number2name 1.0.1

Simple Rust library to convert an index to a name that is as short as possible.
Documentation 
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/// Return the lowercase representation character.
/// Fails if the lowercase representation is not a single character.
pub fn lower(character: char) -> char {
    let mut lc = character.to_lowercase();
    if lc.len() > 1 {
        panic!(
            "character {} is not allowed because its lower-case has more \
        than one unicode code point ({})",
            character, lc
        );
    }
    match lc.next() {
        Some(lc_char) => lc_char,
        None => panic!("no lower-case representation found"),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_valid_lowercase() {
        assert_eq!(lower('A'), 'a');
        assert_eq!(lower('a'), 'a');
    }

    #[test]
    fn test_no_case() {
        assert_eq!(lower('8'), '8');
        assert_eq!(lower('+'), '+');
        assert_eq!(lower('中'), '中');
    }

    #[test]
    #[should_panic]
    fn test_invalid_lowercase() {
        // Example from https://doc.rust-lang.org/std/primitive.char.html#method.to_lowercase
        lower('İ');
    }
}