Well I have no idea what your parent meant, but existential types (types which we assert to exist but don't specify) is in fact already used in Rust
fn goose_iterator() -> impl Iterator<Item=Goose>
That type on the right hand side, impl Iterator<Item=Goose> isn't the name of an actual type, that's just a claim about a property of this type, whatever it is. That's an existential type, the type exists, but we're not naming it.
In Rust this means goose_iterator() has some specific result type, but we're not allowed to know what that type is exactly (the compiler knows but our program does not), the only thing we're promised is that whatever type it is, does implement this trait with these parameters.
You'll see this often with things that return a callable - it's literally not possible to name most of Rust's callables, even though they each have a distinct type you can't utter the name of that type. So we need to use existential types for that case, but you can use them (as I did) to promise only that you're giving back an iterator without nailing down specifically which iterator that is - which would let you swap that iterator in an API without causing an API incompatibility.
Existential types already exist as `impl Trait` for method parameters, unfortunately return position impl traits aren't fully existential because you can only return one type from a function.
I have absolutely no idea what you're trying to say. The RPIT will be bound by the caller and there exists a type such that the declared trait is implemented. Meanwhile an impl parameter will be bound within the method with some type existing such that the trait is implemented. That is the implication of existential, "there exists a type such that...".
1. erased types (have to use behind pointer, so not shallow dynamic sizing) 2. Existential types
So we can build vtables by hand.
Type safe API to any OOP ABI + tons of stuff OOP people never imagined....let's go!