for loop is a powerful upgrade. It
iterates over values from any expression that evaluates into an iterator. What's
an iterator? An iterator is an object that you can ask the question "What's the
next item you have?" until there are no more items.
We'll explore this more in a future chapter. In the meantime, just know Rust makes it easy to create iterators that generate a sequence of integer numbers.
.. operator creates an iterator that generates numbers from a start number up to
but not including an end number.
..= operator creates an iterator that generates numbers from a start number up to
and including an end number.