[DebugInfo] Produce debuginfo for nested types when the outer type is [[clang::standalone_debug]].

Without this, it's impossible to get debuginfo for a nested type that is unused (or that
clang mistakenly believes is unused due to e.g. deficiencies in -debug-info-kind=limited),
even by marking the "unused" nested type itself with [[clang::standalone_debug]].
This makes writing pretty-printers and similar things difficult.

The [[clang::standalone_debug]] attribute is not propagated to the nested type.
But after applying [[clang::standalone_debug]] to the outer type to ensure the nested type
appears in the debuginfo at all, a developer can also additionally apply the attribute to the
nested type if desired.

Author: khuey

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -2075,12 +2075,13 @@ void CGDebugInfo::CollectRecordFields(
 
         // Bump field number for next field.
         ++fieldNo;
-      } else if (CGM.getCodeGenOpts().EmitCodeView) {
+      } else if (CGM.getCodeGenOpts().EmitCodeView ||
+                 record->hasAttr<StandaloneDebugAttr>()) {
         // Debug info for nested types is included in the member list only for
-        // CodeView.
+        // CodeView and types with the standalone_debug attribute.
         if (const auto *nestedType = dyn_cast<TypeDecl>(I)) {
           // MSVC doesn't generate nested type for anonymous struct/union.
-          if (isa<RecordDecl>(I) &&
+          if (CGM.getCodeGenOpts().EmitCodeView && isa<RecordDecl>(I) &&
               cast<RecordDecl>(I)->isAnonymousStructOrUnion())
             continue;
           if (!nestedType->isImplicit() &&

clang/test/CodeGenCXX/standalone-debug-attribute-limited.cpp

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -DSETATTR=0 -triple x86_64-unknown-linux-gnu -emit-llvm -debug-info-kind=limited %s -o - | FileCheck %s --check-prefix=DEBUG
+// RUN: %clang_cc1 -DSETATTR=1 -triple x86_64-unknown-linux-gnu -emit-llvm -debug-info-kind=limited %s -o - | FileCheck %s --check-prefix=WITHATTR
+// Use -debug-info-kind=limited because an unused type will never have a used ctor.
+
+#if SETATTR
+#define STANDALONEDEBUGATTR __attribute__((standalone_debug))
+#else
+#define STANDALONEDEBUGATTR
+#endif
+
+struct STANDALONEDEBUGATTR TypeWithNested {
+  struct Unused1 {
+  };
+  struct STANDALONEDEBUGATTR Unused2 {
+  };
+
+  int value = 0;
+};
+void f(TypeWithNested s) {}
+// DEBUG:  !DICompositeType(tag: DW_TAG_structure_type, name: "TypeWithNested"
+// DEBUG-NOT:  !DICompositeType(tag: DW_TAG_structure_type, name: "Unused1"
+// DEBUG-NOT:  !DICompositeType(tag: DW_TAG_structure_type, name: "Unused2"
+// WITHATTR:  !DICompositeType(tag: DW_TAG_structure_type, name: "TypeWithNested"
+// WITHATTR:  !DICompositeType(tag: DW_TAG_structure_type, name: "Unused1"
+// The STANDALONEDEBUGATTR isn't propagated to the nested type by default, so
+// it should still be a forward declaration.
+// WITHATTR-SAME: DIFlagFwdDecl
+// WITHATTR:  !DICompositeType(tag: DW_TAG_structure_type, name: "Unused2"
+// WITHATTR-NOT: DIFlagFwdDecl