Ah, I was thinking in terms of the actual messages, not compilation. So it should read "Fields are required for compilation by default"?

This leads me to versioning. Imagine you have some old code which won't be upgraded, say an embedded system. Either you don't promote to "required", or you do versioning.

Given the lack of any mention of versioning, I take it that's to be deal with externally? Ie as a separate schema, and detected before handing the rest of the data to the generated code?

edit: Really cool project btw!