compiler/rustc_parse/src/parser/diagnostics.rs

   1 use super::ty::AllowPlus;
   2 use super::TokenType;
   3 use super::{BlockMode, Parser, PathStyle, Restrictions, SemiColonMode, SeqSep, TokenExpectType};
   4
   5 use rustc_ast as ast;
   6 use rustc_ast::ptr::P;
   7 use rustc_ast::token::{self, Lit, LitKind, TokenKind};
   8 use rustc_ast::util::parser::AssocOp;
   9 use rustc_ast::{AngleBracketedArg, AngleBracketedArgs, AnonConst, AttrVec};
  10 use rustc_ast::{BinOpKind, BindingMode, Block, BlockCheckMode, Expr, ExprKind, GenericArg, Item};
  11 use rustc_ast::{ItemKind, Mutability, Param, Pat, PatKind, Path, PathSegment, QSelf, Ty, TyKind};
  12 use rustc_ast_pretty::pprust;
  13 use rustc_data_structures::fx::FxHashSet;
  14 use rustc_errors::{pluralize, struct_span_err};
  15 use rustc_errors::{Applicability, DiagnosticBuilder, Handler, PResult};
  16 use rustc_span::source_map::Spanned;
  17 use rustc_span::symbol::{kw, Ident};
  18 use rustc_span::{MultiSpan, Span, SpanSnippetError, DUMMY_SP};
  19
  20 use tracing::{debug, trace};
  21
  22 const TURBOFISH_SUGGESTION_STR: &str =
  23     "use `::<...>` instead of `<...>` to specify type or const arguments";
  24
  25 /// Creates a placeholder argument.
  26 pub(super) fn dummy_arg(ident: Ident) -> Param {
  27     let pat = P(Pat {
  28         id: ast::DUMMY_NODE_ID,
  29         kind: PatKind::Ident(BindingMode::ByValue(Mutability::Not), ident, None),
  30         span: ident.span,
  31         tokens: None,
  32     });
  33     let ty = Ty { kind: TyKind::Err, span: ident.span, id: ast::DUMMY_NODE_ID, tokens: None };
  34     Param {
  35         attrs: AttrVec::default(),
  36         id: ast::DUMMY_NODE_ID,
  37         pat,
  38         span: ident.span,
  39         ty: P(ty),
  40         is_placeholder: false,
  41     }
  42 }
  43
  44 pub enum Error {
  45     UselessDocComment,
  46 }
  47
  48 impl Error {
  49     fn span_err(self, sp: impl Into<MultiSpan>, handler: &Handler) -> DiagnosticBuilder<'_> {
  50         match self {
  51             Error::UselessDocComment => {
  52                 let mut err = struct_span_err!(
  53                     handler,
  54                     sp,
  55                     E0585,
  56                     "found a documentation comment that doesn't document anything",
  57                 );
  58                 err.help(
  59                     "doc comments must come before what they document, maybe a comment was \
  60                           intended with `//`?",
  61                 );
  62                 err
  63             }
  64         }
  65     }
  66 }
  67
  68 pub(super) trait RecoverQPath: Sized + 'static {
  69     const PATH_STYLE: PathStyle = PathStyle::Expr;
  70     fn to_ty(&self) -> Option<P<Ty>>;
  71     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self;
  72 }
  73
  74 impl RecoverQPath for Ty {
  75     const PATH_STYLE: PathStyle = PathStyle::Type;
  76     fn to_ty(&self) -> Option<P<Ty>> {
  77         Some(P(self.clone()))
  78     }
  79     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
  80         Self {
  81             span: path.span,
  82             kind: TyKind::Path(qself, path),
  83             id: ast::DUMMY_NODE_ID,
  84             tokens: None,
  85         }
  86     }
  87 }
  88
  89 impl RecoverQPath for Pat {
  90     fn to_ty(&self) -> Option<P<Ty>> {
  91         self.to_ty()
  92     }
  93     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
  94         Self {
  95             span: path.span,
  96             kind: PatKind::Path(qself, path),
  97             id: ast::DUMMY_NODE_ID,
  98             tokens: None,
  99         }
 100     }
 101 }
 102
 103 impl RecoverQPath for Expr {
 104     fn to_ty(&self) -> Option<P<Ty>> {
 105         self.to_ty()
 106     }
 107     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
 108         Self {
 109             span: path.span,
 110             kind: ExprKind::Path(qself, path),
 111             attrs: AttrVec::new(),
 112             id: ast::DUMMY_NODE_ID,
 113             tokens: None,
 114         }
 115     }
 116 }
 117
 118 /// Control whether the closing delimiter should be consumed when calling `Parser::consume_block`.
 119 crate enum ConsumeClosingDelim {
 120     Yes,
 121     No,
 122 }
 123
 124 #[derive(Clone, Copy)]
 125 pub enum AttemptLocalParseRecovery {
 126     Yes,
 127     No,
 128 }
 129
 130 impl AttemptLocalParseRecovery {
 131     pub fn yes(&self) -> bool {
 132         match self {
 133             AttemptLocalParseRecovery::Yes => true,
 134             AttemptLocalParseRecovery::No => false,
 135         }
 136     }
 137
 138     pub fn no(&self) -> bool {
 139         match self {
 140             AttemptLocalParseRecovery::Yes => false,
 141             AttemptLocalParseRecovery::No => true,
 142         }
 143     }
 144 }
 145
 146 impl<'a> Parser<'a> {
 147     pub(super) fn span_err<S: Into<MultiSpan>>(&self, sp: S, err: Error) -> DiagnosticBuilder<'a> {
 148         err.span_err(sp, self.diagnostic())
 149     }
 150
 151     pub fn struct_span_err<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> DiagnosticBuilder<'a> {
 152         self.sess.span_diagnostic.struct_span_err(sp, m)
 153     }
 154
 155     pub fn span_bug<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> ! {
 156         self.sess.span_diagnostic.span_bug(sp, m)
 157     }
 158
 159     pub(super) fn diagnostic(&self) -> &'a Handler {
 160         &self.sess.span_diagnostic
 161     }
 162
 163     pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
 164         self.sess.source_map().span_to_snippet(span)
 165     }
 166
 167     pub(super) fn expected_ident_found(&self) -> DiagnosticBuilder<'a> {
 168         let mut err = self.struct_span_err(
 169             self.token.span,
 170             &format!("expected identifier, found {}", super::token_descr(&self.token)),
 171         );
 172         let valid_follow = &[
 173             TokenKind::Eq,
 174             TokenKind::Colon,
 175             TokenKind::Comma,
 176             TokenKind::Semi,
 177             TokenKind::ModSep,
 178             TokenKind::OpenDelim(token::DelimToken::Brace),
 179             TokenKind::OpenDelim(token::DelimToken::Paren),
 180             TokenKind::CloseDelim(token::DelimToken::Brace),
 181             TokenKind::CloseDelim(token::DelimToken::Paren),
 182         ];
 183         match self.token.ident() {
 184             Some((ident, false))
 185                 if ident.is_raw_guess()
 186                     && self.look_ahead(1, |t| valid_follow.contains(&t.kind)) =>
 187             {
 188                 err.span_suggestion(
 189                     ident.span,
 190                     "you can escape reserved keywords to use them as identifiers",
 191                     format!("r#{}", ident.name),
 192                     Applicability::MaybeIncorrect,
 193                 );
 194             }
 195             _ => {}
 196         }
 197         if let Some(token_descr) = super::token_descr_opt(&self.token) {
 198             err.span_label(self.token.span, format!("expected identifier, found {}", token_descr));
 199         } else {
 200             err.span_label(self.token.span, "expected identifier");
 201             if self.token == token::Comma && self.look_ahead(1, |t| t.is_ident()) {
 202                 err.span_suggestion(
 203                     self.token.span,
 204                     "remove this comma",
 205                     String::new(),
 206                     Applicability::MachineApplicable,
 207                 );
 208             }
 209         }
 210         err
 211     }
 212
 213     pub(super) fn expected_one_of_not_found(
 214         &mut self,
 215         edible: &[TokenKind],
 216         inedible: &[TokenKind],
 217     ) -> PResult<'a, bool /* recovered */> {
 218         debug!("expected_one_of_not_found(edible: {:?}, inedible: {:?})", edible, inedible);
 219         fn tokens_to_string(tokens: &[TokenType]) -> String {
 220             let mut i = tokens.iter();
 221             // This might be a sign we need a connect method on `Iterator`.
 222             let b = i.next().map_or_else(String::new, |t| t.to_string());
 223             i.enumerate().fold(b, |mut b, (i, a)| {
 224                 if tokens.len() > 2 && i == tokens.len() - 2 {
 225                     b.push_str(", or ");
 226                 } else if tokens.len() == 2 && i == tokens.len() - 2 {
 227                     b.push_str(" or ");
 228                 } else {
 229                     b.push_str(", ");
 230                 }
 231                 b.push_str(&a.to_string());
 232                 b
 233             })
 234         }
 235
 236         let mut expected = edible
 237             .iter()
 238             .map(|x| TokenType::Token(x.clone()))
 239             .chain(inedible.iter().map(|x| TokenType::Token(x.clone())))
 240             .chain(self.expected_tokens.iter().cloned())
 241             .collect::<Vec<_>>();
 242         expected.sort_by_cached_key(|x| x.to_string());
 243         expected.dedup();
 244
 245         let sm = self.sess.source_map();
 246         let msg = format!("expected `;`, found {}", super::token_descr(&self.token));
 247         let appl = Applicability::MachineApplicable;
 248         if expected.contains(&TokenType::Token(token::Semi)) {
 249             if self.token.span == DUMMY_SP || self.prev_token.span == DUMMY_SP {
 250                 // Likely inside a macro, can't provide meaningful suggestions.
 251             } else if !sm.is_multiline(self.prev_token.span.until(self.token.span)) {
 252                 // The current token is in the same line as the prior token, not recoverable.
 253             } else if [token::Comma, token::Colon].contains(&self.token.kind)
 254                 && self.prev_token.kind == token::CloseDelim(token::Paren)
 255             {
 256                 // Likely typo: The current token is on a new line and is expected to be
 257                 // `.`, `;`, `?`, or an operator after a close delimiter token.
