Anon targets do not support default values for `attrs.dep()`

Basically what the error message says!

This limitation is making it extremely difficult to generate anon targets using complex rules such as `cxx_library`.

Is there a good reason for this limitation to exist, or is it "just a matter of implementation"? If there is a good reason for it, what is the recommended way to generate e.g. anon `cxx_library` targets?