We previously introduced a new $toHashedIndexKey expression via SERVER-49214 but it looks like we did not document it. We should do so as there are use cases for it and there is lack of awareness about the existing expression.

Description of Linked Ticket

When verifying clusters for logical consistency as part of the verifier project, we can speed the process up by computing hashes server-side instead of transferring full documents over the network to the verifier process and computing the hashes there. Verification speed matters, as it's on the critical path for reducing downtime for MongoDB migrations.

One potential option could be to introduce a new operator in the aggregation pipeline.