Comment by Twey

> It is the right tradeoff to write those structs for libraries that absolutely have to avoid dynamic dispatch. In other cases it is better to give a trait object.

IMO it is a hack to use dynamic dispatch (a runtime behaviour with honestly quite limited use cases, like plugin functionality) to get existential types (a type system feature). If you are okay with parametric polymorphism/generics (universal types) you should also be okay with RPIT (existential types), which is the same semantic feature with a different syntax, e.g. you can get the same effect by CPS-encoding except that the syntax makes it untenable.

Because dynamic dispatch is a runtime behaviour it inherits a bunch of limitations that aren't inherent to existential types, a.k.a. Rust's ‘`dyn` safety’ requirements. For example, you can't have (abstract) associated types or functions associated with the type that don't take a magic ‘receiver’ pointer that can be used to look up the vtable.