struct Foo<T, C> where C: Fn() -> T {
    make_t: C,
    value: Option<T>,
}

impl<T, C> Foo<T, C> where C: Fn() -> T {
    fn new(make_t: C) -> Foo<T, C> {
        Foo {
            make_t,
            value: None
        }
    }

    fn value(&mut self) -> &T {
        self.value
            .get_or_insert_with(&self.make_t)
    }
}

fn main() {
    let mut bar = Foo::new(|| 3);

    println!("{}", bar.value());
}