public void emptyStruct(String message) {
    messages.add(message);
    hasEmptyStruct = true;
  }
@Test
  public void testEmptyStruct() {
    CompatibilityReport report = getCompatibilityReport(NestedEmptyStruct.class, NestedEmptyStruct.class);
    assertEquals(
        "encountered an empty struct: required_empty\nencountered an empty struct: optional_empty",
        report.prettyMessages());
    assertTrue(report.hasEmptyStruct());
  }