IDL needs to support OpMsgRequest parsing natively. This is important since fields can be part of a DocumentSequence which is different then a simple BSONObj.

For IDL a new boolean field property will be added called "supports_doc_sequence" do denote the fields that can be found in a DocumentSequence. The only allowed fields are arrays of either BSONObj or IDL structs.

In addition, IDL needs to understand the "$db", and other implicit field semantics.