diff --git a/src/Symfony/Component/Messenger/Command/DebugCommand.php b/src/Symfony/Component/Messenger/Command/DebugCommand.php
index 631bace66ab98..4abfa03e17687 100644
--- a/src/Symfony/Component/Messenger/Command/DebugCommand.php
+++ b/src/Symfony/Component/Messenger/Command/DebugCommand.php
@@ -44,7 +44,7 @@ public function __construct(array $mapping)
     protected function configure()
     {
         $this
-            ->addArgument('bus', InputArgument::OPTIONAL, sprintf('The bus id (one of %s)', implode(', ', array_keys($this->mapping))), null)
+            ->addArgument('bus', InputArgument::OPTIONAL, sprintf('The bus id (one of %s)', implode(', ', array_keys($this->mapping))))
             ->setDescription('Lists messages you can dispatch using the message buses')
             ->setHelp(<<<'EOF'
 The <info>%command.name%</info> command displays all messages that can be
diff --git a/src/Symfony/Component/Messenger/DependencyInjection/MessengerPass.php b/src/Symfony/Component/Messenger/DependencyInjection/MessengerPass.php
index a22a085bd0f50..11d64c1a3f467 100644
--- a/src/Symfony/Component/Messenger/DependencyInjection/MessengerPass.php
+++ b/src/Symfony/Component/Messenger/DependencyInjection/MessengerPass.php
@@ -15,7 +15,6 @@
 use Symfony\Component\DependencyInjection\Argument\RewindableGenerator;
 use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
-use Symfony\Component\DependencyInjection\Compiler\PriorityTaggedServiceTrait;
 use Symfony\Component\DependencyInjection\Compiler\ServiceLocatorTagPass;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Definition;
@@ -33,8 +32,6 @@
  */
 class MessengerPass implements CompilerPassInterface
 {
-    use PriorityTaggedServiceTrait;
-
     private $handlerTag;
     private $busTag;
     private $receiverTag;
@@ -282,7 +279,7 @@ private function registerBusMiddleware(ContainerBuilder $container, string $busI
                 throw new RuntimeException(sprintf('Invalid middleware: service "%s" not found.', $id));
             }
 
-            if (($definition = $container->findDefinition($messengerMiddlewareId))->isAbstract()) {
+            if ($container->findDefinition($messengerMiddlewareId)->isAbstract()) {
                 $childDefinition = new ChildDefinition($messengerMiddlewareId);
                 $childDefinition->setArguments($arguments);
                 $container->setDefinition($messengerMiddlewareId = $busId.'.middleware.'.$id, $childDefinition);
diff --git a/src/Symfony/Component/Messenger/Middleware/TraceableMiddleware.php b/src/Symfony/Component/Messenger/Middleware/TraceableMiddleware.php
index e9a2be37eeeac..771c14633eb83 100644
--- a/src/Symfony/Component/Messenger/Middleware/TraceableMiddleware.php
+++ b/src/Symfony/Component/Messenger/Middleware/TraceableMiddleware.php
@@ -90,7 +90,7 @@ public function next(): MiddlewareInterface
         return $nextMiddleware;
     }
 
-    public function stop()
+    public function stop(): void
     {
         if (null !== $this->currentEvent && $this->stopwatch->isStarted($this->currentEvent)) {
             $this->stopwatch->stop($this->currentEvent);
diff --git a/src/Symfony/Component/Messenger/Tests/MessageBusTest.php b/src/Symfony/Component/Messenger/Tests/MessageBusTest.php
index 6d8311e4759a0..3dedb59c63db5 100644
--- a/src/Symfony/Component/Messenger/Tests/MessageBusTest.php
+++ b/src/Symfony/Component/Messenger/Tests/MessageBusTest.php
@@ -42,7 +42,6 @@ public function testItCallsMiddleware()
     {
         $message = new DummyMessage('Hello');
         $envelope = new Envelope($message);
-        $responseFromDepthMiddleware = 1234;
 
