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(false) {
 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(false) {
 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.token.is_keyword(kw::Union)
 230             && self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace))
 231         {
 232             self.bump();
 233             let (fields, recovered) = self.parse_record_struct_body("union")?;
 234             let span = lo.to(self.prev_token.span);
 235             self.sess.gated_spans.gate(sym::unnamed_fields, span);
 236             TyKind::AnonymousUnion(fields, recovered)
 237         } else if self.eat_keyword(kw::Struct) {
 238             let (fields, recovered) = self.parse_record_struct_body("struct")?;
 239             let span = lo.to(self.prev_token.span);
 240             self.sess.gated_spans.gate(sym::unnamed_fields, span);
 241             TyKind::AnonymousStruct(fields, recovered)
 242         } else if self.is_explicit_dyn_type() {
 243             self.parse_dyn_ty(&mut impl_dyn_multi)?
 244         } else if self.eat_lt() {
 245             // Qualified path
 246             let (qself, path) = self.parse_qpath(PathStyle::Type)?;
 247             TyKind::Path(Some(qself), path)
 248         } else if self.check_path() {
 249             self.parse_path_start_ty(lo, allow_plus)?
 250         } else if self.can_begin_bound() {
 251             self.parse_bare_trait_object(lo, allow_plus)?
 252         } else if self.eat(&token::DotDotDot) {
 253             if allow_c_variadic == AllowCVariadic::Yes {
 254                 TyKind::CVarArgs
 255             } else {
 256                 // FIXME(Centril): Should we just allow `...` syntactically
 257                 // anywhere in a type and use semantic restrictions instead?
 258                 self.error_illegal_c_varadic_ty(lo);
 259                 TyKind::Err
 260             }
 261         } else {
 262             let msg = format!("expected type, found {}", super::token_descr(&self.token));
 263             let mut err = self.struct_span_err(self.token.span, &msg);
 264             err.span_label(self.token.span, "expected type");
 265             self.maybe_annotate_with_ascription(&mut err, true);
 266             return Err(err);
 267         };
 268
 269         let span = lo.to(self.prev_token.span);
 270         let ty = self.mk_ty(span, kind);
 271
 272         // Try to recover from use of `+` with incorrect priority.
 273         self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
 274         self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
 275         self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
 276     }
 277
 278     /// Parses either:
 279     /// - `(TYPE)`, a parenthesized type.
 280     /// - `(TYPE,)`, a tuple with a single field of type TYPE.
 281     fn parse_ty_tuple_or_parens(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 282         let mut trailing_plus = false;
 283         let (ts, trailing) = self.parse_paren_comma_seq(|p| {
 284             let ty = p.parse_ty()?;
 285             trailing_plus = p.prev_token.kind == TokenKind::BinOp(token::Plus);
 286             Ok(ty)
 287         })?;
 288
 289         if ts.len() == 1 && !trailing {
 290             let ty = ts.into_iter().next().unwrap().into_inner();
 291             let maybe_bounds = allow_plus == AllowPlus::Yes && self.token.is_like_plus();
 292             match ty.kind {
 293                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
 294                 TyKind::Path(None, path) if maybe_bounds => {
 295                     self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 296                 }
 297                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
 298                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
 299                 {
 300                     self.parse_remaining_bounds(bounds, true)
 301                 }
 302                 // `(TYPE)`
 303                 _ => Ok(TyKind::Paren(P(ty))),
 304             }
 305         } else {
 306             Ok(TyKind::Tup(ts))
 307         }
 308     }
 309
 310     fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 311         let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
 312         let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
 313         if lt_no_plus {
 314             self.struct_span_err(lo, "lifetime in trait object type must be followed by `+`").emit()
 315         }
 316         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 317     }
 318
 319     fn parse_remaining_bounds_path(
 320         &mut self,
 321         generic_params: Vec<GenericParam>,
 322         path: ast::Path,
 323         lo: Span,
 324         parse_plus: bool,
 325     ) -> PResult<'a, TyKind> {
 326         let poly_trait_ref = PolyTraitRef::new(generic_params, path, lo.to(self.prev_token.span));
 327         let bounds = vec![GenericBound::Trait(poly_trait_ref, TraitBoundModifier::None)];
 328         self.parse_remaining_bounds(bounds, parse_plus)
 329     }
 330
 331     /// Parse the remainder of a bare trait object type given an already parsed list.
