How do I diagnose and fix 'Undefined symbol: _swift_' in Swift/Xcode?

When you encounter the error 'Undefined symbol: _swift_' in Swift/Xcode, it typically indicates a problem with linking your Swift code with the required Swift libraries. Here are some steps to diagnose and fix this error:

Diagnosis Steps

1. Check Build Settings: Ensure that the 'Swift Language Version' is set correctly under Build Settings.
2. Frameworks and Libraries: Make sure all necessary frameworks and libraries are linked to your project.
3. Clean Build Folder: Go to 'Product' in the menu and select 'Clean Build Folder' (or use Shift+Command+K).
4. Check Module Imports: Verify that you're importing any required frameworks and modules in your Swift files.
5. Update Xcode: Ensure you are using the latest version of Xcode, as updates may resolve compatibility issues.

Fixing Steps

1. If using CocoaPods, run `pod install` and reopen the `.xcworkspace` file.
2. For Carthage, ensure you have built the frameworks using `carthage build`. Make sure to add the built frameworks to the 'Embedded Binaries' section.
3. Look for any compilation issues or dependencies that might be missing.