Some background info: Syntax color/highlighting and auto-suggest stopped working correctly. I read a lot of questions about how to fix this but none of them helped me as they were related to Derived Data.
I'm learning Swift from some online tutorials, and noticed my syntax highlighting was different to that of the course teacher's. Then I noticed that when I create an instance of my struct, and use the dot syntax after the instance, Xcode didn't auto-suggest the struct's functions and variables, whereas it does in the downloaded version of the project. After some investigation I noticed that a key difference between my project and the teacher's is that my project has a Deployment Target of 8.1, whereas the teacher's has 8.0. I changed mine to 8.0 and immediately the syntax highlighting was restored and I could access the struct instance's functions and variables.
I'm guessing this may be a bug in Xcode, but perhaps I have something misconfigured in Xcode? I'd be grateful if anyone could explain how/why this bug occurred and what is best practice when choosing the Deployment Target.
The setting is located at Project > General > Deployment Info
I hope the above info may help others who encounter this issue.
Deployment Target is the iOS version that your app is intended for. This does not mean it is incompatible with other versions, simply that it is designed to work best on that version. Generally, using the app on a device with a newer version of the OS is much safer and less likely than using the app on a device with an older version of the OS. There's actually a lot of restrictions around installing an app with a deployment target higher than the OS version on the phone.
The most common deployment target right now is going to be 7.x, as most apps were updated when iOS 7 first came out (there was a pretty big change in UI standards and SDK functionality from 6 to 7), and iOS 8 isn't too terribly different from 7, so there's no need to restrict it and/or write new versions of the app for the new SDK. By having a deployment target of 7.x, you should ensure that the app works well on 7.x and all higher versions, including 8.x. You should also have the lowest deployment target you can that's reasonable - so, a purely iOS 8 app should ideally be targeting iOS 8.0. You'll save yourself a lot of PR and invalid bug reports from users who don't understand versioning.
In your scenario, it sounds like there may be a bug in Xcode when using the iOS 8.1 deployment target. You might want to check and see if your docs are downloaded. Go to Xcode -> Preferences -> Downloads and ensure everything is downloaded. I'm not sure if these are actually used in syntax highlighting and completion code, but it's possible. I would also do clean the build folder (Product -> Hold Alt -> Clean Build Folder). Again, I'm not sure, but I think there's just some small thing that's not clicking here. I'm assuming you cleared Derived Data based on your first paragraph - if not, do that (Window -> Organizer -> Select Project -> Delete).