问如何在Rust中使字符(char)大写

EN

pub fn capitalize_first(input: &str) -> String {
    let mut c = input.chars();
    return match c.next() {
        None => String::new(),
        Some(first) => first.to_uppercase(),
    };
}

  --> exercises/standard_library_types/iterators2.rs:13:24
   |
11 |       return match c.next() {
   |  ____________-
12 | |         None => String::new(),
   | |                 ------------- this is found to be of type `String`
13 | |         Some(first) => first.to_uppercase(),
   | |                        ^^^^^^^^^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
   | |                        |
   | |                        expected struct `String`, found struct `ToUppercase`
14 | |     };
   | |_____- `match` arms have incompatible types

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.