That's totally fair, but that may have something to do with how well you know Rust. I obviously don't know your familiarity level, but once your mental models "snap" out of thinking about moving data around, and start thinking about operations upon the data, it gets really intuitive.
I've seen Rust written in imperative style, and it was clear to me in most of the cases. I think it's more the difference between imperative and functional styles that's throwing people for a loop.
Yeah this is clearly someone who's into FP. If you rewrite this code to replace the map/sum with a for loop it's basically identical in length, while being completely idiomatic.
I am familiar enough with functional programming patterns. I could probably understand this if I had some more experience with Rust. But I think I would be able to understand the snippet in most imperative and functional languages.
My point is not that the snippet is bad, just that Rust feels, looks, and probably is, quite complex. As opposed to, for example, Haskell, which also introduced a bunch of nice, new, ideas.
