Question: XCode 6 Beta 5 and autocomplete stopped working. I thought problem in Beta 5 but I've reinstall to Beta 4 and it's still not work but it did before. I check preferences for "suggests" and I tried to delete Derived Data but it doesn't help

Answer: 
Check if the .m file has been checked in "Target Membership"
Tried to clean/delete derived data multiple times. Autocomplete was disabled in some files, but not App Delegate. After a while I saw red libraries int he project navigator. Those are managed by cocoapods. The solution turned out to be:

* Navigate to the project root folder
* Delete "Pods" directory
* open terminal, type cd
* drag the project folder there to get full path
* run pod install

Upon reinstalling pods, syntax highlighting and code completion is restored in the affected files.