compiler/rustc_parse/src/lexer/mod.rs

   1 use crate::lexer::unicode_chars::UNICODE_ARRAY;
   2 use rustc_ast::ast::{self, AttrStyle};
   3 use rustc_ast::token::{self, CommentKind, Delimiter, Token, TokenKind};
   4 use rustc_ast::tokenstream::TokenStream;
   5 use rustc_ast::util::unicode::contains_text_flow_control_chars;
   6 use rustc_errors::{
   7     error_code, Applicability, DiagnosticBuilder, ErrorGuaranteed, PResult, StashKey,
   8 };
   9 use rustc_lexer::unescape::{self, Mode};
  10 use rustc_lexer::Cursor;
  11 use rustc_lexer::{Base, DocStyle, RawStrError};
  12 use rustc_session::lint::builtin::{
  13     RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX, TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
  14 };
  15 use rustc_session::lint::BuiltinLintDiagnostics;
  16 use rustc_session::parse::ParseSess;
  17 use rustc_span::symbol::{sym, Symbol};
  18 use rustc_span::{edition::Edition, BytePos, Pos, Span};
  19
  20 mod tokentrees;
  21 mod unescape_error_reporting;
  22 mod unicode_chars;
  23
  24 use unescape_error_reporting::{emit_unescape_error, escaped_char};
  25
  26 // This type is used a lot. Make sure it doesn't unintentionally get bigger.
  27 //
  28 // This assertion is in this crate, rather than in `rustc_lexer`, because that
  29 // crate cannot depend on `rustc_data_structures`.
  30 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  31 rustc_data_structures::static_assert_size!(rustc_lexer::Token, 12);
  32
  33 #[derive(Clone, Debug)]
  34 pub struct UnmatchedBrace {
  35     pub expected_delim: Delimiter,
  36     pub found_delim: Option<Delimiter>,
  37     pub found_span: Span,
  38     pub unclosed_span: Option<Span>,
  39     pub candidate_span: Option<Span>,
  40 }
  41
  42 pub(crate) fn parse_token_trees<'a>(
  43     sess: &'a ParseSess,
  44     mut src: &'a str,
  45     mut start_pos: BytePos,
  46     override_span: Option<Span>,
  47 ) -> (PResult<'a, TokenStream>, Vec<UnmatchedBrace>) {
  48     // Skip `#!`, if present.
  49     if let Some(shebang_len) = rustc_lexer::strip_shebang(src) {
  50         src = &src[shebang_len..];
  51         start_pos = start_pos + BytePos::from_usize(shebang_len);
  52     }
  53
  54     let cursor = Cursor::new(src);
  55     let string_reader =
  56         StringReader { sess, start_pos, pos: start_pos, src, cursor, override_span };
  57     tokentrees::TokenTreesReader::parse_all_token_trees(string_reader)
  58 }
  59
  60 struct StringReader<'a> {
  61     sess: &'a ParseSess,
  62     /// Initial position, read-only.
  63     start_pos: BytePos,
  64     /// The absolute offset within the source_map of the current character.
  65     pos: BytePos,
  66     /// Source text to tokenize.
  67     src: &'a str,
  68     /// Cursor for getting lexer tokens.
  69     cursor: Cursor<'a>,
  70     override_span: Option<Span>,
  71 }
  72
  73 impl<'a> StringReader<'a> {
  74     fn mk_sp(&self, lo: BytePos, hi: BytePos) -> Span {
  75         self.override_span.unwrap_or_else(|| Span::with_root_ctxt(lo, hi))
  76     }
  77
  78     /// Returns the next token, paired with a bool indicating if the token was
  79     /// preceded by whitespace.
