fn main() {
    let mut y = 3;
    let f = |x| {
        y = 4;
        y + x
    };
    let x = 3;
    let yy = f(x);
    println!("y={}, yy={}", y, yy);
}

错误：

error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
  --> closures/src/main.rs:28:14
   |
23 |     let f = |x| {
   |         - help: consider changing this to be mutable: `mut f`
...
28 |     let yy = f(x);
   |              ^ cannot borrow as mutable

不明白最后一个错误提示说明啥