 332     fn parse_remaining_bounds(
 333         &mut self,
 334         mut bounds: GenericBounds,
 335         plus: bool,
 336     ) -> PResult<'a, TyKind> {
 337         if plus {
 338             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
 339             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);
 340         }
 341         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
 342     }
 343
 344     /// Parses a raw pointer type: `*[const | mut] $type`.
 345     fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
 346         let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
 347             let span = self.prev_token.span;
 348             let msg = "expected mut or const in raw pointer type";
 349             self.struct_span_err(span, msg)
 350                 .span_label(span, msg)
 351                 .help("use `*mut T` or `*const T` as appropriate")
 352                 .emit();
 353             Mutability::Not
 354         });
 355         let ty = self.parse_ty_no_plus()?;
 356         Ok(TyKind::Ptr(MutTy { ty, mutbl }))
 357     }
 358
 359     /// Parses an array (`[TYPE; EXPR]`) or slice (`[TYPE]`) type.
 360     /// The opening `[` bracket is already eaten.
 361     fn parse_array_or_slice_ty(&mut self) -> PResult<'a, TyKind> {
 362         let elt_ty = match self.parse_ty() {
 363             Ok(ty) => ty,
 364             Err(mut err)
 365                 if self.look_ahead(1, |t| t.kind == token::CloseDelim(token::Bracket))
 366                     | self.look_ahead(1, |t| t.kind == token::Semi) =>
 367             {
 368                 // Recover from `[LIT; EXPR]` and `[LIT]`
 369                 self.bump();
 370                 err.emit();
 371                 self.mk_ty(self.prev_token.span, TyKind::Err)
 372             }
 373             Err(err) => return Err(err),
 374         };
 375
 376         let ty = if self.eat(&token::Semi) {
 377             let mut length = self.parse_anon_const_expr()?;
 378             if let Err(e) = self.expect(&token::CloseDelim(token::Bracket)) {
 379                 // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works
 380                 self.check_mistyped_turbofish_with_multiple_type_params(e, &mut length.value)?;
 381                 self.expect(&token::CloseDelim(token::Bracket))?;
 382             }
 383             TyKind::Array(elt_ty, length)
 384         } else {
 385             self.expect(&token::CloseDelim(token::Bracket))?;
 386             TyKind::Slice(elt_ty)
 387         };
 388
 389         Ok(ty)
 390     }
 391
 392     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
 393         let and_span = self.prev_token.span;
 394         let mut opt_lifetime =
 395             if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
 396         let mut mutbl = self.parse_mutability();
 397         if self.token.is_lifetime() && mutbl == Mutability::Mut && opt_lifetime.is_none() {
 398             // A lifetime is invalid here: it would be part of a bare trait bound, which requires
 399             // it to be followed by a plus, but we disallow plus in the pointee type.
 400             // So we can handle this case as an error here, and suggest `'a mut`.
 401             // If there *is* a plus next though, handling the error later provides better suggestions
 402             // (like adding parentheses)
 403             if !self.look_ahead(1, |t| t.is_like_plus()) {
 404                 let lifetime_span = self.token.span;
 405                 let span = and_span.to(lifetime_span);
 406
 407                 let mut err = self.struct_span_err(span, "lifetime must precede `mut`");
 408                 if let Ok(lifetime_src) = self.span_to_snippet(lifetime_span) {
 409                     err.span_suggestion(
 410                         span,
 411                         "place the lifetime before `mut`",
 412                         format!("&{} mut", lifetime_src),
 413                         Applicability::MaybeIncorrect,
 414                     );
 415                 }
 416                 err.emit();
 417
 418                 opt_lifetime = Some(self.expect_lifetime());
 419             }
 420         } else if self.token.is_keyword(kw::Dyn)
 421             && mutbl == Mutability::Not
 422             && self.look_ahead(1, |t| t.is_keyword(kw::Mut))
 423         {
 424             // We have `&dyn mut ...`, which is invalid and should be `&mut dyn ...`.
