db.coll.aggregate([{$match: {b: 3}}, {$rankFusion: {input: {pipelines: {a: [{$sort: {_id: 1}}]}}}}])

That pipeline works even though it should reject it for $rankFusion not being the first stage of the pipeline.

We'd been relying on validation that $search/$vectorSearch/$geoNear must be the first stage of the pipeline. But if the first input pipeline is a custom-sorted pipeline without one of those stages, it won't have any validation to make sure $rankFusion was first. I believe "StageConstraints" on DocumentSourceRankFusion is unreachable code. We only call constraints() after desugaring