Bug 1631709 - NotNull must not be movable. r=jwalden

Differential Revision: https://phabricator.services.mozilla.com/D71721

mfbt/NotNull.h

@@ -144,6 +144,13 @@ class NotNull {
     return mBasePtr.operator->();
   }
   constexpr decltype(*mBasePtr) operator*() const { return *mBasePtr; }
+
+  // NotNull can be copied, but not moved. Moving a NotNull with a smart base
+  // pointer would leave a nullptr NotNull behind. The move operations must not
+  // be explicitly deleted though, since that would cause overload resolution to
+  // fail in situations where a copy is possible.
+  NotNull(const NotNull&) = default;
+  NotNull& operator=(const NotNull&) = default;
 };
 
 // Specialization for T* to allow adding MOZ_NONNULL_RETURN attributes.

mfbt/tests/TestNotNull.cpp

@@ -288,6 +288,14 @@ void TestNotNullWithRefPtr() {
 
   // At this point the refcount is 4.
 
+  NotNull<RefPtr<MyRefType>> r6 = std::move(r2);
+  mozilla::Unused << r6;
+
+  CHECK(r2.get());
+  CHECK(r6.get());
+
+  // At this point the refcount is 5 again, since NotNull is not movable.
+
   // At function's end all RefPtrs are destroyed and the refcount drops to 0
   // and the MyRefType is destroyed.
 }