The key challenge of rust for me:

"Complex grammar. I've become somewhat infamous about wanting to keep the language LL(1) but the fact is that today one can't parse Rust very easily, much less pretty-print (thus auto-format) it, and this is an actual (and fairly frequent) source of problems. It's easier to work with than C++, but that's fairly faint praise. I lost almost every argument about this, from the angle brackets for type parameters to the pattern-binding ambiguity to the semicolon and brace rules to ... ugh I don't even want to get into it. The grammar is not what I wanted. Sorry."