Remove ViewExecutable::cloneView()

+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/Feed.phpundefined
@@ -244,7 +244,7 @@ public function submitOptionsForm(&$form, &$form_state) {
+  public function attachTo(ViewExecutable $clone, $display_id) {

+++ b/core/modules/views/lib/Drupal/views/ViewExecutable.phpundefined
@@ -1464,7 +1464,9 @@ public function attachDisplays() {
+        $cloned_view = new static($this->storage);

So we have to decide whether it is a ViewExecutable or a storage, but couldn't be both at the same time :) I will write some tests now, just to be sure it will not be broken.

+++ b/core/modules/views/views.moduleundefined
@@ -1418,21 +1413,12 @@ function views_get_applicable_views($type) {
+    foreach ($executable->displayHandlers as $id => $handler) {
+      if (!empty($handler->definition[$type]) && $handler->isEnabled()) {
+        $result[] = array($executable, $id);
       }

The main advantage not checking the definition of the plugin here is, that all views displays (even the ones which come from a view without a page), though i'm not sure we have to care that much, as things will change a lot in the future anyway.

+++ b/core/modules/views/views_ui/admin.incundefined
@@ -1460,8 +1460,7 @@ function views_ui_config_item_form($form, &$form_state) {
-          $temp_view = $view->cloneView();
-          views_ui_cache_set($temp_view);
+          views_ui_cache_set($view);

Wasn't the point of cloning the view here to not contain all the runtime information of the executable. views_ui_cache_set already removes some of the values, but i guess there might be values left?

Tim explained me how that static() works so here comes a small doc improvement.

The tests are added in #1861852: Change notice: Views Attachments aren't rendered anymore so this issues seems to be fine. (not sure whether we want to RTBC/commit a patch which contains parts which are known to be broken)

Nice to get rid of this smelly PHP4 code. :) Great job!

Committed and pushed to 8.x. Thanks!