fn foo<T, F, G>(x: &mut T, mut f: F, g: G)
where
    F: FnMut(&T) -> &T,
    G: Fn(&T) -> &T,
{
    {
        f(x);
    }

    g(x);
}

fn main() {
    let mut x = 5;
    foo(&mut x, |a| a, |a| a);
}