> And its undefined behaviour in release mode.

No, it uses 2’s complement and is well defined in release mode. From [1]:

> When you’re compiling in release mode with the --release flag, Rust does not include checks for integer overflow that cause panics. Instead, if overflow occurs, Rust performs two’s complement wrapping.

[1]: https://doc.rust-lang.org/book/ch03-02-data-types.html#integ...

I'm pretty sure it's not undefined behavior in rust in release mode if it does overflow. It's fully specified behavior, I believe.