diff -r 000000000000 -r 6474c204b198 content/xul/templates/src/nsContentSupportMap.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/xul/templates/src/nsContentSupportMap.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,70 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsContentSupportMap_h__
+#define nsContentSupportMap_h__
+
+#include "pldhash.h"
+#include "nsTemplateMatch.h"
+
+/**
+ * The nsContentSupportMap maintains a mapping from a "resource element"
+ * in the content tree to the nsTemplateMatch that was used to instantiate it. This
+ * is necessary to allow the XUL content to be built lazily. Specifically,
+ * when building "resumes" on a partially-built content element, the builder
+ * will walk upwards in the content tree to find the first element with an
+ * 'id' attribute. This element is assumed to be the "resource element",
+ * and allows the content builder to access the nsTemplateMatch (variable assignments
+ * and rule information).
+ */
+class nsContentSupportMap {
+public:
+    nsContentSupportMap() { Init(); }
+    ~nsContentSupportMap() { Finish(); }
+
+    nsresult Put(nsIContent* aElement, nsTemplateMatch* aMatch) {
+        if (!mMap.ops)
+            return NS_ERROR_NOT_INITIALIZED;
+
+        PLDHashEntryHdr* hdr = PL_DHashTableOperate(&mMap, aElement, PL_DHASH_ADD);
+        if (!hdr)
+            return NS_ERROR_OUT_OF_MEMORY;
+
+        Entry* entry = reinterpret_cast<Entry*>(hdr);
+        NS_ASSERTION(entry->mMatch == nullptr, "over-writing entry");
+        entry->mContent = aElement;
+        entry->mMatch   = aMatch;
+        return NS_OK; }
+
+    bool Get(nsIContent* aElement, nsTemplateMatch** aMatch) {
+        if (!mMap.ops)
+            return false;
+
+        PLDHashEntryHdr* hdr = PL_DHashTableOperate(&mMap, aElement, PL_DHASH_LOOKUP);
+        if (PL_DHASH_ENTRY_IS_FREE(hdr))
+            return false;
+
+        Entry* entry = reinterpret_cast<Entry*>(hdr);
+        *aMatch = entry->mMatch;
+        return true; }
+
+    nsresult Remove(nsIContent* aElement);
+
+    void Clear() { Finish(); Init(); }
+
+protected:
+    PLDHashTable mMap;
+
+    void Init();
+    void Finish();
+
+    struct Entry {
+        PLDHashEntryHdr  mHdr;
+        nsIContent*      mContent;
+        nsTemplateMatch* mMatch;
+    };
+};
+
+#endif