[cxx-interop] Add attribute to hide Swift declarations from interop

This can help work around problems when the names of a C++ declaration
and a Swift declaration would collide, or to temporarily work around
compiler bugs.

rdar://152838988&140802127

Author: Gabor Horvath

include/swift/AST/AttrKind.h

@@ -97,6 +97,7 @@ enum class ENUM_EXTENSIBILITY_ATTR(closed) EffectsKind : uint8_t {
 /// This enum represents the possible values of the @_expose attribute.
 enum class ENUM_EXTENSIBILITY_ATTR(closed) ExposureKind : uint8_t {
   Cxx SWIFT_NAME("cxx"),
+  NotCxx SWIFT_NAME("notcxx"),
   Wasm SWIFT_NAME("wasm"),
   Last_ExposureKind = Wasm
 };

lib/AST/Attr.cpp

@@ -1253,6 +1253,9 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     case ExposureKind::Cxx:
       Printer << "(Cxx";
       break;
+    case ExposureKind::NotCxx:
+      Printer << "(!Cxx";
+      break;
     }
     if (!cast<ExposeAttr>(this)->Name.empty())
       Printer << ", \"" << cast<ExposeAttr>(this)->Name << "\"";

lib/Parse/ParseDecl.cpp

@@ -3064,13 +3064,22 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
                  "Cxx");
         ParseSymbolName = false;
       };
+      bool isNegated = false;
+      if (Tok.is(tok::oper_prefix) && Tok.getText() == "!") {
+        isNegated = true;
+        consumeToken(tok::oper_prefix);
+      }
       if (Tok.isNot(tok::identifier)) {
         diagnoseExpectOption();
         return makeParserSuccess();
       }
       if (Tok.getText() == "Cxx") {
-        ExpKind = ExposureKind::Cxx;
+        ExpKind = isNegated ? ExposureKind::NotCxx : ExposureKind::Cxx;
       } else if (Tok.getText() == "wasm") {
+        if (isNegated) {
+          diagnoseExpectOption();
+          return makeParserSuccess();
+        }
         ExpKind = ExposureKind::Wasm;
       } else {
         diagnoseExpectOption();

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -22,6 +22,7 @@
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTMangler.h"
 #include "swift/AST/ASTVisitor.h"
+#include "swift/AST/Attr.h"
 #include "swift/AST/ClangSwiftTypeCorrespondence.h"
 #include "swift/AST/Comment.h"
 #include "swift/AST/ConformanceLookup.h"
@@ -2950,6 +2951,17 @@ static bool excludeForObjCImplementation(const ValueDecl *VD) {
   return false;
 }
 
+static bool hasExposeNotCxxAttr(const ValueDecl *VD) {
+  for (const auto *attr : VD->getAttrs().getAttributes<ExposeAttr>())
+    if (attr->getExposureKind() == ExposureKind::NotCxx)
+      return true;
+  if (const auto *NMT = dyn_cast<NominalTypeDecl>(VD->getDeclContext()))
+    return hasExposeNotCxxAttr(NMT);
+  if (const auto *ED = dyn_cast<ExtensionDecl>(VD->getDeclContext()))
+    return hasExposeNotCxxAttr(ED->getExtendedNominal());
+  return false;
+}
+
 static bool isExposedToThisModule(const ModuleDecl &M, const ValueDecl *VD,
                                   const llvm::StringSet<> &exposedModules) {
   if (VD->hasClangNode())
@@ -3001,6 +3013,9 @@ bool DeclAndTypePrinter::shouldInclude(const ValueDecl *VD) {
   if (requiresExposedAttribute && !hasExposeAttr(VD))
     return false;
 
+  if (hasExposeNotCxxAttr(VD))
+    return false;
+
   if (!isVisible(VD))
     return false;
 

lib/Sema/TypeCheckAttr.cpp

@@ -23,6 +23,8 @@
 #include "TypeCheckType.h"
 #include "TypeChecker.h"
 #include "swift/AST/ASTVisitor.h"
+#include "swift/AST/Attr.h"
+#include "swift/AST/AttrKind.h"
 #include "swift/AST/AvailabilityInference.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/ConformanceLookup.h"
@@ -2415,6 +2417,12 @@ void AttributeChecker::visitExposeAttr(ExposeAttr *attr) {
       diagnose(attr->getLocation(), diag::expose_wasm_not_at_top_level_func);
     break;
   }
+  case ExposureKind::NotCxx:
+    for (const auto *attr : D->getAttrs().getAttributes<ExposeAttr>())
+      if (attr->getExposureKind() == ExposureKind::Cxx)
+        diagnose(attr->getLocation(), diag::expose_only_non_other_attr,
+                 "@_expose(Cxx)");
+    break;
   case ExposureKind::Cxx: {
     auto *VD = cast<ValueDecl>(D);
     // Expose cannot be mixed with '@_cdecl' declarations.

test/Interop/SwiftToCxx/expose-attr/swift-private-decls.swift

@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -module-name SwiftPrivate -clang-header-expose-decls=all-public -typecheck -verify -emit-clang-header-path %t/swiftprivate.h
+// RUN: %FileCheck %s < %t/swiftprivate.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/swiftprivate.h  -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY)
+
+public struct Exposed {
+    public var x: Int
+    @_expose(!Cxx)
+    public var notExposedField: Int
+}
+
+@_expose(!Cxx)
+public struct NotExposed {
+    
+}
+
+extension NotExposed {
+    func notExposed() {}
+}
+
+@_expose(!Cxx)
+public func NotExposedfunction() {}
+
+// CHECK-NOT: NotExposed
+// CHECK-NOT: notExposed
+// CHECK: Exposed

test/attr/attr_expose.swift

@@ -15,6 +15,12 @@ func incorrectLangSpecifier() {}
 @_expose(Cxx) @_cdecl("test") // expected-error {{'@_expose' cannot be applied to an '@_cdecl' declaration}}
 func cdeclAndExpose() {}
 
+@_expose(Cxx) @_expose(!Cxx)  // expected-error {{'@_expose' cannot be applied to an '@_expose(Cxx)' declaration}}
+func contradictingExpose() {}
+
+@_expose(!Cxx) @_expose(Cxx)  // expected-error {{'@_expose' cannot be applied to an '@_expose(Cxx)' declaration}}
+func contradictingExpose2() {}
+
 func hasNested() {
   @_expose(Cxx) // expected-error{{can only be used in a non-local scope}}
   func nested() { }