Allow passing plugin instances to field

Closes #3464721

src/Plugin/Field/FieldType/PluginReferenceItem.php

@@ -2,7 +2,9 @@
 
 namespace Drupal\pluginreference\Plugin\Field\FieldType;
 
+use Drupal\Component\Plugin\ConfigurableInterface;
 use Drupal\Component\Plugin\PluginBase;
+use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\NestedArray;
@@ -429,6 +431,12 @@ class PluginReferenceItem extends FieldItemBase implements OptionsProviderInterf
    * {@inheritdoc}
    */
   public function setValue($values, $notify = TRUE): void {
+    if ($values instanceof PluginInspectionInterface) {
+      $values = [
+        static::mainPropertyName() => $values->getPluginId(),
+        'configuration' => $values instanceof ConfigurableInterface ? $values->getConfiguration() : [],
+      ];
+    }
     // Treat the values as property value of the main property, if no array is
     // given.
     if (!is_array($values)) {