title: What does the `#[lang = "..."]` attribute do?
body: <p>I'm reading the code at <a href="https://doc.rust-lang.org/1.56.0/src/core/str/mod.rs.html#120-122" rel="nofollow noreferrer">https://doc.rust-lang.org/1.56.0/src/core/str/mod.rs.html#120-122</a>:</p>
  <pre class="lang-rust prettyprint-override"><code>#[lang = &quot;str&quot;]
  #[cfg(not(test))]
  impl str {
  ...
  </code></pre>
  <p>and I can't find a reference to this attribute <code>lang</code>. Does it have something to do with declaring the structure of the primitive type <code>str</code>? And if so, where is the information dealing with the inner structure of <code>str</code>?</p>