[c-interop] Add test for SWIFT_RETURNS_RETAINED annotations in C interop mode

rdar://144976203

Author: j-hui

test/Interop/C/struct/Inputs/module.modulemap

@@ -18,3 +18,7 @@ module StructAsOptionSet {
 module NoncopyableStructs {
   header "noncopyable-struct.h"
 }
+
+module ReturnForeignReference {
+  header "return-foreign-reference.h"
+}

test/Interop/C/struct/Inputs/return-foreign-reference.h

@@ -0,0 +1,33 @@
+#ifndef __RETURN_FOREIGN_REFERENCE_H
+#define __RETURN_FOREIGN_REFERENCE_H
+
+struct
+__attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:FRTRetain")))
+__attribute__((swift_attr("release:FRTRelease")))
+FRT {
+  int value;
+};
+
+struct Value { int value; };
+
+void FRTRetain(struct FRT *x);
+void FRTRelease(struct FRT *x);
+
+struct FRT *getFRTNoAnnotations(void); // expected-note {{annotate}}
+struct FRT *getFRTRetained() __attribute__((swift_attr("returns_retained")));
+struct FRT *getFRTUnretained() __attribute__((swift_attr("returns_unretained")));
+
+struct FRT *getFRTUnretained() __attribute__((swift_attr("returns_unretained")));
+
+struct FRT *getFRTConflictingAnnotations()
+    __attribute__((swift_attr("returns_retained")))
+    __attribute__((swift_attr("returns_unretained")));
+// expected-error@-3 {{cannot be annotated with both}}
+
+struct Value *getValueRetained() __attribute__((swift_attr("returns_retained")));
+// expected-warning@-1 {{should not be annotated}}
+struct Value *getValueUnretained() __attribute__((swift_attr("returns_unretained")));
+// expected-warning@-1 {{should not be annotated}}
+
+#endif /* __RETURN_FOREIGN_REFERENCE_H */

test/Interop/C/struct/return-foreign-reference-typechecker.swift

@@ -0,0 +1,17 @@
+// RUN: %target-typecheck-verify-swift -cxx-interoperability-mode=off \
+// RUN:   -disable-availability-checking \
+// RUN:   -I %S%{fs-sep}Inputs \
+// RUN:   -verify-additional-file %S%{fs-sep}Inputs%{fs-sep}return-foreign-reference.h
+
+import ReturnForeignReference
+
+let _: FRT = getFRTNoAnnotations() // expected-warning {{cannot infer ownership}}
+let _: FRT = getFRTRetained()
+let _: FRT = getFRTUnretained()
+
+
+// These calls to incorrectly annotated functions are necessary to force
+// diagnostics to appear in the header file.
+let _: FRT = getFRTConflictingAnnotations()
+let _ = getValueRetained()
+let _ = getValueUnretained()