Delay tempfile loading.

(See comment for explanation about why we do this).

lib/rspec/matchers/built_in/output.rb

@@ -1,5 +1,4 @@
 require 'stringio'
-require 'tempfile'
 
 module RSpec
   module Matchers
@@ -172,6 +171,14 @@ def self.capture(block)
       # @private
       class CaptureStreamToTempfile < Struct.new(:name, :stream)
         def capture(block)
+          # We delay loading tempfile until it is actually needed because
+          # we want to minimize stdlibs loaded so that users who use a
+          # portion of the stdlib can't have passing specs while forgetting
+          # to load it themselves. `CaptureStreamToTempfile` is rarely used
+          # and `tempfile` pulls in a bunch of things (delegate, tmpdir,
+          # thread, fileutils, etc), so it's worth delaying it until this point.
+          require 'tempfile'
+
           original_stream = stream.clone
           captured_stream = Tempfile.new(name)
 

spec/rspec/expectations_spec.rb

@@ -9,10 +9,8 @@
       'require "rspec/expectations"'
     ],
     :allowed_loaded_feature_regexps => [
-      /stringio/, /tempfile/,         # Used by `output` matcher. Can't be easily avoided.
-      /delegate/, /tmpdir/, /thread/, # required by tempfile
-      /fileutils/, /etc/,             # required by tmpdir
-      /prettyprint.rb/, /pp.rb/       # required by rspec-support
+      /stringio/,               # Used by `output` matcher. Can't be easily avoided.
+      /prettyprint.rb/, /pp.rb/ # required by rspec-support
     ]
 
   it 'does not allow expectation failures to be caught by a bare rescue' do