Insufficient covariance check makes self_cell unsound
All public versions prior to 1.02 used an insufficient check to ensure that users correctly marked the dependent type as either covariant or not_covariant. This allowed users to mark a dependent as covariant even though its type was not covariant but invariant, for certain invariant types involving trait object lifetimes. One example for such a dependent type is type Dependent<'a> = RefCell<Box<dyn fmt::Display + 'a>>. Such a type allowed unsound …