title: Why can LayoutBuilder not have an intrinsic dimensions?
body: <p>In the docs of <code>_RenderLayoutBuilder</code> in <code>layout_builder.dart</code> it says:</p>
  <pre><code>'LayoutBuilder does not support returning intrinsic dimensions.\n'
  'Calculating the intrinsic dimensions would require running the layout '
  'callback speculatively, which might mutate the live render object tree.'
  </code></pre>
  <p>What is the “layout callback” in this case, and why would it have to be called speculatively?</p>
  <p>I am trying to wrap my head around how to implement a custom <code>RenderBox</code> that similarly to <code>LayoutBuilder</code> builds its child using a builder callback and supports intrinsic dimensions.</p>