[cxx-interop] use SWIFT_EXTERN for Swift function prototypes

This will allow these functions to be used from C.

Author: hyp

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -843,7 +843,7 @@ class DeclAndTypePrinter::Implementation
     Mangle::ASTMangler mangler;
     FuncionSwiftABIInformation funcABI(FD, mangler);
 
-    os << "extern \"C\" ";
+    os << "SWIFT_EXTERN ";
     printFunctionDeclAsCFunctionDecl(FD, funcABI.getSymbolName(), resultTy);
     // Swift functions can't throw exceptions, we can only
     // throw them from C++ when emitting C++ inline thunks for the Swift

test/Interop/SwiftToCxx/functions/cdecl.swift

@@ -7,7 +7,7 @@
 // CHECK-LABEL: namespace CdeclFunctions {
 
 // CHECK: namespace _impl {
-// CHECK: extern "C" int cfuncPassTwo(int x, int y) SWIFT_NOEXCEPT;
+// CHECK: SWIFT_EXTERN int cfuncPassTwo(int x, int y) SWIFT_NOEXCEPT;
 // CHECK: }
 
 @_cdecl("cfuncPassTwo")

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

@@ -8,11 +8,11 @@
 
 // CHECK-LABEL: namespace _impl {
 
-// CHECK: extern "C" void $s9Functions16alwaysDeprecatedyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_DEPRECATED; // alwaysDeprecated()
-// CHECK: extern "C" void $s9Functions19alwaysDeprecatedTwoyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_DEPRECATED_MSG("it should not be used"); // alwaysDeprecatedTwo()
-// CHECK: extern "C" void $s9Functions17alwaysUnavailableyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_UNAVAILABLE; // alwaysUnavailable()
-// CHECK: extern "C" void $s9Functions24alwaysUnavailableMessageyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_UNAVAILABLE_MSG("stuff happened"); // alwaysUnavailableMessage()
-// CHECK: extern "C" void $s9Functions22singlePlatAvailabilityyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_AVAILABILITY(macos,introduced=11); // singlePlatAvailability()
+// CHECK: SWIFT_EXTERN void $s9Functions16alwaysDeprecatedyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_DEPRECATED; // alwaysDeprecated()
+// CHECK: SWIFT_EXTERN void $s9Functions19alwaysDeprecatedTwoyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_DEPRECATED_MSG("it should not be used"); // alwaysDeprecatedTwo()
+// CHECK: SWIFT_EXTERN void $s9Functions17alwaysUnavailableyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_UNAVAILABLE; // alwaysUnavailable()
+// CHECK: SWIFT_EXTERN void $s9Functions24alwaysUnavailableMessageyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_UNAVAILABLE_MSG("stuff happened"); // alwaysUnavailableMessage()
+// CHECK: SWIFT_EXTERN void $s9Functions22singlePlatAvailabilityyyF(void) SWIFT_NOEXCEPT SWIFT_CALL SWIFT_AVAILABILITY(macos,introduced=11); // singlePlatAvailability()
 
 // CHECK: }
 

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

@@ -8,11 +8,11 @@
 
 // CHECK-LABEL: namespace _impl {
 
-// CHECK: extern "C" void $s9Functions17passIntReturnVoid1xys5Int32V_tF(int x) SWIFT_NOEXCEPT SWIFT_CALL; // passIntReturnVoid(x:)
-// CHECK: extern "C" int $s9Functions016passTwoIntReturnD01x1ys5Int32VAF_AFtF(int x, int y) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnInt(x:y:)
-// CHECK: extern "C" int $s9Functions016passTwoIntReturnD10NoArgLabelys5Int32VAD_ADtF(int, int) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnIntNoArgLabel(_:_:)
-// CHECK: extern "C" int $s9Functions016passTwoIntReturnD19NoArgLabelParamNameys5Int32VAD_ADtF(int x2, int y2) SWIFT_NOEXCEPT SWIFT_CALL; // passTwoIntReturnIntNoArgLabelParamName(_:_:)
-// CHECK: extern "C" void $s9Functions014passVoidReturnC0yyF(void) SWIFT_NOEXCEPT SWIFT_CALL; // passVoidReturnVoid()
+// CHECK: SWIFT_EXTERN void $s9Functions17passIntReturnVoid1xys5Int32V_tF(int x) SWIFT_NOEXCEPT SWIFT_CALL; // passIntReturnVoid(x:)
+// 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 $s9Functions014passVoidReturnC0yyF(void) SWIFT_NOEXCEPT SWIFT_CALL; // passVoidReturnVoid()
 
 // CHECK: }