For performance reasons, you can no longer call html_safe! on Strings. Instead, all Strings are always not html_safe?. Instead, you can get a SafeBuffer from a String by calling #html_safe, which will SafeBuffer.new(self).

  * Additionally, instead of doing concat("</form>".html_safe), you can do
    safe_concat("</form>"), which will skip both the flag set, and the flag
    check.
  * For the first pass, I converted virtually all #html_safe!s to #html_safe,
    and the tests pass. A further optimization would be to try to use
    #safe_concat as much as possible, reducing the performance impact if
    we know up front that a String is safe.

  This consists of:

  * String#html_safe! a method to mark a string as 'safe'
  * ActionView::SafeBuffer a string subclass which escapes anything unsafe which is concatenated to it
  * Calls to String#html_safe! throughout the rails helpers
  * a 'raw' helper which lets you concatenate trusted HTML from non-safety-aware sources (e.g. presantized strings in the DB)
  * New ERB implementation based on erubis which uses a SafeBuffer instead of a String

Hat tip to Django for the inspiration.