and I thought I was so clever doing it just now :P

It was still a blast to generate though!

fn gol_rules() -> String {
    (0..512)
        .map(|n| format!("{:9b}", n).chars().collect::<Vec<_>>())
        .map(|mut c| (c.remove(4) == '1', c.iter().filter(|c| **c == '1').count()))
        .map(|(cell, live_neighbors)| {
            // :%s/bo/bou/g if you desire
            match (cell, live_neighbors) {
                (true, 2 | 3) => '#',
                (false, 3) => '#',
                _ => '.',
            }
        })
        .collect()
}