Support for magically detecting whether an argument to an XPConnect'ed function is a dom node with a native nsISupports, and unwrapping it properly.

This'll go live as soon as I can persuade joki or vidur to rebuild the DOM stubs.

dom/tools/JSStubGen.cpp

@@ -1210,7 +1210,7 @@ static const char *kJSClassStr =
 "//\n"
 "JSClass %sClass = {\n"
 "  \"%s\", \n"
-"  JSCLASS_HAS_PRIVATE,\n"
+"  JSCLASS_HAS_PRIVATE | JSCLASS_PRIVATE_IS_NSISUPPORTS,\n"
 "  JS_PropertyStub,\n"
 "  JS_PropertyStub,\n"
 "  Get%sProperty,\n"

js/src/jsapi.h

@@ -389,6 +389,7 @@ struct JSClass {
 #define JSCLASS_HAS_PRIVATE     0x01    /* class instances have private slot */
 #define JSCLASS_NEW_ENUMERATE   0x02    /* class has JSNewEnumerateOp method */
 #define JSCLASS_NEW_RESOLVE     0x04    /* class has JSNewResolveOp method */
+#define JSCLASS_PRIVATE_IS_NSISUPPORTS 0x08  /* private slot is nsISupports* */
 
 struct JSObjectOps {
     /* Mandatory non-null function pointer members. */

js/src/xpconnect/src/xpcconvert.cpp

@@ -286,6 +286,26 @@ XPCConvert::NativeData2JS(JSContext* cx, jsval* d, const void* s,
     return JS_TRUE;
 }
 
+static JSBool
+GetISupportsFromJSObject(JSContext* cx, JSObject* obj, nsISupports** iface)
+{
+    JSClass* jsclass =
+#ifdef JS_THREADSAFE
+            JS_GetClass(cx, obj);
+#else
+            JS_GetClass(obj);
+#endif
+    NS_ASSERTION(jsclass, "obj has no class");
+    if(jsclass &&
+       (jsclass->flags & JSCLASS_HAS_PRIVATE) &&
+       (jsclass->flags & JSCLASS_PRIVATE_IS_NSISUPPORTS))
+    {
+        *iface = (nsISupports*) JS_GetPrivate(cx, obj);
+        return JS_TRUE;
+    }
+    return JS_FALSE;
+}
+
 /***************************************************************************/
 // static
 JSBool
@@ -523,6 +543,16 @@ XPCConvert::JSData2Native(JSContext* cx, void* d, jsval s,
                 else
                     iface->QueryInterface(*iid, (void**)&iface);
             }
+            else if(GetISupportsFromJSObject(cx, obj, &iface))
+            {
+                if(iface)
+                    iface->QueryInterface(*iid, (void**)&iface);
+                else
+                {
+                    *((nsISupports**)d) = NULL;
+                    return JS_TRUE;
+                }
+            }
             else
             {
                 // lets try to build a wrapper around the JSObject