[cxx-interop] Do not import zero-sized fields

Zero sized fields are messing up the offset calculations when we import
C++ fields to Swift. We assume that the size of the field is determined
by the type of the field. This is not true for fields marked with
no_unique_address. Those fields can have 0 size while the
sizeof(decltype(field)) is still 1.

rdar://143907490

Author: Gabor Horvath

lib/IRGen/GenStruct.cpp

@@ -1451,6 +1451,9 @@ class ClangRecordLowering {
   /// Place the next struct field at its appropriate offset.
   void addStructField(const clang::FieldDecl *clangField,
                       VarDecl *swiftField, const clang::ASTRecordLayout &layout) {
+    // Skip importing zero-sized fields for now.
+    if (clangField->isZeroSize(ClangContext))
+      return;
     unsigned fieldOffset = layout.getFieldOffset(clangField->getFieldIndex());
     assert(!clangField->isBitField());
     Size offset( SubobjectAdjustment.getValue() + fieldOffset / 8);

test/Interop/Cxx/class/Inputs/member-variables.h

@@ -6,4 +6,11 @@ class MyClass {
   const int const_member = 23;
 };
 
+struct Empty {};
+
+struct HasZeroSizedField {
+  int a;
+  [[no_unique_address]] Empty b;
+};
+
 #endif

test/Interop/Cxx/class/zero-sized-field.swift

@@ -0,0 +1,16 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -enable-experimental-cxx-interop)
+//
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import MemberVariables
+
+var FieldsTestSuite = TestSuite("Generating code with zero sized fields")
+
+FieldsTestSuite.test("Zero sized field") {
+  var s = HasZeroSizedField()
+  s.a = 5
+  expectEqual(s.a, 5)
+}
+
+runAllTests()