The phrase

Unicode string character by character can consume a lot more memory
than  you  expected.  The  string  love: ❤  is  7  characters  long,  requires  12
bytes of storage in a String—and 32 bytes of memory when processed as
individual  characters.  This  may  seem  like  a  small  amount  of  memory,
but if your reader enters the entirety of War and Peace into your program’s
input box, per-character parsing may require more resources than you
expected

gives, in my humble opinion, the wrong idea that iterators allocate memory.
What’s written is true if I do my_string.chars().collect::<Vec<char>>(), but only iterating overs chars won’t consume any memory, the single chars are transmuted from original string bytes.
I suggest to reformulate the example clarifing that iterators won’t allocate memory