get_details (VanillaCon tys) = plusFVs (map get_bang tys)
get_details (InfixCon t1 t2) = get_bang t1 `plusFV` get_bang t2
get_details (RecCon fields) = plusFVs [get_bang t | (_, t) <- fields]
- get_details (NewCon t _) = extractHsTyNames t
get_field (fs,t) | any (`elemNameSet` source_fvs) fs = get_bang t
| otherwise = emptyFVs