Check CAPNPROTO_PRIVATE in arena.h.

c++/src/capnproto/arena.h

@@ -21,8 +21,12 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 // SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// THIS HEADER IS NOT INCLUDABLE BY CLIENTS, even generated code.  It is entirely internal to the
-// library, which means we can safely #include STL stuff.
+#ifndef CAPNPROTO_ARENA_H_
+#define CAPNPROTO_ARENA_H_
+
+#ifndef CAPNPROTO_PRIVATE
+#error "This header is only meant to be included by Cap'n Proto's own source code."
+#endif
 
 #include <vector>
 #include <memory>
@@ -31,9 +35,6 @@
 #include "type-safety.h"
 #include "message.h"
 
-#ifndef CAPNPROTO_ARENA_H_
-#define CAPNPROTO_ARENA_H_
-
 namespace capnproto {
 namespace internal {
 

c++/src/capnproto/layout-test.c++

@@ -21,6 +21,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 // SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+#define CAPNPROTO_PRIVATE
 #include "layout.h"
 #include "message.h"
 #include "arena.h"