Instead of filling the vector with default constructed objects, and then throwing them out by calling std::transform:

https://github.com/mongodb/mongo/blob/r3.5.9/src/mongo/db/repl/oplog_buffer_collection.cpp#L178

Once this done, the default constructor for InsertStatement can also be removed:
https://github.com/mongodb/mongo/blob/1550eb477fcf883e861f63ffc988a6060a2fbfb1/src/mongo/db/catalog/collection.h#L99