 258                 //
 259                 // let a = std::process::Command::new("echo")
 260                 //         .arg("1")
 261                 //         ,arg("2")
 262                 //         ^
 263                 // https://github.com/rust-lang/rust/issues/72253
 264             } else if self.look_ahead(1, |t| {
 265                 t == &token::CloseDelim(token::Brace)
 266                     || t.can_begin_expr() && t.kind != token::Colon
 267             }) && [token::Comma, token::Colon].contains(&self.token.kind)
 268             {
 269                 // Likely typo: `,` → `;` or `:` → `;`. This is triggered if the current token is
 270                 // either `,` or `:`, and the next token could either start a new statement or is a
 271                 // block close. For example:
 272                 //
 273                 //   let x = 32:
 274                 //   let y = 42;
 275                 self.bump();
 276                 let sp = self.prev_token.span;
 277                 self.struct_span_err(sp, &msg)
 278                     .span_suggestion_short(sp, "change this to `;`", ";".to_string(), appl)
 279                     .emit();
 280                 return Ok(false);
 281             } else if self.look_ahead(0, |t| {
 282                 t == &token::CloseDelim(token::Brace)
 283                     || (
 284                         t.can_begin_expr() && t != &token::Semi && t != &token::Pound
 285                         // Avoid triggering with too many trailing `#` in raw string.
 286                     )
 287             }) {
 288                 // Missing semicolon typo. This is triggered if the next token could either start a
 289                 // new statement or is a block close. For example:
 290                 //
 291                 //   let x = 32
 292                 //   let y = 42;
 293                 let sp = self.prev_token.span.shrink_to_hi();
 294                 self.struct_span_err(sp, &msg)
 295                     .span_label(self.token.span, "unexpected token")
 296                     .span_suggestion_short(sp, "add `;` here", ";".to_string(), appl)
 297                     .emit();
 298                 return Ok(false);
 299             }
 300         }
 301
 302         let expect = tokens_to_string(&expected[..]);
 303         let actual = super::token_descr(&self.token);
 304         let (msg_exp, (label_sp, label_exp)) = if expected.len() > 1 {
 305             let short_expect = if expected.len() > 6 {
 306                 format!("{} possible tokens", expected.len())
 307             } else {
 308                 expect.clone()
 309             };
 310             (
 311                 format!("expected one of {}, found {}", expect, actual),
 312                 (self.prev_token.span.shrink_to_hi(), format!("expected one of {}", short_expect)),
 313             )
 314         } else if expected.is_empty() {
 315             (
 316                 format!("unexpected token: {}", actual),
 317                 (self.prev_token.span, "unexpected token after this".to_string()),
 318             )
 319         } else {
 320             (
 321                 format!("expected {}, found {}", expect, actual),
 322                 (self.prev_token.span.shrink_to_hi(), format!("expected {}", expect)),
 323             )
 324         };
 325         self.last_unexpected_token_span = Some(self.token.span);
 326         let mut err = self.struct_span_err(self.token.span, &msg_exp);
 327
 328         // Add suggestion for a missing closing angle bracket if '>' is included in expected_tokens
 329         // there are unclosed angle brackets
 330         if self.unmatched_angle_bracket_count > 0
 331             && self.token.kind == TokenKind::Eq
 332             && expected.iter().any(|tok| matches!(tok, TokenType::Token(TokenKind::Gt)))
 333         {
 334             err.span_label(self.prev_token.span, "maybe try to close unmatched angle bracket");
 335         }
 336
 337         let sp = if self.token == token::Eof {
 338             // This is EOF; don't want to point at the following char, but rather the last token.
 339             self.prev_token.span
 340         } else {
 341             label_sp
 342         };
 343         match self.recover_closing_delimiter(
 344             &expected
 345                 .iter()
 346                 .filter_map(|tt| match tt {
 347                     TokenType::Token(t) => Some(t.clone()),
 348                     _ => None,
 349                 })
 350                 .collect::<Vec<_>>(),
 351             err,
 352         ) {
 353             Err(e) => err = e,
 354             Ok(recovered) => {
 355                 return Ok(recovered);
 356             }
 357         }
 358
 359         if self.check_too_many_raw_str_terminators(&mut err) {
 360             return Err(err);
 361         }
 362
 363         if self.prev_token.span == DUMMY_SP {
 364             // Account for macro context where the previous span might not be
 365             // available to avoid incorrect output (#54841).
 366             err.span_label(self.token.span, label_exp);
 367         } else if !sm.is_multiline(self.token.span.shrink_to_hi().until(sp.shrink_to_lo())) {
 368             // When the spans are in the same line, it means that the only content between
 369             // them is whitespace, point at the found token in that case:
 370             //
 371             // X |     () => { syntax error };
 372             //   |                    ^^^^^ expected one of 8 possible tokens here
 373             //
 374             // instead of having:
 375             //
 376             // X |     () => { syntax error };
 377             //   |                   -^^^^^ unexpected token
 378             //   |                   |
 379             //   |                   expected one of 8 possible tokens here
 380             err.span_label(self.token.span, label_exp);
 381         } else {
 382             err.span_label(sp, label_exp);
 383             err.span_label(self.token.span, "unexpected token");
 384         }
 385         self.maybe_annotate_with_ascription(&mut err, false);
 386         Err(err)
 387     }
 388
 389     fn check_too_many_raw_str_terminators(&mut self, err: &mut DiagnosticBuilder<'_>) -> bool {
 390         match (&self.prev_token.kind, &self.token.kind) {
 391             (
 392                 TokenKind::Literal(Lit {
 393                     kind: LitKind::StrRaw(n_hashes) | LitKind::ByteStrRaw(n_hashes),
 394                     ..
 395                 }),
 396                 TokenKind::Pound,
 397             ) => {
 398                 err.set_primary_message("too many `#` when terminating raw string");
 399                 err.span_suggestion(
 400                     self.token.span,
 401                     "remove the extra `#`",
 402                     String::new(),
 403                     Applicability::MachineApplicable,
 404                 );
 405                 err.note(&format!("the raw string started with {} `#`s", n_hashes));
 406                 true
 407             }
 408             _ => false,
 409         }
 410     }
 411
 412     pub fn maybe_suggest_struct_literal(
 413         &mut self,
 414         lo: Span,
 415         s: BlockCheckMode,
 416     ) -> Option<PResult<'a, P<Block>>> {
 417         if self.token.is_ident() && self.look_ahead(1, |t| t == &token::Colon) {
 418             // We might be having a struct literal where people forgot to include the path:
 419             // fn foo() -> Foo {
 420             //     field: value,
 421             // }
 422             let mut snapshot = self.clone();
 423             let path =
 424                 Path { segments: vec![], span: self.prev_token.span.shrink_to_lo(), tokens: None };
 425             let struct_expr = snapshot.parse_struct_expr(None, path, AttrVec::new(), false);
 426             let block_tail = self.parse_block_tail(lo, s, AttemptLocalParseRecovery::No);
 427             return Some(match (struct_expr, block_tail) {
 428                 (Ok(expr), Err(mut err)) => {
 429                     // We have encountered the following:
 430                     // fn foo() -> Foo {
 431                     //     field: value,
 432                     // }
 433                     // Suggest:
 434                     // fn foo() -> Foo { Path {
 435                     //     field: value,
 436                     // } }
 437                     err.delay_as_bug();
 438                     self.struct_span_err(expr.span, "struct literal body without path")
 439                         .multipart_suggestion(
 440                             "you might have forgotten to add the struct literal inside the block",
 441                             vec![
 442                                 (expr.span.shrink_to_lo(), "{ SomeStruct ".to_string()),
 443                                 (expr.span.shrink_to_hi(), " }".to_string()),
 444                             ],
 445                             Applicability::MaybeIncorrect,
 446                         )
 447                         .emit();
 448                     *self = snapshot;
 449                     let mut tail = self.mk_block(
 450                         vec![self.mk_stmt_err(expr.span)],
 451                         s,
 452                         lo.to(self.prev_token.span),
 453                     );
 454                     tail.could_be_bare_literal = true;
 455                     Ok(tail)
 456                 }
 457                 (Err(mut err), Ok(tail)) => {
 458                     // We have a block tail that contains a somehow valid type ascription expr.
 459                     err.cancel();
 460                     Ok(tail)
 461                 }
 462                 (Err(mut snapshot_err), Err(err)) => {
 463                     // We don't know what went wrong, emit the normal error.
 464                     snapshot_err.cancel();
 465                     self.consume_block(token::Brace, ConsumeClosingDelim::Yes);
 466                     Err(err)
 467                 }
 468                 (Ok(_), Ok(mut tail)) => {
 469                     tail.could_be_bare_literal = true;
 470                     Ok(tail)
 471                 }
 472             });
 473         }
 474         None
 475     }
 476
 477     pub fn maybe_annotate_with_ascription(
 478         &mut self,
 479         err: &mut DiagnosticBuilder<'_>,
 480         maybe_expected_semicolon: bool,
 481     ) {
 482         if let Some((sp, likely_path)) = self.last_type_ascription.take() {
 483             let sm = self.sess.source_map();
 484             let next_pos = sm.lookup_char_pos(self.token.span.lo());
 485             let op_pos = sm.lookup_char_pos(sp.hi());
 486
 487             let allow_unstable = self.sess.unstable_features.is_nightly_build();
 488
 489             if likely_path {
 490                 err.span_suggestion(
 491                     sp,
 492                     "maybe write a path separator here",
 493                     "::".to_string(),
 494                     if allow_unstable {
 495                         Applicability::MaybeIncorrect
 496                     } else {
 497                         Applicability::MachineApplicable
 498                     },
 499                 );
 500                 self.sess.type_ascription_path_suggestions.borrow_mut().insert(sp);
 501             } else if op_pos.line != next_pos.line && maybe_expected_semicolon {
 502                 err.span_suggestion(
 503                     sp,
 504                     "try using a semicolon",
 505                     ";".to_string(),
 506                     Applicability::MaybeIncorrect,
 507                 );
 508             } else if allow_unstable {
 509                 err.span_label(sp, "tried to parse a type due to this type ascription");
 510             } else {
 511                 err.span_label(sp, "tried to parse a type due to this");
 512             }
 513             if allow_unstable {
 514                 // Give extra information about type ascription only if it's a nightly compiler.
