feature: warn when reusable workflow is used as a step

[IlyaGulya]

Using reusable workflow as a step is not supported by Github Actions.
However, actionlint currently does not warn about this behavior.

Now it does 🙂

[rhysd]

I'm not sure what this feature tries to solve. Is it a common mistake to pass a workflow YAML file to uses? How did you confirm that it's common?

[IlyaGulya]

@rhysd I'm not sure how common that is, however, Github currently does not warn if someone does that.
It gives error only once specific step is being run. (https://github.com/IlyaGulya/gh-actions-playground/actions/runs/11211480780/job/31160454101#step:3:1)
I'm in a process of learning the github actions and this one issue I've ran into recently.
And since there's no warning neither from github actions, nor actionlint, it was pretty unpleasant 🙂
Thus, I think that this check at least would not be excess part of actionlint.

[IlyaGulya]

For context, there's examples of people asking about this:
https://stackoverflow.com/questions/75733616/github-actions-how-to-call-a-reusable-workflow-as-a-step
https://www.reddit.com/r/github/comments/155yvdf/reusable_workflow_as_steps_for_caller_workflow/

It's not common, though 🙂

[rhysd]

Okay, I'm not against this feature but I'd like to make false positives as minimal as possible.

One possible false positive is local actions under .github/workflows/ directory. When local actions are put in the directory, for example .github/workflows/actions/my-action, they are used like

- uses: ./.github/workflows/actions/my-action
  with:
    arg: ...

This check will report the valid action usage as invalid reusable workflow usage.

[IlyaGulya]

Sure, I will fix that and try to test as much cases as possible