[Distributed] Allow disabling SerializationRequirement by using Any

It should be possible to disable SerializationRequirement by assigning
Any to it. This effectively disables the checking, since any type
conforms to that.

Previously we would crash trying to check for this conformance; or
rather, emitting diagnostics about un-necessary casts then encountering
incomplete types in CSDiagnostics.

Avoiding the cast entirely sounds like a good solution here.

resolves rdar://159285863

lib/Sema/DerivedConformance/DerivedConformanceDistributedActor.cpp

@@ -332,17 +332,24 @@ deriveBodyDistributed_invokeHandlerOnReturn(AbstractFunctionDecl *afd,
     metatypeVar->setImplicit();
     metatypeVar->setSynthesized();
 
+    // If the SerializationRequirement requires it, we need to emit a cast:
     // metatype as! <<concrete SerializationRequirement.Type>>
+    bool serializationRequirementIsAny =
+        metatypeParam->getInterfaceType()->getMetatypeInstanceType()->isEqual(
+            C.getAnyExistentialType());
+
     auto metatypeRef =
         new (C) DeclRefExpr(ConcreteDeclRef(metatypeParam), dloc, implicit);
-    auto metatypeSRCastExpr = ForcedCheckedCastExpr::createImplicit(
-        C, metatypeRef, serializationRequirementMetaTypeTy);
-
     auto metatypePattern = NamedPattern::createImplicit(C, metatypeVar);
-    auto metatypePB = PatternBindingDecl::createImplicit(
-        C, swift::StaticSpellingKind::None, metatypePattern,
-        /*expr=*/metatypeSRCastExpr, func);
 
+    PatternBindingDecl *metatypePB = serializationRequirementIsAny ?
+      PatternBindingDecl::createImplicit(
+          C, swift::StaticSpellingKind::None, metatypePattern,
+          /*expr=*/metatypeRef, func) :
+      PatternBindingDecl::createImplicit(
+          C, swift::StaticSpellingKind::None, metatypePattern,
+          /*expr=*/ForcedCheckedCastExpr::createImplicit(
+          C, metatypeRef, serializationRequirementMetaTypeTy), func);
     stmts.push_back(metatypePB);
     stmts.push_back(metatypeVar);
   }
@@ -399,7 +406,6 @@ static FuncDecl *deriveDistributedActorSystem_invokeHandlerOnReturn(
   auto system = derived.Nominal;
   auto &C = system->getASTContext();
 
-  // auto serializationRequirementType = getDistributedActorSystemType(decl);
   auto resultHandlerType = getDistributedActorSystemResultHandlerType(system);
   auto unsafeRawPointerType = C.getUnsafeRawPointerType();
   auto anyTypeType = ExistentialMetatypeType::get(C.TheAnyType); // Any.Type
@@ -436,7 +442,7 @@ static FuncDecl *deriveDistributedActorSystem_invokeHandlerOnReturn(
                                /*throws=*/true,
                                /*ThrownType=*/Type(),
                                /*genericParams=*/nullptr, params,
-                               /*returnType*/ TupleType::getEmpty(C), system);
+                               /*returnType=*/TupleType::getEmpty(C), system);
   funcDecl->setSynthesized(true);
   funcDecl->copyFormalAccessFrom(system, /*sourceIsParentContext=*/true);
   funcDecl->setBodySynthesizer(deriveBodyDistributed_invokeHandlerOnReturn);

test/Distributed/distributed_actor_system_serialization_requirement_any.swift

@@ -0,0 +1,97 @@
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.7-abi-triple -I %t 2>&1 %s
+
+// UNSUPPORTED: back_deploy_concurrency
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import Distributed
+
+final class CrashActorSystem: DistributedActorSystem {
+  typealias ActorID = CrashActorID
+  typealias InvocationEncoder = CrashInvocationEncoder
+  typealias InvocationDecoder = CrashInvocationDecoder
+  typealias ResultHandler = CrashResultHandler
+
+  typealias SerializationRequirement = Any
+
+  func assignID<Actor: DistributedActor>(_: Actor.Type) -> ActorID where Actor.ID == ActorID {
+    fatalError()
+  }
+
+  func resignID(_: ActorID) {
+    // nothing
+  }
+
+  func actorReady<Actor: DistributedActor>(_: Actor) where Actor.ID == ActorID { }
+
+  func resolve<Actor: DistributedActor>(id _: ActorID, as _: Actor.Type) throws -> Actor? where Actor.ID == ActorID {
+    return nil
+  }
+
+  func makeInvocationEncoder() -> InvocationEncoder {
+    return InvocationEncoder()
+  }
+
+  func remoteCallVoid<Actor: DistributedActor, Failure: Error>(
+    on _: Actor,
+    target _: RemoteCallTarget,
+    invocation _: inout InvocationEncoder,
+    throwing _: Failure.Type
+  ) async throws where Actor.ID == ActorID {
+    return
+  }
+
+  func remoteCall<Actor: DistributedActor, Failure: Error, Success: SerializationRequirement>(
+    on _: Actor,
+    target _: RemoteCallTarget,
+    invocation _: inout InvocationEncoder,
+    throwing _: Failure.Type,
+    returning _: Success.Type
+  ) async throws -> Success where Actor.ID == ActorID {
+    fatalError()
+  }
+
+}
+
+struct CrashActorID: Hashable {
+  func hash(into hasher: inout Hasher) { }
+}
+
+struct CrashInvocationEncoder: DistributedTargetInvocationEncoder {
+  typealias SerializationRequirement = CrashActorSystem.SerializationRequirement
+
+  func recordGenericSubstitution<GenericSubstitution>(_: GenericSubstitution.Type) throws { }
+  mutating func recordArgument<Argument: SerializationRequirement>(_: RemoteCallArgument<Argument>) throws { }
+  mutating func recordReturnType<Success: SerializationRequirement>(_: Success.Type) throws { }
+  func recordErrorType<Failure: Error>(_: Failure.Type) throws { }
+  func doneRecording() throws { }
+}
+
+struct CrashInvocationDecoder: DistributedTargetInvocationDecoder {
+  typealias SerializationRequirement = CrashActorSystem.SerializationRequirement
+
+  func decodeGenericSubstitutions() throws -> [Any.Type] {
+    return []
+  }
+
+  mutating func decodeNextArgument<Argument: SerializationRequirement>() throws -> Argument {
+    fatalError()
+  }
+
+  func decodeReturnType() throws -> Any.Type? {
+    return nil
+  }
+
+  func decodeErrorType() throws -> Any.Type? {
+    return nil
+  }
+
+}
+
+struct CrashResultHandler: DistributedTargetInvocationResultHandler {
+  typealias SerializationRequirement = CrashActorSystem.SerializationRequirement
+
+  func onReturn<Success: SerializationRequirement>(value _: Success) async throws { }
+  func onReturnVoid() async throws { }
+  func onThrow<Failure: Error>(error _: Failure) async throws { }
+}