Ironic, the most complicated thing for me to understand when I started writing rust was borrowing and lifetimes. The rest of the common features of Rust isn't too complicated to understand.
Understanding the concepts is not the issue with me (as of now). It is the hard to read syntax (my personal opinion!). It puts a lot strain on my mind. Same is the case with C++ with extreme use of STL. I have difficulty in reading that, although with clear mind, I will eventually get the code, it is not fun.