in protobuf/src/internal.rs [46:50]
fn write_message<W: WriterBackend>(&self, w: &mut Writer<W>) -> Result<()> {
w.write_with_tag(10, |w| w.write_string(&**&self.key))?;
w.write_with_tag(18, |w| w.write_string(&**&self.value))?;
Ok(())
}