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URL to the section(s) of the book with this problem:
https://doc.rust-lang.org/book/ch17-05-traits-for-async.html#the-pin-and-unpin-traits

Description of the problem:
Quoting chapter 17.5 from rustbook, section "Pin and Unpin Traits":
"
To make that concrete, think about a String: it has a length and the Unicode characters that make it up.
"
A String consitutes of len, capacity and a pointer to unicode characters. On the other hand, a string slice &str constitutes of pointer and len.

Suggested fix: