[PrintAsObjC] Add return type related attributes to '_cdecl' global functions

Additionally, Print '(void)' instead '()' for functions that takes no arguments.

Author: rintaro

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -649,10 +649,8 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
       = FD->getForeignErrorConvention();
     assert(!FD->getGenericSignature() &&
            "top-level generic functions not supported here");
-    auto resultTy = getForeignResultType(
-        FD,
-        FD->getInterfaceType()->castTo<FunctionType>(),
-        errorConvention);
+    auto funcTy = FD->getInterfaceType()->castTo<FunctionType>();
+    auto resultTy = getForeignResultType(FD, funcTy, errorConvention);
 
     // The result type may be a partial function type we need to close
     // up later.
@@ -666,18 +664,29 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
 
     assert(FD->getParameterLists().size() == 1 && "not a C-compatible func");
     auto params = FD->getParameterLists().back();
-    interleave(*params,
-               [&](const ParamDecl *param) {
-                 print(param->getInterfaceType(), OTK_None, param->getName(),
-                       IsFunctionParam);
-               },
-               [&]{ os << ", "; });
+    if (params->size()) {
+      interleave(*params,
+                 [&](const ParamDecl *param) {
+                   print(param->getInterfaceType(), OTK_None, param->getName(),
+                         IsFunctionParam);
+                 },
+                 [&]{ os << ", "; });
+    } else {
+      os << "void";
+    }
 
     os << ')';
 
     // Finish the result type.
     multiPart.finish();
 
+    if (funcTy->getResult()->isUninhabited()) {
+      os << " SWIFT_NORETURN";
+    } else if (!funcTy->getResult()->isVoid() &&
+               !FD->getAttrs().hasAttribute<DiscardableResultAttr>()) {
+      os << " SWIFT_WARN_UNUSED_RESULT";
+    }
+
     appendAvailabilityAttribute(FD);
 
     os << ';';

test/PrintAsObjC/cdecl.swift

@@ -9,14 +9,14 @@
 // REQUIRES: objc_interop
 
 // CHECK: /// What a nightmare!
-// CHECK-LABEL: double (^ _Nonnull block_nightmare(SWIFT_NOESCAPE float (^ _Nonnull x)(NSInteger)))(char);
+// CHECK-LABEL: double (^ _Nonnull block_nightmare(SWIFT_NOESCAPE float (^ _Nonnull x)(NSInteger)))(char) SWIFT_WARN_UNUSED_RESULT;
 
 /// What a nightmare!
 @_cdecl("block_nightmare")
 public func block_nightmare(x: @convention(block) (Int) -> Float)
   -> @convention(block) (CChar) -> Double { return { _ in 0 } }
 
-// CHECK-LABEL: double (^ _Nonnull block_recurring_nightmare(float (^ _Nonnull x)(SWIFT_NOESCAPE NSInteger (^ _Nonnull)(double))))(SWIFT_NOESCAPE char (^ _Nonnull)(unsigned char));
+// CHECK-LABEL: double (^ _Nonnull block_recurring_nightmare(float (^ _Nonnull x)(SWIFT_NOESCAPE NSInteger (^ _Nonnull)(double))))(SWIFT_NOESCAPE char (^ _Nonnull)(unsigned char)) SWIFT_WARN_UNUSED_RESULT;
 @_cdecl("block_recurring_nightmare")
 public func block_recurring_nightmare(x: @escaping @convention(block) (@convention(block) (Double) -> Int) -> Float)
   -> @convention(block) (_ asdfasdf: @convention(block) (CUnsignedChar) -> CChar) -> Double {
@@ -27,12 +27,12 @@ public func block_recurring_nightmare(x: @escaping @convention(block) (@conventi
 @_cdecl("foo_bar")
 func foo(x: Int, bar y: Int) {}
 
-// CHECK-LABEL: double (* _Nonnull function_pointer_nightmare(SWIFT_NOESCAPE float (* _Nonnull x)(NSInteger)))(char);
+// CHECK-LABEL: double (* _Nonnull function_pointer_nightmare(SWIFT_NOESCAPE float (* _Nonnull x)(NSInteger)))(char) SWIFT_WARN_UNUSED_RESULT;
 @_cdecl("function_pointer_nightmare")
 func function_pointer_nightmare(x: @convention(c) (Int) -> Float)
   -> @convention(c) (CChar) -> Double { return { _ in 0 } }
 
-// CHECK-LABEL: double (* _Nonnull function_pointer_recurring_nightmare(float (* _Nonnull x)(SWIFT_NOESCAPE NSInteger (* _Nonnull)(double))))(SWIFT_NOESCAPE char (* _Nonnull)(unsigned char));
+// CHECK-LABEL: double (* _Nonnull function_pointer_recurring_nightmare(float (* _Nonnull x)(SWIFT_NOESCAPE NSInteger (* _Nonnull)(double))))(SWIFT_NOESCAPE char (* _Nonnull)(unsigned char)) SWIFT_WARN_UNUSED_RESULT;
 @_cdecl("function_pointer_recurring_nightmare")
 public func function_pointer_recurring_nightmare(x: @escaping @convention(c) (@convention(c) (Double) -> Int) -> Float)
   -> @convention(c) (@convention(c) (CUnsignedChar) -> CChar) -> Double {
@@ -42,3 +42,7 @@ public func function_pointer_recurring_nightmare(x: @escaping @convention(c) (@c
 // CHECK-LABEL: void has_keyword_arg_names(NSInteger auto_, NSInteger union_);
 @_cdecl("has_keyword_arg_names")
 func keywordArgNames(auto: Int, union: Int) {}
+
+// CHECK-LABEL: void return_never(void) SWIFT_NORETURN;
+@_cdecl("return_never")
+func returnNever() -> Never { fatalError() }