ref struct Sample
{
    private ReadOnlySpan<char> _str;

    public Sample(ReadOnlySpan<char> str)
    {
        _str = str;
    }

    public void MoveNext()
    {
        var span = _str;
    }
}

ref struct Sample(ReadOnlySpan<char> str)
{
    public void MoveNext()
    {
        // ❌ CS9110 Cannot use primary constructor parameter 'str' that has ref-like type inside an instance member
        var span = str;
    }
}