> you would store the concrete structs continously in Rust as well. Rust would also not waste space in the vector for storing a vtable-pointer, and would instead construct fat pointers dynamically when needed (since it knows the type, it knows which vtable to inject in the fat pointer).

Uh, what? How is the compiler just supposed to magically know which of the structures in the array are of what type, without any additional identifying information? I'm assuming that in this optimized case, there's a hidden type field in each struct, that it would use to index into a table of vtable pointers? If so, there you go, that's yet another level of indirection.