Make #[bindless] in ExtendedMaterial actually enable bindless mode. (#16818)

I forgot to set `BINDLESS_SLOT_COUNT` in `ExtendedMaterial`'s
implementation of `AsBindGroup`, so it didn't actually become bindless.
In fact, it would usually crash with a shader/bind group layout
mismatch, because some parts of Bevy's renderer thought that the
resulting material was bindless while other parts didn't. This commit
corrects the situation.

I had to make `BINDLESS_SLOT_COUNT` a function instead of a constant
because the `ExtendedMaterial` version needs some logic. Unfortunately,
trait methods can't be `const fn`s, so it has to be a runtime function.

crates/bevy_pbr/src/extended_material.rs

@@ -150,6 +150,15 @@ impl<B: Material, E: MaterialExtension> AsBindGroup for ExtendedMaterial<B, E> {
     type Data = (<B as AsBindGroup>::Data, <E as AsBindGroup>::Data);
     type Param = (<B as AsBindGroup>::Param, <E as AsBindGroup>::Param);
 
+    fn bindless_slot_count() -> Option<u32> {
+        match (B::bindless_slot_count(), E::bindless_slot_count()) {
+            (Some(base_bindless_slot_count), Some(extension_bindless_slot_count)) => {
+                Some(base_bindless_slot_count.min(extension_bindless_slot_count))
+            }
+            _ => None,
+        }
+    }
+
     fn unprepared_bind_group(
         &self,
         layout: &BindGroupLayout,
@@ -159,9 +168,7 @@ impl<B: Material, E: MaterialExtension> AsBindGroup for ExtendedMaterial<B, E> {
     ) -> Result<UnpreparedBindGroup<Self::Data>, AsBindGroupError> {
         // Only allow bindless mode if both the base material and the extension
         // support it.
-        force_no_bindless = force_no_bindless
-            || B::BINDLESS_SLOT_COUNT.is_none()
-            || E::BINDLESS_SLOT_COUNT.is_none();
+        force_no_bindless = force_no_bindless || Self::bindless_slot_count().is_none();
 
         // add together the bindings of the base material and the user material
         let UnpreparedBindGroup {
@@ -199,9 +206,7 @@ impl<B: Material, E: MaterialExtension> AsBindGroup for ExtendedMaterial<B, E> {
     {
         // Only allow bindless mode if both the base material and the extension
         // support it.
-        force_no_bindless = force_no_bindless
-            || B::BINDLESS_SLOT_COUNT.is_none()
-            || E::BINDLESS_SLOT_COUNT.is_none();
+        force_no_bindless = force_no_bindless || Self::bindless_slot_count().is_none();
 
         // add together the bindings of the standard material and the user material
         let mut entries = B::bind_group_layout_entries(render_device, force_no_bindless);

crates/bevy_pbr/src/material_bind_groups.rs

@@ -440,7 +440,7 @@ where
 {
     /// Returns a new bind group.
     fn new() -> MaterialBindlessBindGroup<M> {
-        let count = M::BINDLESS_SLOT_COUNT.unwrap_or(1);
+        let count = M::bindless_slot_count().unwrap_or(1);
 
         MaterialBindlessBindGroup {
             bind_group: None,
@@ -789,7 +789,7 @@ pub fn material_uses_bindless_resources<M>(render_device: &RenderDevice) -> bool
 where
     M: Material,
 {
-    M::BINDLESS_SLOT_COUNT.is_some()
+    M::bindless_slot_count().is_some()
         && render_device
             .features()
             .contains(WgpuFeatures::BUFFER_BINDING_ARRAY | WgpuFeatures::TEXTURE_BINDING_ARRAY)

crates/bevy_render/macros/src/as_bind_group.rs

@@ -567,7 +567,11 @@ pub fn derive_as_bind_group(ast: syn::DeriveInput) -> Result<TokenStream> {
     // limitations into account.
     let (bindless_slot_count, actual_bindless_slot_count_declaration) = match attr_bindless_count {
         Some(bindless_count) => (
-            quote! { const BINDLESS_SLOT_COUNT: Option<u32> = Some(#bindless_count); },
+            quote! {
+                fn bindless_slot_count() -> Option<u32> {
+                    Some(#bindless_count)
+                }
+            },
             quote! {
                 let #actual_bindless_slot_count = if render_device.features().contains(
                     #render_path::settings::WgpuFeatures::BUFFER_BINDING_ARRAY |

crates/bevy_render/src/render_resource/bind_group.rs

@@ -337,7 +337,9 @@ pub trait AsBindGroup {
     /// Note that the *actual* slot count may be different from this value, due
     /// to platform limitations. For example, if bindless resources aren't
     /// supported on this platform, the actual slot count will be 1.
-    const BINDLESS_SLOT_COUNT: Option<u32> = None;
+    fn bindless_slot_count() -> Option<u32> {
+        None
+    }
 
     /// label
     fn label() -> Option<&'static str> {