Rust Book chapter 3.2: explain why table.clone() is needed

[iven]

In the book:

Finally, inside of our map()/collect() loop, we call table.clone(). The clone() method on Arc is what bumps up the reference count, and when it goes out of scope, it decrements the count.

I think it's better to add a few lines and let the readers know why bumping up the reference count here is needed.

[steveklabnik]

It's hard to figure out what level of depth to go into in these higher-level docs, you're right.