Relax overly strict maxInterStageShaderVariables validation for fragment stage inputs

CHANGELOG.md

@@ -49,7 +49,7 @@ Bottom level categories:
 
 #### General
 
-- BREAKING: Migrated from the `maxInterStageShaderComponents` limit to `maxInterStageShaderVariables`, which changes validation in a way that should not affect most programs. This follows the latest changes of the WebGPU spec. By @ErichDonGubler in [#8652](https://github.com/gfx-rs/wgpu/pull/8652).
+- BREAKING: Migrated from the `maxInterStageShaderComponents` limit to `maxInterStageShaderVariables`, which changes validation in a way that should not affect most programs. This follows the latest changes of the WebGPU spec. By @ErichDonGubler in [#8652](https://github.com/gfx-rs/wgpu/pull/8652), [#8792](https://github.com/gfx-rs/wgpu/pull/8792).
 - Fixed validation of the texture format in GPUDepthStencilState when neither depth nor stencil is actually enabled. By @andyleiserson in [#8766](https://github.com/gfx-rs/wgpu/pull/8766).
 
 #### GLES

cts_runner/test.lst

@@ -165,6 +165,16 @@ webgpu:api,validation,render_pipeline,depth_stencil_state:stencil_write:*
 webgpu:api,validation,render_pipeline,inter_stage:max_shader_variable_location:isAsync=false;*
 //FAIL: webgpu:api,validation,render_pipeline,inter_stage:max_shader_variable_location:isAsync=true;*
 // https://github.com/gfx-rs/wgpu/pull/8712
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=false;numVariablesDelta=0;useExtraBuiltinInputs=false
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=false;numVariablesDelta=0;useExtraBuiltinInputs=true
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=false;numVariablesDelta=1;useExtraBuiltinInputs=false
+//FAIL: webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=false;numVariablesDelta=-1;useExtraBuiltinInputs=true
+// https://github.com/gpuweb/cts/pull/4546
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=true;numVariablesDelta=0;useExtraBuiltinInputs=false
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=true;numVariablesDelta=0;useExtraBuiltinInputs=true
+webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=true;numVariablesDelta=1;useExtraBuiltinInputs=false
+//FAIL: webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,input:isAsync=true;numVariablesDelta=-1;useExtraBuiltinInputs=true
+// https://github.com/gpuweb/cts/pull/4546
 webgpu:api,validation,render_pipeline,inter_stage:max_variables_count,output:*
 webgpu:api,validation,resource_usages,buffer,in_pass_encoder:*
 // FAIL: 2 other cases in resource_usages,texture,in_pass_encoder. https://github.com/gfx-rs/wgpu/issues/3126

wgpu-core/src/validation.rs

@@ -1597,7 +1597,7 @@ impl Interface {
                 for output in entry_point.inputs.iter() {
                     match *output {
                         Varying::Local { ref iv, location } => {
-                            if location >= max_fragment_shader_input_variables {
+                            if location >= self.limits.max_inter_stage_shader_variables {
                                 return Err(StageError::FragmentInputLocationTooLarge {
                                     location,
                                     var: iv.clone(),

wgpu-core/src/validation/shader_io_deductions.rs

@@ -49,7 +49,7 @@ impl MaxFragmentShaderInputDeduction {
                 | InterStageBuiltIn::ViewIndex
                 | InterStageBuiltIn::PointCoord => 1,
                 InterStageBuiltIn::Barycentric => 3,
-                InterStageBuiltIn::Position => 4,
+                InterStageBuiltIn::Position => 0,
             },
         }
     }
@@ -58,6 +58,7 @@ impl MaxFragmentShaderInputDeduction {
         use naga::BuiltIn;
 
         Some(Self::InterStageBuiltIn(match builtin {
+            BuiltIn::Position { .. } => InterStageBuiltIn::Position,
             BuiltIn::FrontFacing => InterStageBuiltIn::FrontFacing,
             BuiltIn::SampleIndex => InterStageBuiltIn::SampleIndex,
             BuiltIn::SampleMask => InterStageBuiltIn::SampleMask,
@@ -67,7 +68,6 @@ impl MaxFragmentShaderInputDeduction {
 
             BuiltIn::PointCoord => InterStageBuiltIn::PointCoord,
             BuiltIn::Barycentric => InterStageBuiltIn::Barycentric,
-            BuiltIn::Position { .. } => InterStageBuiltIn::Position,
             BuiltIn::ViewIndex => InterStageBuiltIn::ViewIndex,
 
             BuiltIn::BaseInstance
@@ -107,6 +107,7 @@ impl MaxFragmentShaderInputDeduction {
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum InterStageBuiltIn {
     // Standard for WebGPU
+    Position,
     FrontFacing,
     SampleIndex,
     SampleMask,
@@ -117,7 +118,6 @@ pub enum InterStageBuiltIn {
     // Non-standard
     PointCoord,
     Barycentric,
-    Position,
     ViewIndex,
 }
 
@@ -147,9 +147,18 @@ where
             .filter(|(_, effective_deduction)| *effective_deduction > 0);
         if relevant_deductions.clone().next().is_some() {
             writeln!(f, "; note that some deductions apply during validation:")?;
+            let mut wrote_something = false;
             for deduction in deductions {
-                writeln!(f, "\n- {deduction:?}: {}", accessor(deduction))?;
+                let deducted_amount = accessor(deduction);
+                if deducted_amount > 0 {
+                    writeln!(f, "\n- {deduction:?}: {}", accessor(deduction))?;
+                    wrote_something = true;
+                }
             }
+            debug_assert!(
+                wrote_something,
+                "no substantial deductions found in error display"
+            );
         }
         Ok(())
     })