err(span, "`?` may only modify trait bounds, not lifetime bounds")
 646                 .emit();
 647         }
 648     }
 649
 650     /// Recover on `('lifetime)` with `(` already eaten.
 651     fn recover_paren_lifetime(&mut self, lo: Span, inner_lo: Span) -> PResult<'a, ()> {
 652         let inner_span = inner_lo.to(self.prev_token.span);
 653         self.expect(&token::CloseDelim(token::Paren))?;
 654         let mut err = self.struct_span_err(
 655             lo.to(self.prev_token.span),
 656             "parenthesized lifetime bounds are not supported",
 657         );
 658         if let Ok(snippet) = self.span_to_snippet(inner_span) {
 659             err.span_suggestion_short(
 660                 lo.to(self.prev_token.span),
 661                 "remove the parentheses",
 662                 snippet,
 663                 Applicability::MachineApplicable,
 664             );
 665         }
 666         err.emit();
 667         Ok(())
 668     }
 669
 670     /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `?const Trait`.
 671     ///
 672     /// If no modifiers are present, this does not consume any tokens.
 673     ///
 674     /// ```
 675     /// TY_BOUND_MODIFIERS = "?" ["const" ["?"]]
 676     /// ```
 677     fn parse_ty_bound_modifiers(&mut self) -> BoundModifiers {
 678         if !self.eat(&token::Question) {
 679             return BoundModifiers { maybe: None, maybe_const: None };
 680         }
 681
 682         // `? ...`
 683         let first_question = self.prev_token.span;
 684         if !self.eat_keyword(kw::Const) {
 685             return BoundModifiers { maybe: Some(first_question), maybe_const: None };
 686         }
 687
 688         // `?const ...`
 689         let maybe_const = first_question.to(self.prev_token.span);
 690         self.sess.gated_spans.gate(sym::const_trait_bound_opt_out, maybe_const);
 691         if !self.eat(&token::Question) {
 692             return BoundModifiers { maybe: None, maybe_const: Some(maybe_const) };
 693         }
 694
 695         // `?const ? ...`
 696         let second_question = self.prev_token.span;
 697         BoundModifiers { maybe: Some(second_question), maybe_const: Some(maybe_const) }
 698     }
 699
 700     /// Parses a type bound according to:
 701     /// ```
 702     /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
 703     /// TY_BOUND_NOPAREN = [TY_BOUND_MODIFIERS] [for<LT_PARAM_DEFS>] SIMPLE_PATH
 704     /// ```
 705     ///
 706     /// For example, this grammar accepts `?const ?for<'a: 'b> m::Trait<'a>`.
 707     fn parse_generic_ty_bound(
 708         &mut self,
 709         lo: Span,
 710         has_parens: bool,
 711         modifiers: BoundModifiers,
 712     ) -> PResult<'a, GenericBound> {
 713         let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 714         let path = self.parse_path(PathStyle::Type)?;
 715         if has_parens {
 716             self.expect(&token::CloseDelim(token::Paren))?;
 717         }
 718
 719         let modifier = modifiers.to_trait_bound_modifier();
 720         let poly_trait = PolyTraitRef::new(lifetime_defs, path, lo.to(self.prev_token.span));
 721         Ok(GenericBound::Trait(poly_trait, modifier))
 722     }
 723
 724     /// Optionally parses `for<$generic_params>`.
 725     pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
 726         if self.eat_keyword(kw::For) {
 727             self.expect_lt()?;
 728             let params = self.parse_generic_params()?;
 729             self.expect_gt()?;
 730             // We rely on AST validation to rule out invalid cases: There must not be type
 731             // parameters, and the lifetime parameters must not have bounds.
 732             Ok(params)
 733         } else {
 734             Ok(Vec::new())
 735         }
 736     }
 737
 738     pub(super) fn check_lifetime(&mut self) -> bool {
 739         self.expected_tokens.push(TokenType::Lifetime);
 740         self.token.is_lifetime()
 741     }
 742
 743     /// Parses a single lifetime `'a` or panics.
 744     pub(super) fn expect_lifetime(&mut self) -> Lifetime {
 745         if let Some(ident) = self.token.lifetime() {
 746             self.bump();
 747             Lifetime { ident, id: ast::DUMMY_NODE_ID }
 748         } else {
 749             self.span_bug(self.token.span, "not a lifetime")
 750         }
 751     }
 752
 753     pub(super) fn mk_ty(&self, span: Span, kind: TyKind) -> P<Ty> {
 754         P(Ty { kind, span, id: ast::DUMMY_NODE_ID, tokens: None })
 755     }
 756 }