it would be great if they measured the results of opt1_idiomatic with _ => unreachable!(). In theory the compiler would optimize that better than _ => 0.

As a fan of functional programing, it is validating in a way to see the more functional approach being faster. The reason for not wanting to mutate is that it’s easier to reason about pure code. Usually this is for the programmers benefit, but it can be good for the compiler too as we see here. Obviously there are many cases where it is faster to mutate (many data structures can benefit from mutation) but there is this general assumption that fp is slower which isn’t exactly true either.