Rust async issues with local variables

Question

I have a method like this:

    pub async fn build_incoming_stream(&mut self) -> impl Stream<Item = Result<
        SslStream<<tokio::net::tcp::Incoming<'_> as TryStream>::Ok>,
        Box<dyn Error + Send + Sync + 'static>,>,> 
    {
        let addr = self.address.parse::<SocketAddr>()
        .expect("Couldn't parse socket address");

        let mut listener = TcpListener::bind(addr)
                           .await
                           .expect("Couldn't bind address");
        let acceptor = self.configure_acceptor()
                       .await
                       .expect("configuring acceptor failed");

        incoming(listener.incoming(), acceptor.clone()) 
}

Where fn incoming is:

    pub fn incoming<S>(incoming: S,acceptor: SslAcceptor,) -> impl Stream<Item =  
    Result<SslStream<S::Ok>, StreamError>> where
     S: TryStream + Unpin,
     S::Ok: AsyncRead + AsyncWrite + Send + Sync + Debug + Unpin + 'static,
     S::Error: Into<StreamError>,
    {
      let mut incoming = incoming;

      try_stream! {
         while let Some(stream) = incoming.try_next().await? {
            let tls = tokio_openssl::accept(&acceptor, stream).await?;

            let ssl = SslStream {
               inner: tls
            };

             yield ssl;
         }
       }
     }

The compiler keeps complaining that I can't return a value referencing local data, on the last line of build_incomig_stream fn:

cannot return value referencing local data listener returns a value referencing data owned by the current function

I've tried to wrap it in async move block, to capture listener, but with no luck... any ideas how could I resolve or work around this issue?

ktrapez · Accepted Answer · 2020-07-24 09:20:34Z

Seems that tcp listener already implements stream, so this code works:

pub async fn build_incoming_stream(
        &mut self,
    ) -> impl Stream<
        Item = Result<
            SslStream<<tokio::net::tcp::Incoming<'_> as TryStream>::Ok>,
            Box<dyn Error + Send + Sync + 'static>,
        >,
    > {
        let addr = self
            .address
            .parse::<SocketAddr>()
            .expect("Couldn't parse socket address");

        
        let acceptor = self
            .configure_acceptor()
            .await
            .expect("configuring acceptor failed");
       

        incoming(
            TcpListener::bind(addr)
                .await
                .expect("Couldn't bind address"),
            acceptor.clone(),
        )
    }

The issue with code posted in question was in

pub fn incoming(&mut self) -> Incoming<'_> {
        Incoming::new(self)
    }

because incoming takes tcp listener as mutable reference and this was causing trouble.

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