_files conditional custom separator

["Pavel S. Mazurin" <kovpas@xxxxxxxxx>]

I'm trying to create a completion script for buck
<https://github.com/facebook/buck>. In essence, I want to be able to
complete the following format:

buck //path/to/my/buck/file:TargetName

I managed to make completion work for the most part: I can complete
everything up to the TargetName:

buck //path/to/my/buck/file/

with the following script:

if compset -P '//'; then
  _files -/
fi

The problem is that _files adds this / in the end of the path. However, I
want to have a check: if the path contains a BUCK file, I want this slash
to be replaced with a colon:

buck //path/to/my/buck/file:

Is there any way to achieve this?

-Pavel