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
  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                     let c = self.str_from(start).chars().next().unwrap();
 236                     let mut err =
 237                         self.struct_err_span_char(start, self.pos, "unknown start of token", c);
 238                     // FIXME: the lexer could be used to turn the ASCII version of unicode
 239                     // homoglyphs, instead of keeping a table in `check_for_substitution`into the
 240                     // token. Ideally, this should be inside `rustc_lexer`. However, we should
 241                     // first remove compound tokens like `<<` from `rustc_lexer`, and then add
 242                     // fancier error recovery to it, as there will be less overall work to do this
 243                     // way.
 244                     let token = unicode_chars::check_for_substitution(self, start, c, &mut err);
 245                     if c == '\x00' {
 246                         err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
 247                     }
 248                     err.emit();
 249                     if let Some(token) = token {
 250                         token
 251                     } else {
 252                         preceded_by_whitespace = true;
 253                         continue;
 254                     }
 255                 }
 256                 rustc_lexer::TokenKind::Eof => token::Eof,
 257             };
 258             let span = self.mk_sp(start, self.pos);
 259             return (Token::new(kind, span), preceded_by_whitespace);
 260         }
 261     }
 262
 263     /// Report a fatal lexical error with a given span.
 264     fn fatal_span(&self, sp: Span, m: &str) -> ! {
 265         self.sess.span_diagnostic.span_fatal(sp, m)
 266     }
 267
 268     /// Report a lexical error with a given span.
 269     fn err_span(&self, sp: Span, m: &str) {
 270         self.sess.span_diagnostic.struct_span_err(sp, m).emit();
 271     }
 272
 273     /// Report a fatal error spanning [`from_pos`, `to_pos`).
 274     fn fatal_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) -> ! {
 275         self.fatal_span(self.mk_sp(from_pos, to_pos), m)
 276     }
 277
 278     /// Report a lexical error spanning [`from_pos`, `to_pos`).
 279     fn err_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) {
 280         self.err_span(self.mk_sp(from_pos, to_pos), m)
 281     }
 282
 283     fn struct_fatal_span_char(
 284         &self,
 285         from_pos: BytePos,
 286         to_pos: BytePos,
 287         m: &str,
 288         c: char,
 289     ) -> DiagnosticBuilder<'a, !> {
 290         self.sess
 291             .span_diagnostic
 292             .struct_span_fatal(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
 293     }
 294
 295     fn struct_err_span_char(
 296         &self,
 297         from_pos: BytePos,
 298         to_pos: BytePos,
 299         m: &str,
 300         c: char,
 301     ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
 302         self.sess
 303             .span_diagnostic
 304             .struct_span_err(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
 305     }
 306
 307     /// Detect usages of Unicode codepoints changing the direction of the text on screen and loudly
 308     /// complain about it.
 309     fn lint_unicode_text_flow(&self, start: BytePos) {
 310         // Opening delimiter of the length 2 is not included into the comment text.
 311         let content_start = start + BytePos(2);
 312         let content = self.str_from(content_start);
 313         if contains_text_flow_control_chars(content) {
 314             let span = self.mk_sp(start, self.pos);
 315             self.sess.buffer_lint_with_diagnostic(
 316                 &TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
 317                 span,
 318                 ast::CRATE_NODE_ID,
 319                 "unicode codepoint changing visible direction of text present in comment",
 320                 BuiltinLintDiagnostics::UnicodeTextFlow(span, content.to_string()),
 321             );
 322         }
 323     }
 324
 325     fn cook_doc_comment(
 326         &self,
 327         content_start: BytePos,
 328         content: &str,
 329         comment_kind: CommentKind,
 330         doc_style: DocStyle,
 331     ) -> TokenKind {
 332         if content.contains('\r') {
 333             for (idx, _) in content.char_indices().filter(|&(_, c)| c == '\r') {
 334                 self.err_span_(
 335                     content_start + BytePos(idx as u32),
 336                     content_start + BytePos(idx as u32 + 1),
 337                     match comment_kind {
 338                         CommentKind::Line => "bare CR not allowed in doc-comment",
 339                         CommentKind::Block => "bare CR not allowed in block doc-comment",
 340                     },
 341                 );
 342             }
 343         }
 344
 345         let attr_style = match doc_style {
 346             DocStyle::Outer => AttrStyle::Outer,
 347             DocStyle::Inner => AttrStyle::Inner,
 348         };
 349
 350         token::DocComment(comment_kind, attr_style, Symbol::intern(content))
 351     }
 352
 353     fn cook_lexer_literal(
 354         &self,
 355         start: BytePos,
 356         suffix_start: BytePos,
 357         kind: rustc_lexer::LiteralKind,
 358     ) -> (token::LitKind, Symbol) {
 359         // prefix means `"` or `br"` or `r###"`, ...
