When running the program in chapter 8, “Implementing Combat”, the printout Health before attack was never printed so I assumed something was wrong in the call chain.

How would I go about debugging this call chain (ecs.entry_mut(*victim).unwrap()...)?

#[system]
#[read_component(WantsToAttack)]
#[read_component(Health)]
pub fn combat(ecs: &mut SubWorld, commands: &mut CommandBuffer) {
    ...
    victims.iter().for_each(|(message, victim)| {
        if let Ok(mut health) = ecs
            .entry_mut(*victim)
            .unwrap()
            .get_component_mut::<Health>()
        {
            println!("Health before attack: {}", health.current);
            ...
        }
        commands.remove(*message);
    });
}

After some comparing of source code, I found out that I had actually written read_component(Health) instead of write_component(Health).

#[system]
#[read_component(WantsToAttack)]
#[write_component(Health)]
pub fn combat(ecs: &mut SubWorld, commands: &mut CommandBuffer) {
    ...
}

This is my first encounter with Rust but I was surprised that this error slipped by the compiler. I expected similar error as if I hade left out some mut…