[clang-reorder-fields] Check for flexible array member (#160262)

A flexible array member must remain the last field in the struct.

Author: vvuksanovic

clang-tools-extra/clang-reorder-fields/ReorderFieldsAction.cpp

@@ -164,6 +164,22 @@ getNewFieldsOrder(const RecordDecl *Definition,
   return NewFieldsOrder;
 }
 
+static bool isOrderValid(const RecordDecl *RD, ArrayRef<unsigned> FieldOrder) {
+  if (FieldOrder.empty())
+    return false;
+
+  // If there is a flexible array member in the struct, it must remain the last
+  // field.
+  if (RD->hasFlexibleArrayMember() &&
+      FieldOrder.back() != FieldOrder.size() - 1) {
+    llvm::errs()
+        << "Flexible array member must remain the last field in the struct\n";
+    return false;
+  }
+
+  return true;
+}
+
 struct ReorderedStruct {
 public:
   ReorderedStruct(const RecordDecl *Decl, ArrayRef<unsigned> NewFieldsOrder)
@@ -662,7 +678,7 @@ class ReorderingConsumer : public ASTConsumer {
       return;
     SmallVector<unsigned, 4> NewFieldsOrder =
         getNewFieldsOrder(RD, DesiredFieldsOrder);
-    if (NewFieldsOrder.empty())
+    if (!isOrderValid(RD, NewFieldsOrder))
       return;
     ReorderedStruct RS{RD, NewFieldsOrder};
 
@@ -699,7 +715,7 @@ class ReorderingConsumer : public ASTConsumer {
 
 std::unique_ptr<ASTConsumer> ReorderFieldsAction::newASTConsumer() {
   return std::make_unique<ReorderingConsumer>(RecordName, DesiredFieldsOrder,
-                                               Replacements);
+                                              Replacements);
 }
 
 } // namespace reorder_fields

clang-tools-extra/test/clang-reorder-fields/FlexibleArrayMember.c

@@ -0,0 +1,10 @@
+// RUN: clang-reorder-fields -record-name Foo -fields-order z,y,x %s -- 2>&1 | FileCheck --check-prefix=CHECK-BAD %s
+// RUN: clang-reorder-fields -record-name Foo -fields-order y,x,z %s -- | FileCheck --check-prefix=CHECK-GOOD %s
+
+// CHECK-BAD: {{^Flexible array member must remain the last field in the struct}}
+
+struct Foo {
+  int x;   // CHECK-GOOD:      {{^  int y;}}
+  int y;   // CHECK-GOOD-NEXT: {{^  int x;}}
+  int z[]; // CHECK-GOOD-NEXT: {{^  int z\[\];}}
+};