I don’t know much about Rust so I can’t comment much on that. The intended point of my example was that in a purely functional environment, you can’t have a local, low-level cache, because updating a cache is inherently stateful. So either you need something like a mechanism that lets you break the rules locally, like say unsafePerformIO in Haskell, or you need to infect your entire call chain with whatever mechanism you use to manage effects top-down. While that is perfectly reasonable, given the constraints you choose by adopting a purely functional language, I don’t think it’s particularly helpful.
My conclusion was that I’d rather have a hybrid of imperative and functional styles, contrary to the suggestion in the original presentation and in general agreement with stdbrouw’s comment.