Proposal: give Ptr a nominal role

Currently, we have data Ptr a = Ptr Addr# type role Ptr phantom This is weird: accidentally coercing a pointer to a different type is very bad. The only reason Ptr has this role is that without it, castPtr and such may not be free or will involve unsafe coercions. Thankfully, we have enough power to fix this now. data Addr = Ptr_ Addr# newtype Ptr a = Ptr_ Addr type role Ptr nominal pattern Ptr :: Addr# -> Ptr a pattern Ptr a# = Ptr_ (Addr a#) castPtr :: Ptr a -> Ptr b castPtr (Ptr a) = Ptr a ptrCoercible :: ((forall a b. Coercible (Ptr a) (Ptr b)) => r) -> r ptrCoercible r = r ptrCoercion :: Coercion (Ptr a) (Ptr b) ptrCoercion = Coercion I propose that we do this.