[cxx-interop] Fix a crash with [[no_unique_address]]

Swift does not support storing fields in the padding of the previous
fields just yet, so let's not import fields like that from C++.
Represent them as opaque blobs instead.

Fixes #80764

Author: Gabor Horvath

Diff for: lib/ClangImporter/ImportDecl.cpp

@@ -61,6 +61,7 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjCCommon.h"
 #include "clang/AST/PrettyPrinter.h"
+#include "clang/AST/RecordLayout.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Basic/TargetInfo.h"
@@ -4341,6 +4342,18 @@ namespace {
     }
 
     Decl *VisitFieldDecl(const clang::FieldDecl *decl) {
+      if (decl->hasAttr<clang::NoUniqueAddressAttr>()) {
+        if (const auto *rd = decl->getType()->getAsRecordDecl()) {
+          // Clang can store the next field in the padding of this one. Swift
+          // does not support this yet so let's not import the field and
+          // represent it with an opaque blob in codegen.
+          const auto &fieldLayout =
+              decl->getASTContext().getASTRecordLayout(rd);
+          if (!decl->isZeroSize(decl->getASTContext()) &&
+              fieldLayout.getDataSize() != fieldLayout.getSize())
+            return nullptr;
+        }
+      }
       // Fields are imported as variables.
       std::optional<ImportedName> correctSwiftName;
       ImportedName importedName;

Diff for: lib/IRGen/GenStruct.cpp

@@ -16,6 +16,7 @@
 
 #include "GenStruct.h"
 
+#include "IRGen.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/ConformanceLookup.h"
 #include "swift/AST/Decl.h"
@@ -42,6 +43,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
+#include "llvm/Support/Error.h"
 #include <iterator>
 
 #include "GenDecl.h"
@@ -1459,6 +1461,14 @@ class ClangRecordLowering {
     unsigned fieldOffset = layout.getFieldOffset(clangField->getFieldIndex());
     assert(!clangField->isBitField());
     Size offset( SubobjectAdjustment.getValue() + fieldOffset / 8);
+    std::optional<Size> dataSize;
+    if (clangField->hasAttr<clang::NoUniqueAddressAttr>()) {
+      if (const auto *rd = clangField->getType()->getAsRecordDecl()) {
+        // Clang can store the next field in the padding of this one.
+        const auto &fieldLayout = ClangContext.getASTRecordLayout(rd);
+        dataSize = Size(fieldLayout.getDataSize().getQuantity());
+      }
+    }
 
     // If we have a Swift import of this type, use our lowered information.
     if (swiftField) {
@@ -1471,7 +1481,8 @@ class ClangRecordLowering {
 
     // Otherwise, add it as an opaque blob.
     auto fieldSize = isZeroSized ? clang::CharUnits::Zero() : ClangContext.getTypeSizeInChars(clangField->getType());
-    return addOpaqueField(offset, Size(fieldSize.getQuantity()));
+    return addOpaqueField(offset,
+                          dataSize.value_or(Size(fieldSize.getQuantity())));
   }
 
   /// Add opaque storage for bitfields spanning the given range of bits.
@@ -1512,7 +1523,8 @@ class ClangRecordLowering {
   }
 
   /// Add information to track a value field at the current offset.
-  void addFieldInfo(VarDecl *swiftField, const FixedTypeInfo &fieldType, bool isZeroSized) {
+  void addFieldInfo(VarDecl *swiftField, const FixedTypeInfo &fieldType,
+                    bool isZeroSized) {
     bool isLoadableField = isa<LoadableTypeInfo>(fieldType);
     unsigned explosionSize = 0;
     if (isLoadableField)

Diff for: test/Interop/Cxx/class/Inputs/member-variables.h

@@ -1,6 +1,9 @@
 #ifndef TEST_INTEROP_CXX_CLASS_INPUTS_MEMBER_VARIABLES_H
 #define TEST_INTEROP_CXX_CLASS_INPUTS_MEMBER_VARIABLES_H
 
+#include <cstddef>
+#include <optional>
+
 class MyClass {
 public:
   const int const_member = 23;
@@ -24,6 +27,14 @@ struct HasZeroSizedField {
   void set_c(short c) { this->c = c; }
 };
 
+struct ReuseFieldPadding {
+  [[no_unique_address]] std::optional<int> a;
+  char c;
+  char get_c() const { return c; }
+  void set_c(char c) { this->c = c; }
+  int offset() const { return offsetof(ReuseFieldPadding, c); }
+};
+
 inline int takesZeroSizedInCpp(HasZeroSizedField x) {
   return x.a;
 }

Diff for: test/Interop/Cxx/class/zero-sized-field.swift

@@ -24,4 +24,18 @@ FieldsTestSuite.test("Zero sized field") {
   expectEqual(s.b.getNum(), 42)
 }
 
+FieldsTestSuite.test("Field padding reused") {
+  var s = ReuseFieldPadding()
+  s.c = 5
+  expectEqual(s.offset(), MemoryLayout<ReuseFieldPadding>.offset(of: \.c)! as Int32)
+  expectEqual(s.c, 5)
+  expectEqual(s.get_c(), 5)
+  s.set_c(6)
+  expectEqual(s.c, 6)
+  expectEqual(s.get_c(), 6)
+  var s2 = s
+  expectEqual(s2.c, 6)
+  expectEqual(s2.get_c(), 6)
+}
+
 runAllTests()