MemoryToSpan.cs

public sealed unsafe class SpanToMemoryMemoryManager<T> : MemoryManager<T> where T : unmanaged
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static Memory<T> GetMemory(Span<T> span)
    {
        return new SpanToMemoryMemoryManager<T>(span).Memory;
    }

    private readonly T* _ptr;
    private readonly int _length;

    public SpanToMemoryMemoryManager(Span<T> span)
    {
        _ptr = (T*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(span));
        _length = span.Length;
    }

    public override Span<T> GetSpan() => new Span<T>(_ptr, _length);

    public override Memory<T> Memory => CreateMemory(_length);

    public override MemoryHandle Pin(int elementIndex = 0)
    {
        if ((uint)elementIndex < (uint)_length)
            return new MemoryHandle(_ptr + elementIndex);

        throw new ArgumentOutOfRangeException(nameof(elementIndex));
    }

    public override void Unpin() { }

    protected override void Dispose(bool disposing) { }
}