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

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

impl Foo {
    fn foo(&self) {
        self.magic.set(self.magic.get() + 1);
        if let Some(bar) = self.bar.take() {
            bar.doit();
            self.bar.set(Some(bar));
        }
        self.magic.set(self.magic.get() + 1);
    }

    fn wee(&self) {
        assert_eq!(self.magic.get() % 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: Cell::new(0) });
    let bar = Rc::new(Bar { foo: foo.clone() });
    foo.bar.set(Some(bar.clone()));
    foo.foo();
}