bcachefs-tools/rust-src/bch_bindgen/src/opts.rs

#[macro_export]
macro_rules! opt_set {
    ($opts:ident, $n:ident, $v:expr) => {
        bch_bindgen::paste! {
            $opts.$n = $v;
            $opts.[<set_ $n _defined>](1);
        }
    }
}