@Override
          public ImmutableSet<FactoryMethodDescriptor> visitTypeAsInterface(
              TypeElement type, Void p) {
            // applied to the factory interface
            messager.printMessage(
                ERROR, "Auto-factory doesn't support being applied to interfaces.", type, mirror);
            return ImmutableSet.of();
          }
A service that traverses an element and returns the set of factory methods defined therein.

@author Gregory Kick
factory/src/main/java/com/google/auto/factory/processor/FactoryDescriptorGenerator.java