  80     fn next_token(&mut self) -> (Token, bool) {
  81         let mut preceded_by_whitespace = false;
  82         let mut swallow_next_invalid = 0;
  83         // Skip trivial (whitespace & comments) tokens
  84         loop {
  85             let token = self.cursor.advance_token();
  86             let start = self.pos;
  87             self.pos = self.pos + BytePos(token.len);
  88
  89             debug!("next_token: {:?}({:?})", token.kind, self.str_from(start));
  90
  91             // Now "cook" the token, converting the simple `rustc_lexer::TokenKind` enum into a
  92             // rich `rustc_ast::TokenKind`. This turns strings into interned symbols and runs
  93             // additional validation.
  94             let kind = match token.kind {
  95                 rustc_lexer::TokenKind::LineComment { doc_style } => {
  96                     // Skip non-doc comments
  97                     let Some(doc_style) = doc_style else {
  98                         self.lint_unicode_text_flow(start);
  99                         preceded_by_whitespace = true;
 100                         continue;
 101                     };
 102
 103                     // Opening delimiter of the length 3 is not included into the symbol.
 104                     let content_start = start + BytePos(3);
 105                     let content = self.str_from(content_start);
 106                     self.cook_doc_comment(content_start, content, CommentKind::Line, doc_style)
 107                 }
 108                 rustc_lexer::TokenKind::BlockComment { doc_style, terminated } => {
 109                     if !terminated {
 110                         self.report_unterminated_block_comment(start, doc_style);
 111                     }
 112
 113                     // Skip non-doc comments
 114                     let Some(doc_style) = doc_style else {
 115                         self.lint_unicode_text_flow(start);
 116                         preceded_by_whitespace = true;
 117                         continue;
 118                     };
 119
 120                     // Opening delimiter of the length 3 and closing delimiter of the length 2
 121                     // are not included into the symbol.
 122                     let content_start = start + BytePos(3);
 123                     let content_end = self.pos - BytePos(if terminated { 2 } else { 0 });
 124                     let content = self.str_from_to(content_start, content_end);
 125                     self.cook_doc_comment(content_start, content, CommentKind::Block, doc_style)
 126                 }
 127                 rustc_lexer::TokenKind::Whitespace => {
 128                     preceded_by_whitespace = true;
 129                     continue;
 130                 }
 131                 rustc_lexer::TokenKind::Ident => {
 132                     let sym = nfc_normalize(self.str_from(start));
 133                     let span = self.mk_sp(start, self.pos);
 134                     self.sess.symbol_gallery.insert(sym, span);
 135                     token::Ident(sym, false)
 136                 }
 137                 rustc_lexer::TokenKind::RawIdent => {
 138                     let sym = nfc_normalize(self.str_from(start + BytePos(2)));
 139                     let span = self.mk_sp(start, self.pos);
 140                     self.sess.symbol_gallery.insert(sym, span);
 141                     if !sym.can_be_raw() {
 142                         self.err_span(span, &format!("`{}` cannot be a raw identifier", sym));
 143                     }
 144                     self.sess.raw_identifier_spans.borrow_mut().push(span);
 145                     token::Ident(sym, true)
 146                 }
 147                 rustc_lexer::TokenKind::UnknownPrefix => {
 148                     self.report_unknown_prefix(start);
 149                     let sym = nfc_normalize(self.str_from(start));
 150                     let span = self.mk_sp(start, self.pos);
 151                     self.sess.symbol_gallery.insert(sym, span);
 152                     token::Ident(sym, false)
 153                 }
 154                 rustc_lexer::TokenKind::InvalidIdent
 155                     // Do not recover an identifier with emoji if the codepoint is a confusable
 156                     // with a recoverable substitution token, like `➖`.
