Does the default impl of `ProjectField::is_projectable` permit unsound union field projection?

[joshlf]

On second re-reading, I think this is actually unsound as written. Just because a union is valid doesn't mean that any particular field is valid. E.g. you could initialize a union with a field value 2u8 – that doesn't mean that another field of type bool is valid.

Originally posted by @joshlf in #2940 (comment)