in an `if` condition, avoid complex blocks or closures with blocks; instead, move the block or closure higher and bind it with a `let`
writing `&String` instead of `&str` involves a new object where a slice will do. (function parameter)
this `.into_iter()` call is equivalent to `.iter()` and will not move the `slice`
writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used with non-Vec-based slices.