为什么有时可能从不可变数组借用mut引用(&M)?

让我们试着编译以下代码:

trait Bar {
    fn bar(&mut self);
}

fn foo(a1: &mut Bar, j: usize) {
    let a = [a1];
    a[0].bar(); //compilation ok
    a[j % 2].bar();
}

fn main() {}

编译错误:

error[E0596]: cannot borrow immutable local variable `a` as mutable
 --> src/main.rs:8:5
  |
6 |     let a = [a1];
  |         - consider changing this to `mut a`
7 |     a[0].bar(); //compilation ok
8 |     a[j % 2].bar();
  |     ^ cannot borrow mutably

为什么是 a[0].bar() 好的,但是 a[j % 2].bar() 失败?这是编译器错误吗?