add subject variable to expression context

UPGRADE-2.8.md

@@ -448,6 +448,9 @@ FrameworkBundle
 Security
 --------
 
+ * The `object` variable passed to expressions evaluated by the `ExpressionVoter`
+   is deprecated. Instead use the new `subject` variable.
+
  * The `AbstractVoter` class was deprecated. Instead, extend the `Voter` class and
    move your voting logic in the `supports($attribute, $subject)` and
    `voteOnAttribute($attribute, $object, TokenInterface $token)` methods.

src/Symfony/Component/Security/Core/Authorization/Voter/ExpressionVoter.php

@@ -102,6 +102,7 @@ private function getVariables(TokenInterface $token, $object)
             'token' => $token,
             'user' => $token->getUser(),
             'object' => $object,
+            'subject' => $object,
             'roles' => array_map(function ($role) { return $role->getRole(); }, $roles),
             'trust_resolver' => $this->trustResolver,
         );