compiler/rustc_parse/src/parser/diagnostics.rs

   1 use super::pat::Expected;
   2 use super::{
   3     BlockMode, CommaRecoveryMode, Parser, PathStyle, Restrictions, SemiColonMode, SeqSep,
   4     TokenExpectType, TokenType,
   5 };
   6 use crate::errors::{
   7     AmbiguousPlus, AttributeOnParamType, BadQPathStage2, BadTypePlus, BadTypePlusSub,
   8     ComparisonOperatorsCannotBeChained, ComparisonOperatorsCannotBeChainedSugg,
   9     ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentOnParamType,
  10     DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
  11     GenericParamsWithoutAngleBrackets, GenericParamsWithoutAngleBracketsSugg, InInTypo,
  12     IncorrectAwait, IncorrectSemicolon, IncorrectUseOfAwait, ParenthesesInForHead,
  13     ParenthesesInForHeadSugg, PatternMethodParamWithoutBody, QuestionMarkInType,
  14     QuestionMarkInTypeSugg, SelfParamNotFirst, StructLiteralBodyWithoutPath,
  15     StructLiteralBodyWithoutPathSugg, SuggEscapeToUseAsIdentifier, SuggRemoveComma,
  16     UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
  17     UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets, UseEqInstead,
  18 };
  19
  20 use crate::lexer::UnmatchedBrace;
  21 use rustc_ast as ast;
  22 use rustc_ast::ptr::P;
  23 use rustc_ast::token::{self, Delimiter, Lit, LitKind, TokenKind};
  24 use rustc_ast::util::parser::AssocOp;
  25 use rustc_ast::{
  26     AngleBracketedArg, AngleBracketedArgs, AnonConst, AttrVec, BinOpKind, BindingAnnotation, Block,
  27     BlockCheckMode, Expr, ExprKind, GenericArg, Generics, Item, ItemKind, Param, Pat, PatKind,
  28     Path, PathSegment, QSelf, Ty, TyKind,
  29 };
  30 use rustc_ast_pretty::pprust;
  31 use rustc_data_structures::fx::FxHashSet;
  32 use rustc_errors::{
  33     fluent, Applicability, DiagnosticBuilder, DiagnosticMessage, Handler, MultiSpan, PResult,
  34 };
  35 use rustc_errors::{pluralize, Diagnostic, ErrorGuaranteed, IntoDiagnostic};
  36 use rustc_session::errors::ExprParenthesesNeeded;
  37 use rustc_span::source_map::Spanned;
  38 use rustc_span::symbol::{kw, sym, Ident};
  39 use rustc_span::{Span, SpanSnippetError, DUMMY_SP};
  40 use std::ops::{Deref, DerefMut};
  41
  42 use std::mem::take;
  43
  44 use crate::parser;
  45
  46 /// Creates a placeholder argument.
  47 pub(super) fn dummy_arg(ident: Ident) -> Param {
  48     let pat = P(Pat {
  49         id: ast::DUMMY_NODE_ID,
  50         kind: PatKind::Ident(BindingAnnotation::NONE, ident, None),
  51         span: ident.span,
  52         tokens: None,
  53     });
  54     let ty = Ty { kind: TyKind::Err, span: ident.span, id: ast::DUMMY_NODE_ID, tokens: None };
  55     Param {
  56         attrs: AttrVec::default(),
  57         id: ast::DUMMY_NODE_ID,
  58         pat,
  59         span: ident.span,
  60         ty: P(ty),
  61         is_placeholder: false,
  62     }
  63 }
  64
  65 pub(super) trait RecoverQPath: Sized + 'static {
  66     const PATH_STYLE: PathStyle = PathStyle::Expr;
  67     fn to_ty(&self) -> Option<P<Ty>>;
  68     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self;
  69 }
  70
  71 impl RecoverQPath for Ty {
  72     const PATH_STYLE: PathStyle = PathStyle::Type;
  73     fn to_ty(&self) -> Option<P<Ty>> {
  74         Some(P(self.clone()))
  75     }
  76     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
  77         Self {
  78             span: path.span,
  79             kind: TyKind::Path(qself, path),
  80             id: ast::DUMMY_NODE_ID,
  81             tokens: None,
  82         }
  83     }
  84 }
  85
  86 impl RecoverQPath for Pat {
  87     fn to_ty(&self) -> Option<P<Ty>> {
  88         self.to_ty()
  89     }
  90     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
  91         Self {
  92             span: path.span,
  93             kind: PatKind::Path(qself, path),
  94             id: ast::DUMMY_NODE_ID,
  95             tokens: None,
  96         }
  97     }
  98 }
  99
 100 impl RecoverQPath for Expr {
 101     fn to_ty(&self) -> Option<P<Ty>> {
 102         self.to_ty()
 103     }
 104     fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
 105         Self {
 106             span: path.span,
 107             kind: ExprKind::Path(qself, path),
 108             attrs: AttrVec::new(),
 109             id: ast::DUMMY_NODE_ID,
 110             tokens: None,
 111         }
 112     }
 113 }
 114
 115 /// Control whether the closing delimiter should be consumed when calling `Parser::consume_block`.
 116 pub(crate) enum ConsumeClosingDelim {
 117     Yes,
 118     No,
 119 }
 120
 121 #[derive(Clone, Copy)]
 122 pub enum AttemptLocalParseRecovery {
 123     Yes,
 124     No,
 125 }
 126
 127 impl AttemptLocalParseRecovery {
 128     pub fn yes(&self) -> bool {
 129         match self {
 130             AttemptLocalParseRecovery::Yes => true,
 131             AttemptLocalParseRecovery::No => false,
 132         }
 133     }
 134
 135     pub fn no(&self) -> bool {
 136         match self {
 137             AttemptLocalParseRecovery::Yes => false,
 138             AttemptLocalParseRecovery::No => true,
 139         }
 140     }
 141 }
 142
 143 /// Information for emitting suggestions and recovering from
 144 /// C-style `i++`, `--i`, etc.
 145 #[derive(Debug, Copy, Clone)]
 146 struct IncDecRecovery {
 147     /// Is this increment/decrement its own statement?
 148     standalone: IsStandalone,
 149     /// Is this an increment or decrement?
 150     op: IncOrDec,
 151     /// Is this pre- or postfix?
 152     fixity: UnaryFixity,
 153 }
 154
 155 /// Is an increment or decrement expression its own statement?
 156 #[derive(Debug, Copy, Clone)]
 157 enum IsStandalone {
 158     /// It's standalone, i.e., its own statement.
 159     Standalone,
 160     /// It's a subexpression, i.e., *not* standalone.
 161     Subexpr,
 162     /// It's maybe standalone; we're not sure.
 163     Maybe,
 164 }
 165
 166 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 167 enum IncOrDec {
 168     Inc,
 169     // FIXME: `i--` recovery isn't implemented yet
 170     #[allow(dead_code)]
 171     Dec,
 172 }
 173
 174 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 175 enum UnaryFixity {
 176     Pre,
 177     Post,
 178 }
 179
 180 impl IncOrDec {
 181     fn chr(&self) -> char {
 182         match self {
 183             Self::Inc => '+',
 184             Self::Dec => '-',
 185         }
 186     }
 187
 188     fn name(&self) -> &'static str {
 189         match self {
 190             Self::Inc => "increment",
 191             Self::Dec => "decrement",
 192         }
 193     }
 194 }
 195
 196 impl std::fmt::Display for UnaryFixity {
 197     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 198         match self {
 199             Self::Pre => write!(f, "prefix"),
 200             Self::Post => write!(f, "postfix"),
 201         }
 202     }
 203 }
 204
 205 struct MultiSugg {
 206     msg: String,
 207     patches: Vec<(Span, String)>,
 208     applicability: Applicability,
 209 }
 210
 211 impl MultiSugg {
 212     fn emit(self, err: &mut Diagnostic) {
 213         err.multipart_suggestion(&self.msg, self.patches, self.applicability);
 214     }
 215
 216     /// Overrides individual messages and applicabilities.
 217     fn emit_many(
 218         err: &mut Diagnostic,
 219         msg: &str,
 220         applicability: Applicability,
 221         suggestions: impl Iterator<Item = Self>,
 222     ) {
 223         err.multipart_suggestions(msg, suggestions.map(|s| s.patches), applicability);
 224     }
 225 }
 226
 227 // SnapshotParser is used to create a snapshot of the parser
 228 // without causing duplicate errors being emitted when the `Parser`
 229 // is dropped.
 230 pub struct SnapshotParser<'a> {
 231     parser: Parser<'a>,
 232     unclosed_delims: Vec<UnmatchedBrace>,
 233 }
 234
 235 impl<'a> Deref for SnapshotParser<'a> {
 236     type Target = Parser<'a>;
 237
 238     fn deref(&self) -> &Self::Target {
 239         &self.parser
 240     }
 241 }
 242
 243 impl<'a> DerefMut for SnapshotParser<'a> {
 244     fn deref_mut(&mut self) -> &mut Self::Target {
 245         &mut self.parser
 246     }
 247 }
 248
 249 impl<'a> Parser<'a> {
 250     #[rustc_lint_diagnostics]
 251     pub fn struct_span_err<S: Into<MultiSpan>>(
 252         &self,
 253         sp: S,
 254         m: impl Into<DiagnosticMessage>,
 255     ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
 256         self.sess.span_diagnostic.struct_span_err(sp, m)
 257     }
 258
 259     pub fn span_bug<S: Into<MultiSpan>>(&self, sp: S, m: impl Into<DiagnosticMessage>) -> ! {
 260         self.sess.span_diagnostic.span_bug(sp, m)
 261     }
 262
 263     pub(super) fn diagnostic(&self) -> &'a Handler {
 264         &self.sess.span_diagnostic
 265     }
 266
 267     /// Replace `self` with `snapshot.parser` and extend `unclosed_delims` with `snapshot.unclosed_delims`.
 268     /// This is to avoid losing unclosed delims errors `create_snapshot_for_diagnostic` clears.
 269     pub(super) fn restore_snapshot(&mut self, snapshot: SnapshotParser<'a>) {
 270         *self = snapshot.parser;
 271         self.unclosed_delims.extend(snapshot.unclosed_delims);
 272     }
 273
 274     pub fn unclosed_delims(&self) -> &[UnmatchedBrace] {
 275         &self.unclosed_delims
 276     }
 277
 278     /// Create a snapshot of the `Parser`.
 279     pub fn create_snapshot_for_diagnostic(&self) -> SnapshotParser<'a> {
 280         let mut snapshot = self.clone();
 281         let unclosed_delims = self.unclosed_delims.clone();
 282         // Clear `unclosed_delims` in snapshot to avoid
 283         // duplicate errors being emitted when the `Parser`
 284         // is dropped (which may or may not happen, depending
 285         // if the parsing the snapshot is created for is successful)
 286         snapshot.unclosed_delims.clear();
 287         SnapshotParser { parser: snapshot, unclosed_delims }
 288     }
 289
 290     pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
 291         self.sess.source_map().span_to_snippet(span)
 292     }
 293
 294     pub(super) fn expected_ident_found(&self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
 295         let valid_follow = &[
 296             TokenKind::Eq,
 297             TokenKind::Colon,
 298             TokenKind::Comma,
 299             TokenKind::Semi,
 300             TokenKind::ModSep,
 301             TokenKind::OpenDelim(Delimiter::Brace),
 302             TokenKind::OpenDelim(Delimiter::Parenthesis),
 303             TokenKind::CloseDelim(Delimiter::Brace),
 304             TokenKind::CloseDelim(Delimiter::Parenthesis),
 305         ];
 306         let suggest_raw = match self.token.ident() {
 307             Some((ident, false))
 308                 if ident.is_raw_guess()
 309                     && self.look_ahead(1, |t| valid_follow.contains(&t.kind)) =>
 310             {
 311                 Some(SuggEscapeToUseAsIdentifier {
 312                     span: ident.span.shrink_to_lo(),
 313                     // `Symbol::to_string()` is different from `Symbol::into_diagnostic_arg()`,
 314                     // which uses `Symbol::to_ident_string()` and "helpfully" adds an implicit `r#`
 315                     ident_name: ident.name.to_string(),
 316                 })
 317             }
 318             _ => None,
 319         };
 320
 321         let suggest_remove_comma =
 322             if self.token == token::Comma && self.look_ahead(1, |t| t.is_ident()) {
 323                 Some(SuggRemoveComma { span: self.token.span })
 324             } else {
 325                 None
 326             };
 327
 328         let err = ExpectedIdentifier {
 329             span: self.token.span,
 330             token: self.token.clone(),
 331             suggest_raw,
 332             suggest_remove_comma,
 333         };
 334         err.into_diagnostic(&self.sess.span_diagnostic)
 335     }
 336
 337     pub(super) fn expected_one_of_not_found(
 338         &mut self,
 339         edible: &[TokenKind],
 340         inedible: &[TokenKind],
 341     ) -> PResult<'a, bool /* recovered */> {
 342         debug!("expected_one_of_not_found(edible: {:?}, inedible: {:?})", edible, inedible);
 343         fn tokens_to_string(tokens: &[TokenType]) -> String {
 344             let mut i = tokens.iter();
 345             // This might be a sign we need a connect method on `Iterator`.
