diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2645,6 +2645,7 @@
     'PhabricatorSubscriptionsEditor' => 'applications/subscriptions/editor/PhabricatorSubscriptionsEditor.php',
     'PhabricatorSubscriptionsListController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsListController.php',
     'PhabricatorSubscriptionsSubscribeEmailCommand' => 'applications/subscriptions/command/PhabricatorSubscriptionsSubscribeEmailCommand.php',
+    'PhabricatorSubscriptionsSubscribersPolicyRule' => 'applications/subscriptions/policyrule/PhabricatorSubscriptionsSubscribersPolicyRule.php',
     'PhabricatorSubscriptionsTransactionController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsTransactionController.php',
     'PhabricatorSubscriptionsUIEventListener' => 'applications/subscriptions/events/PhabricatorSubscriptionsUIEventListener.php',
     'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php',
@@ -6166,6 +6167,7 @@
     'PhabricatorSubscriptionsEditor' => 'PhabricatorEditor',
     'PhabricatorSubscriptionsListController' => 'PhabricatorController',
     'PhabricatorSubscriptionsSubscribeEmailCommand' => 'MetaMTAEmailTransactionCommand',
+    'PhabricatorSubscriptionsSubscribersPolicyRule' => 'PhabricatorPolicyRule',
     'PhabricatorSubscriptionsTransactionController' => 'PhabricatorController',
     'PhabricatorSubscriptionsUIEventListener' => 'PhabricatorEventListener',
     'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'MetaMTAEmailTransactionCommand',
diff --git a/src/applications/policy/__tests__/PhabricatorPolicyDataTestCase.php b/src/applications/policy/__tests__/PhabricatorPolicyDataTestCase.php
--- a/src/applications/policy/__tests__/PhabricatorPolicyDataTestCase.php
+++ b/src/applications/policy/__tests__/PhabricatorPolicyDataTestCase.php
@@ -208,5 +208,32 @@
         PhabricatorPolicyCapability::CAN_VIEW));
   }
 
+  public function testObjectPolicyRuleSubscribers() {
+    $author = $this->generateNewTestUser();
+
+    $rule = new PhabricatorSubscriptionsSubscribersPolicyRule();
+
+    $task = ManiphestTask::initializeNewTask($author);
+    $task->setViewPolicy($rule->getObjectPolicyFullKey());
+    $task->save();
+
+    $this->assertFalse(
+      PhabricatorPolicyFilter::hasCapability(
+        $author,
+        $task,
+        PhabricatorPolicyCapability::CAN_VIEW));
+
+    id(new PhabricatorSubscriptionsEditor())
+      ->setActor($author)
+      ->setObject($task)
+      ->subscribeExplicit(array($author->getPHID()))
+      ->save();
+
+    $this->assertTrue(
+      PhabricatorPolicyFilter::hasCapability(
+        $author,
+        $task,
+        PhabricatorPolicyCapability::CAN_VIEW));
+  }
 
 }
diff --git a/src/applications/policy/rule/PhabricatorPolicyRule.php b/src/applications/policy/rule/PhabricatorPolicyRule.php
--- a/src/applications/policy/rule/PhabricatorPolicyRule.php
+++ b/src/applications/policy/rule/PhabricatorPolicyRule.php
@@ -97,6 +97,56 @@
   }
 
 
+/* -(  Transaction Hints  )-------------------------------------------------- */
+
+
+  /**
+   * Tell policy rules about upcoming transaction effects.
+   *
+   * Before transaction effects are applied, we try to stop users from making
+   * edits which will lock them out of objects. We can't do this perfectly,
+   * since they can set a policy to "the moon is full" moments before it wanes,
+   * but we try to prevent as many mistakes as possible.
+   *
+   * Some policy rules depend on complex checks against object state which
+   * we can't set up ahead of time. For example, subscriptions require database
+   * writes.
+   *
+   * In cases like this, instead of doing writes, you can pass a hint about an
+   * object to a policy rule. The rule can then look for hints and use them in
+   * rendering a verdict about whether the user will be able to see the object
+   * or not after applying the policy change.
+   *
+   * @param PhabricatorPolicyInterface Object to pass a hint about.
+   * @param PhabricatorPolicyRule Rule to pass hint to.
+   * @param wild Hint.
+   * @return void
+   */
+  public static function passTransactionHintToRule(
+    PhabricatorPolicyInterface $object,
+    PhabricatorPolicyRule $rule,
+    $hint) {
+
+    $cache = PhabricatorCaches::getRequestCache();
+    $cache->setKey(self::getObjectPolicyCacheKey($object, $rule), $hint);
+  }
+
+  protected function getTransactionHint(
+    PhabricatorPolicyInterface $object) {
+
+    $cache = PhabricatorCaches::getRequestCache();
+    return $cache->getKey(self::getObjectPolicyCacheKey($object, $this));
+  }
+
+  private static function getObjectPolicyCacheKey(
+    PhabricatorPolicyInterface $object,
+    PhabricatorPolicyRule $rule) {
+    $hash = spl_object_hash($object);
+    $rule = get_class($rule);
+    return 'policycache.'.$hash.'.'.$rule;
+  }
+
+
 /* -(  Implementing Object Policies  )--------------------------------------- */
 
