Support explicit [vk::location(n)] binding on metal/wgsl. (#5907)

Author: csyonghe

docs/user-guide/a2-02-metal-target-specific.md

@@ -276,6 +276,8 @@ Since metal does not differentiate a constant buffer, a shader resource (read-on
 
 `spaceN` specifiers inside `register` semantics are ignored.
 
+The `[vk::location(N)]` attributes on stage input/output parameters are respected.
+
 ## Specialization Constants
 
 Specialization constants declared with the `[SpecializationConstant]` or `[vk::constant_id]` attribute will be translated into a `function_constant` when generating Metal source.

docs/user-guide/a2-03-wgsl-target-specific.md

@@ -163,6 +163,8 @@ The `[vk::binding(index,set)]` attribute is respected when emitting WGSL code, a
 
 If the `[vk::binding()]` attribute is not specified by a `:register()` semantic is present, Slang will derive the binding from the `register` semantic the same way as the SPIRV and GLSL backends.
 
+The `[vk::location(N)]` attributes on stage input/output parameters are respected.
+
 ## Specialization Constants
 
 Specialization constants declared with the `[SpecializationConstant]` or `[vk::constant_id]` attribute will be translated into a global `override` declaration when generating WGSL source.

source/slang/slang-parameter-binding.cpp

@@ -1909,7 +1909,8 @@ static RefPtr<TypeLayout> processEntryPointVaryingParameterDecl(
     // `location`s in declaration order coincidentally matches
     // the `SV_Target` order.
     //
-    if (isKhronosTarget(context->getTargetRequest()))
+    if (isKhronosTarget(context->getTargetRequest()) ||
+        isMetalTarget(context->getTargetRequest()) || isWGPUTarget(context->getTargetRequest()))
     {
         if (auto locationAttr = decl->findModifier<GLSLLocationAttribute>())
         {

tests/metal/explicit-location.slang

@@ -0,0 +1,17 @@
+//TEST:SIMPLE(filecheck=MTL): -target metal
+//TEST:SIMPLE(filecheck=WGSL): -target wgsl -entry vertMain -stage vertex
+
+// MTL: attribute(3)
+// WGSL: @location(3)
+
+struct Vertex
+{
+    [vk::location(0)] float4 pos;
+    [vk::location(3)] float2 uv;
+}
+
+[shader("vertex")]
+float4 vertMain(Vertex vin) : SV_Position
+{
+    return vin.pos + vin.uv.x;
+}