 360         let (lit_kind, mode, prefix_len, postfix_len) = match kind {
 361             rustc_lexer::LiteralKind::Char { terminated } => {
 362                 if !terminated {
 363                     self.sess.span_diagnostic.span_fatal_with_code(
 364                         self.mk_sp(start, suffix_start),
 365                         "unterminated character literal",
 366                         error_code!(E0762),
 367                     )
 368                 }
 369                 (token::Char, Mode::Char, 1, 1) // ' '
 370             }
 371             rustc_lexer::LiteralKind::Byte { terminated } => {
 372                 if !terminated {
 373                     self.sess.span_diagnostic.span_fatal_with_code(
 374                         self.mk_sp(start + BytePos(1), suffix_start),
 375                         "unterminated byte constant",
 376                         error_code!(E0763),
 377                     )
 378                 }
 379                 (token::Byte, Mode::Byte, 2, 1) // b' '
 380             }
 381             rustc_lexer::LiteralKind::Str { terminated } => {
 382                 if !terminated {
 383                     self.sess.span_diagnostic.span_fatal_with_code(
 384                         self.mk_sp(start, suffix_start),
 385                         "unterminated double quote string",
 386                         error_code!(E0765),
 387                     )
 388                 }
 389                 (token::Str, Mode::Str, 1, 1) // " "
 390             }
 391             rustc_lexer::LiteralKind::ByteStr { terminated } => {
 392                 if !terminated {
 393                     self.sess.span_diagnostic.span_fatal_with_code(
 394                         self.mk_sp(start + BytePos(1), suffix_start),
 395                         "unterminated double quote byte string",
 396                         error_code!(E0766),
 397                     )
 398                 }
 399                 (token::ByteStr, Mode::ByteStr, 2, 1) // b" "
 400             }
 401             rustc_lexer::LiteralKind::RawStr { n_hashes } => {
 402                 if let Some(n_hashes) = n_hashes {
 403                     let n = u32::from(n_hashes);
 404                     (token::StrRaw(n_hashes), Mode::RawStr, 2 + n, 1 + n) // r##" "##
 405                 } else {
 406                     self.report_raw_str_error(start, 1);
 407                 }
 408             }
 409             rustc_lexer::LiteralKind::RawByteStr { n_hashes } => {
 410                 if let Some(n_hashes) = n_hashes {
 411                     let n = u32::from(n_hashes);
 412                     (token::ByteStrRaw(n_hashes), Mode::RawByteStr, 3 + n, 1 + n) // br##" "##
 413                 } else {
 414                     self.report_raw_str_error(start, 2);
 415                 }
 416             }
 417             rustc_lexer::LiteralKind::Int { base, empty_int } => {
 418                 return if empty_int {
 419                     self.sess
 420                         .span_diagnostic
 421                         .struct_span_err_with_code(
 422                             self.mk_sp(start, suffix_start),
 423                             "no valid digits found for number",
 424                             error_code!(E0768),
 425                         )
 426                         .emit();
 427                     (token::Integer, sym::integer(0))
 428                 } else {
 429                     self.validate_int_literal(base, start, suffix_start);
 430                     (token::Integer, self.symbol_from_to(start, suffix_start))
 431                 };
 432             }
 433             rustc_lexer::LiteralKind::Float { base, empty_exponent } => {
 434                 if empty_exponent {
 435                     self.err_span_(start, self.pos, "expected at least one digit in exponent");
 436                 }
 437
 438                 match base {
 439                     Base::Hexadecimal => self.err_span_(
 440                         start,
 441                         suffix_start,
 442                         "hexadecimal float literal is not supported",
 443                     ),
 444                     Base::Octal => {
 445                         self.err_span_(start, suffix_start, "octal float literal is not supported")
 446                     }
 447                     Base::Binary => {
 448                         self.err_span_(start, suffix_start, "binary float literal is not supported")
 449                     }
 450                     _ => (),
 451                 }
 452
 453                 let id = self.symbol_from_to(start, suffix_start);
 454                 return (token::Float, id);
 455             }
 456         };
 457         let content_start = start + BytePos(prefix_len);
 458         let content_end = suffix_start - BytePos(postfix_len);
 459         let id = self.symbol_from_to(content_start, content_end);
 460         self.validate_literal_escape(mode, content_start, content_end, prefix_len, postfix_len);
 461         (lit_kind, id)
 462     }
 463
 464     #[inline]
 465     fn src_index(&self, pos: BytePos) -> usize {
 466         (pos - self.start_pos).to_usize()
 467     }
 468
 469     /// Slice of the source text from `start` up to but excluding `self.pos`,
 470     /// meaning the slice does not include the character `self.ch`.
