Unable to implement tokio_util::codec::{Decoder, Encoder} with mqttrs version 0.4

[svanharmelen]

I updated our app to use tokio 1.x the other day, and with that I also updated mqttrs from 0.3 to 0.4. But since Packet now requires a lifetime, I don't see how I can implement tokio_util::codec::{Decoder, Encoder} using this version.

I tried the following code:

use bytes::BytesMut;
use mqttrs::{Error, Packet};
use tokio_util::codec::{Decoder, Encoder};

pub struct MQTTCodec;

impl MQTTCodec {
   pub fn new() -> MQTTCodec {
       MQTTCodec {}
   }
}

impl<'a> Decoder for MQTTCodec {
   type Item<'a> = Packet<'a>;
   type Error = Error;

   fn decode(&mut self, src: &mut BytesMut) -> Result<Option<Packet<'a>>, Self::Error> {
       mqttrs::decode_slice(src)
   }
}

impl Encoder<Packet<'_>> for MQTTCodec {
   type Error = Error;

   fn encode(&mut self, packet: Packet, buf: &mut BytesMut) -> Result<(), Self::Error> {
       mqttrs::encode_slice(&packet, &mut buf)
   }
}

But this gives me:

error[E0658]: generic associated types are unstable
  --> iot-gateway/tests/mqttbroker/codec.rs:14:5
   |
14 |     type Item<'a> = Packet<'a>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #44265 <https://github.com/rust-lang/rust/issues/44265> for more information

error[E0496]: lifetime name `'a` shadows a lifetime name that is already in scope
  --> iot-gateway/tests/mqttbroker/codec.rs:14:15
   |
13 | impl<'a> Decoder for MQTTCodec {
   |      -- first declared here
14 |     type Item<'a> = Packet<'a>;
   |               ^^ lifetime `'a` already in scope

Do you have any suggestions or pointers on how to make this work again with mqttrs 0.4? Thanks!

[svanharmelen]

@00imvj00 any ideas, pointers or suggestions? Thanks!

[00imvj00]

@svanharmelen apologies for the delayed reply. This is a known issue. @MathiasKoch has integrated this with async, he might be able to help more. I hope. @MathiasKoch any ideas???

[MathiasKoch]

I have not used in any async code at all. All my applications are #![no_std]

I am however currently running with a fork of this, as i am doing some other no-std, no-alloc optimizations for my application, so feel free to disregard me currently. I do hope we can patch stuff together eventually, for a good community mqtt encoder crate that can be used across the entire ecosystem.
If you want to solve this without reverting to std only, i would look into the Managed crate, and how they use it in smoltcp.
smoltcp does no_std, no_alloc with full async support and no rough edges on neither std users, nor no_std.

[stabler]

I recently tried to update mqtt-async-client-rs and ran into the same issue. The reference on Packet makes it very difficult to integrate into applications which process incoming packets in a separate context- especially given the limited tooling to make a copy of the underlying data.

Now that it's been a few months, how are people approaching this? Are there examples of integrations that I could look at?

If I could propose a solution that uses Packet<'a> by default but allows conversion to owned objects for applications that require it, what about something like this:

pub enum Packet<'a> {
  Connect(Connect<'a>),
  Connack(Connack),
  ...
}

pub enum PacketOwned {
  Connect(ConnectOwned),
  Connack(Connack),
  ...
}

impl<'a> Packet<'a> {
  pub fn to_owned(&'a self) -> PacketOwned {
    ...
  }
}