#[derive(Clone)]
struct Nestt(Option<::std::rc::Rc<Nestt>>);

impl Nestt {
    pub fn depth(&self) -> usize {
        let mut q = ::std::borrow::Cow::Borrowed(self);
        let mut ctr = 0;
        loop {
            let cl = if let Some(ref x) = q.as_ref().0 {
                (**x).clone()
            } else {
                break;
            };
            q = ::std::borrow::Cow::Owned(cl);
            ctr+=1;
        }
        ctr
    }
}

fn main() {
    let n = Nestt(
        Some(std::rc::Rc::new(Nestt(
            Some(std::rc::Rc::new(Nestt(
                None
            )))
        )))
    );

    assert_eq!(n.depth(), 2);
}