[rubygems/rubygems] Respect `BUNDLE_USER_CONFIG` if set

https://github.com/rubygems/rubygems/commit/f28ab141af

lib/bundler/settings.rb

@@ -428,6 +428,8 @@ module Bundler
     def global_config_file
       if ENV["BUNDLE_CONFIG"] && !ENV["BUNDLE_CONFIG"].empty?
         Pathname.new(ENV["BUNDLE_CONFIG"])
+      elsif ENV["BUNDLE_USER_CONFIG"] && !ENV["BUNDLE_USER_CONFIG"].empty?
+        Pathname.new(ENV["BUNDLE_USER_CONFIG"])
       elsif Bundler.rubygems.user_home && !Bundler.rubygems.user_home.empty?
         Pathname.new(Bundler.rubygems.user_home).join(".bundle/config")
       end

spec/bundler/commands/config_spec.rb

@@ -76,6 +76,20 @@ RSpec.describe ".bundle/config" do
     end
   end
 
+  describe "config location" do
+    let(:bundle_user_config) { File.join(Dir.home, ".config/bundler") }
+
+    before do
+      Dir.mkdir File.dirname(bundle_user_config)
+    end
+
+    it "can be configured through BUNDLE_USER_CONFIG" do
+      bundle "config set path vendor", :env => { "BUNDLE_USER_CONFIG" => bundle_user_config }
+      bundle "config get path", :env => { "BUNDLE_USER_CONFIG" => bundle_user_config }
+      expect(out).to include("Set for the current user (#{bundle_user_config}): \"vendor\"")
+    end
+  end
+
   describe "global" do
     before(:each) do
       install_gemfile <<-G