I am having the same problem and can see we are not the only ones: https://forums.developer.apple.com/forums/thread/749760

we have the same issue

I have this problem with Xcode 14.3 as well for a large project. For my smaller project everything still works, but for my large one there is no syntax highlightling, not auto-complete, no quick jump or anything. Xcode is basically just a notepad! It started out as problems with showing the compile errors, or they would sometimes show very briefly and then disappear right away. Now it shows the compile errors, but everything else is gone. I have tried deleting Derived Data, switching 'Suggest completions while typing' on/off as suggested in a lot of places, switching IDEIndexDisable on/off, and I have also tried to reinstall xcode. Nothing is working!