Make RenderStage::Extract run on the render world (#4402)

# Objective

- Currently, the `Extract` `RenderStage` is executed on the main world, with the render world available as a resource.
- However, when needing access to resources in the render world (e.g. to mutate them), the only way to do so was to get exclusive access to the whole `RenderWorld` resource.
- This meant that effectively only one extract which wrote to resources could run at a time.
- We didn't previously make `Extract`ing writing to the world a non-happy path, even though we want to discourage that.

## Solution

- Move the extract stage to run on the render world.
- Add the main world as a `MainWorld` resource.
- Add an `Extract` `SystemParam` as a convenience to access a (read only) `SystemParam` in the main world during `Extract`.

## Future work

It should be possible to avoid needing to use `get_or_spawn` for the render commands, since now the `Commands`' `Entities` matches up with the world being executed on.
We need to determine how this interacts with #3519
It's theoretically possible to remove the need for the `value` method on `Extract`. However, that requires slightly changing the `SystemParam` interface, which would make it more complicated. That would probably mess up the `SystemState` api too.

## Todo
I still need to add doc comments to `Extract`.

---

## Changelog

### Changed
- The `Extract` `RenderStage` now runs on the render world (instead of the main world as before).
   You must use the `Extract` `SystemParam` to access the main world during the extract phase.
   Resources on the render world can now be accessed using `ResMut` during extract.

### Removed
- `Commands::spawn_and_forget`. Use `Commands::get_or_spawn(e).insert_bundle(bundle)` instead

## Migration Guide

The `Extract` `RenderStage` now runs on the render world (instead of the main world as before).
You must use the `Extract` `SystemParam` to access the main world during the extract phase. `Extract` takes a single type parameter, which is any system parameter (such as `Res`, `Query` etc.). It will extract this from the main world, and returns the result of this extraction when `value` is called on it.

For example, if previously your extract system looked like:
```rust
fn extract_clouds(mut commands: Commands, clouds: Query<Entity, With<Cloud>>) {
    for cloud in clouds.iter() {
        commands.get_or_spawn(cloud).insert(Cloud);
    }
}
```
the new version would be:
```rust
fn extract_clouds(mut commands: Commands, mut clouds: Extract<Query<Entity, With<Cloud>>>) {
    for cloud in clouds.value().iter() {
        commands.get_or_spawn(cloud).insert(Cloud);
    }
}
```
The diff is:
```diff
--- a/src/clouds.rs
+++ b/src/clouds.rs
@@ -1,5 +1,5 @@
-fn extract_clouds(mut commands: Commands, clouds: Query<Entity, With<Cloud>>) {
-    for cloud in clouds.iter() {
+fn extract_clouds(mut commands: Commands, mut clouds: Extract<Query<Entity, With<Cloud>>>) {
+    for cloud in clouds.value().iter() {
         commands.get_or_spawn(cloud).insert(Cloud);
     }
 }
```
You can now also access resources from the render world using the normal system parameters during `Extract`:
```rust
fn extract_assets(mut render_assets: ResMut<MyAssets>, source_assets: Extract<Res<MyAssets>>) {
     *render_assets = source_assets.clone();
}
```
Please note that all existing extract systems need to be updated to match this new style; even if they currently compile they will not run as expected. A warning will be emitted on a best-effort basis if this is not met.

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_core_pipeline/src/core_2d/mod.rs

@@ -25,7 +25,7 @@ use bevy_render::{
         DrawFunctionId, DrawFunctions, EntityPhaseItem, PhaseItem, RenderPhase,
     },
     render_resource::CachedRenderPipelineId,
-    RenderApp, RenderStage,
+    Extract, RenderApp, RenderStage,
 };
 use bevy_utils::FloatOrd;
 use std::ops::Range;
@@ -123,7 +123,7 @@ impl BatchedPhaseItem for Transparent2d {
 
 pub fn extract_core_2d_camera_phases(
     mut commands: Commands,
-    cameras_2d: Query<(Entity, &Camera), With<Camera2d>>,
+    cameras_2d: Extract<Query<(Entity, &Camera), With<Camera2d>>>,
 ) {
     for (entity, camera) in cameras_2d.iter() {
         if camera.is_active {

crates/bevy_core_pipeline/src/core_3d/mod.rs

@@ -34,7 +34,7 @@ use bevy_render::{
     renderer::RenderDevice,
     texture::TextureCache,
     view::ViewDepthTexture,
-    RenderApp, RenderStage,
+    Extract, RenderApp, RenderStage,
 };
 use bevy_utils::{FloatOrd, HashMap};
 