 346             let b = i.next().map_or_else(String::new, |t| t.to_string());
 347             i.enumerate().fold(b, |mut b, (i, a)| {
 348                 if tokens.len() > 2 && i == tokens.len() - 2 {
 349                     b.push_str(", or ");
 350                 } else if tokens.len() == 2 && i == tokens.len() - 2 {
 351                     b.push_str(" or ");
 352                 } else {
 353                     b.push_str(", ");
 354                 }
 355                 b.push_str(&a.to_string());
 356                 b
 357             })
 358         }
 359
 360         let mut expected = edible
 361             .iter()
 362             .map(|x| TokenType::Token(x.clone()))
 363             .chain(inedible.iter().map(|x| TokenType::Token(x.clone())))
 364             .chain(self.expected_tokens.iter().cloned())
 365             .filter_map(|token| {
 366                 // filter out suggestions which suggest the same token which was found and deemed incorrect
 367                 fn is_ident_eq_keyword(found: &TokenKind, expected: &TokenType) -> bool {
 368                     if let TokenKind::Ident(current_sym, _) = found {
 369                         if let TokenType::Keyword(suggested_sym) = expected {
 370                             return current_sym == suggested_sym;
 371                         }
 372                     }
 373                     false
 374                 }
 375                 if token != parser::TokenType::Token(self.token.kind.clone()) {
 376                     let eq = is_ident_eq_keyword(&self.token.kind, &token);
 377                     // if the suggestion is a keyword and the found token is an ident,
 378                     // the content of which are equal to the suggestion's content,
 379                     // we can remove that suggestion (see the return None statement below)
 380
 381                     // if this isn't the case however, and the suggestion is a token the
 382                     // content of which is the same as the found token's, we remove it as well
 383                     if !eq {
 384                         if let TokenType::Token(kind) = &token {
 385                             if kind == &self.token.kind {
 386                                 return None;
 387                             }
 388                         }
 389                         return Some(token);
 390                     }
 391                 }
 392                 return None;
 393             })
 394             .collect::<Vec<_>>();
 395         expected.sort_by_cached_key(|x| x.to_string());
 396         expected.dedup();
 397
 398         let sm = self.sess.source_map();
 399
 400         // Special-case "expected `;`" errors
 401         if expected.contains(&TokenType::Token(token::Semi)) {
 402             if self.token.span == DUMMY_SP || self.prev_token.span == DUMMY_SP {
 403                 // Likely inside a macro, can't provide meaningful suggestions.
 404             } else if !sm.is_multiline(self.prev_token.span.until(self.token.span)) {
 405                 // The current token is in the same line as the prior token, not recoverable.
 406             } else if [token::Comma, token::Colon].contains(&self.token.kind)
 407                 && self.prev_token.kind == token::CloseDelim(Delimiter::Parenthesis)
 408             {
 409                 // Likely typo: The current token is on a new line and is expected to be
 410                 // `.`, `;`, `?`, or an operator after a close delimiter token.
 411                 //
 412                 // let a = std::process::Command::new("echo")
 413                 //         .arg("1")
 414                 //         ,arg("2")
 415                 //         ^
 416                 // https://github.com/rust-lang/rust/issues/72253
 417             } else if self.look_ahead(1, |t| {
 418                 t == &token::CloseDelim(Delimiter::Brace)
 419                     || t.can_begin_expr() && t.kind != token::Colon
 420             }) && [token::Comma, token::Colon].contains(&self.token.kind)
 421             {
 422                 // Likely typo: `,` → `;` or `:` → `;`. This is triggered if the current token is
 423                 // either `,` or `:`, and the next token could either start a new statement or is a
 424                 // block close. For example:
 425                 //
 426                 //   let x = 32:
 427                 //   let y = 42;
 428                 self.sess.emit_err(ExpectedSemi {
 429                     span: self.token.span,
 430                     token: self.token.clone(),
 431                     unexpected_token_label: None,
 432                     sugg: ExpectedSemiSugg::ChangeToSemi(self.token.span),
 433                 });
 434                 self.bump();
 435                 return Ok(true);
 436             } else if self.look_ahead(0, |t| {
 437                 t == &token::CloseDelim(Delimiter::Brace)
 438                     || ((t.can_begin_expr() || t.can_begin_item())
 439                         && t != &token::Semi
 440                         && t != &token::Pound)
 441                     // Avoid triggering with too many trailing `#` in raw string.
 442                     || (sm.is_multiline(
 443                         self.prev_token.span.shrink_to_hi().until(self.token.span.shrink_to_lo()),
 444                     ) && t == &token::Pound)
 445             }) && !expected.contains(&TokenType::Token(token::Comma))
 446             {
 447                 // Missing semicolon typo. This is triggered if the next token could either start a
 448                 // new statement or is a block close. For example:
 449                 //
 450                 //   let x = 32
 451                 //   let y = 42;
 452                 let span = self.prev_token.span.shrink_to_hi();
 453                 self.sess.emit_err(ExpectedSemi {
 454                     span,
 455                     token: self.token.clone(),
 456                     unexpected_token_label: Some(self.token.span),
 457                     sugg: ExpectedSemiSugg::AddSemi(span),
 458                 });
 459                 return Ok(true);
 460             }
 461         }
 462
 463         if self.token.kind == TokenKind::EqEq
 464             && self.prev_token.is_ident()
 465             && expected.iter().any(|tok| matches!(tok, TokenType::Token(TokenKind::Eq)))
 466         {
 467             // Likely typo: `=` → `==` in let expr or enum item
 468             return Err(self.sess.create_err(UseEqInstead { span: self.token.span }));
 469         }
 470
 471         let expect = tokens_to_string(&expected);
 472         let actual = super::token_descr(&self.token);
 473         let (msg_exp, (label_sp, label_exp)) = if expected.len() > 1 {
 474             let short_expect = if expected.len() > 6 {
 475                 format!("{} possible tokens", expected.len())
 476             } else {
 477                 expect.clone()
 478             };
 479             (
 480                 format!("expected one of {expect}, found {actual}"),
 481                 (self.prev_token.span.shrink_to_hi(), format!("expected one of {short_expect}")),
 482             )
 483         } else if expected.is_empty() {
 484             (
 485                 format!("unexpected token: {actual}"),
 486                 (self.prev_token.span, "unexpected token after this".to_string()),
 487             )
 488         } else {
 489             (
 490                 format!("expected {expect}, found {actual}"),
 491                 (self.prev_token.span.shrink_to_hi(), format!("expected {expect}")),
 492             )
 493         };
 494         self.last_unexpected_token_span = Some(self.token.span);
 495         // FIXME: translation requires list formatting (for `expect`)
 496         let mut err = self.struct_span_err(self.token.span, &msg_exp);
 497
 498         if let TokenKind::Ident(symbol, _) = &self.prev_token.kind {
 499             if ["def", "fun", "func", "function"].contains(&symbol.as_str()) {
 500                 err.span_suggestion_short(
 501                     self.prev_token.span,
 502                     &format!("write `fn` instead of `{symbol}` to declare a function"),
 503                     "fn",
 504                     Applicability::MachineApplicable,
 505                 );
 506             }
 507         }
 508
 509         // `pub` may be used for an item or `pub(crate)`
 510         if self.prev_token.is_ident_named(sym::public)
 511             && (self.token.can_begin_item()
 512                 || self.token.kind == TokenKind::OpenDelim(Delimiter::Parenthesis))
 513         {
 514             err.span_suggestion_short(
 515                 self.prev_token.span,
 516                 "write `pub` instead of `public` to make the item public",
 517                 "pub",
 518                 Applicability::MachineApplicable,
 519             );
 520         }
 521
 522         // Add suggestion for a missing closing angle bracket if '>' is included in expected_tokens
 523         // there are unclosed angle brackets
 524         if self.unmatched_angle_bracket_count > 0
 525             && self.token.kind == TokenKind::Eq
 526             && expected.iter().any(|tok| matches!(tok, TokenType::Token(TokenKind::Gt)))
 527         {
 528             err.span_label(self.prev_token.span, "maybe try to close unmatched angle bracket");
 529         }
 530
 531         let sp = if self.token == token::Eof {
 532             // This is EOF; don't want to point at the following char, but rather the last token.
 533             self.prev_token.span
 534         } else {
 535             label_sp
 536         };
 537         match self.recover_closing_delimiter(
 538             &expected
 539                 .iter()
 540                 .filter_map(|tt| match tt {
 541                     TokenType::Token(t) => Some(t.clone()),
 542                     _ => None,
 543                 })
 544                 .collect::<Vec<_>>(),
 545             err,
 546         ) {
 547             Err(e) => err = e,
 548             Ok(recovered) => {
 549                 return Ok(recovered);
 550             }
 551         }
 552
 553         if self.check_too_many_raw_str_terminators(&mut err) {
 554             if expected.contains(&TokenType::Token(token::Semi)) && self.eat(&token::Semi) {
 555                 err.emit();
 556                 return Ok(true);
 557             } else {
 558                 return Err(err);
 559             }
 560         }
 561
 562         if self.prev_token.span == DUMMY_SP {
 563             // Account for macro context where the previous span might not be
 564             // available to avoid incorrect output (#54841).
 565             err.span_label(self.token.span, label_exp);
 566         } else if !sm.is_multiline(self.token.span.shrink_to_hi().until(sp.shrink_to_lo())) {
 567             // When the spans are in the same line, it means that the only content between
 568             // them is whitespace, point at the found token in that case:
 569             //
 570             // X |     () => { syntax error };
 571             //   |                    ^^^^^ expected one of 8 possible tokens here
 572             //
 573             // instead of having:
 574             //
 575             // X |     () => { syntax error };
 576             //   |                   -^^^^^ unexpected token
 577             //   |                   |
 578             //   |                   expected one of 8 possible tokens here
 579             err.span_label(self.token.span, label_exp);
 580         } else {
 581             err.span_label(sp, label_exp);
 582             err.span_label(self.token.span, "unexpected token");
 583         }
 584         self.maybe_annotate_with_ascription(&mut err, false);
 585         Err(err)
 586     }
 587
 588     fn check_too_many_raw_str_terminators(&mut self, err: &mut Diagnostic) -> bool {
 589         let sm = self.sess.source_map();
 590         match (&self.prev_token.kind, &self.token.kind) {
 591             (
 592                 TokenKind::Literal(Lit {
 593                     kind: LitKind::StrRaw(n_hashes) | LitKind::ByteStrRaw(n_hashes),
 594                     ..
 595                 }),
 596                 TokenKind::Pound,
 597             ) if !sm.is_multiline(
 598                 self.prev_token.span.shrink_to_hi().until(self.token.span.shrink_to_lo()),
 599             ) =>
 600             {
 601                 let n_hashes: u8 = *n_hashes;
 602                 err.set_primary_message("too many `#` when terminating raw string");
 603                 let str_span = self.prev_token.span;
 604                 let mut span = self.token.span;
 605                 let mut count = 0;
 606                 while self.token.kind == TokenKind::Pound
 607                     && !sm.is_multiline(span.shrink_to_hi().until(self.token.span.shrink_to_lo()))
 608                 {
 609                     span = span.with_hi(self.token.span.hi());
 610                     self.bump();
 611                     count += 1;
 612                 }
 613                 err.set_span(span);
 614                 err.span_suggestion(
 615                     span,
 616                     &format!("remove the extra `#`{}", pluralize!(count)),
 617                     "",
 618                     Applicability::MachineApplicable,
 619                 );
 620                 err.span_label(
 621                     str_span,
 622                     &format!("this raw string started with {n_hashes} `#`{}", pluralize!(n_hashes)),
 623                 );
 624                 true
 625             }
 626             _ => false,
 627         }
 628     }
 629
 630     pub fn maybe_suggest_struct_literal(
 631         &mut self,
 632         lo: Span,
 633         s: BlockCheckMode,
 634     ) -> Option<PResult<'a, P<Block>>> {
 635         if self.token.is_ident() && self.look_ahead(1, |t| t == &token::Colon) {
 636             // We might be having a struct literal where people forgot to include the path:
 637             // fn foo() -> Foo {
 638             //     field: value,
 639             // }
 640             let mut snapshot = self.create_snapshot_for_diagnostic();
 641             let path =
 642                 Path { segments: vec![], span: self.prev_token.span.shrink_to_lo(), tokens: None };
 643             let struct_expr = snapshot.parse_struct_expr(None, path, false);
 644             let block_tail = self.parse_block_tail(lo, s, AttemptLocalParseRecovery::No);
 645             return Some(match (struct_expr, block_tail) {
 646                 (Ok(expr), Err(mut err)) => {
 647                     // We have encountered the following:
 648                     // fn foo() -> Foo {
 649                     //     field: value,
 650                     // }
 651                     // Suggest:
 652                     // fn foo() -> Foo { Path {
 653                     //     field: value,
 654                     // } }
 655                     err.delay_as_bug();
 656                     self.sess.emit_err(StructLiteralBodyWithoutPath {
 657                         span: expr.span,
 658                         sugg: StructLiteralBodyWithoutPathSugg {
 659                             before: expr.span.shrink_to_lo(),
 660                             after: expr.span.shrink_to_hi(),
 661                         },
 662                     });
 663                     self.restore_snapshot(snapshot);
 664                     let mut tail = self.mk_block(
 665                         vec![self.mk_stmt_err(expr.span)],
 666                         s,
 667                         lo.to(self.prev_token.span),
 668                     );
 669                     tail.could_be_bare_literal = true;
 670                     Ok(tail)
 671                 }
 672                 (Err(err), Ok(tail)) => {
 673                     // We have a block tail that contains a somehow valid type ascription expr.
