Purity isn't as big of a deal in Rust as it is in other languages. We used to have it, but it wasn't very useful.
Right, purity aka referential transparency is basically required when you have lazy evaluation by default (as in Haskell).
Since Rust is a strictly evaluated language, it's easy to reason about the order statements and expressions will be executed in and when side effects will happen, so purity is not generally necessary.