$result = (new Validator())->validate(
    data: ['a' => 1, 'b' => 2],
    rules: [
        'a' => new Callback(function(){
            $result = new Result();
            $result->addError('message', valuePath: ['b']);
            return $result;
        })
    ],
);

// Errors:
// ['a.b' => ['message']]