@Value.Lazy
    public Optional<DataMirror> datatypeMarker() {
      Optional<AbstractDeclaring> provider = datatypeProvider();
      if (provider.isPresent()) {
        return provider.get().datatypeEnabled();
      }
      return Optional.absent();
    }
The class, which is annotated to be a {@code @Value.Immutable}, {@code @Value.Include} or
{@code @Value.Enclosing}.
@return declaring type
value-processor/src/org/immutables/value/processor/meta/Proto.java