trait Barable {
    fn new() -> Self;
    // etc.
}

struct Bar;

impl Barable for Bar {
    fn new() -> Self {
        Bar
    }
}

struct Foo<B :Barable> {
    bar: B,
    callback: fn(&B),
}

impl<B: Barable> Foo<B> {
    fn new(callback: fn(&B)) -> Self {
        Foo{
            bar: B::new(),
            callback: callback
        }
    }
}

fn example_callback(bar: &Bar) {
}

fn main() {
    let foo = Foo::new(example_callback);
}