 515                 err.note(
 516                     "`#![feature(type_ascription)]` lets you annotate an expression with a type: \
 517                      `<expr>: <type>`",
 518                 );
 519                 if !likely_path {
 520                     // Avoid giving too much info when it was likely an unrelated typo.
 521                     err.note(
 522                         "see issue #23416 <https://github.com/rust-lang/rust/issues/23416> \
 523                         for more information",
 524                     );
 525                 }
 526             }
 527         }
 528     }
 529
 530     /// Eats and discards tokens until one of `kets` is encountered. Respects token trees,
 531     /// passes through any errors encountered. Used for error recovery.
 532     pub(super) fn eat_to_tokens(&mut self, kets: &[&TokenKind]) {
 533         if let Err(ref mut err) =
 534             self.parse_seq_to_before_tokens(kets, SeqSep::none(), TokenExpectType::Expect, |p| {
 535                 Ok(p.parse_token_tree())
 536             })
 537         {
 538             err.cancel();
 539         }
 540     }
 541
 542     /// This function checks if there are trailing angle brackets and produces
 543     /// a diagnostic to suggest removing them.
 544     ///
 545     /// ```ignore (diagnostic)
 546     /// let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>();
 547     ///                                                        ^^ help: remove extra angle brackets
 548     /// ```
 549     ///
 550     /// If `true` is returned, then trailing brackets were recovered, tokens were consumed
 551     /// up until one of the tokens in 'end' was encountered, and an error was emitted.
 552     pub(super) fn check_trailing_angle_brackets(
 553         &mut self,
 554         segment: &PathSegment,
 555         end: &[&TokenKind],
 556     ) -> bool {
 557         // This function is intended to be invoked after parsing a path segment where there are two
 558         // cases:
 559         //
 560         // 1. A specific token is expected after the path segment.
 561         //    eg. `x.foo(`, `x.foo::<u32>(` (parenthesis - method call),
 562         //        `Foo::`, or `Foo::<Bar>::` (mod sep - continued path).
 563         // 2. No specific token is expected after the path segment.
 564         //    eg. `x.foo` (field access)
 565         //
 566         // This function is called after parsing `.foo` and before parsing the token `end` (if
 567         // present). This includes any angle bracket arguments, such as `.foo::<u32>` or
 568         // `Foo::<Bar>`.
 569
 570         // We only care about trailing angle brackets if we previously parsed angle bracket
 571         // arguments. This helps stop us incorrectly suggesting that extra angle brackets be
 572         // removed in this case:
 573         //
 574         // `x.foo >> (3)` (where `x.foo` is a `u32` for example)
 575         //
 576         // This case is particularly tricky as we won't notice it just looking at the tokens -
 577         // it will appear the same (in terms of upcoming tokens) as below (since the `::<u32>` will
 578         // have already been parsed):
 579         //
 580         // `x.foo::<u32>>>(3)`
 581         let parsed_angle_bracket_args =
 582             segment.args.as_ref().map_or(false, |args| args.is_angle_bracketed());
 583
 584         debug!(
 585             "check_trailing_angle_brackets: parsed_angle_bracket_args={:?}",
 586             parsed_angle_bracket_args,
 587         );
 588         if !parsed_angle_bracket_args {
 589             return false;
 590         }
 591
 592         // Keep the span at the start so we can highlight the sequence of `>` characters to be
 593         // removed.
 594         let lo = self.token.span;
 595
 596         // We need to look-ahead to see if we have `>` characters without moving the cursor forward
 597         // (since we might have the field access case and the characters we're eating are
 598         // actual operators and not trailing characters - ie `x.foo >> 3`).
 599         let mut position = 0;
 600
 601         // We can encounter `>` or `>>` tokens in any order, so we need to keep track of how
 602         // many of each (so we can correctly pluralize our error messages) and continue to
 603         // advance.
 604         let mut number_of_shr = 0;
 605         let mut number_of_gt = 0;
 606         while self.look_ahead(position, |t| {
 607             trace!("check_trailing_angle_brackets: t={:?}", t);
 608             if *t == token::BinOp(token::BinOpToken::Shr) {
 609                 number_of_shr += 1;
 610                 true
 611             } else if *t == token::Gt {
 612                 number_of_gt += 1;
 613                 true
 614             } else {
 615                 false
 616             }
 617         }) {
 618             position += 1;
 619         }
 620
 621         // If we didn't find any trailing `>` characters, then we have nothing to error about.
 622         debug!(
 623             "check_trailing_angle_brackets: number_of_gt={:?} number_of_shr={:?}",
 624             number_of_gt, number_of_shr,
 625         );
 626         if number_of_gt < 1 && number_of_shr < 1 {
 627             return false;
 628         }
 629
 630         // Finally, double check that we have our end token as otherwise this is the
 631         // second case.
 632         if self.look_ahead(position, |t| {
 633             trace!("check_trailing_angle_brackets: t={:?}", t);
 634             end.contains(&&t.kind)
 635         }) {
 636             // Eat from where we started until the end token so that parsing can continue
 637             // as if we didn't have those extra angle brackets.
 638             self.eat_to_tokens(end);
 639             let span = lo.until(self.token.span);
 640
 641             let total_num_of_gt = number_of_gt + number_of_shr * 2;
 642             self.struct_span_err(
 643                 span,
 644                 &format!("unmatched angle bracket{}", pluralize!(total_num_of_gt)),
 645             )
 646             .span_suggestion(
 647                 span,
 648                 &format!("remove extra angle bracket{}", pluralize!(total_num_of_gt)),
 649                 String::new(),
 650                 Applicability::MachineApplicable,
 651             )
 652             .emit();
 653             return true;
 654         }
 655         false
 656     }
 657
 658     /// Check if a method call with an intended turbofish has been written without surrounding
 659     /// angle brackets.
 660     pub(super) fn check_turbofish_missing_angle_brackets(&mut self, segment: &mut PathSegment) {
 661         if token::ModSep == self.token.kind && segment.args.is_none() {
 662             let snapshot = self.clone();
 663             self.bump();
 664             let lo = self.token.span;
 665             match self.parse_angle_args() {
 666                 Ok(args) => {
 667                     let span = lo.to(self.prev_token.span);
 668                     // Detect trailing `>` like in `x.collect::Vec<_>>()`.
 669                     let mut trailing_span = self.prev_token.span.shrink_to_hi();
 670                     while self.token.kind == token::BinOp(token::Shr)
 671                         || self.token.kind == token::Gt
 672                     {
 673                         trailing_span = trailing_span.to(self.token.span);
 674                         self.bump();
 675                     }
 676                     if self.token.kind == token::OpenDelim(token::Paren) {
 677                         // Recover from bad turbofish: `foo.collect::Vec<_>()`.
 678                         let args = AngleBracketedArgs { args, span }.into();
 679                         segment.args = args;
 680
 681                         self.struct_span_err(
 682                             span,
 683                             "generic parameters without surrounding angle brackets",
 684                         )
 685                         .multipart_suggestion(
 686                             "surround the type parameters with angle brackets",
 687                             vec![
 688                                 (span.shrink_to_lo(), "<".to_string()),
 689                                 (trailing_span, ">".to_string()),
 690                             ],
 691                             Applicability::MachineApplicable,
 692                         )
 693                         .emit();
 694                     } else {
 695                         // This doesn't look like an invalid turbofish, can't recover parse state.
 696                         *self = snapshot;
 697                     }
 698                 }
 699                 Err(mut err) => {
 700                     // We couldn't parse generic parameters, unlikely to be a turbofish. Rely on
 701                     // generic parse error instead.
 702                     err.cancel();
 703                     *self = snapshot;
 704                 }
 705             }
 706         }
 707     }
 708
 709     /// When writing a turbofish with multiple type parameters missing the leading `::`, we will
 710     /// encounter a parse error when encountering the first `,`.
 711     pub(super) fn check_mistyped_turbofish_with_multiple_type_params(
 712         &mut self,
 713         mut e: DiagnosticBuilder<'a>,
 714         expr: &mut P<Expr>,
 715     ) -> PResult<'a, ()> {
 716         if let ExprKind::Binary(binop, _, _) = &expr.kind {
 717             if let ast::BinOpKind::Lt = binop.node {
 718                 if self.eat(&token::Comma) {
 719                     let x = self.parse_seq_to_before_end(
 720                         &token::Gt,
 721                         SeqSep::trailing_allowed(token::Comma),
 722                         |p| p.parse_generic_arg(),
 723                     );
 724                     match x {
 725                         Ok((_, _, false)) => {
 726                             if self.eat(&token::Gt) {
 727                                 match self.parse_expr() {
 728                                     Ok(_) => {
 729                                         e.span_suggestion_verbose(
 730                                             binop.span.shrink_to_lo(),
 731                                             TURBOFISH_SUGGESTION_STR,
 732                                             "::".to_string(),
 733                                             Applicability::MaybeIncorrect,
 734                                         );
 735                                         e.emit();
 736                                         *expr =
 737                                             self.mk_expr_err(expr.span.to(self.prev_token.span));
 738                                         return Ok(());
 739                                     }
 740                                     Err(mut err) => {
 741                                         err.cancel();
 742                                     }
 743                                 }
 744                             }
 745                         }
 746                         Err(mut err) => {
 747                             err.cancel();
 748                         }
 749                         _ => {}
 750                     }
 751                 }
 752             }
 753         }
 754         Err(e)
 755     }
 756
 757     /// Check to see if a pair of chained operators looks like an attempt at chained comparison,
 758     /// e.g. `1 < x <= 3`. If so, suggest either splitting the comparison into two, or
 759     /// parenthesising the leftmost comparison.
 760     fn attempt_chained_comparison_suggestion(
 761         &mut self,
 762         err: &mut DiagnosticBuilder<'_>,
 763         inner_op: &Expr,
 764         outer_op: &Spanned<AssocOp>,
 765     ) -> bool /* advanced the cursor */ {
 766         if let ExprKind::Binary(op, ref l1, ref r1) = inner_op.kind {
 767             if let ExprKind::Field(_, ident) = l1.kind {
 768                 if ident.as_str().parse::<i32>().is_err() && !matches!(r1.kind, ExprKind::Lit(_)) {
 769                     // The parser has encountered `foo.bar<baz`, the likelihood of the turbofish
 770                     // suggestion being the only one to apply is high.