 674                     err.cancel();
 675                     Ok(tail)
 676                 }
 677                 (Err(snapshot_err), Err(err)) => {
 678                     // We don't know what went wrong, emit the normal error.
 679                     snapshot_err.cancel();
 680                     self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
 681                     Err(err)
 682                 }
 683                 (Ok(_), Ok(mut tail)) => {
 684                     tail.could_be_bare_literal = true;
 685                     Ok(tail)
 686                 }
 687             });
 688         }
 689         None
 690     }
 691
 692     pub fn maybe_annotate_with_ascription(
 693         &mut self,
 694         err: &mut Diagnostic,
 695         maybe_expected_semicolon: bool,
 696     ) {
 697         if let Some((sp, likely_path)) = self.last_type_ascription.take() {
 698             let sm = self.sess.source_map();
 699             let next_pos = sm.lookup_char_pos(self.token.span.lo());
 700             let op_pos = sm.lookup_char_pos(sp.hi());
 701
 702             let allow_unstable = self.sess.unstable_features.is_nightly_build();
 703
 704             if likely_path {
 705                 err.span_suggestion(
 706                     sp,
 707                     "maybe write a path separator here",
 708                     "::",
 709                     if allow_unstable {
 710                         Applicability::MaybeIncorrect
 711                     } else {
 712                         Applicability::MachineApplicable
 713                     },
 714                 );
 715                 self.sess.type_ascription_path_suggestions.borrow_mut().insert(sp);
 716             } else if op_pos.line != next_pos.line && maybe_expected_semicolon {
 717                 err.span_suggestion(
 718                     sp,
 719                     "try using a semicolon",
 720                     ";",
 721                     Applicability::MaybeIncorrect,
 722                 );
 723             } else if allow_unstable {
 724                 err.span_label(sp, "tried to parse a type due to this type ascription");
 725             } else {
 726                 err.span_label(sp, "tried to parse a type due to this");
 727             }
 728             if allow_unstable {
 729                 // Give extra information about type ascription only if it's a nightly compiler.
 730                 err.note(
 731                     "`#![feature(type_ascription)]` lets you annotate an expression with a type: \
 732                      `<expr>: <type>`",
 733                 );
 734                 if !likely_path {
 735                     // Avoid giving too much info when it was likely an unrelated typo.
 736                     err.note(
 737                         "see issue #23416 <https://github.com/rust-lang/rust/issues/23416> \
 738                         for more information",
 739                     );
 740                 }
 741             }
 742         }
 743     }
 744
 745     /// Eats and discards tokens until one of `kets` is encountered. Respects token trees,
 746     /// passes through any errors encountered. Used for error recovery.
 747     pub(super) fn eat_to_tokens(&mut self, kets: &[&TokenKind]) {
 748         if let Err(err) =
 749             self.parse_seq_to_before_tokens(kets, SeqSep::none(), TokenExpectType::Expect, |p| {
 750                 Ok(p.parse_token_tree())
 751             })
 752         {
 753             err.cancel();
 754         }
 755     }
 756
 757     /// This function checks if there are trailing angle brackets and produces
 758     /// a diagnostic to suggest removing them.
 759     ///
 760     /// ```ignore (diagnostic)
 761     /// let _ = [1, 2, 3].into_iter().collect::<Vec<usize>>>>();
 762     ///                                                    ^^ help: remove extra angle brackets
 763     /// ```
 764     ///
 765     /// If `true` is returned, then trailing brackets were recovered, tokens were consumed
 766     /// up until one of the tokens in 'end' was encountered, and an error was emitted.
 767     pub(super) fn check_trailing_angle_brackets(
 768         &mut self,
 769         segment: &PathSegment,
 770         end: &[&TokenKind],
 771     ) -> bool {
 772         // This function is intended to be invoked after parsing a path segment where there are two
 773         // cases:
 774         //
 775         // 1. A specific token is expected after the path segment.
 776         //    eg. `x.foo(`, `x.foo::<u32>(` (parenthesis - method call),
 777         //        `Foo::`, or `Foo::<Bar>::` (mod sep - continued path).
 778         // 2. No specific token is expected after the path segment.
 779         //    eg. `x.foo` (field access)
 780         //
 781         // This function is called after parsing `.foo` and before parsing the token `end` (if
 782         // present). This includes any angle bracket arguments, such as `.foo::<u32>` or
 783         // `Foo::<Bar>`.
 784
 785         // We only care about trailing angle brackets if we previously parsed angle bracket
 786         // arguments. This helps stop us incorrectly suggesting that extra angle brackets be
 787         // removed in this case:
 788         //
 789         // `x.foo >> (3)` (where `x.foo` is a `u32` for example)
 790         //
 791         // This case is particularly tricky as we won't notice it just looking at the tokens -
 792         // it will appear the same (in terms of upcoming tokens) as below (since the `::<u32>` will
 793         // have already been parsed):
 794         //
 795         // `x.foo::<u32>>>(3)`
 796         let parsed_angle_bracket_args =
 797             segment.args.as_ref().map_or(false, |args| args.is_angle_bracketed());
 798
 799         debug!(
 800             "check_trailing_angle_brackets: parsed_angle_bracket_args={:?}",
 801             parsed_angle_bracket_args,
 802         );
 803         if !parsed_angle_bracket_args {
 804             return false;
 805         }
 806
 807         // Keep the span at the start so we can highlight the sequence of `>` characters to be
 808         // removed.
 809         let lo = self.token.span;
 810
 811         // We need to look-ahead to see if we have `>` characters without moving the cursor forward
 812         // (since we might have the field access case and the characters we're eating are
 813         // actual operators and not trailing characters - ie `x.foo >> 3`).
 814         let mut position = 0;
 815
 816         // We can encounter `>` or `>>` tokens in any order, so we need to keep track of how
 817         // many of each (so we can correctly pluralize our error messages) and continue to
 818         // advance.
 819         let mut number_of_shr = 0;
 820         let mut number_of_gt = 0;
 821         while self.look_ahead(position, |t| {
 822             trace!("check_trailing_angle_brackets: t={:?}", t);
 823             if *t == token::BinOp(token::BinOpToken::Shr) {
 824                 number_of_shr += 1;
 825                 true
 826             } else if *t == token::Gt {
 827                 number_of_gt += 1;
 828                 true
 829             } else {
 830                 false
 831             }
 832         }) {
 833             position += 1;
 834         }
 835
 836         // If we didn't find any trailing `>` characters, then we have nothing to error about.
 837         debug!(
 838             "check_trailing_angle_brackets: number_of_gt={:?} number_of_shr={:?}",
 839             number_of_gt, number_of_shr,
 840         );
 841         if number_of_gt < 1 && number_of_shr < 1 {
 842             return false;
 843         }
 844
 845         // Finally, double check that we have our end token as otherwise this is the
 846         // second case.
 847         if self.look_ahead(position, |t| {
 848             trace!("check_trailing_angle_brackets: t={:?}", t);
 849             end.contains(&&t.kind)
 850         }) {
 851             // Eat from where we started until the end token so that parsing can continue
 852             // as if we didn't have those extra angle brackets.
 853             self.eat_to_tokens(end);
 854             let span = lo.until(self.token.span);
 855
 856             let num_extra_brackets = number_of_gt + number_of_shr * 2;
 857             self.sess.emit_err(UnmatchedAngleBrackets { span, num_extra_brackets });
 858             return true;
 859         }
 860         false
 861     }
 862
 863     /// Check if a method call with an intended turbofish has been written without surrounding
 864     /// angle brackets.
 865     pub(super) fn check_turbofish_missing_angle_brackets(&mut self, segment: &mut PathSegment) {
 866         if token::ModSep == self.token.kind && segment.args.is_none() {
 867             let snapshot = self.create_snapshot_for_diagnostic();
 868             self.bump();
 869             let lo = self.token.span;
 870             match self.parse_angle_args(None) {
 871                 Ok(args) => {
 872                     let span = lo.to(self.prev_token.span);
 873                     // Detect trailing `>` like in `x.collect::Vec<_>>()`.
 874                     let mut trailing_span = self.prev_token.span.shrink_to_hi();
 875                     while self.token.kind == token::BinOp(token::Shr)
 876                         || self.token.kind == token::Gt
 877                     {
 878                         trailing_span = trailing_span.to(self.token.span);
 879                         self.bump();
 880                     }
 881                     if self.token.kind == token::OpenDelim(Delimiter::Parenthesis) {
 882                         // Recover from bad turbofish: `foo.collect::Vec<_>()`.
 883                         let args = AngleBracketedArgs { args, span }.into();
 884                         segment.args = args;
 885
 886                         self.sess.emit_err(GenericParamsWithoutAngleBrackets {
 887                             span,
 888                             sugg: GenericParamsWithoutAngleBracketsSugg {
 889                                 left: span.shrink_to_lo(),
 890                                 right: trailing_span,
 891                             },
 892                         });
 893                     } else {
 894                         // This doesn't look like an invalid turbofish, can't recover parse state.
 895                         self.restore_snapshot(snapshot);
 896                     }
 897                 }
 898                 Err(err) => {
 899                     // We couldn't parse generic parameters, unlikely to be a turbofish. Rely on
 900                     // generic parse error instead.
 901                     err.cancel();
 902                     self.restore_snapshot(snapshot);
 903                 }
 904             }
 905         }
 906     }
 907
 908     /// When writing a turbofish with multiple type parameters missing the leading `::`, we will
 909     /// encounter a parse error when encountering the first `,`.
 910     pub(super) fn check_mistyped_turbofish_with_multiple_type_params(
 911         &mut self,
 912         mut e: DiagnosticBuilder<'a, ErrorGuaranteed>,
 913         expr: &mut P<Expr>,
 914     ) -> PResult<'a, ()> {
 915         if let ExprKind::Binary(binop, _, _) = &expr.kind
 916             && let ast::BinOpKind::Lt = binop.node
 917             && self.eat(&token::Comma)
 918         {
 919             let x = self.parse_seq_to_before_end(
 920                 &token::Gt,
 921                 SeqSep::trailing_allowed(token::Comma),
 922                 |p| p.parse_generic_arg(None),
 923             );
 924             match x {
 925                 Ok((_, _, false)) => {
 926                     if self.eat(&token::Gt) {
 927                         e.span_suggestion_verbose(
 928                             binop.span.shrink_to_lo(),
 929                             fluent::parser::sugg_turbofish_syntax,
 930                             "::",
 931                             Applicability::MaybeIncorrect,
 932                         )
 933                         .emit();
 934                         match self.parse_expr() {
 935                             Ok(_) => {
 936                                 *expr =
 937                                     self.mk_expr_err(expr.span.to(self.prev_token.span));
 938                                 return Ok(());
 939                             }
 940                             Err(err) => {
 941                                 *expr = self.mk_expr_err(expr.span);
 942                                 err.cancel();
 943                             }
 944                         }
 945                     }
 946                 }
 947                 Err(err) => {
 948                     err.cancel();
 949                 }
 950                 _ => {}
 951             }
 952         }
 953         Err(e)
 954     }
 955
 956     /// Check to see if a pair of chained operators looks like an attempt at chained comparison,
 957     /// e.g. `1 < x <= 3`. If so, suggest either splitting the comparison into two, or
 958     /// parenthesising the leftmost comparison.
 959     fn attempt_chained_comparison_suggestion(
 960         &mut self,
 961         err: &mut ComparisonOperatorsCannotBeChained,
 962         inner_op: &Expr,
 963         outer_op: &Spanned<AssocOp>,
 964     ) -> bool /* advanced the cursor */ {
 965         if let ExprKind::Binary(op, ref l1, ref r1) = inner_op.kind {
 966             if let ExprKind::Field(_, ident) = l1.kind
 967                 && ident.as_str().parse::<i32>().is_err()
 968                 && !matches!(r1.kind, ExprKind::Lit(_))
 969             {
 970                 // The parser has encountered `foo.bar<baz`, the likelihood of the turbofish
 971                 // suggestion being the only one to apply is high.
 972                 return false;
 973             }
 974             return match (op.node, &outer_op.node) {
 975                 // `x == y == z`
 976                 (BinOpKind::Eq, AssocOp::Equal) |
 977                 // `x < y < z` and friends.
 978                 (BinOpKind::Lt, AssocOp::Less | AssocOp::LessEqual) |
 979                 (BinOpKind::Le, AssocOp::LessEqual | AssocOp::Less) |
 980                 // `x > y > z` and friends.
