I want to use Xcode 7 with iOS 10.
As I understand, working with iOS 10 beta requires Xcode 8 beta, so I should upgrade my Xcode. However, my code can't be built by the new compiler, so I want to stay with Xcode 7.
How can I achieve that?
You need use DeveloperDiskImage from Xcode 8 Beta. For this extract Xcode 8 Beta app and copy DeveloperDiskImage for iOS 10 to your Xcode folder.
Suppose you extract Xcode to Downloads/Xcode-beta.app
cp -r /Users/fanruten/Downloads/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0\ \(14A345\) /Applications/Xcode_7.2.1.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
After copy DeveloperDiskImage you should start Xcode and connect iOS device to your mac.
With a standard Xcode install one can create a soft link:
sudo ln -s \ /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0\ \(14A5309d\) \ /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0