diff --git a/src/google/protobuf/compiler/BUILD.bazel b/src/google/protobuf/compiler/BUILD.bazel
index f98c31785bec..45afef2a933a 100644
--- a/src/google/protobuf/compiler/BUILD.bazel
+++ b/src/google/protobuf/compiler/BUILD.bazel
@@ -164,6 +164,7 @@ cc_binary(
     copts = COPTS,
     visibility = [
         "//src/google/protobuf:__subpackages__",
+        "//upb:__subpackages__",
     ],
     deps = [
         ":command_line_interface",
diff --git a/upb/BUILD b/upb/BUILD
index 1e52a8092b14..ff5d6defc704 100644
--- a/upb/BUILD
+++ b/upb/BUILD
@@ -406,6 +406,7 @@ upb_amalgamation(
         ":mem_internal",
         ":message",
         ":message_accessors",
+        ":message_copy",
         ":message_internal",
         ":message_internal_types",
         ":message_tagged_ptr",
@@ -456,6 +457,7 @@ upb_amalgamation(
         ":mem_internal",
         ":message",
         ":message_accessors",
+        ":message_copy",
         ":message_internal",
         ":message_internal_types",
         ":message_tagged_ptr",
@@ -506,6 +508,7 @@ upb_amalgamation(
         ":mem_internal",
         ":message",
         ":message_accessors",
+        ":message_copy",
         ":message_internal",
         ":message_internal_types",
         ":message_tagged_ptr",
diff --git a/upb/port/def.inc b/upb/port/def.inc
index f32082138463..aa335c8c869b 100644
--- a/upb/port/def.inc
+++ b/upb/port/def.inc
@@ -323,6 +323,11 @@ void __asan_unpoison_memory_region(void const volatile *addr, size_t size);
 
 #if defined(UPB_IS_GOOGLE3) && !defined(UPB_BOOTSTRAP_STAGE0)
 #define UPB_DESC(sym) proto2_##sym
+#define UPB_DESC_MINITABLE(sym) &proto2__##sym##_msg_init
+#elif defined(UPB_BOOTSTRAP_STAGE0)
+#define UPB_DESC(sym) google_protobuf_##sym
+#define UPB_DESC_MINITABLE(sym) google__protobuf__##sym##_msg_init()
 #else
 #define UPB_DESC(sym) google_protobuf_##sym
+#define UPB_DESC_MINITABLE(sym) &google__protobuf__##sym##_msg_init
 #endif
diff --git a/upb/port/undef.inc b/upb/port/undef.inc
index eb0e1654f8fb..5a1429e34c49 100644
--- a/upb/port/undef.inc
+++ b/upb/port/undef.inc
@@ -49,6 +49,7 @@
 #undef UPB_GNUC_MIN
 #undef UPB_DESCRIPTOR_UPB_H_FILENAME
 #undef UPB_DESC
+#undef UPB_DESC_MINITABLE
 #undef UPB_IS_GOOGLE3
 #undef UPB_ATOMIC
 #undef UPB_USE_C11_ATOMICS
diff --git a/upb/reflection/BUILD b/upb/reflection/BUILD
index bf0c0e4dbeb7..f6ea369d0661 100644
--- a/upb/reflection/BUILD
+++ b/upb/reflection/BUILD
@@ -5,6 +5,15 @@
 # license that can be found in the LICENSE file or at
 # https://developers.google.com/open-source/licenses/bsd
 
+load(
+    "//upb/cmake:build_defs.bzl",
+    "staleness_test",
+)
+load(
+    "//src/google/protobuf/editions:defaults.bzl",
+    "compile_edition_defaults",
+    "embed_edition_defaults",
+)
 load("//bazel:build_defs.bzl", "UPB_DEFAULT_COPTS")
 load("//bazel:upb_minitable_proto_library.bzl", "upb_minitable_proto_library")
 load("//bazel:upb_proto_library.bzl", "upb_proto_reflection_library")
@@ -108,6 +117,7 @@ bootstrap_cc_library(
         "internal/method_def.h",
         "internal/oneof_def.h",
         "internal/service_def.h",
+        "internal/upb_edition_defaults.h",
         "message.h",
         "message.hpp",
         "message_def.h",
@@ -125,11 +135,13 @@ bootstrap_cc_library(
         "//upb:mem",
         "//upb:message",
         "//upb:message_accessors",
+        "//upb:message_copy",
         "//upb:message_value",
         "//upb:mini_descriptor",
         "//upb:mini_descriptor_internal",
         "//upb:mini_table",
         "//upb:port",
+        "//upb/base:internal",
     ],
 )
 
@@ -171,6 +183,30 @@ cc_test(
     ],
 )
 
+compile_edition_defaults(
+    name = "upb_edition_defaults",
+    srcs = [
+        "//:descriptor_proto",
+    ],
+    maximum_edition = "2023",
+    minimum_edition = "PROTO2",
+)
+
+embed_edition_defaults(
+    name = "embedded_upb_edition_defaults_generate",
+    defaults = "upb_edition_defaults",
+    output = "generated/internal/upb_edition_defaults.h",
+    placeholder = "DEFAULTS_VALUE",
+    template = "internal/upb_edition_defaults.h.template",
+)
+
+staleness_test(
+    name = "bootstrap_upb_defaults_staleness_test",
+    outs = ["internal/upb_edition_defaults.h"],
+    generated_pattern = "generated/%s",
+    target_files = ["internal/upb_edition_defaults.h"],
+)
+
 # begin:github_only
 filegroup(
     name = "source_files",
diff --git a/upb/reflection/def_pool.c b/upb/reflection/def_pool.c
index 90d3beadf57e..88e42ded15b1 100644
--- a/upb/reflection/def_pool.c
+++ b/upb/reflection/def_pool.c
@@ -17,6 +17,7 @@
 #include "upb/reflection/internal/file_def.h"
 #include "upb/reflection/internal/message_def.h"
 #include "upb/reflection/internal/service_def.h"
+#include "upb/reflection/internal/upb_edition_defaults.h"
 
 // Must be last.
 #include "upb/port/def.inc"
@@ -27,6 +28,7 @@ struct upb_DefPool {
   upb_strtable files;  // file_name -> (upb_FileDef*)
   upb_inttable exts;   // (upb_MiniTableExtension*) -> (upb_FieldDef*)
   upb_ExtensionRegistry* extreg;
+  const UPB_DESC(FeatureSetDefaults) * feature_set_defaults;
   upb_MiniTablePlatform platform;
   void* scratch_data;
   size_t scratch_size;
@@ -39,6 +41,8 @@ void upb_DefPool_Free(upb_DefPool* s) {
   upb_gfree(s);
 }
 
+static const char serialized_defaults[] = UPB_INTERNAL_UPB_EDITION_DEFAULTS;
+
 upb_DefPool* upb_DefPool_New(void) {
   upb_DefPool* s = upb_gmalloc(sizeof(*s));
   if (!s) return NULL;
@@ -58,6 +62,10 @@ upb_DefPool* upb_DefPool_New(void) {
   if (!s->extreg) goto err;
 
   s->platform = kUpb_MiniTablePlatform_Native;
+  s->feature_set_defaults = UPB_DESC(FeatureSetDefaults_parse)(
+      serialized_defaults, sizeof(serialized_defaults) - 1, s->arena);
+
+  if (!s->feature_set_defaults) goto err;
 
   return s;
 
@@ -66,6 +74,11 @@ upb_DefPool* upb_DefPool_New(void) {
   return NULL;
 }
 
+const UPB_DESC(FeatureSetDefaults) *
+    upb_DefPool_FeatureSetDefaults(const upb_DefPool* s) {
+  return s->feature_set_defaults;
+}
+
 bool _upb_DefPool_InsertExt(upb_DefPool* s, const upb_MiniTableExtension* ext,
                             const upb_FieldDef* f) {
   return upb_inttable_insert(&s->exts, (uintptr_t)ext, upb_value_constptr(f),
@@ -279,7 +292,11 @@ static const upb_FileDef* upb_DefBuilder_AddFileToPool(
       remove_filedef(s, builder->file);
       builder->file = NULL;
     }
-  } else if (!builder->arena || !builder->tmp_arena) {
+  } else if (!builder->arena || !builder->tmp_arena ||
+             !upb_strtable_init(&builder->feature_cache, 16,
+                                builder->tmp_arena) ||
+             !(builder->legacy_features =
+                   UPB_DESC(FeatureSet_new)(builder->tmp_arena))) {
     _upb_DefBuilder_OomErr(builder);
   } else {
     _upb_FileDef_Create(builder, file_proto);
@@ -312,6 +329,8 @@ static const upb_FileDef* _upb_DefPool_AddFile(
 
   upb_DefBuilder ctx = {
       .symtab = s,
+      .tmp_buf = NULL,
+      .tmp_buf_size = 0,
       .layout = layout,
       .platform = s->platform,
       .msg_count = 0,
diff --git a/upb/reflection/def_pool.h b/upb/reflection/def_pool.h
index f21a0dbd2db7..63cce8a08074 100644
--- a/upb/reflection/def_pool.h
+++ b/upb/reflection/def_pool.h
@@ -26,6 +26,9 @@ UPB_API void upb_DefPool_Free(upb_DefPool* s);
 