 771                     return false;
 772                 }
 773             }
 774             let mut enclose = |left: Span, right: Span| {
 775                 err.multipart_suggestion(
 776                     "parenthesize the comparison",
 777                     vec![
 778                         (left.shrink_to_lo(), "(".to_string()),
 779                         (right.shrink_to_hi(), ")".to_string()),
 780                     ],
 781                     Applicability::MaybeIncorrect,
 782                 );
 783             };
 784             return match (op.node, &outer_op.node) {
 785                 // `x == y == z`
 786                 (BinOpKind::Eq, AssocOp::Equal) |
 787                 // `x < y < z` and friends.
 788                 (BinOpKind::Lt, AssocOp::Less | AssocOp::LessEqual) |
 789                 (BinOpKind::Le, AssocOp::LessEqual | AssocOp::Less) |
 790                 // `x > y > z` and friends.
 791                 (BinOpKind::Gt, AssocOp::Greater | AssocOp::GreaterEqual) |
 792                 (BinOpKind::Ge, AssocOp::GreaterEqual | AssocOp::Greater) => {
 793                     let expr_to_str = |e: &Expr| {
 794                         self.span_to_snippet(e.span)
 795                             .unwrap_or_else(|_| pprust::expr_to_string(&e))
 796                     };
 797                     err.span_suggestion_verbose(
 798                         inner_op.span.shrink_to_hi(),
 799                         "split the comparison into two",
 800                         format!(" && {}", expr_to_str(&r1)),
 801                         Applicability::MaybeIncorrect,
 802                     );
 803                     false // Keep the current parse behavior, where the AST is `(x < y) < z`.
 804                 }
 805                 // `x == y < z`
 806                 (BinOpKind::Eq, AssocOp::Less | AssocOp::LessEqual | AssocOp::Greater | AssocOp::GreaterEqual) => {
 807                     // Consume `z`/outer-op-rhs.
 808                     let snapshot = self.clone();
 809                     match self.parse_expr() {
 810                         Ok(r2) => {
 811                             // We are sure that outer-op-rhs could be consumed, the suggestion is
 812                             // likely correct.
 813                             enclose(r1.span, r2.span);
 814                             true
 815                         }
 816                         Err(mut expr_err) => {
 817                             expr_err.cancel();
 818                             *self = snapshot;
 819                             false
 820                         }
 821                     }
 822                 }
 823                 // `x > y == z`
 824                 (BinOpKind::Lt | BinOpKind::Le | BinOpKind::Gt | BinOpKind::Ge, AssocOp::Equal) => {
 825                     let snapshot = self.clone();
 826                     // At this point it is always valid to enclose the lhs in parentheses, no
 827                     // further checks are necessary.
 828                     match self.parse_expr() {
 829                         Ok(_) => {
 830                             enclose(l1.span, r1.span);
 831                             true
 832                         }
 833                         Err(mut expr_err) => {
 834                             expr_err.cancel();
 835                             *self = snapshot;
 836                             false
 837                         }
 838                     }
 839                 }
 840                 _ => false,
 841             };
 842         }
 843         false
 844     }
 845
 846     /// Produces an error if comparison operators are chained (RFC #558).
 847     /// We only need to check the LHS, not the RHS, because all comparison ops have same
 848     /// precedence (see `fn precedence`) and are left-associative (see `fn fixity`).
 849     ///
 850     /// This can also be hit if someone incorrectly writes `foo<bar>()` when they should have used
 851     /// the turbofish (`foo::<bar>()`) syntax. We attempt some heuristic recovery if that is the
 852     /// case.
 853     ///
 854     /// Keep in mind that given that `outer_op.is_comparison()` holds and comparison ops are left
 855     /// associative we can infer that we have:
 856     ///
 857     /// ```text
 858     ///           outer_op
 859     ///           /   \
 860     ///     inner_op   r2
 861     ///        /  \
 862     ///      l1    r1
 863     /// ```
 864     pub(super) fn check_no_chained_comparison(
 865         &mut self,
 866         inner_op: &Expr,
 867         outer_op: &Spanned<AssocOp>,
 868     ) -> PResult<'a, Option<P<Expr>>> {
 869         debug_assert!(
 870             outer_op.node.is_comparison(),
 871             "check_no_chained_comparison: {:?} is not comparison",
 872             outer_op.node,
 873         );
 874
 875         let mk_err_expr =
 876             |this: &Self, span| Ok(Some(this.mk_expr(span, ExprKind::Err, AttrVec::new())));
 877
 878         match inner_op.kind {
 879             ExprKind::Binary(op, ref l1, ref r1) if op.node.is_comparison() => {
 880                 let mut err = self.struct_span_err(
 881                     vec![op.span, self.prev_token.span],
 882                     "comparison operators cannot be chained",
 883                 );
 884
 885                 let suggest = |err: &mut DiagnosticBuilder<'_>| {
 886                     err.span_suggestion_verbose(
 887                         op.span.shrink_to_lo(),
 888                         TURBOFISH_SUGGESTION_STR,
 889                         "::".to_string(),
 890                         Applicability::MaybeIncorrect,
 891                     );
 892                 };
 893
 894                 // Include `<` to provide this recommendation even in a case like
 895                 // `Foo<Bar<Baz<Qux, ()>>>`
 896                 if op.node == BinOpKind::Lt && outer_op.node == AssocOp::Less
 897                     || outer_op.node == AssocOp::Greater
 898                 {
 899                     if outer_op.node == AssocOp::Less {
 900                         let snapshot = self.clone();
 901                         self.bump();
 902                         // So far we have parsed `foo<bar<`, consume the rest of the type args.
 903                         let modifiers =
 904                             [(token::Lt, 1), (token::Gt, -1), (token::BinOp(token::Shr), -2)];
 905                         self.consume_tts(1, &modifiers[..]);
 906
 907                         if !&[token::OpenDelim(token::Paren), token::ModSep]
 908                             .contains(&self.token.kind)
 909                         {
 910                             // We don't have `foo< bar >(` or `foo< bar >::`, so we rewind the
 911                             // parser and bail out.
 912                             *self = snapshot.clone();
 913                         }
 914                     }
 915                     return if token::ModSep == self.token.kind {
 916                         // We have some certainty that this was a bad turbofish at this point.
 917                         // `foo< bar >::`
 918                         suggest(&mut err);
 919
 920                         let snapshot = self.clone();
 921                         self.bump(); // `::`
 922
 923                         // Consume the rest of the likely `foo<bar>::new()` or return at `foo<bar>`.
 924                         match self.parse_expr() {
 925                             Ok(_) => {
 926                                 // 99% certain that the suggestion is correct, continue parsing.
 927                                 err.emit();
 928                                 // FIXME: actually check that the two expressions in the binop are
 929                                 // paths and resynthesize new fn call expression instead of using
 930                                 // `ExprKind::Err` placeholder.
 931                                 mk_err_expr(self, inner_op.span.to(self.prev_token.span))
 932                             }
 933                             Err(mut expr_err) => {
 934                                 expr_err.cancel();
 935                                 // Not entirely sure now, but we bubble the error up with the
 936                                 // suggestion.
 937                                 *self = snapshot;
 938                                 Err(err)
 939                             }
 940                         }
 941                     } else if token::OpenDelim(token::Paren) == self.token.kind {
 942                         // We have high certainty that this was a bad turbofish at this point.
 943                         // `foo< bar >(`
 944                         suggest(&mut err);
 945                         // Consume the fn call arguments.
 946                         match self.consume_fn_args() {
 947                             Err(()) => Err(err),
 948                             Ok(()) => {
 949                                 err.emit();
 950                                 // FIXME: actually check that the two expressions in the binop are
 951                                 // paths and resynthesize new fn call expression instead of using
 952                                 // `ExprKind::Err` placeholder.
 953                                 mk_err_expr(self, inner_op.span.to(self.prev_token.span))
 954                             }
 955                         }
 956                     } else {
 957                         if !matches!(l1.kind, ExprKind::Lit(_))
 958                             && !matches!(r1.kind, ExprKind::Lit(_))
 959                         {
 960                             // All we know is that this is `foo < bar >` and *nothing* else. Try to
 961                             // be helpful, but don't attempt to recover.
 962                             err.help(TURBOFISH_SUGGESTION_STR);
 963                             err.help("or use `(...)` if you meant to specify fn arguments");
 964                         }
 965
 966                         // If it looks like a genuine attempt to chain operators (as opposed to a
 967                         // misformatted turbofish, for instance), suggest a correct form.
 968                         if self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op)
 969                         {
 970                             err.emit();
 971                             mk_err_expr(self, inner_op.span.to(self.prev_token.span))
 972                         } else {
 973                             // These cases cause too many knock-down errors, bail out (#61329).
 974                             Err(err)
 975                         }
 976                     };
 977                 }
 978                 let recover =
 979                     self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op);
 980                 err.emit();
 981                 if recover {
 982                     return mk_err_expr(self, inner_op.span.to(self.prev_token.span));
 983                 }
 984             }
 985             _ => {}
 986         }
 987         Ok(None)
 988     }
 989
 990     fn consume_fn_args(&mut self) -> Result<(), ()> {
 991         let snapshot = self.clone();
 992         self.bump(); // `(`
 993
 994         // Consume the fn call arguments.
 995         let modifiers =
 996             [(token::OpenDelim(token::Paren), 1), (token::CloseDelim(token::Paren), -1)];
 997         self.consume_tts(1, &modifiers[..]);
 998
 999         if self.token.kind == token::Eof {
1000             // Not entirely sure that what we consumed were fn arguments, rollback.
1001             *self = snapshot;
1002             Err(())
1003         } else {
1004             // 99% certain that the suggestion is correct, continue parsing.
1005             Ok(())
1006         }
1007     }
1008
1009     pub(super) fn maybe_report_ambiguous_plus(
1010         &mut self,
1011         allow_plus: AllowPlus,
1012         impl_dyn_multi: bool,
1013         ty: &Ty,
1014     ) {
1015         if matches!(allow_plus, AllowPlus::No) && impl_dyn_multi {
1016             let sum_with_parens = format!("({})", pprust::ty_to_string(&ty));
1017             self.struct_span_err(ty.span, "ambiguous `+` in a type")
1018                 .span_suggestion(
1019                     ty.span,
1020                     "use parentheses to disambiguate",
1021                     sum_with_parens,
1022                     Applicability::MachineApplicable,
1023                 )
1024                 .emit();
1025         }
1026     }
1027
1028     pub(super) fn maybe_recover_from_bad_type_plus(
1029         &mut self,
1030         allow_plus: AllowPlus,
1031         ty: &Ty,
1032     ) -> PResult<'a, ()> {
1033         // Do not add `+` to expected tokens.
