let file = (square.0 & 0b0000_0111) as i8;
let rank = (square.0 >> 3) as i8;

for &(i, j) in &[(-2, -1), (-2, 1), (-1, -2), (-1, 2),
                    (1, -2), (1, 2), (2, -1), (2, 1)] {
        if file + i < 0 || file + i >= 8 || rank + j < 0 || rank + j >= 8 {
            continue;
        }
        let dest_square = Square(((rank + j) * 8 + file + i) as u8)
        if board[dest_square].color() != Some(board.to_move) {
                add_if_legal(board, ChessMove::new(square, dest_square), moves);
        }
}