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E0502: cannot borrow as mutable because it is also borrowed as immutable

不変参照が生きている（後で使われる）間に、同じ変数を可変で借用しようとするとE0502エラーになります。

 1  fn main() {
 2      let mut nums = vec![10, 20, 30];
 3      let head = &nums[0];
 4      _______________
               ^
 5      println!("{}", head);
 6  }