Pubsub only allows setting global MsgIdFunction, which applies to all the topics. Still, It would be useful to have the ability to set custom per topic msg id generation, which, e.g., can be dependant on the msg format of some topic. For this to be implemented using the global MsgIdFunction, it would need to be aware of all the msg types of each topic and do some ugly switching to determine what MsgIdFunction to use for what msg.

Fortunately, an unused TopicOpt can set custom values while joining the topic, so some per topic customizations are conceived by design, and the described feature can be implemented.