enum Command = {
    name: option(Ident),
    op: Op
};

./dbl.rs:15:16: 15:17 error: expecting ',' but found '('
./dbl.rs:15     name: option(Ident),
                            ^

The errors says it was looking for a comma where the open paren is. I stared at at this for a long time wondering why it thought there should be a comma there. Eventually I realized that, even though it 'expected' a comma, there were other possible characters that it would have been happy with as well - in this case <.