1034         if matches!(allow_plus, AllowPlus::No) || !self.token.is_like_plus() {
1035             return Ok(());
1036         }
1037
1038         self.bump(); // `+`
1039         let bounds = self.parse_generic_bounds(None)?;
1040         let sum_span = ty.span.to(self.prev_token.span);
1041
1042         let mut err = struct_span_err!(
1043             self.sess.span_diagnostic,
1044             sum_span,
1045             E0178,
1046             "expected a path on the left-hand side of `+`, not `{}`",
1047             pprust::ty_to_string(ty)
1048         );
1049
1050         match ty.kind {
1051             TyKind::Rptr(ref lifetime, ref mut_ty) => {
1052                 let sum_with_parens = pprust::to_string(|s| {
1053                     s.s.word("&");
1054                     s.print_opt_lifetime(lifetime);
1055                     s.print_mutability(mut_ty.mutbl, false);
1056                     s.popen();
1057                     s.print_type(&mut_ty.ty);
1058                     s.print_type_bounds(" +", &bounds);
1059                     s.pclose()
1060                 });
1061                 err.span_suggestion(
1062                     sum_span,
1063                     "try adding parentheses",
1064                     sum_with_parens,
1065                     Applicability::MachineApplicable,
1066                 );
1067             }
1068             TyKind::Ptr(..) | TyKind::BareFn(..) => {
1069                 err.span_label(sum_span, "perhaps you forgot parentheses?");
1070             }
1071             _ => {
1072                 err.span_label(sum_span, "expected a path");
1073             }
1074         }
1075         err.emit();
1076         Ok(())
1077     }
1078
1079     /// Tries to recover from associated item paths like `[T]::AssocItem` / `(T, U)::AssocItem`.
1080     /// Attempts to convert the base expression/pattern/type into a type, parses the `::AssocItem`
1081     /// tail, and combines them into a `<Ty>::AssocItem` expression/pattern/type.
1082     pub(super) fn maybe_recover_from_bad_qpath<T: RecoverQPath>(
1083         &mut self,
1084         base: P<T>,
1085         allow_recovery: bool,
1086     ) -> PResult<'a, P<T>> {
1087         // Do not add `::` to expected tokens.
1088         if allow_recovery && self.token == token::ModSep {
1089             if let Some(ty) = base.to_ty() {
1090                 return self.maybe_recover_from_bad_qpath_stage_2(ty.span, ty);
1091             }
1092         }
1093         Ok(base)
1094     }
1095
1096     /// Given an already parsed `Ty`, parses the `::AssocItem` tail and
1097     /// combines them into a `<Ty>::AssocItem` expression/pattern/type.
1098     pub(super) fn maybe_recover_from_bad_qpath_stage_2<T: RecoverQPath>(
1099         &mut self,
1100         ty_span: Span,
1101         ty: P<Ty>,
1102     ) -> PResult<'a, P<T>> {
1103         self.expect(&token::ModSep)?;
1104
1105         let mut path = ast::Path { segments: Vec::new(), span: DUMMY_SP, tokens: None };
1106         self.parse_path_segments(&mut path.segments, T::PATH_STYLE)?;
1107         path.span = ty_span.to(self.prev_token.span);
1108
1109         let ty_str = self.span_to_snippet(ty_span).unwrap_or_else(|_| pprust::ty_to_string(&ty));
1110         self.struct_span_err(path.span, "missing angle brackets in associated item path")
1111             .span_suggestion(
1112                 // This is a best-effort recovery.
1113                 path.span,
1114                 "try",
1115                 format!("<{}>::{}", ty_str, pprust::path_to_string(&path)),
1116                 Applicability::MaybeIncorrect,
1117             )
1118             .emit();
1119
1120         let path_span = ty_span.shrink_to_hi(); // Use an empty path since `position == 0`.
1121         Ok(P(T::recovered(Some(QSelf { ty, path_span, position: 0 }), path)))
1122     }
1123
1124     pub(super) fn maybe_consume_incorrect_semicolon(&mut self, items: &[P<Item>]) -> bool {
1125         if self.eat(&token::Semi) {
1126             let mut err = self.struct_span_err(self.prev_token.span, "expected item, found `;`");
1127             err.span_suggestion_short(
1128                 self.prev_token.span,
1129                 "remove this semicolon",
1130                 String::new(),
1131                 Applicability::MachineApplicable,
1132             );
1133             if !items.is_empty() {
1134                 let previous_item = &items[items.len() - 1];
1135                 let previous_item_kind_name = match previous_item.kind {
1136                     // Say "braced struct" because tuple-structs and
1137                     // braceless-empty-struct declarations do take a semicolon.
1138                     ItemKind::Struct(..) => Some("braced struct"),
1139                     ItemKind::Enum(..) => Some("enum"),
1140                     ItemKind::Trait(..) => Some("trait"),
1141                     ItemKind::Union(..) => Some("union"),
1142                     _ => None,
1143                 };
1144                 if let Some(name) = previous_item_kind_name {
1145                     err.help(&format!("{} declarations are not followed by a semicolon", name));
1146                 }
1147             }
1148             err.emit();
1149             true
1150         } else {
1151             false
1152         }
1153     }
1154
1155     /// Creates a `DiagnosticBuilder` for an unexpected token `t` and tries to recover if it is a
1156     /// closing delimiter.
1157     pub(super) fn unexpected_try_recover(
1158         &mut self,
1159         t: &TokenKind,
1160     ) -> PResult<'a, bool /* recovered */> {
1161         let token_str = pprust::token_kind_to_string(t);
1162         let this_token_str = super::token_descr(&self.token);
1163         let (prev_sp, sp) = match (&self.token.kind, self.subparser_name) {
1164             // Point at the end of the macro call when reaching end of macro arguments.
1165             (token::Eof, Some(_)) => {
1166                 let sp = self.sess.source_map().next_point(self.prev_token.span);
1167                 (sp, sp)
1168             }
1169             // We don't want to point at the following span after DUMMY_SP.
1170             // This happens when the parser finds an empty TokenStream.
1171             _ if self.prev_token.span == DUMMY_SP => (self.token.span, self.token.span),
1172             // EOF, don't want to point at the following char, but rather the last token.
1173             (token::Eof, None) => (self.prev_token.span, self.token.span),
1174             _ => (self.prev_token.span.shrink_to_hi(), self.token.span),
1175         };
1176         let msg = format!(
1177             "expected `{}`, found {}",
1178             token_str,
1179             match (&self.token.kind, self.subparser_name) {
1180                 (token::Eof, Some(origin)) => format!("end of {}", origin),
1181                 _ => this_token_str,
1182             },
1183         );
1184         let mut err = self.struct_span_err(sp, &msg);
1185         let label_exp = format!("expected `{}`", token_str);
1186         match self.recover_closing_delimiter(&[t.clone()], err) {
1187             Err(e) => err = e,
1188             Ok(recovered) => {
1189                 return Ok(recovered);
1190             }
1191         }
1192         let sm = self.sess.source_map();
1193         if !sm.is_multiline(prev_sp.until(sp)) {
1194             // When the spans are in the same line, it means that the only content
1195             // between them is whitespace, point only at the found token.
1196             err.span_label(sp, label_exp);
1197         } else {
1198             err.span_label(prev_sp, label_exp);
1199             err.span_label(sp, "unexpected token");
1200         }
1201         Err(err)
1202     }
1203
1204     pub(super) fn expect_semi(&mut self) -> PResult<'a, ()> {
1205         if self.eat(&token::Semi) {
1206             return Ok(());
1207         }
1208         self.expect(&token::Semi).map(drop) // Error unconditionally
1209     }
1210
1211     /// Consumes alternative await syntaxes like `await!(<expr>)`, `await <expr>`,
1212     /// `await? <expr>`, `await(<expr>)`, and `await { <expr> }`.
1213     pub(super) fn recover_incorrect_await_syntax(
1214         &mut self,
1215         lo: Span,
1216         await_sp: Span,
1217         attrs: AttrVec,
1218     ) -> PResult<'a, P<Expr>> {
1219         let (hi, expr, is_question) = if self.token == token::Not {
1220             // Handle `await!(<expr>)`.
1221             self.recover_await_macro()?
1222         } else {
1223             self.recover_await_prefix(await_sp)?
1224         };
1225         let sp = self.error_on_incorrect_await(lo, hi, &expr, is_question);
1226         let kind = match expr.kind {
1227             // Avoid knock-down errors as we don't know whether to interpret this as `foo().await?`
1228             // or `foo()?.await` (the very reason we went with postfix syntax 😅).
1229             ExprKind::Try(_) => ExprKind::Err,
1230             _ => ExprKind::Await(expr),
1231         };
1232         let expr = self.mk_expr(lo.to(sp), kind, attrs);
1233         self.maybe_recover_from_bad_qpath(expr, true)
1234     }
1235
1236     fn recover_await_macro(&mut self) -> PResult<'a, (Span, P<Expr>, bool)> {
1237         self.expect(&token::Not)?;
1238         self.expect(&token::OpenDelim(token::Paren))?;
1239         let expr = self.parse_expr()?;
1240         self.expect(&token::CloseDelim(token::Paren))?;
1241         Ok((self.prev_token.span, expr, false))
1242     }
1243
1244     fn recover_await_prefix(&mut self, await_sp: Span) -> PResult<'a, (Span, P<Expr>, bool)> {
1245         let is_question = self.eat(&token::Question); // Handle `await? <expr>`.
1246         let expr = if self.token == token::OpenDelim(token::Brace) {
1247             // Handle `await { <expr> }`.
1248             // This needs to be handled separately from the next arm to avoid
1249             // interpreting `await { <expr> }?` as `<expr>?.await`.
