Check for a validate function to enable an interposition step in JSG_…

…STRUCT

src/workerd/api/http.c++

@@ -1076,8 +1076,6 @@ jsg::Ref<Request> Request::constructor(
         }
 
         KJ_IF_SOME(c, initDict.cache) {
-          JSG_REQUIRE(FeatureFlags::get(js).getCacheOptionEnabled(), Error, kj::str(
-            "The 'cache' field on 'RequestInitializerDict' is not implemented."));
           cacheMode = getCacheModeFromName(c);
         }
 
@@ -1200,6 +1198,13 @@ kj::Maybe<kj::String> Request::serializeCfBlobJson(jsg::Lock& js) {
   return cf.serialize(js);
 }
 
+void RequestInitializerDict::validate() {
+  if(cache != kj::none) {
+    JSG_REQUIRE(Worker::Api::current().getFeatureFlags().getCacheOptionEnabled(), Error, kj::str(
+      "The 'cache' field on 'RequestInitializerDict' is not implemented."));
+  }
+}
+
 void Request::serialize(
     jsg::Lock& js, jsg::Serializer& serializer,
     const jsg::TypeHandler<RequestInitializerDict>& initDictHandler) {

src/workerd/api/http.h

@@ -752,6 +752,8 @@ struct RequestInitializerDict {
   // jsg::Optional<kj::String> priority;
   // TODO(conform): Might support later?
 
+  void validate();
+
   JSG_STRUCT(method, headers, body, redirect, fetcher, cf, mode, credentials, cache,
              referrer, referrerPolicy, integrity, signal);
   JSG_STRUCT_TS_OVERRIDE(RequestInit<Cf = CfProperties> {

src/workerd/jsg/struct.h

@@ -118,10 +118,15 @@ class StructWrapper<Self, T, TypeTuple<FieldWrappers...>, kj::_::Indexes<indices
     //   it prescribes lexicographically-ordered member initialization, with base members ordered
     //   before derived members. Objects with mutating getters might be broken by this, but it
     //   doesn't seem worth fixing absent a compelling use case.
-
-    return T {
+    auto t = T {
       kj::get<indices>(fields).unwrap(static_cast<Self&>(*this), isolate, context, in)...
     };
+
+    if constexpr (requires { t.validate(); }) {
+      t.validate();
+    }
+
+    return t;
   }
 
   void newContext() = delete;