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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 prices = vec![100, 200];
 3      let cheapest = &prices[0];
 4      ________________
                ^
 5      println!("{}", cheapest);
 6  }