Function chars_needed_to_complete

pub fn chars_needed_to_complete<'needle>(
    needle: &'needle str,
    haystack: &str,
) -> Option<&'needle str>

What characters would need to be added to haystack to get to needle? returns None if wouldn’t be possible.

use parse_up::util::chars_needed_to_complete;
assert_eq!(chars_needed_to_complete("tag", "ta" ), Some("g"));
assert_eq!(chars_needed_to_complete("tag", ""   ), Some("tag"));
assert_eq!(chars_needed_to_complete("tag", "tag"), Some("")); // Special case
assert_eq!(chars_needed_to_complete("tag", "tar"), None);
assert_eq!(chars_needed_to_complete(""   , "foo"), None);