BUNDLE_WITHOUT is not applied when deploying app on Heroku - heroku

Question

My goal is to deploy an app on Heroku for the environment 'heroku_production'.
My Gemfile looks like this:
source 'https://rubygems.org'
gem 'rails', '3.2.9'
# Bundle edge Rails instead:
# gem 'rails', :git => 'git://github.com/rails/rails.git'
group :development, :test, :production do
gem 'sqlite3'
end
# This is needed in case this app is deployed in Heroku - in order to make the Heroku deployment independent from the production environment
group :heroku_production do
gem 'pg'
end
...
I set the BUNDLE_WITHOUT variable to "development:test:production". Here is the output when calling heroku config --app my_app:
=== my_app Config Vars
BUNDLE_WITHOUT: development:test:production
DATABASE_URL: postgres://XXXXXXX....
HEROKU_POSTGRESQL_CYAN_URL: postgres://XXXXXXX....
RACK_ENV: heroku_production
RAILS_ENV: heroku_production
But when I push my stuff to Heroku the BUNDLE_WITHOUT var is not applied as you can see in the following error message (line 3: --without development:test):
-----> Ruby/Rails app detected
-----> Installing dependencies using Bundler version 1.3.2
Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Using rake (10.0.4)
...
Using pg (0.14.1)
Installing sqlite3 (1.3.7)
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
/usr/local/bin/ruby extconf.rb
checking for sqlite3.h... no
sqlite3.h is missing. Try 'port install sqlite3 +universal'
or 'yum install sqlite-devel' and check your shared library search path (the
location where your sqlite3 shared library is located).
*** extconf.rb failed ***
An error occurred while installing sqlite3 (1.3.7), and Bundler cannot continue.
Make sure that `gem install sqlite3 -v '1.3.7'` succeeds before bundling.
!
! Failed to install gems via Bundler.
!
! Heroku push rejected, failed to compile Ruby/rails app
To git#heroku.com:my_app.git
! [remote rejected] 3dac67374c268d3bf9e051a6db39efc56ae81c76 -> master (pre-receive hook declined)
Why is Heroku doing that?
Googling didn't help as well.

Answer 1

Please note - for the time being, the Cedar stack does not support BUNDLE_WITHOUT.
From: https://devcenter.heroku.com/articles/bundler#specifying-gems-and-groups

Answer 2

I recently contacted Heroku about this issue, and they say that BUNDLE_WITHOUT now works!
Here's the response I got:
Hi,
user_env_compile is deprecated, but the functionality of this just became default for the Ruby buildpack - so you can use BUNDLE_WITHOUT config var.
I believe that you saw this article, but is that correct? https://devcenter.heroku.com/articles/bundler#specifying-gems-and-groups
Seems like we need to update that document!
Best,
[name redacted]
Haven't tried it yet but will soon.