One thing I like about Rust a lot is that it lets you choose which hit you want to take when it comes to polymorphism. You can manually (and without much difficulty!) prefer dynamic dispatch if it's important to you to keep your binary size small, but you can also choose static dispatch and allow some replicated copies of your parametric code if that's what you want to optimize for.
It's also worth noting that if you are using polymorphic functions that are only ever called in your code with a single known type parameter, then your program should be just as efficient as if you wrote a monomorphic version with that fixed type in both runtime _and_ binary size, which means the only disadvantage to the polymorphism there is compile time.
It would be nice if Box<dyn Trait> implemented Trait. Then we would be able to write the function only once with parametric polymorphism and then at callsite decide if we want to monomorphize for the given type or not.
You can provide this implementation yourself easily enough though. I agree it's maybe not ideal that this needs to be done for every Trait you want this behavior for.
I’ve been using C and C++ (sorry Rust), like, forever, and I think should hate them with all fibers of my soul. A high-level programming language is “supposed” to let me forget, for the higher being’s sake, all machine-specific details and focus on the logic of the problem at hand. (Hello FORTRAN.)
If you, as the programmer, would prefer not to know and remember machine specific details, you can use rust or c++ easily. Just don't think you will always get the best performance possible for your hardware. And I think that is justified and reasonable.
It's also worth noting that if you are using polymorphic functions that are only ever called in your code with a single known type parameter, then your program should be just as efficient as if you wrote a monomorphic version with that fixed type in both runtime _and_ binary size, which means the only disadvantage to the polymorphism there is compile time.