Hot reloading for system code

[julhe]

Hot reloading of binaries seems to be a tricky topic (not only in Rust), but the massive iteration time reductions would be worth it.

My idea would be to take advantage of the ECS architecture here. When working with bevy, I noticed that most of my changes would involve systems. My plan would be to separate a bevy project into two different parts/binaries:
* bevy + user components + runner
* main + user systems (hot re loadable)

With this design, all entities, assets, states, etc. would be persistent across sessions. I also think that this wouldn't cause much problems on system side, since they are (mostly?) stateless. Only if the user changes component code, he would need to restart. For release builds, both parts could be statically linked.

After some discussions on the discord, I figured that systems indeed could have (important) states within them (entity change detection, event reader). That would make my initial idea far more difficult.

But the question remains: can we offer any support for hot reloading?

Happy to hear your comments!