@jsq@mattiem The Swift 6 compiler will still support the Swift 5 language mode, just as the Swift 5 compilers still support the Swift 4 and 4.2 language modes. The purpose of the Swift 6 language mode is enforcing data race safety by default, so if you don’t want that, you’ll continue to use Swift 5 mode. If you want to enable a specific source breaking change from Swift 6 in Swift 5 mode (say, full error type inference for typed throws), you can do that with upcoming feature flags.