(step 1 from post) Rebuilding Bevy system functions

playground.rs

// ------------------
// The game code
// ------------------

fn main() {
    App::new().add_system(example_system).run();
}

fn example_system() {
    println!("foo");
}

// ------------------
// App boilerplate
// ------------------


struct App {
    systems: Vec<Box<dyn System>>,
}

impl App {
    fn new() -> App {
        App {
            systems: Vec::new(),
        }
    }

    fn add_system<S: System>(mut self, function: S) -> Self {
        self.systems.push(Box::new(function));
        self
    }

    fn run(&mut self) {
        for system in &mut self.systems {
            system.run();
        }
    }
}

// ------------------
// Systems magic
// ------------------

trait System: 'static {
    fn run(&mut self);
}

impl<F> System for F
where
    F: Fn() -> () + 'static,
{
    fn run(&mut self) {
        self();
    }
}