 981                 (BinOpKind::Gt, AssocOp::Greater | AssocOp::GreaterEqual) |
 982                 (BinOpKind::Ge, AssocOp::GreaterEqual | AssocOp::Greater) => {
 983                     let expr_to_str = |e: &Expr| {
 984                         self.span_to_snippet(e.span)
 985                             .unwrap_or_else(|_| pprust::expr_to_string(&e))
 986                     };
 987                     err.chaining_sugg = Some(ComparisonOperatorsCannotBeChainedSugg::SplitComparison {
 988                         span: inner_op.span.shrink_to_hi(),
 989                         middle_term: expr_to_str(&r1),
 990                     });
 991                     false // Keep the current parse behavior, where the AST is `(x < y) < z`.
 992                 }
 993                 // `x == y < z`
 994                 (BinOpKind::Eq, AssocOp::Less | AssocOp::LessEqual | AssocOp::Greater | AssocOp::GreaterEqual) => {
 995                     // Consume `z`/outer-op-rhs.
 996                     let snapshot = self.create_snapshot_for_diagnostic();
 997                     match self.parse_expr() {
 998                         Ok(r2) => {
 999                             // We are sure that outer-op-rhs could be consumed, the suggestion is
1000                             // likely correct.
1001                             err.chaining_sugg = Some(ComparisonOperatorsCannotBeChainedSugg::Parenthesize {
1002                                 left: r1.span.shrink_to_lo(),
1003                                 right: r2.span.shrink_to_hi(),
1004                             });
1005                             true
1006                         }
1007                         Err(expr_err) => {
1008                             expr_err.cancel();
1009                             self.restore_snapshot(snapshot);
1010                             false
1011                         }
1012                     }
1013                 }
1014                 // `x > y == z`
1015                 (BinOpKind::Lt | BinOpKind::Le | BinOpKind::Gt | BinOpKind::Ge, AssocOp::Equal) => {
1016                     let snapshot = self.create_snapshot_for_diagnostic();
1017                     // At this point it is always valid to enclose the lhs in parentheses, no
1018                     // further checks are necessary.
1019                     match self.parse_expr() {
1020                         Ok(_) => {
1021                             err.chaining_sugg = Some(ComparisonOperatorsCannotBeChainedSugg::Parenthesize {
1022                                 left: l1.span.shrink_to_lo(),
1023                                 right: r1.span.shrink_to_hi(),
1024                             });
1025                             true
1026                         }
1027                         Err(expr_err) => {
1028                             expr_err.cancel();
1029                             self.restore_snapshot(snapshot);
1030                             false
1031                         }
1032                     }
1033                 }
1034                 _ => false,
1035             };
1036         }
1037         false
1038     }
1039
1040     /// Produces an error if comparison operators are chained (RFC #558).
1041     /// We only need to check the LHS, not the RHS, because all comparison ops have same
1042     /// precedence (see `fn precedence`) and are left-associative (see `fn fixity`).
1043     ///
1044     /// This can also be hit if someone incorrectly writes `foo<bar>()` when they should have used
1045     /// the turbofish (`foo::<bar>()`) syntax. We attempt some heuristic recovery if that is the
1046     /// case.
1047     ///
1048     /// Keep in mind that given that `outer_op.is_comparison()` holds and comparison ops are left
1049     /// associative we can infer that we have:
1050     ///
1051     /// ```text
1052     ///           outer_op
1053     ///           /   \
1054     ///     inner_op   r2
1055     ///        /  \
1056     ///      l1    r1
1057     /// ```
1058     pub(super) fn check_no_chained_comparison(
1059         &mut self,
1060         inner_op: &Expr,
1061         outer_op: &Spanned<AssocOp>,
1062     ) -> PResult<'a, Option<P<Expr>>> {
1063         debug_assert!(
1064             outer_op.node.is_comparison(),
1065             "check_no_chained_comparison: {:?} is not comparison",
1066             outer_op.node,
1067         );
1068
1069         let mk_err_expr = |this: &Self, span| Ok(Some(this.mk_expr(span, ExprKind::Err)));
1070
1071         match inner_op.kind {
1072             ExprKind::Binary(op, ref l1, ref r1) if op.node.is_comparison() => {
1073                 let mut err = ComparisonOperatorsCannotBeChained {
1074                     span: vec![op.span, self.prev_token.span],
1075                     suggest_turbofish: None,
1076                     help_turbofish: None,
1077                     chaining_sugg: None,
1078                 };
1079
1080                 // Include `<` to provide this recommendation even in a case like
1081                 // `Foo<Bar<Baz<Qux, ()>>>`
1082                 if op.node == BinOpKind::Lt && outer_op.node == AssocOp::Less
1083                     || outer_op.node == AssocOp::Greater
1084                 {
1085                     if outer_op.node == AssocOp::Less {
1086                         let snapshot = self.create_snapshot_for_diagnostic();
1087                         self.bump();
1088                         // So far we have parsed `foo<bar<`, consume the rest of the type args.
1089                         let modifiers =
1090                             [(token::Lt, 1), (token::Gt, -1), (token::BinOp(token::Shr), -2)];
1091                         self.consume_tts(1, &modifiers);
1092
1093                         if !&[token::OpenDelim(Delimiter::Parenthesis), token::ModSep]
1094                             .contains(&self.token.kind)
1095                         {
1096                             // We don't have `foo< bar >(` or `foo< bar >::`, so we rewind the
1097                             // parser and bail out.
1098                             self.restore_snapshot(snapshot);
1099                         }
1100                     }
1101                     return if token::ModSep == self.token.kind {
1102                         // We have some certainty that this was a bad turbofish at this point.
1103                         // `foo< bar >::`
1104                         err.suggest_turbofish = Some(op.span.shrink_to_lo());
1105
1106                         let snapshot = self.create_snapshot_for_diagnostic();
1107                         self.bump(); // `::`
1108
1109                         // Consume the rest of the likely `foo<bar>::new()` or return at `foo<bar>`.
1110                         match self.parse_expr() {
1111                             Ok(_) => {
1112                                 // 99% certain that the suggestion is correct, continue parsing.
1113                                 self.sess.emit_err(err);
1114                                 // FIXME: actually check that the two expressions in the binop are
1115                                 // paths and resynthesize new fn call expression instead of using
1116                                 // `ExprKind::Err` placeholder.
1117                                 mk_err_expr(self, inner_op.span.to(self.prev_token.span))
1118                             }
1119                             Err(expr_err) => {
1120                                 expr_err.cancel();
1121                                 // Not entirely sure now, but we bubble the error up with the
1122                                 // suggestion.
1123                                 self.restore_snapshot(snapshot);
1124                                 Err(err.into_diagnostic(&self.sess.span_diagnostic))
1125                             }
1126                         }
1127                     } else if token::OpenDelim(Delimiter::Parenthesis) == self.token.kind {
1128                         // We have high certainty that this was a bad turbofish at this point.
1129                         // `foo< bar >(`
1130                         err.suggest_turbofish = Some(op.span.shrink_to_lo());
1131                         // Consume the fn call arguments.
1132                         match self.consume_fn_args() {
1133                             Err(()) => Err(err.into_diagnostic(&self.sess.span_diagnostic)),
1134                             Ok(()) => {
1135                                 self.sess.emit_err(err);
1136                                 // FIXME: actually check that the two expressions in the binop are
1137                                 // paths and resynthesize new fn call expression instead of using
1138                                 // `ExprKind::Err` placeholder.
1139                                 mk_err_expr(self, inner_op.span.to(self.prev_token.span))
1140                             }
1141                         }
1142                     } else {
1143                         if !matches!(l1.kind, ExprKind::Lit(_))
1144                             && !matches!(r1.kind, ExprKind::Lit(_))
1145                         {
1146                             // All we know is that this is `foo < bar >` and *nothing* else. Try to
1147                             // be helpful, but don't attempt to recover.
1148                             err.help_turbofish = Some(());
1149                         }
1150
1151                         // If it looks like a genuine attempt to chain operators (as opposed to a
1152                         // misformatted turbofish, for instance), suggest a correct form.
1153                         if self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op)
1154                         {
1155                             self.sess.emit_err(err);
1156                             mk_err_expr(self, inner_op.span.to(self.prev_token.span))
1157                         } else {
1158                             // These cases cause too many knock-down errors, bail out (#61329).
1159                             Err(err.into_diagnostic(&self.sess.span_diagnostic))
1160                         }
1161                     };
1162                 }
1163                 let recover =
1164                     self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op);
1165                 self.sess.emit_err(err);
1166                 if recover {
1167                     return mk_err_expr(self, inner_op.span.to(self.prev_token.span));
1168                 }
1169             }
1170             _ => {}
1171         }
1172         Ok(None)
1173     }
1174
1175     fn consume_fn_args(&mut self) -> Result<(), ()> {
1176         let snapshot = self.create_snapshot_for_diagnostic();
1177         self.bump(); // `(`
1178
1179         // Consume the fn call arguments.
1180         let modifiers = [
1181             (token::OpenDelim(Delimiter::Parenthesis), 1),
1182             (token::CloseDelim(Delimiter::Parenthesis), -1),
1183         ];
1184         self.consume_tts(1, &modifiers);
1185
1186         if self.token.kind == token::Eof {
1187             // Not entirely sure that what we consumed were fn arguments, rollback.
1188             self.restore_snapshot(snapshot);
1189             Err(())
1190         } else {
1191             // 99% certain that the suggestion is correct, continue parsing.
1192             Ok(())
1193         }
1194     }
1195
1196     pub(super) fn maybe_report_ambiguous_plus(&mut self, impl_dyn_multi: bool, ty: &Ty) {
1197         if impl_dyn_multi {
1198             self.sess.emit_err(AmbiguousPlus { sum_ty: pprust::ty_to_string(&ty), span: ty.span });
1199         }
1200     }
1201
1202     /// Swift lets users write `Ty?` to mean `Option<Ty>`. Parse the construct and recover from it.
1203     pub(super) fn maybe_recover_from_question_mark(&mut self, ty: P<Ty>) -> P<Ty> {
1204         if self.token == token::Question {
1205             self.bump();
1206             self.sess.emit_err(QuestionMarkInType {
1207                 span: self.prev_token.span,
1208                 sugg: QuestionMarkInTypeSugg {
1209                     left: ty.span.shrink_to_lo(),
1210                     right: self.prev_token.span,
1211                 },
1212             });
1213             self.mk_ty(ty.span.to(self.prev_token.span), TyKind::Err)
1214         } else {
1215             ty
1216         }
1217     }
1218
1219     pub(super) fn maybe_recover_from_bad_type_plus(&mut self, ty: &Ty) -> PResult<'a, ()> {
1220         // Do not add `+` to expected tokens.