 UPB_API upb_DefPool* upb_DefPool_New(void);
 
+UPB_API const UPB_DESC(FeatureSetDefaults) *
+    upb_DefPool_FeatureSetDefaults(const upb_DefPool* s);
+
 UPB_API const upb_MessageDef* upb_DefPool_FindMessageByName(
     const upb_DefPool* s, const char* sym);
 
diff --git a/upb/reflection/enum_def.c b/upb/reflection/enum_def.c
index 97fc63e9acea..8ba894f21d66 100644
--- a/upb/reflection/enum_def.c
+++ b/upb/reflection/enum_def.c
@@ -23,7 +23,8 @@
 #include "upb/port/def.inc"
 
 struct upb_EnumDef {
-  const UPB_DESC(EnumOptions) * opts;
+  const UPB_DESC(EnumOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   const upb_MiniTableEnum* layout;  // Only for proto2.
   const upb_FileDef* file;
   const upb_MessageDef* containing_type;  // Could be merged with "file".
@@ -37,8 +38,10 @@ struct upb_EnumDef {
   int res_range_count;
   int res_name_count;
   int32_t defaultval;
-  bool is_closed;
   bool is_sorted;  // Whether all of the values are defined in ascending order.
+#if UINTPTR_MAX == 0xffffffff
+  uint32_t padding;  // Increase size to a multiple of 8.
+#endif
 };
 
 upb_EnumDef* _upb_EnumDef_At(const upb_EnumDef* e, int i) {
@@ -140,7 +143,11 @@ const upb_EnumValueDef* upb_EnumDef_Value(const upb_EnumDef* e, int i) {
   return _upb_EnumValueDef_At(e->values, i);
 }
 
-bool upb_EnumDef_IsClosed(const upb_EnumDef* e) { return e->is_closed; }
+bool upb_EnumDef_IsClosed(const upb_EnumDef* e) {
+  if (UPB_TREAT_PROTO2_ENUMS_LIKE_PROTO3) return false;
+  return UPB_DESC(FeatureSet_enum_type)(e->resolved_features) ==
+         UPB_DESC(FeatureSet_CLOSED);
+}
 
 bool upb_EnumDef_MiniDescriptorEncode(const upb_EnumDef* e, upb_Arena* a,
                                       upb_StringView* out) {
@@ -209,6 +216,7 @@ static upb_StringView* _upb_EnumReservedNames_New(
 
 static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
                            const UPB_DESC(EnumDescriptorProto) * enum_proto,
+                           const UPB_DESC(FeatureSet*) parent_features,
                            upb_EnumDef* e) {
   const UPB_DESC(EnumValueDescriptorProto)* const* values;
   const UPB_DESC(EnumDescriptorProto_EnumReservedRange)* const* res_ranges;
@@ -216,6 +224,10 @@ static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
   upb_StringView name;
   size_t n_value, n_res_range, n_res_name;
 
+  UPB_DEF_SET_OPTIONS(e->opts, EnumDescriptorProto, EnumOptions, enum_proto);
+  e->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(EnumOptions_features)(e->opts));
+
   // Must happen before _upb_DefBuilder_Add()
   e->file = _upb_DefBuilder_File(ctx);
 
@@ -225,9 +237,6 @@ static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
   _upb_DefBuilder_Add(ctx, e->full_name,
                       _upb_DefType_Pack(e, UPB_DEFTYPE_ENUM));
 
-  e->is_closed = (!UPB_TREAT_PROTO2_ENUMS_LIKE_PROTO3) &&
-                 (upb_FileDef_Syntax(e->file) == kUpb_Syntax_Proto2);
-
   values = UPB_DESC(EnumDescriptorProto_value)(enum_proto, &n_value);
 
   bool ok = upb_strtable_init(&e->ntoi, n_value, ctx->arena);
@@ -238,8 +247,8 @@ static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
 
   e->defaultval = 0;
   e->value_count = n_value;
-  e->values =
-      _upb_EnumValueDefs_New(ctx, prefix, n_value, values, e, &e->is_sorted);
+  e->values = _upb_EnumValueDefs_New(ctx, prefix, n_value, values,
+                                     e->resolved_features, e, &e->is_sorted);
 
   if (n_value == 0) {
     _upb_DefBuilder_Errf(ctx, "enums must contain at least one value (%s)",
@@ -256,11 +265,9 @@ static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
   e->res_name_count = n_res_name;
   e->res_names = _upb_EnumReservedNames_New(ctx, n_res_name, res_names);
 
-  UPB_DEF_SET_OPTIONS(e->opts, EnumDescriptorProto, EnumOptions, enum_proto);
-
   upb_inttable_compact(&e->iton, ctx->arena);
 
-  if (e->is_closed) {
+  if (upb_EnumDef_IsClosed(e)) {
     if (ctx->layout) {
       UPB_ASSERT(ctx->enum_count < ctx->layout->enum_count);
       e->layout = ctx->layout->enums[ctx->enum_count++];
@@ -272,10 +279,11 @@ static void create_enumdef(upb_DefBuilder* ctx, const char* prefix,
   }
 }
 
-upb_EnumDef* _upb_EnumDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(EnumDescriptorProto) * const* protos,
-    const upb_MessageDef* containing_type) {
+upb_EnumDef* _upb_EnumDefs_New(upb_DefBuilder* ctx, int n,
+                               const UPB_DESC(EnumDescriptorProto*)
+                                   const* protos,
+                               const UPB_DESC(FeatureSet*) parent_features,
+                               const upb_MessageDef* containing_type) {
   _upb_DefType_CheckPadding(sizeof(upb_EnumDef));
 
   // If a containing type is defined then get the full name from that.
@@ -285,7 +293,7 @@ upb_EnumDef* _upb_EnumDefs_New(
 
   upb_EnumDef* e = _upb_DefBuilder_Alloc(ctx, sizeof(upb_EnumDef) * n);
   for (int i = 0; i < n; i++) {
-    create_enumdef(ctx, name, protos[i], &e[i]);
+    create_enumdef(ctx, name, protos[i], parent_features, &e[i]);
     e[i].containing_type = containing_type;
   }
   return e;
diff --git a/upb/reflection/enum_def.h b/upb/reflection/enum_def.h
index 6f406f54bb07..a81ce5fd1171 100644
--- a/upb/reflection/enum_def.h
+++ b/upb/reflection/enum_def.h
@@ -40,6 +40,7 @@ bool upb_EnumDef_MiniDescriptorEncode(const upb_EnumDef* e, upb_Arena* a,
 
 const char* upb_EnumDef_Name(const upb_EnumDef* e);
 const UPB_DESC(EnumOptions) * upb_EnumDef_Options(const upb_EnumDef* e);
+const UPB_DESC(FeatureSet) * upb_EnumDef_ResolvedFeatures(const upb_EnumDef* e);
 
 upb_StringView upb_EnumDef_ReservedName(const upb_EnumDef* e, int i);
 int upb_EnumDef_ReservedNameCount(const upb_EnumDef* e);
diff --git a/upb/reflection/enum_value_def.c b/upb/reflection/enum_value_def.c
index d06c9331075c..d250daade6e5 100644
--- a/upb/reflection/enum_value_def.c
+++ b/upb/reflection/enum_value_def.c
@@ -7,19 +7,27 @@
 
 #include "upb/reflection/internal/enum_value_def.h"
 
+#include <stdint.h>
+
 #include "upb/reflection/def_type.h"
+#include "upb/reflection/enum_def.h"
+#include "upb/reflection/enum_value_def.h"
+#include "upb/reflection/file_def.h"
 #include "upb/reflection/internal/def_builder.h"
 #include "upb/reflection/internal/enum_def.h"
-#include "upb/reflection/internal/file_def.h"
 
