I just downloaded the GM release of Xcode 6 and it won't compile with this error:
Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc failed with exit code 1
This error can happen for numerous reasons, so this is meant to be a debugging hint. You may want to try using
xcodebuild in the command line. It will give you details as to what files are the culprits.
To do this, open Terminal and go to your project folder. Once there, type in
xcodebuild -project YourProject.xcodeproj -scheme YourScheme
or if you're working in a workspace
xcodebuild -workspace YourProject.xcworkspace -scheme YourScheme
You may see A LOT of messages pop up, but at the very end of the output you should see the specific files that are causing the crash. Back in XCode, go into those files and start playing with some of the Swift syntax to see what's going on. In my case, it had to do with the setAttributeString function, but I've seen other people have issues with ! and ?.
Hopefully that will get you headed in the right direction.