If objects have the same type, which is statically known, there is no need for virtual functions because the compiler can resolve the specific method implementation at compile-time.

> or you could make a union of all applicable objects (thus guaranteed to be constant sized, at the size of the largest object) and store those in the array/vector, switching on a type enum

You can do this in both C++ and Rust easily, with equivalent efficiency in both. In Rust you would just use an enum type. This design is generally highly discouraged though, because it requires callers to be aware of all possible "derived classes".

My comments were only about the case where you are using true language-level polymorphism.