Rule source on GitHub

DowngradeClassOnObjectToGetClassRector

Change $object::class to get_class($object)

 class SomeClass
 {
     public function run($object)
     {
-        return $object::class;
+        return get_class($object);
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\ClassConstFetch\DowngradeClassOnObjectToGetClassRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeClassOnObjectToGetClassRector::class,
    ]);