rust_basic_code/rust_code/1-4/borrow/src/main.rs

fn main() {
    let mut s = String::from("hello");
    {
        let r1 = &s;
        let r2 = &s;
        println!("Hello, world!{},{}", r1, r2);
    }
    let r3 = &mut s;
    println!("Hello, world!{}", r3);
}