1250             self.parse_block_expr(None, self.token.span, BlockCheckMode::Default, AttrVec::new())
1251         } else {
1252             self.parse_expr()
1253         }
1254         .map_err(|mut err| {
1255             err.span_label(await_sp, "while parsing this incorrect await expression");
1256             err
1257         })?;
1258         Ok((expr.span, expr, is_question))
1259     }
1260
1261     fn error_on_incorrect_await(&self, lo: Span, hi: Span, expr: &Expr, is_question: bool) -> Span {
1262         let expr_str =
1263             self.span_to_snippet(expr.span).unwrap_or_else(|_| pprust::expr_to_string(&expr));
1264         let suggestion = format!("{}.await{}", expr_str, if is_question { "?" } else { "" });
1265         let sp = lo.to(hi);
1266         let app = match expr.kind {
1267             ExprKind::Try(_) => Applicability::MaybeIncorrect, // `await <expr>?`
1268             _ => Applicability::MachineApplicable,
1269         };
1270         self.struct_span_err(sp, "incorrect use of `await`")
1271             .span_suggestion(sp, "`await` is a postfix operation", suggestion, app)
1272             .emit();
1273         sp
1274     }
1275
1276     /// If encountering `future.await()`, consumes and emits an error.
1277     pub(super) fn recover_from_await_method_call(&mut self) {
1278         if self.token == token::OpenDelim(token::Paren)
1279             && self.look_ahead(1, |t| t == &token::CloseDelim(token::Paren))
1280         {
1281             // future.await()
1282             let lo = self.token.span;
1283             self.bump(); // (
1284             let sp = lo.to(self.token.span);
1285             self.bump(); // )
1286             self.struct_span_err(sp, "incorrect use of `await`")
1287                 .span_suggestion(
1288                     sp,
1289                     "`await` is not a method call, remove the parentheses",
1290                     String::new(),
1291                     Applicability::MachineApplicable,
1292                 )
1293                 .emit();
1294         }
1295     }
1296
1297     pub(super) fn try_macro_suggestion(&mut self) -> PResult<'a, P<Expr>> {
1298         let is_try = self.token.is_keyword(kw::Try);
1299         let is_questionmark = self.look_ahead(1, |t| t == &token::Not); //check for !
1300         let is_open = self.look_ahead(2, |t| t == &token::OpenDelim(token::Paren)); //check for (
1301
1302         if is_try && is_questionmark && is_open {
1303             let lo = self.token.span;
1304             self.bump(); //remove try
1305             self.bump(); //remove !
1306             let try_span = lo.to(self.token.span); //we take the try!( span
1307             self.bump(); //remove (
1308             let is_empty = self.token == token::CloseDelim(token::Paren); //check if the block is empty
1309             self.consume_block(token::Paren, ConsumeClosingDelim::No); //eat the block
1310             let hi = self.token.span;
1311             self.bump(); //remove )
1312             let mut err = self.struct_span_err(lo.to(hi), "use of deprecated `try` macro");
1313             err.note("in the 2018 edition `try` is a reserved keyword, and the `try!()` macro is deprecated");
1314             let prefix = if is_empty { "" } else { "alternatively, " };
1315             if !is_empty {
1316                 err.multipart_suggestion(
1317                     "you can use the `?` operator instead",
1318                     vec![(try_span, "".to_owned()), (hi, "?".to_owned())],
1319                     Applicability::MachineApplicable,
1320                 );
1321             }
1322             err.span_suggestion(lo.shrink_to_lo(), &format!("{}you can still access the deprecated `try!()` macro using the \"raw identifier\" syntax", prefix), "r#".to_string(), Applicability::MachineApplicable);
1323             err.emit();
1324             Ok(self.mk_expr_err(lo.to(hi)))
1325         } else {
1326             Err(self.expected_expression_found()) // The user isn't trying to invoke the try! macro
1327         }
1328     }
1329
1330     /// Recovers a situation like `for ( $pat in $expr )`
1331     /// and suggest writing `for $pat in $expr` instead.
1332     ///
1333     /// This should be called before parsing the `$block`.
1334     pub(super) fn recover_parens_around_for_head(
1335         &mut self,
1336         pat: P<Pat>,
1337         expr: &Expr,
1338         begin_paren: Option<Span>,
1339     ) -> P<Pat> {
1340         match (&self.token.kind, begin_paren) {
1341             (token::CloseDelim(token::Paren), Some(begin_par_sp)) => {
1342                 self.bump();
1343
1344                 let pat_str = self
1345                     // Remove the `(` from the span of the pattern:
1346                     .span_to_snippet(pat.span.trim_start(begin_par_sp).unwrap())
1347                     .unwrap_or_else(|_| pprust::pat_to_string(&pat));
1348
1349                 self.struct_span_err(self.prev_token.span, "unexpected closing `)`")
1350                     .span_label(begin_par_sp, "opening `(`")
1351                     .span_suggestion(
1352                         begin_par_sp.to(self.prev_token.span),
1353                         "remove parenthesis in `for` loop",
1354                         format!("{} in {}", pat_str, pprust::expr_to_string(&expr)),
1355                         // With e.g. `for (x) in y)` this would replace `(x) in y)`
1356                         // with `x) in y)` which is syntactically invalid.
1357                         // However, this is prevented before we get here.
1358                         Applicability::MachineApplicable,
1359                     )
1360                     .emit();
1361
1362                 // Unwrap `(pat)` into `pat` to avoid the `unused_parens` lint.
1363                 pat.and_then(|pat| match pat.kind {
1364                     PatKind::Paren(pat) => pat,
1365                     _ => P(pat),
1366                 })
1367             }
1368             _ => pat,
1369         }
1370     }
1371
1372     pub(super) fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
1373         (self.token == token::Lt && // `foo:<bar`, likely a typoed turbofish.
1374             self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident()))
1375             || self.token.is_ident() &&
1376             matches!(node, ast::ExprKind::Path(..) | ast::ExprKind::Field(..)) &&
1377             !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
1378             self.look_ahead(1, |t| t == &token::OpenDelim(token::Paren))
1379             || self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace)) // `foo:bar {`
1380             || self.look_ahead(1, |t| t == &token::Colon) &&     // `foo:bar::<baz`
1381             self.look_ahead(2, |t| t == &token::Lt) &&
1382             self.look_ahead(3, |t| t.is_ident())
1383             || self.look_ahead(1, |t| t == &token::Colon) &&  // `foo:bar:baz`
1384             self.look_ahead(2, |t| t.is_ident())
1385             || self.look_ahead(1, |t| t == &token::ModSep)
1386                 && (self.look_ahead(2, |t| t.is_ident()) ||   // `foo:bar::baz`
1387             self.look_ahead(2, |t| t == &token::Lt)) // `foo:bar::<baz>`
1388     }
1389
1390     pub(super) fn recover_seq_parse_error(
1391         &mut self,
1392         delim: token::DelimToken,
1393         lo: Span,
1394         result: PResult<'a, P<Expr>>,
1395     ) -> P<Expr> {
1396         match result {
1397             Ok(x) => x,
1398             Err(mut err) => {
1399                 err.emit();
1400                 // Recover from parse error, callers expect the closing delim to be consumed.
1401                 self.consume_block(delim, ConsumeClosingDelim::Yes);
1402                 self.mk_expr(lo.to(self.prev_token.span), ExprKind::Err, AttrVec::new())
1403             }
1404         }
1405     }
1406
1407     pub(super) fn recover_closing_delimiter(
1408         &mut self,
1409         tokens: &[TokenKind],
1410         mut err: DiagnosticBuilder<'a>,
1411     ) -> PResult<'a, bool> {
1412         let mut pos = None;
1413         // We want to use the last closing delim that would apply.
1414         for (i, unmatched) in self.unclosed_delims.iter().enumerate().rev() {
1415             if tokens.contains(&token::CloseDelim(unmatched.expected_delim))
1416                 && Some(self.token.span) > unmatched.unclosed_span
1417             {
1418                 pos = Some(i);
1419             }
1420         }
1421         match pos {
1422             Some(pos) => {
1423                 // Recover and assume that the detected unclosed delimiter was meant for
1424                 // this location. Emit the diagnostic and act as if the delimiter was
1425                 // present for the parser's sake.
1426
1427                 // Don't attempt to recover from this unclosed delimiter more than once.
1428                 let unmatched = self.unclosed_delims.remove(pos);
1429                 let delim = TokenType::Token(token::CloseDelim(unmatched.expected_delim));
1430                 if unmatched.found_delim.is_none() {
1431                     // We encountered `Eof`, set this fact here to avoid complaining about missing
1432                     // `fn main()` when we found place to suggest the closing brace.
1433                     *self.sess.reached_eof.borrow_mut() = true;
1434                 }
1435
1436                 // We want to suggest the inclusion of the closing delimiter where it makes
1437                 // the most sense, which is immediately after the last token:
1438                 //
1439                 //  {foo(bar {}}
1440                 //      ^      ^
1441                 //      |      |
1442                 //      |      help: `)` may belong here
1443                 //      |
1444                 //      unclosed delimiter
1445                 if let Some(sp) = unmatched.unclosed_span {
1446                     let mut primary_span: Vec<Span> =
1447                         err.span.primary_spans().iter().cloned().collect();
1448                     primary_span.push(sp);
1449                     let mut primary_span: MultiSpan = primary_span.into();
1450                     for span_label in err.span.span_labels() {
1451                         if let Some(label) = span_label.label {
1452                             primary_span.push_span_label(span_label.span, label);
1453                         }
1454                     }
1455                     err.set_span(primary_span);
1456                     err.span_label(sp, "unclosed delimiter");
1457                 }
1458                 // Backticks should be removed to apply suggestions.
1459                 let mut delim = delim.to_string();
1460                 delim.retain(|c| c != '`');
1461                 err.span_suggestion_short(
1462                     self.prev_token.span.shrink_to_hi(),
1463                     &format!("`{}` may belong here", delim),
1464                     delim,
1465                     Applicability::MaybeIncorrect,
1466                 );
1467                 if unmatched.found_delim.is_none() {
1468                     // Encountered `Eof` when lexing blocks. Do not recover here to avoid knockdown
1469                     // errors which would be emitted elsewhere in the parser and let other error
1470                     // recovery consume the rest of the file.
1471                     Err(err)
1472                 } else {
1473                     err.emit();
1474                     self.expected_tokens.clear(); // Reduce the number of errors.
1475                     Ok(true)
1476                 }
1477             }
1478             _ => Err(err),
1479         }
1480     }
1481
1482     /// Eats tokens until we can be relatively sure we reached the end of the
1483     /// statement. This is something of a best-effort heuristic.
