Rust has no notion of purity. This would require something like an effect system. const functions can't directly do any IO or even allocation - at the moment.

Rust has no notion of purity. This would require something like an effect system. const functions can't directly do any IO or even allocation - at the moment. But this can be easily circumvented, eg by using a proc macro that does IO. Sidenote: even in Haskell the function signature doesn't guarantee purity, due to unsafePerformIO.