commit 53a2c0b introduced a change to ensure gsub returns an unsafe string.

When gsub is used in block form, variables such as $1, $2, $`, $&, and $’ will be set appropriately. Now it seems that SafeBuffer's intervention loses the context and these variables are no longer available in the block.

This is the simplest test I can make to demonstrate the issue (which currently fails on 3.0-stable)...

class SafeBufferTest < ActiveSupport::TestCase
  def setup
    @buffer = ActiveSupport::SafeBuffer.new
  end
  [..]
  test "Should not break gsub block form match variables" do
    @buffer << 'matchme'
    @buffer.gsub(/(matchme)/) { assert_equal 'matchme', $1 }
  end

So far it's know that this breaks escape_javascript since it uses $1 matcher (see #1553 ). I presume there may be others also affected.

Investigating what a good fix would be.. seems awful heavy handed to have to capture and reset all those variables, but on the other hand, its bad form to leave a ruby core method different from it's documented behaviour.