At least two years ago, Google seemed to be driving an effort to move away from Piper and to Mercurial - not just a frontend (https://groups.google.com/forum/#!

At least two years ago, Google seemed to be driving an effort to move away from Piper and to Mercurial - not just a frontend (https://groups.google.com/forum/#!topic/mozilla.dev.version-control/hh8-l0I2b-0 https://groups.google.com/forum/#!topic/mozilla.dev.version-...). They would need to build a bunch of custom extensions for that to work, and they've open sourced some. But I have no idea what came of that effort.