1 files changed, 24 insertions, 12 deletions
diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index 8b8aa2b380..c56ac16203 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -76,10 +76,24 @@ end
 module ActiveSupport #:nodoc:
   class SafeBuffer < String
     UNSAFE_STRING_METHODS = ["capitalize", "chomp", "chop", "delete", "downcase", "gsub", "lstrip", "next", "reverse", "rstrip", "slice", "squeeze", "strip", "sub", "succ", "swapcase", "tr", "tr_s", "upcase"].freeze
+
+    # TODO: Should safe_concat check if the current buffer is dirty or not?
+    # We should probably raise as it would mean we are adding concatenating
+    # to something that is safe but it actually isn't.
     alias safe_concat concat
 
+    def initialize(*)
+      @dirty = false
+      super
+    end
+
+    def initialize_copy(other)
+      super
+      @dirty = other.dirty?
+    end
+
     def concat(value)
-      if value.html_safe?
+      if dirty? || value.html_safe?
         super(value)
       else
         super(ERB::Util.h(value))
@@ -92,11 +106,7 @@ module ActiveSupport #:nodoc:
     end
 
     def html_safe?
-      true
-    end
-
-    def html_safe
-      self
+      !dirty?
     end
 
     def to_s
@@ -113,7 +123,6 @@ module ActiveSupport #:nodoc:
 
     def to_yaml(*args)
       return super() if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
-
       to_str.to_yaml(*args)
     end
 
@@ -124,18 +133,21 @@ module ActiveSupport #:nodoc:
         end
 
         def #{unsafe_method}!(*args)
-          raise TypeError, "Cannot modify SafeBuffer in place"
+          @dirty = true
+          super
         end
       EOT
     end
+
+    protected
+
+    def dirty?
+      @dirty
+    end
   end
 end
 
 class String
-  def html_safe!
-    raise "You can't call html_safe! on a String"
-  end
-
   def html_safe
     ActiveSupport::SafeBuffer.new(self)
   end