symfony
diff --git a/‎src/Symfony/Bundle/FrameworkBundle/Test/MailerAssertionsTrait.php
+10 b/‎src/Symfony/Bundle/FrameworkBundle/Test/MailerAssertionsTrait.php
+10
diff --git a/‎src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php
+2 b/‎src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php
+2
diff --git a/‎src/Symfony/Component/Mime/Test/Constraint/EmailSubjectContains.php
+55 b/‎src/Symfony/Component/Mime/Test/Constraint/EmailSubjectContains.php
+55
@@ -90,6 +90,16 @@ public static function assertEmailAddressContains(RawMessage $email, string $hea
         self::assertThat($email, new MimeConstraint\EmailAddressContains($headerName, $expectedValue), $message);
     }
 
+    public static function assertEmailSubjectContains(RawMessage $email, string $expectedValue, string $message = ''): void
+    {
+        self::assertThat($email, new MimeConstraint\EmailSubjectContains($expectedValue), $message);
+    }
+
+    public static function assertEmailSubjectNotContains(RawMessage $email, string $expectedValue, string $message = ''): void
+    {
+        self::assertThat($email, new LogicalNot(new MimeConstraint\EmailSubjectContains($expectedValue)), $message);
+    }
+
     /**
      * @return MessageEvent[]
      */
 
@@ -104,6 +104,8 @@ public function testMailerAssertions()
         $this->assertEmailAttachmentCount($email, 1);
 
         $email = $this->getMailerMessage($second);
+        $this->assertEmailSubjectContains($email, 'Foo');
+        $this->assertEmailSubjectNotContains($email, 'Bar');
         $this->assertEmailAddressContains($email, 'To', 'fabien@symfony.com');
         $this->assertEmailAddressContains($email, 'To', 'thomas@symfony.com');
         $this->assertEmailAddressContains($email, 'Reply-To', 'me@symfony.com');
 
@@ -0,0 +1,55 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Test\Constraint;
+
+use PHPUnit\Framework\Constraint\Constraint;
+use Symfony\Component\Mime\Email;
+
+final class EmailSubjectContains extends Constraint
+{
+    private $expectedSubjectValue;
+
+    public function __construct(string $expectedSubjectValue)
+    {
+        $this->expectedSubjectValue = $expectedSubjectValue;
+    }
+
+    public function toString(): string
+    {
+        return sprintf('contains subject with value "%s"', $this->expectedSubjectValue);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function matches($other): bool
+    {
+        if (!$other instanceof Email) {
+            throw new \LogicException('Can only test a message subject on a Email instance.');
+        }
+
+        return str_contains((string) $other->getSubject(), $this->expectedSubjectValue);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function failureDescription($other): string
+    {
+        $message = 'The email subject ' . $this->toString();
+        if ($other instanceof Email) {
+            $message .= sprintf('. The subject was: "%s"', $other->getSubject() ?? '<empty>');
+        }
+
+        return $message;
+    }
+}