Replacement of %q when escape characters are present can break code

[tonymarklove]

The rule that replaces %q with a double quote string literal is not correct in the presence of both an escaped backslash and an escaped single quote.

Our real use case was to replace 'hello' (quotes are part of the string) with \'hello\'.

---

Expected behavior

No change.

Actual behavior

diff --git a/example.rb b/example.rb
index ef2b914..380c92a 100644
--- a/example.rb
+++ b/example.rb
@@ -7,7 +7,7 @@ class TestSample < Test::Unit::TestCase
     target = 'foobar'
     expected = "\\'bar"
     assert do
-      target.gsub('foo', %q(\\\')) == expected
+      target.gsub('foo', "\\\'") == expected
     end
   end
 end

Steps to reproduce the problem

example.rb

# frozen_string_literal: true
require 'test/unit'

# comment
class TestSample < Test::Unit::TestCase
  test 'replace' do
    target = 'foobar'
    expected = "\\'bar"
    assert do
      target.gsub('foo', %q(\\\')) == expected
    end
  end
end

$ ruby example.rb
Loaded suite example
Started
.

Finished in 0.000961 seconds.
---------------------------------------------------------------------------------------------------------
1 tests, 1 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
---------------------------------------------------------------------------------------------------------

$ rubocop -a example.rb
Inspecting 1 file
C

Offenses:

example.rb:10:26: C: [Corrected] Use %q only for strings that contain both single quotes and double quotes.
      target.gsub('foo', %q(\\\')) == expected
                         ^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

ruby example.rb
Loaded suite example
Started
F
=========================================================================================================
Failure:
        target.gsub('foo', "\\\'") == expected
        |      |                   |  |
        |      |                   |  "\\'bar"
        |      |                   false
        |      "barbar"
        "foobar"
test: replace(TestSample)
/Users/tony/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/power_assert-0.2.6/lib/power_assert.rb:36:in `start'
example.rb:9:in `block in <class:TestSample>'
      6:   test 'replace' do
      7:     target = 'foobar'
      8:     expected = "\\'bar"
  =>  9:     assert do
     10:       target.gsub('foo', "\\\'") == expected
     11:     end
     12:   end
=========================================================================================================


Finished in 0.005932 seconds.
---------------------------------------------------------------------------------------------------------
1 tests, 1 assertions, 1 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
0% passed
---------------------------------------------------------------------------------------------------------

RuboCop version

$ rubocop -V
0.41.1 (using Parser 2.3.1.2, running on ruby 2.3.1 x86_64-darwin15)