refactor(parser): reduce work parsing regexps (#1999)

#1926 produced a small performance regression because when parsing a
regexp, some work is repeated.

crates/oxc_parser/src/cursor.rs

@@ -1,5 +1,6 @@
 //! Code related to navigating `Token`s from the lexer
 
+use oxc_ast::ast::RegExpFlags;
 use oxc_diagnostics::Result;
 use oxc_span::Span;
 
@@ -200,8 +201,10 @@ impl<'a> Parser<'a> {
     }
 
     /// Tell lexer to read a regex
-    pub(crate) fn read_regex(&mut self) {
-        self.token = self.lexer.next_regex(self.cur_kind());
+    pub(crate) fn read_regex(&mut self) -> (u32, RegExpFlags) {
+        let (token, pattern_end, flags) = self.lexer.next_regex(self.cur_kind());
+        self.token = token;
+        (pattern_end, flags)
     }
 
     /// Tell lexer to read a template substitution tail

crates/oxc_parser/src/js/expression.rs

@@ -180,7 +180,6 @@ impl<'a> Parser<'a> {
             }
             Kind::LParen => self.parse_parenthesized_expression(span),
             Kind::Slash | Kind::SlashEq => {
-                self.read_regex();
                 let literal = self.parse_literal_regexp();
                 Ok(self.ast.literal_regexp_expression(literal))
             }
@@ -320,22 +319,10 @@ impl<'a> Parser<'a> {
     pub(crate) fn parse_literal_regexp(&mut self) -> RegExpLiteral {
         let span = self.start_span();
 
-        // split out the flag part of `/regex/flag` by looking for `/` from the end
-        let regex_src = self.cur_src();
-        let mut flags = RegExpFlags::empty();
-
-        let mut split_index = None;
-        for (i, c) in regex_src.char_indices().rev() {
-            if let Ok(flag) = RegExpFlags::try_from(c) {
-                flags |= flag;
-            } else {
-                split_index.replace(i);
-                break;
-            }
-        }
-
-        // `/` are omitted from the pattern
-        let pattern = split_index.map_or(regex_src, |i| regex_src.get(1..i).unwrap_or(""));
+        // split out pattern
+        let (pattern_end, flags) = self.read_regex();
+        let pattern_start = self.cur_token().start + 1; // +1 to exclude `/`
+        let pattern = &self.source_text[pattern_start as usize..pattern_end as usize];
 
         self.bump_any();
         self.ast.reg_exp_literal(self.end_span(span), pattern, flags)

crates/oxc_parser/src/lexer/mod.rs

@@ -192,16 +192,17 @@ impl<'a> Lexer<'a> {
     ///   where a `RegularExpressionLiteral` is permitted
     /// Which means the parser needs to re-tokenize on `PrimaryExpression`,
     /// `RegularExpressionLiteral` only appear on the right hand side of `PrimaryExpression`
-    pub fn next_regex(&mut self, kind: Kind) -> Token {
+    pub fn next_regex(&mut self, kind: Kind) -> (Token, u32, RegExpFlags) {
         self.current.token.start = self.offset()
             - match kind {
                 Kind::Slash => 1,
                 Kind::SlashEq => 2,
                 _ => unreachable!(),
             };
-        let kind = self.read_regex();
+        let (pattern_end, flags) = self.read_regex();
         self.lookahead.clear();
-        self.finish_next(kind)
+        let token = self.finish_next(Kind::RegExp);
+        (token, pattern_end, flags)
     }
 
     pub fn next_right_angle(&mut self) -> Token {
@@ -828,18 +829,20 @@ impl<'a> Lexer<'a> {
     }
 
     /// 12.9.5 Regular Expression Literals
-    fn read_regex(&mut self) -> Kind {
+    fn read_regex(&mut self) -> (u32, RegExpFlags) {
         let mut in_escape = false;
         let mut in_character_class = false;
         loop {
             match self.current.chars.next() {
                 None => {
                     self.error(diagnostics::UnterminatedRegExp(self.unterminated_range()));
-                    return Kind::Undetermined;
+                    return (self.offset(), RegExpFlags::empty());
                 }
                 Some(c) if is_line_terminator(c) => {
                     self.error(diagnostics::UnterminatedRegExp(self.unterminated_range()));
-                    return Kind::Undetermined;
+                    #[allow(clippy::cast_possible_truncation)]
+                    let pattern_end = self.offset() - c.len_utf8() as u32;
+                    return (pattern_end, RegExpFlags::empty());
                 }
                 Some(c) => {
                     if in_escape {
@@ -857,28 +860,29 @@ impl<'a> Lexer<'a> {
             }
         }
 
+        let pattern_end = self.offset() - 1; // -1 to exclude `/`
         let mut flags = RegExpFlags::empty();
 
         while let Some(ch @ ('$' | '_' | 'a'..='z' | 'A'..='Z' | '0'..='9')) = self.peek() {
             self.current.chars.next();
             if !ch.is_ascii_lowercase() {
                 self.error(diagnostics::RegExpFlag(ch, self.current_offset()));
-                return Kind::Undetermined;
+                break;
             }
             let flag = if let Ok(flag) = RegExpFlags::try_from(ch) {
                 flag
             } else {
                 self.error(diagnostics::RegExpFlag(ch, self.current_offset()));
-                return Kind::Undetermined;
+                break;
             };
             if flags.contains(flag) {
                 self.error(diagnostics::RegExpFlagTwice(ch, self.current_offset()));
-                return Kind::Undetermined;
+                break;
             }
             flags |= flag;
         }
 
-        Kind::RegExp
+        (pattern_end, flags)
     }
 
     /// 12.8.6 Template Literal Lexical Components