[cxx-interop] Prevent Swift from importing fields marked with [[no_unique_address]]

Author: susmonteiro

include/swift/AST/DiagnosticsClangImporter.def

@@ -373,5 +373,8 @@ NOTE(ptr_to_nonescapable,none,
      "pointer to non-escapable type %0 cannot be imported",
      (const clang::Type*))
 
+WARNING(unsupported_attribute, none,
+        "Swift doesn't support fields marked with %0", (StringRef))
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/ClangImporter/ImportDecl.cpp

@@ -4383,6 +4383,16 @@ namespace {
       std::optional<ImportedName> correctSwiftName;
       ImportedName importedName;
 
+      if (decl->hasAttr<clang::NoUniqueAddressAttr>()) {
+        // FIXME: Swift doesn't support fields marked with [[no_unique_address]] rdar://148437848
+
+        Impl.addImportDiagnostic(
+            decl,
+            Diagnostic(diag::unsupported_attribute, "[[no_unique_address]]"),
+            decl->getLocation());
+        return nullptr;
+      }
+
       std::tie(importedName, correctSwiftName) = importFullName(decl);
       if (!importedName) {
         return nullptr;

test/Interop/Cxx/stdlib/print-std-string.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -o %t/strings -Xfrontend -enable-experimental-cxx-interop
+// RUN: %target-codesign %t/strings
+// RUN: %target-run %t/strings
+
+// REQUIRES: executable_test
+
+import CxxStdlib
+
+public func test() {
+    let cxxString: std.string = "Hello"
+    print(cxxString)
+    let swiftString = String(cxxString)
+    print(swiftString)
+}
+