@@ -208,7 +208,7 @@ impl CachedRenderPipelinePhaseItem for Transparent3d {
 
 pub fn extract_core_3d_camera_phases(
     mut commands: Commands,
-    cameras_3d: Query<(Entity, &Camera), With<Camera3d>>,
+    cameras_3d: Extract<Query<(Entity, &Camera), With<Camera3d>>>,
 ) {
     for (entity, camera) in cameras_3d.iter() {
         if camera.is_active {

crates/bevy_ecs/src/schedule/stage.rs

@@ -12,7 +12,10 @@ use crate::{
     },
     world::{World, WorldId},
 };
-use bevy_utils::{tracing::info, HashMap, HashSet};
+use bevy_utils::{
+    tracing::{info, warn},
+    HashMap, HashSet,
+};
 use downcast_rs::{impl_downcast, Downcast};
 use fixedbitset::FixedBitSet;
 use std::fmt::Debug;
@@ -88,6 +91,7 @@ pub struct SystemStage {
     last_tick_check: u32,
     /// If true, buffers will be automatically applied at the end of the stage. If false, buffers must be manually applied.
     apply_buffers: bool,
+    must_read_resource: Option<ComponentId>,
 }
 
 impl SystemStage {
@@ -110,6 +114,7 @@ impl SystemStage {
             uninitialized_at_end: vec![],
             last_tick_check: Default::default(),
             apply_buffers: true,
+            must_read_resource: None,
         }
     }
 
@@ -139,6 +144,10 @@ impl SystemStage {
         self.executor = executor;
     }
 
+    pub fn set_must_read_resource(&mut self, resource_id: ComponentId) {
+        self.must_read_resource = Some(resource_id);
+    }
+
     #[must_use]
     pub fn with_system<Params>(mut self, system: impl IntoSystemDescriptor<Params>) -> Self {
         self.add_system(system);
@@ -563,6 +572,20 @@ impl SystemStage {
         }
     }
 
+    fn check_uses_resource(&self, resource_id: ComponentId, world: &World) {
+        debug_assert!(!self.systems_modified);
+        for system in &self.parallel {
+            let access = system.component_access().unwrap();
+            if !access.has_read(resource_id) {
+                let component_name = world.components().get_info(resource_id).unwrap().name();
+                warn!(
+                    "System {} doesn't access resource {component_name}, despite being required to",
+                    system.name()
+                );
+            }
+        }
+    }
+
     /// All system and component change ticks are scanned once the world counter has incremented
     /// at least [`CHECK_TICK_THRESHOLD`](crate::change_detection::CHECK_TICK_THRESHOLD)
     /// times since the previous `check_tick` scan.
@@ -782,6 +805,9 @@ impl Stage for SystemStage {
             if world.contains_resource::<ReportExecutionOrderAmbiguities>() {
                 self.report_ambiguities(world);
             }
+            if let Some(resource_id) = self.must_read_resource {
+                self.check_uses_resource(resource_id, world);
+            }
         } else if self.executor_modified {
             self.executor.rebuild_cached_data(&self.parallel);
             self.executor_modified = false;

crates/bevy_ecs/src/system/commands/mod.rs

@@ -145,12 +145,6 @@ impl<'w, 's> Commands<'w, 's> {
         }
     }
 
-    /// Spawns a [`Bundle`] without pre-allocating an [`Entity`]. The [`Entity`] will be allocated
-    /// when this [`Command`] is applied.
-    pub fn spawn_and_forget(&mut self, bundle: impl Bundle) {
-        self.queue.push(Spawn { bundle });
-    }
-
     /// Creates a new entity with the components contained in `bundle`.
     ///
     /// This returns an [`EntityCommands`] builder, which enables inserting more components and

