type RenderData = u32;

struct Manager {
    #[allow(dead_code)]
    num_entities: usize,
    render_data: Vec<Option<RenderData>>
}

pub fn main() {

    let mut entity_manager = Manager {
        num_entities: 3,
        render_data: vec![Some(5), Some(2), None]
    };

    // Inside my game loop...
    for data in entity_manager.render_data.iter_mut().filter_map(Option::as_mut) {
         *data += 1;
         println!("{:?}", data);
    }
}