How to resolve counterintuitive CS9091

[andanteyk]

I'm currently having trouble resolving warning CS9091.
Is there any solution?

Sharplab

public readonly partial struct UniBigInt
{
    private readonly v256[]? m_Value;
    private readonly int m_Sign;

    public static unsafe NativeResource operator +(in UniBigInt a, in UniBigInt b)
    {
        NativeResource result;

        fixed (v256* aPtr = a.m_Value, bPtr = b.m_Value)
        {
            var aSpan = new ReadOnlyBurstSpan<v256>(aPtr, a.m_Value?.Length ?? 0);
            var bSpan = new ReadOnlyBurstSpan<v256>(bPtr, b.m_Value?.Length ?? 0);

            // Here, aSpan and bSpan issue CS9091
            // "This returns local by reference but it is not a ref local"
            Add(aSpan, a.m_Sign, bSpan, b.m_Sign, out result);
        }

        return result;
    }

    // The BurstCompile specification does not allow you to change the signature.
    // [BurstCompile]
    static void Add(in ReadOnlyBurstSpan<v256> aValue, int aSign, in ReadOnlyBurstSpan<v256> bValue, int bSign, out NativeResource result)
    {
        // Do Something (omitted)
        result = default;
    }
}

internal unsafe readonly struct ReadOnlyBurstSpan<T> where T : unmanaged
{
    private readonly T* m_Ptr;
    private readonly int m_Length;

    public ReadOnlyBurstSpan(T* ptr, int length)
    {
        m_Ptr = ptr;
        m_Length = length;
    }
}

public readonly ref struct NativeResource
{
    // omitted
}

public struct v256
{
    // omitted (defined in unity)
}

• It says This returns local by reference but it is not a ref local. However, since this is an argument and not a return value, I don't understand the meaning of the warning text.
• This can be resolved by deleting the argument in, but due to Unity/Burst, in cannot be deleted.
• Changing the out argument NativeResource from a ref struct to a normal struct would fix the warning (why!?), but I want to keep it as a ref struct.
• This code seems to work as expected.

[huoyaoyuan]

out NativeResource result is treated as return value, and since NativeResource is a ref struct, it's applicable to lifetime tracking.

in ReadOnlyBurstSpan<v256> aValue should be scoped to indicate that its won't be re-assigned to other variables.

[andanteyk]

I didn't know about scoped, this solved the problem.
Thank you so much for the quick explanation!