Add warning when action/controller style links used (#483)

Support for this format was dropped in version 5 and upgrading/reverting
is covered in the linked release notes.

* [x] Add specs

app/helpers/govuk_link_helper.rb

@@ -88,12 +88,18 @@ def button_attributes(disabled)
   end
 
   def extract_link_args(new_tab: false, inverse: false, muted: false, no_underline: false, no_visited_state: false, text_colour: false, **kwargs)
+    Rails.logger.warn(actions_warning_message(kwargs.fetch(:action))) if kwargs.key?(:action)
+    Rails.logger.warn(controller_warning_message(kwargs.fetch(:controller))) if kwargs.key?(:controller)
+
     link_classes = extract_link_classes(inverse: inverse, muted: muted, no_underline: no_underline, no_visited_state: no_visited_state, text_colour: text_colour)
 
     { **link_classes, **new_tab_args(new_tab) }.deep_merge_html_attributes(kwargs)
   end
 
   def extract_button_link_args(new_tab: false, disabled: false, inverse: false, secondary: false, warning: false, **kwargs)
+    Rails.logger.warn(actions_warning_message(kwargs.fetch(:action))) if kwargs.key?(:action)
+    Rails.logger.warn(controller_warning_message(kwargs.fetch(:controller))) if kwargs.key?(:controller)
+
     button_classes = extract_button_classes(inverse: inverse, secondary: secondary, warning: warning)
 
     { **button_classes, **button_attributes(disabled), **new_tab_args(new_tab) }.deep_merge_html_attributes(kwargs)
@@ -139,6 +145,14 @@ def build_text(text, visually_hidden_prefix:, visually_hidden_suffix:)
 
     safe_join([govuk_visually_hidden(prefix), text, govuk_visually_hidden(suffix)].compact)
   end
+
+  def actions_warning_message(value)
+    "action: '#{value}' parameter detected Support for old style controller/action links has been removed. See https://github.com/x-govuk/govuk-components/releases/tag/v5.0.0"
+  end
+
+  def controller_warning_message(value)
+    "controller: '#{value}' parameter detected. Support for old style controller/action links has been removed. See https://github.com/x-govuk/govuk-components/releases/tag/v5.0.0"
+  end
 end
 
 ActiveSupport.on_load(:action_view) { include GovukLinkHelper }

spec/helpers/govuk_link_helper_spec.rb

@@ -141,6 +141,28 @@ def url_for(path)
         expect(subject).to have_tag("a", text: "hello", with: { href: "/world", class: "govuk-link", lang: "en-GB", dir: "ltr" })
       end
     end
+
+    context "when legacy arguments are passed" do
+      before { allow(Rails.logger).to receive(:warn).and_return(true) }
+
+      describe "actions" do
+        let(:kwargs) { { action: "some-action" } }
+
+        specify "triggers a warning about using legacy action: param" do
+          subject
+          expect(Rails.logger).to have_received(:warn).once.with(/action: 'some-action' parameter detected/)
+        end
+      end
+
+      describe "controller" do
+        let(:kwargs) { { controller: "some-controller" } }
+
+        specify "triggers a warning about using legacy controller: param" do
+          subject
+          expect(Rails.logger).to have_received(:warn).once.with(/controller: 'some-controller' parameter detected/)
+        end
+      end
+    end
   end
 
   describe "govuk_mail_to" do