 471     fn str_from(&self, start: BytePos) -> &str {
 472         self.str_from_to(start, self.pos)
 473     }
 474
 475     /// As symbol_from, with an explicit endpoint.
 476     fn symbol_from_to(&self, start: BytePos, end: BytePos) -> Symbol {
 477         debug!("taking an ident from {:?} to {:?}", start, end);
 478         Symbol::intern(self.str_from_to(start, end))
 479     }
 480
 481     /// Slice of the source text spanning from `start` up to but excluding `end`.
 482     fn str_from_to(&self, start: BytePos, end: BytePos) -> &str {
 483         &self.src[self.src_index(start)..self.src_index(end)]
 484     }
 485
 486     fn report_raw_str_error(&self, start: BytePos, prefix_len: u32) -> ! {
 487         match rustc_lexer::validate_raw_str(self.str_from(start), prefix_len) {
 488             Err(RawStrError::InvalidStarter { bad_char }) => {
 489                 self.report_non_started_raw_string(start, bad_char)
 490             }
 491             Err(RawStrError::NoTerminator { expected, found, possible_terminator_offset }) => self
 492                 .report_unterminated_raw_string(start, expected, possible_terminator_offset, found),
 493             Err(RawStrError::TooManyDelimiters { found }) => {
 494                 self.report_too_many_hashes(start, found)
 495             }
 496             Ok(()) => panic!("no error found for supposedly invalid raw string literal"),
 497         }
 498     }
 499
 500     fn report_non_started_raw_string(&self, start: BytePos, bad_char: char) -> ! {
 501         self.struct_fatal_span_char(
 502             start,
 503             self.pos,
 504             "found invalid character; only `#` is allowed in raw string delimitation",
 505             bad_char,
 506         )
 507         .emit()
 508     }
 509
 510     fn report_unterminated_raw_string(
 511         &self,
 512         start: BytePos,
 513         n_hashes: u32,
 514         possible_offset: Option<u32>,
 515         found_terminators: u32,
 516     ) -> ! {
 517         let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
 518             self.mk_sp(start, start),
 519             "unterminated raw string",
 520             error_code!(E0748),
 521         );
 522
 523         err.span_label(self.mk_sp(start, start), "unterminated raw string");
 524
 525         if n_hashes > 0 {
 526             err.note(&format!(
 527                 "this raw string should be terminated with `\"{}`",
 528                 "#".repeat(n_hashes as usize)
 529             ));
 530         }
 531
 532         if let Some(possible_offset) = possible_offset {
 533             let lo = start + BytePos(possible_offset as u32);
 534             let hi = lo + BytePos(found_terminators as u32);
 535             let span = self.mk_sp(lo, hi);
 536             err.span_suggestion(
 537                 span,
 538                 "consider terminating the string here",
 539                 "#".repeat(n_hashes as usize),
 540                 Applicability::MaybeIncorrect,
 541             );
 542         }
 543
 544         err.emit()
 545     }
 546
 547     fn report_unterminated_block_comment(&self, start: BytePos, doc_style: Option<DocStyle>) {
 548         let msg = match doc_style {
 549             Some(_) => "unterminated block doc-comment",
 550             None => "unterminated block comment",
 551         };
 552         let last_bpos = self.pos;
 553         let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
 554             self.mk_sp(start, last_bpos),
 555             msg,
 556             error_code!(E0758),
 557         );
 558         let mut nested_block_comment_open_idxs = vec![];
 559         let mut last_nested_block_comment_idxs = None;
 560         let mut content_chars = self.str_from(start).char_indices().peekable();
 561
 562         while let Some((idx, current_char)) = content_chars.next() {
 563             match content_chars.peek() {
 564                 Some((_, '*')) if current_char == '/' => {
 565                     nested_block_comment_open_idxs.push(idx);
 566                 }
 567                 Some((_, '/')) if current_char == '*' => {
 568                     last_nested_block_comment_idxs =
 569                         nested_block_comment_open_idxs.pop().map(|open_idx| (open_idx, idx));
 570                 }
 571                 _ => {}
 572             };
 573         }
 574
 575         if let Some((nested_open_idx, nested_close_idx)) = last_nested_block_comment_idxs {
 576             err.span_label(self.mk_sp(start, start + BytePos(2)), msg)
 577                 .span_label(
 578                     self.mk_sp(
 579                         start + BytePos(nested_open_idx as u32),
 580                         start + BytePos(nested_open_idx as u32 + 2),
 581                     ),
 582                     "...as last nested comment starts here, maybe you want to close this instead?",
 583                 )
 584                 .span_label(
 585                     self.mk_sp(
 586                         start + BytePos(nested_close_idx as u32),
 587                         start + BytePos(nested_close_idx as u32 + 2),
 588                     ),
 589                     "...and last nested comment terminates here.",
 590                 );
 591         }
 592
 593         err.emit();
 594     }
 595
 596     // RFC 3101 introduced the idea of (reserved) prefixes. As of Rust 2021,
 597     // using a (unknown) prefix is an error. In earlier editions, however, they
 598     // only result in a (allowed by default) lint, and are treated as regular
 599     // identifier tokens.
