use std::rc::Rc;
use std::cell::Cell;

struct Foo {
    bar: Cell<Option<Rc<Bar>>>,
    magic: usize
}

impl Foo {
    fn foo(&mut self) {
        self.magic +=1;
        let tempvalue = self.bar.take();
        tempvalue.unwrap().doit();
        self.bar.set(tempvalue);
        self.magic +=1;
    }

    fn wee(&self) {
        assert_eq!(self.magic %2, 0);
    }
}

struct Bar {
    foo: Rc<Foo>,
}

impl Bar {
    fn doit(self) {
        self.foo.wee();
    }
}

fn main() {
    let foo = Rc::new(Foo { bar: Cell::new(None), magic:0 });
    let bar = Rc::new(Bar { foo: foo.clone() });
    foo.bar.set(Some(bar.clone()));
    foo.foo();
}