observe on WorldEntityMut may cause the reference to be invalidated

[nakedible]

Bevy version

0.15.0-rc.2 (present in 0.14.0) as well

What you did

    #[derive(Event)]
    struct Boom;

    #[derive(Event)]
    struct BoomTomorrow;

    fn boom(trigger: Trigger<Boom>, mut commands: Commands) {
        println!("boom, despawning entity: {:?}", trigger.entity());
        commands.entity(trigger.entity()).despawn();
    }

    fn boom_tomorrow(trigger: Trigger<Boom>, mut commands: Commands) {
        // boom tomorrow
    }

    #[test]
    fn test_boom() {
        let mut app = App::new();

        let mut id = app.world_mut().spawn(Name::new("a")).observe(boom).id();
        let mut id2 = app.world_mut().spawn(Name::new("b")).id();
        
        app.world_mut().flush();
        println!("flushed");
        
        let mut a = app.world_mut().entity_mut(id);
        a.trigger(Boom);
        a.observe(boom_tomorrow);
        let name = a.get::<Name>().unwrap();
        assert_eq!(name.as_str(), "b");
    }

Result

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.00s

What went wrong

• WorldEntityMut depends on the fact that the EntityLocation it caches does not change while it is alive, or changes when it itself expects it to.
• This means that there must be no way to flush the command queue while WorldEntityMut lives, as this might cause queued despawns to be run.
• observe internally calls world.spawn().
• world.spawn() implicitly has a flush() in it, as evidenced in bug report Commands apply at unexpected times with exclusive World access #14621.
• Hence, entity gets despawned while WorldEntityMut exists, which means any accesses will use outdated EntityLocation

Additional information

Also related as context #16034.