Fix inconsistent behavior from String#first/#last

While calling String#first or String#last with zero or a Fixnum < the
string's length returns a new string, a Fixnum >= the string's length
returns the string itself. This inconsistency can lead to inadvertent
mutation of a string.

1 files changed, 12 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index ea12f1ced5..f8d4d0f0dc 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -296,6 +296,12 @@ class StringAccessTest < ActiveSupport::TestCase
     assert_equal 'x', 'x'.first(4)
   end
 
+  test "#first with Fixnum >= string length still returns a new string" do
+    string = "hello"
+    different_string = string.first(5)
+    assert_not_same different_string, string
+  end
+
   test "#last returns the last character" do
     assert_equal "o", "hello".last
     assert_equal 'x', 'x'.last
@@ -308,6 +314,12 @@ class StringAccessTest < ActiveSupport::TestCase
     assert_equal 'x', 'x'.last(4)
   end
 
+  test "#last with Fixnum >= string length still returns a new string" do
+    string = "hello"
+    different_string = string.last(5)
+    assert_not_same different_string, string
+  end
+
   test "access returns a real string" do
     hash = {}
     hash["h"] = true