The overhead isn't always 8 bytes. When the type uses the naive representation, the overhead will generally match the alignment. For example an Option<u8> occup

The overhead isn't always 8 bytes. When the type uses the naive representation, the overhead will generally match the alignment. For example an Option<u8> occupies 2 bytes, 1 of which is for the tag. And if the type has some illegal values, these can sometimes be used to discriminate the type. When that's possible, the union will have the same size as the largest underlying type with no overhead for a tag. For example Option<&T> matches the size of &T, since it uses the illegal null value to represent None.