Add feature flag for switch to basic search

ee/app/helpers/ee/search_helper.rb

@@ -70,14 +70,13 @@ module EE
       search_path(search_params)
     end
 
-    # Switching to basic search should only be possible for a subset of
-    # scopes. This method assumes you're already allowed to search within
-    # this scope but it's just here to determine if you can switch to basic
-    # search.
-    def switch_to_basic_searchable_tab?(scope)
+    def show_switch_to_basic_search?(search_service)
+      return false unless ::Feature.enabled?(:switch_to_basic_search, default_enabled: false)
+      return false unless search_service.use_elasticsearch?
+
       return true if @project
 
-      scope.in?(SWITCH_TO_BASIC_SEARCHABLE_TABS)
+      search_service.scope.in?(SWITCH_TO_BASIC_SEARCHABLE_TABS)
     end
 
     private

ee/app/views/search/_form_revert_to_basic.html.haml

-- if search_service.use_elasticsearch? && switch_to_basic_searchable_tab?(search_service.scope)
+- if show_switch_to_basic_search?(search_service)
   - begin_link = '<a href="%{url}">'.html_safe % { url: revert_to_basic_search_filter_url }
   - end_link = '</a>'.html_safe
   %p.text-center

ee/spec/helpers/search_helper_spec.rb

@@ -176,9 +176,29 @@ describe SearchHelper do
     end
   end
 
-  describe '#switch_to_basic_searchable_tab?' do
+  describe '#show_switch_to_basic_search?' do
+    let(:use_elasticsearch) { true }
     let(:scope) { 'commits' }
-    subject { switch_to_basic_searchable_tab?(scope) }
+    let(:search_service) { instance_double(Search::GlobalService, use_elasticsearch?: use_elasticsearch, scope: scope) }
+    subject { show_switch_to_basic_search?(search_service) }
+
+    before do
+      stub_feature_flags(switch_to_basic_search: true)
+    end
+
+    context 'when :switch_to_basic_search feature is disabled' do
+      before do
+        stub_feature_flags(switch_to_basic_search: false)
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when not currently using elasticsearch' do
+      let(:use_elasticsearch) { false }
+
+      it { is_expected.to eq(false) }
+    end
 
     context 'when project scope' do
       before do