github_actions_formatter.rb

# Use it like so:
# > bundle exec {rubocop, standardrb} --require ./lib/github_actions_formatter.rb --format GithubActionsFormatter

class GithubActionsFormatter < RuboCop::Formatter::BaseFormatter
  def file_finished(file, offenses)
    return unless (uncorrected_offenses = offenses.reject(&:corrected?)).any?

    uncorrected_offenses.each do |o|
      output.printf("::error file=%s,line=%d,col=%d::%s\n", path_to(file), o.line, o.real_column, o.message.tr("\n", " "))
    end
  end

  private

  def path_to(file)
    Pathname.new(file).relative_path_from(Pathname.new(Dir.pwd))
  end
end