Currently we have the macro REGISTER_AGG_STAGE_MAPPING taking 3 parameters, e.g.,:

REGISTER_AGG_STAGE_MAPPING(querySettingsStage,
DocumentSourceQuerySettings::id,
documentSourceQuerySettingsToStageFn);

We can reduce the number of parameters to just 1 (QO stage name), e.g.:

REGISTER_AGG_STAGE_MAPPING(DocumentSourceQuerySettings);

This modified macro will expect QO-stage-name-derived function defined, e.g.,

boost::intrusive_ptr<exec::agg::Stage> mapDocumentSourceQuerySettingsToStage(
const DocumentSource& ds)

This will enforce unified function and initialization naming.

If this function needs access to the document source's private members we can provide an additional macro

FRIEND_AGG_STAGE_MAPPING(DocumentSourceQuerySettings); to avoid specifying the full function signature.