wtf_wtf.rs

    pub fn make_mut<'b>(this: &'b mut RcThunk<'a, T>) -> &'b mut T where T: Clone {
        // No, moving it into a temp doesn't help. We just have to trust the CSE
        // pass here.
        if Rc::get_mut(&mut this.0).is_some() {
            return &mut **Rc::get_mut(&mut this.0).unwrap();
        }

        let new_rc = Rc::new(Thunk::computed((*this.0).clone()));
        this.0 = new_rc;
        RcThunk::get_mut(this).unwrap()
    }