When the BSON library's doc! macro was first written, the ":" character was not allowed in macro patterns, so the syntax for a key-value pair was implemented as "key" => value rather than "key": value. Later, when Rust added support for using ":" in macro patterns, the old syntax for doc! was kept around for backwards compatibility reasons. Since it's been a couple of years since that change was made, we should consider finally getting rid of the legacy syntax before 1.0.