The BufReaderWriter<RW> is a convenience struct that facilitates automatic switching between buffered reading and writing from a single underlying Read + Write + Seek instance (generally applicable for std::fs::File). BufReaderWriter moves the underlying reader/writer between a BufReader and BufWriter as needed.
The reader/writer needs to be seekable as switching from reading to writing involves discarding the read buffer and seeking the underlying reader/writer back to the current position of the BufReader.