It was recently revealed that Google's Allo chat app will receive its long-awaited desktop client at some point in the near future, and new data pulled from version 9.0 of the app seems to indicate that the mobile and desktop clients will be able to connect with one another in two ways – either with a code sent through a text message or a QR code that can be scanned on a mobile device. Unlike previous results of APK mining of this version, 9to5Google have actually managed to get these features into a somewhat functional state in the mobile app, meaning that their code is already (close to being) complete.
Though the feature obviously doesn't have much use at this time since the desktop client has yet to be released, it's quite plain to see that the app is offering options to use a text code or a QR code to pair with the desktop client. Without a desktop client to test on, there's no way to tell just how complete the features are at this point; the source got them to work via direct modification of the app's code inside the APK file. The features are by no means officially ready, and despite their seemingly nearly-there state, they could still end up getting scrapped in favor of a cloud link based on your Google login, similar to Hangouts, or something different entirely.
Allo is shaping up to be a fairly interesting chat app and is taking cues from other apps for some of its features. One example is incognito conversations; they mimic the many encrypted chat apps out there, but Allo will likely be one of very few services to offer a group incognito chat app. Likewise, the linking features seen here today are quite similar to how WhatsApp pairs its mobile and desktop clients. Google seems to be positioning Allo and Duo to completely replace Hangouts, at least in the consumer market, and features like these are bringing the two apps closer to that goal. In any case, more details on upcoming features of Allo will likely be available in the coming weeks.