[InlineArray(256)]
public struct BufferStruct : IBufferInterface
{
    private byte _data;

    [UnscopedRef]
    public ReadOnlySpan<byte> Data => this;
}


interface IBufferInterface
{
    [UnscopedRef] //<--commenting this out removes the CS8156 error below
    public ReadOnlySpan<byte> Data { get; }
}

struct Foo<T> 
    where T : struct, IBufferInterface
{
    T genericBuffer;
    BufferStruct directBuffer;
    IBufferInterface interfaceBuffer;
    

    //[UnscopedRef] //<-- Has no effect
    public ReadOnlySpan<byte> GetGenericBuffer()
    {
        //error CS8156: An expression cannot be used in this 
        //context because it may not be passed or returned by reference
        return genericBuffer.Data;
    }

    [UnscopedRef]//this is required to fix CS8156
    public ReadOnlySpan<byte> GetDirectBuffer()
    {
        //was an error, but it's fixed by [UnscopedRef]
        return directBuffer.Data;
    }

    public ReadOnlySpan<byte> GetInterfaceBuffer()
    {
        return interfaceBuffer.Data;
    }
}