t_wの輪郭

fn main() {
    let mut s = String::from("hello");

    let r1 = &mut s;
    let r2 = &mut s;    //error

    println!("{}, {}", r1, r2);
}

  |
4 |     let r1 = &mut s;
  |              ------ first mutable borrow occurs here
5 |     let r2 = &mut s;    //error
  |              ^^^^^^ second mutable borrow occurs here
6 | 
7 |     println!("{}, {}", r1, r2);
  |                        -- first borrow later used here