Xcode 9 code completion and highlighting not working

Question

I have seen many threads about this online, but not for Xcode 9. For some reason code completion and syntax highlighting is no longer working. Last week it was fine.

Using the GM build.

Anyone have this issue or know how to resolve it? A restart of Xcode and my computer did not help.

Answer 1

In Finder I navigated to user->Library->Developer->Xcode->DerivedData and deleted the subfolder of the project in question. After next build of the project the syntax highlighting and code completion returned.

Answer 2

In my situation, there are a few of .swift files not working. I've overcome by removing these files from project and re-adding them back.