FromVariant derive has incorrect type on fields field

[mfreeborn]

Looking at the documentation here, it suggests that the type for the fields field is Option<darling::ast::Fields<__>>.

Having just tried to implement this (using darling v0.14.1), I get an error stating that the true type is actually darling::ast::Fields<_>.

Testing it on a plain enum without tuple/field variants, fields returns an empty vector. I think it would be better if the behaviour was as per the documentation, and that non-tuple/field variants return None and others return Some(*at least one field*).

I also think it would be clear in the docs to mention that darling::ast::Fields<_> something which implements FromField e.g. darling::ast::Fields<T: impl darling::FromField> just to save a bit of digging around in the documentation/chasing error messages.

[TedDriggs]

Changing it to be an Option would break lots of existing code, so I'll update the docs to accurately capture the true type.

[mfreeborn]

That's much clearer, thanks!