Does `TagEncoding::Direct` need to worry about generators

[danielsn]

AFAIK, we can only have this case for Enums. Generators use a different field naming scheme, so we would need to specalize this operation for Enum/Generator, if there is a case where it happens for generators. Since we can't come up with one, we can't test doing this. If you find a case that does need it, please post it on this issue!