t_wの輪郭

ベクタの要素を不変変数に借用した場合は、そのベクタの要素を変更・追加できなくなる 

let mut v = vec![1, 2, 3, 4, 5];
let first = &v[0];
v.push(6);
println!("The first element is: {}", first);
/*
error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
  --> src\main.rs:23:5
   |
21 |     let first = &v[0];
   |                  - immutable borrow occurs here
22 | 
23 |     v.push(6);
   |     ^^^^^^^^^ mutable borrow occurs here
24 | 
25 |     println!("The first element is: {}", first);
   |                                          ----- immutable borrow later used here
*/