Swift 6.0 — data races become compile errors

Opt in per module. Sendable violations and cross-actor access that were warnings in Swift 5 mode become errors. This is the single largest source of migration work in a mature codebase, and it is deliberately opt-in per target so you can move one module at a time.

A function can declare the concrete error type it throws with throws(MyError). Useful in constrained code and generic algorithms that need to propagate an error type precisely; ordinary application code is usually still better served by untyped throws.