Freeze options so we raise an error when people modify it in place.

activemodel/lib/active_model/validations.rb

@@ -118,11 +118,13 @@ module ActiveModel
       #   end
       #
       def validate(*args, &block)
-        options = args.last
-        if options.is_a?(Hash) && options.key?(:on)
+        options = args.extract_options!
+        if options.key?(:on)
+          options = options.dup
           options[:if] = Array.wrap(options[:if])
           options[:if] << "validation_context == :#{options[:on]}"
         end
+        args << options
         set_callback(:validate, *args, &block)
       end
 

activemodel/lib/active_model/validator.rb

@@ -111,7 +111,7 @@ module ActiveModel #:nodoc:
 
     # Accepts options that will be made available through the +options+ reader.
     def initialize(options)
-      @options = options
+      @options = options.freeze
     end
 
     # Return the kind for this validator.