One-shot systems as Commands -- allow commands to use SystemParams

[joseph-gio]

Objective

Fixes #2192.

Currently, anonymous one-shot commands can be defined as a closure accepting &mut World. Working with exclusive world access is quite cumbersome, and is usually only necessary if a command makes changes to a world's archetypes. In cases where a command simply needs to access and mutate world data, it is much more ergonomic to do so via SystemParams such as ResMut<> or Query.

Solution

• Add the trait CommandSystemParam, which is a subset of SystemParam that excludes types that don't make sense for one-shot systems, such as Local<> or Commands.
• Any systems consisting of CommandSystemParams may be used as a command.

Example

// One-shot systems are run by calling `Commands::add`.
commands.add(|main_character: Res<MainCharacter>,
              mut characters: Query<&mut Character>| {
    let main_character = characters.get_mut(main_character.id()).unwrap();
    main_character.do_thing();
});

---

TODO: Docs. I'd like to see the response to this PR before putting in the effort to write full documentation.

TODO: Look into storing "command systems" in a registry resource, to avoid duplicate initialization code. This will require benchmarking.

Changelog

TODO

Migration Guide

TODO

[joseph-gio]

@alice-i-cecile I don't need a full review or approval yet, but I'd like to get your opinion on this PR before I move forward with it.

[alice-i-cecile]

I agree that we want a commands-powered API for one-shot systems :) Concerns with this approach though:

1. This doesn't cache state, which means a) serious efficiency cost b) no change detection. This was a dealbreaker for Cart in my early experiments, which is what led to the design in Fast and correct one-shot systems with a system registry resource #4090.
2. I think you really want a dedicated collection for systems that are supposed to run at the next command flush, in order to unlock future parallelism.
3. I don't like the addition of yet another trait with CommandSystemParam. With 1 resolved, I don't think it's needed.

Excited to see work towards one-shot systems in one form or another, but I'm not sold on this approach.

[joseph-gio]

1. I think that the efficiency cost is overstated -- this approach has no overhead compared to using SystemState manually inside of a |&mut World| command. Also, I don't think change detection even makes sense within a one-shot system, so this isn't a loss IMO.
2. This is probably a cleaner solution.

[alice-i-cecile]

I think that the efficiency cost is overstated

Generally agreed, but not my requirement :p

Also, I don't think change detection even makes sense within a one-shot system, so this isn't a loss IMO.

Once you start looking at using one shot systems for UI callbacks this makes a lot of sense to me :)