3 files changed, 131 insertions, 1 deletions
diff --git a/activesupport/lib/active_support/core_ext/string.rb b/activesupport/lib/active_support/core_ext/string.rb
index d06a5a32fb..6c52f12712 100644
--- a/activesupport/lib/active_support/core_ext/string.rb
+++ b/activesupport/lib/active_support/core_ext/string.rb
@@ -7,4 +7,5 @@ require 'active_support/core_ext/string/access'
 require 'active_support/core_ext/string/iterators'
 require 'active_support/core_ext/string/xchar'
 require 'active_support/core_ext/string/behavior'
-require 'active_support/core_ext/string/interpolation'
-\ No newline at end of file
+require 'active_support/core_ext/string/interpolation'
+require 'active_support/core_ext/string/output_safety'
+\ No newline at end of file
diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
new file mode 100644
index 0000000000..2cca4763f4
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -0,0 +1,43 @@
+class String
+  def html_safe?
+    defined?(@_rails_html_safe) && @_rails_html_safe
+  end
+
+  def html_safe!
+    @_rails_html_safe = true
+    self
+  end
+  
+  def html_safe
+    dup.html_safe!
+  end
+  
+  alias original_plus +
+  def +(other)
+    result = original_plus(other)
+    if html_safe? && also_html_safe?(other)
+      result.html_safe!
+    else
+      result
+    end
+  end
+  
+  alias original_concat <<
+  def <<(other)
+    result = original_concat(other)
+    unless html_safe? && also_html_safe?(other)
+      @_rails_html_safe = false
+    end
+    result
+  end
+  
+  def concat(other)
+    self << other
+  end
+  
+  private
+    def also_html_safe?(other)
+      other.respond_to?(:html_safe?) && other.html_safe?
+    end
+  
+end
+\ No newline at end of file
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index db9073e298..584a41b631 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -356,3 +356,89 @@ class StringBytesizeTest < Test::Unit::TestCase
     assert_equal 3, 'foo'.bytesize
   end
 end
+
+class OutputSafetyTest < ActiveSupport::TestCase
+  def setup
+    @string = "hello"
+  end
+
+  test "A string is unsafe by default" do
+    assert !@string.html_safe?
+  end
+
+  test "A string can be marked safe" do
+    @string.html_safe!
+    assert @string.html_safe?
+  end
+
+  test "Marking a string safe returns the string" do
+    assert_equal @string, @string.html_safe!
+  end
+
+  test "Adding a safe string to another safe string returns a safe string" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+    @combination = @other_string + @string
+
+    assert_equal "otherhello", @combination
+    assert @combination.html_safe?
+  end
+
+  test "Adding an unsafe string to a safe string returns an unsafe string" do
+    @other_string = "other".html_safe!
+    @combination = @other_string + @string
+    @other_combination = @string + @other_string
+
+    assert_equal "otherhello", @combination
+    assert_equal "helloother", @other_combination
+
+    assert !@combination.html_safe?
+    assert !@other_combination.html_safe?
+  end
+
+  test "Concatting safe onto unsafe yields unsafe" do
+    @other_string = "other"
+    @string.html_safe!
+
+    @other_string.concat(@string)
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting unsafe onto safe yields unsafe" do
+    @other_string = "other".html_safe!
+
+    @other_string.concat(@string)
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting safe onto safe yields safe" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+
+    @other_string.concat(@string)
+    assert @other_string.html_safe?
+  end
+
+  test "Concatting safe onto unsafe with << yields unsafe" do
+    @other_string = "other"
+    @string.html_safe!
+
+    @other_string << @string
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting unsafe onto safe with << yields unsafe" do
+    @other_string = "other".html_safe!
+
+    @other_string << @string
+    assert !@other_string.html_safe?
+  end
+
+  test "Concatting safe onto safe with << yields safe" do
+    @other_string = "other".html_safe!
+    @string.html_safe!
+
+    @other_string << @string
+    assert @other_string.html_safe?
+  end
+end