Formal arg with ellipsis

[dingxiangfei2009]

This PR will

• add support for ellipsis marker in formal argument lists
• (TODO) test on formal argument parsing

[dingxiangfei2009]

Oh, shoot. I was planning to open this as a draft PR.

Nevertheless, I would like to gather early feedback since I think it will be great to know the best practices from you. 😄

[ebkalderon]

Thanks for the pull request! Your code seems correct, but it doesn't appear to work for me, unfortunately, likely due to a subtle issue with separated_list_partial() unintentionally hard bailing due to it consuming the trailing , and failing to parse the subsequent ellipsis.

You should be able to reproduce the failure by running the following command:

echo '{ foo, ... }: foo' | cargo run -p nix-parser --example viewer

[ebkalderon]

I think the closure could just as easily be replaced with:

map(preceded(tokens::comma, tokens::ellipsis), Some),

[ebkalderon]

By the way, commit c807b89 has updated util::error_expr_if() to no longer require a &str argument anymore. The method can now deduce the found token description using recognize(). I'm very sorry for the inconvenience. It should be easy to fix the conflicts, though; just need to remove the string arguments wherever they occur. Please let me know if you need any assistance!