layout (std430, binding = 0) buffer indices {
    int j[];
};

RWBuffer indices
{
    int j[];
}

RWBuffer<{int j[];}> indices;

struct index
{
    //requires explicit length, which is why this doesn't work
    int j[];
}
RWStructuredBuffer<index> indices;

//id is SV_DispatchThreadID
indices.j[int(id.y)]