The X macro pattern used by mongoc-rpc-private.h and mongoc-rpc.c to define structs and their corresponding gather/printf/swab_(to/from)_le/scatter functions has been a continued source of frustration when attempting to debug RPC function behavior. As we continue to improve OP_MSG support and move away from legacy opcodes, it may be worth refactoring this component to improve readability and the debugging experience.