 // Must be last.
 #include "upb/port/def.inc"
 
 struct upb_EnumValueDef {
-  const UPB_DESC(EnumValueOptions) * opts;
+  const UPB_DESC(EnumValueOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   const upb_EnumDef* parent;
   const char* full_name;
   int32_t number;
+#if UINTPTR_MAX == 0xffffffff
+  uint32_t padding;  // Increase size to a multiple of 8.
+#endif
 };
 
 upb_EnumValueDef* _upb_EnumValueDef_At(const upb_EnumValueDef* v, int i) {
@@ -76,9 +84,15 @@ uint32_t upb_EnumValueDef_Index(const upb_EnumValueDef* v) {
 }
 
 static void create_enumvaldef(upb_DefBuilder* ctx, const char* prefix,
-                              const UPB_DESC(EnumValueDescriptorProto) *
+                              const UPB_DESC(EnumValueDescriptorProto*)
                                   val_proto,
+                              const UPB_DESC(FeatureSet*) parent_features,
                               upb_EnumDef* e, upb_EnumValueDef* v) {
+  UPB_DEF_SET_OPTIONS(v->opts, EnumValueDescriptorProto, EnumValueOptions,
+                      val_proto);
+  v->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(EnumValueOptions_features)(v->opts));
+
   upb_StringView name = UPB_DESC(EnumValueDescriptorProto_name)(val_proto);
 
   v->parent = e;  // Must happen prior to _upb_DefBuilder_Add()
@@ -87,17 +101,32 @@ static void create_enumvaldef(upb_DefBuilder* ctx, const char* prefix,
   _upb_DefBuilder_Add(ctx, v->full_name,
                       _upb_DefType_Pack(v, UPB_DEFTYPE_ENUMVAL));
 
-  UPB_DEF_SET_OPTIONS(v->opts, EnumValueDescriptorProto, EnumValueOptions,
-                      val_proto);
-
   bool ok = _upb_EnumDef_Insert(e, v, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 }
 
+static void _upb_EnumValueDef_CheckZeroValue(upb_DefBuilder* ctx,
+                                             const upb_EnumDef* e,
+                                             const upb_EnumValueDef* v, int n) {
+  if (upb_EnumDef_IsClosed(e) || n == 0 || v[0].number == 0) return;
+
+  // When the special UPB_TREAT_PROTO2_ENUMS_LIKE_PROTO3 is enabled, we have to
+  // exempt proto2 enums from this check, even when we are treating them as
+  // open.
+  if (UPB_TREAT_PROTO2_ENUMS_LIKE_PROTO3 &&
+      upb_FileDef_Syntax(upb_EnumDef_File(e)) == kUpb_Syntax_Proto2) {
+    return;
+  }
+
+  _upb_DefBuilder_Errf(ctx, "for open enums, the first value must be zero (%s)",
+                       upb_EnumDef_FullName(e));
+}
+
 // Allocate and initialize an array of |n| enum value defs owned by |e|.
 upb_EnumValueDef* _upb_EnumValueDefs_New(
     upb_DefBuilder* ctx, const char* prefix, int n,
-    const UPB_DESC(EnumValueDescriptorProto) * const* protos, upb_EnumDef* e,
+    const UPB_DESC(EnumValueDescriptorProto*) const* protos,
+    const UPB_DESC(FeatureSet*) parent_features, upb_EnumDef* e,
     bool* is_sorted) {
   _upb_DefType_CheckPadding(sizeof(upb_EnumValueDef));
 
@@ -107,19 +136,14 @@ upb_EnumValueDef* _upb_EnumValueDefs_New(
   *is_sorted = true;
   uint32_t previous = 0;
   for (int i = 0; i < n; i++) {
-    create_enumvaldef(ctx, prefix, protos[i], e, &v[i]);
+    create_enumvaldef(ctx, prefix, protos[i], parent_features, e, &v[i]);
 
     const uint32_t current = v[i].number;
     if (previous > current) *is_sorted = false;
     previous = current;
   }
 
-  if (upb_FileDef_Syntax(ctx->file) == kUpb_Syntax_Proto3 && n > 0 &&
-      v[0].number != 0) {
-    _upb_DefBuilder_Errf(ctx,
-                         "for proto3, the first enum value must be zero (%s)",
-                         upb_EnumDef_FullName(e));
-  }
+  _upb_EnumValueDef_CheckZeroValue(ctx, e, v, n);
 
   return v;
 }
diff --git a/upb/reflection/enum_value_def.h b/upb/reflection/enum_value_def.h
index e95232847a16..4fbbade19d02 100644
--- a/upb/reflection/enum_value_def.h
+++ b/upb/reflection/enum_value_def.h
@@ -27,6 +27,8 @@ UPB_API const char* upb_EnumValueDef_Name(const upb_EnumValueDef* v);
 UPB_API int32_t upb_EnumValueDef_Number(const upb_EnumValueDef* v);
 const UPB_DESC(EnumValueOptions) *
     upb_EnumValueDef_Options(const upb_EnumValueDef* v);
+const UPB_DESC(FeatureSet) *
+    upb_EnumValueDef_ResolvedFeatures(const upb_EnumValueDef* e);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/extension_range.c b/upb/reflection/extension_range.c
index b088ba92ede6..658a0a41e82f 100644
--- a/upb/reflection/extension_range.c
+++ b/upb/reflection/extension_range.c
@@ -7,6 +7,9 @@
 
 #include "upb/reflection/internal/extension_range.h"
 
+#include <stdint.h>
+
+#include "upb/reflection/extension_range.h"
 #include "upb/reflection/field_def.h"
 #include "upb/reflection/internal/def_builder.h"
 #include "upb/reflection/message_def.h"
@@ -16,6 +19,7 @@
 
 struct upb_ExtensionRange {
   const UPB_DESC(ExtensionRangeOptions) * opts;
+  const UPB_DESC(FeatureSet) * resolved_features;
   int32_t start;
   int32_t end;
 };
@@ -41,12 +45,18 @@ int32_t upb_ExtensionRange_End(const upb_ExtensionRange* r) { return r->end; }
 
 upb_ExtensionRange* _upb_ExtensionRanges_New(
     upb_DefBuilder* ctx, int n,
-    const UPB_DESC(DescriptorProto_ExtensionRange) * const* protos,
-    const upb_MessageDef* m) {
+    const UPB_DESC(DescriptorProto_ExtensionRange*) const* protos,
+    const UPB_DESC(FeatureSet*) parent_features, const upb_MessageDef* m) {
   upb_ExtensionRange* r =
       _upb_DefBuilder_Alloc(ctx, sizeof(upb_ExtensionRange) * n);
 
   for (int i = 0; i < n; i++) {
+    UPB_DEF_SET_OPTIONS(r[i].opts, DescriptorProto_ExtensionRange,
+                        ExtensionRangeOptions, protos[i]);
+    r[i].resolved_features = _upb_DefBuilder_ResolveFeatures(
+        ctx, parent_features,
+        UPB_DESC(ExtensionRangeOptions_features)(r[i].opts));
+
     const int32_t start =
         UPB_DESC(DescriptorProto_ExtensionRange_start)(protos[i]);
     const int32_t end = UPB_DESC(DescriptorProto_ExtensionRange_end)(protos[i]);
@@ -66,8 +76,6 @@ upb_ExtensionRange* _upb_ExtensionRanges_New(
 
     r[i].start = start;
     r[i].end = end;
-    UPB_DEF_SET_OPTIONS(r[i].opts, DescriptorProto_ExtensionRange,
-                        ExtensionRangeOptions, protos[i]);
   }
 
   return r;
diff --git a/upb/reflection/extension_range.h b/upb/reflection/extension_range.h
index 7c3e19f005b1..3ddd5340e740 100644
--- a/upb/reflection/extension_range.h
+++ b/upb/reflection/extension_range.h
@@ -25,6 +25,8 @@ int32_t upb_ExtensionRange_End(const upb_ExtensionRange* r);
 bool upb_ExtensionRange_HasOptions(const upb_ExtensionRange* r);
 const UPB_DESC(ExtensionRangeOptions) *
     upb_ExtensionRange_Options(const upb_ExtensionRange* r);
+const UPB_DESC(FeatureSet) *
+    upb_ExtensionRange_ResolvedFeatures(const upb_ExtensionRange* e);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/field_def.c b/upb/reflection/field_def.c
index 537d05ef85a7..5b67e209ccb3 100644
--- a/upb/reflection/field_def.c
+++ b/upb/reflection/field_def.c
@@ -12,6 +12,7 @@
 #include <stdbool.h>
 
