Added QueryElementAt() which combines GetElementAt() and QueryInterface(). Rationale: combining operations saves a call to AddRef/Release, since it's done by QI. Approved by dp.

mailnews/base/util/nsMsgFolder.h

@@ -56,6 +56,9 @@ public:
   NS_IMETHOD GetElementAt(PRUint32 i, nsISupports* *result) {
     return mSubFolders->GetElementAt(i, result);
   }
+  NS_IMETHOD  QueryElementAt(PRUint32 i, const nsIID & iid, void * *result) {
+    return mSubFolders->QueryElementAt(i, iid, result);
+  }
   NS_IMETHOD SetElementAt(PRUint32 i, nsISupports* value) {
     return mSubFolders->SetElementAt(i, value);
   }

xpcom/ds/nsICollection.idl

@@ -25,6 +25,8 @@ interface nsICollection : nsISupports
 
   PRUint32 Count();
   nsISupports GetElementAt(in PRUint32 index);
+  void QueryElementAt(in PRUint32 index, in nsIIDRef uuid, 
+                      [iid_is(uuid),retval] out nsQIResult result);
   void SetElementAt(in PRUint32 index, in nsISupports item);
   void AppendElement(in nsISupports item);
   void RemoveElement(in nsISupports item);

xpcom/ds/nsSupportsArray.h

@@ -39,6 +39,14 @@ public:
     *result = ElementAt(aIndex);
     return NS_OK;
   }
+  NS_IMETHOD QueryElementAt(PRUint32 aIndex, const nsIID & aIID, void * *aResult) {
+    if (aIndex < mCount) {
+      nsISupports* element = mArray[aIndex];
+      if (nsnull != element)
+        return element->QueryInterface(aIID, aResult);
+    }
+    return NS_ERROR_FAILURE;
+  }
   NS_IMETHOD SetElementAt(PRUint32 aIndex, nsISupports* value) {
     PRBool ok = ReplaceElementAt(value, aIndex);
     return ok ? NS_OK : NS_ERROR_FAILURE;