Tested with:

diff --git a/examples/record_wav.rs b/examples/record_wav.rs
index 9127123..01f7c2d 100644
--- a/examples/record_wav.rs
+++ b/examples/record_wav.rs
@@ -13,9 +13,9 @@ fn main() -> Result<(), failure::Error> {
     let host = cpal::default_host();

     // Setup the default input device and stream with the default input format.
-    let device = host.default_input_device().expect("Failed to get default input device");
+    let device = host.default_output_device().expect("Failed to get default input device");
     println!("Default input device: {}", device.name()?);
-    let format = device.default_input_format().expect("Failed to get default input format");
+    let format = device.default_output_format().expect("Failed to get default input format");
     println!("Default input format: {:?}", format);
     let event_loop = host.event_loop();
     let stream_id = event_loop.build_input_stream(&device, &format)?;

I think this is okay, but the current API design makes it a very hidden feature. Would you like to add some docs?

Though I think we might change the Device API to differ between input and output devices, someday. At that time we might need to add a Windows-specific function to convert an output to "loopback" input.

What's a good place to document this?

I think you can document it as a part of WasapiHost.

Is there a way I can help push this forward? I have a project here, that would really benefit from this being merged. I currently have a vendored version of cpal with hacked together loopback support, it would be awesome if I could use a upstream version.

Merging as per @ishitatsuyuki 's approval, there seem to be docs now.

Awesome! Thank y'all for the continued work on cpal!

Is there any document or example about how to use loopback in RustAudio/cpal?

It used to be as simple as just creating an input stream and sticking that into your output stream. The cpal API changed tho and I didn't had the time to wrap my head around it so I can't provide example code. But I guess the general idea didn't change here, just the function names.