 425             let span = and_span.to(self.look_ahead(1, |t| t.span));
 426             let mut err = self.struct_span_err(span, "`mut` must precede `dyn`");
 427             err.span_suggestion(
 428                 span,
 429                 "place `mut` before `dyn`",
 430                 "&mut dyn".to_string(),
 431                 Applicability::MachineApplicable,
 432             );
 433             err.emit();
 434
 435             // Recovery
 436             mutbl = Mutability::Mut;
 437             let (dyn_tok, dyn_tok_sp) = (self.token.clone(), self.token_spacing);
 438             self.bump();
 439             self.bump_with((dyn_tok, dyn_tok_sp));
 440         }
 441         let ty = self.parse_ty_no_plus()?;
 442         Ok(TyKind::Rptr(opt_lifetime, MutTy { ty, mutbl }))
 443     }
 444
 445     // Parses the `typeof(EXPR)`.
 446     // To avoid ambiguity, the type is surrounded by parenthesis.
 447     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
 448         self.expect(&token::OpenDelim(token::Paren))?;
 449         let expr = self.parse_anon_const_expr()?;
 450         self.expect(&token::CloseDelim(token::Paren))?;
 451         Ok(TyKind::Typeof(expr))
 452     }
 453
 454     /// Parses a function pointer type (`TyKind::BareFn`).
 455     /// ```
 456     /// [unsafe] [extern "ABI"] fn (S) -> T
 457     ///  ^~~~~^          ^~~~^     ^~^    ^
 458     ///    |               |        |     |
 459     ///    |               |        |   Return type
 460     /// Function Style    ABI  Parameter types
 461     /// ```
 462     /// We actually parse `FnHeader FnDecl`, but we error on `const` and `async` qualifiers.
 463     fn parse_ty_bare_fn(
 464         &mut self,
 465         lo: Span,
 466         params: Vec<GenericParam>,
 467         recover_return_sign: RecoverReturnSign,
 468     ) -> PResult<'a, TyKind> {
 469         let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
 470         let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
 471         let whole_span = lo.to(self.prev_token.span);
 472         if let ast::Const::Yes(span) = constness {
 473             self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
 474         }
 475         if let ast::Async::Yes { span, .. } = asyncness {
 476             self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
 477         }
 478         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
 479     }
 480
 481     /// Emit an error for the given bad function pointer qualifier.
 482     fn error_fn_ptr_bad_qualifier(&self, span: Span, qual_span: Span, qual: &str) {
 483         self.struct_span_err(span, &format!("an `fn` pointer type cannot be `{}`", qual))
 484             .span_label(qual_span, format!("`{}` because of this", qual))
 485             .span_suggestion_short(
 486                 qual_span,
 487                 &format!("remove the `{}` qualifier", qual),
 488                 String::new(),
 489                 Applicability::MaybeIncorrect,
 490             )
 491             .emit();
 492     }
 493
 494     /// Parses an `impl B0 + ... + Bn` type.
 495     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 496         // Always parse bounds greedily for better error recovery.
 497         let bounds = self.parse_generic_bounds(None)?;
 498         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 499         Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
 500     }
 501
 502     /// Is a `dyn B0 + ... + Bn` type allowed here?
 503     fn is_explicit_dyn_type(&mut self) -> bool {
 504         self.check_keyword(kw::Dyn)
 505             && (!self.token.uninterpolated_span().rust_2015()
 506                 || self.look_ahead(1, |t| {
 507                     t.can_begin_bound() && !can_continue_type_after_non_fn_ident(t)
 508                 }))
 509     }
 510
 511     /// Parses a `dyn B0 + ... + Bn` type.
 512     ///
 513     /// Note that this does *not* parse bare trait objects.
 514     fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
 515         self.bump(); // `dyn`
 516         // Always parse bounds greedily for better error recovery.
 517         let bounds = self.parse_generic_bounds(None)?;
 518         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
 519         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn))
 520     }
 521
 522     /// Parses a type starting with a path.
