For example, a for loop from 0-through-N (N:i32) will execute N times in C, but N-or-infinity times in Rust. That small difference of infinity controls how the loop condition is tested, whether it can be turned into a simple counter or not.

Eh, not quite true. If N is a 64-bit or an unsigned integer but your loop index is a signed 32-bit integer, then you will have this behavior. In Rust, the idiomatic for loop will have the index variable always be the same size as the condition variable, which prevents the infinity case from happening. In C, if you have a 64-bit condition but a 32-bit unsigned loop index variable, you get the N-or-infinity case as well.

In other words, if your language is subtly redesigned to make you jump through hoops to do things like have mismatched integer types in for loops, then the need to rely on undefined behavior to reclaim that performance is lessened.