Doctrine annotations for custom field formatters in Drupal 8
I confused PHP array syntax for annotation syntax. Those =&gt; should be = and the trailing commas need to be removed from lines that are the last in their set. So it should be:

    /**
     * Plugin implementation of the 'views_hybrid' formatter.
     *
     * @FieldFormatter(
     *   id = "views_hybrid",
     *   label = @Translation("Views hybrid"),
     *   field_types = {"entity_reference"},
     *   settings = {
     *      "items_count" = "5",
     *      "view" = "frontpage",
     *      "display" = "linked_title_only",
     *      "classes" = {},
     *      "view_mode" = "teaser",
     *      "arguments" = ""
     *   }
     * )
     */