Partition Pruning not included in optimizer tracing

There is a feature called "Partition Pruning" ( entry point:
prune_partitions()) where the optimizer figures out the set of
partitions it will need to read for a given table when running the
query.

One can see its result in "partitions" column in EXPLAIN PARTITIONS or
in EXPLAIN FORMAT=JSON.

Nothing is printed in the optimizer trace about this.