1221         if !self.token.is_like_plus() {
1222             return Ok(());
1223         }
1224
1225         self.bump(); // `+`
1226         let bounds = self.parse_generic_bounds(None)?;
1227         let sum_span = ty.span.to(self.prev_token.span);
1228
1229         let sub = match ty.kind {
1230             TyKind::Rptr(ref lifetime, ref mut_ty) => {
1231                 let sum_with_parens = pprust::to_string(|s| {
1232                     s.s.word("&");
1233                     s.print_opt_lifetime(lifetime);
1234                     s.print_mutability(mut_ty.mutbl, false);
1235                     s.popen();
1236                     s.print_type(&mut_ty.ty);
1237                     if !bounds.is_empty() {
1238                         s.word(" + ");
1239                         s.print_type_bounds(&bounds);
1240                     }
1241                     s.pclose()
1242                 });
1243
1244                 BadTypePlusSub::AddParen { sum_with_parens, span: sum_span }
1245             }
1246             TyKind::Ptr(..) | TyKind::BareFn(..) => BadTypePlusSub::ForgotParen { span: sum_span },
1247             _ => BadTypePlusSub::ExpectPath { span: sum_span },
1248         };
1249
1250         self.sess.emit_err(BadTypePlus { ty: pprust::ty_to_string(ty), span: sum_span, sub });
1251
1252         Ok(())
1253     }
1254
1255     pub(super) fn recover_from_prefix_increment(
1256         &mut self,
1257         operand_expr: P<Expr>,
1258         op_span: Span,
1259         prev_is_semi: bool,
1260     ) -> PResult<'a, P<Expr>> {
1261         let standalone =
1262             if prev_is_semi { IsStandalone::Standalone } else { IsStandalone::Subexpr };
1263         let kind = IncDecRecovery { standalone, op: IncOrDec::Inc, fixity: UnaryFixity::Pre };
1264
1265         self.recover_from_inc_dec(operand_expr, kind, op_span)
1266     }
1267
1268     pub(super) fn recover_from_postfix_increment(
1269         &mut self,
1270         operand_expr: P<Expr>,
1271         op_span: Span,
1272     ) -> PResult<'a, P<Expr>> {
1273         let kind = IncDecRecovery {
1274             standalone: IsStandalone::Maybe,
1275             op: IncOrDec::Inc,
1276             fixity: UnaryFixity::Post,
1277         };
1278
1279         self.recover_from_inc_dec(operand_expr, kind, op_span)
1280     }
1281
1282     fn recover_from_inc_dec(
1283         &mut self,
1284         base: P<Expr>,
1285         kind: IncDecRecovery,
1286         op_span: Span,
1287     ) -> PResult<'a, P<Expr>> {
1288         let mut err = self.struct_span_err(
1289             op_span,
1290             &format!("Rust has no {} {} operator", kind.fixity, kind.op.name()),
1291         );
1292         err.span_label(op_span, &format!("not a valid {} operator", kind.fixity));
1293
1294         let help_base_case = |mut err: DiagnosticBuilder<'_, _>, base| {
1295             err.help(&format!("use `{}= 1` instead", kind.op.chr()));
1296             err.emit();
1297             Ok(base)
1298         };
1299
1300         // (pre, post)
1301         let spans = match kind.fixity {
1302             UnaryFixity::Pre => (op_span, base.span.shrink_to_hi()),
1303             UnaryFixity::Post => (base.span.shrink_to_lo(), op_span),
1304         };
1305
1306         match kind.standalone {
1307             IsStandalone::Standalone => self.inc_dec_standalone_suggest(kind, spans).emit(&mut err),
1308             IsStandalone::Subexpr => {
1309                 let Ok(base_src) = self.span_to_snippet(base.span)
1310                     else { return help_base_case(err, base) };
1311                 match kind.fixity {
1312                     UnaryFixity::Pre => {
1313                         self.prefix_inc_dec_suggest(base_src, kind, spans).emit(&mut err)
1314                     }
1315                     UnaryFixity::Post => {
1316                         self.postfix_inc_dec_suggest(base_src, kind, spans).emit(&mut err)
1317                     }
1318                 }
1319             }
1320             IsStandalone::Maybe => {
1321                 let Ok(base_src) = self.span_to_snippet(base.span)
1322                     else { return help_base_case(err, base) };
1323                 let sugg1 = match kind.fixity {
1324                     UnaryFixity::Pre => self.prefix_inc_dec_suggest(base_src, kind, spans),
1325                     UnaryFixity::Post => self.postfix_inc_dec_suggest(base_src, kind, spans),
1326                 };
1327                 let sugg2 = self.inc_dec_standalone_suggest(kind, spans);
1328                 MultiSugg::emit_many(
1329                     &mut err,
1330                     "use `+= 1` instead",
1331                     Applicability::Unspecified,
1332                     [sugg1, sugg2].into_iter(),
1333                 )
1334             }
1335         }
1336         Err(err)
1337     }
1338
1339     fn prefix_inc_dec_suggest(
1340         &mut self,
1341         base_src: String,
1342         kind: IncDecRecovery,
1343         (pre_span, post_span): (Span, Span),
1344     ) -> MultiSugg {
1345         MultiSugg {
1346             msg: format!("use `{}= 1` instead", kind.op.chr()),
1347             patches: vec![
1348                 (pre_span, "{ ".to_string()),
1349                 (post_span, format!(" {}= 1; {} }}", kind.op.chr(), base_src)),
1350             ],
1351             applicability: Applicability::MachineApplicable,
1352         }
1353     }
1354
1355     fn postfix_inc_dec_suggest(
1356         &mut self,
1357         base_src: String,
1358         kind: IncDecRecovery,
1359         (pre_span, post_span): (Span, Span),
1360     ) -> MultiSugg {
1361         let tmp_var = if base_src.trim() == "tmp" { "tmp_" } else { "tmp" };
1362         MultiSugg {
1363             msg: format!("use `{}= 1` instead", kind.op.chr()),
1364             patches: vec![
1365                 (pre_span, format!("{{ let {tmp_var} = ")),
1366                 (post_span, format!("; {} {}= 1; {} }}", base_src, kind.op.chr(), tmp_var)),
1367             ],
1368             applicability: Applicability::HasPlaceholders,
1369         }
1370     }
1371
1372     fn inc_dec_standalone_suggest(
1373         &mut self,
1374         kind: IncDecRecovery,
1375         (pre_span, post_span): (Span, Span),
1376     ) -> MultiSugg {
1377         let mut patches = Vec::new();
1378
1379         if !pre_span.is_empty() {
1380             patches.push((pre_span, String::new()));
1381         }
1382
1383         patches.push((post_span, format!(" {}= 1", kind.op.chr())));
1384
1385         MultiSugg {
1386             msg: format!("use `{}= 1` instead", kind.op.chr()),
1387             patches,
1388             applicability: Applicability::MachineApplicable,
1389         }
1390     }
1391
1392     /// Tries to recover from associated item paths like `[T]::AssocItem` / `(T, U)::AssocItem`.
1393     /// Attempts to convert the base expression/pattern/type into a type, parses the `::AssocItem`
1394     /// tail, and combines them into a `<Ty>::AssocItem` expression/pattern/type.
1395     pub(super) fn maybe_recover_from_bad_qpath<T: RecoverQPath>(
1396         &mut self,
1397         base: P<T>,
1398     ) -> PResult<'a, P<T>> {
1399         // Do not add `::` to expected tokens.
1400         if self.token == token::ModSep {
1401             if let Some(ty) = base.to_ty() {
1402                 return self.maybe_recover_from_bad_qpath_stage_2(ty.span, ty);
1403             }
1404         }
1405         Ok(base)
1406     }
1407
1408     /// Given an already parsed `Ty`, parses the `::AssocItem` tail and
1409     /// combines them into a `<Ty>::AssocItem` expression/pattern/type.
1410     pub(super) fn maybe_recover_from_bad_qpath_stage_2<T: RecoverQPath>(
1411         &mut self,
1412         ty_span: Span,
1413         ty: P<Ty>,
1414     ) -> PResult<'a, P<T>> {
1415         self.expect(&token::ModSep)?;
1416
1417         let mut path = ast::Path { segments: Vec::new(), span: DUMMY_SP, tokens: None };
1418         self.parse_path_segments(&mut path.segments, T::PATH_STYLE, None)?;
1419         path.span = ty_span.to(self.prev_token.span);
1420
1421         let ty_str = self.span_to_snippet(ty_span).unwrap_or_else(|_| pprust::ty_to_string(&ty));
1422         self.sess.emit_err(BadQPathStage2 {
1423             span: path.span,
1424             ty: format!("<{}>::{}", ty_str, pprust::path_to_string(&path)),
1425         });
1426
1427         let path_span = ty_span.shrink_to_hi(); // Use an empty path since `position == 0`.
1428         Ok(P(T::recovered(Some(QSelf { ty, path_span, position: 0 }), path)))
1429     }
1430
1431     pub fn maybe_consume_incorrect_semicolon(&mut self, items: &[P<Item>]) -> bool {
1432         if self.token.kind == TokenKind::Semi {
1433             self.bump();
1434
1435             let mut err =
1436                 IncorrectSemicolon { span: self.prev_token.span, opt_help: None, name: "" };
1437
1438             if !items.is_empty() {
1439                 let previous_item = &items[items.len() - 1];
1440                 let previous_item_kind_name = match previous_item.kind {
1441                     // Say "braced struct" because tuple-structs and
1442                     // braceless-empty-struct declarations do take a semicolon.
1443                     ItemKind::Struct(..) => Some("braced struct"),
1444                     ItemKind::Enum(..) => Some("enum"),
1445                     ItemKind::Trait(..) => Some("trait"),
1446                     ItemKind::Union(..) => Some("union"),
1447                     _ => None,
1448                 };
1449                 if let Some(name) = previous_item_kind_name {
1450                     err.opt_help = Some(());
1451                     err.name = name;
1452                 }
1453             }
1454             self.sess.emit_err(err);
1455             true
1456         } else {
1457             false
1458         }
1459     }
1460
1461     /// Creates a `DiagnosticBuilder` for an unexpected token `t` and tries to recover if it is a
1462     /// closing delimiter.
1463     pub(super) fn unexpected_try_recover(
1464         &mut self,
1465         t: &TokenKind,
1466     ) -> PResult<'a, bool /* recovered */> {
1467         let token_str = pprust::token_kind_to_string(t);
1468         let this_token_str = super::token_descr(&self.token);
1469         let (prev_sp, sp) = match (&self.token.kind, self.subparser_name) {
1470             // Point at the end of the macro call when reaching end of macro arguments.
1471             (token::Eof, Some(_)) => {
1472                 let sp = self.sess.source_map().next_point(self.prev_token.span);
1473                 (sp, sp)
1474             }
1475             // We don't want to point at the following span after DUMMY_SP.
1476             // This happens when the parser finds an empty TokenStream.
1477             _ if self.prev_token.span == DUMMY_SP => (self.token.span, self.token.span),
1478             // EOF, don't want to point at the following char, but rather the last token.
1479             (token::Eof, None) => (self.prev_token.span, self.token.span),
1480             _ => (self.prev_token.span.shrink_to_hi(), self.token.span),
1481         };
1482         let msg = format!(
1483             "expected `{}`, found {}",
1484             token_str,
1485             match (&self.token.kind, self.subparser_name) {
1486                 (token::Eof, Some(origin)) => format!("end of {origin}"),
1487                 _ => this_token_str,
1488             },
1489         );
1490         let mut err = self.struct_span_err(sp, &msg);
1491         let label_exp = format!("expected `{token_str}`");
1492         match self.recover_closing_delimiter(&[t.clone()], err) {
1493             Err(e) => err = e,
1494             Ok(recovered) => {
1495                 return Ok(recovered);
1496             }
1497         }
1498         let sm = self.sess.source_map();
1499         if !sm.is_multiline(prev_sp.until(sp)) {
1500             // When the spans are in the same line, it means that the only content
1501             // between them is whitespace, point only at the found token.
1502             err.span_label(sp, label_exp);
1503         } else {
1504             err.span_label(prev_sp, label_exp);
1505             err.span_label(sp, "unexpected token");
1506         }
1507         Err(err)
1508     }
1509
1510     pub(super) fn expect_semi(&mut self) -> PResult<'a, ()> {
1511         if self.eat(&token::Semi) {
1512             return Ok(());
1513         }
1514         self.expect(&token::Semi).map(drop) // Error unconditionally
1515     }
1516
1517     /// Consumes alternative await syntaxes like `await!(<expr>)`, `await <expr>`,
1518     /// `await? <expr>`, `await(<expr>)`, and `await { <expr> }`.
1519     pub(super) fn recover_incorrect_await_syntax(
1520         &mut self,
1521         lo: Span,
1522         await_sp: Span,
1523     ) -> PResult<'a, P<Expr>> {
1524         let (hi, expr, is_question) = if self.token == token::Not {
1525             // Handle `await!(<expr>)`.
1526             self.recover_await_macro()?
1527         } else {
1528             self.recover_await_prefix(await_sp)?
1529         };
1530         let sp = self.error_on_incorrect_await(lo, hi, &expr, is_question);
1531         let kind = match expr.kind {
1532             // Avoid knock-down errors as we don't know whether to interpret this as `foo().await?`
1533             // or `foo()?.await` (the very reason we went with postfix syntax 😅).
1534             ExprKind::Try(_) => ExprKind::Err,
1535             _ => ExprKind::Await(expr),
1536         };
1537         let expr = self.mk_expr(lo.to(sp), kind);
1538         self.maybe_recover_from_bad_qpath(expr)
1539     }
1540
1541     fn recover_await_macro(&mut self) -> PResult<'a, (Span, P<Expr>, bool)> {
1542         self.expect(&token::Not)?;
1543         self.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
1544         let expr = self.parse_expr()?;
1545         self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
1546         Ok((self.prev_token.span, expr, false))
1547     }
1548
1549     fn recover_await_prefix(&mut self, await_sp: Span) -> PResult<'a, (Span, P<Expr>, bool)> {
1550         let is_question = self.eat(&token::Question); // Handle `await? <expr>`.
1551         let expr = if self.token == token::OpenDelim(Delimiter::Brace) {
1552             // Handle `await { <expr> }`.
1553             // This needs to be handled separately from the next arm to avoid
1554             // interpreting `await { <expr> }?` as `<expr>?.await`.
1555             self.parse_block_expr(None, self.token.span, BlockCheckMode::Default)
1556         } else {
1557             self.parse_expr()
1558         }
1559         .map_err(|mut err| {
1560             err.span_label(await_sp, "while parsing this incorrect await expression");
1561             err
1562         })?;
1563         Ok((expr.span, expr, is_question))
1564     }
1565
1566     fn error_on_incorrect_await(&self, lo: Span, hi: Span, expr: &Expr, is_question: bool) -> Span {
1567         let span = lo.to(hi);
1568         let applicability = match expr.kind {
1569             ExprKind::Try(_) => Applicability::MaybeIncorrect, // `await <expr>?`
1570             _ => Applicability::MachineApplicable,
1571         };
1572
1573         self.sess.emit_err(IncorrectAwait {
1574             span,
1575             sugg_span: (span, applicability),
1576             expr: self.span_to_snippet(expr.span).unwrap_or_else(|_| pprust::expr_to_string(&expr)),
1577             question_mark: if is_question { "?" } else { "" },
1578         });
1579
1580         span
1581     }
1582
1583     /// If encountering `future.await()`, consumes and emits an error.
