Bug 1123907 - Part 2: Add MOZ_NEEDS_NO_VTABLE_TYPE to Attributes.h, and use it to verify the EntryType argument of nsTHashtable; r=ehsan

mfbt/Attributes.h

@@ -454,6 +454,9 @@
  * MOZ_MUST_USE: Applies to type declarations.  Makes it a compile time error to not
  *   use the return value of a function which has this type.  This is intended to be
  *   used with types which it is an error to not use.
+ * MOZ_NEEDS_NO_VTABLE_TYPE: Applies to template class declarations.  Makes it
+ *   a compile time error to instantiate this template with a type parameter which
+ *   has a VTable.
  */
 #ifdef MOZ_CLANG_PLUGIN
 #  define MOZ_MUST_OVERRIDE __attribute__((annotate("moz_must_override")))
@@ -474,6 +477,7 @@
 #  define MOZ_UNSAFE_REF(reason) __attribute__((annotate("moz_weak_ref")))
 #  define MOZ_NO_ADDREF_RELEASE_ON_RETURN __attribute__((annotate("moz_no_addref_release_on_return")))
 #  define MOZ_MUST_USE __attribute__((annotate("moz_must_use")))
+#  define MOZ_NEEDS_NO_VTABLE_TYPE __attribute__((annotate("moz_needs_no_vtable_type")))
 /*
  * It turns out that clang doesn't like void func() __attribute__ {} without a
  * warning, so use pragmas to disable the warning. This code won't work on GCC
@@ -498,6 +502,7 @@
 #  define MOZ_UNSAFE_REF(reason) /* nothing */
 #  define MOZ_NO_ADDREF_RELEASE_ON_RETURN /* nothing */
 #  define MOZ_MUST_USE /* nothing */
+#  define MOZ_NEEDS_NO_VTABLE_TYPE /* nothing */
 #endif /* MOZ_CLANG_PLUGIN */
 
 #endif /* __cplusplus */

xpcom/glue/nsTHashtable.h

@@ -16,6 +16,7 @@
 #include "mozilla/Move.h"
 #include "mozilla/fallible.h"
 #include "mozilla/PodOperations.h"
+#include "mozilla/Attributes.h"
 
 #include <new>
 
@@ -82,7 +83,7 @@ enum PLDHashOperator
 };
 
 template<class EntryType>
-class nsTHashtable
+class MOZ_NEEDS_NO_VTABLE_TYPE nsTHashtable
 {
   typedef mozilla::fallible_t fallible_t;