code: private static String formatFoundRequired(
        String description,
        JSType found,
        JSType required,
        Set<String> missing,
        Set<String> mismatch) {
      String foundStr = found.toString();
      String requiredStr = required.toString();
      if (foundStr.equals(requiredStr)) {
        foundStr = found.toAnnotationString(Nullability.IMPLICIT);
        requiredStr = required.toAnnotationString(Nullability.IMPLICIT);
      }
      String missingStr = "";
      String mismatchStr = "";
      if (missing != null && !missing.isEmpty()) {
        missingStr = Joiner.on(",").join(missing);
      }
      if (mismatch != null && !mismatch.isEmpty()) {
        mismatchStr = Joiner.on(",").join(mismatch);
      }
       if (missingStr.length() > 0 || mismatchStr.length() > 0) {
        return MessageFormat.format(
            FOUND_REQUIRED_MISSING, description, foundStr, requiredStr, missingStr, mismatchStr);
      } else {
        return MessageFormat.format(FOUND_REQUIRED, description, foundStr, requiredStr);
      }
    }
docs: Formats a found/required error message.