This now works on macOS too and is ready for review.
https://github.com/eregon/setup-ruby-test/runs/231525062

TruffleRuby 19.3.0 has been released, and no longer depends on a system-installed LLVM, which simplifies this quite a bit. The new release also handles macOS not having system headers anymore automatically.

@damccorm Could you review this?

I'd like to add some documentation for this in the README, but I think we should merge #27 first.

Working example usage:
https://github.com/eregon/setup-ruby-test/blob/master/.github/workflows/ci.yml

Any chances to merge this in?

@bryanmacfarlane Could you review this?

In the meantime, https://github.com/eregon/use-ruby-action supports Ruby, JRuby and TruffleRuby.

@eregon this is not a fork of this repo, isn't it? And it only supports the latest stable versions, right?

@pkuczynski https://github.com/eregon/use-ruby-action is another action, and currently I think it's a superset of setup-ruby in terms of versions offered. It could probably support more versions if needed.

Anyway, this PR would add TruffleRuby support to setup-ruby, but given the maintainers have been rather inactive I don't know how long it will take to be merged. That other action supports truffleruby currently (and more recent versions), that's why I mentioned it here.

Yeah, I ended up using your package in my build rubyconfig/config#266. Much more up to date. Let's hope it will stay like this :)

I'll close this, it's growing conflicts and it probably make little sense to add before this action supports more prebuilt MRI versions.

If someone wants to test with TruffleRuby, JRuby or more Ruby versions, I suggest to use https://github.com/ruby/setup-ruby