[orange_fritter]

Every attempt at escaping mutability basically kills the language in the mainstream because so much of "real" programming is just bit-twiddling that gets too verbose when immutability is involved. It's a good question whether Rust nudges the world toward functional/declarative spiritual purity by placing constraints on mutation. I'm betting that No, it doesn't.

[vacuity]

An explicit goal of Rust is to be "low level", which is an admittedly vague phrase. While you could certainly write a Rust library that clones linked lists left and right, and maybe someone would prefer that, someone else should be able to write a library that does in-place mutation with that delicious imperative goodness (badness?). To GP's point about environment variables, I think that's more of an issue with the fact that Rust tries to be compatible with existing C conventions in the OS. I don't think Rust can do much about it, since it can interface with C libraries that don't care about any Rust constraints on environment variables.