My big open issue with iOS right now is bitcode.
Has anyone come up with a way to output proper LLVM bitcode for iOS? With rustup it's easy (as of 1.11) to target all iOS platforms, but I produce staticlib's without bitcode. Trying to create a lib with bitcode inside, you end up with a non-compatible bitcode between LLVM versions.
Some people have mentioned swapping out the default Rust LLVM version for the Apple one, but I personally haven't gotten that far.