add utils to FragmentState

similar utils for layout etc should be added later as needed

crates/bevy_core_pipeline/src/bloom/downsampling_pipeline.rs

@@ -124,7 +124,10 @@ impl Specializer<RenderPipeline> for BloomDownsamplingSpecializer {
             "bloom_downsampling_pipeline".into()
         });
 
-        let fragment = descriptor.fragment.get_or_insert_default();
+        // TODO: should this error?
+        let Some(fragment) = &mut descriptor.fragment else {
+            return Ok(key);
+        };
 
         fragment.entry_point = Some(if key.first_downsample {
             "downsample_first".into()

crates/bevy_core_pipeline/src/bloom/upsampling_pipeline.rs

@@ -109,6 +109,10 @@ impl Specializer<RenderPipeline> for BloomUpsamplingSpecializer {
         key: Self::Key,
         descriptor: &mut RenderPipelineDescriptor,
     ) -> Result<Canonical<Self::Key>, BevyError> {
+        let Some(fragment) = &mut descriptor.fragment else {
+            return Ok(key);
+        };
+
         let texture_format = if key.final_pipeline {
             ViewTarget::TEXTURE_FORMAT_HDR
         } else {
@@ -147,17 +151,23 @@ impl Specializer<RenderPipeline> for BloomUpsamplingSpecializer {
             },
         };
 
-        let fragment = descriptor.fragment.get_or_insert_default();
+        let target = ColorTargetState {
+            format: texture_format,
+            blend: Some(BlendState {
+                // placeholder
+                color: color_blend,
+                alpha: BlendComponent {
+                    src_factor: BlendFactor::Zero,
+                    dst_factor: BlendFactor::One,
+                    operation: BlendOperation::Add,
+                },
+            }),
+            write_mask: ColorWrites::ALL,
+        };
 
-        if let Some(Some(color_target)) = fragment.targets.first_mut()
-            && let Some(blend_state) = &mut color_target.blend
-        {
-            blend_state.color = color_blend;
-            color_target.format = texture_format;
-            Ok(key)
-        } else {
-            Err("color target state or blend state missing".into())
-        }
+        fragment.set_target(0, target);
+
+        Ok(key)
     }
 }
 

crates/bevy_render/src/render_resource/pipeline.rs

@@ -7,7 +7,7 @@ use crate::{
 use alloc::borrow::Cow;
 use bevy_asset::Handle;
 use bevy_utils::WgpuWrapper;
-use core::ops::Deref;
+use core::{iter, ops::Deref};
 use wgpu::{
     ColorTargetState, DepthStencilState, MultisampleState, PrimitiveState, PushConstantRange,
 };
@@ -137,6 +137,12 @@ pub struct FragmentState {
     pub targets: Vec<Option<ColorTargetState>>,
 }
 
+impl FragmentState {
+    pub fn set_target(&mut self, index: usize, target: ColorTargetState) {
+        filling_insert_at(&mut self.targets, index, None, Some(target));
+    }
+}
+
 /// Describes a compute pipeline.
 #[derive(Clone, Debug, PartialEq, Eq, Default)]
 pub struct ComputePipelineDescriptor {
@@ -153,3 +159,9 @@ pub struct ComputePipelineDescriptor {
     /// If this is false, reading from workgroup variables before writing to them will result in garbage values.
     pub zero_initialize_workgroup_memory: bool,
 }
+
+fn filling_insert_at<T: Clone>(vec: &mut Vec<T>, index: usize, filler: T, value: T) {
+    let num_to_fill = index.saturating_sub(vec.len() - 1);
+    vec.extend(iter::repeat_n(filler, num_to_fill));
+    vec[index] = value;
+}