Bug 1192130 - Part 2: Use MOZ_NON_AUTOABLE to validate the usage of already_AddRefed, r=ehsan

mfbt/AlreadyAddRefed.h

@@ -29,7 +29,7 @@ struct unused_t;
  * because of the sheer number of usages of already_AddRefed.
  */
 template<class T>
-struct MOZ_MUST_USE already_AddRefed
+struct MOZ_MUST_USE MOZ_NON_AUTOABLE already_AddRefed
 {
   /*
    * We want to allow returning nullptr from functions returning

mfbt/Attributes.h

@@ -475,6 +475,10 @@
  *   declarations where an instance of the template should be considered, for
  *   static analysis purposes, to inherit any type annotations (such as
  *   MOZ_MUST_USE and MOZ_STACK_CLASS) from its template arguments.
+ * MOZ_NON_AUTOABLE: Applies to class declarations. Makes it a compile time error to
+ *   use `auto` in place of this type in variable declarations.  This is intended to
+ *   be used with types which are intended to be implicitly constructed into other
+ *   other types before being assigned to variables.
  */
 #ifdef MOZ_CLANG_PLUGIN
 #  define MOZ_MUST_OVERRIDE __attribute__((annotate("moz_must_override")))
@@ -499,8 +503,9 @@
 #  define MOZ_NEEDS_NO_VTABLE_TYPE __attribute__((annotate("moz_needs_no_vtable_type")))
 #  define MOZ_NON_MEMMOVABLE __attribute__((annotate("moz_non_memmovable")))
 #  define MOZ_NEEDS_MEMMOVABLE_TYPE __attribute__((annotate("moz_needs_memmovable_type")))
-#  define MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS               \
+#  define MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS \
     __attribute__((annotate("moz_inherit_type_annotations_from_template_args")))
+#  define MOZ_NON_AUTOABLE __attribute__((annotate("moz_non_autoable")))
 /*
  * 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
@@ -530,6 +535,7 @@
 #  define MOZ_NON_MEMMOVABLE /* nothing */
 #  define MOZ_NEEDS_MEMMOVABLE_TYPE /* nothing */
 #  define MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS /* nothing */
+#  define MOZ_NON_AUTOABLE /* nothing */
 #endif /* MOZ_CLANG_PLUGIN */
 
 /*

toolkit/devtools/server/HeapSnapshot.cpp

@@ -649,15 +649,14 @@ ThreadSafeChromeUtils::ReadHeapSnapshot(GlobalObject& global,
   if (rv.Failed())
     return nullptr;
 
-  auto snapshot = HeapSnapshot::Create(cx, global,
-                                       reinterpret_cast<const uint8_t*>(mm.address()),
-                                       mm.size(), rv);
+  nsRefPtr<HeapSnapshot> snapshot = HeapSnapshot::Create(
+      cx, global, reinterpret_cast<const uint8_t*>(mm.address()), mm.size(), rv);
 
   if (!rv.Failed())
     Telemetry::AccumulateTimeDelta(Telemetry::DEVTOOLS_READ_HEAP_SNAPSHOT_MS,
                                    start);
 
-  return snapshot;
+  return snapshot.forget();
 }
 
 } // namespace dom