[clang] [Clang] add user-level sizeless attribute (PR #71894)

[Timm Baeder via cfe-commits]

================
@@ -2400,7 +2402,66 @@ bool Type::isWebAssemblyTableType() const {
   return false;
 }
 
-bool Type::isSizelessType() const { return isSizelessBuiltinType(); }
+bool Type::isSizelessType() const {
+  // Check if this type or any of its constituents are sizeless, due to
+  // being a builtin type or individually having the user attribute.
+  // As structs can be recursive, we iterate through without repeats.
+  SmallVector<const Type *, 1> todo = {this};
+  llvm::SmallPtrSet<const Type *, 1> done;
+
+  while (todo.size()) {
+    auto current = todo.pop_back_val();
+    if (done.count(current))
+      continue;
+    done.insert(current);
+
+    // If either this is a known sizeless type from being a builtin
+    // or as marked by the user, this is a sizeless type.
+    if (current->isSizelessBuiltinType())
+      return true;
+    if (current->hasAttr(attr::SizelessType))
+      return true;
+
+    // Otherwise return true if any inner types are sizeless.
+    switch (current->CanonicalType->getTypeClass()) {
+    default:
+      break;
+    case Record: {
+      // A struct with sizeless types is itself sizeless.
+      RecordDecl *Rec = cast<RecordType>(current->CanonicalType)->getDecl();
+
+      // skip incomplete structs
+      if (!Rec->isCompleteDefinition())
+        break;
+
+      // a struct marked sizeless explicitly is sizeless
+      if (Rec->hasAttr<clang::SizelessTypeAttr>())
+        return true;
+
+      // A struct is sizeless if it contains a sizeless field
+      for (auto field : Rec->fields())
+        todo.push_back(field->getType().getTypePtr());
+
+      // A class is sizeless if it contains a sizeless base
+      if (auto CXXRec = dyn_cast<CXXRecordDecl>(Rec))
----------------
tbaederr wrote:

```suggestion
      if (const auto *CXXRec = dyn_cast<CXXRecordDecl>(Rec))
```

https://github.com/llvm/llvm-project/pull/71894