The original design for the operations layer refactor in v1.1 included a code generation component called drivergen. Some of the files defined in x/mongo/driver/operation are auto-generated, but the codegen tool didn't supply all of the functionality we needed so many of them are hand-written. We've also updated many of them by hand to include new features (e.g. rewriting the "count" command as an aggregate pipeline for 4.9+). We should remove the code generation component from the driver altogether.