$xml = '<?xml version="1.0" encoding="UTF-8"?>
<main>
    <items>
        <foo cool="true"></foo>
        <bar other="true"></bar>
    </items>
</main>
';

$data = $this->serializer->deserialize($xml, Main::class, 'xml');

class Main {
    /**
     * @var array<Foo|Bar>
     */
    public array $items = [];
}
class Foo {
    public bool $cool;
}
class Bar {
    public bool $other;
}