In C Driver 1.4 we started fixing CDRIVER-725 "mongoc_collection_create_index should return index document" by creating a new function mongoc_collection_create_index_2 which returned the index document, but then in CDRIVER-1322 we superseded that function with mongoc_collection_create_index_with_write_concern, which both accepts a write concern and returns the index document.

Since part of the new function's purpose it to support a MongoDB 3.4 feature (DRIVERS-290), we'll put it off until the C Driver release 1.5, which is the release for MongoDB 3.4 support in general.