macro_rules! generate_conversions {
(u8, $representation:ty, $target_type:ty) => { ... };
(u16, $representation:ty, $target_type:ty) => { ... };
(u32, $representation:ty, $target_type:ty) => { ... };
(u64, $representation:ty, $target_type:ty) => { ... };
($to:ty, $representation:ty, $target_type:ty, $_:expr) => { ... };
}