1 files changed, 39 insertions, 14 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 20e40fe40f..a19b5c95c7 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 +76,33 @@ 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
-    alias safe_concat concat
+
+    alias_method :original_concat, :concat
+    private :original_concat
+
+    class SafeConcatError < StandardError
+      def initialize
+        super "Could not concatenate to the buffer because it is not html safe."
+      end
+    end
+
+    def safe_concat(value)
+      raise SafeConcatError if dirty?
+      original_concat(value)
+    end
+
+    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))
@@ -92,15 +115,15 @@ module ActiveSupport #:nodoc:
     end
 
     def html_safe?
-      true
+      !dirty?
     end
 
-    def html_safe
+    def to_s
       self
     end
 
-    def to_s
-      self
+    def to_param
+      to_str
     end
 
     def encode_with(coder)
@@ -109,29 +132,31 @@ module ActiveSupport #:nodoc:
 
     def to_yaml(*args)
       return super() if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
-
       to_str.to_yaml(*args)
     end
 
     UNSAFE_STRING_METHODS.each do |unsafe_method|
       class_eval <<-EOT, __FILE__, __LINE__
-        def #{unsafe_method}(*args)
-          super.to_str
+        def #{unsafe_method}(*args, &block)
+          to_str.#{unsafe_method}(*args, &block)
         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