Make Bundler.setup not make Kernel#gem public in Bundler 2

lib/bundler/feature_flag.rb

@@ -38,6 +38,7 @@ def self.settings_method(name, key, &default)
     settings_flag(:only_update_to_newer_versions) { bundler_2_mode? }
     settings_flag(:plugins) { @bundler_version >= Gem::Version.new("1.14") }
     settings_flag(:prefer_gems_rb) { bundler_2_mode? }
+    settings_flag(:setup_makes_kernel_gem_public) { !bundler_2_mode? }
     settings_flag(:skip_default_git_sources) { bundler_2_mode? }
     settings_flag(:specific_platform) { bundler_2_mode? }
     settings_flag(:suppress_install_using_messages) { bundler_2_mode? }

lib/bundler/rubygems_integration.rb

@@ -393,9 +393,8 @@ def replace_gem(specs, specs_by_name)
           raise e
         end
 
-        # TODO: delete this in 2.0, it's a backwards compatibility shim
-        # see https://github.com/bundler/bundler/issues/5102
-        kernel_class.send(:public, :gem)
+        # backwards compatibility shim, see https://github.com/bundler/bundler/issues/5102
+        kernel_class.send(:public, :gem) if Bundler.feature_flag.setup_makes_kernel_gem_public?
       end
     end
 

lib/bundler/settings.rb

@@ -35,6 +35,7 @@ class Settings
       only_update_to_newer_versions
       plugins
       prefer_gems_rb
+      setup_makes_kernel_gem_public
       silence_root_warning
       skip_default_git_sources
       specific_platform

spec/runtime/setup_spec.rb

@@ -1338,17 +1338,51 @@ def lock_with(ruby_version = nil)
   end
 
   describe "after setup" do
-    it "allows calling #gem on random objects" do
+    it "allows calling #gem on random objects", :bundler => "< 2" do
       install_gemfile <<-G
         source "file:#{gem_repo1}"
         gem "rack"
       G
+
       ruby! <<-RUBY
         require "bundler/setup"
         Object.new.gem "rack"
         puts Gem.loaded_specs["rack"].full_name
       RUBY
+
       expect(out).to eq("rack-1.0.0")
     end
+
+    it "keeps Kernel#gem private", :bundler => "2" do
+      install_gemfile! <<-G
+        source "file:#{gem_repo1}"
+        gem "rack"
+      G
+
+      ruby <<-RUBY
+        require "bundler/setup"
+        Object.new.gem "rack"
+        puts "FAIL"
+      RUBY
+
+      expect(last_command.stdboth).not_to include "FAIL"
+      expect(last_command.stderr).to include "private method `gem'"
+    end
+
+    it "keeps Kernel#require private" do
+      install_gemfile! <<-G
+        source "file:#{gem_repo1}"
+        gem "rack"
+      G
+
+      ruby <<-RUBY
+        require "bundler/setup"
+        Object.new.require "rack"
+        puts "FAIL"
+      RUBY
+
+      expect(last_command.stdboth).not_to include "FAIL"
+      expect(last_command.stderr).to include "private method `require'"
+    end
   end
 end