 
diff --git a/src/applications/subscriptions/policyrule/PhabricatorSubscriptionsSubscribersPolicyRule.php b/src/applications/subscriptions/policyrule/PhabricatorSubscriptionsSubscribersPolicyRule.php
new file mode 100644
--- /dev/null
+++ b/src/applications/subscriptions/policyrule/PhabricatorSubscriptionsSubscribersPolicyRule.php
@@ -0,0 +1,121 @@
+<?php
+
+final class PhabricatorSubscriptionsSubscribersPolicyRule
+  extends PhabricatorPolicyRule {
+
+  private $subscribed = array();
+  private $sourcePHIDs = array();
+
+  public function getObjectPolicyKey() {
+    return 'subscriptions.subscribers';
+  }
+
+  public function getObjectPolicyName() {
+    return pht('Subscribers');
+  }
+
+  public function getPolicyExplanation() {
+    return pht('Subscribers can take this action.');
+  }
+
+  public function getRuleDescription() {
+    return pht('subscribers');
+  }
+
+  public function canApplyToObject(PhabricatorPolicyInterface $object) {
+    return ($object instanceof PhabricatorSubscribableInterface);
+  }
+
+  public function willApplyRules(
+    PhabricatorUser $viewer,
+    array $values,
+    array $objects) {
+
+    // We want to let the user see the object if they're a subscriber or
+    // a member of any project which is a subscriber. Additionally, because
+    // subscriber state is complex, we need to read hints passed from
+    // the TransactionEditor to predict policy state after transactions apply.
+
+    $viewer_phid = $viewer->getPHID();
+    if (!$viewer_phid) {
+      return;
+    }
+
+    if (empty($this->subscribed[$viewer_phid])) {
+      $this->subscribed[$viewer_phid] = array();
+    }
+
+    // Load the project PHIDs the user is a member of.
+    if (!isset($this->sourcePHIDs[$viewer_phid])) {
+      $source_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
+        $viewer_phid,
+        PhabricatorProjectMemberOfProjectEdgeType::EDGECONST);
+      $source_phids[] = $viewer_phid;
+      $this->sourcePHIDs[$viewer_phid] = $source_phids;
+    }
+
+    // Look for transaction hints.
+    foreach ($objects as $key => $object) {
+      $cache = $this->getTransactionHint($object);
+      if ($cache === null) {
+        // We don't have a hint for this object, so we'll deal with it below.
+        continue;
+      }
+
+      // We have a hint, so use that as the source of truth.
+      unset($objects[$key]);
+
+      foreach ($this->sourcePHIDs[$viewer_phid] as $source_phid) {
+        if (isset($cache[$source_phid])) {
+          $this->subscribed[$viewer_phid][$object->getPHID()] = true;
+          break;
+        }
+      }
+    }
+
+    $phids = mpull($objects, 'getPHID');
+    if (!$phids) {
+      return;
+    }
+
+    $edge_query = id(new PhabricatorEdgeQuery())
+      ->withSourcePHIDs($this->sourcePHIDs[$viewer_phid])
+      ->withEdgeTypes(
+        array(
+          PhabricatorSubscribedToObjectEdgeType::EDGECONST,
+        ))
+      ->withDestinationPHIDs($phids);
+
+    $edge_query->execute();
+
+    $subscribed = $edge_query->getDestinationPHIDs();
+    if (!$subscribed) {
+      return;
+    }
+
+    $this->subscribed[$viewer_phid] += array_fill_keys($subscribed, true);
+  }
+
+  public function applyRule(
+    PhabricatorUser $viewer,
+    $value,
+    PhabricatorPolicyInterface $object) {
+
+    $viewer_phid = $viewer->getPHID();
+    if (!$viewer_phid) {
+      return false;
+    }
+
+    if ($object->isAutomaticallySubscribed($viewer_phid)) {
+      return true;
+    }
+
+    $subscribed = idx($this->subscribed, $viewer_phid);
+    return isset($subscribed[$object->getPHID()]);
+  }
+
+  public function getValueControlType() {
+    return self::CONTROL_TYPE_NONE;
+  }
+
+}
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -2087,6 +2087,19 @@
 
     foreach ($xactions as $xaction) {
       switch ($xaction->getTransactionType()) {
+        case PhabricatorTransactions::TYPE_SUBSCRIBERS:
+          $clone_xaction = clone $xaction;
+          $clone_xaction->setOldValue(array_values($this->subscribers));
+          $clone_xaction->setNewValue(
+            $this->getPHIDTransactionNewValue(
+              $clone_xaction));
+
+          PhabricatorPolicyRule::passTransactionHintToRule(
+            $copy,
+            new PhabricatorSubscriptionsSubscribersPolicyRule(),
+            array_fuse($clone_xaction->getNewValue()));
+
+          break;
         case PhabricatorTransactions::TYPE_SPACE:
           $space_phid = $this->getTransactionNewValue($object, $xaction);
           $copy->setSpacePHID($space_phid);