Untagged unit variant fails to deserialize from visit_none()

[H2CO3]

I'm writing a serialization format that uses explicit optionals. Similarly to serde_json::Value, my Value enum has a Null variant representing the null value. When asked to deserialize_any(), this null value calls visit_none() on the supplied visitor. However, UntaggedUnitVisitor doesn't implement visit_none(), only visit_unit(), so this fails.

It seems to me that UntaggedUnitVisitor should ideally implement visit_none() because OptionVisitor treats unit and None as the same, so this would be the consistent behavior.

Another reason is that although I could fix the error by making Value::deserialize_any() call visitor.visit_unit() instead of visitor.visit_none(), I'd rather not change this behavior, because untagged enums are what I would consider more of a less-used special case, whereas Option has very regular structure and semantics, and it is used a lot more frequently too.