 600     fn report_unknown_prefix(&self, start: BytePos) {
 601         let prefix_span = self.mk_sp(start, self.pos);
 602         let prefix_str = self.str_from_to(start, self.pos);
 603         let msg = format!("prefix `{}` is unknown", prefix_str);
 604
 605         let expn_data = prefix_span.ctxt().outer_expn_data();
 606
 607         if expn_data.edition >= Edition::Edition2021 {
 608             // In Rust 2021, this is a hard error.
 609             let mut err = self.sess.span_diagnostic.struct_span_err(prefix_span, &msg);
 610             err.span_label(prefix_span, "unknown prefix");
 611             if prefix_str == "rb" {
 612                 err.span_suggestion_verbose(
 613                     prefix_span,
 614                     "use `br` for a raw byte string",
 615                     "br",
 616                     Applicability::MaybeIncorrect,
 617                 );
 618             } else if expn_data.is_root() {
 619                 err.span_suggestion_verbose(
 620                     prefix_span.shrink_to_hi(),
 621                     "consider inserting whitespace here",
 622                     " ",
 623                     Applicability::MaybeIncorrect,
 624                 );
 625             }
 626             err.note("prefixed identifiers and literals are reserved since Rust 2021");
 627             err.emit();
 628         } else {
 629             // Before Rust 2021, only emit a lint for migration.
 630             self.sess.buffer_lint_with_diagnostic(
 631                 &RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
 632                 prefix_span,
 633                 ast::CRATE_NODE_ID,
 634                 &msg,
 635                 BuiltinLintDiagnostics::ReservedPrefix(prefix_span),
 636             );
 637         }
 638     }
 639
 640     fn report_too_many_hashes(&self, start: BytePos, found: u32) -> ! {
 641         self.fatal_span_(
 642             start,
 643             self.pos,
 644             &format!(
 645                 "too many `#` symbols: raw strings may be delimited \
 646                 by up to 255 `#` symbols, but found {}",
 647                 found
 648             ),
 649         )
 650     }
 651
 652     fn validate_literal_escape(
 653         &self,
 654         mode: Mode,
 655         content_start: BytePos,
 656         content_end: BytePos,
 657         prefix_len: u32,
 658         postfix_len: u32,
 659     ) {
 660         let lit_content = self.str_from_to(content_start, content_end);
 661         unescape::unescape_literal(lit_content, mode, &mut |range, result| {
 662             // Here we only check for errors. The actual unescaping is done later.
 663             if let Err(err) = result {
 664                 let span_with_quotes = self
 665                     .mk_sp(content_start - BytePos(prefix_len), content_end + BytePos(postfix_len));
 666                 let (start, end) = (range.start as u32, range.end as u32);
 667                 let lo = content_start + BytePos(start);
 668                 let hi = lo + BytePos(end - start);
 669                 let span = self.mk_sp(lo, hi);
 670                 emit_unescape_error(
 671                     &self.sess.span_diagnostic,
 672                     lit_content,
 673                     span_with_quotes,
 674                     span,
 675                     mode,
 676                     range,
 677                     err,
 678                 );
 679             }
 680         });
 681     }
 682
 683     fn validate_int_literal(&self, base: Base, content_start: BytePos, content_end: BytePos) {
 684         let base = match base {
 685             Base::Binary => 2,
 686             Base::Octal => 8,
 687             _ => return,
 688         };
 689         let s = self.str_from_to(content_start + BytePos(2), content_end);
 690         for (idx, c) in s.char_indices() {
 691             let idx = idx as u32;
 692             if c != '_' && c.to_digit(base).is_none() {
 693                 let lo = content_start + BytePos(2 + idx);
 694                 let hi = content_start + BytePos(2 + idx + c.len_utf8() as u32);
 695                 self.err_span_(lo, hi, &format!("invalid digit for a base {} literal", base));
 696             }
 697         }
 698     }
 699 }
 700
 701 pub fn nfc_normalize(string: &str) -> Symbol {
 702     use unicode_normalization::{is_nfc_quick, IsNormalized, UnicodeNormalization};
 703     match is_nfc_quick(string.chars()) {
 704         IsNormalized::Yes => Symbol::intern(string),
 705         _ => {
 706             let normalized_str: String = string.chars().nfc().collect();
 707             Symbol::intern(&normalized_str)
 708         }
 709     }
 710 }