 523     ///
 524     /// This can be:
 525     /// 1. a type macro, `mac!(...)`,
 526     /// 2. a bare trait object, `B0 + ... + Bn`,
 527     /// 3. or a path, `path::to::MyType`.
 528     fn parse_path_start_ty(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
 529         // Simple path
 530         let path = self.parse_path(PathStyle::Type)?;
 531         if self.eat(&token::Not) {
 532             // Macro invocation in type position
 533             Ok(TyKind::MacCall(MacCall {
 534                 path,
 535                 args: self.parse_mac_args()?,
 536                 prior_type_ascription: self.last_type_ascription,
 537             }))
 538         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
 539             // `Trait1 + Trait2 + 'a`
 540             self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
 541         } else {
 542             // Just a type path.
 543             Ok(TyKind::Path(None, path))
 544         }
 545     }
 546
 547     fn error_illegal_c_varadic_ty(&self, lo: Span) {
 548         struct_span_err!(
 549             self.sess.span_diagnostic,
 550             lo.to(self.prev_token.span),
 551             E0743,
 552             "C-variadic type `...` may not be nested inside another type",
 553         )
 554         .emit();
 555     }
 556
 557     pub(super) fn parse_generic_bounds(
 558         &mut self,
 559         colon_span: Option<Span>,
 560     ) -> PResult<'a, GenericBounds> {
 561         self.parse_generic_bounds_common(AllowPlus::Yes, colon_span)
 562     }
 563
 564     /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
 565     ///
 566     /// See `parse_generic_bound` for the `BOUND` grammar.
 567     fn parse_generic_bounds_common(
 568         &mut self,
 569         allow_plus: AllowPlus,
 570         colon_span: Option<Span>,
 571     ) -> PResult<'a, GenericBounds> {
 572         let mut bounds = Vec::new();
 573         let mut negative_bounds = Vec::new();
 574
 575         while self.can_begin_bound() || self.token.is_keyword(kw::Dyn) {
 576             if self.token.is_keyword(kw::Dyn) {
 577                 // Account for `&dyn Trait + dyn Other`.
 578                 self.struct_span_err(self.token.span, "invalid `dyn` keyword")
 579                     .help("`dyn` is only needed at the start of a trait `+`-separated list")
 580                     .span_suggestion(
 581                         self.token.span,
 582                         "remove this keyword",
 583                         String::new(),
 584                         Applicability::MachineApplicable,
 585                     )
 586                     .emit();
 587                 self.bump();
 588             }
 589             match self.parse_generic_bound()? {
 590                 Ok(bound) => bounds.push(bound),
 591                 Err(neg_sp) => negative_bounds.push(neg_sp),
 592             }
 593             if allow_plus == AllowPlus::No || !self.eat_plus() {
 594                 break;
 595             }
 596         }
 597
 598         if !negative_bounds.is_empty() {
 599             self.error_negative_bounds(colon_span, &bounds, negative_bounds);
 600         }
 601
 602         Ok(bounds)
 603     }
 604
 605     /// Can the current token begin a bound?
 606     fn can_begin_bound(&mut self) -> bool {
 607         // This needs to be synchronized with `TokenKind::can_begin_bound`.
 608         self.check_path()
 609         || self.check_lifetime()
 610         || self.check(&token::Not) // Used for error reporting only.
