Yes. The compiler kept track of which source locations were impacted by which types of "method upgrades". Was something inlined? Dead code eliminated? Converted

Yes. The compiler kept track of which source locations were impacted by which types of "method upgrades". Was something inlined? Dead code eliminated? Converted to a direct call to an internal entry point (avoiding dispatch)? Converted to a direct slot accessor? All of those could be colored differently. I'm thinking of writing a blog post along the lines of "So, you thought that was a function call, eh?" as Dylan treats everything as a function call at the syntax level, but can go and optimize it into something better. That'll be a pretty complicated post though with a lot of Dylan + generated code examples and I know that I lost some people today by showing C in the type system overview post.