Potential approach for rspec-2 matchers using Capybara::string

capybara_string_matchers.rb

  module CapybaraStringMatchers
    extend RSpec::Matchers::DSL

    matcher :have_selector do |selector|
      match do |html|
        simplified(html).has_selector?(selector)
      end

      failure_message_for_should do |html|
        "expected to find selector #{selector.inspect} in:\n#{html.to_s}"
      end

      match_for_should_not do |_|
        raise "`should_not have_selector` is not supported. Use `should have_no_selector` instead"
      end

      def simplified(html)
        Capybara.string(html)
      end
    end

    matcher :have_no_selector do |selector|
      match do |html|
        simplified(html).has_no_selector?(selector)
      end

      failure_message_for_should do |html|
        "expected not to find selector #{selector.inspect} in:\n#{html.to_s}"
      end

      match_for_should_not do |_|
        raise "`should_not have_no_selector` is not supported. Use `should have_selector` instead"
      end

      def simplified(html)
        Capybara.string(html)
      end
    end
  end

failure_output.txt

Failures:

  1) things/edit.html.erb renders the edit thing form
     Failure/Error: rendered.should have_selector("form input#thing_names")
       expected to find selector "form input#thing_names" in:
       <h1>Editing thing</h1>
       
       <form accept-charset="UTF-8" action="/things/1001" class="edit_thing" id="edit_thing_1001" method="post"><div style="margin:0;padding:0;display:inline"><input name="utf8" type="hidden" value="&#x2713;" /><input name="_method" type="hidden" value="put" /></div>
       
         <div class="field">
           <label for="thing_name">Name</label><br />
           <input id="thing_name" name="thing[name]" size="30" type="text" value="MyString" />
         </div>
         <div class="actions">
           <input id="thing_submit" name="commit" type="submit" value="Update Thing" />
         </div>
       </form>
       
       <a href="/things/1001">Show</a> |
       <a href="/things">Back</a>
     # ./spec/views/things/edit.html.erb_spec.rb:31:in `block (2 levels) in <top (required)>'