+<?php
+
+namespace Drupal\drupalmoduleupgrader\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines a Plugin annotation object for DMU indexer plugins.
+ *
+ * Indexers scan a target module to determine what's in it so that other plugins
+ * can use that information. All available indexers are always run before any
+ * other plugin type. Indexers are responsible for cataloguing things like:
+ *
+ * - What hooks a module implements, and where those implementations reside (i.e.,
+ * which files)
+ * - Classes defined by a module
+ * - Functions defined by a module
+ * - Tests defined by a module, and what kind of tests they are
+ * - Which functions are called by the module, and when
+ *
+ * Any information gathered by an indexer is available to other plugin types.
+ * Essentially, indexers build a "map" of a target module, which is stored in
+ * an index backend (by default, an SQLite database that lives only in memory).
+ *
+ * Plugin Namespace: Plugin\DMU\Indexer
+ *
+ * @Annotation
+ */
+class Indexer extends Plugin {
+
+ /**
+ * The plugin ID.
+ *
+ * @var string
+ */
+ public $id;
+
+}