 #include "upb/base/descriptor_constants.h"
+#include "upb/message/accessors.h"
 #include "upb/mini_descriptor/decode.h"
 #include "upb/mini_descriptor/internal/modifiers.h"
 #include "upb/reflection/def.h"
@@ -35,7 +36,8 @@ typedef struct {
 } str_t;
 
 struct upb_FieldDef {
-  const UPB_DESC(FieldOptions) * opts;
+  const UPB_DESC(FieldOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   const upb_FileDef* file;
   const upb_MessageDef* msgdef;
   const char* full_name;
@@ -65,13 +67,9 @@ struct upb_FieldDef {
   bool has_json_name;
   bool has_presence;
   bool is_extension;
-  bool is_packed;
   bool is_proto3_optional;
   upb_FieldType type_;
   upb_Label label_;
-#if UINTPTR_MAX == 0xffffffff
-  uint32_t padding;  // Increase size to a multiple of 8.
-#endif
 };
 
 upb_FieldDef* _upb_FieldDef_At(const upb_FieldDef* f, int i) {
@@ -140,7 +138,9 @@ bool _upb_FieldDef_IsPackable(const upb_FieldDef* f) {
 }
 
 bool upb_FieldDef_IsPacked(const upb_FieldDef* f) {
-  return _upb_FieldDef_IsPackable(f) && f->is_packed;
+  return _upb_FieldDef_IsPackable(f) &&
+         UPB_DESC(FeatureSet_repeated_field_encoding(f->resolved_features)) ==
+             UPB_DESC(FeatureSet_PACKED);
 }
 
 const char* upb_FieldDef_Name(const upb_FieldDef* f) {
@@ -253,44 +253,21 @@ bool _upb_FieldDef_IsProto3Optional(const upb_FieldDef* f) {
 
 int _upb_FieldDef_LayoutIndex(const upb_FieldDef* f) { return f->layout_index; }
 
-// begin:google_only
-// static bool _upb_FieldDef_EnforceUtf8Option(const upb_FieldDef* f) {
-// #if defined(UPB_BOOTSTRAP_STAGE0)
-//   return true;
-// #else
-//   return UPB_DESC(FieldOptions_enforce_utf8)(f->opts);
-// #endif
-// }
-// end:google_only
-
-// begin:github_only
-static bool _upb_FieldDef_EnforceUtf8Option(const upb_FieldDef* f) {
-  return true;
-}
-// end:github_only
-
 bool _upb_FieldDef_ValidateUtf8(const upb_FieldDef* f) {
   if (upb_FieldDef_Type(f) != kUpb_FieldType_String) return false;
-  return upb_FileDef_Syntax(upb_FieldDef_File(f)) == kUpb_Syntax_Proto3
-             ? _upb_FieldDef_EnforceUtf8Option(f)
-             : false;
+  return UPB_DESC(FeatureSet_utf8_validation(f->resolved_features)) ==
+         UPB_DESC(FeatureSet_VERIFY);
 }
 
 uint64_t _upb_FieldDef_Modifiers(const upb_FieldDef* f) {
   uint64_t out = upb_FieldDef_IsPacked(f) ? kUpb_FieldModifier_IsPacked : 0;
 
-  switch (f->label_) {
-    case kUpb_Label_Optional:
-      if (!upb_FieldDef_HasPresence(f)) {
-        out |= kUpb_FieldModifier_IsProto3Singular;
-      }
-      break;
-    case kUpb_Label_Repeated:
-      out |= kUpb_FieldModifier_IsRepeated;
-      break;
-    case kUpb_Label_Required:
-      out |= kUpb_FieldModifier_IsRequired;
-      break;
+  if (upb_FieldDef_IsRepeated(f)) {
+    out |= kUpb_FieldModifier_IsRepeated;
+  } else if (upb_FieldDef_IsRequired(f)) {
+    out |= kUpb_FieldModifier_IsRequired;
+  } else if (!upb_FieldDef_HasPresence(f)) {
+    out |= kUpb_FieldModifier_IsProto3Singular;
   }
 
   if (_upb_FieldDef_IsClosedEnum(f)) {
@@ -330,7 +307,8 @@ bool upb_FieldDef_IsRepeated(const upb_FieldDef* f) {
 }
 
 bool upb_FieldDef_IsRequired(const upb_FieldDef* f) {
-  return upb_FieldDef_Label(f) == kUpb_Label_Required;
+  return UPB_DESC(FeatureSet_field_presence)(f->resolved_features) ==
+         UPB_DESC(FeatureSet_LEGACY_REQUIRED);
 }
 
 bool upb_FieldDef_IsString(const upb_FieldDef* f) {
@@ -554,19 +532,63 @@ static void set_default_default(upb_DefBuilder* ctx, upb_FieldDef* f) {
   }
 }
 
+static bool _upb_FieldDef_InferLegacyFeatures(
+    upb_DefBuilder* ctx, upb_FieldDef* f,
+    const UPB_DESC(FieldDescriptorProto*) proto,
+    const UPB_DESC(FieldOptions*) options, upb_Syntax syntax,
+    UPB_DESC(FeatureSet*) features) {
+  bool ret = false;
+
+  if (UPB_DESC(FieldDescriptorProto_label)(proto) == kUpb_Label_Required) {
+    if (syntax == kUpb_Syntax_Proto3) {
+      _upb_DefBuilder_Errf(ctx, "proto3 fields cannot be required (%s)",
+                           f->full_name);
+    }
+    int val = UPB_DESC(FeatureSet_LEGACY_REQUIRED);
+    UPB_DESC(FeatureSet_set_field_presence(features, val));
+    ret = true;
+  }
+
+  if (UPB_DESC(FieldDescriptorProto_type)(proto) == kUpb_FieldType_Group) {
+    int val = UPB_DESC(FeatureSet_DELIMITED);
+    UPB_DESC(FeatureSet_set_message_encoding(features, val));
+    ret = true;
+  }
+
+  if (UPB_DESC(FieldOptions_has_packed)(options)) {
+    int val = UPB_DESC(FieldOptions_packed)(options)
+                  ? UPB_DESC(FeatureSet_PACKED)
+                  : UPB_DESC(FeatureSet_EXPANDED);
+    UPB_DESC(FeatureSet_set_repeated_field_encoding(features, val));
+    ret = true;
+  }
+
+// begin:google_only
+// #ifndef UPB_BOOTSTRAP_STAGE0
+//   if (syntax == kUpb_Syntax_Proto3 &&
+//       UPB_DESC(FieldOptions_has_enforce_utf8)(options) &&
+//       !UPB_DESC(FieldOptions_enforce_utf8)(options)) {
+//     int val = UPB_DESC(FeatureSet_UNVERIFIED);
+//     UPB_DESC(FeatureSet_set_utf8_validation(features, val));
+//     ret = true;
+//   }
+// #endif
+//   // clang-format off
+// end:google_only
+  // clang-format on
+
+  return ret;
+}
+
 static void _upb_FieldDef_Create(upb_DefBuilder* ctx, const char* prefix,
-                                 const UPB_DESC(FieldDescriptorProto) *
+                                 const UPB_DESC(FeatureSet*) parent_features,
+                                 const UPB_DESC(FieldDescriptorProto*)
                                      field_proto,
                                  upb_MessageDef* m, upb_FieldDef* f) {
   // Must happen before _upb_DefBuilder_Add()
   f->file = _upb_DefBuilder_File(ctx);
 
-  if (!UPB_DESC(FieldDescriptorProto_has_name)(field_proto)) {
-    _upb_DefBuilder_Errf(ctx, "field has no name");
-  }
-
   const upb_StringView name = UPB_DESC(FieldDescriptorProto_name)(field_proto);
-
   f->full_name = _upb_DefBuilder_MakeFullName(ctx, prefix, name);
   f->label_ = (int)UPB_DESC(FieldDescriptorProto_label)(field_proto);
   f->number_ = UPB_DESC(FieldDescriptorProto_number)(field_proto);
@@ -575,6 +597,29 @@ static void _upb_FieldDef_Create(upb_DefBuilder* ctx, const char* prefix,
   f->msgdef = m;
   f->scope.oneof = NULL;
 
+  UPB_DEF_SET_OPTIONS(f->opts, FieldDescriptorProto, FieldOptions, field_proto);
+
+  upb_Syntax syntax = upb_FileDef_Syntax(f->file);
+  const UPB_DESC(FeatureSet*) unresolved_features =
+      UPB_DESC(FieldOptions_features)(f->opts);
+  bool implicit = false;
+
+  if (syntax != kUpb_Syntax_Editions) {
+    upb_Message_Clear(ctx->legacy_features, UPB_DESC_MINITABLE(FeatureSet));
+    if (_upb_FieldDef_InferLegacyFeatures(ctx, f, field_proto, f->opts, syntax,
+                                          ctx->legacy_features)) {
+      implicit = true;
+      unresolved_features = ctx->legacy_features;
+    }
+  }
+
+  f->resolved_features = _upb_DefBuilder_DoResolveFeatures(
+      ctx, parent_features, unresolved_features, implicit);
+
+  if (!UPB_DESC(FieldDescriptorProto_has_name)(field_proto)) {
+    _upb_DefBuilder_Errf(ctx, "field has no name");
+  }
+
   f->has_json_name = UPB_DESC(FieldDescriptorProto_has_json_name)(field_proto);
   if (f->has_json_name) {
     const upb_StringView sv =
@@ -630,12 +675,6 @@ static void _upb_FieldDef_Create(upb_DefBuilder* ctx, const char* prefix,
    * to the field_proto until later when we can properly resolve it. */
   f->sub.unresolved = field_proto;
 
