storage.in_memory: use attr classes in the (internal) SubStorage.
No change to the interface yet, it's a preliminary change needed to do it.
Migrated from D4108 (view on Phabricator)
Staging instance, all changes can be removed at any time
No change to the interface yet, it's a preliminary change needed to do it.
Migrated from D4108 (view on Phabricator)