it is the first time I ask for something so sorry if I\'m not enough precise!
I tried to upload my rails app on heroku using enabling the user-env-compile to manage
You don't have to do anything (the labs is deprecated and you should delete it), by default now heroku is exporting your environement variables during slug compilation.
Be careful, it is not recommended to use the environement variable during slug compilation (you can check http://12factor.net/ for more informations)
Hope it helps.