You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: utf8.rs
    path of file: ./repos/phaser/phaser/vendor/regex/src
    
    the code of the file until where you have to start completion: /// A few elementary UTF-8 encoding and decoding functions used by the matching
/// engines.
///
/// In an ideal world, the matching engines operate on `&str` and we can just
/// lean on the standard library for all our UTF-8 needs. However, to support
/// byte based regexes (that can match on arbitrary bytes which may contain
/// UTF-8), we need to be capable
    
    fn reject_invalid_last() {
        // Invalid start byte
        assert_eq!(decode_last_utf8(&[0xFF]), None);
        // Surrogate pair
        assert_eq!(decode_last_utf8(&[0xED, 0xA0, 0x81]), None);
        // Bad lengths
        assert_eq!(decode_last_utf8(&[0xC3]), None); // 2 bytes
        assert_eq!(decode_last_utf8(&[0xEF, 0xBF]), None); // 3 bytes
        assert_eq!(decode_last_utf8(&[0xF4, 0x8F, 0xBF]), None); // 4 bytes
                                                                 // Not a minimal UTF-8 sequence
        assert_eq!(decode_last_utf8(&[TAG_TWO, TAG_CONT | b'a']), None);
        assert_eq!(
            decode_last_utf8(&[TAG_THREE, TAG_CONT, TAG_CONT | b'a',]),
            None
        );
        assert_eq!(
            decode_last_utf8(
                &[TAG_FOUR, TAG_CONT, TAG_CONT, TAG_CONT | b'a',]
            ),
            None
        );
    }