add better error message when labelling ShrinkwrapMut without Shrinkwrap
Right now, if a user specified ShrinkwrapMut
without Shrinkwrap
, a bunch of unhelpful messages about not having Deref
impl'd for DerefMut
, etc., will pop up, without it being a shrinkwraprs specific message. This should be fixed; we'll need to check for attributes on the struct itself.