> I saw a proof of concept of something like this in Haskell a few years back, but it's amazing it see it (probably) making it into the core of a mainstream language.

Probably Conal Elliott’s work, eg in Vertigo (http://conal.net/Vertigo/, circa 2005)? There he was using it for normal computations used in pixel shading, pretty cool stuff. He is still active in this field, and has a lot of new papers that are more ML focused. I do wonder if “general” AD support will be useful for computer graphics as well as ML?