#![allow(unused_variables)]
fn main() {
    use std::rc::Rc;
    let mut data = Rc::new(75);
    let weak = Rc::downgrade(&data);
    assert!(75 == * data);
    assert!(75 == * weak.upgrade().unwrap());
    *Rc::make_mut(&mut data) += 1;
    assert!(76 == * data);
    assert!(weak.upgrade().is_none());
}