Check that all vertex outputs are consumed by the fragment shader (#2704)

wgpu-core/src/validation.rs

@@ -263,6 +263,8 @@ pub enum StageError {
         #[source]
         error: InputError,
     },
+    #[error("location[{location}] is provided by the previous stage output but is not consumed as input by this stage.")]
+    InputNotConsumed { location: wgt::ShaderLocation },
 }
 
 fn map_storage_format_to_naga(format: wgt::TextureFormat) -> Option<naga::StorageFormat> {
@@ -1158,6 +1160,21 @@ impl Interface {
             }
         }
 
+        // Check all vertex outputs and make sure the fragment shader consumes them.
+        if shader_stage == naga::ShaderStage::Fragment {
+            for &index in inputs.keys() {
+                // This is a linear scan, but the count should be low enough that this should be fine.
+                let found = entry_point.inputs.iter().any(|v| match *v {
+                    Varying::Local { location, .. } => location == index,
+                    Varying::BuiltIn(_) => false,
+                });
+
+                if !found {
+                    return Err(StageError::InputNotConsumed { location: index });
+                }
+            }
+        }
+
         if shader_stage == naga::ShaderStage::Vertex {
             for output in entry_point.outputs.iter() {
                 //TODO: count builtins towards the limit?

wgpu/examples/cube/shader.wgsl

@@ -30,6 +30,6 @@ fn fs_main(vertex: VertexOutput) -> @location(0) vec4<f32> {
 }
 
 @fragment
-fn fs_wire() -> @location(0) vec4<f32> {
+fn fs_wire(vertex: VertexOutput) -> @location(0) vec4<f32> {
     return vec4<f32>(0.0, 0.5, 0.0, 0.5);
 }