1484     ///
1485     /// We terminate when we find an unmatched `}` (without consuming it).
1486     pub(super) fn recover_stmt(&mut self) {
1487         self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore)
1488     }
1489
1490     /// If `break_on_semi` is `Break`, then we will stop consuming tokens after
1491     /// finding (and consuming) a `;` outside of `{}` or `[]` (note that this is
1492     /// approximate -- it can mean we break too early due to macros, but that
1493     /// should only lead to sub-optimal recovery, not inaccurate parsing).
1494     ///
1495     /// If `break_on_block` is `Break`, then we will stop consuming tokens
1496     /// after finding (and consuming) a brace-delimited block.
1497     pub(super) fn recover_stmt_(
1498         &mut self,
1499         break_on_semi: SemiColonMode,
1500         break_on_block: BlockMode,
1501     ) {
1502         let mut brace_depth = 0;
1503         let mut bracket_depth = 0;
1504         let mut in_block = false;
1505         debug!("recover_stmt_ enter loop (semi={:?}, block={:?})", break_on_semi, break_on_block);
1506         loop {
1507             debug!("recover_stmt_ loop {:?}", self.token);
1508             match self.token.kind {
1509                 token::OpenDelim(token::DelimToken::Brace) => {
1510                     brace_depth += 1;
1511                     self.bump();
1512                     if break_on_block == BlockMode::Break && brace_depth == 1 && bracket_depth == 0
1513                     {
1514                         in_block = true;
1515                     }
1516                 }
1517                 token::OpenDelim(token::DelimToken::Bracket) => {
1518                     bracket_depth += 1;
1519                     self.bump();
1520                 }
1521                 token::CloseDelim(token::DelimToken::Brace) => {
1522                     if brace_depth == 0 {
1523                         debug!("recover_stmt_ return - close delim {:?}", self.token);
1524                         break;
1525                     }
1526                     brace_depth -= 1;
1527                     self.bump();
1528                     if in_block && bracket_depth == 0 && brace_depth == 0 {
1529                         debug!("recover_stmt_ return - block end {:?}", self.token);
1530                         break;
1531                     }
1532                 }
1533                 token::CloseDelim(token::DelimToken::Bracket) => {
1534                     bracket_depth -= 1;
1535                     if bracket_depth < 0 {
1536                         bracket_depth = 0;
1537                     }
1538                     self.bump();
1539                 }
1540                 token::Eof => {
1541                     debug!("recover_stmt_ return - Eof");
1542                     break;
1543                 }
1544                 token::Semi => {
1545                     self.bump();
1546                     if break_on_semi == SemiColonMode::Break
1547                         && brace_depth == 0
1548                         && bracket_depth == 0
1549                     {
1550                         debug!("recover_stmt_ return - Semi");
1551                         break;
1552                     }
1553                 }
1554                 token::Comma
1555                     if break_on_semi == SemiColonMode::Comma
1556                         && brace_depth == 0
1557                         && bracket_depth == 0 =>
1558                 {
1559                     debug!("recover_stmt_ return - Semi");
1560                     break;
1561                 }
1562                 _ => self.bump(),
1563             }
1564         }
1565     }
1566
1567     pub(super) fn check_for_for_in_in_typo(&mut self, in_span: Span) {
1568         if self.eat_keyword(kw::In) {
1569             // a common typo: `for _ in in bar {}`
1570             self.struct_span_err(self.prev_token.span, "expected iterable, found keyword `in`")
1571                 .span_suggestion_short(
1572                     in_span.until(self.prev_token.span),
1573                     "remove the duplicated `in`",
1574                     String::new(),
1575                     Applicability::MachineApplicable,
1576                 )
1577                 .emit();
1578         }
1579     }
1580
1581     pub(super) fn eat_incorrect_doc_comment_for_param_type(&mut self) {
1582         if let token::DocComment(..) = self.token.kind {
1583             self.struct_span_err(
1584                 self.token.span,
1585                 "documentation comments cannot be applied to a function parameter's type",
1586             )
1587             .span_label(self.token.span, "doc comments are not allowed here")
1588             .emit();
1589             self.bump();
1590         } else if self.token == token::Pound
1591             && self.look_ahead(1, |t| *t == token::OpenDelim(token::Bracket))
1592         {
1593             let lo = self.token.span;
1594             // Skip every token until next possible arg.
1595             while self.token != token::CloseDelim(token::Bracket) {
1596                 self.bump();
1597             }
1598             let sp = lo.to(self.token.span);
1599             self.bump();
1600             self.struct_span_err(sp, "attributes cannot be applied to a function parameter's type")
1601                 .span_label(sp, "attributes are not allowed here")
1602                 .emit();
1603         }
1604     }
1605
1606     pub(super) fn parameter_without_type(
1607         &mut self,
1608         err: &mut DiagnosticBuilder<'_>,
1609         pat: P<ast::Pat>,
1610         require_name: bool,
1611         first_param: bool,
1612     ) -> Option<Ident> {
1613         // If we find a pattern followed by an identifier, it could be an (incorrect)
1614         // C-style parameter declaration.
1615         if self.check_ident()
1616             && self.look_ahead(1, |t| *t == token::Comma || *t == token::CloseDelim(token::Paren))
1617         {
1618             // `fn foo(String s) {}`
1619             let ident = self.parse_ident().unwrap();
1620             let span = pat.span.with_hi(ident.span.hi());
1621
1622             err.span_suggestion(
1623                 span,
1624                 "declare the type after the parameter binding",
1625                 String::from("<identifier>: <type>"),
1626                 Applicability::HasPlaceholders,
1627             );
1628             return Some(ident);
1629         } else if require_name
1630             && (self.token == token::Comma
1631                 || self.token == token::Lt
1632                 || self.token == token::CloseDelim(token::Paren))
1633         {
1634             let rfc_note = "anonymous parameters are removed in the 2018 edition (see RFC 1685)";
1635
1636             let (ident, self_sugg, param_sugg, type_sugg, self_span, param_span, type_span) =
1637                 match pat.kind {
1638                     PatKind::Ident(_, ident, _) => (
1639                         ident,
1640                         "self: ".to_string(),
1641                         ": TypeName".to_string(),
1642                         "_: ".to_string(),
1643                         pat.span.shrink_to_lo(),
1644                         pat.span.shrink_to_hi(),
1645                         pat.span.shrink_to_lo(),
1646                     ),
1647                     // Also catches `fn foo(&a)`.
1648                     PatKind::Ref(ref inner_pat, mutab)
1649                         if matches!(inner_pat.clone().into_inner().kind, PatKind::Ident(..)) =>
1650                     {
1651                         match inner_pat.clone().into_inner().kind {
1652                             PatKind::Ident(_, ident, _) => {
1653                                 let mutab = mutab.prefix_str();
1654                                 (
1655                                     ident,
1656                                     "self: ".to_string(),
1657                                     format!("{}: &{}TypeName", ident, mutab),
1658                                     "_: ".to_string(),
1659                                     pat.span.shrink_to_lo(),
1660                                     pat.span,
1661                                     pat.span.shrink_to_lo(),
1662                                 )
1663                             }
1664                             _ => unreachable!(),
1665                         }
1666                     }
1667                     _ => {
1668                         // Otherwise, try to get a type and emit a suggestion.
1669                         if let Some(ty) = pat.to_ty() {
1670                             err.span_suggestion_verbose(
1671                                 pat.span,
1672                                 "explicitly ignore the parameter name",
1673                                 format!("_: {}", pprust::ty_to_string(&ty)),
1674                                 Applicability::MachineApplicable,
1675                             );
1676                             err.note(rfc_note);
1677                         }
1678
1679                         return None;
1680                     }
1681                 };
1682
1683             // `fn foo(a, b) {}`, `fn foo(a<x>, b<y>) {}` or `fn foo(usize, usize) {}`
1684             if first_param {
1685                 err.span_suggestion(
1686                     self_span,
1687                     "if this is a `self` type, give it a parameter name",
1688                     self_sugg,
1689                     Applicability::MaybeIncorrect,
1690                 );
1691             }
1692             // Avoid suggesting that `fn foo(HashMap<u32>)` is fixed with a change to
1693             // `fn foo(HashMap: TypeName<u32>)`.
1694             if self.token != token::Lt {
1695                 err.span_suggestion(
1696                     param_span,
1697                     "if this is a parameter name, give it a type",
1698                     param_sugg,
1699                     Applicability::HasPlaceholders,
1700                 );
1701             }
1702             err.span_suggestion(
1703                 type_span,
1704                 "if this is a type, explicitly ignore the parameter name",
1705                 type_sugg,
1706                 Applicability::MachineApplicable,
1707             );
1708             err.note(rfc_note);
1709
1710             // Don't attempt to recover by using the `X` in `X<Y>` as the parameter name.
1711             return if self.token == token::Lt { None } else { Some(ident) };
1712         }
1713         None
1714     }
1715
1716     pub(super) fn recover_arg_parse(&mut self) -> PResult<'a, (P<ast::Pat>, P<ast::Ty>)> {
1717         let pat = self.parse_pat_no_top_alt(Some("argument name"))?;
1718         self.expect(&token::Colon)?;
1719         let ty = self.parse_ty()?;
1720
1721         struct_span_err!(
1722             self.diagnostic(),
1723             pat.span,
1724             E0642,
1725             "patterns aren't allowed in methods without bodies",
1726         )
1727         .span_suggestion_short(
1728             pat.span,
1729             "give this argument a name or use an underscore to ignore it",
1730             "_".to_owned(),
1731             Applicability::MachineApplicable,
1732         )
1733         .emit();
1734
1735         // Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
1736         let pat =
1737             P(Pat { kind: PatKind::Wild, span: pat.span, id: ast::DUMMY_NODE_ID, tokens: None });
1738         Ok((pat, ty))
1739     }
1740
1741     pub(super) fn recover_bad_self_param(&mut self, mut param: Param) -> PResult<'a, Param> {
1742         let sp = param.pat.span;
1743         param.ty.kind = TyKind::Err;
1744         self.struct_span_err(sp, "unexpected `self` parameter in function")
1745             .span_label(sp, "must be the first parameter of an associated function")
1746             .emit();
1747         Ok(param)
1748     }
1749
1750     pub(super) fn consume_block(
1751         &mut self,
1752         delim: token::DelimToken,
1753         consume_close: ConsumeClosingDelim,
1754     ) {
1755         let mut brace_depth = 0;
1756         loop {
1757             if self.eat(&token::OpenDelim(delim)) {
1758                 brace_depth += 1;
1759             } else if self.check(&token::CloseDelim(delim)) {
1760                 if brace_depth == 0 {
1761                     if let ConsumeClosingDelim::Yes = consume_close {
1762                         // Some of the callers of this method expect to be able to parse the
1763                         // closing delimiter themselves, so we leave it alone. Otherwise we advance
1764                         // the parser.
