private static String whichIsField(@NotNull @NonNls String field, @NotNull Object existing, @NotNull Object fresh, @Nullable String msg) {
    return msg == null ? null : appendDetail(msg, "which is `." + field + "` of " + existing + " and " + fresh);
  }
private static String whichIsField(@NotNull @NonNls String field, @NotNull Object existing, @NotNull Object fresh, @Nullable String msg)