Get payload size in a simpler way

[redeexpressos]

hi. im trying to get the payload size using deku so I can write to a connection (asyncwrite). I dont have the to_bytes function in my message type so then I could use len(). (see FIXME)
Am I doing something wrong? I created the create_packet function to workaround but seems weird...

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
pub struct KeepAlive {}

#[derive(Debug, Clone, Copy, PartialEq, DekuRead, DekuWrite)]
#[deku(endian = "endianness", ctx = "endianness: deku::ctx::Endian")]
pub struct MessageHeader {
    pub message_id: u32,
    pub len: u32,
}

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[deku(endian = "big")]
pub struct MessagePacket {
    pub header: MessageHeader,

    #[deku(ctx = "*header")]
    pub payload: Payload,
}

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[deku(ctx = "_: deku::ctx::Endian, header: MessageHeader")]
pub struct Payload {
    #[deku(
        bytes_read = "header.len",
        assert = "header.len <= 8192",
        ctx = "header.message_id"
    )]
    pub message_with_limit: MessageWithLimit,
}

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[deku(id = "message_id", ctx = "message_id: u32")]
pub enum Message {
    #[deku(id = 0x0000_00FF)]
    KeepAlive(messages::KeepAlive),

   // ...
}


fn create_keep_alive_message() -> MessagePacket {
    let message = Message::KeepAlive(KeepAlive {});
    create_packet(message)
}

#[must_use]
fn create_packet(message: Message) -> MessagePacket {
    let message_id = message.deku_id().unwrap();

    // FIXME: weird solution to get payload size of Message to set remaining_bytes...??
    let mut buf = vec![];
    let mut cursor = Cursor::new(&mut buf);
    let mut writer = Writer::new(&mut cursor);
    message.to_writer(&mut writer, message_id).unwrap();
    let remaining_bytes = cursor.get_ref().len() as u32;

    let header = MessageHeader {  message_id, remaining_bytes };
    let message_with_limit = MessageWithLimit { message };
    let payload = ControlPayload { message_with_limit };

    MessagePacket { header, payload }
}

TLDR: how to get payload length of a Message (which is an enum)

[carlocorradini]

@redeexpressos
You can get the number of bits_written from writer (see https://docs.rs/deku/latest/deku/writer/struct.Writer.html#structfield.bits_written):

let bytes_written = writer.bits_written / 8usize; // Check remainder if message is not byte-aligned