 611         || self.check(&token::Question)
 612         || self.check_keyword(kw::For)
 613         || self.check(&token::OpenDelim(token::Paren))
 614     }
 615
 616     fn error_negative_bounds(
 617         &self,
 618         colon_span: Option<Span>,
 619         bounds: &[GenericBound],
 620         negative_bounds: Vec<Span>,
 621     ) {
 622         let negative_bounds_len = negative_bounds.len();
 623         let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
 624         let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
 625         err.span_label(last_span, "negative bounds are not supported");
 626         if let Some(bound_list) = colon_span {
 627             let bound_list = bound_list.to(self.prev_token.span);
 628             let mut new_bound_list = String::new();
 629             if !bounds.is_empty() {
 630                 let mut snippets = bounds.iter().map(|bound| self.span_to_snippet(bound.span()));
 631                 while let Some(Ok(snippet)) = snippets.next() {
 632                     new_bound_list.push_str(" + ");
 633                     new_bound_list.push_str(&snippet);
 634                 }
 635                 new_bound_list = new_bound_list.replacen(" +", ":", 1);
 636             }
 637             err.tool_only_span_suggestion(
 638                 bound_list,
 639                 &format!("remove the bound{}", pluralize!(negative_bounds_len)),
 640                 new_bound_list,
 641                 Applicability::MachineApplicable,
 642             );
 643         }
 644         err.emit();
 645     }
 646
 647     /// Parses a bound according to the grammar:
 648     /// ```
 649     /// BOUND = TY_BOUND | LT_BOUND
 650     /// ```
 651     fn parse_generic_bound(&mut self) -> PResult<'a, Result<GenericBound, Span>> {
 652         let anchor_lo = self.prev_token.span;
 653         let lo = self.token.span;
 654         let has_parens = self.eat(&token::OpenDelim(token::Paren));
 655         let inner_lo = self.token.span;
 656         let is_negative = self.eat(&token::Not);
 657
 658         let modifiers = self.parse_ty_bound_modifiers();
 659         let bound = if self.token.is_lifetime() {
 660             self.error_lt_bound_with_modifiers(modifiers);
 661             self.parse_generic_lt_bound(lo, inner_lo, has_parens)?
 662         } else {
 663             self.parse_generic_ty_bound(lo, has_parens, modifiers)?
 664         };
 665
 666         Ok(if is_negative { Err(anchor_lo.to(self.prev_token.span)) } else { Ok(bound) })
 667     }
 668
 669     /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
 670     /// ```
 671     /// LT_BOUND = LIFETIME
 672     /// ```
 673     fn parse_generic_lt_bound(
 674         &mut self,
 675         lo: Span,
 676         inner_lo: Span,
 677         has_parens: bool,
 678     ) -> PResult<'a, GenericBound> {
 679         let bound = GenericBound::Outlives(self.expect_lifetime());
 680         if has_parens {
 681             // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
 682             // possibly introducing `GenericBound::Paren(P<GenericBound>)`?
 683             self.recover_paren_lifetime(lo, inner_lo)?;
 684         }
 685         Ok(bound)
 686     }
 687
 688     /// Emits an error if any trait bound modifiers were present.
 689     fn error_lt_bound_with_modifiers(&self, modifiers: BoundModifiers) {
 690         if let Some(span) = modifiers.maybe_const {
 691             self.struct_span_err(
 692                 span,
 693                 "`?const` may only modify trait bounds, not lifetime bounds",
 694             )
 695             .emit();
 696         }
 697
 698         if let Some(span) = modifiers.maybe {
 699             self.struct_span_err(span, "`?` may only modify trait bounds, not lifetime bounds")
 700                 .emit();
 701         }
 702     }
 703
 704     /// Recover on `('lifetime)` with `(` already eaten.
 705     fn recover_paren_lifetime(&mut self, lo: Span, inner_lo: Span) -> PResult<'a, ()> {
 706         let inner_span = inner_lo.to(self.prev_token.span);
 707         self.expect(&token::CloseDelim(token::Paren))?;
 708         let mut err = self.struct_span_err(
 709             lo.to(self.prev_token.span),
 710             "parenthesized lifetime bounds are not supported",
 711         );
 712         if let Ok(snippet) = self.span_to_snippet(inner_span) {
 713             err.span_suggestion_short(
 714                 lo.to(self.prev_token.span),
 715                 "remove the parentheses",
 716                 snippet,
 717                 Applicability::MachineApplicable,
 718             );
 719         }
 720         err.emit();
 721         Ok(())
 722     }
 723
 724     /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `?const Trait`.
 725     ///
 726     /// If no modifiers are present, this does not consume any tokens.
 727     ///
 728     /// ```
 729     /// TY_BOUND_MODIFIERS = "?" ["const" ["?"]]
