compiler/rustc_parse/src/parser/diagnostics.rs

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