RSpec self-generating example groups

with_zones_spec.rb

RSpec.shared_examples 'with different zones', :within_time_zones do
  block = metadata[:block] # the block passed to `context 'some test'`
  parent_example_group = superclass # the `describe 'root'` one

  # around { } # skip running examples in `context 'some test'`
  # or
  # metadata[:skip] = true # skip the whole `context 'some test'`
  # or
  parent_example_group.children.delete(self)

  metadata[:within_time_zones].each do |time_zone|
    # generates a group in the parent
    group = parent_example_group.context "with tz=#{time_zone}", &block

    group.around do |example|
      $tz = time_zone
      example.run
      $tz = nil
    end

    group.let(:tz) { time_zone } # optional
  end
end

RSpec.describe 'root' do
  describe 'inner' do
    context 'some test', within_time_zones: ['Madrid', 'Minsk'] do
      it { expect(tz).to eq(tz) }
      it { expect($tz).to eq($tz) }
    end
    context '2' do
      it '22' do
        expect(22).to eq 22
      end
    end
  end
end