Define __SWIFT_ATTR_SUPPORTS_MACROS when the MacrosOnImports features is available

Author: DougGregor

lib/ClangImporter/ClangImporter.cpp

@@ -622,6 +622,11 @@ void importer::getNormalInvocationArguments(
     if (clangSupportsPragmaAttributeWithSwiftAttr())
       invocationArgStrs.push_back("-D__SWIFT_ATTR_SUPPORTS_SENDABLE_DECLS=1");
 
+    // Indicate that the compiler will respect macros applied to imported
+    // declarations via '__attribute__((swift_attr("@...")))'.
+    if (LangOpts.hasFeature(Feature::MacrosOnImports))
+      invocationArgStrs.push_back("-D__SWIFT_ATTR_SUPPORTS_MACROS=1");
+
     if (triple.isXROS()) {
       // FIXME: This is a gnarly hack until some macros get adjusted in the SDK.
       invocationArgStrs.insert(invocationArgStrs.end(), {

test/Inputs/clang-importer-sdk/usr/include/completion_handler_globals.h

@@ -1,11 +1,16 @@
-void async_divide(double x, double y, void (* _Nonnull completionHandler)(double x))
-  __attribute__((swift_attr("@macro_library.AddAsync")));
+#if __SWIFT_ATTR_SUPPORTS_MACROS
+#define ADD_ASYNC __attribute__((swift_attr("@macro_library.AddAsync")))
+#else
+#define ADD_ASYNC
+#endif
+
+void async_divide(double x, double y, void (* _Nonnull completionHandler)(double x)) ADD_ASYNC;
 
 typedef struct SlowComputer {
 } SlowComputer;
 
 void computer_divide(const SlowComputer *computer, double x, double y, void (* _Nonnull completionHandler)(double x))
-  __attribute__((swift_attr("@macro_library.AddAsync")))
+  ADD_ASYNC
   __attribute__((swift_name("SlowComputer.divide(self:_:_:completionHandler:)")));
 
 
@@ -14,7 +19,7 @@ void computer_divide(const SlowComputer *computer, double x, double y, void (* _
 
 @interface Computer: NSObject
 -(void)multiply:(double)x by:(double)y afterDone:(void (^ _Nonnull)(double x))afterDone
-  __attribute__((swift_attr("@macro_library.AddAsync")))
+  ADD_ASYNC
   __attribute__((swift_async(none)));
 @end
 #endif