In theory, it should be possible to use previous Swift toolchains with new Xcode versions, so that you can continue to compiler your code before having to upgrade the Swift version. You can download toolchain version at https://swift.org/download.
However, while apps do compile after switching the toolchain in Xcode preferences and using Xcode 10.2 for the command line tools - running them results in this error on launch:
dyld: Symbol not found: _OBJC_CLASS_$__TtCs12_SwiftObject
Referenced from: ~/Library/Developer/CoreSimulator/Devices/CAAB28E5-9B4A-41A3-ADED-A481A62842BA/data/Containers/Bundle/Application/<Redacted>/Frameworks/PhoneNumberKit.framework/PhoneNumberKit
Expected in: /Library/Developer/Toolchains/swift-5.0.1-RELEASE.xctoolchain/usr/lib/swift/iphonesimulator/libswiftCore.dylib
in ~/Library/Developer/CoreSimulator/Devices/CAAB28E5-9B4A-41A3-ADED-A481A62842BA/data/Containers/Bundle/Application/<Redacted>/Frameworks/PhoneNumberKit.framework/PhoneNumberKit
There's some discussion around a similar error in previous Xcode betas, on GitHub, Apple forums, and elsewhere that suggest it's either and issue with Carthage or the beta - and possibly just the third-party lib I'm using itself.
Has anyone encountered this before? Is it possible to use old toolchains with newer Xcode versions?