escape_once works on frozen strings.

That way, it works the same way as #html_escape and
ERB::Util.html_escape_once for which it acts as a replacement in Rails.

Furthermore, it is not obvious from the method name that the string
given as an argument is being changed.

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Haml Changelog
 
+* Helpers#escape_once works on frozen strings (as does
+  ERB::Util.html_escape_once for which it acts as a replacement in
+  Rails.
+
 ## 4.0.3
 
 Released May 21, 2013 ([diff](https://github.com/haml/haml/compare/4.0.2...4.0.3)).

lib/haml/helpers.rb

@@ -564,14 +564,12 @@ MESSAGE
       # @return [String] The sanitized string
       def escape_once(text)
         text = text.to_s
-        text.gsub!(HTML_ESCAPE_ONCE_REGEX, HTML_ESCAPE)
-        text
+        text.gsub(HTML_ESCAPE_ONCE_REGEX, HTML_ESCAPE)
       end
     else
       def escape_once(text)
         text = text.to_s
-        text.gsub!(HTML_ESCAPE_ONCE_REGEX){|s| HTML_ESCAPE[s]}
-        text
+        text.gsub(HTML_ESCAPE_ONCE_REGEX){|s| HTML_ESCAPE[s]}
       end
     end
 

test/helper_test.rb

@@ -553,5 +553,13 @@ HAML
     $stderr = old_stderr
   end
 
+  def test_escape_once_should_work_on_frozen_strings
+    begin
+      Haml::Helpers.escape_once('foo'.freeze)
+    rescue => e
+      flunk e.message
+    end
+  end
+
 end