only in patch2:
unchanged:
--- a/core/modules/migrate/src/Plugin/MigrateSourceInterface.php
+++ b/core/modules/migrate/src/Plugin/MigrateSourceInterface.php
@@ -44,10 +44,14 @@ public function prepareRow(Row $row);
   public function __toString();
 
   /**
-   * Get the source ids.
+   * Defines the source fields uniquely identifying a source row. None of these
+   * fields should contain a NULL value - if necessary, use prepareRow() or
+   * hook_migrate_prepare_row() to rewrite NULL values to appropriate empty
+   * values (such as '' or 0).
    *
    * @return array
-   *   The source ids.
+   *   Array keyed by source field name, with values being a schema array
+   *   describing the field (such as ['type' => 'string]).
    */
   public function getIds();