-  if (f->label_ == kUpb_Label_Required &&
-      upb_FileDef_Syntax(f->file) == kUpb_Syntax_Proto3) {
-    _upb_DefBuilder_Errf(ctx, "proto3 fields cannot be required (%s)",
-                         f->full_name);
-  }
-
   if (UPB_DESC(FieldDescriptorProto_has_oneof_index)(field_proto)) {
     int oneof_index = UPB_DESC(FieldDescriptorProto_oneof_index)(field_proto);
 
@@ -659,27 +698,21 @@ static void _upb_FieldDef_Create(upb_DefBuilder* ctx, const char* prefix,
     _upb_OneofDef_Insert(ctx, oneof, f, name.data, name.size);
   }
 
-  UPB_DEF_SET_OPTIONS(f->opts, FieldDescriptorProto, FieldOptions, field_proto);
-
-  if (UPB_DESC(FieldOptions_has_packed)(f->opts)) {
-    f->is_packed = UPB_DESC(FieldOptions_packed)(f->opts);
-  } else {
-    f->is_packed = upb_FileDef_Syntax(f->file) == kUpb_Syntax_Proto3;
-  }
-
   f->has_presence =
       (!upb_FieldDef_IsRepeated(f)) &&
       (f->type_ == kUpb_FieldType_Message || f->type_ == kUpb_FieldType_Group ||
        upb_FieldDef_ContainingOneof(f) ||
-       (upb_FileDef_Syntax(f->file) == kUpb_Syntax_Proto2));
+       UPB_DESC(FeatureSet_field_presence)(f->resolved_features) !=
+           UPB_DESC(FeatureSet_IMPLICIT));
 }
 
 static void _upb_FieldDef_CreateExt(upb_DefBuilder* ctx, const char* prefix,
-                                    const UPB_DESC(FieldDescriptorProto) *
+                                    const UPB_DESC(FeatureSet*) parent_features,
+                                    const UPB_DESC(FieldDescriptorProto*)
                                         field_proto,
                                     upb_MessageDef* m, upb_FieldDef* f) {
   f->is_extension = true;
-  _upb_FieldDef_Create(ctx, prefix, field_proto, m, f);
+  _upb_FieldDef_Create(ctx, prefix, parent_features, field_proto, m, f);
 
   if (UPB_DESC(FieldDescriptorProto_has_oneof_index)(field_proto)) {
     _upb_DefBuilder_Errf(ctx, "oneof_index provided for extension field (%s)",
@@ -696,11 +729,13 @@ static void _upb_FieldDef_CreateExt(upb_DefBuilder* ctx, const char* prefix,
 }
 
 static void _upb_FieldDef_CreateNotExt(upb_DefBuilder* ctx, const char* prefix,
-                                       const UPB_DESC(FieldDescriptorProto) *
+                                       const UPB_DESC(FeatureSet*)
+                                           parent_features,
+                                       const UPB_DESC(FieldDescriptorProto*)
                                            field_proto,
                                        upb_MessageDef* m, upb_FieldDef* f) {
   f->is_extension = false;
-  _upb_FieldDef_Create(ctx, prefix, field_proto, m, f);
+  _upb_FieldDef_Create(ctx, prefix, parent_features, field_proto, m, f);
 
   if (!UPB_DESC(FieldDescriptorProto_has_oneof_index)(field_proto)) {
     if (f->is_proto3_optional) {
@@ -714,10 +749,11 @@ static void _upb_FieldDef_CreateNotExt(upb_DefBuilder* ctx, const char* prefix,
   _upb_MessageDef_InsertField(ctx, m, f);
 }
 
-upb_FieldDef* _upb_Extensions_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(FieldDescriptorProto) * const* protos, const char* prefix,
-    upb_MessageDef* m) {
+upb_FieldDef* _upb_Extensions_New(upb_DefBuilder* ctx, int n,
+                                  const UPB_DESC(FieldDescriptorProto*)
+                                      const* protos,
+                                  const UPB_DESC(FeatureSet*) parent_features,
+                                  const char* prefix, upb_MessageDef* m) {
   _upb_DefType_CheckPadding(sizeof(upb_FieldDef));
   upb_FieldDef* defs =
       (upb_FieldDef*)_upb_DefBuilder_Alloc(ctx, sizeof(upb_FieldDef) * n);
@@ -725,17 +761,19 @@ upb_FieldDef* _upb_Extensions_New(
   for (int i = 0; i < n; i++) {
     upb_FieldDef* f = &defs[i];
 
-    _upb_FieldDef_CreateExt(ctx, prefix, protos[i], m, f);
+    _upb_FieldDef_CreateExt(ctx, prefix, parent_features, protos[i], m, f);
     f->index_ = i;
   }
 
   return defs;
 }
 
-upb_FieldDef* _upb_FieldDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(FieldDescriptorProto) * const* protos, const char* prefix,
-    upb_MessageDef* m, bool* is_sorted) {
+upb_FieldDef* _upb_FieldDefs_New(upb_DefBuilder* ctx, int n,
+                                 const UPB_DESC(FieldDescriptorProto*)
+                                     const* protos,
+                                 const UPB_DESC(FeatureSet*) parent_features,
+                                 const char* prefix, upb_MessageDef* m,
+                                 bool* is_sorted) {
   _upb_DefType_CheckPadding(sizeof(upb_FieldDef));
   upb_FieldDef* defs =
       (upb_FieldDef*)_upb_DefBuilder_Alloc(ctx, sizeof(upb_FieldDef) * n);
@@ -744,7 +782,7 @@ upb_FieldDef* _upb_FieldDefs_New(
   for (int i = 0; i < n; i++) {
     upb_FieldDef* f = &defs[i];
 
-    _upb_FieldDef_CreateNotExt(ctx, prefix, protos[i], m, f);
+    _upb_FieldDef_CreateNotExt(ctx, prefix, parent_features, protos[i], m, f);
     f->index_ = i;
     if (!ctx->layout) {
       // Speculate that the def fields are sorted.  We will always sort the
diff --git a/upb/reflection/field_def.h b/upb/reflection/field_def.h
index b3305bc1f29d..35c3e702ede5 100644
--- a/upb/reflection/field_def.h
+++ b/upb/reflection/field_def.h
@@ -61,6 +61,8 @@ const upb_MiniTableField* upb_FieldDef_MiniTable(const upb_FieldDef* f);
 UPB_API const char* upb_FieldDef_Name(const upb_FieldDef* f);
 UPB_API uint32_t upb_FieldDef_Number(const upb_FieldDef* f);
 const UPB_DESC(FieldOptions) * upb_FieldDef_Options(const upb_FieldDef* f);
+const UPB_DESC(FeatureSet) *
+    upb_FieldDef_ResolvedFeatures(const upb_FieldDef* f);
 UPB_API const upb_OneofDef* upb_FieldDef_RealContainingOneof(
     const upb_FieldDef* f);
 UPB_API upb_FieldType upb_FieldDef_Type(const upb_FieldDef* f);
diff --git a/upb/reflection/file_def.c b/upb/reflection/file_def.c
index bd4b254cac43..abdbd3523e84 100644
--- a/upb/reflection/file_def.c
+++ b/upb/reflection/file_def.c
@@ -7,6 +7,8 @@
 
 #include "upb/reflection/internal/file_def.h"
 
+#include <stddef.h>
+
 #include "upb/reflection/def_pool.h"
 #include "upb/reflection/internal/def_builder.h"
 #include "upb/reflection/internal/enum_def.h"
@@ -19,7 +21,8 @@
 #include "upb/port/def.inc"
 
 struct upb_FileDef {
-  const UPB_DESC(FileOptions) * opts;
+  const UPB_DESC(FileOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   const char* name;
   const char* package;
   UPB_DESC(Edition) edition;
@@ -49,6 +52,11 @@ const UPB_DESC(FileOptions) * upb_FileDef_Options(const upb_FileDef* f) {
   return f->opts;
 }
 
+const UPB_DESC(FeatureSet) *
+    upb_FileDef_ResolvedFeatures(const upb_FileDef* f) {
+  return f->resolved_features;
+}
+
 bool upb_FileDef_HasOptions(const upb_FileDef* f) {
   return f->opts != (void*)kUpbDefOptDefault;
 }
@@ -165,6 +173,34 @@ static int count_exts_in_msg(const UPB_DESC(DescriptorProto) * msg_proto) {
   return ext_count;
 }
 
