state desired/required tox plugins and warn/fail on missing ones

[RonnyPfannschmidt]

tox should have a way to specify desired/required tox plugins for use with a tox.ini

use case i have in mind:

• create tox-linting plugin that handles checks for python 2/3 using flake8, rst-linter and/or other tools
• let that plugin add its own envs to the envlist
• make tox warn if its missing, potentially fail on CI if its not missing and unrelated environments are not selected

[gaborbernat]

This has been done via #936

[RonnyPfannschmidt]

after review of the pr i disagree, #936 doesnt even solve half of this

[gaborbernat]

It has the fail/warn part. Plugins can freely modify the envlist so nothing do there. What else am I missing?

[RonnyPfannschmidt]

@gaborbernat all we have is fail - there is no warning mechanism, no running of working envs and i'm not sure if there is the plugin can add env`s to the envlist implemnted, so from my pov its not even half

[gaborbernat]

I don't think we should warn. If a dependency is missing a hard fail is the appropriate solution. While there's no plugin implemented I don't think there's any core changes needed for anyone to implement one.

[gaborbernat]

If no one does it in the meantime I'll implement one once #998 is done.

[RonnyPfannschmidt]

#998 would make the considerations wrt warning and graceful degradation irrelevant as it could always be done correctly

a initial api to have a canonical way to have plugins add plugin related env's might be nice to have as a followup - that way i would consider this one as done