Another KJ_WARN_UNUSED_RESULT.

c++/src/capnp/rpc.h

@@ -275,7 +275,7 @@ public:
     // Wait for a message to be received and return it.  If the read stream cleanly terminates,
     // return null.  If any other problem occurs, throw an exception.
 
-    virtual kj::Promise<void> shutdown() = 0;
+    virtual kj::Promise<void> shutdown() KJ_WARN_UNUSED_RESULT = 0;
     // Waits until all outgoing messages have been sent, then shuts down the outgoing stream. The
     // returned promise resolves after shutdown is complete.
   };