 157                     if !UNICODE_ARRAY
 158                         .iter()
 159                         .any(|&(c, _, _)| {
 160                             let sym = self.str_from(start);
 161                             sym.chars().count() == 1 && c == sym.chars().next().unwrap()
 162                         }) =>
 163                 {
 164                     let sym = nfc_normalize(self.str_from(start));
 165                     let span = self.mk_sp(start, self.pos);
 166                     self.sess.bad_unicode_identifiers.borrow_mut().entry(sym).or_default()
 167                         .push(span);
 168                     token::Ident(sym, false)
 169                 }
 170                 rustc_lexer::TokenKind::Literal { kind, suffix_start } => {
 171                     let suffix_start = start + BytePos(suffix_start);
 172                     let (kind, symbol) = self.cook_lexer_literal(start, suffix_start, kind);
 173                     let suffix = if suffix_start < self.pos {
 174                         let string = self.str_from(suffix_start);
 175                         if string == "_" {
 176                             self.sess
 177                                 .span_diagnostic
 178                                 .struct_span_err(
 179                                     self.mk_sp(suffix_start, self.pos),
 180                                     "underscore literal suffix is not allowed",
 181                                 )
 182                                 .emit();
 183                             None
 184                         } else {
 185                             Some(Symbol::intern(string))
 186                         }
 187                     } else {
 188                         None
 189                     };
 190                     token::Literal(token::Lit { kind, symbol, suffix })
 191                 }
 192                 rustc_lexer::TokenKind::Lifetime { starts_with_number } => {
 193                     // Include the leading `'` in the real identifier, for macro
 194                     // expansion purposes. See #12512 for the gory details of why
 195                     // this is necessary.
 196                     let lifetime_name = self.str_from(start);
 197                     if starts_with_number {
 198                         let span = self.mk_sp(start, self.pos);
 199                         let mut diag = self.sess.struct_err("lifetimes cannot start with a number");
 200                         diag.set_span(span);
 201                         diag.stash(span, StashKey::LifetimeIsChar);
 202                     }
 203                     let ident = Symbol::intern(lifetime_name);
 204                     token::Lifetime(ident)
 205                 }
 206                 rustc_lexer::TokenKind::Semi => token::Semi,
 207                 rustc_lexer::TokenKind::Comma => token::Comma,
 208                 rustc_lexer::TokenKind::Dot => token::Dot,
 209                 rustc_lexer::TokenKind::OpenParen => token::OpenDelim(Delimiter::Parenthesis),
 210                 rustc_lexer::TokenKind::CloseParen => token::CloseDelim(Delimiter::Parenthesis),
 211                 rustc_lexer::TokenKind::OpenBrace => token::OpenDelim(Delimiter::Brace),
 212                 rustc_lexer::TokenKind::CloseBrace => token::CloseDelim(Delimiter::Brace),
 213                 rustc_lexer::TokenKind::OpenBracket => token::OpenDelim(Delimiter::Bracket),
 214                 rustc_lexer::TokenKind::CloseBracket => token::CloseDelim(Delimiter::Bracket),
 215                 rustc_lexer::TokenKind::At => token::At,
 216                 rustc_lexer::TokenKind::Pound => token::Pound,
 217                 rustc_lexer::TokenKind::Tilde => token::Tilde,
 218                 rustc_lexer::TokenKind::Question => token::Question,
 219                 rustc_lexer::TokenKind::Colon => token::Colon,
 220                 rustc_lexer::TokenKind::Dollar => token::Dollar,
 221                 rustc_lexer::TokenKind::Eq => token::Eq,
 222                 rustc_lexer::TokenKind::Bang => token::Not,
 223                 rustc_lexer::TokenKind::Lt => token::Lt,
 224                 rustc_lexer::TokenKind::Gt => token::Gt,
 225                 rustc_lexer::TokenKind::Minus => token::BinOp(token::Minus),
 226                 rustc_lexer::TokenKind::And => token::BinOp(token::And),
 227                 rustc_lexer::TokenKind::Or => token::BinOp(token::Or),
 228                 rustc_lexer::TokenKind::Plus => token::BinOp(token::Plus),
 229                 rustc_lexer::TokenKind::Star => token::BinOp(token::Star),
 230                 rustc_lexer::TokenKind::Slash => token::BinOp(token::Slash),
 231                 rustc_lexer::TokenKind::Caret => token::BinOp(token::Caret),
 232                 rustc_lexer::TokenKind::Percent => token::BinOp(token::Percent),
 233
 234                 rustc_lexer::TokenKind::Unknown | rustc_lexer::TokenKind::InvalidIdent => {
 235                     // Don't emit diagnostics for sequences of the same invalid token
 236                     if swallow_next_invalid > 0 {
 237                         swallow_next_invalid -= 1;
 238                         continue;
 239                     }
 240                     let mut it = self.str_from_to_end(start).chars();
 241                     let c = it.next().unwrap();
 242                     let repeats = it.take_while(|c1| *c1 == c).count();
 243                     let mut err =
 244                         self.struct_err_span_char(start, self.pos + Pos::from_usize(repeats * c.len_utf8()), "unknown start of token", c);
 245                     // FIXME: the lexer could be used to turn the ASCII version of unicode
 246                     // homoglyphs, instead of keeping a table in `check_for_substitution`into the
 247                     // token. Ideally, this should be inside `rustc_lexer`. However, we should
 248                     // first remove compound tokens like `<<` from `rustc_lexer`, and then add
 249                     // fancier error recovery to it, as there will be less overall work to do this
 250                     // way.
