debug_unreachable::debug_unreachable!

macro_rules! debug_unreachable {
    () => { debug_unreachable!("entered unreachable code") };
    ($e:expr) => {
        if cfg!(ndebug) {
            $crate::__unreachable()
        } else {
            panic!($e);
        }
    }
}

panic!() in debug builds, optimization hint in release.