crates/bevy_pbr/src/material.rs

@@ -34,7 +34,7 @@ use bevy_render::{
     renderer::RenderDevice,
     texture::FallbackImage,
     view::{ExtractedView, Msaa, VisibleEntities},
-    RenderApp, RenderStage,
+    Extract, RenderApp, RenderStage,
 };
 use bevy_utils::{tracing::error, HashMap, HashSet};
 use std::hash::Hash;
@@ -455,15 +455,15 @@ pub type RenderMaterials<T> = HashMap<Handle<T>, PreparedMaterial<T>>;
 /// into the "render world".
 fn extract_materials<M: Material>(
     mut commands: Commands,
-    mut events: EventReader<AssetEvent<M>>,
-    assets: Res<Assets<M>>,
+    mut events: Extract<EventReader<AssetEvent<M>>>,
+    assets: Extract<Res<Assets<M>>>,
 ) {
     let mut changed_assets = HashSet::default();
     let mut removed = Vec::new();
     for event in events.iter() {
         match event {
             AssetEvent::Created { handle } | AssetEvent::Modified { handle } => {
-                changed_assets.insert(handle);
+                changed_assets.insert(handle.clone_weak());
             }
             AssetEvent::Removed { handle } => {
                 changed_assets.remove(handle);
@@ -474,8 +474,8 @@ fn extract_materials<M: Material>(
 
     let mut extracted_assets = Vec::new();
     for handle in changed_assets.drain() {
-        if let Some(asset) = assets.get(handle) {
-            extracted_assets.push((handle.clone_weak(), asset.clone()));
+        if let Some(asset) = assets.get(&handle) {
+            extracted_assets.push((handle, asset.clone()));
         }
     }
 

crates/bevy_pbr/src/render/light.rs

@@ -28,6 +28,7 @@ use bevy_render::{
     view::{
         ExtractedView, ViewUniform, ViewUniformOffset, ViewUniforms, Visibility, VisibleEntities,
     },
+    Extract,
 };
 use bevy_transform::components::GlobalTransform;
 use bevy_utils::FloatOrd;
@@ -386,7 +387,10 @@ pub struct ExtractedClustersPointLights {
     data: Vec<VisiblePointLights>,
 }
 
-pub fn extract_clusters(mut commands: Commands, views: Query<(Entity, &Clusters), With<Camera>>) {
+pub fn extract_clusters(
+    mut commands: Commands,
+    views: Extract<Query<(Entity, &Clusters), With<Camera>>>,
+) {
     for (entity, clusters) in views.iter() {
         commands.get_or_spawn(entity).insert_bundle((
             ExtractedClustersPointLights {
@@ -404,20 +408,22 @@ pub fn extract_clusters(mut commands: Commands, views: Query<(Entity, &Clusters)
 #[allow(clippy::too_many_arguments)]
 pub fn extract_lights(
     mut commands: Commands,
-    point_light_shadow_map: Res<PointLightShadowMap>,
-    directional_light_shadow_map: Res<DirectionalLightShadowMap>,
-    global_point_lights: Res<GlobalVisiblePointLights>,
-    mut point_lights: Query<(&PointLight, &mut CubemapVisibleEntities, &GlobalTransform)>,
-    mut spot_lights: Query<(&SpotLight, &mut VisibleEntities, &GlobalTransform)>,
-    mut directional_lights: Query<
-        (
-            Entity,
-            &DirectionalLight,
-            &mut VisibleEntities,
-            &GlobalTransform,
-            &Visibility,
-        ),
-        Without<SpotLight>,
+    point_light_shadow_map: Extract<Res<PointLightShadowMap>>,
+    directional_light_shadow_map: Extract<Res<DirectionalLightShadowMap>>,
+    global_point_lights: Extract<Res<GlobalVisiblePointLights>>,
+    point_lights: Extract<Query<(&PointLight, &CubemapVisibleEntities, &GlobalTransform)>>,
+    spot_lights: Extract<Query<(&SpotLight, &VisibleEntities, &GlobalTransform)>>,
+    directional_lights: Extract<
+        Query<
+            (
+                Entity,
+                &DirectionalLight,
+                &VisibleEntities,
+                &GlobalTransform,
+                &Visibility,
+            ),
+            Without<SpotLight>,
+        >,
     >,
     mut previous_point_lights_len: Local<usize>,
     mut previous_spot_lights_len: Local<usize>,
@@ -441,10 +447,10 @@ pub fn extract_lights(
 
