If rust did add const generic expressions I mean. It still would only generate code for the used instantiations.

Ah, I wasn't really talking about Rust.

Rust already does have some level of const generic expressions, but they are indeed only possible to instantiate with values known at compile time, like C++.

The difficulty of type checking them symbolically still applies regardless of how they're instantiated, but OTOH it doesn't look like Rust is really trying to go that direction.