Objective

• While working on Extract extract to bevy_extract #22852 , I found AppLabel needed multiple additional bounds ( the largest being L: AppLabel + Default + Clone + Eq + Copy , in total 44 times) - this greatly reduces the readability of that PR

e.g. I want to change ExtractComponentPlugin to be generic over L: AppLabel + Default, so that I can do the following:

-        if let Some(render_app) = app.get_sub_app_mut(RenderApp) {
+        if let Some(render_app) = app.get_sub_app_mut(L::default()) {

Eq is needed to by EntityEquivalent to make

unsafe impl<L: AppLabel> EntityEquivalent for RenderEntity<L> {}

Clone is needed to make SyncToRenderWorld generic

Copy is needed to make Plugin for SyncWorldPlugin generic

If its desired, I can remove the extra constraints, and just have pub trait AppLabel: AppLabelInterior {} as a placeholder, and add requirements as... required.

• Note: I tried adding this to AppLabel directly, but AppLabel is created by bevy_ecs::define_label! , and I was unable to add the above there

Solution

• Use AppLabelInterior as a derived label, that have a super trait AppLabel with the additional constraints.

Testing

• CI