span class='kw'>let</span> <span class='ident'>z</span> <span class='op'>=</span> <span class='ident'>x</span>.<span class='ident'>borrow_mut</span>();</pre>

<!-- This will in fact panic, at runtime. This is what `RefCell` does: it enforces -->

<!-- Rust’s borrowing rules at runtime, and `panic!`s if they’re violated. This -->

<!-- allows us to get around another aspect of Rust’s mutability rules. Let’s talk -->

<!-- about it first. -->

<p>実際に、このコードは実行時にパニック