+const UPB_DESC(FeatureSet*)
+    _upb_FileDef_FindEdition(upb_DefBuilder* ctx, int edition) {
+  const UPB_DESC(FeatureSetDefaults)* defaults =
+      upb_DefPool_FeatureSetDefaults(ctx->symtab);
+
+  int min = UPB_DESC(FeatureSetDefaults_minimum_edition)(defaults);
+  int max = UPB_DESC(FeatureSetDefaults_maximum_edition)(defaults);
+  if (edition < min || edition > max) {
+    _upb_DefBuilder_Errf(ctx,
+                         "Edition %d is outside the supported range [%d, %d] "
+                         "given in the defaults",
+                         edition, min, max);
+  }
+
+  size_t n;
+  const UPB_DESC(FeatureSetDefaults_FeatureSetEditionDefault)* const* d =
+      UPB_DESC(FeatureSetDefaults_defaults)(defaults, &n);
+  const UPB_DESC(FeatureSet)* ret = NULL;
+  for (size_t i = 0; i < n; i++) {
+    if (UPB_DESC(FeatureSetDefaults_FeatureSetEditionDefault_edition)(d[i]) >
+        edition) {
+      break;
+    }
+    ret = UPB_DESC(FeatureSetDefaults_FeatureSetEditionDefault_features)(d[i]);
+  }
+  return ret;
+}
+
 // Allocate and initialize one file def, and add it to the context object.
 void _upb_FileDef_Create(upb_DefBuilder* ctx,
                          const UPB_DESC(FileDescriptorProto) * file_proto) {
@@ -233,21 +269,33 @@ void _upb_FileDef_Create(upb_DefBuilder* ctx,
 
     if (streql_view(syntax, "proto2")) {
       file->syntax = kUpb_Syntax_Proto2;
+      file->edition = UPB_DESC(EDITION_PROTO2);
     } else if (streql_view(syntax, "proto3")) {
       file->syntax = kUpb_Syntax_Proto3;
+      file->edition = UPB_DESC(EDITION_PROTO3);
     } else if (streql_view(syntax, "editions")) {
       file->syntax = kUpb_Syntax_Editions;
+      file->edition = UPB_DESC(FileDescriptorProto_edition)(file_proto);
     } else {
       _upb_DefBuilder_Errf(ctx, "Invalid syntax '" UPB_STRINGVIEW_FORMAT "'",
                            UPB_STRINGVIEW_ARGS(syntax));
     }
   } else {
     file->syntax = kUpb_Syntax_Proto2;
+    file->edition = UPB_DESC(EDITION_PROTO2);
   }
 
   // Read options.
   UPB_DEF_SET_OPTIONS(file->opts, FileDescriptorProto, FileOptions, file_proto);
 
+  // Resolve features.
+  const UPB_DESC(FeatureSet*) edition_defaults =
+      _upb_FileDef_FindEdition(ctx, file->edition);
+  const UPB_DESC(FeatureSet*) unresolved =
+      UPB_DESC(FileOptions_features)(file->opts);
+  file->resolved_features =
+      _upb_DefBuilder_ResolveFeatures(ctx, edition_defaults, unresolved);
+
   // Verify dependencies.
   strs = UPB_DESC(FileDescriptorProto_dependency)(file_proto, &n);
   file->dep_count = n;
@@ -293,22 +341,26 @@ void _upb_FileDef_Create(upb_DefBuilder* ctx,
   // Create enums.
   enums = UPB_DESC(FileDescriptorProto_enum_type)(file_proto, &n);
   file->top_lvl_enum_count = n;
-  file->top_lvl_enums = _upb_EnumDefs_New(ctx, n, enums, NULL);
+  file->top_lvl_enums =
+      _upb_EnumDefs_New(ctx, n, enums, file->resolved_features, NULL);
 
   // Create extensions.
   exts = UPB_DESC(FileDescriptorProto_extension)(file_proto, &n);
   file->top_lvl_ext_count = n;
-  file->top_lvl_exts = _upb_Extensions_New(ctx, n, exts, file->package, NULL);
+  file->top_lvl_exts = _upb_Extensions_New(
+      ctx, n, exts, file->resolved_features, file->package, NULL);
 
   // Create messages.
   msgs = UPB_DESC(FileDescriptorProto_message_type)(file_proto, &n);
   file->top_lvl_msg_count = n;
-  file->top_lvl_msgs = _upb_MessageDefs_New(ctx, n, msgs, NULL);
+  file->top_lvl_msgs =
+      _upb_MessageDefs_New(ctx, n, msgs, file->resolved_features, NULL);
 
   // Create services.
   services = UPB_DESC(FileDescriptorProto_service)(file_proto, &n);
   file->service_count = n;
-  file->services = _upb_ServiceDefs_New(ctx, n, services);
+  file->services =
+      _upb_ServiceDefs_New(ctx, n, services, file->resolved_features);
 
   // Now that all names are in the table, build layouts and resolve refs.
 
diff --git a/upb/reflection/file_def.h b/upb/reflection/file_def.h
index f338c25f92e5..912d0081d986 100644
--- a/upb/reflection/file_def.h
+++ b/upb/reflection/file_def.h
@@ -24,6 +24,7 @@ int upb_FileDef_DependencyCount(const upb_FileDef* f);
 bool upb_FileDef_HasOptions(const upb_FileDef* f);
 UPB_API const char* upb_FileDef_Name(const upb_FileDef* f);
 const UPB_DESC(FileOptions) * upb_FileDef_Options(const upb_FileDef* f);
+const UPB_DESC(FeatureSet) * upb_FileDef_ResolvedFeatures(const upb_FileDef* f);
 const char* upb_FileDef_Package(const upb_FileDef* f);
 UPB_DESC(Edition) upb_FileDef_Edition(const upb_FileDef* f);
 UPB_API const upb_DefPool* upb_FileDef_Pool(const upb_FileDef* f);
diff --git a/upb/reflection/internal/def_builder.c b/upb/reflection/internal/def_builder.c
index 2ab16f27deef..c0b5b65da7bb 100644
--- a/upb/reflection/internal/def_builder.c
+++ b/upb/reflection/internal/def_builder.c
@@ -9,9 +9,12 @@
 
 #include <string.h>
 
+#include "upb/base/internal/log2.h"
+#include "upb/message/copy.h"
 #include "upb/reflection/def_pool.h"
 #include "upb/reflection/def_type.h"
 #include "upb/reflection/field_def.h"
+#include "upb/reflection/file_def.h"
 #include "upb/reflection/internal/strdup2.h"
 
 // Must be last.
@@ -337,3 +340,74 @@ void _upb_DefBuilder_CheckIdentSlow(upb_DefBuilder* ctx, upb_StringView name,
   // We should never reach this point.
   UPB_ASSERT(false);
 }
+
+upb_StringView _upb_DefBuilder_MakeKey(upb_DefBuilder* ctx,
+                                       const UPB_DESC(FeatureSet*) parent,
+                                       upb_StringView key) {
+  size_t need = key.size + sizeof(void*);
+  if (ctx->tmp_buf_size < need) {
+    ctx->tmp_buf_size = UPB_MAX(64, upb_Log2Ceiling(need));
+    ctx->tmp_buf = upb_Arena_Malloc(ctx->tmp_arena, ctx->tmp_buf_size);
+    if (!ctx->tmp_buf) _upb_DefBuilder_OomErr(ctx);
+  }
+
+  memcpy(ctx->tmp_buf, &parent, sizeof(void*));
+  memcpy(ctx->tmp_buf + 8, key.data, key.size);
+  return upb_StringView_FromDataAndSize(ctx->tmp_buf, need);
+}
+
+bool _upb_DefBuilder_GetOrCreateFeatureSet(upb_DefBuilder* ctx,
+                                           const UPB_DESC(FeatureSet*) parent,
+                                           upb_StringView key,
+                                           UPB_DESC(FeatureSet**) set) {
+  upb_StringView k = _upb_DefBuilder_MakeKey(ctx, parent, key);
+  upb_value v;
+  if (upb_strtable_lookup2(&ctx->feature_cache, k.data, k.size, &v)) {
+    *set = upb_value_getptr(v);
+    return false;
+  }
+
+  *set =
+      upb_Message_DeepClone(parent, UPB_DESC_MINITABLE(FeatureSet), ctx->arena);
+  if (!*set) _upb_DefBuilder_OomErr(ctx);
+
+  v = upb_value_ptr(*set);
+  if (!upb_strtable_insert(&ctx->feature_cache, k.data, k.size, v,
+                           ctx->tmp_arena)) {
+    _upb_DefBuilder_OomErr(ctx);
+  }
+
+  return true;
+}
+
+const UPB_DESC(FeatureSet*)
+    _upb_DefBuilder_DoResolveFeatures(upb_DefBuilder* ctx,
+                                      const UPB_DESC(FeatureSet*) parent,
+                                      const UPB_DESC(FeatureSet*) child,
+                                      bool is_implicit) {
+  assert(parent);
+  if (!child) return parent;
+
+  if (child && !is_implicit &&
+      upb_FileDef_Syntax(ctx->file) != kUpb_Syntax_Editions) {
+    _upb_DefBuilder_Errf(ctx, "Features can only be specified for editions");
+  }
+
+  UPB_DESC(FeatureSet*) resolved;
+  size_t child_size;
+  const char* child_bytes =
+      UPB_DESC(FeatureSet_serialize)(child, ctx->tmp_arena, &child_size);
+  if (!child_bytes) _upb_DefBuilder_OomErr(ctx);
+
+  upb_StringView key = upb_StringView_FromDataAndSize(child_bytes, child_size);
+  if (!_upb_DefBuilder_GetOrCreateFeatureSet(ctx, parent, key, &resolved)) {
+    return resolved;
+  }
+
+  upb_DecodeStatus dec_status =
+      upb_Decode(child_bytes, child_size, resolved,
+                 UPB_DESC_MINITABLE(FeatureSet), NULL, 0, ctx->arena);
+  if (dec_status != kUpb_DecodeStatus_Ok) _upb_DefBuilder_OomErr(ctx);
+
+  return resolved;
+}
diff --git a/upb/reflection/internal/def_builder.h b/upb/reflection/internal/def_builder.h
index 584553214dea..b9a0bff5f8f7 100644
--- a/upb/reflection/internal/def_builder.h
+++ b/upb/reflection/internal/def_builder.h
@@ -36,6 +36,10 @@ extern "C" {
 