     let mut point_lights_values = Vec::with_capacity(*previous_point_lights_len);
     for entity in global_point_lights.iter().copied() {
-        if let Ok((point_light, cubemap_visible_entities, transform)) = point_lights.get_mut(entity)
-        {
-            let render_cubemap_visible_entities =
-                std::mem::take(cubemap_visible_entities.into_inner());
+        if let Ok((point_light, cubemap_visible_entities, transform)) = point_lights.get(entity) {
+            // TODO: This is very much not ideal. We should be able to re-use the vector memory.
+            // However, since exclusive access to the main world in extract is ill-advised, we just clone here.
+            let render_cubemap_visible_entities = cubemap_visible_entities.clone();
             point_lights_values.push((
                 entity,
                 (
@@ -475,8 +481,10 @@ pub fn extract_lights(
 
     let mut spot_lights_values = Vec::with_capacity(*previous_spot_lights_len);
     for entity in global_point_lights.iter().copied() {
-        if let Ok((spot_light, visible_entities, transform)) = spot_lights.get_mut(entity) {
-            let render_visible_entities = std::mem::take(visible_entities.into_inner());
+        if let Ok((spot_light, visible_entities, transform)) = spot_lights.get(entity) {
+            // TODO: This is very much not ideal. We should be able to re-use the vector memory.
+            // However, since exclusive access to the main world in extract is ill-advised, we just clone here.
+            let render_visible_entities = visible_entities.clone();
             let texel_size =
                 2.0 * spot_light.outer_angle.tan() / directional_light_shadow_map.size as f32;
 
@@ -512,7 +520,7 @@ pub fn extract_lights(
     commands.insert_or_spawn_batch(spot_lights_values);
 
     for (entity, directional_light, visible_entities, transform, visibility) in
-        directional_lights.iter_mut()
+        directional_lights.iter()
     {
         if !visibility.is_visible {
             continue;
@@ -530,7 +538,8 @@ pub fn extract_lights(
             );
         let directional_light_texel_size =
             largest_dimension / directional_light_shadow_map.size as f32;
-        let render_visible_entities = std::mem::take(visible_entities.into_inner());
+        // TODO: As above
+        let render_visible_entities = visible_entities.clone();
         commands.get_or_spawn(entity).insert_bundle((
             ExtractedDirectionalLight {
                 color: directional_light.color,

crates/bevy_pbr/src/render/mesh.rs

@@ -25,7 +25,7 @@ use bevy_render::{
         BevyDefault, DefaultImageSampler, GpuImage, Image, ImageSampler, TextureFormatPixelInfo,
     },
     view::{ComputedVisibility, ViewUniform, ViewUniformOffset, ViewUniforms},
-    RenderApp, RenderStage,
+    Extract, RenderApp, RenderStage,
 };
 use bevy_transform::components::GlobalTransform;
 use std::num::NonZeroU64;
@@ -118,14 +118,16 @@ pub fn extract_meshes(
     mut commands: Commands,
     mut prev_caster_commands_len: Local<usize>,
     mut prev_not_caster_commands_len: Local<usize>,
-    meshes_query: Query<(
-        Entity,
-        &ComputedVisibility,
-        &GlobalTransform,
-        &Handle<Mesh>,
-        Option<With<NotShadowReceiver>>,
-        Option<With<NotShadowCaster>>,
-    )>,
+    meshes_query: Extract<
+        Query<(
+            Entity,
+            &ComputedVisibility,
+            &GlobalTransform,
+            &Handle<Mesh>,
+            Option<With<NotShadowReceiver>>,
+            Option<With<NotShadowCaster>>,
+        )>,
+    >,
 ) {
     let mut caster_commands = Vec::with_capacity(*prev_caster_commands_len);
     let mut not_caster_commands = Vec::with_capacity(*prev_not_caster_commands_len);
@@ -202,12 +204,12 @@ impl SkinnedMeshJoints {
 }
 