 251                     let token = unicode_chars::check_for_substitution(self, start, c, &mut err, repeats+1);
 252                     if c == '\x00' {
 253                         err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
 254                     }
 255                     if repeats > 0 {
 256                         if repeats == 1 {
 257                             err.note(format!("character appears once more"));
 258                         } else {
 259                             err.note(format!("character appears {repeats} more times"));
 260                         }
 261                         swallow_next_invalid = repeats;
 262                     }
 263                     err.emit();
 264                     if let Some(token) = token {
 265                         token
 266                     } else {
 267                         preceded_by_whitespace = true;
 268                         continue;
 269                     }
 270                 }
 271                 rustc_lexer::TokenKind::Eof => token::Eof,
 272             };
 273             let span = self.mk_sp(start, self.pos);
 274             return (Token::new(kind, span), preceded_by_whitespace);
 275         }
 276     }
 277
 278     /// Report a fatal lexical error with a given span.
 279     fn fatal_span(&self, sp: Span, m: &str) -> ! {
 280         self.sess.span_diagnostic.span_fatal(sp, m)
 281     }
 282
 283     /// Report a lexical error with a given span.
 284     fn err_span(&self, sp: Span, m: &str) {
 285         self.sess.span_diagnostic.struct_span_err(sp, m).emit();
 286     }
 287
 288     /// Report a fatal error spanning [`from_pos`, `to_pos`).
 289     fn fatal_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) -> ! {
 290         self.fatal_span(self.mk_sp(from_pos, to_pos), m)
 291     }
 292
 293     /// Report a lexical error spanning [`from_pos`, `to_pos`).
 294     fn err_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) {
 295         self.err_span(self.mk_sp(from_pos, to_pos), m)
 296     }
 297
 298     fn struct_fatal_span_char(
 299         &self,
 300         from_pos: BytePos,
 301         to_pos: BytePos,
 302         m: &str,
 303         c: char,
 304     ) -> DiagnosticBuilder<'a, !> {
 305         self.sess
 306             .span_diagnostic
 307             .struct_span_fatal(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
 308     }
 309
 310     fn struct_err_span_char(
 311         &self,
 312         from_pos: BytePos,
 313         to_pos: BytePos,
 314         m: &str,
 315         c: char,
 316     ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
 317         self.sess
 318             .span_diagnostic
 319             .struct_span_err(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
 320     }
 321
 322     /// Detect usages of Unicode codepoints changing the direction of the text on screen and loudly
 323     /// complain about it.
 324     fn lint_unicode_text_flow(&self, start: BytePos) {
 325         // Opening delimiter of the length 2 is not included into the comment text.
