Fourth.rs note, and printing.

[X01XX]

In fourth.rs, my cargo, (edition = "2018"), wants Node prev and next Boxed. It was not too hard to make it work. How about printing? Something like:

impl<T: std::fmt::Display> fmt::Display for List<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut rc_str = String::from("(");

        let mut cur_link = &self.head;
        let mut start = true;

        while let Some(boxed_node) = cur_link {
            if start {
            } else {
                rc_str.push_str(", ");
            }
            start = false;
            rc_str.push_str(&format!("{}", boxed_node.elem));
            cur_link = &boxed_node.next;
        }
        rc_str.push_str(")");

        write!(f, "{}", rc_str)
    }
}

[X01XX]

Improved code formatting.