You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.rs
    path of file: ./repos/rust-esp32-std-demo/src
    
    the code of the file until where you have to start completion: #![allow(unused_imports)]
#![allow(clippy::single_component_path_imports)]
//#![feature(backtrace)]

#[cfg(all(feature = "qemu", not(esp32)))]
compile_error!("The `qemu` feature can only be built for the `xtensa-esp32-espidf` target.");

#[cfg(all(feature = "ip101", not(esp32)))]
compile_error!("The `ip101` feature can only be built for the `xtensa-esp32-espidf` target.");

#[cfg(all(feature = "kaluga", not(esp32s2)))]
compile_error!("The `kaluga` feature can only be built for the `xtensa-esp32s2-espidf` target.");

#[cfg(all(feature = "ttgo", not(esp32)))]
compile_error!("The `ttgo` feature can only be built for the `xtensa-esp32-espidf` target.");

#[cfg(all(feature = "heltec", not(esp32)))]
compile_error!("The `heltec` feature can only be built for the `xtensa-esp32-espidf` target.");

#[cfg(all(feature = "esp32s3_usb_otg", not(esp32s3)))]
compile_error!(
    "The `esp32s3_usb_otg` feature can only be built for the `xtensa-esp32s3-espidf` target."
);

use core::cell::RefCell;
use core::ffi::{self, CStr};
use core::fmt::{self, Debug};
use core::sync::atomic::*;

use std::fs;
use std::io::{Read as _, Write as _};
use std::net::{TcpListener, TcpStream, ToSocketAddrs};

    
fn test_mqtt_client() -> Result<EspMqttClient<'static>> {
    info!("About to start MQTT client");

    let conf = MqttClientConfiguration {
        client_id: Some("rust-esp32-std-demo"),
        crt_bundle_attach: Some(esp_idf_svc::sys::esp_crt_bundle_attach),

        ..Default::default()
    };

    let (mut client, mut connection) = EspMqttClient::new("mqtts://broker.emqx.io:8883", &conf)?;

    info!("MQTT client started");

    // Need to immediately start pumping the connection for messages, or else subscribe() and publish() below will not work
    // Note that when using the alternative constructor - `EspMqttClient::new` - you don't need to
    // spawn a new thread, as the messages will be pumped with a backpressure into the callback you provide.
    // Yet, you still need to efficiently process each message in the callback without blocking for too long.
    //
    // Note also that if you go to http://tools.emqx.io/ and then connect and send a message to topic
    // "rust-esp32-std-demo", the client configured here should receive it.
    thread::spawn(move || {
        info!("MQTT Listening for messages");

        while let Ok(event) = connection.next() {
            info!("MQTT Event: {}", event.payload());
        }

        info!("MQTT connection loop exit");
    });

    client.subscribe("rust-esp32-std-demo", QoS::AtMostOnce)?;

    info!("Subscribed to all topics (rust-esp32-std-demo)");

    client.publish(
        "rust-esp32-std-demo",
        QoS::AtMostOnce,
        false,
        "Hello from rust-esp32-std-demo!".as_bytes(),
    )?;

    info!("Published a hello message to topic \"rust-esp32-std-demo\"");

    Ok(client)
}