<?php

namespace Symfony\Bundle\FrameworkBundle\Test;

trait BrowserKitAssertionsTrait
{
    public static function assertResponseIsUnprocessable(string $message = ''): void
    {
        self::assertThatForResponse(new ResponseConstraint\ResponseIsUnprocessable(), $message);
    }
    // ...
}

<?php

namespace Symfony\Component\HttpFoundation\Test\Constraint;

use PHPUnit\Framework\Constraint\Constraint;
use Symfony\Component\HttpFoundation\Response;

final class ResponseIsUnprocessable extends Constraint
{
    /**
     * {@inheritdoc}
     */
    public function toString(): string
    {
        return 'is unprocessable';
    }

    /**
     * @param Response $response
     *
     * {@inheritdoc}
     */
    protected function matches($response): bool
    {
        return Response::HTTP_UNPROCESSABLE_ENTITY === $response->getStatusCode();
    }

    /**
     * @param Response $response
     *
     * {@inheritdoc}
     */
    protected function failureDescription($response): string
    {
        return 'the Response '.$this->toString();
    }

    /**
     * @param Response $response
     *
     * {@inheritdoc}
     */
    protected function additionalFailureDescription($response): string
    {
        return (string) $response;
    }
}