[Distributed] add note explaining DefaultDistributedActorSystem

Author: ktoso

include/swift/AST/DiagnosticsSema.def

@@ -4496,11 +4496,12 @@ ERROR(actor_protocol_illegal_inheritance,none,
       "non-actor type %0 cannot conform to the 'Actor' protocol",
       (DeclName))
 ERROR(distributed_actor_conformance_missing_system_type,none,
-      "distributed actor %0 does not declare ActorSystem it can be used with. "
-      "Declare the associated actor system with a 'typealias ActorSystem = MyActorSystem' "
-      "inside the actor, or module-wide by declaring a global 'typealias "
-      "DefaultDistributedActorSystem = MyActorSystem'.",
+      "distributed actor %0 does not declare ActorSystem it can be used with.",
       (DeclName))
+NOTE(note_distributed_actor_system_can_be_defined_using_defaultdistributedactorsystem,none,
+     "you can provide a module-wide default actor system by declaring:\n"
+     "typealias DefaultDistributedActorSystem = <#ConcreteActorSystem#>\n",
+     ())
 ERROR(distributed_actor_protocol_illegal_inheritance,none,
       "non-distributed actor type %0 cannot conform to the 'DistributedActor' protocol",
       (DeclName))

lib/Sema/TypeCheckProtocol.cpp

@@ -29,6 +29,7 @@
 #include "swift/AST/AccessScope.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/Decl.h"
+#include "swift/AST/DistributedDecl.h"
 #include "swift/AST/Effects.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericEnvironment.h"
@@ -5486,17 +5487,13 @@ void swift::diagnoseConformanceFailure(Type T,
       return;
 
     // If it is missing the ActorSystem type, suggest adding it:
-    // FIXME(distributed): use getDistributedActorSystemType(nominal); once merged
-    auto &C = nominal->getASTContext();
-    auto DA = C.getDistributedActorDecl();
-    Type selfType = nominal->getSelfInterfaceType();
-    auto conformance = nominal->getParentModule()->lookupConformance(selfType, DA);
-    auto systemTy = conformance.getTypeWitnessByName(selfType, C.Id_ActorSystem);
-
+    auto systemTy = getDistributedActorSystemType(/*actor=*/nominal);
     if (!systemTy || systemTy->hasError()) {
       diags.diagnose(ComplainLoc,
                      diag::distributed_actor_conformance_missing_system_type,
                      nominal->getName());
+      diags.diagnose(nominal->getStartLoc(),
+                     diag::note_distributed_actor_system_can_be_defined_using_defaultdistributedactorsystem);
       return;
     }
   }

test/Distributed/distributed_actor_system_missing.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -typecheck -verify -enable-experimental-distributed -disable-availability-checking -I %t 2>&1 %s
+
+// UNSUPPORTED: back_deploy_concurrency
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import _Distributed
+
+distributed actor DA {
+  // expected-error@-1{{distributed actor 'DA' does not declare ActorSystem it can be used with.}}
+
+  // expected-note@-3{{you can provide a module-wide default actor system by declaring:}}
+
+  // Note to add the typealias is diagnosed on the protocol:
+  // _Distributed.DistributedActor:3:20: note: diagnostic produced elsewhere: protocol requires nested type 'ActorSystem'; do you want to add it?
+}