[json] Try to not generate AutoLoader destructors (#30501)

BUILD

@@ -7271,6 +7271,7 @@ grpc_cc_library(
         "json",
         "json_args",
         "json_object_loader",
+        "no_destruct",
         "time",
     ],
 )

src/core/lib/json/json_object_loader.h

@@ -119,7 +119,7 @@ class LoaderInterface {
                         ErrorList* errors) const = 0;
 
  protected:
-  virtual ~LoaderInterface() = default;
+  ~LoaderInterface() = default;
 };
 
 // Loads a scalar (string or number).
@@ -129,7 +129,7 @@ class LoadScalar : public LoaderInterface {
                 ErrorList* errors) const override;
 
  protected:
-  ~LoadScalar() override = default;
+  ~LoadScalar() = default;
 
  private:
   // true if we're loading a number, false if we're loading a string.
@@ -144,7 +144,7 @@ class LoadScalar : public LoaderInterface {
 // Load a string.
 class LoadString : public LoadScalar {
  protected:
-  ~LoadString() override = default;
+  ~LoadString() = default;
 
  private:
   bool IsNumber() const override;
@@ -155,7 +155,7 @@ class LoadString : public LoadScalar {
 // Load a Duration.
 class LoadDuration : public LoadScalar {
  protected:
-  ~LoadDuration() override = default;
+  ~LoadDuration() = default;
 
  private:
   bool IsNumber() const override;
@@ -166,7 +166,7 @@ class LoadDuration : public LoadScalar {
 // Load a number.
 class LoadNumber : public LoadScalar {
  protected:
-  ~LoadNumber() override = default;
+  ~LoadNumber() = default;
 
  private:
   bool IsNumber() const override;
@@ -176,7 +176,7 @@ class LoadNumber : public LoadScalar {
 template <typename T>
 class TypedLoadSignedNumber : public LoadNumber {
  protected:
-  ~TypedLoadSignedNumber() override = default;
+  ~TypedLoadSignedNumber() = default;
 
  private:
   void LoadInto(const std::string& value, void* dst,
@@ -191,7 +191,7 @@ class TypedLoadSignedNumber : public LoadNumber {
 template <typename T>
 class TypedLoadUnsignedNumber : public LoadNumber {
  protected:
-  ~TypedLoadUnsignedNumber() override = default;
+  ~TypedLoadUnsignedNumber() = default;
 
  private:
   void LoadInto(const std::string& value, void* dst,
@@ -205,7 +205,7 @@ class TypedLoadUnsignedNumber : public LoadNumber {
 // Load a float.
 class LoadFloat : public LoadNumber {
  protected:
-  ~LoadFloat() override = default;
+  ~LoadFloat() = default;
 
  private:
   void LoadInto(const std::string& value, void* dst,
@@ -219,7 +219,7 @@ class LoadFloat : public LoadNumber {
 // Load a double.
 class LoadDouble : public LoadNumber {
  protected:
-  ~LoadDouble() override = default;
+  ~LoadDouble() = default;
 
  private:
   void LoadInto(const std::string& value, void* dst,
@@ -251,7 +251,7 @@ class LoadVector : public LoaderInterface {
                 ErrorList* errors) const override;
 
  protected:
-  ~LoadVector() override = default;
+  ~LoadVector() = default;
 
  private:
   virtual void* EmplaceBack(void* dst) const = 0;
@@ -265,7 +265,7 @@ class LoadMap : public LoaderInterface {
                 ErrorList* errors) const override;
 
  protected:
-  ~LoadMap() override = default;
+  ~LoadMap() = default;
 
  private:
   virtual void* Insert(const std::string& name, void* dst) const = 0;
@@ -279,7 +279,7 @@ class LoadOptional : public LoaderInterface {
                 ErrorList* errors) const override;
 
  protected:
-  ~LoadOptional() override = default;
+  ~LoadOptional() = default;
 
  private:
   virtual void* Emplace(void* dst) const = 0;
@@ -301,34 +301,68 @@ class AutoLoader final : public LoaderInterface {
                 ErrorList* errors) const override {
     T::JsonLoader(args)->LoadInto(json, args, dst, errors);
   }
+
+ private:
+  ~AutoLoader() = default;
 };
 
