String

string-str

when str and string share in ... AsRef

fn is_hello<T: AsRef<str>>(s: T) {
   assert_eq!("hello", s.as_ref());
}

let s = "hello";
is_hello(s);

let s = "hello".to_string();
is_hello(s);

#![allow(unused)]
fn main() {
let s = String::from("hello");

let b = s.into_boxed_str();
println!("{}",b);
}

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

//Selcect number form string, it will be a usize.
String:String = ss.chars().filter(|x| x.is_digit(10)).collect();
//Selcect number form string, it will be a float.
String:String = ss.chars().filter(|x| x.is_digit(10)||x='.').collect();