 326         let content_start = start + BytePos(2);
 327         let content = self.str_from(content_start);
 328         if contains_text_flow_control_chars(content) {
 329             let span = self.mk_sp(start, self.pos);
 330             self.sess.buffer_lint_with_diagnostic(
 331                 &TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
 332                 span,
 333                 ast::CRATE_NODE_ID,
 334                 "unicode codepoint changing visible direction of text present in comment",
 335                 BuiltinLintDiagnostics::UnicodeTextFlow(span, content.to_string()),
 336             );
 337         }
 338     }
 339
 340     fn cook_doc_comment(
 341         &self,
 342         content_start: BytePos,
 343         content: &str,
 344         comment_kind: CommentKind,
 345         doc_style: DocStyle,
 346     ) -> TokenKind {
 347         if content.contains('\r') {
 348             for (idx, _) in content.char_indices().filter(|&(_, c)| c == '\r') {
 349                 self.err_span_(
 350                     content_start + BytePos(idx as u32),
 351                     content_start + BytePos(idx as u32 + 1),
 352                     match comment_kind {
 353                         CommentKind::Line => "bare CR not allowed in doc-comment",
 354                         CommentKind::Block => "bare CR not allowed in block doc-comment",
 355                     },
 356                 );
 357             }
 358         }
 359
 360         let attr_style = match doc_style {
 361             DocStyle::Outer => AttrStyle::Outer,
 362             DocStyle::Inner => AttrStyle::Inner,
 363         };
 364
 365         token::DocComment(comment_kind, attr_style, Symbol::intern(content))
 366     }
 367
 368     fn cook_lexer_literal(
 369         &self,
 370         start: BytePos,
 371         end: BytePos,
 372         kind: rustc_lexer::LiteralKind,
 373     ) -> (token::LitKind, Symbol) {
 374         match kind {
 375             rustc_lexer::LiteralKind::Char { terminated } => {
 376                 if !terminated {
 377                     self.sess.span_diagnostic.span_fatal_with_code(
 378                         self.mk_sp(start, end),
 379                         "unterminated character literal",
 380                         error_code!(E0762),
 381                     )
 382                 }
 383                 self.cook_quoted(token::Char, Mode::Char, start, end, 1, 1) // ' '
 384             }
 385             rustc_lexer::LiteralKind::Byte { terminated } => {
 386                 if !terminated {
 387                     self.sess.span_diagnostic.span_fatal_with_code(
 388                         self.mk_sp(start + BytePos(1), end),
 389                         "unterminated byte constant",
 390                         error_code!(E0763),
 391                     )
 392                 }
 393                 self.cook_quoted(token::Byte, Mode::Byte, start, end, 2, 1) // b' '
 394             }
 395             rustc_lexer::LiteralKind::Str { terminated } => {
 396                 if !terminated {
 397                     self.sess.span_diagnostic.span_fatal_with_code(
 398                         self.mk_sp(start, end),
 399                         "unterminated double quote string",
 400                         error_code!(E0765),
 401                     )
 402                 }
 403                 self.cook_quoted(token::Str, Mode::Str, start, end, 1, 1) // " "
 404             }
 405             rustc_lexer::LiteralKind::ByteStr { terminated } => {
 406                 if !terminated {
 407                     self.sess.span_diagnostic.span_fatal_with_code(
 408                         self.mk_sp(start + BytePos(1), end),
 409                         "unterminated double quote byte string",
 410                         error_code!(E0766),
 411                     )
 412                 }
 413                 self.cook_quoted(token::ByteStr, Mode::ByteStr, start, end, 2, 1) // b" "
 414             }
 415             rustc_lexer::LiteralKind::RawStr { n_hashes } => {
 416                 if let Some(n_hashes) = n_hashes {
 417                     let n = u32::from(n_hashes);
 418                     let kind = token::StrRaw(n_hashes);
