test.rs

mod config {
    pub enum Node<T> {
        Directory(~str, ~[Node<T>]),
        Entry(~str, ~T)
    }

    pub fn add_entry<'a, T>(node: &'a mut Node<T>, entry: Node<T>) -> Option<&'a mut Node<T>> {
        if match node {
            &Directory(_, ref mut list) => { list.push(entry); true }
            _ => false
        } { Some(node) } else { None }
        
    }
}

fn main() {
    let entry = ~config::Entry(~"test", ~0.0);
    let mut dir = ~config::Directory(~"dir", ~[]);

    let dir2 = config::add_entry(dir, entry);
}