Add RunSystem

[Zeenobit]

Add a RunSystem extension trait to allow for immediate execution of systems on a World for debugging and/or testing purposes.

Objective

Fixes #6184

Initially, I made this CL as ApplyCommands. After a discussion with @cart , we decided a more generic implementation would be better to support all systems. This is the new revised CL. Sorry for the long delay! 😅

This CL allows users to do this:

use bevy::prelude::*;
use bevy::ecs::system::RunSystem;

struct T(usize);

impl Resource for T {}

fn system(In(n): In<usize>, mut commands: Commands) -> usize {
    commands.insert_resource(T(n));
    n + 1
}

let mut world = World::default();
let n = world.run_system_with(1, system);
assert_eq!(n, 2);
assert_eq!(world.resource::<T>().0, 1);

Solution

This is implemented as a trait extension and not included in any preludes to ensure it's being used consciously.
Internally, it just initializes and runs a systems, and applies any deferred parameters all "in place".
The trait has 2 functions (one of which calls the other by default):

• run_system_with is the general implementation, which allows user to pass system input parameters
• run_system is the ergonomic wrapper for systems with no input parameter (to avoid having the user pass () as input).

Additionally, this trait is also implemented for &mut App. I added this mainly for ergonomics (app.run_system vs. app.world.run_system). (Removed based on feedback)

[killercup]

Looks like a neat helper for tests.

[hymm]

I like this api much better than the previous pr.

Not a huge fan of implementing on &mut App just for decreased verbosity. A little worse ergonomics for tests is fine and we're polluting the public api. But not worth blocking on.

[Zeenobit]

Not a huge fan of implementing on &mut App just for decreased verbosity

I was debating that myself. Especially because compared to the previous PR, with this one you don't really need the app anymore to test systems, and in the few cases where we do want the app, using app.world.run_system vs. app.run_system isn't that different.

So I think you're right. I'll remove the implementation for &mut App.