wgpu-core: Only produce StageError::InputNotConsumed on DX11/DX12

This error only exists due to an issue with naga's HLSL support:
gfx-rs/naga#1945
The WGPU spec itself allows vertex shader outputs that are
not consumed by the fragment shader.

Until the issue is fixed, we can allow unconsumed outputs on
all platforms other than DX11/DX12.

wgpu-core/src/device/resource.rs

@@ -1286,7 +1286,7 @@ impl<A: HalApi> Device<A> {
                     inner: Box::new(inner),
                 })
             })?;
-        let interface = validation::Interface::new(&module, &info, self.limits.clone());
+        let interface = validation::Interface::new(&module, &info, self.limits.clone(), A::VARIANT);
         let hal_shader = hal::ShaderInput::Naga(hal::NagaShader { module, info });
 
         let hal_desc = hal::ShaderModuleDescriptor {

wgpu-core/src/validation.rs

@@ -121,6 +121,7 @@ struct EntryPoint {
 #[derive(Debug)]
 pub struct Interface {
     limits: wgt::Limits,
+    backend: wgt::Backend,
     resources: naga::Arena<Resource>,
     entry_points: FastHashMap<(naga::ShaderStage, String), EntryPoint>,
 }
@@ -829,7 +830,12 @@ impl Interface {
         list.push(varying);
     }
 
-    pub fn new(module: &naga::Module, info: &naga::valid::ModuleInfo, limits: wgt::Limits) -> Self {
+    pub fn new(
+        module: &naga::Module,
+        info: &naga::valid::ModuleInfo,
+        limits: wgt::Limits,
+        backend: wgt::Backend,
+    ) -> Self {
         let mut resources = naga::Arena::new();
         let mut resource_mapping = FastHashMap::default();
         for (var_handle, var) in module.global_variables.iter() {
@@ -910,6 +916,7 @@ impl Interface {
 
         Self {
             limits,
+            backend,
             resources,
             entry_points,
         }
@@ -1119,7 +1126,11 @@ impl Interface {
         }
 
         // Check all vertex outputs and make sure the fragment shader consumes them.
-        if shader_stage == naga::ShaderStage::Fragment {
+        // This is only needed for HLSL shaders (DX11 and DX12) due to a naga HLSL issue:
+        // https://github.com/gfx-rs/naga/issues/1945
+        if shader_stage == naga::ShaderStage::Fragment
+            && matches!(self.backend, wgt::Backend::Dx11 | wgt::Backend::Dx12)
+        {
             for &index in inputs.keys() {
                 // This is a linear scan, but the count should be low enough
                 // that this should be fine.