`_i` instead
  |
  = note: #[warn(unused_variables)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.50s
</code></pre>
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The warning suggests that we use `_i` as a name instead: the underscore
indicates that we intend for this variable to be unused. We can automatically
apply that suggestion using the `rustfix` tool by running the command `cargo
fix`:
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<p>警告は、変数名に<code>_i</code>を使ってはどうかと提案しています：アンダーバーはその変数