��味しない:
「外側のミュータビリティ(exterior mutability)」を表します。例として、<a href="../std/sync/struct.Arc.html"><code>Arc&lt;T&gt;</code></a> を考えます:</p>

<span class='rusttest'>fn main() {
    use std::sync::Arc;

let x = Arc::new(5);
let y = x.clone();
}</span><pre class='rust rust-example-rendered'>
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>sync</span>::<span class='ident'>Arc</span>;

<span class='kw'>let</span> <span clas