With the appropriate compilation options the C++ compiler will not make any optimizations that depend on the undefined beahvior for overflow.
The only defect here is that the options to do unsafe optimizations are default and you have to give explicitly an option to forbid them, instead of the safe option being default.
Since the overflow check is also controlled by a compilation option in Rust and the default for release builds is no checking, there is no difference. Even if Rust might have done less optimizations, an unchecked overflow will still cause a bug.
