Mojo: Force folks to handle the return value of MessageReceiver::Accept{WithResponder}

mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 
 {%- macro pass_params(parameters) %}
 {%-   for param in parameters %}
 {%-     if param.kind|is_object_kind -%}
 mojo::internal::Wrap(params->{{param.name}}())
 {%-     elif param.kind|is_handle_kind -%}
 mojo::MakePassable(params->{{param.name}}()).Pass()
@@ skipping 96 matching lines @@
 {%- endif %}
 
   {{build_message(params_name, method.parameters)}}
 
 {%- if method.response_parameters != None %}
   mojo::MessageReceiver* responder =
       new {{class_name}}_{{method.name}}_ForwardToCallback(callback);
   if (!receiver_->AcceptWithResponder(&message, responder))
     delete responder;
 {%- else %}
-  receiver_->Accept(&message);
+  bool ok MOJO_ALLOW_UNUSED = receiver_->Accept(&message);
+  // This return value may be ignored as !ok implies the Connector has
+  // encountered an error, which will be visible through other means.
 {%- endif %}
 }
 {%- endfor %}
 
 {#--- ProxyToResponder definition #}
 {%- for method in interface.methods -%}
 {%-   if method.response_parameters != None %}
 {%-     set message_name =
             "internal::k%s_%s_Name"|format(interface.name, method.name) %}
 {%-     set params_name =
@@ skipping 18 matching lines @@
   uint64_t request_id_;
   mutable mojo::MessageReceiver* responder_;
   MOJO_DISALLOW_COPY_AND_ASSIGN({{class_name}}_{{method.name}}_ProxyToResponder);
 };
 void {{class_name}}_{{method.name}}_ProxyToResponder::Run(
     {{interface_macros.declare_params("in_", method.response_parameters)}}) const {
   {{compute_payload_size(params_name, method.response_parameters)}}
   mojo::internal::ResponseMessageBuilder builder(
       {{message_name}}, payload_size, request_id_);
   {{build_message(params_name, method.response_parameters)}}
-  responder_->Accept(&message);
+  bool ok MOJO_ALLOW_UNUSED = responder_->Accept(&message);
+  // TODO(darin): !ok returned here indicates a malformed message, and that may
+  // be good reason to close the connection. However, we don't have a way to do
+  // that from here. We should add a way.
   delete responder_;
   responder_ = NULL;
 }
 {%-   endif -%}
 {%- endfor %}
 
 {{class_name}}Stub::{{class_name}}Stub({{class_name}}* sink)
     : sink_(sink) {
 }
 
@@ skipping 47 matching lines @@
       return true;
 {%-     else %}
       break;
 {%-     endif %}
     }
 {%-   endfor %}
   }
 {%- endif %}
   return false;
 }