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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 v = vec![1, 2, 3];
 3      let first = &v[0];
 4      ____________
              ^
 5      println!("{}", first);
 6  }