Why does assumeIsolated() now require T: Sendable?

[tgoyne (Thomas Goyne)]

@SE-0414: Region Based Isolation

actor Actor1 {
    var x: NonSendableType

    nonisolated func get() -> NonSendableType {
        self.assumeIsolated { isolatedSelf in
            return isolatedSelf.x
        }
    }
}

I do not understand this example. This is calling assumeIsolated in a context which is not isolated to that actor. How is this not just a runtime fatal error?

@John_McCall

The better long-term fix is usually to reduce your use of assumeIsolated and find a way to statically propagate the isolation you’re relying on.

The two places we use assumeIsolated are after a hop through obj-c or c++ (which cannot propagate isolated parameters unless there's some attribute I missed), and in implementations of DynamicProperty.update() which is a synchronous function that needs to be able to share access to mutable data with @MainActor things, and so with the previous "don't use locks" messaging the only way to implement it was to rely on the fact that it happened to always be called on the main thread.