1584     pub(super) fn recover_from_await_method_call(&mut self) {
1585         if self.token == token::OpenDelim(Delimiter::Parenthesis)
1586             && self.look_ahead(1, |t| t == &token::CloseDelim(Delimiter::Parenthesis))
1587         {
1588             // future.await()
1589             let lo = self.token.span;
1590             self.bump(); // (
1591             let span = lo.to(self.token.span);
1592             self.bump(); // )
1593
1594             self.sess.emit_err(IncorrectUseOfAwait { span });
1595         }
1596     }
1597
1598     pub(super) fn try_macro_suggestion(&mut self) -> PResult<'a, P<Expr>> {
1599         let is_try = self.token.is_keyword(kw::Try);
1600         let is_questionmark = self.look_ahead(1, |t| t == &token::Not); //check for !
1601         let is_open = self.look_ahead(2, |t| t == &token::OpenDelim(Delimiter::Parenthesis)); //check for (
1602
1603         if is_try && is_questionmark && is_open {
1604             let lo = self.token.span;
1605             self.bump(); //remove try
1606             self.bump(); //remove !
1607             let try_span = lo.to(self.token.span); //we take the try!( span
1608             self.bump(); //remove (
1609             let is_empty = self.token == token::CloseDelim(Delimiter::Parenthesis); //check if the block is empty
1610             self.consume_block(Delimiter::Parenthesis, ConsumeClosingDelim::No); //eat the block
1611             let hi = self.token.span;
1612             self.bump(); //remove )
1613             let mut err = self.struct_span_err(lo.to(hi), "use of deprecated `try` macro");
1614             err.note("in the 2018 edition `try` is a reserved keyword, and the `try!()` macro is deprecated");
1615             let prefix = if is_empty { "" } else { "alternatively, " };
1616             if !is_empty {
1617                 err.multipart_suggestion(
1618                     "you can use the `?` operator instead",
1619                     vec![(try_span, "".to_owned()), (hi, "?".to_owned())],
1620                     Applicability::MachineApplicable,
1621                 );
1622             }
1623             err.span_suggestion(lo.shrink_to_lo(), &format!("{prefix}you can still access the deprecated `try!()` macro using the \"raw identifier\" syntax"), "r#", Applicability::MachineApplicable);
1624             err.emit();
1625             Ok(self.mk_expr_err(lo.to(hi)))
1626         } else {
1627             Err(self.expected_expression_found()) // The user isn't trying to invoke the try! macro
1628         }
1629     }
1630
1631     /// Recovers a situation like `for ( $pat in $expr )`
1632     /// and suggest writing `for $pat in $expr` instead.
1633     ///
1634     /// This should be called before parsing the `$block`.
1635     pub(super) fn recover_parens_around_for_head(
1636         &mut self,
1637         pat: P<Pat>,
1638         begin_paren: Option<Span>,
1639     ) -> P<Pat> {
1640         match (&self.token.kind, begin_paren) {
1641             (token::CloseDelim(Delimiter::Parenthesis), Some(begin_par_sp)) => {
1642                 self.bump();
1643
1644                 self.sess.emit_err(ParenthesesInForHead {
1645                     span: vec![begin_par_sp, self.prev_token.span],
1646                     // With e.g. `for (x) in y)` this would replace `(x) in y)`
1647                     // with `x) in y)` which is syntactically invalid.
1648                     // However, this is prevented before we get here.
1649                     sugg: ParenthesesInForHeadSugg {
1650                         left: begin_par_sp,
1651                         right: self.prev_token.span,
1652                     },
1653                 });
1654
1655                 // Unwrap `(pat)` into `pat` to avoid the `unused_parens` lint.
1656                 pat.and_then(|pat| match pat.kind {
1657                     PatKind::Paren(pat) => pat,
1658                     _ => P(pat),
1659                 })
1660             }
1661             _ => pat,
1662         }
1663     }
1664
1665     pub(super) fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
1666         (self.token == token::Lt && // `foo:<bar`, likely a typoed turbofish.
1667             self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident()))
1668             || self.token.is_ident() &&
1669             matches!(node, ast::ExprKind::Path(..) | ast::ExprKind::Field(..)) &&
1670             !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
1671             self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Parenthesis))
1672             || self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Brace)) // `foo:bar {`
1673             || self.look_ahead(1, |t| t == &token::Colon) &&     // `foo:bar::<baz`
1674             self.look_ahead(2, |t| t == &token::Lt) &&
1675             self.look_ahead(3, |t| t.is_ident())
1676             || self.look_ahead(1, |t| t == &token::Colon) &&  // `foo:bar:baz`
1677             self.look_ahead(2, |t| t.is_ident())
1678             || self.look_ahead(1, |t| t == &token::ModSep)
1679                 && (self.look_ahead(2, |t| t.is_ident()) ||   // `foo:bar::baz`
1680             self.look_ahead(2, |t| t == &token::Lt)) // `foo:bar::<baz>`
1681     }
1682
1683     pub(super) fn recover_seq_parse_error(
1684         &mut self,
1685         delim: Delimiter,
1686         lo: Span,
1687         result: PResult<'a, P<Expr>>,
1688     ) -> P<Expr> {
1689         match result {
1690             Ok(x) => x,
1691             Err(mut err) => {
1692                 err.emit();
1693                 // Recover from parse error, callers expect the closing delim to be consumed.
1694                 self.consume_block(delim, ConsumeClosingDelim::Yes);
1695                 self.mk_expr(lo.to(self.prev_token.span), ExprKind::Err)
1696             }
1697         }
1698     }
1699
1700     pub(super) fn recover_closing_delimiter(
1701         &mut self,
1702         tokens: &[TokenKind],
1703         mut err: DiagnosticBuilder<'a, ErrorGuaranteed>,
1704     ) -> PResult<'a, bool> {
1705         let mut pos = None;
1706         // We want to use the last closing delim that would apply.
1707         for (i, unmatched) in self.unclosed_delims.iter().enumerate().rev() {
1708             if tokens.contains(&token::CloseDelim(unmatched.expected_delim))
1709                 && Some(self.token.span) > unmatched.unclosed_span
1710             {
1711                 pos = Some(i);
1712             }
1713         }
1714         match pos {
1715             Some(pos) => {
1716                 // Recover and assume that the detected unclosed delimiter was meant for
1717                 // this location. Emit the diagnostic and act as if the delimiter was
1718                 // present for the parser's sake.
1719
1720                 // Don't attempt to recover from this unclosed delimiter more than once.
1721                 let unmatched = self.unclosed_delims.remove(pos);
1722                 let delim = TokenType::Token(token::CloseDelim(unmatched.expected_delim));
1723                 if unmatched.found_delim.is_none() {
1724                     // We encountered `Eof`, set this fact here to avoid complaining about missing
1725                     // `fn main()` when we found place to suggest the closing brace.
1726                     *self.sess.reached_eof.borrow_mut() = true;
1727                 }
1728
1729                 // We want to suggest the inclusion of the closing delimiter where it makes
1730                 // the most sense, which is immediately after the last token:
1731                 //
1732                 //  {foo(bar {}}
1733                 //      ^      ^
1734                 //      |      |
1735                 //      |      help: `)` may belong here
1736                 //      |
1737                 //      unclosed delimiter
1738                 if let Some(sp) = unmatched.unclosed_span {
1739                     let mut primary_span: Vec<Span> =
1740                         err.span.primary_spans().iter().cloned().collect();
1741                     primary_span.push(sp);
1742                     let mut primary_span: MultiSpan = primary_span.into();
1743                     for span_label in err.span.span_labels() {
1744                         if let Some(label) = span_label.label {
1745                             primary_span.push_span_label(span_label.span, label);
1746                         }
1747                     }
1748                     err.set_span(primary_span);
1749                     err.span_label(sp, "unclosed delimiter");
1750                 }
1751                 // Backticks should be removed to apply suggestions.
1752                 let mut delim = delim.to_string();
1753                 delim.retain(|c| c != '`');
1754                 err.span_suggestion_short(
1755                     self.prev_token.span.shrink_to_hi(),
1756                     &format!("`{delim}` may belong here"),
1757                     delim,
1758                     Applicability::MaybeIncorrect,
1759                 );
1760                 if unmatched.found_delim.is_none() {
1761                     // Encountered `Eof` when lexing blocks. Do not recover here to avoid knockdown
1762                     // errors which would be emitted elsewhere in the parser and let other error
1763                     // recovery consume the rest of the file.
1764                     Err(err)
1765                 } else {
1766                     err.emit();
1767                     self.expected_tokens.clear(); // Reduce the number of errors.
1768                     Ok(true)
1769                 }
1770             }
1771             _ => Err(err),
1772         }
1773     }
1774
1775     /// Eats tokens until we can be relatively sure we reached the end of the
1776     /// statement. This is something of a best-effort heuristic.
1777     ///
1778     /// We terminate when we find an unmatched `}` (without consuming it).
1779     pub(super) fn recover_stmt(&mut self) {
1780         self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore)
1781     }
1782
1783     /// If `break_on_semi` is `Break`, then we will stop consuming tokens after
1784     /// finding (and consuming) a `;` outside of `{}` or `[]` (note that this is
1785     /// approximate -- it can mean we break too early due to macros, but that
1786     /// should only lead to sub-optimal recovery, not inaccurate parsing).
1787     ///
1788     /// If `break_on_block` is `Break`, then we will stop consuming tokens
1789     /// after finding (and consuming) a brace-delimited block.
1790     pub(super) fn recover_stmt_(
1791         &mut self,
1792         break_on_semi: SemiColonMode,
1793         break_on_block: BlockMode,
1794     ) {
1795         let mut brace_depth = 0;
1796         let mut bracket_depth = 0;
1797         let mut in_block = false;
1798         debug!("recover_stmt_ enter loop (semi={:?}, block={:?})", break_on_semi, break_on_block);
1799         loop {
1800             debug!("recover_stmt_ loop {:?}", self.token);
1801             match self.token.kind {
1802                 token::OpenDelim(Delimiter::Brace) => {
1803                     brace_depth += 1;
1804                     self.bump();
1805                     if break_on_block == BlockMode::Break && brace_depth == 1 && bracket_depth == 0
1806                     {
1807                         in_block = true;
1808                     }
1809                 }
1810                 token::OpenDelim(Delimiter::Bracket) => {
1811                     bracket_depth += 1;
1812                     self.bump();
1813                 }
1814                 token::CloseDelim(Delimiter::Brace) => {
1815                     if brace_depth == 0 {
1816                         debug!("recover_stmt_ return - close delim {:?}", self.token);
1817                         break;
1818                     }
1819                     brace_depth -= 1;
1820                     self.bump();
1821                     if in_block && bracket_depth == 0 && brace_depth == 0 {
1822                         debug!("recover_stmt_ return - block end {:?}", self.token);
1823                         break;
1824                     }
1825                 }
1826                 token::CloseDelim(Delimiter::Bracket) => {
1827                     bracket_depth -= 1;
1828                     if bracket_depth < 0 {
1829                         bracket_depth = 0;
1830                     }
1831                     self.bump();
1832                 }
1833                 token::Eof => {
1834                     debug!("recover_stmt_ return - Eof");
1835                     break;
1836                 }
1837                 token::Semi => {
1838                     self.bump();
1839                     if break_on_semi == SemiColonMode::Break
1840                         && brace_depth == 0
1841                         && bracket_depth == 0
1842                     {
1843                         debug!("recover_stmt_ return - Semi");
1844                         break;
1845                     }
1846                 }
1847                 token::Comma
1848                     if break_on_semi == SemiColonMode::Comma
1849                         && brace_depth == 0
1850                         && bracket_depth == 0 =>
1851                 {
1852                     debug!("recover_stmt_ return - Semi");
1853                     break;
1854                 }
1855                 _ => self.bump(),
1856             }
1857         }
1858     }
1859
1860     pub(super) fn check_for_for_in_in_typo(&mut self, in_span: Span) {
1861         if self.eat_keyword(kw::In) {
1862             // a common typo: `for _ in in bar {}`
1863             self.sess.emit_err(InInTypo {
1864                 span: self.prev_token.span,
1865                 sugg_span: in_span.until(self.prev_token.span),
1866             });
1867         }
1868     }
1869
1870     pub(super) fn eat_incorrect_doc_comment_for_param_type(&mut self) {
1871         if let token::DocComment(..) = self.token.kind {
1872             self.sess.emit_err(DocCommentOnParamType { span: self.token.span });
1873             self.bump();
1874         } else if self.token == token::Pound
1875             && self.look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Bracket))
1876         {
1877             let lo = self.token.span;
1878             // Skip every token until next possible arg.
1879             while self.token != token::CloseDelim(Delimiter::Bracket) {
1880                 self.bump();
1881             }
1882             let sp = lo.to(self.token.span);
1883             self.bump();
1884             self.sess.emit_err(AttributeOnParamType { span: sp });
1885         }
1886     }
1887
1888     pub(super) fn parameter_without_type(
1889         &mut self,
1890         err: &mut Diagnostic,
1891         pat: P<ast::Pat>,
1892         require_name: bool,
1893         first_param: bool,
1894     ) -> Option<Ident> {
1895         // If we find a pattern followed by an identifier, it could be an (incorrect)
1896         // C-style parameter declaration.
