-is_array($foo) || $foo instanceof Countable;
+is_countable($foo);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\Php73\Rector\BooleanOr\IsCountableRector;

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

SETS:  PHP 7.3 Polyfills