[Distributed] AST: Make invocation decoder discoverable given distributed actor

This is going to be used by IRGen later on to make it possible to
decode arguments inside of distributed target accessor instead of
allocating additional storage.

Author: xedin

include/swift/AST/ASTContext.h

@@ -1345,6 +1345,10 @@ class ASTContext final {
   /// alternative specified via the -entry-point-function-name frontend flag.
   std::string getEntryPointFunctionName() const;
 
+  /// Find the concrete invocation decoder associated with the given actor.
+  NominalTypeDecl *
+  getDistributedActorInvocationDecoder(NominalTypeDecl *);
+
 private:
   friend Decl;
 

include/swift/AST/KnownIdentifiers.def

@@ -289,6 +289,7 @@ IDENTIFIER(system)
 IDENTIFIER(target)
 IDENTIFIER(throwing)
 IDENTIFIER(using)
+IDENTIFIER(InvocationDecoder)
 IDENTIFIER(whenLocal)
 
 #undef IDENTIFIER

include/swift/AST/TypeCheckRequests.h

@@ -1091,6 +1091,24 @@ class GetDistributedActorIDPropertyRequest :
     bool isCached() const { return true; }
 };
 
+/// Obtain the invocation decoder associated with the given distributed actor.
+class GetDistributedActorInvocationDecoderRequest :
+  public SimpleRequest<GetDistributedActorInvocationDecoderRequest,
+                       NominalTypeDecl *(NominalTypeDecl *),
+                       RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  NominalTypeDecl *evaluate(Evaluator &evaluator, NominalTypeDecl *actor) const;
+
+public:
+  // Caching
+  bool isCached() const { return true; }
+};
+
 /// Retrieve the static "shared" property within a global actor that provides
 /// the actor instance representing the global actor.
 ///

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -112,6 +112,9 @@ SWIFT_REQUEST(TypeChecker, GetDistributedActorIDPropertyRequest, VarDecl *(Nomin
               Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, GetDistributedActorSystemPropertyRequest, VarDecl *(NominalTypeDecl *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, GetDistributedActorInvocationDecoderRequest,
+              NominalTypeDecl *(NominalTypeDecl *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, GlobalActorInstanceRequest,
               VarDecl *(NominalTypeDecl *),
               Cached, NoLocationInfo)

lib/Sema/TypeCheckDistributed.cpp

@@ -427,3 +427,49 @@ Type swift::getDistributedActorIDType(NominalTypeDecl *actor) {
 
   return t;
 }
+
+NominalTypeDecl *
+ASTContext::getDistributedActorInvocationDecoder(NominalTypeDecl *actor) {
+  if (!actor->isDistributedActor())
+    return nullptr;
+
+  return evaluateOrDefault(
+      evaluator, GetDistributedActorInvocationDecoderRequest{actor}, nullptr);
+}
+
+NominalTypeDecl *
+GetDistributedActorInvocationDecoderRequest::evaluate(Evaluator &evaluator,
+                                                      NominalTypeDecl *actor) const {
+    assert(actor->isDistributedActor());
+  auto &ctx = actor->getASTContext();
+
+  auto actorProtocol = ctx.getProtocol(KnownProtocolKind::DistributedActor);
+  if (!actorProtocol)
+    return nullptr;
+
+  AssociatedTypeDecl *actorSystemDecl =
+      actorProtocol->getAssociatedType(ctx.Id_ActorSystem);
+  if (!actorSystemDecl)
+    return nullptr;
+
+  auto actorSystemProtocol = ctx.getProtocol(KnownProtocolKind::DistributedActorSystem);
+  if (!actorSystemProtocol)
+    return nullptr;
+
+  AssociatedTypeDecl *decoderAssocTypeDecl =
+      actorSystemProtocol->getAssociatedType(ctx.Id_InvocationDecoder);
+  if (!decoderAssocTypeDecl)
+    return nullptr;
+
+  auto module = actor->getParentModule();
+  Type selfType = actor->getSelfInterfaceType();
+  auto conformance = module->lookupConformance(selfType, actorProtocol);
+  Type dependentType = actorProtocol->getSelfInterfaceType();
+  dependentType = DependentMemberType::get(dependentType, actorSystemDecl);
+  dependentType = DependentMemberType::get(dependentType, decoderAssocTypeDecl);
+  auto t = dependentType.subst(
+      SubstitutionMap::getProtocolSubstitutions(
+        actorProtocol, selfType, conformance));
+
+  return t->getAnyNominal();
+}