 730     /// ```
 731     fn parse_ty_bound_modifiers(&mut self) -> BoundModifiers {
 732         if !self.eat(&token::Question) {
 733             return BoundModifiers { maybe: None, maybe_const: None };
 734         }
 735
 736         // `? ...`
 737         let first_question = self.prev_token.span;
 738         if !self.eat_keyword(kw::Const) {
 739             return BoundModifiers { maybe: Some(first_question), maybe_const: None };
 740         }
 741
 742         // `?const ...`
 743         let maybe_const = first_question.to(self.prev_token.span);
 744         self.sess.gated_spans.gate(sym::const_trait_bound_opt_out, maybe_const);
 745         if !self.eat(&token::Question) {
 746             return BoundModifiers { maybe: None, maybe_const: Some(maybe_const) };
 747         }
 748
 749         // `?const ? ...`
 750         let second_question = self.prev_token.span;
 751         BoundModifiers { maybe: Some(second_question), maybe_const: Some(maybe_const) }
 752     }
 753
 754     /// Parses a type bound according to:
 755     /// ```
 756     /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
 757     /// TY_BOUND_NOPAREN = [TY_BOUND_MODIFIERS] [for<LT_PARAM_DEFS>] SIMPLE_PATH
 758     /// ```
 759     ///
 760     /// For example, this grammar accepts `?const ?for<'a: 'b> m::Trait<'a>`.
 761     fn parse_generic_ty_bound(
 762         &mut self,
 763         lo: Span,
 764         has_parens: bool,
 765         modifiers: BoundModifiers,
 766     ) -> PResult<'a, GenericBound> {
 767         let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
 768         let path = self.parse_path(PathStyle::Type)?;
 769         if has_parens {
 770             if self.token.is_like_plus() {
 771                 // Someone has written something like `&dyn (Trait + Other)`. The correct code
 772                 // would be `&(dyn Trait + Other)`, but we don't have access to the appropriate
 773                 // span to suggest that. When written as `&dyn Trait + Other`, an appropriate
 774                 // suggestion is given.
 775                 let bounds = vec![];
 776                 self.parse_remaining_bounds(bounds, true)?;
 777                 self.expect(&token::CloseDelim(token::Paren))?;
 778                 let sp = vec![lo, self.prev_token.span];
 779                 let sugg: Vec<_> = sp.iter().map(|sp| (*sp, String::new())).collect();
 780                 self.struct_span_err(sp, "incorrect braces around trait bounds")
 781                     .multipart_suggestion(
 782                         "remove the parentheses",
 783                         sugg,
 784                         Applicability::MachineApplicable,
 785                     )
 786                     .emit();
 787             } else {
 788                 self.expect(&token::CloseDelim(token::Paren))?;
 789             }
 790         }
 791
 792         let modifier = modifiers.to_trait_bound_modifier();
 793         let poly_trait = PolyTraitRef::new(lifetime_defs, path, lo.to(self.prev_token.span));
 794         Ok(GenericBound::Trait(poly_trait, modifier))
 795     }
 796
 797     /// Optionally parses `for<$generic_params>`.
 798     pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
 799         if self.eat_keyword(kw::For) {
 800             self.expect_lt()?;
 801             let params = self.parse_generic_params()?;
 802             self.expect_gt()?;
 803             // We rely on AST validation to rule out invalid cases: There must not be type
 804             // parameters, and the lifetime parameters must not have bounds.
 805             Ok(params)
 806         } else {
 807             Ok(Vec::new())
 808         }
 809     }
 810
 811     pub(super) fn check_lifetime(&mut self) -> bool {
 812         self.expected_tokens.push(TokenType::Lifetime);
 813         self.token.is_lifetime()
 814     }
 815
 816     /// Parses a single lifetime `'a` or panics.
 817     pub(super) fn expect_lifetime(&mut self) -> Lifetime {
 818         if let Some(ident) = self.token.lifetime() {
 819             self.bump();
 820             Lifetime { ident, id: ast::DUMMY_NODE_ID }
 821         } else {
 822             self.span_bug(self.token.span, "not a lifetime")
 823         }
 824     }
 825
 826     pub(super) fn mk_ty(&self, span: Span, kind: TyKind) -> P<Ty> {
 827         P(Ty { kind, span, id: ast::DUMMY_NODE_ID, tokens: None })
 828     }
 829 }