This recipe is incredibly important. No matter what you do, or which library you use, it's going to use iterators somewhere. All of the operations presented can be used on any collection and all types that implement the iterator trait.
In the first section, we looked at different ways to create iterators. I mention that ranges are limited because, in order to be iterable, the range-type has to implement Step. char doesn't, so you wouldn't be able to use 'A'..'D' as an iterator. For this reason, in line [209], we iterate over the characters as bytes:
let alphabet: Vec<_> = (b'A' .. b'z' + 1) // Start as u8
.map(|c| c as char) // Convert all to chars
.filter(|c| c.is_alphabetic()) // Filter only alphabetic chars
.collect(); // Collect as Vec<char>
We have to set the limit of the range to b'z' + 1, because ranges are non-inclusive. You might have noticed that...