Only those with "administer users" can link actor

opencase.module

@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Access\AccessResult;
 use Drupal\opencase\EntityTypeRelationsWidget;
 
 
@@ -30,12 +31,11 @@ function opencase_block_access(\Drupal\block\Entity\Block $block, $operation, \D
       'entity.oc_activity.edit_form',
       'entity.oc_activity.add_form'
     ];
-    return \Drupal\Core\Access\AccessResult::forbiddenIf(!in_array($route_name, $routes_where_it_should_be_shown))
-      ->addCacheableDependency($block);
+    return AccessResult::forbiddenIf(!in_array($route_name, $routes_where_it_should_be_shown))->addCacheableDependency($block);
   }
 
   // No opinion.
-  return \Drupal\Core\Access\AccessResult::neutral();
+  return AccessResult::neutral();
 }
 
 
@@ -155,3 +155,13 @@ function opencase_views_pre_render($view) {
     $view->exposed_widgets = NULL;
   }
 }
+
+
+function opencase_entity_field_access($operation, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $account, $items = NULL) {
+  if ($field_definition->getName() == 'field_linked_opencase_actor'
+      && $operation == 'edit'
+      && !$account->hasPermission('administer users')) {
+    return AccessResult::forbidden();
+  }
+  return AccessResult::neutral();
+}