using instance_exec to change scope of can blocks to instance of ability, this is a bit ugly so I may end up using methods instead

lib/cancan.rb

@@ -1,2 +1,3 @@
 $:.unshift(File.dirname(__FILE__))
 require 'cancan/ability'
+require 'cancan/instance_exec'

lib/cancan/ability.rb

@@ -1,5 +1,7 @@
 module CanCan
   module Ability
+    attr_accessor :user
+    
     def self.included(base)
       base.extend ClassMethods
       base.alias_action :index, :show, :to => :read
@@ -18,7 +20,7 @@ module CanCan
               block_args << action if can_action == :manage
               block_args << (target.class == Class ? target : target.class) if can_target == :all
               block_args << (target.class == Class ? nil : target)
-              return can_block.call(*block_args)
+              return instance_exec(*block_args, &can_block)
             end
           end
         end
@@ -48,6 +50,12 @@ module CanCan
         target = args.pop[:to]
         @aliased_actions[target] = args
       end
+      
+      def for_user(user)
+        ability = new
+        ability.user = user
+        ability
+      end
     end
   end
 end

lib/cancan/instance_exec.rb

@@ -0,0 +1,15 @@
+# see http://eigenclass.org/hiki.rb?instance_exec
+class Object
+  module InstanceExecHelper; end
+  include InstanceExecHelper
+  def instance_exec(*args, &block) # !> method redefined; discarding old instance_exec
+    mname = "__instance_exec_#{Thread.current.object_id.abs}_#{object_id.abs}"
+    InstanceExecHelper.module_eval{ define_method(mname, &block) }
+    begin
+      ret = send(mname, *args)
+    ensure
+      InstanceExecHelper.module_eval{ undef_method(mname) } rescue nil
+    end
+    ret
+  end
+end

spec/cancan/ability_spec.rb

@@ -77,4 +77,11 @@ describe CanCan::Ability do
     @ability.can?(:new, 123).should == :create_called
     @ability.can?(:edit, 123).should == :update_called
   end
+  
+  it "should be able to access given user" do
+    @ability_class.can(:preview, :all) { user }
+    ability = @ability_class.for_user(:some_user)
+    ability.user.should == :some_user
+    ability.can?(:preview, 123).should == :some_user
+  end
 end