 // Specializations of AutoLoader for basic types.
 template <>
-class AutoLoader<std::string> final : public LoadString {};
+class AutoLoader<std::string> final : public LoadString {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<Duration> final : public LoadDuration {};
+class AutoLoader<Duration> final : public LoadDuration {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<int32_t> final : public TypedLoadSignedNumber<int32_t> {};
+class AutoLoader<int32_t> final : public TypedLoadSignedNumber<int32_t> {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<int64_t> final : public TypedLoadSignedNumber<int64_t> {};
+class AutoLoader<int64_t> final : public TypedLoadSignedNumber<int64_t> {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<uint32_t> final : public TypedLoadUnsignedNumber<uint32_t> {};
+class AutoLoader<uint32_t> final : public TypedLoadUnsignedNumber<uint32_t> {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<uint64_t> final : public TypedLoadUnsignedNumber<uint64_t> {};
+class AutoLoader<uint64_t> final : public TypedLoadUnsignedNumber<uint64_t> {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<float> final : public LoadFloat {};
+class AutoLoader<float> final : public LoadFloat {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<double> final : public LoadDouble {};
+class AutoLoader<double> final : public LoadDouble {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<bool> final : public LoadBool {};
+class AutoLoader<bool> final : public LoadBool {
+ private:
+  ~AutoLoader() = default;
+};
 template <>
-class AutoLoader<Json::Object> final : public LoadUnprocessedJsonObject {};
+class AutoLoader<Json::Object> final : public LoadUnprocessedJsonObject {
+ private:
+  ~AutoLoader() = default;
+};
 
 // Specializations of AutoLoader for vectors.
 template <typename T>
 class AutoLoader<std::vector<T>> final : public LoadVector {
  private:
+  ~AutoLoader() = default;
   void* EmplaceBack(void* dst) const final {
     auto* vec = static_cast<std::vector<T>*>(dst);
     vec->emplace_back();
@@ -348,6 +382,9 @@ class AutoLoader<std::vector<bool>> final : public LoaderInterface {
  public:
   void LoadInto(const Json& json, const JsonArgs& args, void* dst,
                 ErrorList* errors) const override;
+
+ private:
+  ~AutoLoader() = default;
 };
 
 // Specializations of AutoLoader for maps.
@@ -362,6 +399,9 @@ class AutoLoader<std::map<std::string, T>> final : public LoadMap {
   const LoaderInterface* ElementLoader() const final {
     return LoaderForType<T>();
   }
+
+ private:
+  ~AutoLoader() = default;
 };
 
 // Specializations of AutoLoader for absl::optional<>.
@@ -374,6 +414,9 @@ class AutoLoader<absl::optional<T>> final : public LoadOptional {
   const LoaderInterface* ElementLoader() const final {
     return LoaderForType<T>();
   }
+
+ private:
+  ~AutoLoader() = default;
 };
 
 // Implementation of aforementioned LoaderForType.

src/core/lib/json/json_util.cc

@@ -20,16 +20,17 @@
 
 #include "src/core/lib/json/json_util.h"
 
+#include "src/core/lib/gprpp/no_destruct.h"
 #include "src/core/lib/json/json_args.h"
 #include "src/core/lib/json/json_object_loader.h"
 
 namespace grpc_core {
 
 bool ParseDurationFromJson(const Json& field, Duration* duration) {
-  json_detail::AutoLoader<Duration> loader;
   ErrorList errors;
-  static_cast<json_detail::LoaderInterface&>(loader).LoadInto(
-      field, JsonArgs(), duration, &errors);
+  static_cast<json_detail::LoaderInterface*>(
+      NoDestructSingleton<json_detail::AutoLoader<Duration>>::Get())
+      ->LoadInto(field, JsonArgs(), duration, &errors);
   return errors.ok();
 }