title: Adding a value-dependent data attribute to a simple_form checkbox collection
body: <p>I'm generating a list of checkboxes for a single collection like so:</p>
  
  <pre><code>= f.input :parts, as:check_boxes, collection: @parts_list
  </code></pre>
  
  <p>I want some checkboxes in the collection to disappear/reappear depending on the value of a select widget higher up in the form. (e.g. choosing "Tracker Robot" from the Robot select means that the "Legs" part checkbox disappears and the "Wheels" checkbox appears, etc.)</p>
  
  <p>What I'd like to do is attach a computed data attribute to each individual Part checkbox, with the attribute value listing the Robots that can use that Part; then some JS will do the work of hiding/showing the checkboxes. However, I don't know how I can generate those data attributes using simple_form. </p>
  
  <p>I would normally create a custom "parts" input, but there seems to be a problem with making custom collection inputs; it looks for a named method (collection_parts) inside form_builder.rb, which won't exist, and if I try and extend the FormBuilder it sends me down a major rabbit hole.</p>
  
  <p>I could write some JS to load the data attrs into the generated HTML, but then I have to generate custom JS based on my Rails data, and that feels like the wrong way to do it.</p>