Explain how to upgrade bin/ for Rails 4

railties/lib/rails/app_rails_loader.rb

@@ -12,22 +12,43 @@ def self.exec_app_rails
       exe ||= find_executable_in_parent_path
       return unless exe
 
-      if File.read(exe) =~ /(APP|ENGINE)_PATH/
-        # This is a Rails-generated binstub, let's use it
+      contents = File.read(exe)
+
+      # This is the Rails executable, let's use it
+      if contents =~ /(APP|ENGINE)_PATH/
         exec RUBY, exe, *ARGV if find_executable
         Dir.chdir("..") do
           # Recurse in a chdir block: if the search fails we want to be sure
           # the application is generated in the original working directory.
           exec_app_rails unless cwd == Dir.pwd
         end
-      elsif exe.match(%r(bin/rails$))
-        # this is a Bundler binstub, so we load the app ourselves
+
+      # This is a Bundler binstub. Stop and explain how to upgrade.
+      elsif exe =~ /bin\/rails$/ && contents =~ /This file was generated by Bundler/
+        $stderr.puts <<-end_bin_upgrade_warning
+Looks like your app's ./bin/rails is a stub that was generated by Bundler.
+
+In Rails 4, your app's bin/ directory contains executables that are versioned
+like any other source code, rather than stubs that are generated on demand.
+
+Here's how to upgrade:
+
+  bundle config --delete bin    # Turn off Bundler's stub generator
+  rake rails:update:bin         # Use the new Rails 4 executables
+  git add bin                   # Add bin/ to source control
+
+You may need to remove bin/ from your .gitignore as well.
+
+When you install a gem whose executable you want to use in your app,
+generate it and add it to source control:
+
+  bundle binstubs some-gem-name
+  git add bin/new-executable
+
+        end_bin_upgrade_warning
+
         Object.const_set(:APP_PATH, File.expand_path('config/application',  Dir.pwd))
         require File.expand_path('../boot', APP_PATH)
-        puts "Rails 4 no longer supports Bundler's --binstubs option. You " \
-          "will need to disable it and update your bin/rails file.\n" \
-          "Please run: `bundle config --delete bin && rm -rf bin`, then " \
-          "`rake rails:update:bin` and add the resulting bin/ to git."
         require 'rails/commands'
       end
     rescue SystemCallError