fix: bound the lexer mutator loop instead of a broken guard

The "saw mutator twice" guard could never fire: its seen map was
declared after the restart label, so every goto reset it. It also
cannot simply be hoisted, because splicing nested includes copies
rules and legitimately reprocesses the same mutator pointer in
other states. Replace it with an iteration bound, which also
catches include cycles that previously span forever.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: alecthomas

regexp.go

@@ -353,16 +353,17 @@ func (r *RegexLexer) maybeCompile() (err error) {
 			}
 		}
 	}
+	// Mutators can't be tracked individually as include splicing duplicates rules.
+	const mutatorLimit = 10000
+	restarts := 0
 restart:
-	seen := map[LexerMutator]bool{}
+	if restarts++; restarts > mutatorLimit {
+		return fmt.Errorf("lexer mutators did not converge after %d iterations; a LexerMutator may have failed to remove itself", mutatorLimit)
+	}
 	for state := range r.rules {
 		for i := range len(r.rules[state]) {
 			rule := r.rules[state][i]
 			if compile, ok := rule.Mutator.(LexerMutator); ok {
-				if seen[compile] {
-					return fmt.Errorf("saw mutator %T twice; this should not happen", compile)
-				}
-				seen[compile] = true
 				if err := compile.MutateLexer(r.rules, state, i); err != nil {
 					return err
 				}