Add AsyncEntryPoint SILDeclRef type

The AsyncEntryPoint represents the thunk that is wrapped in a task. This
thunk is used to ensure that the main function explicitly calls "exit",
and to properly unwrap and report any unhandled errors returned from the
user-written main. The function takes on the name `@async_main` in the
emitted SIL.

include/swift/SIL/SILDeclRef.h

@@ -156,8 +156,11 @@ struct SILDeclRef {
     /// The main entry-point function. This may reference a SourceFile for a
     /// top-level main, or a decl for e.g an @main decl.
     EntryPoint,
+
+    /// The asynchronous main entry-point function.
+    AsyncEntryPoint,
   };
-  
+
   /// The AST node represented by this SILDeclRef.
   Loc loc;
   /// The Kind of this SILDeclRef.
@@ -231,6 +234,9 @@ struct SILDeclRef {
   /// Produces a SILDeclRef for a synthetic main entry-point such as @main.
   static SILDeclRef getMainDeclEntryPoint(ValueDecl *decl);
 
+  /// Produces a SILDeclRef for the synthesized async main entry-point
+  static SILDeclRef getAsyncMainDeclEntryPoint(ValueDecl *decl);
+
   /// Produces a SILDeclRef for the entry-point of a main FileUnit.
   static SILDeclRef getMainFileEntryPoint(FileUnit *file);
 

lib/IRGen/GenObjC.cpp

@@ -474,6 +474,7 @@ namespace {
       case SILDeclRef::Kind::PropertyWrapperBackingInitializer:
       case SILDeclRef::Kind::PropertyWrapperInitFromProjectedValue:
       case SILDeclRef::Kind::EntryPoint:
+      case SILDeclRef::Kind::AsyncEntryPoint:
         llvm_unreachable("Method does not have a selector");
 
       case SILDeclRef::Kind::Destroyer:

lib/SIL/IR/SILDeclRef.cpp

@@ -273,6 +273,8 @@ SILLinkage SILDeclRef::getLinkage(ForDefinition_t forDefinition) const {
   // The main entry-point is public.
   if (kind == Kind::EntryPoint)
     return SILLinkage::Public;
+  if (kind == Kind::AsyncEntryPoint)
+    return SILLinkage::Hidden;
 
   // Add External to the linkage (e.g. Public -> PublicExternal) if this is a
   // declaration not a definition.
@@ -448,6 +450,15 @@ SILDeclRef SILDeclRef::getMainDeclEntryPoint(ValueDecl *decl) {
   return result;
 }
 
+SILDeclRef SILDeclRef::getAsyncMainDeclEntryPoint(ValueDecl *decl) {
+  auto *file = cast<FileUnit>(decl->getDeclContext()->getModuleScopeContext());
+  assert(file->getMainDecl() == decl);
+  SILDeclRef result;
+  result.loc = decl;
+  result.kind = Kind::AsyncEntryPoint;
+  return result;
+}
+
 SILDeclRef SILDeclRef::getMainFileEntryPoint(FileUnit *file) {
   assert(file->hasEntryPoint() && !file->getMainDecl());
   SILDeclRef result;
@@ -539,7 +550,7 @@ IsSerialized_t SILDeclRef::isSerialized() const {
     return IsNotSerialized;
   }
 
-  if (kind == Kind::EntryPoint)
+  if (kind == Kind::EntryPoint || kind == Kind::AsyncEntryPoint)
     return IsNotSerialized;
 
   if (isIVarInitializerOrDestroyer())
@@ -925,6 +936,9 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
     return mangler.mangleInitFromProjectedValueEntity(cast<VarDecl>(getDecl()),
                                                       SKind);
 
+  case SILDeclRef::Kind::AsyncEntryPoint: {
+    return "async_Main";
+  }
   case SILDeclRef::Kind::EntryPoint: {
     return getASTContext().getEntryPointFunctionName();
   }
@@ -1257,7 +1271,8 @@ unsigned SILDeclRef::getParameterListCount() const {
     return 1;
 
   // Always uncurried even if the underlying function is curried.
-  if (kind == Kind::DefaultArgGenerator || kind == Kind::EntryPoint)
+  if (kind == Kind::DefaultArgGenerator || kind == Kind::EntryPoint ||
+      kind == Kind::AsyncEntryPoint)
     return 1;
 
   auto *vd = getDecl();

lib/SIL/IR/SILFunctionType.cpp

@@ -2526,6 +2526,9 @@ static CanSILFunctionType getNativeSILFunctionType(
     case SILDeclRef::Kind::Deallocator:
       return getSILFunctionTypeForConventions(DeallocatorConventions());
 
+    case SILDeclRef::Kind::AsyncEntryPoint:
+      return getSILFunctionTypeForConventions(
+          DefaultConventions(NormalParameterConvention::Guaranteed));
     case SILDeclRef::Kind::EntryPoint:
       llvm_unreachable("Handled by getSILFunctionTypeForAbstractCFunction");
     }
@@ -3071,6 +3074,7 @@ static ObjCSelectorFamily getObjCSelectorFamily(SILDeclRef c) {
   case SILDeclRef::Kind::PropertyWrapperBackingInitializer:
   case SILDeclRef::Kind::PropertyWrapperInitFromProjectedValue:
   case SILDeclRef::Kind::EntryPoint:
+  case SILDeclRef::Kind::AsyncEntryPoint:
     llvm_unreachable("Unexpected Kind of foreign SILDeclRef");
   }
 
