I'm a Rust noob. At first it's daunting and nothing will compile. But it's getting easier and I feel like getting over that initial difficulty is mainly a matter of internalizing a few basic rules, after which it feels more natural. So from what I've seen so far, I wouldn't rate Rust as especially difficult. It certainly feels easier than C++.

For a really challenging language I'd suggest Haskell. That one nearly broke my brain because thinking functionally is really very different if you're used to more procedural languages. Rust, it seems, teaches you new discipline, but Haskell teaches you a different way of thinking.

Anyway, that's how it feels as a relative noob in both.