Just curious, why did they make the fancy f count as 2?

  assert_eq!(get_middle("ƒoo"), "o");

I suggest to add this casetest too because the string "ƒoo" contains a non-ASCII character, which may behave differently than ASCII characters in certain operations.

for example the len of ƒoo is 4 not 3.

  assert_eq!("ƒoo".len(), 4); // fancy f!