This is a subtle point. It's not the array assignment that's the race here, but the update to the shared (by closure capture) `a` variable. You'd get the same data race if `a` were an `Int`, and we don't say that integers are non-thread-safe. If each closure had its own reference to the array, it would be copied on write.

I believe this is pretty good evidence for my claim that understanding the thread safety is confusing.