diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index f29fc6f..a7f75b8 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -131,9 +131,17 @@ function edit_library_info() {
  * Implements hook_field_formatter_info_alter().
  *
  * Edit extends the @FieldFormatter annotation with the following keys:
- * - edit: I have no idea what this does. Could be 'direct' I think.
+ * - edit: currently only contains one subkey 'editor' which indicates which
+ *   in-place editor should be used. Possible values are 'form', 'direct' or
+ *   'disabled'.
  */
 function edit_field_formatter_info_alter(&$info) {
+  foreach ($info as $formatter_info) {
+    // Set in-place editor to form if none is supplied.
+    if (empty($formatter_info['edit'])) {
+      $formatter_info['edit'] = array('editor' => 'form');
+    }
+  }
 }
 
 /**
diff --git a/core/modules/edit/lib/Drupal/edit/EditorSelector.php b/core/modules/edit/lib/Drupal/edit/EditorSelector.php
index c42cb02..232a52f 100644
--- a/core/modules/edit/lib/Drupal/edit/EditorSelector.php
+++ b/core/modules/edit/lib/Drupal/edit/EditorSelector.php
@@ -63,7 +63,7 @@ public function getEditor($formatter_type, FieldInstance $instance, array $items
     // 'form' editor, since that can work for any field. Formatter definitions
     // can use 'disabled' to explicitly opt out of in-place editing.
     $formatter_info = field_info_formatter_types($formatter_type);
-    $editor_id = isset($formatter_info['edit']['editor']) ? $formatter_info['edit']['editor'] : 'form';
+    $editor_id = $formatter_info['edit']['editor'];
     if ($editor_id === 'disabled') {
       return;
     }
diff --git a/core/modules/field/lib/Drupal/field/Annotation/FieldFormatter.php b/core/modules/field/lib/Drupal/field/Annotation/FieldFormatter.php
index 7fac2bf..f72b2e0 100644
--- a/core/modules/field/lib/Drupal/field/Annotation/FieldFormatter.php
+++ b/core/modules/field/lib/Drupal/field/Annotation/FieldFormatter.php
@@ -16,6 +16,9 @@
  * called by the Field Attach API field_attach_prepare_view() and
  * field_attach_view() functions.
  *
+ * Additional annotation keys form formatters can be defined in
+ * hook_field_formatter_info_alter().
+ *
  * @Annotation
  *
  * @see \Drupal\field\Plugin\Type\Formatter\FormatterPluginManager