struct Foo {
    a: Option<String>,
}

fn bar(a: &mut Foo, b: &mut String) {
    unimplemented!()
}

impl Foo {
    fn bar(&mut self) {
        if let Some(mut a) = self.a.take() {
            bar(self, &mut a);
            self.a.replace(a);
        }
    }
}