// The callback we'll use to pass to the Stream. It will write the input directly to the output.
let f = Box::new(move |input: &[f32], _: Settings,
                       output: &mut[f32], _: Settings,
                       dt: f64, _: CallbackFlags| {
    for (output_sample, input_sample) in o.iter_mut().zip(i.iter()) {
        *output_sample = *input_sample;
    }
    CallbackResult::Continue
});

// Run our callback on the default, duplex, non-blocking stream.
let stream = SoundStream::new()
    .duplex(StreamParams::new(), StreamParams::new())
    .run_callback(f)
    .unwrap();