For most queries, metadata dependency validation happens here when we are building the executor for the find layer. Pipelines that don't require an input document source don't go through this path - we have attempted to cover these cases through this special check in run_aggregate. 

This doesn't cover all cases, though. Non-search source stages (e.g. $documents) also skip the pipeline_d path, but won't trigger the special run_aggregate check.

Probably more relevant: there are no checks for any source stages on the $lookup/$unionWith paths, so even search validation doesn't happen there.