 419                     self.cook_quoted(kind, Mode::RawStr, start, end, 2 + n, 1 + n) // r##" "##
 420                 } else {
 421                     self.report_raw_str_error(start, 1);
 422                 }
 423             }
 424             rustc_lexer::LiteralKind::RawByteStr { n_hashes } => {
 425                 if let Some(n_hashes) = n_hashes {
 426                     let n = u32::from(n_hashes);
 427                     let kind = token::ByteStrRaw(n_hashes);
 428                     self.cook_quoted(kind, Mode::RawByteStr, start, end, 3 + n, 1 + n) // br##" "##
 429                 } else {
 430                     self.report_raw_str_error(start, 2);
 431                 }
 432             }
 433             rustc_lexer::LiteralKind::Int { base, empty_int } => {
 434                 if empty_int {
 435                     self.sess
 436                         .span_diagnostic
 437                         .struct_span_err_with_code(
 438                             self.mk_sp(start, end),
 439                             "no valid digits found for number",
 440                             error_code!(E0768),
 441                         )
 442                         .emit();
 443                     (token::Integer, sym::integer(0))
 444                 } else {
 445                     if matches!(base, Base::Binary | Base::Octal) {
 446                         let base = base as u32;
 447                         let s = self.str_from_to(start + BytePos(2), end);
 448                         for (idx, c) in s.char_indices() {
 449                             if c != '_' && c.to_digit(base).is_none() {
 450                                 self.err_span_(
 451                                     start + BytePos::from_usize(2 + idx),
 452                                     start + BytePos::from_usize(2 + idx + c.len_utf8()),
 453                                     &format!("invalid digit for a base {} literal", base),
 454                                 );
 455                             }
 456                         }
 457                     }
 458                     (token::Integer, self.symbol_from_to(start, end))
 459                 }
 460             }
 461             rustc_lexer::LiteralKind::Float { base, empty_exponent } => {
 462                 if empty_exponent {
 463                     self.err_span_(start, self.pos, "expected at least one digit in exponent");
 464                 }
 465                 match base {
 466                     Base::Hexadecimal => {
 467                         self.err_span_(start, end, "hexadecimal float literal is not supported")
 468                     }
 469                     Base::Octal => {
 470                         self.err_span_(start, end, "octal float literal is not supported")
 471                     }
 472                     Base::Binary => {
 473                         self.err_span_(start, end, "binary float literal is not supported")
 474                     }
 475                     _ => {}
 476                 }
 477                 (token::Float, self.symbol_from_to(start, end))
 478             }
 479         }
 480     }
 481
 482     #[inline]
 483     fn src_index(&self, pos: BytePos) -> usize {
 484         (pos - self.start_pos).to_usize()
 485     }
 486
 487     /// Slice of the source text from `start` up to but excluding `self.pos`,
 488     /// meaning the slice does not include the character `self.ch`.
 489     fn str_from(&self, start: BytePos) -> &str {
 490         self.str_from_to(start, self.pos)
 491     }
 492
 493     /// As symbol_from, with an explicit endpoint.
 494     fn symbol_from_to(&self, start: BytePos, end: BytePos) -> Symbol {
 495         debug!("taking an ident from {:?} to {:?}", start, end);
 496         Symbol::intern(self.str_from_to(start, end))
 497     }
 498
 499     /// Slice of the source text spanning from `start` up to but excluding `end`.
 500     fn str_from_to(&self, start: BytePos, end: BytePos) -> &str {
 501         &self.src[self.src_index(start)..self.src_index(end)]
 502     }
 503
 504     /// Slice of the source text spanning from `start` until the end
 505     fn str_from_to_end(&self, start: BytePos) -> &str {
 506         &self.src[self.src_index(start)..]
