Your code that uses Async_extra.Udp won't build anymore? Same here!
Jane Street accidentally dropped the Async_extra.Udp module from their v0.12 release of async_extra. 😆
You can get it back by doing:
opam pin add async_udp git+https://github.com/mbacarella/async_udp
Say yes if it asks to create a new package.
Add async_udp to your project's dune libraries list.
You'll have to change stuff like Async_extra.Udp to Async_udp in your code. Hopefully this is all you need to do.
Some discussion here: