Because there's no possible choice for T. You can't use u8, because then you would allow non-utf8 data. You can't use char, because a &[char] uses four bytes pe

Because there's no possible choice for T. You can't use u8, because then you would allow non-utf8 data. You can't use char, because a &[char] uses four bytes per character, whereas a &str stores the characters in utf-8, which is a variable-width encoding. A &str is really a different kind of thing from other slices. In any other slice, each element in the slice takes up a constant number of bytes, but this is not the case for a &str.