1765                         self.bump();
1766                     }
1767                     return;
1768                 } else {
1769                     self.bump();
1770                     brace_depth -= 1;
1771                     continue;
1772                 }
1773             } else if self.token == token::Eof || self.eat(&token::CloseDelim(token::NoDelim)) {
1774                 return;
1775             } else {
1776                 self.bump();
1777             }
1778         }
1779     }
1780
1781     pub(super) fn expected_expression_found(&self) -> DiagnosticBuilder<'a> {
1782         let (span, msg) = match (&self.token.kind, self.subparser_name) {
1783             (&token::Eof, Some(origin)) => {
1784                 let sp = self.sess.source_map().next_point(self.prev_token.span);
1785                 (sp, format!("expected expression, found end of {}", origin))
1786             }
1787             _ => (
1788                 self.token.span,
1789                 format!("expected expression, found {}", super::token_descr(&self.token),),
1790             ),
1791         };
1792         let mut err = self.struct_span_err(span, &msg);
1793         let sp = self.sess.source_map().start_point(self.token.span);
1794         if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&sp) {
1795             self.sess.expr_parentheses_needed(&mut err, *sp);
1796         }
1797         err.span_label(span, "expected expression");
1798         err
1799     }
1800
1801     fn consume_tts(
1802         &mut self,
1803         mut acc: i64, // `i64` because malformed code can have more closing delims than opening.
1804         // Not using `FxHashMap` due to `token::TokenKind: !Eq + !Hash`.
1805         modifier: &[(token::TokenKind, i64)],
1806     ) {
1807         while acc > 0 {
1808             if let Some((_, val)) = modifier.iter().find(|(t, _)| *t == self.token.kind) {
1809                 acc += *val;
1810             }
1811             if self.token.kind == token::Eof {
1812                 break;
1813             }
1814             self.bump();
1815         }
1816     }
1817
1818     /// Replace duplicated recovered parameters with `_` pattern to avoid unnecessary errors.
1819     ///
1820     /// This is necessary because at this point we don't know whether we parsed a function with
1821     /// anonymous parameters or a function with names but no types. In order to minimize
1822     /// unnecessary errors, we assume the parameters are in the shape of `fn foo(a, b, c)` where
1823     /// the parameters are *names* (so we don't emit errors about not being able to find `b` in
1824     /// the local scope), but if we find the same name multiple times, like in `fn foo(i8, i8)`,
1825     /// we deduplicate them to not complain about duplicated parameter names.
1826     pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut Vec<Param>) {
1827         let mut seen_inputs = FxHashSet::default();
1828         for input in fn_inputs.iter_mut() {
1829             let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err) =
1830                 (&input.pat.kind, &input.ty.kind)
1831             {
1832                 Some(*ident)
1833             } else {
1834                 None
1835             };
1836             if let Some(ident) = opt_ident {
1837                 if seen_inputs.contains(&ident) {
1838                     input.pat.kind = PatKind::Wild;
1839                 }
1840                 seen_inputs.insert(ident);
1841             }
1842         }
1843     }
1844
1845     /// Handle encountering a symbol in a generic argument list that is not a `,` or `>`. In this
1846     /// case, we emit an error and try to suggest enclosing a const argument in braces if it looks
1847     /// like the user has forgotten them.
1848     pub fn handle_ambiguous_unbraced_const_arg(
1849         &mut self,
1850         args: &mut Vec<AngleBracketedArg>,
1851     ) -> PResult<'a, bool> {
1852         // If we haven't encountered a closing `>`, then the argument is malformed.
1853         // It's likely that the user has written a const expression without enclosing it
1854         // in braces, so we try to recover here.
1855         let arg = args.pop().unwrap();
1856         // FIXME: for some reason using `unexpected` or `expected_one_of_not_found` has
1857         // adverse side-effects to subsequent errors and seems to advance the parser.
1858         // We are causing this error here exclusively in case that a `const` expression
1859         // could be recovered from the current parser state, even if followed by more
1860         // arguments after a comma.
1861         let mut err = self.struct_span_err(
1862             self.token.span,
1863             &format!("expected one of `,` or `>`, found {}", super::token_descr(&self.token)),
1864         );
1865         err.span_label(self.token.span, "expected one of `,` or `>`");
1866         match self.recover_const_arg(arg.span(), err) {
1867             Ok(arg) => {
1868                 args.push(AngleBracketedArg::Arg(arg));
1869                 if self.eat(&token::Comma) {
1870                     return Ok(true); // Continue
1871                 }
1872             }
1873             Err(mut err) => {
1874                 args.push(arg);
1875                 // We will emit a more generic error later.
1876                 err.delay_as_bug();
1877             }
1878         }
1879         return Ok(false); // Don't continue.
1880     }
1881
1882     /// Attempt to parse a generic const argument that has not been enclosed in braces.
1883     /// There are a limited number of expressions that are permitted without being encoded
1884     /// in braces:
1885     /// - Literals.
1886     /// - Single-segment paths (i.e. standalone generic const parameters).
1887     /// All other expressions that can be parsed will emit an error suggesting the expression be
1888     /// wrapped in braces.
1889     pub fn handle_unambiguous_unbraced_const_arg(&mut self) -> PResult<'a, P<Expr>> {
1890         let start = self.token.span;
1891         let expr = self.parse_expr_res(Restrictions::CONST_EXPR, None).map_err(|mut err| {
1892             err.span_label(
1893                 start.shrink_to_lo(),
1894                 "while parsing a const generic argument starting here",
1895             );
1896             err
1897         })?;
1898         if !self.expr_is_valid_const_arg(&expr) {
1899             self.struct_span_err(
1900                 expr.span,
1901                 "expressions must be enclosed in braces to be used as const generic \
1902                     arguments",
1903             )
1904             .multipart_suggestion(
1905                 "enclose the `const` expression in braces",
1906                 vec![
1907                     (expr.span.shrink_to_lo(), "{ ".to_string()),
1908                     (expr.span.shrink_to_hi(), " }".to_string()),
1909                 ],
1910                 Applicability::MachineApplicable,
1911             )
1912             .emit();
1913         }
1914         Ok(expr)
1915     }
1916
1917     /// Try to recover from possible generic const argument without `{` and `}`.
1918     ///
1919     /// When encountering code like `foo::< bar + 3 >` or `foo::< bar - baz >` we suggest
1920     /// `foo::<{ bar + 3 }>` and `foo::<{ bar - baz }>`, respectively. We only provide a suggestion
1921     /// if we think that that the resulting expression would be well formed.
1922     pub fn recover_const_arg(
1923         &mut self,
1924         start: Span,
1925         mut err: DiagnosticBuilder<'a>,
1926     ) -> PResult<'a, GenericArg> {
1927         let is_op = AssocOp::from_token(&self.token)
1928             .and_then(|op| {
1929                 if let AssocOp::Greater
1930                 | AssocOp::Less
1931                 | AssocOp::ShiftRight
1932                 | AssocOp::GreaterEqual
1933                 // Don't recover from `foo::<bar = baz>`, because this could be an attempt to
1934                 // assign a value to a defaulted generic parameter.
1935                 | AssocOp::Assign
1936                 | AssocOp::AssignOp(_) = op
1937                 {
1938                     None
1939                 } else {
1940                     Some(op)
1941                 }
1942             })
1943             .is_some();
1944         // This will be true when a trait object type `Foo +` or a path which was a `const fn` with
1945         // type params has been parsed.
1946         let was_op =
1947             matches!(self.prev_token.kind, token::BinOp(token::Plus | token::Shr) | token::Gt);
1948         if !is_op && !was_op {
1949             // We perform these checks and early return to avoid taking a snapshot unnecessarily.
1950             return Err(err);
1951         }
1952         let snapshot = self.clone();
1953         if is_op {
1954             self.bump();
1955         }
1956         match self.parse_expr_res(Restrictions::CONST_EXPR, None) {
1957             Ok(expr) => {
1958                 if token::Comma == self.token.kind || self.token.kind.should_end_const_arg() {
1959                     // Avoid the following output by checking that we consumed a full const arg:
1960                     // help: expressions must be enclosed in braces to be used as const generic
1961                     //       arguments
1962                     //    |
1963                     // LL |     let sr: Vec<{ (u32, _, _) = vec![] };
1964                     //    |                 ^                      ^
1965                     err.multipart_suggestion(
1966                         "expressions must be enclosed in braces to be used as const generic \
1967                          arguments",
1968                         vec![
1969                             (start.shrink_to_lo(), "{ ".to_string()),
1970                             (expr.span.shrink_to_hi(), " }".to_string()),
1971                         ],
1972                         Applicability::MaybeIncorrect,
1973                     );
1974                     let value = self.mk_expr_err(start.to(expr.span));
1975                     err.emit();
1976                     return Ok(GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value }));
1977                 }
1978             }
1979             Err(mut err) => {
1980                 err.cancel();
1981             }
1982         }
1983         *self = snapshot;
1984         Err(err)
1985     }
1986
1987     /// Get the diagnostics for the cases where `move async` is found.
1988     ///
1989     /// `move_async_span` starts at the 'm' of the move keyword and ends with the 'c' of the async keyword
1990     pub(super) fn incorrect_move_async_order_found(
1991         &self,
1992         move_async_span: Span,
1993     ) -> DiagnosticBuilder<'a> {
1994         let mut err =
1995             self.struct_span_err(move_async_span, "the order of `move` and `async` is incorrect");
1996         err.span_suggestion_verbose(
1997             move_async_span,
1998             "try switching the order",
1999             "async move".to_owned(),
2000             Applicability::MaybeIncorrect,
2001         );
2002         err
2003     }
2004 }