diff --git a/content/html/content/src/nsGenericHTMLElement.cpp b/content/html/content/src/nsGenericHTMLElement.cpp
index 7d7b77d5c52..7829c9a860e 100644
--- a/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/content/html/content/src/nsGenericHTMLElement.cpp
@@ -3154,7 +3154,7 @@ nsGenericHTMLElement::SetItemValue(JSContext* aCx, JS::Value aValue,
     return;
   }
 
-  FakeDependentString string;
+  binding_detail::FakeDependentString string;
   JS::Rooted<JS::Value> value(aCx, aValue);
   if (!ConvertJSValueToString(aCx, value, &value, eStringify, eStringify, string)) {
     aError.Throw(NS_ERROR_UNEXPECTED);
diff --git a/dom/bindings/BindingDeclarations.h b/dom/bindings/BindingDeclarations.h
index 996cd033ded..fce3a5605eb 100644
--- a/dom/bindings/BindingDeclarations.h
+++ b/dom/bindings/BindingDeclarations.h
@@ -322,7 +322,9 @@ public:
 // internal strings.  So we just have to forward-declare it and reimplement its
 // ToAStringPtr.
 
+namespace binding_detail {
 struct FakeDependentString;
+} // namespace binding_detail
 
 template<>
 class Optional<nsAString>
@@ -344,7 +346,7 @@ public:
 
   // If this code ever goes away, remove the comment pointing to it in the
   // FakeDependentString class in BindingUtils.h.
-  void operator=(const FakeDependentString* str)
+  void operator=(const binding_detail::FakeDependentString* str)
   {
     MOZ_ASSERT(str);
     mStr = reinterpret_cast<const nsDependentString*>(str);
diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index 820158f1df4..4bb9c60c6bb 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -1456,6 +1456,8 @@ AppendNamedPropertyIds(JSContext* cx, JS::Handle<JSObject*> proxy,
                        nsTArray<nsString>& names,
                        bool shadowPrototypeProperties, JS::AutoIdVector& props);
 
+namespace binding_detail {
+
 // A struct that has the same layout as an nsDependentString but much
 // faster constructor and destructor behavior
 struct FakeDependentString {
@@ -1533,6 +1535,8 @@ private:
   };
 };
 
+} // namespace binding_detail
+
 enum StringificationBehavior {
   eStringify,
   eEmpty,
@@ -1545,7 +1549,7 @@ ConvertJSValueToString(JSContext* cx, JS::Handle<JS::Value> v,
                        JS::MutableHandle<JS::Value> pval,
                        StringificationBehavior nullBehavior,
                        StringificationBehavior undefinedBehavior,
-                       FakeDependentString& result)
+                       binding_detail::FakeDependentString& result)
 {
   JSString *s;
   if (v.isString()) {
diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index 803e910bf33..27e83f5ffb7 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -3636,7 +3636,7 @@ for (uint32_t i = 0; i < length; ++i) {
                 assignString = "${declName} = str"
             return JSToNativeConversionInfo(
                 "{\n"
-                "  FakeDependentString str;\n"
+                "  binding_detail::FakeDependentString str;\n"
                 "%s\n"
                 "  %s;\n"
                 "}\n" % (
@@ -3646,16 +3646,20 @@ for (uint32_t i = 0; i < length; ++i) {
 
         if isOptional:
             declType = "Optional<nsAString>"
+            holderType = CGGeneric("binding_detail::FakeDependentString")
+            conversionCode = ("%s\n"
+                              "${declName} = &${holderName};" %
+                              getConversionCode("${holderName}"))
         else:
-            declType = "NonNull<nsAString>"
+            declType = "binding_detail::FakeDependentString"
+            holderType = None
+            conversionCode = getConversionCode("${declName}")
 
         # No need to deal with optional here; we handled it already
         return JSToNativeConversionInfo(
-            ("%s\n"
-             "${declName} = &${holderName};" %
-             getConversionCode("${holderName}")),
+            conversionCode,
             declType=CGGeneric(declType),
-            holderType=CGGeneric("FakeDependentString"))
+            holderType=holderType)
 
     if type.isByteString():
         assert not isEnforceRange and not isClamp
@@ -6942,7 +6946,7 @@ class CGUnionStruct(CGThing):
                                 [Argument("const nsString::char_type*", "aData"),
                                  Argument("nsString::size_type", "aLength")],
                                 inline=True, bodyInHeader=True,
-                                body="mValue.mString.Value().Assign(aData, aLength);"))
+                                body="SetAsString().Assign(aData, aLength);"))
 
             body = string.Template('MOZ_ASSERT(Is${name}(), "Wrong type!");\n'
                                    'mValue.m${name}.Destroy();\n'
@@ -7141,7 +7145,7 @@ class CGUnionConversionStruct(CGThing):
                                      [Argument("const nsDependentString::char_type*", "aData"),
                                       Argument("nsDependentString::size_type", "aLength")],
                                      inline=True, bodyInHeader=True,
-                                     body="mStringHolder.SetData(aData, aLength);"))
+                                     body="SetAsString().SetData(aData, aLength);"))
 
             if vars["holderType"] is not None:
                 members.append(ClassMember("m%sHolder" % vars["name"],
@@ -7965,7 +7969,7 @@ class CGProxyNamedOperation(CGProxySpecialOperation):
         # seems like probable overkill.
         return ("JS::Rooted<JS::Value> nameVal(cx);\n" +
                 idDecl +
-                ("FakeDependentString %s;\n" % argName) +
+                ("binding_detail::FakeDependentString %s;\n" % argName) +
                 unwrapString +
                 ("\n"
                  "\n"
@@ -11601,6 +11605,8 @@ struct PrototypeTraits;
         includes.add("mozilla/dom/OwningNonNull.h")
         includes.add("mozilla/dom/UnionMember.h")
         includes.add("mozilla/dom/BindingDeclarations.h")
+        # Need BindingUtils.h for FakeDependentString
+        includes.add("mozilla/dom/BindingUtils.h")
         implincludes.add("mozilla/dom/PrimitiveConversions.h")
 
         # Wrap all of that in our namespaces.
diff --git a/dom/events/nsJSEventListener.cpp b/dom/events/nsJSEventListener.cpp
index b68b312e47c..62fceedbdb3 100644
--- a/dom/events/nsJSEventListener.cpp
+++ b/dom/events/nsJSEventListener.cpp
@@ -173,7 +173,7 @@ nsJSEventListener::HandleEvent(nsIDOMEvent* aEvent)
         (scriptEvent->message == NS_LOAD_ERROR ||
          scriptEvent->typeString.EqualsLiteral("error"))) {
       errorMsg = scriptEvent->errorMsg;
-      msgOrEvent.SetAsString() = static_cast<nsAString*>(&errorMsg);
+      msgOrEvent.SetAsString().SetData(errorMsg.Data(), errorMsg.Length());
 
       file = scriptEvent->fileName;
       fileName = &file;