 struct upb_DefBuilder {
   upb_DefPool* symtab;
+  upb_strtable feature_cache;             // Caches features by identity.
+  UPB_DESC(FeatureSet*) legacy_features;  // For computing legacy features.
+  char* tmp_buf;                          // Temporary buffer in tmp_arena.
+  size_t tmp_buf_size;                    // Size of temporary buffer.
   upb_FileDef* file;                 // File we are building.
   upb_Arena* arena;                  // Allocate defs here.
   upb_Arena* tmp_arena;              // For temporary allocations.
@@ -128,6 +132,25 @@ UPB_INLINE void _upb_DefBuilder_CheckIdentFull(upb_DefBuilder* ctx,
   if (!good) _upb_DefBuilder_CheckIdentSlow(ctx, name, true);
 }
 
+// Returns true if the returned feature set is new and must be populated.
+bool _upb_DefBuilder_GetOrCreateFeatureSet(upb_DefBuilder* ctx,
+                                           const UPB_DESC(FeatureSet*) parent,
+                                           upb_StringView key,
+                                           UPB_DESC(FeatureSet**) set);
+
+const UPB_DESC(FeatureSet*)
+    _upb_DefBuilder_DoResolveFeatures(upb_DefBuilder* ctx,
+                                      const UPB_DESC(FeatureSet*) parent,
+                                      const UPB_DESC(FeatureSet*) child,
+                                      bool is_implicit);
+
+UPB_INLINE const UPB_DESC(FeatureSet*)
+    _upb_DefBuilder_ResolveFeatures(upb_DefBuilder* ctx,
+                                    const UPB_DESC(FeatureSet*) parent,
+                                    const UPB_DESC(FeatureSet*) child) {
+  return _upb_DefBuilder_DoResolveFeatures(ctx, parent, child, false);
+}
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif
diff --git a/upb/reflection/internal/enum_def.h b/upb/reflection/internal/enum_def.h
index 8f6b22011203..d8912a08c4d4 100644
--- a/upb/reflection/internal/enum_def.h
+++ b/upb/reflection/internal/enum_def.h
@@ -22,10 +22,11 @@ bool _upb_EnumDef_Insert(upb_EnumDef* e, upb_EnumValueDef* v, upb_Arena* a);
 const upb_MiniTableEnum* _upb_EnumDef_MiniTable(const upb_EnumDef* e);
 
 // Allocate and initialize an array of |n| enum defs.
-upb_EnumDef* _upb_EnumDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(EnumDescriptorProto) * const* protos,
-    const upb_MessageDef* containing_type);
+upb_EnumDef* _upb_EnumDefs_New(upb_DefBuilder* ctx, int n,
+                               const UPB_DESC(EnumDescriptorProto*)
+                                   const* protos,
+                               const UPB_DESC(FeatureSet*) parent_features,
+                               const upb_MessageDef* containing_type);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/internal/enum_reserved_range.h b/upb/reflection/internal/enum_reserved_range.h
index fe30604a1a86..6b988ee2458d 100644
--- a/upb/reflection/internal/enum_reserved_range.h
+++ b/upb/reflection/internal/enum_reserved_range.h
@@ -23,7 +23,7 @@ upb_EnumReservedRange* _upb_EnumReservedRange_At(const upb_EnumReservedRange* r,
 // Allocate and initialize an array of |n| reserved ranges owned by |e|.
 upb_EnumReservedRange* _upb_EnumReservedRanges_New(
     upb_DefBuilder* ctx, int n,
-    const UPB_DESC(EnumDescriptorProto_EnumReservedRange) * const* protos,
+    const UPB_DESC(EnumDescriptorProto_EnumReservedRange*) const* protos,
     const upb_EnumDef* e);
 
 #ifdef __cplusplus
diff --git a/upb/reflection/internal/enum_value_def.h b/upb/reflection/internal/enum_value_def.h
index 2484567febac..e75e48a6e25a 100644
--- a/upb/reflection/internal/enum_value_def.h
+++ b/upb/reflection/internal/enum_value_def.h
@@ -22,7 +22,8 @@ upb_EnumValueDef* _upb_EnumValueDef_At(const upb_EnumValueDef* v, int i);
 // Allocate and initialize an array of |n| enum value defs owned by |e|.
 upb_EnumValueDef* _upb_EnumValueDefs_New(
     upb_DefBuilder* ctx, const char* prefix, int n,
-    const UPB_DESC(EnumValueDescriptorProto) * const* protos, upb_EnumDef* e,
+    const UPB_DESC(EnumValueDescriptorProto*) const* protos,
+    const UPB_DESC(FeatureSet*) parent_features, upb_EnumDef* e,
     bool* is_sorted);
 
 const upb_EnumValueDef** _upb_EnumValueDefs_Sorted(const upb_EnumValueDef* v,
diff --git a/upb/reflection/internal/extension_range.h b/upb/reflection/internal/extension_range.h
index 09901b41ff81..2432701c5f9c 100644
--- a/upb/reflection/internal/extension_range.h
+++ b/upb/reflection/internal/extension_range.h
@@ -22,8 +22,8 @@ upb_ExtensionRange* _upb_ExtensionRange_At(const upb_ExtensionRange* r, int i);
 // Allocate and initialize an array of |n| extension ranges owned by |m|.
 upb_ExtensionRange* _upb_ExtensionRanges_New(
     upb_DefBuilder* ctx, int n,
-    const UPB_DESC(DescriptorProto_ExtensionRange) * const* protos,
-    const upb_MessageDef* m);
+    const UPB_DESC(DescriptorProto_ExtensionRange*) const* protos,
+    const UPB_DESC(FeatureSet*) parent_features, const upb_MessageDef* m);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/internal/field_def.h b/upb/reflection/internal/field_def.h
index 838f3b607028..f41a8226a637 100644
--- a/upb/reflection/internal/field_def.h
+++ b/upb/reflection/internal/field_def.h
@@ -31,16 +31,19 @@ void _upb_FieldDef_BuildMiniTableExtension(upb_DefBuilder* ctx,
                                            const upb_FieldDef* f);
 
 // Allocate and initialize an array of |n| extensions (field defs).
-upb_FieldDef* _upb_Extensions_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(FieldDescriptorProto) * const* protos, const char* prefix,
-    upb_MessageDef* m);
+upb_FieldDef* _upb_Extensions_New(upb_DefBuilder* ctx, int n,
+                                  const UPB_DESC(FieldDescriptorProto*)
+                                      const* protos,
+                                  const UPB_DESC(FeatureSet*) parent_features,
+                                  const char* prefix, upb_MessageDef* m);
 
 // Allocate and initialize an array of |n| field defs.
-upb_FieldDef* _upb_FieldDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(FieldDescriptorProto) * const* protos, const char* prefix,
-    upb_MessageDef* m, bool* is_sorted);
+upb_FieldDef* _upb_FieldDefs_New(upb_DefBuilder* ctx, int n,
+                                 const UPB_DESC(FieldDescriptorProto*)
+                                     const* protos,
+                                 const UPB_DESC(FeatureSet*) parent_features,
+                                 const char* prefix, upb_MessageDef* m,
+                                 bool* is_sorted);
 
