[assume-mainactor] Only perform the change for items that are within the current module.

I also added a small runtime test just as a sanity check. We do not change any
codegen here since the change is at the Sema level... but I thought it would be
prudent to at least have a small smoke test.

rdar://140439795

Author: gottesmm

lib/Sema/TypeCheckConcurrency.cpp

@@ -5627,9 +5627,10 @@ InferredActorIsolation ActorIsolationRequest::evaluate(
   ActorIsolation defaultIsolation = ActorIsolation::forUnspecified();
   IsolationSource defaultIsolationSource;
 
-  // If we are supposed to infer main actor isolation by default, make our
-  // default isolation main actor.
-  if (ctx.LangOpts.hasFeature(Feature::UnspecifiedMeansMainActorIsolated)) {
+  // If we are supposed to infer main actor isolation by default for entities
+  // within our module, make our default isolation main actor.
+  if (ctx.LangOpts.hasFeature(Feature::UnspecifiedMeansMainActorIsolated) &&
+      value->getModuleContext() == ctx.MainModule) {
     defaultIsolation = ActorIsolation::forMainActor(ctx);
   }
 

test/Concurrency/Runtime/unspecified_is_main_actor.swift

@@ -0,0 +1,92 @@
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -swift-version 6 -g -import-objc-header %S/Inputs/RunOnMainActor.h %import-libdispatch -enable-experimental-feature UnspecifiedMeansMainActorIsolated )
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: concurrency_runtime
+// REQUIRES: libdispatch
+// REQUIRES: asserts
+// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
+
+// UNSUPPORTED: freestanding
+
+// For now we do not support back deployment or use os stdlib
+// UNSUPPORTED: back_deployment_concurrency
+// UNSUPPORTED: use_os_stdlib
+
+// Just a runtime test as a sanity check.
+
+import StdlibUnittest
+import Dispatch
+
+////////////////////////
+// MARK: Declarations //
+////////////////////////
+
+@_silgen_name("dispatch_assert_queue")
+nonisolated func dispatch_assertQueue(_ ptr: UnsafeRawPointer)
+
+nonisolated func checkIfOnMainQueue() {
+  dispatch_assertQueue(getDispatchMain())
+}
+
+actor Custom {
+}
+
+@globalActor
+struct CustomActor {
+  static nonisolated var shared: Custom {
+    return Custom()
+  }
+}
+
+///////////////////////////////////////////
+// MARK: Scaffolding/Testing Scaffolding //
+///////////////////////////////////////////
+
+let tests = TestSuite("UnspecifiedIsMainActor")
+
+tests.test("checkIfOnMainQueue does not crash on the main queue") { @MainActor () -> () in
+  // Why do we crash if this is synchronous.
+  expectCrashLater()
+  checkIfOnMainQueue()
+}
+
+tests.test("checkIfOnMainQueue does not crash on the main queue") { @MainActor () async -> () in
+  checkIfOnMainQueue()
+}
+
+tests.test("checkIfOnMainQueue crashes off the main queue") {
+  expectCrashLater()
+  await { @CustomActor in
+    print("=> checkIfOnMainQueue crashes off the main queue")
+    checkIfOnMainQueue()
+  }()
+}
+
+tests.test("checkIfOnMainQueue crashes off the main queue 2") { @CustomActor () async -> () in
+  expectCrashLater()
+  print("=> checkIfOnMainQueue crashes off the main queue 2")
+  checkIfOnMainQueue()
+}
+
+/////////////////
+// MARK: Tests //
+/////////////////
+
+class Klass {}
+
+struct MainActorIsolated {
+  init() {}
+
+  func test() async {
+    checkIfOnMainQueue()
+  }
+};
+
+tests.test("callNominalType") { @CustomActor () -> () in
+  let x = MainActorIsolated()
+  // We would crash without hopping here.
+  await x.test()
+}
+
+await runAllTestsAsync()

test/Concurrency/assume_mainactor.swift

@@ -70,7 +70,7 @@ struct NonIsolatedStructContainingKlass {
 
 @globalActor
 actor CustomActor {
-  static let shared = CustomActor()
+  static nonisolated let shared = CustomActor()
 }
 
 // CHECK: // unspecifiedAsync<A>(_:)
@@ -199,3 +199,9 @@ actor MyActor2 {
   @CustomActor
   init(x: ()) {}
 }
+
+@CustomActor func validateThatPrintIsStillNonIsolated() {
+  // Since we are in a CustomActor, we can only call this if print is
+  // NonIsolated and not if print was inferred to be main actor.
+  print("123")
+}