rector/bin/bootstrap.php

<?php declare(strict_types=1);

// very dummy way to detect rector dev repository, didn't find anything better - improve if you like
$isDevRepository = file_exists(__DIR__ . '/../README.md') || \file_exists(__DIR__ . '/../../build');

// load the project with Prefixed Rector
if (defined('RECTOR_PREFIXED') && ! $isDevRepository) {
    $projectAutoload = getcwd() . '/vendor/autoload.php';
    if (is_file($projectAutoload)) {
        require_once $projectAutoload;
    }
}

// autoload project even from non-root use
$autoloadInWorkingDirectory = getcwd() . '/vendor/autoload.php';
if (is_file($autoloadInWorkingDirectory)) {
    require_once $autoloadInWorkingDirectory;
}

if (class_exists('Rector\DependencyInjection\RectorKernel')) {
    return;
}

$possibleAutoloadPaths = [
    // dev repository or prefixed rector
    __DIR__ . '/../vendor/autoload.php',
    // composer require
    __DIR__ . '/../../../../vendor/autoload.php',
];

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

die(sprintf(
    'Composer autoload.php was not found in paths "%s". Have you ran "composer update"?',
    implode('", "', $possibleAutoloadPaths)
));