Our internal Command type uses a generic type T for the body of the command. This theoretically allows us to use structs to model the shape of specific commands, but in practice we use either Document or RawDocumentBuf for T for the majority of our operations. We should remove the generic type and instead use RawDocumentBuf across the board to simplify the implementation of the Command type. See this comment for more context: https://github.com/mongodb/mongo-rust-driver/pull/1009#discussion_r1457685402