 507     }
 508
 509     fn report_raw_str_error(&self, start: BytePos, prefix_len: u32) -> ! {
 510         match rustc_lexer::validate_raw_str(self.str_from(start), prefix_len) {
 511             Err(RawStrError::InvalidStarter { bad_char }) => {
 512                 self.report_non_started_raw_string(start, bad_char)
 513             }
 514             Err(RawStrError::NoTerminator { expected, found, possible_terminator_offset }) => self
 515                 .report_unterminated_raw_string(start, expected, possible_terminator_offset, found),
 516             Err(RawStrError::TooManyDelimiters { found }) => {
 517                 self.report_too_many_hashes(start, found)
 518             }
 519             Ok(()) => panic!("no error found for supposedly invalid raw string literal"),
 520         }
 521     }
 522
 523     fn report_non_started_raw_string(&self, start: BytePos, bad_char: char) -> ! {
 524         self.struct_fatal_span_char(
 525             start,
 526             self.pos,
 527             "found invalid character; only `#` is allowed in raw string delimitation",
 528             bad_char,
 529         )
 530         .emit()
 531     }
 532
 533     fn report_unterminated_raw_string(
 534         &self,
 535         start: BytePos,
 536         n_hashes: u32,
 537         possible_offset: Option<u32>,
 538         found_terminators: u32,
 539     ) -> ! {
 540         let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
 541             self.mk_sp(start, start),
 542             "unterminated raw string",
 543             error_code!(E0748),
 544         );
 545
 546         err.span_label(self.mk_sp(start, start), "unterminated raw string");
 547
 548         if n_hashes > 0 {
 549             err.note(&format!(
 550                 "this raw string should be terminated with `\"{}`",
 551                 "#".repeat(n_hashes as usize)
 552             ));
 553         }
 554
 555         if let Some(possible_offset) = possible_offset {
 556             let lo = start + BytePos(possible_offset as u32);
 557             let hi = lo + BytePos(found_terminators as u32);
 558             let span = self.mk_sp(lo, hi);
 559             err.span_suggestion(
 560                 span,
 561                 "consider terminating the string here",
 562                 "#".repeat(n_hashes as usize),
 563                 Applicability::MaybeIncorrect,
 564             );
 565         }
 566
 567         err.emit()
 568     }
 569
 570     fn report_unterminated_block_comment(&self, start: BytePos, doc_style: Option<DocStyle>) {
 571         let msg = match doc_style {
 572             Some(_) => "unterminated block doc-comment",
 573             None => "unterminated block comment",
 574         };
 575         let last_bpos = self.pos;
 576         let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
 577             self.mk_sp(start, last_bpos),
 578             msg,
 579             error_code!(E0758),
 580         );
 581         let mut nested_block_comment_open_idxs = vec![];
 582         let mut last_nested_block_comment_idxs = None;
 583         let mut content_chars = self.str_from(start).char_indices().peekable();
 584
 585         while let Some((idx, current_char)) = content_chars.next() {
 586             match content_chars.peek() {
 587                 Some((_, '*')) if current_char == '/' => {
 588                     nested_block_comment_open_idxs.push(idx);
 589                 }
 590                 Some((_, '/')) if current_char == '*' => {
 591                     last_nested_block_comment_idxs =
 592                         nested_block_comment_open_idxs.pop().map(|open_idx| (open_idx, idx));
 593                 }
 594                 _ => {}
 595             };
 596         }
 597
 598         if let Some((nested_open_idx, nested_close_idx)) = last_nested_block_comment_idxs {
 599             err.span_label(self.mk_sp(start, start + BytePos(2)), msg)
 600                 .span_label(
 601                     self.mk_sp(
 602                         start + BytePos(nested_open_idx as u32),
 603                         start + BytePos(nested_open_idx as u32 + 2),
 604                     ),
 605                     "...as last nested comment starts here, maybe you want to close this instead?",
 606                 )
 607                 .span_label(
 608                     self.mk_sp(
 609                         start + BytePos(nested_close_idx as u32),
 610                         start + BytePos(nested_close_idx as u32 + 2),
 611                     ),
 612                     "...and last nested comment terminates here.",
 613                 );
 614         }
 615
 616         err.emit();
 617     }
 618
 619     // RFC 3101 introduced the idea of (reserved) prefixes. As of Rust 2021,
 620     // using a (unknown) prefix is an error. In earlier editions, however, they
 621     // only result in a (allowed by default) lint, and are treated as regular
 622     // identifier tokens.
