getSourceEditingRestrictions example

    // 2. Even though `sourceEditing` may not be enabled before this update, it     // must be after, at least if image uploads are disabled: extra mark-up will     // be added to its configuration to avoid breaking backwards compatibility.     if (!$image_uploads_are_enabled) {
      if (!$source_editing_is_already_enabled) {
        $this->assertNotContains('sourceEditing', $editor_before->getSettings()['toolbar']['items']);
      }
      $this->assertContains('sourceEditing', $editor_after->getSettings()['toolbar']['items']);
      $source_editing_before = $source_editing_is_already_enabled
        ? static::getSourceEditingRestrictions($editor_before)
        : HTMLRestrictions::emptySet();
      $source_editing_after = static::getSourceEditingRestrictions($editor_after);
      if ($source_editing_is_already_enabled) {
        // Nothing has been removed from the allowed source editing tags.         $this->assertFalse($source_editing_before->allowsNothing());
        $this->assertTrue($source_editing_before->diff($source_editing_after)
          ->allowsNothing());
      }
      $this->assertSame($expected_source_editing_additions, $source_editing_after->diff($source_editing_before)
        ->toCKEditor5ElementsArray());
    }