[Concurrency/Distributed] nonisolated-nonsending by default breaks distributed thunks

the new NonisolatedNonsendingByDefault upcoming feature breaks remote
calls in distributed actors, because the expected isolation doesn't
match and the runtime swift_distributed_execute_target_resume will
crash.

This is a short term fix to unblock adopters, however preferably we
should mark the thunks as nonisolated(nonsending), though that seems to
be more involved.

resolves rdar://159247975

include/swift/AST/Decl.h

@@ -3004,6 +3004,13 @@ public:
   /// `distributed var get { }` accessors.
   bool isDistributedGetAccessor() const;
 
+  /// Is this a 'distributed thunk'?
+  ///
+  /// Distributed thunks are synthesized functions which perform the "is remote?"
+  /// check, before dispatching to a 'system.remoteCall' (if actor was remote).
+  /// They are always 'async' and 'throws'.
+  bool isDistributedThunk() const;
+
   bool hasName() const { return bool(Name); }
   bool isOperator() const { return Name.isOperator(); }
 

lib/AST/DistributedDecl.cpp

@@ -1369,6 +1369,13 @@ bool ValueDecl::isDistributedGetAccessor() const {
   return false;
 }
 
+bool ValueDecl::isDistributedThunk() const {
+  if (auto func = dyn_cast<AbstractFunctionDecl>(this)) {
+    return func->isDistributedThunk();
+  }
+  return false;
+}
+
 ConstructorDecl *
 NominalTypeDecl::getDistributedRemoteCallTargetInitFunction() const {
   auto mutableThis = const_cast<NominalTypeDecl *>(this);

lib/Sema/CodeSynthesisDistributedActor.cpp

@@ -737,6 +737,7 @@ static FuncDecl *createSameSignatureDistributedThunkDecl(DeclContext *DC,
 
   thunk->setSynthesized(true);
   thunk->setDistributedThunk(true);
+  // TODO(distributed): These would benefit from becoming nonisolated(nonsending)
   thunk->getAttrs().add(NonisolatedAttr::createImplicit(C));
 
   return thunk;

lib/Sema/TypeCheckConcurrency.cpp

@@ -5231,6 +5231,13 @@ getIsolationFromAttributes(const Decl *decl, bool shouldDiagnose = true,
     if (decl->getASTContext().LangOpts.hasFeature(
             Feature::NonisolatedNonsendingByDefault)) {
       if (auto *value = dyn_cast<ValueDecl>(decl)) {
+        // TODO(distributed): make distributed thunks nonisolated(nonsending) and remove this if
+        if (value->isAsync() && value->isDistributedThunk()) {
+          // don't change isolation of distributed thunks until we make them nonisolated(nonsending),
+          // since the runtime calling them assumes they're just nonisolated right now.
+          return ActorIsolation::forNonisolated(nonisolatedAttr->isUnsafe());
+        }
+
         if (value->isAsync() &&
             value->getModuleContext() == decl->getASTContext().MainModule) {
           return ActorIsolation::forCallerIsolationInheriting();

test/Distributed/Runtime/distributed_actor_remoteCall_roundtrip.swift

@@ -4,8 +4,6 @@
 // RUN: %target-codesign %t/a.out
 // RUN: %target-run %t/a.out | %FileCheck %s --enable-var-scope
 
-// X: %target-run-simple-swift( -Xfrontend -module-name=main -target %target-swift-5.7-abi-triple  -parse-as-library -Xfrontend -I -Xfrontend %t ) | %FileCheck %s
-
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // REQUIRES: distributed

test/Distributed/Runtime/distributed_actor_remoteCall_roundtrip_nonisolated_nonsending_by_default.swift

@@ -0,0 +1,73 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend-emit-module -emit-module-path %t/FakeDistributedActorSystems.swiftmodule \
+// RUN:     -module-name FakeDistributedActorSystems -target %target-swift-5.7-abi-triple \
+// RUN:     %S/../Inputs/FakeDistributedActorSystems.swift
+
+// RUN: %target-build-swift -module-name main -enable-upcoming-feature NonisolatedNonsendingByDefault \
+// RUN:     -target %target-swift-5.7-abi-triple -j2 -parse-as-library -I %t %s \
+// RUN:     %S/../Inputs/FakeDistributedActorSystems.swift -o %t/a.out
+
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s --enable-var-scope
+
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+// rdar://76038845
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+// FIXME(distributed): Distributed actors currently have some issues on windows, isRemote always returns false. rdar://82593574
+// UNSUPPORTED: OS=windows-msvc
+
+import Distributed
+import FakeDistributedActorSystems
+
+typealias DefaultDistributedActorSystem = FakeRoundtripActorSystem
+
+distributed actor Greeter: CustomStringConvertible {
+  distributed func echo(name: String) -> String {
+    return "Echo: \(name) (impl on: \(self.id))"
+  }
+
+  distributed func error() throws -> String {
+    throw SomeError()
+  }
+
+  nonisolated var description: String {
+    "\(Self.self)(\(id))"
+  }
+}
+
+struct SomeError: Error {}
+
+// ==== Test -------------------------------------------------------------------
+
+func test() async throws {
+  let system = DefaultDistributedActorSystem()
+
+  let local = Greeter(actorSystem: system)
+  let ref = try Greeter.resolve(id: local.id, using: system)
+
+  let reply = try await ref.echo(name: "Caplin")
+  // CHECK: > encode argument name:name, value: Caplin
+  // CHECK-NOT: > encode error type
+  // CHECK: > encode return type: Swift.String
+  // CHECK: > done recording
+  // CHECK: >> remoteCall
+  // CHECK: > decode return type: Swift.String
+  // CHECK: > decode argument: Caplin
+  // CHECK: << onReturn: Echo: Caplin (impl on: ActorAddress(address: "<unique-id>"))
+
+  print("got: \(reply)")
+  // CHECK: got: Echo: Caplin (impl on: ActorAddress(address: "<unique-id>"))
+}
+
+@main struct Main {
+  static func main() async {
+    try! await test()
+  }
+}