remove some evals from callback conditionals

activemodel/lib/active_model/callbacks.rb

@@ -135,7 +135,10 @@ module ActiveModel
       klass.define_singleton_method("after_#{callback}") do |*args, &block|
         options = args.extract_options!
         options[:prepend] = true
-        options[:if] = Array(options[:if]) << "value != false"
+        conditional = ActiveSupport::Callbacks::Conditionals::Value.new { |v|
+          v != false
+        }
+        options[:if] = Array(options[:if]) << conditional
         set_callback(:"#{callback}", :after, *(args << options), &block)
       end
     end

activemodel/lib/active_model/validations.rb

@@ -142,7 +142,9 @@ module ActiveModel
         if options.key?(:on)
           options = options.dup
           options[:if] = Array(options[:if])
-          options[:if].unshift("validation_context == :#{options[:on]}")
+          options[:if].unshift lambda { |o|
+            o.validation_context == options[:on]
+          }
         end
         args << options
         set_callback(:validate, *args, &block)

activesupport/lib/active_support/callbacks.rb

@@ -94,6 +94,15 @@ module ActiveSupport
     def halted_callback_hook(filter)
     end
 
+    module Conditionals # :nodoc:
+      class Value
+        def initialize(&block)
+          @block = block
+        end
+        def call(target, value); @block.call(value); end
+      end
+    end
+
     module Filters
       Environment = Struct.new(:target, :halted, :value, :run_block)
 
@@ -415,6 +424,7 @@ module ActiveSupport
         when String
           l = eval "lambda { |value| #{filter} }"
           lambda { |target, value| target.instance_exec(value, &l) }
+        when Conditionals::Value then filter
         when ::Proc
           if filter.arity > 1
             return lambda { |target, _, &block|