struct Foo {
    n: u32,
}

impl Foo {
    fn bar(&mut self) {
        println!("{}", self.n)
    }
}

struct Caller<F, G: FnMut(&mut F)> {
    f: G,
    _ghost: std::marker::PhantomData<F>
}

fn main() {
    let f = Foo { n: 100 };
    let c = Caller::<Foo, _>{ f: |f| f.bar(), _ghost: std::marker::PhantomData };
}