diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 7efd2902d097..42032a6baea3 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -3139,6 +3139,12 @@ nsIDocument::GetLastModified(nsAString& aLastModified) const
   }
 }
 
+static void
+IncrementExpandoGeneration(nsIDocument& aDoc)
+{
+  ++static_cast<nsDocument&>(aDoc).mExpandoAndGeneration.generation;
+}
+
 void
 nsIDocument::AddToNameTable(Element* aElement, nsAtom* aName)
 {
@@ -3152,7 +3158,7 @@ nsIDocument::AddToNameTable(Element* aElement, nsAtom* aName)
   if (entry) {
     if (!entry->HasNameElement() &&
         !entry->HasIdElementExposedAsHTMLDocumentProperty()) {
-      ++mExpandoAndGeneration.generation;
+      IncrementExpandoGeneration(*this);
     }
     entry->AddNameElement(this, aElement);
   }
@@ -3172,7 +3178,7 @@ nsIDocument::RemoveFromNameTable(Element* aElement, nsAtom* aName)
   entry->RemoveNameElement(aElement);
   if (!entry->HasNameElement() &&
       !entry->HasIdElementExposedAsHTMLDocumentProperty()) {
-    ++mExpandoAndGeneration.generation;
+    IncrementExpandoGeneration(*this);
   }
 }
 
@@ -3185,7 +3191,7 @@ nsIDocument::AddToIdTable(Element* aElement, nsAtom* aId)
     if (nsGenericHTMLElement::ShouldExposeIdAsHTMLDocumentProperty(aElement) &&
         !entry->HasNameElement() &&
         !entry->HasIdElementExposedAsHTMLDocumentProperty()) {
-      ++mExpandoAndGeneration.generation;
+      IncrementExpandoGeneration(*this);
     }
     entry->AddIdElement(aElement);
   }
@@ -3209,7 +3215,7 @@ nsIDocument::RemoveFromIdTable(Element* aElement, nsAtom* aId)
   if (nsGenericHTMLElement::ShouldExposeIdAsHTMLDocumentProperty(aElement) &&
       !entry->HasNameElement() &&
       !entry->HasIdElementExposedAsHTMLDocumentProperty()) {
-    ++mExpandoAndGeneration.generation;
+    IncrementExpandoGeneration(*this);
   }
   if (entry->IsEmpty()) {
     mIdentifierMap.RemoveEntry(entry);
@@ -8747,7 +8753,7 @@ nsIDocument::DestroyElementMaps()
 #endif
   mStyledLinks.Clear();
   mIdentifierMap.Clear();
-  ++mExpandoAndGeneration.generation;
+  IncrementExpandoGeneration(*this);
 }
 
 void
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 41d9f8c0a2d2..c61c898a197c 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -13,6 +13,7 @@
 
 #include "nsIDocument.h"
 
+#include "jsfriendapi.h"
 #include "nsCOMPtr.h"
 #include "nsAutoPtr.h"
 #include "nsCRT.h"
@@ -377,6 +378,14 @@ protected:
   void EnsureOnloadBlocker();
 
 public:
+  // FIXME(emilio): This needs to be here instead of in nsIDocument because Rust
+  // can't represent alignas(8) values on 32-bit architectures, which would
+  // cause nsIDocument's layout to be wrong in the Rust side.
+  //
+  // This can be fixed after updating to rust 1.25 and updating bindgen to
+  // include https://github.com/rust-lang-nursery/rust-bindgen/pull/1271.
+  js::ExpandoAndGeneration mExpandoAndGeneration;
+
   RefPtr<mozilla::EventListenerManager> mListenerManager;
 
   nsClassHashtable<nsStringHashKey, nsRadioGroupStruct> mRadioGroups;
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 8f2f01c27a40..185beeb8255d 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -6,7 +6,6 @@
 #ifndef nsIDocument_h___
 #define nsIDocument_h___
 
-#include "jsfriendapi.h"
 #include "mozilla/FlushType.h"           // for enum
 #include "nsAttrAndChildArray.h"
 #include "nsAutoPtr.h"                   // for member
@@ -4423,11 +4422,6 @@ protected:
   // 2)  We haven't had Destroy() called on us yet.
   nsCOMPtr<nsILayoutHistoryState> mLayoutHistoryState;
 
-public:
-  js::ExpandoAndGeneration mExpandoAndGeneration;
-
-protected:
-
   nsTArray<RefPtr<mozilla::StyleSheet>> mOnDemandBuiltInUASheets;
   nsTArray<RefPtr<mozilla::StyleSheet>> mAdditionalSheets[AdditionalSheetTypeCount];