struct Rec {
    children: Vec<Rec>,
}

fn append_first(rec: &mut Rec) {
    let mut children = &mut rec.children;
    while children.is_empty() {
        children = &mut {children}[0].children;
    }
    children.push(Rec { children: Vec::new() });
}

fn main() {
    let mut rec = Rec { children: vec![Rec { children: Vec::new() }] };
    append_first(&mut rec);
}