@@ -3343,6 +3347,8 @@ TypeConverter::getDeclRefRepresentation(SILDeclRef c) {
     case SILDeclRef::Kind::IVarDestroyer:
       return SILFunctionTypeRepresentation::Method;
 
+    case SILDeclRef::Kind::AsyncEntryPoint:
+      return SILFunctionTypeRepresentation::Thin;
     case SILDeclRef::Kind::EntryPoint:
       return SILFunctionTypeRepresentation::CFunctionPointer;
   }
@@ -4170,6 +4176,7 @@ static AbstractFunctionDecl *getBridgedFunction(SILDeclRef declRef) {
   case SILDeclRef::Kind::IVarInitializer:
   case SILDeclRef::Kind::IVarDestroyer:
   case SILDeclRef::Kind::EntryPoint:
+  case SILDeclRef::Kind::AsyncEntryPoint:
     return nullptr;
   }
   llvm_unreachable("bad SILDeclRef kind");

lib/SIL/IR/SILPrinter.cpp

@@ -316,6 +316,7 @@ void SILDeclRef::print(raw_ostream &OS) const {
   switch (kind) {
   case SILDeclRef::Kind::Func:
   case SILDeclRef::Kind::EntryPoint:
+  case SILDeclRef::Kind::AsyncEntryPoint:
     break;
   case SILDeclRef::Kind::Allocator:
     OS << "!allocator";

lib/SIL/IR/TypeLowering.cpp

@@ -2556,6 +2556,32 @@ getFunctionInterfaceTypeWithCaptures(TypeConverter &TC,
       innerExtInfo);
 }
 
+static CanAnyFunctionType getAsyncEntryPoint(ASTContext &C) {
+
+  // @main struct Main {
+  //    static func main() async throws {}
+  //    static func $main() async throws { try await main() }
+  //  }
+  //
+  // func @async_main() async -> Void {
+  //   do {
+  //      try await Main.$main()
+  //      exit(0)
+  //   } catch {
+  //      _emitErrorInMain(error)
+  //   }
+  // }
+  //
+  // This generates the type signature for @async_main
+  // TODO: 'Never' return type would be more accurate.
+
+  CanType returnType = C.getVoidType()->getCanonicalType();
+  FunctionType::ExtInfo extInfo =
+      FunctionType::ExtInfoBuilder().withAsync(true).withThrows(false).build();
+  return CanAnyFunctionType::get(/*genericSig*/ nullptr, {}, returnType,
+                                 extInfo);
+}
+
 static CanAnyFunctionType getEntryPointInterfaceType(ASTContext &C) {
   // Use standard library types if we have them; otherwise, fall back to
   // builtins.
@@ -2675,6 +2701,8 @@ CanAnyFunctionType TypeConverter::makeConstantInterfaceType(SILDeclRef c) {
   case SILDeclRef::Kind::IVarDestroyer:
     return getIVarInitDestroyerInterfaceType(cast<ClassDecl>(vd),
                                              c.isForeign, true);
+  case SILDeclRef::Kind::AsyncEntryPoint:
+    return getAsyncEntryPoint(Context);
   case SILDeclRef::Kind::EntryPoint:
     return getEntryPointInterfaceType(Context);
   }
@@ -2729,6 +2757,7 @@ TypeConverter::getConstantGenericSignature(SILDeclRef c) {
   case SILDeclRef::Kind::StoredPropertyInitializer:
     return vd->getDeclContext()->getGenericSignatureOfContext();
   case SILDeclRef::Kind::EntryPoint:
+  case SILDeclRef::Kind::AsyncEntryPoint:
     llvm_unreachable("Doesn't have generic signature");
   }
 

lib/SILGen/SILGen.cpp

@@ -1039,6 +1039,7 @@ void SILGenModule::emitFunctionDefinition(SILDeclRef constant, SILFunction *f) {
     postEmitFunction(constant, f);
     return;
   }
+  case SILDeclRef::Kind::AsyncEntryPoint:
   case SILDeclRef::Kind::EntryPoint: {
     f->setBare(IsBare);
 

lib/SILGen/SILGenFunction.cpp

@@ -141,6 +141,7 @@ DeclName SILGenModule::getMagicFunctionName(SILDeclRef ref) {
   case SILDeclRef::Kind::EnumElement:
     return getMagicFunctionName(cast<EnumElementDecl>(ref.getDecl())
                                   ->getDeclContext());
+  case SILDeclRef::Kind::AsyncEntryPoint:
   case SILDeclRef::Kind::EntryPoint:
     auto *file = ref.getDecl()->getDeclContext()->getParentSourceFile();
     return getMagicFunctionName(file);

lib/SILGen/SILGenFunction.h

@@ -638,6 +638,9 @@ public:
   /// application based on a main type and optionally a main type.
   void emitArtificialTopLevel(Decl *mainDecl);
 
+  /// Generate code into @main for starting the async main on the main thread.
+  void emitAsyncMainThreadStart(SILDeclRef entryPoint);
+
   /// Generates code for a class deallocating destructor. This
   /// calls the destroying destructor and then deallocates 'self'.
   void emitDeallocatingDestructor(DestructorDecl *dd);