use std::marker::PhantomData;

struct Inner<'a> {
    _phantom: PhantomData<&'a ()>,
}
impl<'a> Inner<'a> {
    fn do_a_thing(&mut self) {}
}

struct Outer<'a> {
    x: &'a mut Inner<'a>,
}
impl<'a> Outer<'a> {
    fn act_on_inner(&mut self) {
        self.x.do_a_thing();
    }
}

fn f<'a>(x: &'a mut Inner<'a>) {
    let mut outer = Outer { x: x };
    outer.act_on_inner();
}

fn main() {
    let mut x = Inner {
        _phantom: PhantomData,
    };
    f(&mut x);
    f(&mut x);
}