ClosureReturnTypeRector

Add return type to closures based on known return values

-function () {
+function (): int {
     return 100;
 };

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\TypeDeclaration\Rector\Closure\ClosureReturnTypeRector;

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