[Concurrency] Deprecate and obsolete AnyActor.

This marker protocol isn't useful for abstracting over actors and distributed
actors because it doesn't have any runtime requirements, so there's no way
to switch to the given actor. Instead, you should use `isolated (any Actor)?`
parameters, and the compiler will compute a local actor value from a
distributed actor using `DistributedActor.asLocalActor`.

Author: hborla

stdlib/public/Concurrency/Actor.swift

@@ -28,6 +28,8 @@ import Swift
 /// impossible for one to refine the other.
 @_marker
 @available(SwiftStdlib 5.1, *)
+@available(*, deprecated, message: "Use 'any Actor' with 'DistributedActor.asLocalActor' instead")
+@available(swift, obsoleted: 6.0, message: "Use 'any Actor' with 'DistributedActor.asLocalActor' instead")
 public protocol AnyActor: AnyObject, Sendable {}
 
 /// Common protocol to which all actors conform.

test/Concurrency/isolated_parameters.swift

@@ -356,6 +356,7 @@ func isolated_generic_bad_2<T: Equatable>(_ t: isolated T) {}
 // expected-error@-1 {{'isolated' parameter type 'T' does not conform to 'Actor' or 'DistributedActor'}}
 func isolated_generic_bad_3<T: AnyActor>(_ t: isolated T) {}
 // expected-error@-1 {{'isolated' parameter type 'T' does not conform to 'Actor' or 'DistributedActor'}}
+// expected-warning@-2 {{'AnyActor' is deprecated: Use 'any Actor' with 'DistributedActor.asLocalActor' instead}}
 
 func isolated_generic_bad_4<T>(_ t: isolated Array<T>) {}
 // expected-error@-1 {{'isolated' parameter type 'Array<T>' does not conform to 'Actor' or 'DistributedActor'}}
@@ -475,7 +476,7 @@ nonisolated func fromNonisolated(ns: NotSendable) async -> NotSendable {
   await pass(value: ns, isolation: nil)
 }
 
-func invalidIsolatedClosureParam<A: AnyActor> (
+func invalidIsolatedClosureParam<A: AnyActor> ( // expected-warning {{'AnyActor' is deprecated: Use 'any Actor' with 'DistributedActor.asLocalActor' instead}}
   _: (isolated A) async throws -> Void // expected-error {{'isolated' parameter type 'A' does not conform to 'Actor' or 'DistributedActor'}}
 ) {}