Resources as Components Tracking Issue

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This issue does not exist to argue the merits of Resources as Components, for some of the rationale we refer to this HackMD (this really should've been an RFC, huh?). This issue exists to track the progress on implementation and to reference in code when it is only partially working. If there are steps you feel like should be added, or if there are problems with this approach, please let me know.

Implementation Plan

This is a tracking issue for the Resources as Components. This outline is roughly copied from #17485.

• Test robustness, to be less reliant on specific entity counts (Improve test robustness #20248, Adjust benches to be more robust #20207) . We deprecate world.iter_entities() and world._iter_entities_mut() as part of that effort Deprecate iter_entities and iter_entities_mut #20260.
• Reintroduce iter_entities() Revive iter_entities() when you only have immutable World access ❤️‍🩹  #22864.
• Implement resources as components through a custom #[derive(Resource)] macro Store Resources as components on singleton entities #20934.
• Gut ReflectResource Store Resources as components on singleton entities #20934.
• Rename non-send resources to non-send data Store Resources as components on singleton entities #20934.
• Remove resource/component redundancies in component registration Remove component/resource redundancies #22930.
• Add component features to the resource macro, Resource hooks & immutable resources #24164.
• Change clear_all to also remove non-send data clear_all() clears all (non-send data) #22911 .
• Eliminate resources from Access, Remove resources from Access #22910 .
• Speed up resource lookup, Store resources on sparse sets #24077, Cut down on register_component codegen #22919, Components as entities #23988, Resource storage #24058.
• Remove resources from bevy_scene Remove resources from scenes #22968 .
• Create a cleaner component_scope / resource_scope implementation.

Issues

• Resource trait API is dysfunctional when the trait is implemented manually #24686 Manually implementing Resource on top of Component causes unexpected behaviour.
• Types can no longer be effectively used as both resources and components #24592 Resources can no longer be used as Components, which can be usefull.
• Inserting a resource as a component should not make it available as a resource #24591 Using the component API on resources can cause confusing behaviour.
  • Example from Discord: world.spawn((ResourceA::default(), ResourceB::default())
• Components that are added to resource entities, are not serialized (bevy_world_serialization), as per Remove resources from scenes #22968.