Feature-flag the postit trampoline, disabled by default

Set ENV["BUNDLE_ENABLE_TRAMPOLINE"] to enable it

lib/bundler/postit_trampoline.rb

@@ -1,62 +1,65 @@
 # frozen_string_literal: true
+if ENV["BUNDLE_ENABLE_TRAMPOLINE"]
 
-module BundlerVendoredPostIt; end
-require "bundler/vendor/postit/lib/postit"
-require "rubygems"
+  module BundlerVendoredPostIt; end
+  require "bundler/vendor/postit/lib/postit"
+  require "rubygems"
 
-environment = BundlerVendoredPostIt::PostIt::Environment.new([])
-version = Gem::Requirement.new(environment.bundler_version)
+  environment = BundlerVendoredPostIt::PostIt::Environment.new([])
+  version = Gem::Requirement.new(environment.bundler_version)
 
-installed_version =
-  if defined?(Bundler::VERSION)
-    Bundler::VERSION
-  else
-    File.read(File.expand_path("../version.rb", __FILE__)) =~ /VERSION = "(.+)"/
-    $1
-  end
-installed_version &&= Gem::Version.new(installed_version)
-
-if !version.satisfied_by?(installed_version)
-  begin
-    installer = BundlerVendoredPostIt::PostIt::Installer.new(version)
-    installer.install!
-  rescue => e
-    abort <<-EOS.strip
+  installed_version =
+    if defined?(Bundler::VERSION)
+      Bundler::VERSION
+    else
+      File.read(File.expand_path("../version.rb", __FILE__)) =~ /VERSION = "(.+)"/
+      $1
+    end
+  installed_version &&= Gem::Version.new(installed_version)
+
+  if !version.satisfied_by?(installed_version)
+    begin
+      installer = BundlerVendoredPostIt::PostIt::Installer.new(version)
+      installer.install!
+    rescue => e
+      abort <<-EOS.strip
 Installing the inferred bundler version (#{version}) failed.
 If you'd like to update to the current bundler version (#{installed_version}) in this project, run `bundle update --bundler`.
 The error was: #{e}
     EOS
-  end
+    end
 
-  if deleted_spec = Gem.loaded_specs.delete("bundler")
-    deleted_spec.full_require_paths.each {|path| $:.delete(path) }
+    if deleted_spec = Gem.loaded_specs.delete("bundler")
+      deleted_spec.full_require_paths.each {|path| $:.delete(path) }
+    else
+      $:.delete(File.expand_path("../..", __FILE__))
+    end
+    gem "bundler", version
   else
-    $:.delete(File.expand_path("../..", __FILE__))
+    begin
+      gem "bundler", version
+    rescue LoadError
+      $:.unshift(File.expand_path("../..", __FILE__))
+    end
   end
-  gem "bundler", version
-else
-  begin
-    gem "bundler", version
-  rescue LoadError
-    $:.unshift(File.expand_path("../..", __FILE__))
+
+  running_version = begin
+    require "bundler/version"
+    Bundler::VERSION
+  rescue LoadError, NameError
+    nil
   end
-end
-
-running_version = begin
-  require "bundler/version"
-  Bundler::VERSION
-rescue LoadError, NameError
-  nil
-end
-
-if Gem::Requirement.new(">= 1.13.pre".dup).satisfied_by?(Gem::Version.new(running_version))
-  ENV["BUNDLE_POSTIT_TRAMPOLINING_VERSION"] = installed_version.to_s
-elsif ARGV.empty? || ARGV.any? {|a| %w(install i).include? a }
-  puts <<-WARN.strip
+
+  if Gem::Requirement.new(">= 1.13.pre".dup).satisfied_by?(Gem::Version.new(running_version))
+    ENV["BUNDLE_POSTIT_TRAMPOLINING_VERSION"] = installed_version.to_s
+  elsif ARGV.empty? || ARGV.any? {|a| %w(install i).include? a }
+    puts <<-WARN.strip
 You're running Bundler #{installed_version} but this project uses #{running_version}. To update, run `bundle update --bundler`.
   WARN
-end
+  end
+
+  if !Gem::Version.correct?(running_version.to_s) || !version.satisfied_by?(Gem::Version.create(running_version))
+    abort "The running bundler (#{running_version}) does not match the required `#{version}`"
+  end
 
-if !Gem::Version.correct?(running_version.to_s) || !version.satisfied_by?(Gem::Version.create(running_version))
-  abort "The running bundler (#{running_version}) does not match the required `#{version}`"
-end
+end # unless ENV["BUNDLE_ENABLE_TRAMPOLINE"]

spec/other/trampoline_spec.rb

@@ -4,6 +4,7 @@
 describe "bundler version trampolining" do
   before do
     ENV["BUNDLE_DISABLE_POSTIT"] = nil
+    ENV["BUNDLE_ENABLE_TRAMPOLINE"] = "true"
     FileUtils.rm_rf(system_gem_path)
     FileUtils.cp_r(base_system_gems, system_gem_path)
   end