rector/config/set/laravel51.php

<?php

declare(strict_types=1);

use Rector\Renaming\Rector\Name\RenameClassRector;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

# see: https://laravel.com/docs/5.1/upgrade
return static function (ContainerConfigurator $containerConfigurator): void {
    $services = $containerConfigurator->services();

    $services->set(RenameClassRector::class)
        ->call('configure', [[
            RenameClassRector::OLD_TO_NEW_CLASSES => [
                'Illuminate\Validation\Validator' => 'Illuminate\Contracts\Validation\Validator',
            ],
        ]]);
};