struct Callback<B: Extend<String>>(pub fn(buffer: &mut B));

fn force_only_extend<T, E: Extend<T>>(e: &mut E) -> &mut impl Extend<T> {
    e
}

fn foo(buffer: &mut impl Extend<String>) {
    buffer.extend(Some("foo".to_string()))
}

fn bar(buffer: &mut impl Extend<String>) {
    buffer.extend(Some("bar".to_string()))
}

fn main() {
    let cb = Callback(foo);
    let mut v = vec!["hello".to_string()];
    cb.0(force_only_extend(&mut v));
    dbg!(&v);
    let cb = Callback(bar);
    cb.0(force_only_extend(&mut v));
    dbg!(&v);
}