Reject fragment shader output locations > max_color_attachments limit

CHANGELOG.md

@@ -54,6 +54,12 @@ This is a breaking change
 
 By @R-Cramer4 in [#8230](https://github.com/gfx-rs/wgpu/pull/8230)
 
+### 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_hal::MAX_COLOR_ATTACHMENTS;
+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({MAX_COLOR_ATTACHMENTS}) vec4f {{
+    return vec4(1.0, 0.0, 0.0, 1.0);
+}}
+"
+    );
+
+    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 +313,11 @@ pub enum StageError {
     MultipleEntryPointsFound,
     #[error(transparent)]
     InvalidResource(#[from] InvalidResourceError),
+    #[error(
+        "Location[{location}] {var}'s index exceeds `max_color_attachments` ({})",
+        hal::MAX_COLOR_ATTACHMENTS
+    )]
+    ColorAttachmentLocationExceedsLimit { location: u32, var: InterfaceVar },
 }
 
 impl WebGpuError for StageError {
@@ -334,7 +339,8 @@ impl WebGpuError for StageError {
             | Self::TooManyVaryings { .. }
             | Self::MissingEntryPoint(..)
             | Self::NoEntryPointFound
-            | Self::MultipleEntryPointsFound => return ErrorType::Validation,
+            | Self::MultipleEntryPointsFound
+            | Self::ColorAttachmentLocationExceedsLimit { .. } => return ErrorType::Validation,
         };
         e.webgpu_error_type()
     }
@@ -1317,29 +1323,54 @@ impl Interface {
             }
         }
 
-        if shader_stage == naga::ShaderStage::Vertex {
-            for output in entry_point.outputs.iter() {
-                //TODO: count builtins towards the limit?
-                inter_stage_components += match *output {
-                    Varying::Local { ref iv, .. } => iv.ty.dim.num_components(),
-                    Varying::BuiltIn(_) => 0,
-                };
-
-                if let Some(
-                    cmp @ wgt::CompareFunction::Equal | cmp @ wgt::CompareFunction::NotEqual,
-                ) = compare_function
-                {
-                    if let Varying::BuiltIn(naga::BuiltIn::Position { invariant: false }) = *output
+        match shader_stage {
+            naga::ShaderStage::Vertex => {
+                for output in entry_point.outputs.iter() {
+                    //TODO: count builtins towards the limit?
+                    inter_stage_components += match *output {
+                        Varying::Local { ref iv, .. } => iv.ty.dim.num_components(),
+                        Varying::BuiltIn(_) => 0,
+                    };
+
+                    if let Some(
+                        cmp @ wgt::CompareFunction::Equal | cmp @ wgt::CompareFunction::NotEqual,
+                    ) = compare_function
                     {
-                        log::warn!(
-                            "Vertex shader with entry point {entry_point_name} outputs a @builtin(position) without the @invariant \
-                            attribute and is used in a pipeline with {cmp:?}. On some machines, this can cause bad artifacting as {cmp:?} assumes \
-                            the values output from the vertex shader exactly match the value in the depth buffer. The @invariant attribute on the \
-                            @builtin(position) vertex output ensures that the exact same pixel depths are used every render."
-                        );
+                        if let Varying::BuiltIn(naga::BuiltIn::Position { invariant: false }) =
+                            *output
+                        {
+                            log::warn!(
+                                concat!(
+                                    "Vertex shader with entry point {} outputs a ",
+                                    "@builtin(position) without the @invariant attribute and ",
+                                    "is used in a pipeline with {cmp:?}. On some machines, ",
+                                    "this can cause bad artifacting as {cmp:?} assumes the ",
+                                    "values output from the vertex shader exactly match the ",
+                                    "value in the depth buffer. The @invariant attribute on the ",
+                                    "@builtin(position) vertex output ensures that the exact ",
+                                    "same pixel depths are used every render."
+                                ),
+                                entry_point_name,
+                                cmp = cmp
+                            );
+                        }
                     }
                 }
             }
+            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::ColorAttachmentLocationExceedsLimit {
+                            location,
+                            var: iv.clone(),
+                        });
+                    }
+                }
+            }
+            _ => (),
         }
 
         if inter_stage_components > self.limits.max_inter_stage_shader_components {
@@ -1357,6 +1388,7 @@ impl Interface {
                 Varying::BuiltIn(_) => None,
             })
             .collect();
+
         Ok(outputs)
     }