I haven't dug into the details, but here's how Scala went about adding string interpolation: https://docs.google.com/document/d/1NdxNxZYodPA-c4MLr33KzwzK...

That's what I was thinking, but now that you mention it, all that is needed is to have introspection gain the ability to examine local variables up the stack, so I guess it's do-able w/ back compat after all.

I wouldn't hold my breath, though.