         $firstMiddleware = $this->getMockBuilder(MiddlewareInterface::class)->getMock();
         $firstMiddleware->expects($this->once())
diff --git a/src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php b/src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php
index a2c4e0b8c6bf4..371915d9539f3 100644
--- a/src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php
+++ b/src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php
@@ -29,7 +29,7 @@ public function testItTracesDispatch()
 
         $traceableBus = new TraceableMessageBus($bus);
         $line = __LINE__ + 1;
-        $this->assertInstanceOf(Envelope::class, $traceableBus->dispatch($message));
+        $traceableBus->dispatch($message);
         $this->assertCount(1, $tracedMessages = $traceableBus->getDispatchedMessages());
         $this->assertArraySubset(array(
             'message' => $message,
@@ -52,7 +52,7 @@ public function testItTracesDispatchWithEnvelope()
 
         $traceableBus = new TraceableMessageBus($bus);
         $line = __LINE__ + 1;
-        $this->assertInstanceOf(Envelope::class, $traceableBus->dispatch($envelope));
+        $traceableBus->dispatch($envelope);
         $this->assertCount(1, $tracedMessages = $traceableBus->getDispatchedMessages());
         $this->assertArraySubset(array(
             'message' => $message,
diff --git a/src/Symfony/Component/Messenger/Tests/Transport/AmqpExt/AmqpExtIntegrationTest.php b/src/Symfony/Component/Messenger/Tests/Transport/AmqpExt/AmqpExtIntegrationTest.php
index f0476524a011b..511965cc01899 100644
--- a/src/Symfony/Component/Messenger/Tests/Transport/AmqpExt/AmqpExtIntegrationTest.php
+++ b/src/Symfony/Component/Messenger/Tests/Transport/AmqpExt/AmqpExtIntegrationTest.php
@@ -56,7 +56,7 @@ public function testItSendsAndReceivesMessages()
 
         $receivedMessages = 0;
         $receiver->receive(function (?Envelope $envelope) use ($receiver, &$receivedMessages, $first, $second) {
-            $this->assertEquals(0 == $receivedMessages ? $first : $second, $envelope);
+            $this->assertEquals(0 === $receivedMessages ? $first : $second, $envelope);
 
             if (2 === ++$receivedMessages) {
                 $receiver->stop();
diff --git a/src/Symfony/Component/Messenger/Tests/Transport/Sender/SendersLocatorTest.php b/src/Symfony/Component/Messenger/Tests/Transport/Sender/SendersLocatorTest.php
index b703f4481ef5a..f756121e094b4 100644
--- a/src/Symfony/Component/Messenger/Tests/Transport/Sender/SendersLocatorTest.php
+++ b/src/Symfony/Component/Messenger/Tests/Transport/Sender/SendersLocatorTest.php
@@ -28,6 +28,6 @@ public function testItReturnsTheSenderBasedOnTheMessageClass()
         ));
 
         $this->assertSame(array($sender), iterator_to_array($locator->getSenders(new Envelope(new DummyMessage('a')))));
-        $this->assertSame(array(), iterator_to_array($locator->getSenders(new Envelope(new SecondMessage('b')))));
+        $this->assertSame(array(), iterator_to_array($locator->getSenders(new Envelope(new SecondMessage()))));
     }
 }
diff --git a/src/Symfony/Component/Messenger/Transport/Serialization/Serializer.php b/src/Symfony/Component/Messenger/Transport/Serialization/Serializer.php
index dbfc352c24553..430a5fca0606c 100644
--- a/src/Symfony/Component/Messenger/Transport/Serialization/Serializer.php
+++ b/src/Symfony/Component/Messenger/Transport/Serialization/Serializer.php
@@ -99,7 +99,7 @@ public function encode(Envelope $envelope): array
         );
     }
 
-    private function decodeStamps($encodedEnvelope)
+    private function decodeStamps(array $encodedEnvelope): array
     {
         $stamps = array();
         foreach ($encodedEnvelope['headers'] as $name => $value) {
@@ -113,7 +113,7 @@ private function decodeStamps($encodedEnvelope)
         return $stamps;
     }
 
-    private function encodeStamps(Envelope $envelope)
+    private function encodeStamps(Envelope $envelope): array
     {
         if (!$stamps = $envelope->all()) {
             return array();