[WGSL] Add support for accessing field of primitive structs at consta…

…nt time

https://bugs.webkit.org/show_bug.cgi?id=264420
rdar://118126948

Reviewed by Mike Wyrzykowski.

While type checking, also compute the constant result of accessing a field from
a constant primitive struct (e.g. as produced by frexp)

* Source/WebGPU/WGSL/TypeCheck.cpp:
(WGSL::TypeChecker::visit):
* Source/WebGPU/WGSL/tests/valid/constants.wgsl:

Canonical link: https://commits.webkit.org/270439@main

Source/WebGPU/WGSL/TypeCheck.cpp

@@ -754,6 +754,10 @@ void TypeChecker::visit(AST::FieldAccessExpression& access)
                 typeError(access.span(), "struct '", *baseType, "' does not have a member called '", access.fieldName(), "'");
                 return nullptr;
             }
+            if (auto constant = access.base().constantValue()) {
+                auto& constantStruct = std::get<ConstantStruct>(*constant);
+                access.setConstantValue(constantStruct.fields.get(access.fieldName().id()));
+            }
             return primitiveStruct->values[*key];
         }
 

Source/WebGPU/WGSL/tests/valid/constants.wgsl

@@ -80,6 +80,13 @@ fn testMixedConstantValue()
     _ = x;
 }
 
+fn testPrimitiveStructAccess()
+{
+  const f = frexp(1.25);
+  const fract = f.fract;
+  const exp = f.exp;
+}
+
 // Attribute constants
 const group = 0;
 const binding = 1;