What was the hardest coming from C++ to Rust?

To you who came from a C++ background, what was the hardest things about learning and using Rust? I've seen some people mention that you can't define classes and standard OOP paradigms, and some that say Rust's strict rules become restrictive and make GUI development hard. Have any of you had the same experience?

I'm particularly interested in GUI development, and I'm wondering if Rust would be one step forward and two steps backwards in that sense. Of course I get that because the language is much younger there aren't many mature GUI libraries, but I'm thinking if the fundamental ideology of the language makes GUI development harder.