 pub fn extract_skinned_meshes(
-    query: Query<(Entity, &ComputedVisibility, &SkinnedMesh)>,
-    inverse_bindposes: Res<Assets<SkinnedMeshInverseBindposes>>,
-    joint_query: Query<&GlobalTransform>,
     mut commands: Commands,
     mut previous_len: Local<usize>,
     mut previous_joint_len: Local<usize>,
+    query: Extract<Query<(Entity, &ComputedVisibility, &SkinnedMesh)>>,
+    inverse_bindposes: Extract<Res<Assets<SkinnedMeshInverseBindposes>>>,
+    joint_query: Extract<Query<&GlobalTransform>>,
 ) {
     let mut values = Vec::with_capacity(*previous_len);
     let mut joints = Vec::with_capacity(*previous_joint_len);

crates/bevy_render/src/camera/camera.rs

@@ -4,6 +4,7 @@ use crate::{
     render_asset::RenderAssets,
     render_resource::TextureView,
     view::{ExtractedView, ExtractedWindows, VisibleEntities},
+    Extract,
 };
 use bevy_asset::{AssetEvent, Assets, Handle};
 use bevy_derive::{Deref, DerefMut};
@@ -393,13 +394,15 @@ pub struct ExtractedCamera {
 
 pub fn extract_cameras(
     mut commands: Commands,
-    query: Query<(
-        Entity,
-        &Camera,
-        &CameraRenderGraph,
-        &GlobalTransform,
-        &VisibleEntities,
-    )>,
+    query: Extract<
+        Query<(
+            Entity,
+            &Camera,
+            &CameraRenderGraph,
+            &GlobalTransform,
+            &VisibleEntities,
+        )>,
+    >,
 ) {
     for (entity, camera, camera_render_graph, transform, visible_entities) in query.iter() {
         if !camera.is_active {

crates/bevy_render/src/extract_component.rs

@@ -2,15 +2,15 @@ use crate::{
     render_resource::{encase::internal::WriteInto, DynamicUniformBuffer, ShaderType},
     renderer::{RenderDevice, RenderQueue},
     view::ComputedVisibility,
-    RenderApp, RenderStage,
+    Extract, RenderApp, RenderStage,
 };
 use bevy_app::{App, Plugin};
 use bevy_asset::{Asset, Handle};
 use bevy_ecs::{
     component::Component,
     prelude::*,
-    query::{QueryItem, WorldQuery},
-    system::{lifetimeless::Read, StaticSystemParam},
+    query::{QueryItem, ReadOnlyWorldQuery, WorldQuery},
+    system::lifetimeless::Read,
 };
 use std::{marker::PhantomData, ops::Deref};
 
@@ -34,9 +34,9 @@ impl<C: Component> DynamicUniformIndex<C> {
 /// in the [`RenderStage::Extract`](crate::RenderStage::Extract) step.
 pub trait ExtractComponent: Component {
     /// ECS [`WorldQuery`] to fetch the components to extract.
-    type Query: WorldQuery;
+    type Query: WorldQuery + ReadOnlyWorldQuery;
     /// Filters the entities with additional constraints.
-    type Filter: WorldQuery;
+    type Filter: WorldQuery + ReadOnlyWorldQuery;
     /// Defines how the component is transferred into the "render world".
     fn extract_component(item: QueryItem<Self::Query>) -> Self;
 }
@@ -182,7 +182,7 @@ impl<T: Asset> ExtractComponent for Handle<T> {
 fn extract_components<C: ExtractComponent>(
     mut commands: Commands,
     mut previous_len: Local<usize>,
-    mut query: StaticSystemParam<Query<(Entity, C::Query), C::Filter>>,
+    mut query: Extract<Query<(Entity, C::Query), C::Filter>>,
 ) {
     let mut values = Vec::with_capacity(*previous_len);
     for (entity, query_item) in query.iter_mut() {
@@ -196,7 +196,7 @@ fn extract_components<C: ExtractComponent>(
 fn extract_visible_components<C: ExtractComponent>(
     mut commands: Commands,
     mut previous_len: Local<usize>,
-    mut query: StaticSystemParam<Query<(Entity, Read<ComputedVisibility>, C::Query), C::Filter>>,
+    mut query: Extract<Query<(Entity, &ComputedVisibility, C::Query), C::Filter>>,
 ) {
     let mut values = Vec::with_capacity(*previous_len);
     for (entity, computed_visibility, query_item) in query.iter_mut() {