The Google Pixel 2 XL supports USB Power Delivery to charge quickly with the Google 18W USB-C Power Adapter that was originally supplied with the phone. But what happens when you try to charge the Google Pixel 2 XL from other USB Power Delivery enabled products like the Apple MacBook (Early 2016)?
Once we connected the Google Pixel 2 XL through USB Type-C to the Apple MacBook (Early 2016), we can see using Granite River Labs GRL-USB-PD-A1 power analyzer that Vendor Defined Messages initially happen to adjust voltage and current. When completed, the Apple MacBook (Early 2016) initiates USB Power Delivery communication to repeatedly advertise it can provide only 7.5 watts (5 volts @ 1.5 amps) when 90% charged.
With 58% battery left, the Google Pixel 2 XL then requests this 7.5 watts (5 volts @ 1.5 amps) which is accepted by the Mac.
Later we can see the same power cycle gets repeated over 7.5W followed by the Mac and phone exchanging Discover Identity protocol.
Here we can confirm that when plugged in, the Google Pixel 2 XL gets around 7.4 watts (4.9 volts @ 1.5 amps) charging from the 90% charged Apple MacBook (Early 2016), starting from 58% battery level when the phone is on. This power is more similar to the 7.5 watts allowed from USB Battery Charging 1.2 spec which provides a decent level of charging but certainly not to the level that would be considered needing for rapid charging nowadays.
Despite the slower charging than when using the original Google 18W charger, you can still power the phone using the Mac if you have no access to the Google charger.