Some ideas that came out of SERVER-99121 to improve DocumentSourceMatch and DocumentSourceLookup:

• Currently, DocumentSourceLookup::_matchSrc is a pointer to any DocumentSourceMatch(s) that the $lookup was able to absorb during the LUM ($lookup, $unwind, $match) rewrite. It would make more sense for _matchSrc to directly be a pointer to the MatchExpression instead, since we only use _matchSrc to generate the additional filter for the lookup. Note that to do this we would need to add a function that releases the MatchExpression/backing BSON from the DocumentSourceMatch in the pipeline, as described here.
• Since _matchSrc is now a MatchExpression, and the main contents of DocumentSourceMatch::joinMatchWith operate over MatchExpressions (as of SERVER-99121), we could pull out the work currently done in the addPredicates lambda and either put it in expression_algo.h or make it a function of the MatchExpression class.
• Similarly, DocumentSourceMatch::descendMatchOnPath is only used in this LUM rewrite and operates over the input MatchExpression. It would make more sense to either put it in expression_algo.h or make it a function of the MatchExpression class.