[Rust] function with parameters and return value

fn capitalize(s: &str) -> String {
    let mut c = s.chars();
    match c.next() {
        None => String::new(),
        Some(f) => f.to_uppercase().collect::<String>() + c.as_str(),
    }
}

fn main() {
    assert_eq!(capitalize("apple"), "Apple");
    assert_eq!(capitalize("banana"), "Banana");
    assert_eq!(capitalize("orange"), "Orange");
}