OP here, what I had in mind was simple propagation where only functions marked impure could call impure functions. While I maintain that in the abstract, one could design a language around that feature without incurring a huge amount of complexity, Graydon was right doing it bluntly with annotations is a very blunt tool and anything more granular would have been quite complicated. I never interpreted it as a burn, rather someone who knew far more about Rust educating an experienced Haskeller with essentially no direct Rust experience on some of their design decisions.