#[derive(Clone, Copy)]
#[repr(u8)]
pub enum Len {
    Zero = 0,
    One,
    Two,
    Three,
}

#[no_mangle]
pub fn convert_len(len: &Len) -> Option<usize> {
    match *len {
        // Len::Zero => Some(0), // This produces optimal code.
        Len::Zero => None, // This does not: two table lookups, one being identity table.
        Len::One => Some(1),
        Len::Two => Some(2),
        Len::Three => Some(3),
    }
}

// Manual optimal code.
#[no_mangle]
pub fn convert_len_optimal(len: &Len) -> Option<usize> {
    match *len {
        Len::Zero => None,
        len => Some(unsafe { std::mem::transmute::<Len, u8>(len) } as usize),
    }
}