1897         if self.check_ident()
1898             && self.look_ahead(1, |t| {
1899                 *t == token::Comma || *t == token::CloseDelim(Delimiter::Parenthesis)
1900             })
1901         {
1902             // `fn foo(String s) {}`
1903             let ident = self.parse_ident().unwrap();
1904             let span = pat.span.with_hi(ident.span.hi());
1905
1906             err.span_suggestion(
1907                 span,
1908                 "declare the type after the parameter binding",
1909                 "<identifier>: <type>",
1910                 Applicability::HasPlaceholders,
1911             );
1912             return Some(ident);
1913         } else if require_name
1914             && (self.token == token::Comma
1915                 || self.token == token::Lt
1916                 || self.token == token::CloseDelim(Delimiter::Parenthesis))
1917         {
1918             let rfc_note = "anonymous parameters are removed in the 2018 edition (see RFC 1685)";
1919
1920             let (ident, self_sugg, param_sugg, type_sugg, self_span, param_span, type_span) =
1921                 match pat.kind {
1922                     PatKind::Ident(_, ident, _) => (
1923                         ident,
1924                         "self: ",
1925                         ": TypeName".to_string(),
1926                         "_: ",
1927                         pat.span.shrink_to_lo(),
1928                         pat.span.shrink_to_hi(),
1929                         pat.span.shrink_to_lo(),
1930                     ),
1931                     // Also catches `fn foo(&a)`.
1932                     PatKind::Ref(ref inner_pat, mutab)
1933                         if matches!(inner_pat.clone().into_inner().kind, PatKind::Ident(..)) =>
1934                     {
1935                         match inner_pat.clone().into_inner().kind {
1936                             PatKind::Ident(_, ident, _) => {
1937                                 let mutab = mutab.prefix_str();
1938                                 (
1939                                     ident,
1940                                     "self: ",
1941                                     format!("{ident}: &{mutab}TypeName"),
1942                                     "_: ",
1943                                     pat.span.shrink_to_lo(),
1944                                     pat.span,
1945                                     pat.span.shrink_to_lo(),
1946                                 )
1947                             }
1948                             _ => unreachable!(),
1949                         }
1950                     }
1951                     _ => {
1952                         // Otherwise, try to get a type and emit a suggestion.
1953                         if let Some(ty) = pat.to_ty() {
1954                             err.span_suggestion_verbose(
1955                                 pat.span,
1956                                 "explicitly ignore the parameter name",
1957                                 format!("_: {}", pprust::ty_to_string(&ty)),
1958                                 Applicability::MachineApplicable,
1959                             );
1960                             err.note(rfc_note);
1961                         }
1962
1963                         return None;
1964                     }
1965                 };
1966
1967             // `fn foo(a, b) {}`, `fn foo(a<x>, b<y>) {}` or `fn foo(usize, usize) {}`
1968             if first_param {
1969                 err.span_suggestion(
1970                     self_span,
1971                     "if this is a `self` type, give it a parameter name",
1972                     self_sugg,
1973                     Applicability::MaybeIncorrect,
1974                 );
1975             }
1976             // Avoid suggesting that `fn foo(HashMap<u32>)` is fixed with a change to
1977             // `fn foo(HashMap: TypeName<u32>)`.
1978             if self.token != token::Lt {
1979                 err.span_suggestion(
1980                     param_span,
1981                     "if this is a parameter name, give it a type",
1982                     param_sugg,
1983                     Applicability::HasPlaceholders,
1984                 );
1985             }
1986             err.span_suggestion(
1987                 type_span,
1988                 "if this is a type, explicitly ignore the parameter name",
1989                 type_sugg,
1990                 Applicability::MachineApplicable,
1991             );
1992             err.note(rfc_note);
1993
1994             // Don't attempt to recover by using the `X` in `X<Y>` as the parameter name.
1995             return if self.token == token::Lt { None } else { Some(ident) };
1996         }
1997         None
1998     }
1999
2000     pub(super) fn recover_arg_parse(&mut self) -> PResult<'a, (P<ast::Pat>, P<ast::Ty>)> {
2001         let pat = self.parse_pat_no_top_alt(Some("argument name"))?;
2002         self.expect(&token::Colon)?;
2003         let ty = self.parse_ty()?;
2004
2005         self.sess.emit_err(PatternMethodParamWithoutBody { span: pat.span });
2006
2007         // Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
2008         let pat =
2009             P(Pat { kind: PatKind::Wild, span: pat.span, id: ast::DUMMY_NODE_ID, tokens: None });
2010         Ok((pat, ty))
2011     }
2012
2013     pub(super) fn recover_bad_self_param(&mut self, mut param: Param) -> PResult<'a, Param> {
2014         let span = param.pat.span;
2015         param.ty.kind = TyKind::Err;
2016         self.sess.emit_err(SelfParamNotFirst { span });
2017         Ok(param)
2018     }
2019
2020     pub(super) fn consume_block(&mut self, delim: Delimiter, consume_close: ConsumeClosingDelim) {
2021         let mut brace_depth = 0;
2022         loop {
2023             if self.eat(&token::OpenDelim(delim)) {
2024                 brace_depth += 1;
2025             } else if self.check(&token::CloseDelim(delim)) {
2026                 if brace_depth == 0 {
2027                     if let ConsumeClosingDelim::Yes = consume_close {
2028                         // Some of the callers of this method expect to be able to parse the
2029                         // closing delimiter themselves, so we leave it alone. Otherwise we advance
2030                         // the parser.
2031                         self.bump();
2032                     }
2033                     return;
2034                 } else {
2035                     self.bump();
2036                     brace_depth -= 1;
2037                     continue;
2038                 }
2039             } else if self.token == token::Eof {
2040                 return;
2041             } else {
2042                 self.bump();
2043             }
2044         }
2045     }
2046
2047     pub(super) fn expected_expression_found(&self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
2048         let (span, msg) = match (&self.token.kind, self.subparser_name) {
2049             (&token::Eof, Some(origin)) => {
2050                 let sp = self.sess.source_map().next_point(self.prev_token.span);
2051                 (sp, format!("expected expression, found end of {origin}"))
2052             }
2053             _ => (
2054                 self.token.span,
2055                 format!("expected expression, found {}", super::token_descr(&self.token),),
2056             ),
2057         };
2058         let mut err = self.struct_span_err(span, &msg);
2059         let sp = self.sess.source_map().start_point(self.token.span);
2060         if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&sp) {
2061             err.subdiagnostic(ExprParenthesesNeeded::surrounding(*sp));
2062         }
2063         err.span_label(span, "expected expression");
2064         err
2065     }
2066
2067     fn consume_tts(
2068         &mut self,
2069         mut acc: i64, // `i64` because malformed code can have more closing delims than opening.
2070         // Not using `FxHashMap` due to `token::TokenKind: !Eq + !Hash`.
2071         modifier: &[(token::TokenKind, i64)],
2072     ) {
2073         while acc > 0 {
2074             if let Some((_, val)) = modifier.iter().find(|(t, _)| *t == self.token.kind) {
2075                 acc += *val;
2076             }
2077             if self.token.kind == token::Eof {
2078                 break;
2079             }
2080             self.bump();
2081         }
2082     }
2083
2084     /// Replace duplicated recovered parameters with `_` pattern to avoid unnecessary errors.
2085     ///
2086     /// This is necessary because at this point we don't know whether we parsed a function with
2087     /// anonymous parameters or a function with names but no types. In order to minimize
2088     /// unnecessary errors, we assume the parameters are in the shape of `fn foo(a, b, c)` where
2089     /// the parameters are *names* (so we don't emit errors about not being able to find `b` in
2090     /// the local scope), but if we find the same name multiple times, like in `fn foo(i8, i8)`,
2091     /// we deduplicate them to not complain about duplicated parameter names.
2092     pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut Vec<Param>) {
2093         let mut seen_inputs = FxHashSet::default();
2094         for input in fn_inputs.iter_mut() {
2095             let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err) =
2096                 (&input.pat.kind, &input.ty.kind)
2097             {
2098                 Some(*ident)
2099             } else {
2100                 None
2101             };
2102             if let Some(ident) = opt_ident {
2103                 if seen_inputs.contains(&ident) {
2104                     input.pat.kind = PatKind::Wild;
2105                 }
2106                 seen_inputs.insert(ident);
2107             }
2108         }
2109     }
2110
2111     /// Handle encountering a symbol in a generic argument list that is not a `,` or `>`. In this
2112     /// case, we emit an error and try to suggest enclosing a const argument in braces if it looks
2113     /// like the user has forgotten them.
2114     pub fn handle_ambiguous_unbraced_const_arg(
2115         &mut self,
2116         args: &mut Vec<AngleBracketedArg>,
2117     ) -> PResult<'a, bool> {
2118         // If we haven't encountered a closing `>`, then the argument is malformed.
2119         // It's likely that the user has written a const expression without enclosing it
2120         // in braces, so we try to recover here.
2121         let arg = args.pop().unwrap();
2122         // FIXME: for some reason using `unexpected` or `expected_one_of_not_found` has
2123         // adverse side-effects to subsequent errors and seems to advance the parser.
2124         // We are causing this error here exclusively in case that a `const` expression
2125         // could be recovered from the current parser state, even if followed by more
2126         // arguments after a comma.
2127         let mut err = self.struct_span_err(
2128             self.token.span,
2129             &format!("expected one of `,` or `>`, found {}", super::token_descr(&self.token)),
2130         );
2131         err.span_label(self.token.span, "expected one of `,` or `>`");
2132         match self.recover_const_arg(arg.span(), err) {
2133             Ok(arg) => {
2134                 args.push(AngleBracketedArg::Arg(arg));
2135                 if self.eat(&token::Comma) {
2136                     return Ok(true); // Continue
2137                 }
2138             }
2139             Err(mut err) => {
2140                 args.push(arg);
2141                 // We will emit a more generic error later.
2142                 err.delay_as_bug();
2143             }
2144         }
2145         return Ok(false); // Don't continue.
2146     }
2147
2148     /// Attempt to parse a generic const argument that has not been enclosed in braces.
2149     /// There are a limited number of expressions that are permitted without being encoded
2150     /// in braces:
2151     /// - Literals.
2152     /// - Single-segment paths (i.e. standalone generic const parameters).
2153     /// All other expressions that can be parsed will emit an error suggesting the expression be
2154     /// wrapped in braces.
2155     pub fn handle_unambiguous_unbraced_const_arg(&mut self) -> PResult<'a, P<Expr>> {
2156         let start = self.token.span;
2157         let expr = self.parse_expr_res(Restrictions::CONST_EXPR, None).map_err(|mut err| {
2158             err.span_label(
2159                 start.shrink_to_lo(),
2160                 "while parsing a const generic argument starting here",
2161             );
2162             err
2163         })?;
2164         if !self.expr_is_valid_const_arg(&expr) {
2165             self.sess.emit_err(ConstGenericWithoutBraces {
2166                 span: expr.span,
2167                 sugg: ConstGenericWithoutBracesSugg {
2168                     left: expr.span.shrink_to_lo(),
2169                     right: expr.span.shrink_to_hi(),
2170                 },
2171             });
2172         }
2173         Ok(expr)
2174     }
2175
2176     fn recover_const_param_decl(&mut self, ty_generics: Option<&Generics>) -> Option<GenericArg> {
2177         let snapshot = self.create_snapshot_for_diagnostic();
2178         let param = match self.parse_const_param(AttrVec::new()) {
2179             Ok(param) => param,
2180             Err(err) => {
2181                 err.cancel();
2182                 self.restore_snapshot(snapshot);
2183                 return None;
2184             }
2185         };
2186
2187         let ident = param.ident.to_string();
2188         let sugg = match (ty_generics, self.sess.source_map().span_to_snippet(param.span())) {
2189             (Some(Generics { params, span: impl_generics, .. }), Ok(snippet)) => {
2190                 Some(match &params[..] {
2191                     [] => UnexpectedConstParamDeclarationSugg::AddParam {
2192                         impl_generics: *impl_generics,
2193                         incorrect_decl: param.span(),
2194                         snippet,
2195                         ident,
2196                     },
2197                     [.., generic] => UnexpectedConstParamDeclarationSugg::AppendParam {
2198                         impl_generics_end: generic.span().shrink_to_hi(),
2199                         incorrect_decl: param.span(),
2200                         snippet,
2201                         ident,
2202                     },
2203                 })
2204             }
2205             _ => None,
2206         };
2207         self.sess.emit_err(UnexpectedConstParamDeclaration { span: param.span(), sugg });
2208
2209         let value = self.mk_expr_err(param.span());
2210         Some(GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value }))
2211     }
2212
2213     pub fn recover_const_param_declaration(
2214         &mut self,
2215         ty_generics: Option<&Generics>,
2216     ) -> PResult<'a, Option<GenericArg>> {
2217         // We have to check for a few different cases.
2218         if let Some(arg) = self.recover_const_param_decl(ty_generics) {
2219             return Ok(Some(arg));
2220         }
2221
2222         // We haven't consumed `const` yet.
2223         let start = self.token.span;
2224         self.bump(); // `const`
2225
2226         // Detect and recover from the old, pre-RFC2000 syntax for const generics.
2227         let mut err = UnexpectedConstInGenericParam { span: start, to_remove: None };
2228         if self.check_const_arg() {
2229             err.to_remove = Some(start.until(self.token.span));
2230             self.sess.emit_err(err);
2231             Ok(Some(GenericArg::Const(self.parse_const_arg()?)))
2232         } else {
2233             let after_kw_const = self.token.span;
2234             self.recover_const_arg(after_kw_const, err.into_diagnostic(&self.sess.span_diagnostic))
2235                 .map(Some)
2236         }
2237     }
2238
2239     /// Try to recover from possible generic const argument without `{` and `}`.
