struct OnDrop<F: FnOnce()>(Option<F>);
impl<F: FnOnce()> Drop for OnDrop<F> {
    fn drop(&mut self) { if let Some(close) = self.0.take() { close(); } }
}
fn demo(open: impl FnOnce(&str, i32, i32, i32) -> bool, close: impl FnOnce()) {
    if !open("demo", 640, 480, 0) { return; }
    let _cleanup = OnDrop(Some(close));
    // Window work goes here; normal scope exits run the guard.
}

fn main() {
    let closed = std::cell::Cell::new(0);
    demo(|title,w,h,flags| { assert_eq!((title,w,h,flags),("demo",640,480,0)); true }, || closed.set(closed.get()+1));
    assert_eq!(closed.get(),1);
    demo(|_,_,_,_|false, || closed.set(closed.get()+1));
    assert_eq!(closed.get(),1);
}