My point was that you don't need to erase types to get parametricity. It may be that my terminology is faulty, and that in fact what Rust is doing here does constitute "erasing" the types, in that case what describes the distinction between say a Rust function which is polymorphic over a function to be invoked, and a Rust function which merely takes a function pointer as a parameter and then invokes it ? I would say the latter is type erased.