[Distributed] onReturn ad-hoc req must not be mutating

Author: ktoso

lib/AST/DistributedDecl.cpp

@@ -1166,6 +1166,11 @@ AbstractFunctionDecl::isDistributedTargetInvocationResultHandlerOnReturn() const
       return false;
     }
 
+    // --- must not be mutating
+    if (func->isMutating()) {
+      return false;
+    }
+
     // === Check generics
     if (!isGeneric()) {
       return false;

test/Distributed/distributed_actor_system_missing_adhoc_requirement_impls.swift

@@ -816,6 +816,7 @@ struct BadResultHandler_missingOnReturn: DistributedTargetInvocationResultHandle
   func onReturnVoid() async throws {}
   func onThrow<Err: Error>(error: Err) async throws {}
 }
+
 struct BadResultHandler_missingRequirement: DistributedTargetInvocationResultHandler {
   // expected-error@-1{{struct 'BadResultHandler_missingRequirement' is missing witness for protocol requirement 'onReturn'}}
   // expected-note@-2{{protocol 'BadResultHandler_missingRequirement' requires function 'onReturn' with signature:}}
@@ -826,6 +827,16 @@ struct BadResultHandler_missingRequirement: DistributedTargetInvocationResultHan
   func onThrow<Err: Error>(error: Err) async throws {}
 }
 
+struct BadResultHandler_mutatingButShouldNotBe: DistributedTargetInvocationResultHandler {
+  // expected-error@-1{{struct 'BadResultHandler_missingRequirement' is missing witness for protocol requirement 'onReturn'}}
+  // expected-note@-2{{protocol 'BadResultHandler_missingRequirement' requires function 'onReturn' with signature:}}
+  typealias SerializationRequirement = Codable
+
+  mutating func onReturn<Success: Codable>(value: Success) async throws {} // WRONG: can't be mutating
+  func onReturnVoid() async throws {}
+  func onThrow<Err: Error>(error: Err) async throws {}
+}
+
 public struct PublicFakeResultHandler: DistributedTargetInvocationResultHandler {
   public typealias SerializationRequirement = Codable