Finding vendor-dir from Drupal using Composer

CheckedResult.php

<?php
declare(strict_types=1);

namespace Drupal\upgrade_status;

/**
 * CheckedResult provides a format to return both a result and an error.
 *
 * When the error is not NULL, the result is to be ignored
 */
class CheckedResult {

  protected $result;

  protected ?\Exception $exception;

  public function __construct($result, ?\Exception $exception = NULL) {
    $this->result = $result;
    $this->exception = $exception;
  }

  public function __toString(): string {
    if (!$this->isValid()) {
      return $this->exception->getMessage();
    }
    return (string) $this->result;
  }

  public function result(): mixed {
    if (!$this->isValid()) {
      return NULL;
    }
    return $this->result;
  }

  public function isValid(): bool {
    return $this->exception === NULL;
  }
}

PhpstanFinder.php

<?php
declare(strict_types=1);

namespace Drupal\upgrade_status;

use Composer\Autoload\ClassLoader;
use Symfony\Component\ClassLoader\ApcClassLoader;

/**
 * PhpStanFinder provides identification of the path to PHPstan.
 */
class PhpstanFinder {

  protected $classLoader;

  public function __construct($class_loader) {
    $this->classLoader = $class_loader;
  }

  /**
   * Strip deprecated ApcClassLoader if found.
   */
  protected function stripApc() {
    $apcLoader = $this->classLoader;
    if (!($apcLoader instanceof ApcClassLoader)) {
      return;
    }
    $rc = new \ReflectionClass($apcLoader);
    $rp = $rc->getProperty('decorated');
    $rp->setAccessible(true);
    $decorated = $rp->getValue($apcLoader);
    $this->classLoader = $decorated;
  }

  /**
   * @return \Drupal\upgrade_status\CheckedResult
   *   The vendor dir and/or an error.  
   */
  public function getVendorDir(): CheckedResult {
    $this->stripApc();
    if (!($this->classLoader instanceof ClassLoader)) {
      return new CheckedResult('', new \Exception("autoloader is not Composer"));
    }
    $rc = new \ReflectionClass($this->classLoader);
    $file = $rc->getFileName();
    $vendorDir = dirname($file, 2);
    return new CheckedResult($vendorDir);
  }

}

upgrade_status.service.yml

# ...snip...
  upgrade_status.phpstan_finder:
    class: Drupal\upgrade_status\PhpstanFinder
    arguments:
      - '@class_loader'

UsageFragment.php

$f = Drupal::service('upgrade_status.phpstan_finder'); 
echo $f->getVendorDir()->__toString()