[cxx-interop] Enable foreign reference types in C interop

Most of the logic for C++ foreign reference types can be applied to C types as well. Swift had a compiler flag `-Xfrontend -experimental-c-foreign-reference-types` for awhile now which enables foreign reference types without having to enable C++ interop. This change makes it the default behavior.

Since we don't expect anyone to pass `experimental-c-foreign-reference-types` currently, this also removes the frontend flag.

rdar://150308819
(cherry picked from commit 9178af3)

Author: egorzhdan

include/swift/Basic/LangOptions.h

@@ -353,8 +353,6 @@ namespace swift {
       return CXXStdlib == PlatformDefaultCXXStdlib;
     }
 
-    bool CForeignReferenceTypes = false;
-
     /// Imports getters and setters as computed properties.
     bool CxxInteropGettersSettersAsProperties = false;
 

include/swift/Option/Options.td

@@ -871,7 +871,7 @@ def cxx_interoperability_mode :
 def experimental_c_foreign_reference_types :
   Flag<["-"], "experimental-c-foreign-reference-types">,
   Flags<[FrontendOption, HelpHidden, ModuleInterfaceOption]>,
-  HelpText<"Enable experimental C foreign references types (with reference counting).">;
+  HelpText<"Enable experimental C foreign references types. Deprecated, has no effect.">;
 
 def experimental_hermetic_seal_at_link:
   Flag<["-"], "experimental-hermetic-seal-at-link">,

lib/ClangImporter/ImportDecl.cpp

@@ -168,10 +168,6 @@ void ClangImporter::Implementation::makeComputed(AbstractStorageDecl *storage,
 bool importer::recordHasReferenceSemantics(
     const clang::RecordDecl *decl,
     ClangImporter::Implementation *importerImpl) {
-  if (!isa<clang::CXXRecordDecl>(decl) &&
-      !importerImpl->SwiftContext.LangOpts.CForeignReferenceTypes)
-    return false;
-
   // At this point decl might not be fully imported into Swift yet, which
   // means we might not have asked Clang to generate its implicit members, such
   // as copy or move constructors. This would cause CxxRecordSemanticsRequest to

lib/Frontend/CompilerInvocation.cpp

@@ -1582,8 +1582,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       Args.hasFlag(OPT_enable_objc_interop, OPT_disable_objc_interop,
                    Target.isOSDarwin() && !Opts.hasFeature(Feature::Embedded));
 
-  Opts.CForeignReferenceTypes =
-      Args.hasArg(OPT_experimental_c_foreign_reference_types);
+  if (Args.hasArg(OPT_experimental_c_foreign_reference_types))
+    Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
+                   "-experimental-c-foreign-reference-types");
 
   Opts.CxxInteropGettersSettersAsProperties = Args.hasArg(OPT_cxx_interop_getters_setters_as_properties);
   Opts.RequireCxxInteropToImportCxxInteropModule =

test/Interop/C/struct/foreign-reference-flag-deprecated.swift

@@ -0,0 +1,5 @@
+// RUN: %target-build-swift %s -I %S/Inputs/ -Xfrontend -experimental-c-foreign-reference-types 2>&1 | %FileCheck %s
+
+// CHECK: <unknown>:0: warning: flag '-experimental-c-foreign-reference-types' is deprecated
+
+import ForeignReference

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

@@ -1,9 +1,10 @@
-// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -validate-tbd-against-ir=none -Xfrontend -experimental-c-foreign-reference-types -Onone -D NO_OPTIMIZATIONS)
-// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -validate-tbd-against-ir=none -Xfrontend -experimental-c-foreign-reference-types -O)
+// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -validate-tbd-against-ir=none -Onone -D NO_OPTIMIZATIONS)
+// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -validate-tbd-against-ir=none -O)
 //
 // REQUIRES: executable_test
-// TODO: This should work without ObjC interop in the future rdar://97497120
-// REQUIRES: objc_interop
+
+// XFAIL: OS=windows-msvc
+// FIXME: Runtime support for C++ foreign reference types is missing on Windows (https://github.com/swiftlang/swift/issues/82643)
 
 import StdlibUnittest
 import ForeignReference