Add -enable-cxx-interop flag and support for extern "C" {}

- Updated some swift headers to build with c++ and c.
- Added -enable-cxx-interop flag.
- Added support for 'extern "C"'.

Author: pschuh

include/swift/Basic/LangOptions.h

@@ -139,6 +139,11 @@ namespace swift {
     /// configuration options.
     bool EnableObjCInterop = true;
 
+    /// Enable C++ interop code generation and build configuration
+    /// options.
+    /// FIXME: Disabled by default until this is fully baked.
+    bool EnableCXXInterop = false;
+
     /// On Darwin platforms, use the pre-stable ABI's mark bit for Swift
     /// classes instead of the stable ABI's bit. This is needed when
     /// targeting OSes prior to macOS 10.14.4 and iOS 12.2, where

include/swift/Option/Options.td

@@ -841,6 +841,11 @@ def enable_private_imports : Flag<["-"], "enable-private-imports">,
   Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,
   HelpText<"Allows this module's internal and private API to be accessed">;
 
+def enable_cxx_interop :
+  Flag<["-"], "enable-cxx-interop">,
+  HelpText<"Enable C++ interop code generation and config directives">,
+  Flags<[FrontendOption]>;
+
 def sanitize_EQ : CommaJoined<["-"], "sanitize=">,
   Flags<[FrontendOption, NoInteractiveOption]>, MetaVarName<"<check>">,
   HelpText<"Turn on runtime checks for erroneous behavior.">;

lib/ClangImporter/ClangImporter.cpp

@@ -518,7 +518,12 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
       });
 
   } else {
-    invocationArgStrs.insert(invocationArgStrs.end(), {"-x", "c", "-std=gnu11"});
+    bool EnableCXXInterop = LangOpts.EnableCXXInterop;
+    invocationArgStrs.insert(invocationArgStrs.end(), {
+        "-x",
+        EnableCXXInterop ? "c++" : "c",
+        EnableCXXInterop ? "-std=c++17" : "-std=gnu11"
+    });
   }
 
   // Set C language options.

lib/ClangImporter/SwiftLookupTable.cpp

@@ -81,6 +81,10 @@ class SwiftLookupTableWriter : public clang::ModuleFileExtensionWriter {
                               llvm::BitstreamWriter &stream) override;
 
   void populateTable(SwiftLookupTable &table, NameImporter &);
+
+  void populateTableWithDecl(SwiftLookupTable &table,
+                             NameImporter &nameImporter,
+                             clang::Decl* decl);
 };
 
 /// Module file extension reader for the Swift lookup tables.
@@ -1793,21 +1797,35 @@ void importer::finalizeLookupTable(SwiftLookupTable &table,
   }
 }
 
+void SwiftLookupTableWriter::populateTableWithDecl(SwiftLookupTable &table,
+                                                   NameImporter &nameImporter,
+                                                   clang::Decl* decl) {
+  // Skip anything from an AST file.
+  if (decl->isFromASTFile())
+    return;
+
+  // Iterate into extern "C" {} type declarations.
+  if (auto linkageDecl = dyn_cast<clang::LinkageSpecDecl>(decl)) {
+    for (auto* decl : linkageDecl->noload_decls()) {
+      populateTableWithDecl(table, nameImporter, decl);
+    }
+    return;
+  }
+
+  // Skip non-named declarations.
+  auto named = dyn_cast<clang::NamedDecl>(decl);
+  if (!named)
+    return;
+
+  // Add this entry to the lookup table.
+  addEntryToLookupTable(table, named, nameImporter);
+}
+
 void SwiftLookupTableWriter::populateTable(SwiftLookupTable &table,
                                            NameImporter &nameImporter) {
   auto &sema = nameImporter.getClangSema();
   for (auto decl : sema.Context.getTranslationUnitDecl()->noload_decls()) {
-    // Skip anything from an AST file.
-    if (decl->isFromASTFile())
-      continue;
-
-    // Skip non-named declarations.
-    auto named = dyn_cast<clang::NamedDecl>(decl);
-    if (!named)
-      continue;
-
-    // Add this entry to the lookup table.
-    addEntryToLookupTable(table, named, nameImporter);
+    populateTableWithDecl(table, nameImporter, decl);
   }
 
   // Add macros to the lookup table.

lib/Driver/ToolChains.cpp

@@ -188,6 +188,7 @@ static void addCommonFrontendArgs(const ToolChain &TC, const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_enable_library_evolution);
   inputArgs.AddLastArg(arguments, options::OPT_enable_testing);
   inputArgs.AddLastArg(arguments, options::OPT_enable_private_imports);
+  inputArgs.AddLastArg(arguments, options::OPT_enable_cxx_interop);
   inputArgs.AddLastArg(arguments, options::OPT_g_Group);
   inputArgs.AddLastArg(arguments, options::OPT_debug_info_format);
   inputArgs.AddLastArg(arguments, options::OPT_import_underlying_module);

lib/Frontend/CompilerInvocation.cpp

@@ -470,6 +470,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     TargetArg = A->getValue();
   }
 
+  Opts.EnableCXXInterop |= Args.hasArg(OPT_enable_cxx_interop);
   Opts.EnableObjCInterop =
       Args.hasFlag(OPT_enable_objc_interop, OPT_disable_objc_interop,
                    Target.isOSDarwin());

stdlib/public/SwiftShims/GlobalObjects.h

@@ -24,7 +24,10 @@
 #include "Visibility.h"
 
 #ifdef __cplusplus
-namespace swift { extern "C" {
+#ifndef __swift__
+namespace swift {
+#endif
+extern "C" {
 #endif
 
 struct _SwiftArrayBodyStorage {
@@ -102,7 +105,10 @@ static_assert(
     4 * sizeof(__swift_intptr_t) + sizeof(__swift_int64_t),
   "_SwiftSetBodyStorage has unexpected size");
 
-}} // extern "C", namespace swift
+} // extern "C"
+#ifndef __swift__
+} // namespace swift
+#endif
 #endif
 
 #endif

stdlib/public/SwiftShims/HeapObject.h

@@ -20,7 +20,7 @@
 #define SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_64 16
 #define SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_32 8
 
-#ifdef __cplusplus
+#ifndef __swift__
 #include <type_traits>
 #include "swift/Basic/type_traits.h"
 
@@ -48,7 +48,7 @@ struct HeapObject {
 
   SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS;
 
-#ifdef __cplusplus
+#ifndef __swift__
   HeapObject() = default;
 
   // Initialize a HeapObject header as appropriate for a newly-allocated object.
@@ -68,7 +68,7 @@ struct HeapObject {
   void dump() const LLVM_ATTRIBUTE_USED;
 #endif
 
-#endif // __cplusplus
+#endif // __swift__
 };
 
 #ifdef __cplusplus
@@ -92,7 +92,7 @@ __swift_size_t swift_weakRetainCount(HeapObject *obj);
 } // extern "C"
 #endif
 
-#ifdef __cplusplus
+#ifndef __swift__
 static_assert(std::is_trivially_destructible<HeapObject>::value,
               "HeapObject must be trivially destructible");
 

stdlib/public/SwiftShims/RefCount.h

@@ -21,7 +21,7 @@ typedef struct {
   __swift_uintptr_t refCounts SWIFT_ATTRIBUTE_UNAVAILABLE;
 } InlineRefCountsPlaceholder;
 
-#if !defined(__cplusplus)
+#if defined(__swift__)
 
 typedef InlineRefCountsPlaceholder InlineRefCounts;