what is the reason to use write_idx and an additional idx in `Block`?

[cccc123-cpu]

seems to me either one works

[g302ge]

I think this struct is copy from MPMC queue directly. As we know that the MPMC quque need a token semnatics field to making producer and consumer blocking on the queue is full

[g302ge]

And there is an assumption, size_t is alwasys enough in practical situatiion, so the WriteIndex should always grater than ReadIndex