After quite some time, I am ready to share this plugin -
https://galaxy.ansible.com/ui/repo/published/capable/core212/content/action/validate_vars/
There is an issue with galaxy - so documentation is not properly rendered for plugins. I am sorry for that.
Main use case for this plugin is to validate variables, but there is a nice bonus - you can have asserts with it like so:
- capable.core212.validate_vars:
any:
type: any
custom_validation:
- condition: variable1 | int > 100
error_message: "variable1 must be greater than 100."
- condition: variable2 | int < 0
error_message: "variable2 must be less than 0."
And even more, plugin supports mutually_exclusive, required_together, required_one_of, required_if, required_by but for ansible variables (not module arguments)