[PrintAsClang] Fix thunks for throwing Never funcs

Extend the previous commit’s support for functions that return Never to also properly generate code for *throwing* Never functions. This is a little subtle because:

• At the SWIFT_CALL level, throwing Never functions are *not* noreturn
• At the thunk level, throwing Never functions are noreturn *only* if you’re using exceptions; if you’re using swift::Expected, they should throw
• In either case, the compiler cannot statically prove that thunks are noreturn except on the error path, so we need to add an abort() call on the success path

Author: beccadax

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -1410,7 +1410,10 @@ class DeclAndTypePrinter::Implementation
   /// Print C or C++ trailing attributes for a function declaration.
   void printFunctionClangAttributes(FuncDecl *FD, AnyFunctionType *funcTy) {
     if (funcTy->getResult()->isUninhabited()) {
-      os << " SWIFT_NORETURN";
+      if (funcTy->isThrowing())
+        os << " SWIFT_NORETURN_EXCEPT_ERRORS";
+      else
+        os << " SWIFT_NORETURN";
     } else if (!funcTy->getResult()->isVoid() &&
                !FD->getAttrs().hasAttribute<DiscardableResultAttr>()) {
       os << " SWIFT_WARN_UNUSED_RESULT";
@@ -1489,12 +1492,15 @@ class DeclAndTypePrinter::Implementation
     // Swift functions can't throw exceptions, we can only
     // throw them from C++ when emitting C++ inline thunks for the Swift
     // functions.
-    if (!funcTy->isThrowing())
+    if (!funcTy->isThrowing()) {
       os << " SWIFT_NOEXCEPT";
+      // Lowered Never-returning functions *are* considered to return when they
+      // throw, so only use SWIFT_NORETURN on non-throwing functions.
+      if (funcTy->getResult()->isUninhabited())
+        os << " SWIFT_NORETURN";
+    }
     if (!funcABI.useCCallingConvention())
       os << " SWIFT_CALL";
-    if (funcTy->getResult()->isUninhabited())
-      os << " SWIFT_NORETURN";
     printAvailability(FD);
     os << ';';
     if (funcABI.useMangledSymbolName()) {

lib/PrintAsClang/PrintClangFunction.cpp

@@ -1444,6 +1444,8 @@ void DeclAndTypeClangFunctionPrinter::printCxxThunkBody(
     if (resultTy->isVoid()) {
       os << "    return swift::Expected<void>(swift::Error(opaqueError));\n";
       os << "#endif\n";
+      if (FD->getInterfaceType()->castTo<FunctionType>()->getResult()->isUninhabited())
+        os << "  abort();\n";
     } else {
       auto directResultType = signature.getDirectResultType();
       printDirectReturnOrParamCType(

lib/PrintAsClang/_SwiftStdlibCxxOverlay.h

@@ -413,12 +413,14 @@ template<class T>
 using ThrowingResult = T;
 
 #define SWIFT_RETURN_THUNK(T, v) v
+#define SWIFT_NORETURN_EXCEPT_ERRORS SWIFT_NORETURN
 
 #else
 
 template <class T> using ThrowingResult = swift::Expected<T>;
 
 #define SWIFT_RETURN_THUNK(T, v) swift::Expected<T>(v)
+#define SWIFT_NORETURN_EXCEPT_ERRORS
 
 #endif
 

test/Interop/SwiftToCxx/functions/swift-expected-execution.cpp

@@ -89,6 +89,16 @@ int main() {
     valueError.getMessage();
   }
 
+  auto expectedResult2 = Functions::throwFunctionWithNeverReturn();
+  if (!expectedResult2.has_value()) {
+    auto error = expectedResult2.error();
+    auto optionalError = error.as<Functions::NaiveErrors>();
+    assert(optionalError.isSome());
+    auto valueError = optionalError.get();
+    assert(valueError == Functions::NaiveErrors::returnError);
+    valueError.getMessage();
+  }
+
   // Test get T's Value (const)
   const auto valueExp = testIntValue;
   if (valueExp.value() == 42)
@@ -116,6 +126,8 @@ int main() {
 // CHECK-NEXT: returnError
 // CHECK-NEXT: passThrowFunctionWithPossibleReturn
 // CHECK-NEXT: returnError
+// CHECK-NEXT: passThrowFunctionWithNeverReturn
+// CHECK-NEXT: returnError
 // CHECK-NEXT: Test get T's Value (const)
 // CHECK-NEXT: Test get T's Value
 // CHECK-NEXT: testIntValue has a value

