I have not found Rust difficult to learn. I am familiar with all of the concepts applied in Rust from other languages (C++, Scala, C#) and it's internals, except borrow checker. Although, it can be also counted as known to some extent concept of static analysis for variable's lifetimes and 'single at a time' mutation owner, albeit Rust is game changer in the quality and reliability of this analysis.
What can make an expressive language hard is ability to read and interpret it's expressiveness easily using only source code. Remember Perl, which was named by someone as write-only language 
So far so good, I only started few days ago. The main corner stones so far are difficulty to find 'implicitly' applied converters (see more here Why Box<Future<...>> is type equivalent to just Future<...>) and understand where lifetime of a variable comes from and how to control it (i.e. best design practices on expanding it's lifetime, except using Rc<> or Gc<> every time).