[cxx-interop] Restrict the uses of anonymous types

Make sure they are excluded from the reflection metadata (although in
the future we want to make sure indirect fields are included). Make sure
the users cannot refer to the anonymous field, only its members.

Author: Gabor Horvath

lib/ClangImporter/ImportDecl.cpp

@@ -4546,6 +4546,10 @@ namespace {
         // FIXME: Temporarily unreachable because of check above.
         markAsVariant(result, *correctSwiftName);
 
+      if (decl->isAnonymousStructOrUnion())
+        Impl.markUnavailable(
+            result, "refer to the members of the anonymous type instead");
+
       return result;
     }
 

lib/IRGen/StructLayout.cpp

@@ -589,10 +589,17 @@ unsigned irgen::getNumFields(const NominalTypeDecl *target) {
 }
 
 bool irgen::isExportableField(Field field) {
-  if (field.getKind() == Field::Kind::Var &&
-      field.getVarDecl()->getClangDecl() &&
-      field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
-    return false;
+  if (field.getKind() == Field::Kind::Var) {
+    if (field.getVarDecl()->getClangDecl() &&
+        field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
+      return false;
+    // We should not be able to refer to anonymous types.
+    if (const auto *vd = dyn_cast_or_null<clang::ValueDecl>(
+            field.getVarDecl()->getClangDecl()))
+      if (const auto *rd = vd->getType()->getAsRecordDecl())
+        if (rd->isAnonymousStructOrUnion())
+          return false;
+  }
   // All other fields are exportable
   return true;
 }

test/Interop/Cxx/class/Inputs/simple-structs.h

@@ -17,6 +17,15 @@ struct HasPublicFieldsOnly {
   HasPublicFieldsOnly(int i1, int i2) : publ1(i1), publ2(i2) {}
 };
 
+struct HasAnonymousType {
+  HasAnonymousType(int a, int b, int c) : a(a), b(b), c(c) {}
+
+  struct {
+    int a, b;
+  };
+  int c;
+};
+
 struct HasPrivatePublicProtectedFields {
 private:
   int priv1;

test/Interop/Cxx/class/access-anonymous-field.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -swift-version 6 -cxx-interoperability-mode=upcoming-swift
+
+// CHECK: Foobar
+
+import SimpleStructs
+
+let s = HasAnonymousType(1, 2, 3)
+let _ = s.__Anonymous_field0 // expected-error {{'__Anonymous_field0' is unavailable: refer to the members of the anonymous type instead}}
+// Referring to the members of the anonymous type directly.
+let _ = s.a
+let _ = s.b
+let _ = s.c

test/Interop/Cxx/class/print-simple-structs.swift

@@ -24,6 +24,11 @@ func printCxxStructNested() {
     print(s)
 }
 
+func printCxxStructWithAnonType() {
+    let s = HasAnonymousType(1, 2, 3)
+    print(s)
+}
+
 printCxxStructPrivateFields() 
 // CHECK: HasPrivateFieldsOnly()
 
@@ -35,3 +40,6 @@ printCxxStructPrivatePublicProtectedFields()
 
 printCxxStructNested()
 // CHECK: Outer(publStruct: {{.*}}.HasPrivatePublicProtectedFields(publ1: 8, publ2: 12))
+
+printCxxStructWithAnonType()
+// CHECK: HasAnonymousType(c: 3)