test/Interop/SwiftToCxx/functions/swift-functions-errors-execution.cpp

@@ -46,6 +46,11 @@ int main() {
   } catch (swift::Error& e) {
      printf("Exception\n");
   }
+  try {
+    Functions::throwFunctionWithNeverReturn();
+  } catch (swift::Error& e) {
+     printf("Exception\n");
+  }
   try {
     Functions::testDestroyedError();
   } catch(const swift::Error &e) { }
@@ -58,4 +63,6 @@ int main() {
 // CHECK-NEXT: throwError
 // CHECK-NEXT: passThrowFunctionWithReturn
 // CHECK-NEXT: Exception
+// CHECK-NEXT: passThrowFunctionWithNeverReturn
+// CHECK-NEXT: Exception
 // CHECK-NEXT: Test destroyed

test/Interop/SwiftToCxx/functions/swift-functions-errors.swift

@@ -14,6 +14,7 @@
 // CHECK: SWIFT_EXTERN void $s9Functions18emptyThrowFunctionyyKF(SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // emptyThrowFunction()
 // CHECK: SWIFT_EXTERN void $s9Functions18testDestroyedErroryyKF(SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // testDestroyedError()
 // CHECK: SWIFT_EXTERN void $s9Functions13throwFunctionyyKF(SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // throwFunction()
+// CHECK: SWIFT_EXTERN void $s9Functions28throwFunctionWithNeverReturns0E0OyKF(SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // throwFunctionWithNeverReturn()
 // CHECK: SWIFT_EXTERN ptrdiff_t $s9Functions31throwFunctionWithPossibleReturnyS2iKF(ptrdiff_t a, SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // throwFunctionWithPossibleReturn(_:)
 // CHECK: SWIFT_EXTERN ptrdiff_t $s9Functions23throwFunctionWithReturnSiyKF(SWIFT_CONTEXT void * _Nonnull _ctx, SWIFT_ERROR_RESULT void * _Nullable * _Nullable _error) SWIFT_CALL; // throwFunctionWithReturn()
 
@@ -89,6 +90,25 @@ public func throwFunction() throws {
 // CHECK: #endif
 // CHECK: }
 
+@_expose(Cxx)
+public func throwFunctionWithNeverReturn() throws -> Never {
+    print("passThrowFunctionWithNeverReturn")
+    throw NaiveErrors.returnError
+}
+
+// CHECK: SWIFT_INLINE_THUNK swift::ThrowingResult<void> throwFunctionWithNeverReturn() SWIFT_SYMBOL("s:9Functions28throwFunctionWithNeverReturns0E0OyKF") SWIFT_NORETURN_EXCEPT_ERRORS {
+// CHECK-NEXT: void* opaqueError = nullptr;
+// CHECK-NEXT: void* _ctx = nullptr;
+// CHECK-NEXT: _impl::$s9Functions28throwFunctionWithNeverReturns0E0OyKF(_ctx, &opaqueError);
+// CHECK-NEXT: if (opaqueError != nullptr)
+// CHECK-NEXT: #ifdef __cpp_exceptions
+// CHECK-NEXT: throw (swift::Error(opaqueError));
+// CHECK-NEXT: #else
+// CHECK-NEXT: return swift::Expected<void>(swift::Error(opaqueError));
+// CHECK-NEXT: #endif
+// CHECK-NEXT: abort();
+// CHECK-NEXT: }
+
 @_expose(Cxx)
 public func throwFunctionWithPossibleReturn(_ a: Int) throws -> Int {
     print("passThrowFunctionWithPossibleReturn")

test/Interop/SwiftToCxx/functions/swift-functions.swift

@@ -12,7 +12,7 @@
 // CHECK: SWIFT_EXTERN int $s9Functions016passTwoIntReturnD01x1ys5Int32VAF_AFtF(int x, int y) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnInt(x:y:)
 // CHECK: SWIFT_EXTERN int $s9Functions016passTwoIntReturnD10NoArgLabelys5Int32VAD_ADtF(int, int) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnIntNoArgLabel(_:_:)
 // CHECK: SWIFT_EXTERN int $s9Functions016passTwoIntReturnD19NoArgLabelParamNameys5Int32VAD_ADtF(int x2, int y2) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnIntNoArgLabelParamName(_:_:)
-// CHECK: SWIFT_EXTERN void $s9Functions19passVoidReturnNevers0E0OyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_NORETURN; // passVoidReturnNever()
+// CHECK: SWIFT_EXTERN void $s9Functions19passVoidReturnNevers0E0OyF(void) SWIFT_NOEXCEPT SWIFT_NORETURN SWIFT_CALL; // passVoidReturnNever()
 // CHECK: SWIFT_EXTERN void $s9Functions014passVoidReturnC0yyF(void) SWIFT_NOEXCEPT SWIFT_CALL; // passVoidReturnVoid()
 
 // CHECK: }