rector/bin/rector_bootstrap.php

<?php declare(strict_types=1);

/**
 * This allows to load "vendor/autoload.php" both from
 * "composer create-project ..." and "composer require" installation.
 */
$possibleAutoloadPaths = [
    // repository
    __DIR__ . '/../vendor/autoload.php',
    // composer create-project
    __DIR__ . '/../vendor/autoload.php',
    // composer require
    __DIR__ . '/../../../../vendor/autoload.php',
];

foreach ($possibleAutoloadPaths as $possibleAutoloadPath) {
    if (is_file($possibleAutoloadPath)) {
        require_once $possibleAutoloadPath;

        break;
    }
}