2240     ///
2241     /// When encountering code like `foo::< bar + 3 >` or `foo::< bar - baz >` we suggest
2242     /// `foo::<{ bar + 3 }>` and `foo::<{ bar - baz }>`, respectively. We only provide a suggestion
2243     /// if we think that the resulting expression would be well formed.
2244     pub fn recover_const_arg(
2245         &mut self,
2246         start: Span,
2247         mut err: DiagnosticBuilder<'a, ErrorGuaranteed>,
2248     ) -> PResult<'a, GenericArg> {
2249         let is_op_or_dot = AssocOp::from_token(&self.token)
2250             .and_then(|op| {
2251                 if let AssocOp::Greater
2252                 | AssocOp::Less
2253                 | AssocOp::ShiftRight
2254                 | AssocOp::GreaterEqual
2255                 // Don't recover from `foo::<bar = baz>`, because this could be an attempt to
2256                 // assign a value to a defaulted generic parameter.
2257                 | AssocOp::Assign
2258                 | AssocOp::AssignOp(_) = op
2259                 {
2260                     None
2261                 } else {
2262                     Some(op)
2263                 }
2264             })
2265             .is_some()
2266             || self.token.kind == TokenKind::Dot;
2267         // This will be true when a trait object type `Foo +` or a path which was a `const fn` with
2268         // type params has been parsed.
2269         let was_op =
2270             matches!(self.prev_token.kind, token::BinOp(token::Plus | token::Shr) | token::Gt);
2271         if !is_op_or_dot && !was_op {
2272             // We perform these checks and early return to avoid taking a snapshot unnecessarily.
2273             return Err(err);
2274         }
2275         let snapshot = self.create_snapshot_for_diagnostic();
2276         if is_op_or_dot {
2277             self.bump();
2278         }
2279         match self.parse_expr_res(Restrictions::CONST_EXPR, None) {
2280             Ok(expr) => {
2281                 // Find a mistake like `MyTrait<Assoc == S::Assoc>`.
2282                 if token::EqEq == snapshot.token.kind {
2283                     err.span_suggestion(
2284                         snapshot.token.span,
2285                         "if you meant to use an associated type binding, replace `==` with `=`",
2286                         "=",
2287                         Applicability::MaybeIncorrect,
2288                     );
2289                     let value = self.mk_expr_err(start.to(expr.span));
2290                     err.emit();
2291                     return Ok(GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value }));
2292                 } else if token::Colon == snapshot.token.kind
2293                     && expr.span.lo() == snapshot.token.span.hi()
2294                     && matches!(expr.kind, ExprKind::Path(..))
2295                 {
2296                     // Find a mistake like "foo::var:A".
2297                     err.span_suggestion(
2298                         snapshot.token.span,
2299                         "write a path separator here",
2300                         "::",
2301                         Applicability::MaybeIncorrect,
2302                     );
2303                     err.emit();
2304                     return Ok(GenericArg::Type(self.mk_ty(start.to(expr.span), TyKind::Err)));
2305                 } else if token::Comma == self.token.kind || self.token.kind.should_end_const_arg()
2306                 {
2307                     // Avoid the following output by checking that we consumed a full const arg:
2308                     // help: expressions must be enclosed in braces to be used as const generic
2309                     //       arguments
2310                     //    |
2311                     // LL |     let sr: Vec<{ (u32, _, _) = vec![] };
2312                     //    |                 ^                      ^
2313                     return Ok(self.dummy_const_arg_needs_braces(err, start.to(expr.span)));
2314                 }
2315             }
2316             Err(err) => {
2317                 err.cancel();
2318             }
2319         }
2320         self.restore_snapshot(snapshot);
2321         Err(err)
2322     }
2323
2324     /// Creates a dummy const argument, and reports that the expression must be enclosed in braces
2325     pub fn dummy_const_arg_needs_braces(
2326         &self,
2327         mut err: DiagnosticBuilder<'a, ErrorGuaranteed>,
2328         span: Span,
2329     ) -> GenericArg {
2330         err.multipart_suggestion(
2331             "expressions must be enclosed in braces to be used as const generic \
2332              arguments",
2333             vec![(span.shrink_to_lo(), "{ ".to_string()), (span.shrink_to_hi(), " }".to_string())],
2334             Applicability::MaybeIncorrect,
2335         );
2336         let value = self.mk_expr_err(span);
2337         err.emit();
2338         GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value })
2339     }
2340
2341     /// Some special error handling for the "top-level" patterns in a match arm,
2342     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
2343     pub(crate) fn maybe_recover_colon_colon_in_pat_typo(
2344         &mut self,
2345         mut first_pat: P<Pat>,
2346         expected: Expected,
2347     ) -> P<Pat> {
2348         if token::Colon != self.token.kind {
2349             return first_pat;
2350         }
2351         if !matches!(first_pat.kind, PatKind::Ident(_, _, None) | PatKind::Path(..))
2352             || !self.look_ahead(1, |token| token.is_ident() && !token.is_reserved_ident())
2353         {
2354             return first_pat;
2355         }
2356         // The pattern looks like it might be a path with a `::` -> `:` typo:
2357         // `match foo { bar:baz => {} }`
2358         let span = self.token.span;
2359         // We only emit "unexpected `:`" error here if we can successfully parse the
2360         // whole pattern correctly in that case.
2361         let snapshot = self.create_snapshot_for_diagnostic();
2362
2363         // Create error for "unexpected `:`".
2364         match self.expected_one_of_not_found(&[], &[]) {
2365             Err(mut err) => {
2366                 self.bump(); // Skip the `:`.
2367                 match self.parse_pat_no_top_alt(expected) {
2368                     Err(inner_err) => {
2369                         // Carry on as if we had not done anything, callers will emit a
2370                         // reasonable error.
2371                         inner_err.cancel();
2372                         err.cancel();
2373                         self.restore_snapshot(snapshot);
2374                     }
2375                     Ok(mut pat) => {
2376                         // We've parsed the rest of the pattern.
2377                         let new_span = first_pat.span.to(pat.span);
2378                         let mut show_sugg = false;
2379                         // Try to construct a recovered pattern.
2380                         match &mut pat.kind {
2381                             PatKind::Struct(qself @ None, path, ..)
2382                             | PatKind::TupleStruct(qself @ None, path, _)
2383                             | PatKind::Path(qself @ None, path) => match &first_pat.kind {
2384                                 PatKind::Ident(_, ident, _) => {
2385                                     path.segments.insert(0, PathSegment::from_ident(*ident));
2386                                     path.span = new_span;
2387                                     show_sugg = true;
2388                                     first_pat = pat;
2389                                 }
2390                                 PatKind::Path(old_qself, old_path) => {
2391                                     path.segments = old_path
2392                                         .segments
2393                                         .iter()
2394                                         .cloned()
2395                                         .chain(take(&mut path.segments))
2396                                         .collect();
2397                                     path.span = new_span;
2398                                     *qself = old_qself.clone();
2399                                     first_pat = pat;
2400                                     show_sugg = true;
2401                                 }
2402                                 _ => {}
2403                             },
2404                             PatKind::Ident(BindingAnnotation::NONE, ident, None) => {
2405                                 match &first_pat.kind {
2406                                     PatKind::Ident(_, old_ident, _) => {
2407                                         let path = PatKind::Path(
2408                                             None,
2409                                             Path {
2410                                                 span: new_span,
2411                                                 segments: vec![
2412                                                     PathSegment::from_ident(*old_ident),
2413                                                     PathSegment::from_ident(*ident),
2414                                                 ],
2415                                                 tokens: None,
2416                                             },
2417                                         );
2418                                         first_pat = self.mk_pat(new_span, path);
2419                                         show_sugg = true;
2420                                     }
2421                                     PatKind::Path(old_qself, old_path) => {
2422                                         let mut segments = old_path.segments.clone();
2423                                         segments.push(PathSegment::from_ident(*ident));
2424                                         let path = PatKind::Path(
2425                                             old_qself.clone(),
2426                                             Path { span: new_span, segments, tokens: None },
2427                                         );
2428                                         first_pat = self.mk_pat(new_span, path);
2429                                         show_sugg = true;
2430                                     }
2431                                     _ => {}
2432                                 }
2433                             }
2434                             _ => {}
2435                         }
2436                         if show_sugg {
2437                             err.span_suggestion(
2438                                 span,
2439                                 "maybe write a path separator here",
2440                                 "::",
2441                                 Applicability::MaybeIncorrect,
2442                             );
2443                         } else {
2444                             first_pat = self.mk_pat(new_span, PatKind::Wild);
2445                         }
2446                         err.emit();
2447                     }
2448                 }
2449             }
2450             _ => {
2451                 // Carry on as if we had not done anything. This should be unreachable.
2452                 self.restore_snapshot(snapshot);
2453             }
2454         };
2455         first_pat
2456     }
2457
2458     pub(crate) fn maybe_recover_unexpected_block_label(&mut self) -> bool {
2459         let Some(label) = self.eat_label().filter(|_| {
2460             self.eat(&token::Colon) && self.token.kind == token::OpenDelim(Delimiter::Brace)
2461         }) else {
2462             return false;
2463         };
2464         let span = label.ident.span.to(self.prev_token.span);
2465         let mut err = self.struct_span_err(span, "block label not supported here");
2466         err.span_label(span, "not supported here");
2467         err.tool_only_span_suggestion(
2468             label.ident.span.until(self.token.span),
2469             "remove this block label",
2470             "",
2471             Applicability::MachineApplicable,
2472         );
2473         err.emit();
2474         true
2475     }
2476
2477     /// Some special error handling for the "top-level" patterns in a match arm,
2478     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
2479     pub(crate) fn maybe_recover_unexpected_comma(
2480         &mut self,
2481         lo: Span,
2482         rt: CommaRecoveryMode,
2483     ) -> PResult<'a, ()> {
2484         if self.token != token::Comma {
2485             return Ok(());
2486         }
2487
2488         // An unexpected comma after a top-level pattern is a clue that the
2489         // user (perhaps more accustomed to some other language) forgot the
2490         // parentheses in what should have been a tuple pattern; return a
2491         // suggestion-enhanced error here rather than choking on the comma later.
2492         let comma_span = self.token.span;
2493         self.bump();
2494         if let Err(err) = self.skip_pat_list() {
2495             // We didn't expect this to work anyway; we just wanted to advance to the
2496             // end of the comma-sequence so we know the span to suggest parenthesizing.
2497             err.cancel();
2498         }
2499         let seq_span = lo.to(self.prev_token.span);
2500         let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
2501         if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
2502             err.multipart_suggestion(
2503                 &format!(
2504                     "try adding parentheses to match on a tuple{}",
2505                     if let CommaRecoveryMode::LikelyTuple = rt { "" } else { "..." },
2506                 ),
2507                 vec![
2508                     (seq_span.shrink_to_lo(), "(".to_string()),
2509                     (seq_span.shrink_to_hi(), ")".to_string()),
2510                 ],
2511                 Applicability::MachineApplicable,
2512             );
2513             if let CommaRecoveryMode::EitherTupleOrPipe = rt {
2514                 err.span_suggestion(
2515                     seq_span,
2516                     "...or a vertical bar to match on multiple alternatives",
2517                     seq_snippet.replace(',', " |"),
2518                     Applicability::MachineApplicable,
2519                 );
2520             }
2521         }
2522         Err(err)
2523     }
2524
2525     pub(crate) fn maybe_recover_bounds_doubled_colon(&mut self, ty: &Ty) -> PResult<'a, ()> {
2526         let TyKind::Path(qself, path) = &ty.kind else { return Ok(()) };
2527         let qself_position = qself.as_ref().map(|qself| qself.position);
2528         for (i, segments) in path.segments.windows(2).enumerate() {
2529             if qself_position.map(|pos| i < pos).unwrap_or(false) {
2530                 continue;
2531             }
2532             if let [a, b] = segments {
2533                 let (a_span, b_span) = (a.span(), b.span());
2534                 let between_span = a_span.shrink_to_hi().to(b_span.shrink_to_lo());
2535                 if self.span_to_snippet(between_span).as_ref().map(|a| &a[..]) == Ok(":: ") {
2536                     return Err(DoubleColonInBound {
2537                         span: path.span.shrink_to_hi(),
2538                         between: between_span,
2539                     }
2540                     .into_diagnostic(&self.sess.span_diagnostic));
2541                 }
2542             }
2543         }
2544         Ok(())
2545     }
2546
2547     /// Parse and throw away a parenthesized comma separated
2548     /// sequence of patterns until `)` is reached.
2549     fn skip_pat_list(&mut self) -> PResult<'a, ()> {
2550         while !self.check(&token::CloseDelim(Delimiter::Parenthesis)) {
2551             self.parse_pat_no_top_alt(None)?;
2552             if !self.eat(&token::Comma) {
2553                 return Ok(());
2554             }
2555         }
2556         Ok(())
2557     }
2558 }