fix: reject fragment shader output locations > max_color_attachments limit

Author: ErichDonGubler

CHANGELOG.md

@@ -73,6 +73,12 @@ SamplerDescriptor {
 }
 ```
 
+### Bug Fixes
+
+#### General
+
+- Reject fragment shader output `location`s > `max_color_attachments` limit. By @ErichDonGubler in [#8316](https://github.com/gfx-rs/wgpu/pull/8316).
+
 ## v27.0.2 (2025-10-03)
 
 ### Bug Fixes

tests/tests/wgpu-validation/api/mod.rs

@@ -7,4 +7,5 @@ mod device;
 mod experimental;
 mod external_texture;
 mod instance;
+mod render_pipeline;
 mod texture;

tests/tests/wgpu-validation/api/render_pipeline.rs

@@ -0,0 +1,70 @@
+//! Tests of [`wgpu::RenderPipeline`] and related.
+
+use wgpu_test::fail;
+
+#[test]
+fn reject_fragment_shader_output_over_max_color_attachments() {
+    let (device, _queue) = wgpu::Device::noop(&Default::default());
+
+    // NOTE: Vertex shader is a boring quad. The fragment shader is the interesting part.
+    let source = format!(
+        "\
+@vertex
+fn vert(@builtin(vertex_index) vertex_index : u32) -> @builtin(position) vec4f {{
+    var pos = array<vec2f, 3>(
+        vec2(0.0, 0.5),
+        vec2(-0.5, -0.5),
+        vec2(0.5, -0.5)
+    );
+    return vec4f(pos[vertex_index], 0.0, 1.0);
+}}
+
+@fragment
+fn frag() -> @location({}) vec4f {{
+    return vec4(1.0, 0.0, 0.0, 1.0);
+}}
+",
+        device.limits().max_color_attachments
+    );
+
+    let module = device.create_shader_module(wgpu::ShaderModuleDescriptor {
+        label: None,
+        source: wgpu::ShaderSource::Wgsl(source.into()),
+    });
+    let module = &module;
+
+    fail(
+        &device,
+        || {
+            device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
+                layout: None,
+                label: None,
+                vertex: wgpu::VertexState {
+                    module,
+                    entry_point: None,
+                    compilation_options: Default::default(),
+                    buffers: &[],
+                },
+                fragment: Some(wgpu::FragmentState {
+                    module,
+                    entry_point: None,
+                    compilation_options: Default::default(),
+                    targets: &[Some(wgpu::ColorTargetState {
+                        format: wgpu::TextureFormat::Rgba8Unorm,
+                        blend: None,
+                        write_mask: Default::default(),
+                    })],
+                }),
+                primitive: Default::default(),
+                depth_stencil: None,
+                multisample: Default::default(),
+                multiview: None,
+                cache: None,
+            })
+        },
+        Some(concat!(
+            "Location[8] Float32x4 interpolated as Some(Perspective) ",
+            "with sampling Some(Center)'s index exceeds `max_color_attachments` (8)"
+        )),
+    );
+}

wgpu-core/src/validation.rs

@@ -313,6 +313,14 @@ pub enum StageError {
     MultipleEntryPointsFound,
     #[error(transparent)]
     InvalidResource(#[from] InvalidResourceError),
+    #[error(
+        "Location[{location}] {var}'s index exceeds the `max_color_attachments` limit ({limit})"
+    )]
+    ColorAttachmentLocationTooLarge {
+        location: u32,
+        var: InterfaceVar,
+        limit: u32,
+    },
 }
 
 impl WebGpuError for StageError {
@@ -334,7 +342,8 @@ impl WebGpuError for StageError {
             | Self::TooManyVaryings { .. }
             | Self::MissingEntryPoint(..)
             | Self::NoEntryPointFound
-            | Self::MultipleEntryPointsFound => return ErrorType::Validation,
+            | Self::MultipleEntryPointsFound
+            | Self::ColorAttachmentLocationTooLarge { .. } => return ErrorType::Validation,
         };
         e.webgpu_error_type()
     }
@@ -1317,7 +1326,6 @@ impl Interface {
             }
         }
 
-        #[expect(clippy::single_match)]
         match shader_stage {
             naga::ShaderStage::Vertex => {
                 for output in entry_point.outputs.iter() {
@@ -1352,6 +1360,20 @@ impl Interface {
                     }
                 }
             }
+            naga::ShaderStage::Fragment => {
+                for output in &entry_point.outputs {
+                    let &Varying::Local { location, ref iv } = output else {
+                        continue;
+                    };
+                    if location >= self.limits.max_color_attachments {
+                        return Err(StageError::ColorAttachmentLocationTooLarge {
+                            location,
+                            var: iv.clone(),
+                            limit: self.limits.max_color_attachments,
+                        });
+                    }
+                }
+            }
             _ => (),
         }
 
@@ -1370,6 +1392,7 @@ impl Interface {
                 Varying::BuiltIn(_) => None,
             })
             .collect();
+
         Ok(outputs)
     }