=str Avoid subMaterialization when the provided recover source is empty.

[He-Pin]

Extracted from pr #31639
onTop of #31675

1. avoid the materialization when the provided recovered to source is emtpy
2. make use of NotApplied.

[johanandren]

Something I didn't think of before, doesn't the optimization currently the mapMatVal is not invoked?

mySource.etc.recoverWith { case _: Throwable => Source.empty.mapMaterializedValue(something) } }

Not entirely sure if that is fine because source is empty or if we need to check just like we do for the singleSource optimizations.

[He-Pin]

@johanandren For empty Source this kind of optimization should be fine as always return a new Source. And for singleSource, there is an l.traversalSoFar eq EmptyTraversal guard, so that should be fine too?

[johanandren]

Ah, ofc. Yes, that's good, the optimization only works with no attributes etc added on empty but that's fine. 👍

[He-Pin]

@patriknw ping, in case this miss the release 2.7.0

[patriknw]

sorry, we have already closed the door for 2.7.0, but can be included later

[He-Pin]

@patriknw is there any ETA this get merged?

[patriknw]

LGTM