Rust prohibits shared mutable state as part of the basic language (or rather its 'safe' subset), relegating it to special-cased "interior mutability" constructs. This is essentially "as immutable as you can get" in a low-level, systems programming language. (Other languages can thread state mutations explicitly as part of a generally "immutable" design, but that doesn't give you support for the expected low-level features, so instead it's part of the language in Rust.)