private void validateIdentifier(Shape shape, Shape target, List<ValidationEvent> events) {
        if (target.getType() != ShapeType.STRING && target.getType() != ShapeType.ENUM) {
            events.add(badType(shape, target, RelationshipType.IDENTIFIER, ShapeType.STRING));
        }
    }
Validates that neighbors target resolvable shapes of the correct type.
smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/TargetValidator.java