title: Compile Error len is not yet stable as a const fn
body: <p>I get an error from one of my dependencies <a href="https://crates.io/crates/bytes" rel="noreferrer"><code>bytes</code></a> <code>0.5.2</code>. Here a code example of the error:</p>
  
  <pre class="lang-rust prettyprint-override"><code>pub const fn foo(foo: &amp;'static [u8]) -&gt; usize {
      foo.len()
  }
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>error: `core::slice::&lt;impl [T]&gt;::len` is not yet stable as a const fn
   --&gt; &lt;source&gt;:2:5
    |
  2 |     foo.len()
    |     ^^^^^^^^^
  error: aborting due to previous error
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>active toolchain
  ----------------
  
  stable-x86_64-pc-windows-msvc (default) rustc 1.38.0 (625451e37 2019-09-23)
  </code></pre>