implement :label built-in selector partially via expression filter

lib/capybara/selector.rb

@@ -429,21 +429,25 @@ end
 #
 Capybara.add_selector(:label) do
   label "label"
-  xpath do |locator|
+  xpath(:for) do |locator, options|
     xpath = XPath.descendant(:label)
     xpath = xpath[XPath.string.n.is(locator.to_s) | XPath.attr(:id).equals(locator.to_s)] unless locator.nil?
+    if options.has_key?(:for) && !options[:for].is_a?(Capybara::Node::Element)
+      xpath = xpath[XPath.attr(:for).equals(options[:for].to_s).or((~XPath.attr(:for)).and(XPath.descendant()[XPath.attr(:id).equals(options[:for].to_s)]))]
+    end
     xpath
   end
 
   filter(:for) do |node, field_or_value|
     if field_or_value.is_a? Capybara::Node::Element
-      if field_or_value[:id] && (field_or_value[:id] == node[:for])
-        true
+      if node[:for]
+        field_or_value[:id] == node[:for]
       else
         field_or_value.find_xpath('./ancestor::label[1]').include? node.base
       end
     else
-      node[:for] == field_or_value.to_s
+      #Non element values were handled through the expression filter
+      true
     end
   end
 

lib/capybara/spec/session/selectors_spec.rb

@@ -12,7 +12,11 @@ Capybara::SpecHelper.spec Capybara::Selector do
       expect(@session.find(:label, for: 'form_other_title')['for']).to eq 'form_other_title'
     end
 
-    it "finds a label from nested input using :for filter" do
+    it "finds a label from nested input using :for filter with id string" do
+      expect(@session.find(:label, for: 'nested_label').text).to eq 'Nested Label'
+    end
+
+    it "finds a label from nested input using :for filter with element" do
       input = @session.find(:id, 'nested_label')
       expect(@session.find(:label, for: input).text).to eq 'Nested Label'
     end