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: feedback.rs
    path of file: ./repos/cpal/examples
    
    the code of the file until where you have to start completion: //! Feeds back the input stream directly into the output stream.
//!
//! Assumes that the input and output devices can use the same stream configuration and that they
//! support the f32 sample format.
//!
//! Uses a delay of `LATENCY_MS` milliseconds in case the default input and output streams are not
//! precisely synchronised.

use clap::Parser;
use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
use ringbuf::HeapRb;

#[derive(Parser, Debug)]
#[command(version, about = "CPAL feedback example", long_about = None)]
struct Opt {
    /// The input audio device to use
    #[arg(short, long, value_name = "IN", default_value_t = String::from("default"))]
    input_device: String,

    /// The output audio device to use
    #[arg(short, long, value_name = "OUT", default_value_t = String::from("default"))]
    output_device: String,

    /// Specify the delay between input and output
    #[arg(short, long, value_name = "DELAY_MS", default_value_t = 150.0)]
    latency: f32,

    /// Use the JACK host
    #[cfg(all(
        any(
            target_os = "linux",
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "netbsd"
        ),
        feature = "jack"
    ))]
    #[arg(short, long)]
    #[allow(dead_code)]
    jack: bool,
}

fn main() -> anyhow::Result<()> {
    let opt = Opt::parse();

    // Conditionally compile with jack if the feature is specified.
    #[cfg(all(
        any(
            target_os = "linux",
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "netbsd"
        ),
        feature = "jack"
    ))]
    
fn main() -> anyhow::Result<()> {
    let opt = Opt::parse();

    // Conditionally compile with jack if the feature is specified.
    #[cfg(all(
        any(
            target_os = "linux",
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "netbsd"
        ),
        feature = "jack"
    ))]
    // Manually check for flags. Can be passed through cargo with -- e.g.
    // cargo run --release --example beep --features jack -- --jack
    let host = if opt.jack {
        cpal::host_from_id(cpal::available_hosts()
            .into_iter()
            .find(|id| *id == cpal::HostId::Jack)
            .expect(
                "make sure --features jack is specified. only works on OSes where jack is available",
            )).expect("jack host unavailable")
    } else {
        cpal::default_host()
    };

    #[cfg(any(
        not(any(
            target_os = "linux",
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "netbsd"
        )),
        not(feature = "jack")
    ))]
    let host = cpal::default_host();

    // Find devices.
    let input_device = if opt.input_device == "default" {
        host.default_input_device()
    } else {
        host.input_devices()?
            .find(|x| x.name().map(|y| y == opt.input_device).unwrap_or(false))
    }
    .expect("failed to find input device");

    let output_device = if opt.output_device == "default" {
        host.default_output_device()
    } else {
        host.output_devices()?
            .find(|x| x.name().map(|y| y == opt.output_device).unwrap_or(false))
    }
    .expect("failed to find output device");

    println!("Using input device: \"{}\"", input_device.name()?);
    println!("Using output device: \"{}\"", output_device.name()?);

    // We'll try and use the same configuration between streams to keep it simple.
    let config: cpal::StreamConfig = input_device.default_input_config()?.into();

    // Create a delay in case the input and output devices aren't synced.
    let latency_frames = (opt.latency / 1_000.0) * config.sample_rate.0 as f32;
    let latency_samples = latency_frames as usize * config.channels as usize;

    // The buffer to share samples
    let ring = HeapRb::<f32>::new(latency_samples * 2);
    let (mut producer, mut consumer) = ring.split();

    // Fill the samples with 0.0 equal to the length of the delay.
    for _ in 0..latency_samples {
        // The ring buffer has twice as much space as necessary to add latency here,
        // so this should never fail
        producer.push(0.0).unwrap();
    }

    let input_data_fn = move |data: &[f32], _: &cpal::InputCallbackInfo| {
        let mut output_fell_behind = false;
        for &sample in data {
            if producer.push(sample).is_err() {
                output_fell_behind = true;
            }
        }
        if output_fell_behind {
            eprintln!("output stream fell behind: try increasing latency");
        }
    };

    let output_data_fn = move |data: &mut [f32], _: &cpal::OutputCallbackInfo| {
        let mut input_fell_behind = false;
        for sample in data {
            *sample = match consumer.pop() {
                Some(s) => s,
                None => {
                    input_fell_behind = true;
                    0.0
                }
            };
        }
        if input_fell_behind {
            eprintln!("input stream fell behind: try increasing latency");
        }
    };

    // Build streams.
    println!(
        "Attempting to build both streams with f32 samples and `{:?}`.",
        config
    );
    let input_stream = input_device.build_input_stream(&config, input_data_fn, err_fn, None)?;
    let output_stream = output_device.build_output_stream(&config, output_data_fn, err_fn, None)?;
    println!("Successfully built streams.");

    // Play the streams.
    println!(
        "Starting the input and output streams with `{}` milliseconds of latency.",
        opt.latency
    );
    input_stream.play()?;
    output_stream.play()?;

    // Run for 3 seconds before closing.
    println!("Playing for 3 seconds... ");
    std::thread::sleep(std::time::Duration::from_secs(3));
    drop(input_stream);
    drop(output_stream);
    println!("Done!");
    Ok(())
}