a smart
pointer of type `Weak<T>`. Instead of increasing the `strong_count` in the
`Rc<T>` instance by 1, calling `Rc::downgrade` increases the `weak_count` by 1.
The `Rc<T>` type uses `weak_count` to keep track of how many `Weak<T>`
references exist, similar to `strong_count`. The difference is the `weak_count`
doesn’t need to be 0 for the `Rc<T>` instance to be cleaned up.
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<p>ここまで、<code>Rc::clone</code>を呼び出すと<code>Rc&lt;T&gt;</code>インスタンスの<code>strong_count</code>