Thank you! this is helpful.

Ah, see, I'd go straight for Entry in the hash case. Which might be what you're referring to.

yeah, that later one isn't NLL: it's that

  line_result?.split_whitespace()
             ^                   ^

the ? will return your String, but then split_whitespace returns &strs to that String. Since it's not bound anywhere, the String would be deallocated at the end of the line, which would make the iterator dangling. NLL won't fix that.