Rails guy turned rustacean here. If I have a piece of code that looks gnarly, I'll write a comment above it explaining what it does. Elegance != readability always. This can often happen with complicated map/filter/etc chains (both in ruby and in rust!) that are compact but are complicated enough that your eyes glaze over when you read them. Anything eye-glaze-over-ey is probably worth a comment in my opinion.

Other times to comment include when there is some sort of dirty hack, TODO annotations, etc.