[Distributed] better diagnostics for missing actor system typealias

include/swift/AST/DiagnosticsSema.def

@@ -4495,6 +4495,13 @@ ERROR(actor_inheritance,none,
 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.",
+      (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"
@@ -5475,6 +5476,28 @@ void swift::diagnoseConformanceFailure(Type T,
     return;
   }
 
+  // Special case: a distributed actor conformance often can fail because of
+  // a missing ActorSystem (or DefaultDistributedActorSystem) typealias.
+  // In this case, the "normal" errors are an avalanche of errors related to
+  // missing things in the actor that don't help users diagnose the root problem.
+  // Instead, we want to suggest adding the typealias.
+  if (Proto->isSpecificProtocol(KnownProtocolKind::DistributedActor)) {
+    auto nominal = T->getNominalOrBoundGenericNominal();
+    if (!nominal)
+      return;
+
+    // If it is missing the ActorSystem type, suggest adding it:
+    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;
+    }
+  }
+
   // Special case: for enums with a raw type, explain that the failing
   // conformance to RawRepresentable was inferred.
   if (auto enumDecl = T->getEnumOrBoundGenericEnum()) {

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?
+}