"Beyond" in the sense of "after". Languages that support separate compilation usually have some sort of artifacts that do not need to be type-checked anymore. Say, you compile an Ocaml module. The result is a compiled (bytecode or native code) file that represents the implementation of the module and doesn't need to be generated ever again. Essentially, the compiler abstracts over a module after it dealt with it. You could the

If you do the same in rust, you need access to the module's implementation for monomorphization. You might skip the type checking of a function of the implementation or even cache the generated code for a monomorphization as optimizations, but generally you have to treat every function as often as it is used. So the best rust could do is akin to C++ precompiled headers whereas a language like OCaml can simply compile every module exactly once.

Note: OCaml pays for this elegance. Its uniform object representation is less efficient than what rust does. It's a trade-off that the rust designers consciously made, I think.