private ValidationEvent emit(MemberShape memberShape) {
        return ValidationEvent.builder()
                .id(getEventId())
                .shape(memberShape)
                .message("Adding a new member with the `required` trait "
                        + "but not the `default` trait is backwards-incompatible.")
                .severity(Severity.ERROR)
                .build();
    }
Validates that no members are newly created with the required trait
(but no default trait) in existing structures.
smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/AddedRequiredMember.java