Fix dependency of shadow mapping on the optional PrepassPlugin (#7878)

# Objective

Unfortunately, there are three issues with my changes introduced by #7784.

1.  The changes left some dead code. This is already taken care of here: #7875.
2. Disabling prepass causes failures because the shadow mapping relies on the `PrepassPlugin` now.
3. Custom materials use the `prepass.wgsl` shader, but this does not always define a fragment entry point.

This PR fixes 2. and 3. and resolves #7879.

## Solution

- Add a regression test with disabled prepass.
- Split `PrepassPlugin` into two plugins:
  - `PrepassPipelinePlugin` contains the part that is required for the shadow mapping to work and is unconditionally added.
  - `PrepassPlugin` now only adds the systems and resources required for the "real" prepasses.
- Add a noop fragment entry point to `prepass.wgsl`, used if `NORMAL_PASS` is not defined.


Co-authored-by: Edgar Geier <geieredgar@gmail.com>

Cargo.toml

@@ -651,6 +651,13 @@ description = "Showcases wireframe rendering"
 category = "3D Rendering"
 wasm = true
 
+[[example]]
+name = "no_prepass"
+path = "tests/3d/no_prepass.rs"
+
+[package.metadata.example.no_prepass]
+hidden = true
+
 # Animation
 [[example]]
 name = "animated_fox"

crates/bevy_pbr/src/material.rs

@@ -1,6 +1,7 @@
 use crate::{
     render, AlphaMode, DrawMesh, DrawPrepass, EnvironmentMapLight, MeshPipeline, MeshPipelineKey,
-    MeshUniform, PrepassPlugin, RenderLightSystems, SetMeshBindGroup, SetMeshViewBindGroup, Shadow,
+    MeshUniform, PrepassPipelinePlugin, PrepassPlugin, RenderLightSystems, SetMeshBindGroup,
+    SetMeshViewBindGroup, Shadow,
 };
 use bevy_app::{App, IntoSystemAppConfig, Plugin};
 use bevy_asset::{AddAsset, AssetEvent, AssetServer, Assets, Handle};
@@ -207,6 +208,9 @@ where
                 .add_system(queue_material_meshes::<M>.in_set(RenderSet::Queue));
         }
 
+        // PrepassPipelinePlugin is required for shadow mapping and the optional PrepassPlugin
+        app.add_plugin(PrepassPipelinePlugin::<M>::default());
+
         if self.prepass_enabled {
             app.add_plugin(PrepassPlugin::<M>::default());
         }

crates/bevy_pbr/src/prepass/mod.rs

@@ -59,15 +59,18 @@ pub const PREPASS_BINDINGS_SHADER_HANDLE: HandleUntyped =
 pub const PREPASS_UTILS_SHADER_HANDLE: HandleUntyped =
     HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 4603948296044544);
 
-pub struct PrepassPlugin<M: Material>(PhantomData<M>);
+/// Sets up everything required to use the prepass pipeline.
+///
+/// This does not add the actual prepasses, see [`PrepassPlugin`] for that.
+pub struct PrepassPipelinePlugin<M: Material>(PhantomData<M>);
 
-impl<M: Material> Default for PrepassPlugin<M> {
+impl<M: Material> Default for PrepassPipelinePlugin<M> {
     fn default() -> Self {
         Self(Default::default())
     }
 }
 
-impl<M: Material> Plugin for PrepassPlugin<M>
+impl<M: Material> Plugin for PrepassPipelinePlugin<M>
 where
     M::Data: PartialEq + Eq + Hash + Clone,
 {
@@ -93,6 +96,34 @@ where
             Shader::from_wgsl
         );
 
+        let Ok(render_app) = app.get_sub_app_mut(RenderApp) else {
+                return;
+            };
+
+        render_app
+            .add_system(queue_prepass_view_bind_group::<M>.in_set(RenderSet::Queue))
+            .init_resource::<PrepassPipeline<M>>()
+            .init_resource::<PrepassViewBindGroup>()
+            .init_resource::<SpecializedMeshPipelines<PrepassPipeline<M>>>();
+    }
+}
+
+/// Sets up the prepasses for a [`Material`].
+///
+/// This depends on the [`PrepassPipelinePlugin`].
+pub struct PrepassPlugin<M: Material>(PhantomData<M>);
+
+impl<M: Material> Default for PrepassPlugin<M> {
+    fn default() -> Self {
+        Self(Default::default())
+    }
+}
+
+impl<M: Material> Plugin for PrepassPlugin<M>
+where
+    M::Data: PartialEq + Eq + Hash + Clone,
+{
+    fn build(&self, app: &mut bevy_app::App) {
         let Ok(render_app) = app.get_sub_app_mut(RenderApp) else {
             return;
         };
@@ -104,15 +135,11 @@ where
                     .in_set(RenderSet::Prepare)
                     .after(bevy_render::view::prepare_windows),
             )
-            .add_system(queue_prepass_view_bind_group::<M>.in_set(RenderSet::Queue))
             .add_system(queue_prepass_material_meshes::<M>.in_set(RenderSet::Queue))
             .add_system(sort_phase_system::<Opaque3dPrepass>.in_set(RenderSet::PhaseSort))
             .add_system(sort_phase_system::<AlphaMask3dPrepass>.in_set(RenderSet::PhaseSort))
-            .init_resource::<PrepassPipeline<M>>()
             .init_resource::<DrawFunctions<Opaque3dPrepass>>()
             .init_resource::<DrawFunctions<AlphaMask3dPrepass>>()
-            .init_resource::<PrepassViewBindGroup>()
-            .init_resource::<SpecializedMeshPipelines<PrepassPipeline<M>>>()
             .add_render_command::<Opaque3dPrepass, DrawPrepass<M>>()
             .add_render_command::<AlphaMask3dPrepass, DrawPrepass<M>>();
     }
@@ -254,11 +281,12 @@ where
 
         let vertex_buffer_layout = layout.get_layout(&vertex_attributes)?;
 
-        // The fragment shader is only used when the normal prepass is enabled or the material uses alpha cutoff values
-        let fragment = if key
-            .mesh_key
-            .intersects(MeshPipelineKey::NORMAL_PREPASS | MeshPipelineKey::ALPHA_MASK)
-            || blend_key == MeshPipelineKey::BLEND_PREMULTIPLIED_ALPHA
+        // The fragment shader is only used when the normal prepass is enabled
+        // or the material uses alpha cutoff values and doesn't rely on the standard prepass shader
+        let fragment = if key.mesh_key.contains(MeshPipelineKey::NORMAL_PREPASS)
+            || ((key.mesh_key.contains(MeshPipelineKey::ALPHA_MASK)
+                || blend_key == MeshPipelineKey::BLEND_PREMULTIPLIED_ALPHA)
+                && self.material_fragment_shader.is_some())
         {
             // Use the fragment shader from the material if present
             let frag_shader_handle = if let Some(handle) = &self.material_fragment_shader {

tests/3d/no_prepass.rs

@@ -0,0 +1,11 @@
+//! A test to confirm that `bevy` allows disabling the prepass of the standard material.
+//! This is run in CI to ensure that this doesn't regress again.
+use bevy::{pbr::PbrPlugin, prelude::*};
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins.set(PbrPlugin {
+            prepass_enabled: false,
+        }))
+        .run();
+}