It's worth noting that Haskell has "shared xor mutable" via the ST Monad. It effectively providing mutable memory cells that aren't allowed to "escape" a scope

It's worth noting that Haskell has "shared xor mutable" via the ST Monad. It effectively providing mutable memory cells that aren't allowed to "escape" a scope in much the same way as `&mut`. Also having actual purity knowledge (something Rust punted on before 1.0) can sometimes be useful. Although honestly 99% of the time it's only for the benefit of the compiler, which isn't a big deal if you have the tools to write code that's efficient from the get-go. e.g. list fusion is enabled by purity, but is largely uninteresting in Rust because lazy iterator chaining already orders operations and avoids intermediate lists just like list fusion.