 623     fn report_unknown_prefix(&self, start: BytePos) {
 624         let prefix_span = self.mk_sp(start, self.pos);
 625         let prefix_str = self.str_from_to(start, self.pos);
 626         let msg = format!("prefix `{}` is unknown", prefix_str);
 627
 628         let expn_data = prefix_span.ctxt().outer_expn_data();
 629
 630         if expn_data.edition >= Edition::Edition2021 {
 631             // In Rust 2021, this is a hard error.
 632             let mut err = self.sess.span_diagnostic.struct_span_err(prefix_span, &msg);
 633             err.span_label(prefix_span, "unknown prefix");
 634             if prefix_str == "rb" {
 635                 err.span_suggestion_verbose(
 636                     prefix_span,
 637                     "use `br` for a raw byte string",
 638                     "br",
 639                     Applicability::MaybeIncorrect,
 640                 );
 641             } else if expn_data.is_root() {
 642                 err.span_suggestion_verbose(
 643                     prefix_span.shrink_to_hi(),
 644                     "consider inserting whitespace here",
 645                     " ",
 646                     Applicability::MaybeIncorrect,
 647                 );
 648             }
 649             err.note("prefixed identifiers and literals are reserved since Rust 2021");
 650             err.emit();
 651         } else {
 652             // Before Rust 2021, only emit a lint for migration.
 653             self.sess.buffer_lint_with_diagnostic(
 654                 &RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
 655                 prefix_span,
 656                 ast::CRATE_NODE_ID,
 657                 &msg,
 658                 BuiltinLintDiagnostics::ReservedPrefix(prefix_span),
 659             );
 660         }
 661     }
 662
 663     fn report_too_many_hashes(&self, start: BytePos, found: u32) -> ! {
 664         self.fatal_span_(
 665             start,
 666             self.pos,
 667             &format!(
 668                 "too many `#` symbols: raw strings may be delimited \
 669                 by up to 255 `#` symbols, but found {}",
 670                 found
 671             ),
 672         )
 673     }
 674
 675     fn cook_quoted(
 676         &self,
 677         kind: token::LitKind,
 678         mode: Mode,
 679         start: BytePos,
 680         end: BytePos,
 681         prefix_len: u32,
 682         postfix_len: u32,
 683     ) -> (token::LitKind, Symbol) {
 684         let mut has_fatal_err = false;
 685         let content_start = start + BytePos(prefix_len);
 686         let content_end = end - BytePos(postfix_len);
 687         let lit_content = self.str_from_to(content_start, content_end);
 688         unescape::unescape_literal(lit_content, mode, &mut |range, result| {
 689             // Here we only check for errors. The actual unescaping is done later.
 690             if let Err(err) = result {
 691                 let span_with_quotes = self.mk_sp(start, end);
 692                 let (start, end) = (range.start as u32, range.end as u32);
 693                 let lo = content_start + BytePos(start);
 694                 let hi = lo + BytePos(end - start);
 695                 let span = self.mk_sp(lo, hi);
 696                 if err.is_fatal() {
 697                     has_fatal_err = true;
 698                 }
 699                 emit_unescape_error(
 700                     &self.sess.span_diagnostic,
 701                     lit_content,
 702                     span_with_quotes,
 703                     span,
 704                     mode,
 705                     range,
 706                     err,
 707                 );
 708             }
 709         });
 710
 711         // We normally exclude the quotes for the symbol, but for errors we
 712         // include it because it results in clearer error messages.
 713         if !has_fatal_err {
 714             (kind, Symbol::intern(lit_content))
 715         } else {
 716             (token::Err, self.symbol_from_to(start, end))
 717         }
 718     }
 719 }
 720
 721 pub fn nfc_normalize(string: &str) -> Symbol {
 722     use unicode_normalization::{is_nfc_quick, IsNormalized, UnicodeNormalization};
 723     match is_nfc_quick(string.chars()) {
 724         IsNormalized::Yes => Symbol::intern(string),
 725         _ => {
 726             let normalized_str: String = string.chars().nfc().collect();
 727             Symbol::intern(&normalized_str)
 728         }
 729     }
 730 }