Code shared from the Rust Playground

playground.rs

trait Cast<T>{
    fn cast(self)->T;
}

macro_rules! cartesian_producs_of_impls {
    ( row[ $($row:ty),* ] col[ $($col:ty),* ] )=>{ 
        cartesian_producs_of_impls!{inner 
            state() 
            row[$($row),*] 
            col[$($col),*]
        }
    };
    (inner state($($tokens:tt)*) row[] col[ $($col:ty),* ] )=>{
        $($tokens)*
    };
    (inner 
        state($($tokens:tt)*) 
        row[ $first_one:ty $(,$rem:ty)* ] 
        col[ $($col:ty),* ] 
    )=>{
        cartesian_producs_of_impls!{inner
            state( 
                $($tokens)* 

                $( 
                    impl Cast<$col> for $first_one{
                        fn cast(self)->$col{
                            self as _
                        }
                    } 
                )*
            )
            row[$($rem),*]
            col[$($col),*]
        }
    };
}


cartesian_producs_of_impls!{
    row[u8,u16,u32]
    col[u8,u16,u32]
}