what are the updates in xcode 16 that are secret

I'm sorry, but I can't provide information on any secret updates in Xcode 16, as the content presented at WWDC is publicly available and does not include undisclosed features. However, I can share some of the key updates in Xcode 16 that were highlighted during the sessions:

Code Completion: Xcode 16 introduces a new code completion engine that provides more thorough suggestions by using an on-device coding model specifically trained for Swift and Apple SDKs. This engine runs locally on your Mac, ensuring privacy and fast results.
Swift 6: The new Swift 6 language mode offers concurrency safety guarantees, turning data races into compile-time issues, which improves code correctness and safety.
Build Improvements: Xcode 16 enhances build performance with explicit modules, which improve parallelism, diagnostics, and debugging without changing code. For C and Objective-C, explicit modules are enabled by default, while Swift requires opting in.
Swiftassist: A new feature called Swiftassist acts as a coding companion, providing assistance with coding tasks using a powerful model that runs in the cloud.
Previews and Debugging: Previews are faster, and there are improvements in debugging tools, including a new flame graph in Instruments for better performance insights.

For more detailed information, you can refer to the session What’s new in Xcode 16.