[stefs]

rust doesn't guarantee anything if you opt out of the guarantees. two examples that come to mind: unsafe and maybe bounds checks in release mode.

fastmath is probably different anyway, as the "breaking" is on a floating point logic level, as in: results become imprecise, but not exactly "wrong" - as in undefined behaviour. but i don't know fastmath, so i might be wrong.

[steveklabnik]

(bounds checks are not removed in release mode, you have to use unsafe to not have the bounds check)

On the /r/rust thread, folks provided examples of why fastmath would produce UB in safe Rust.

[stefs]

ah, i think i meant overflow checks. and thanks for the pointer, i'll have a look.

[steveklabnik]

ah yes, overflow checks are not in release mode today. They may be in the future. And overflow isn't UB, it's two's compliment wrapping.