scanner: parse multiple comments and long strings without a stackoverflow

vlib/v/scanner/scanner.v

@@ -614,6 +614,13 @@ fn (s Scanner) look_ahead(n int) byte {
 }
 
 fn (mut s Scanner) text_scan() token.Token {
+	// The for loop here is so that instead of doing
+	// `return s.scan()` (which will use a new call stack frame),
+	// text_scan can just do continue, keeping
+	// memory & stack usage low.
+	// That optimization mostly matters for long sections
+	// of comments and string literals.
+	for {
 		// if s.comments_mode == .parse_comments {
 		// println('\nscan()')
 		// }
@@ -794,7 +801,7 @@ fn (mut s Scanner) text_scan() token.Token {
 			`{` {
 				// Skip { in `${` in strings
 				if s.is_inside_string {
-				return s.scan()
+					continue
 				}
 				return s.new_token(.lcbr, '', 1)
 			}
@@ -944,7 +951,7 @@ fn (mut s Scanner) text_scan() token.Token {
 					// treat shebang line (#!) as a comment
 					s.line_comment = s.text[start + 1..s.pos].trim_space()
 					// s.fgenln('// shebang line "$s.line_comment"')
-				return s.scan()
+					continue
 				}
 				hash := s.text[start..s.pos].trim_space()
 				return s.new_token(.hash, hash, hash.len)
@@ -1060,7 +1067,7 @@ fn (mut s Scanner) text_scan() token.Token {
 					}
 					// s.fgenln('// ${s.prev_tok.str()} "$s.line_comment"')
 					// Skip the comment (return the next token)
-				return s.scan()
+					continue
 				}
 				// Multiline comments
 				if nextc == `*` {
@@ -1091,7 +1098,7 @@ fn (mut s Scanner) text_scan() token.Token {
 						return s.new_token(.comment, comment, comment.len + 4)
 					}
 					// Skip if not in fmt mode
-				return s.scan()
+					continue
 				}
 				return s.new_token(.div, '', 1)
 			}
@@ -1103,6 +1110,8 @@ fn (mut s Scanner) text_scan() token.Token {
 			}
 		}
 		s.error('invalid character `$c.str()`')
+		break
+	}
 	return s.end_of_file()
 }