Emit a reason for why a field was unsupported.

PiperOrigin-RevId: 578846947

src/google/protobuf/compiler/rust/accessors/accessor_generator.h

@@ -9,6 +9,8 @@
 #define GOOGLE_PROTOBUF_COMPILER_RUST_ACCESSORS_ACCESSOR_GENERATOR_H__
 
 #include <memory>
+#include <string>
+#include <utility>
 
 #include "absl/log/absl_check.h"
 #include "google/protobuf/compiler/rust/context.h"
@@ -96,8 +98,12 @@ class RepeatedScalar final : public AccessorGenerator {
 
 class UnsupportedField final : public AccessorGenerator {
  public:
+  explicit UnsupportedField(std::string reason) : reason_(std::move(reason)) {}
   ~UnsupportedField() override = default;
   void InMsgImpl(Context<FieldDescriptor> field) const override;
+
+ private:
+  std::string reason_;
 };
 
 }  // namespace rust

src/google/protobuf/compiler/rust/accessors/accessors.cc

@@ -9,6 +9,7 @@
 
 #include <memory>
 
+#include "absl/log/absl_log.h"
 #include "google/protobuf/compiler/rust/accessors/accessor_generator.h"
 #include "google/protobuf/compiler/rust/context.h"
 #include "google/protobuf/descriptor.h"
@@ -26,7 +27,8 @@ std::unique_ptr<AccessorGenerator> AccessorGeneratorFor(
   // TODO: We do not support [ctype=FOO] (used to set the field
   // type in C++ to cord or string_piece) in V0.6 API.
   if (desc.options().has_ctype()) {
-    return std::make_unique<UnsupportedField>();
+    return std::make_unique<UnsupportedField>(
+        "fields with ctype not supported");
   }
 
   switch (desc.type()) {
@@ -50,18 +52,23 @@ std::unique_ptr<AccessorGenerator> AccessorGeneratorFor(
     case FieldDescriptor::TYPE_BYTES:
     case FieldDescriptor::TYPE_STRING:
       if (desc.is_repeated()) {
-        return std::make_unique<UnsupportedField>();
+        return std::make_unique<UnsupportedField>("repeated str not supported");
       }
       return std::make_unique<SingularString>();
     case FieldDescriptor::TYPE_MESSAGE:
       if (desc.is_repeated()) {
-        return std::make_unique<UnsupportedField>();
+        return std::make_unique<UnsupportedField>("repeated msg not supported");
       }
       return std::make_unique<SingularMessage>();
 
-    default:
-      return std::make_unique<UnsupportedField>();
+    case FieldDescriptor::TYPE_ENUM:
+      return std::make_unique<UnsupportedField>("enum not supported");
+
+    case FieldDescriptor::TYPE_GROUP:
+      return std::make_unique<UnsupportedField>("group not supported");
   }
+
+  ABSL_LOG(FATAL) << "Unexpected field type: " << desc.type();
 }
 
 }  // namespace

src/google/protobuf/compiler/rust/accessors/unsupported_field.cc

@@ -16,8 +16,8 @@ namespace compiler {
 namespace rust {
 
 void UnsupportedField::InMsgImpl(Context<FieldDescriptor> field) const {
-  field.Emit(R"rs(
-    // Unsupported! :(
+  field.Emit({{"reason", reason_}}, R"rs(
+    // Unsupported! :( Reason: $reason$
     )rs");
   field.printer().PrintRaw("\n");
 }