 // Allocate and return a list of pointers to the |n| field defs in |ff|,
 // sorted by field number.
diff --git a/upb/reflection/internal/message_def.h b/upb/reflection/internal/message_def.h
index 6cc9686a7415..a8bcec244f48 100644
--- a/upb/reflection/internal/message_def.h
+++ b/upb/reflection/internal/message_def.h
@@ -30,9 +30,12 @@ void _upb_MessageDef_LinkMiniTable(upb_DefBuilder* ctx,
 void _upb_MessageDef_Resolve(upb_DefBuilder* ctx, upb_MessageDef* m);
 
 // Allocate and initialize an array of |n| message defs.
-upb_MessageDef* _upb_MessageDefs_New(
-    upb_DefBuilder* ctx, int n, const UPB_DESC(DescriptorProto) * const* protos,
-    const upb_MessageDef* containing_type);
+upb_MessageDef* _upb_MessageDefs_New(upb_DefBuilder* ctx, int n,
+                                     const UPB_DESC(DescriptorProto*)
+                                         const* protos,
+                                     const UPB_DESC(FeatureSet*)
+                                         parent_features,
+                                     const upb_MessageDef* containing_type);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/internal/method_def.h b/upb/reflection/internal/method_def.h
index a7e8ef8a37a1..b2b628d64227 100644
--- a/upb/reflection/internal/method_def.h
+++ b/upb/reflection/internal/method_def.h
@@ -20,9 +20,11 @@ extern "C" {
 upb_MethodDef* _upb_MethodDef_At(const upb_MethodDef* m, int i);
 
 // Allocate and initialize an array of |n| method defs owned by |s|.
-upb_MethodDef* _upb_MethodDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(MethodDescriptorProto) * const* protos, upb_ServiceDef* s);
+upb_MethodDef* _upb_MethodDefs_New(upb_DefBuilder* ctx, int n,
+                                   const UPB_DESC(MethodDescriptorProto*)
+                                       const* protos,
+                                   const UPB_DESC(FeatureSet*) parent_features,
+                                   upb_ServiceDef* s);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/internal/oneof_def.h b/upb/reflection/internal/oneof_def.h
index 9cd6713efd32..7d803d592cd8 100644
--- a/upb/reflection/internal/oneof_def.h
+++ b/upb/reflection/internal/oneof_def.h
@@ -22,9 +22,11 @@ void _upb_OneofDef_Insert(upb_DefBuilder* ctx, upb_OneofDef* o,
                           const upb_FieldDef* f, const char* name, size_t size);
 
 // Allocate and initialize an array of |n| oneof defs owned by |m|.
-upb_OneofDef* _upb_OneofDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(OneofDescriptorProto) * const* protos, upb_MessageDef* m);
+upb_OneofDef* _upb_OneofDefs_New(upb_DefBuilder* ctx, int n,
+                                 const UPB_DESC(OneofDescriptorProto*)
+                                     const* protos,
+                                 const UPB_DESC(FeatureSet*) parent_features,
+                                 upb_MessageDef* m);
 
 size_t _upb_OneofDefs_Finalize(upb_DefBuilder* ctx, upb_MessageDef* m);
 
diff --git a/upb/reflection/internal/service_def.h b/upb/reflection/internal/service_def.h
index 2845b879070c..b95b53b842f7 100644
--- a/upb/reflection/internal/service_def.h
+++ b/upb/reflection/internal/service_def.h
@@ -20,9 +20,11 @@ extern "C" {
 upb_ServiceDef* _upb_ServiceDef_At(const upb_ServiceDef* s, int i);
 
 // Allocate and initialize an array of |n| service defs.
-upb_ServiceDef* _upb_ServiceDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(ServiceDescriptorProto) * const* protos);
+upb_ServiceDef* _upb_ServiceDefs_New(upb_DefBuilder* ctx, int n,
+                                     const UPB_DESC(ServiceDescriptorProto*)
+                                         const* protos,
+                                     const UPB_DESC(FeatureSet*)
+                                         parent_features);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/internal/upb_edition_defaults.h b/upb/reflection/internal/upb_edition_defaults.h
new file mode 100644
index 000000000000..587d3b13d184
--- /dev/null
+++ b/upb/reflection/internal/upb_edition_defaults.h
@@ -0,0 +1,20 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2023 Google LLC.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+#ifndef UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
+#define UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
+
+// This file contains the serialized FeatureSetDefaults object for
+// language-independent features and (possibly at some point) for upb-specific
+// features. This is used for feature resolution under Editions.
+// NOLINTBEGIN
+// clang-format off
+#define UPB_INTERNAL_UPB_EDITION_DEFAULTS "\n\021\022\014\010\001\020\002\030\002 \001(\0010\002\030\346\007\n\021\022\014\010\002\020\001\030\001 \002(\0010\001\030\347\007\n\021\022\014\010\001\020\001\030\001 \002(\0010\001\030\350\007 \346\007(\350\007"
+// clang-format on
+// NOLINTEND
+
+#endif  // UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
diff --git a/upb/reflection/internal/upb_edition_defaults.h.template b/upb/reflection/internal/upb_edition_defaults.h.template
new file mode 100644
index 000000000000..58ff7f1eb39f
--- /dev/null
+++ b/upb/reflection/internal/upb_edition_defaults.h.template
@@ -0,0 +1,20 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2023 Google LLC.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+#ifndef UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
+#define UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
+
+// This file contains the serialized FeatureSetDefaults object for
+// language-independent features and (possibly at some point) for upb-specific
+// features. This is used for feature resolution under Editions.
+// NOLINTBEGIN
+// clang-format off
+#define UPB_INTERNAL_UPB_EDITION_DEFAULTS "DEFAULTS_VALUE"
+// clang-format on
+// NOLINTEND
+
+#endif  // UPB_REFLECTION_UPB_EDITION_DEFAULTS_H_
diff --git a/upb/reflection/message_def.c b/upb/reflection/message_def.c
index 9e55d612a4ca..96037d18633d 100644
--- a/upb/reflection/message_def.c
+++ b/upb/reflection/message_def.c
@@ -28,7 +28,8 @@
 #include "upb/port/def.inc"
 
 struct upb_MessageDef {
-  const UPB_DESC(MessageOptions) * opts;
+  const UPB_DESC(MessageOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   const upb_MiniTable* layout;
   const upb_FileDef* file;
   const upb_MessageDef* containing_type;
@@ -66,6 +67,9 @@ struct upb_MessageDef {
   bool in_message_set;
   bool is_sorted;
   upb_WellKnown well_known_type;
+#if UINTPTR_MAX == 0xffffffff
+  uint32_t padding;  // Increase size to a multiple of 8.
+#endif
 };
 
 static void assign_msg_wellknowntype(upb_MessageDef* m) {
@@ -397,10 +401,9 @@ void _upb_MessageDef_InsertField(upb_DefBuilder* ctx, upb_MessageDef* m,
       _upb_MessageDef_Insert(m, shortname, shortnamelen, field_v, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
-  // TODO: Once editions is supported this should turn into a
-  // check on LEGACY_BEST_EFFORT
   if (strcmp(shortname, json_name) != 0 &&
-      upb_FileDef_Syntax(m->file) == kUpb_Syntax_Proto3 &&
+      UPB_DESC(FeatureSet_json_format)(m->resolved_features) ==
+          UPB_DESC(FeatureSet_ALLOW) &&
       upb_strtable_lookup(&m->ntof, json_name, &v)) {
     _upb_DefBuilder_Errf(
         ctx, "duplicate json_name for (%s) with original field name (%s)",
@@ -517,7 +520,8 @@ static bool _upb_MessageDef_ValidateUtf8(const upb_MessageDef* m) {
 static uint64_t _upb_MessageDef_Modifiers(const upb_MessageDef* m) {
   uint64_t out = 0;
 
-  if (upb_FileDef_Syntax(m->file) == kUpb_Syntax_Proto3) {
+  if (UPB_DESC(FeatureSet_repeated_field_encoding(m->resolved_features)) ==
+      UPB_DESC(FeatureSet_PACKED)) {
     out |= kUpb_MessageModifier_DefaultIsPacked;
   }
 
@@ -631,7 +635,8 @@ static upb_StringView* _upb_ReservedNames_New(upb_DefBuilder* ctx, int n,
 }
 
 static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
-                          const UPB_DESC(DescriptorProto) * msg_proto,
+                          const UPB_DESC(DescriptorProto*) msg_proto,
+                          const UPB_DESC(FeatureSet*) parent_features,
                           const upb_MessageDef* containing_type,
                           upb_MessageDef* m) {
   const UPB_DESC(OneofDescriptorProto)* const* oneofs;
@@ -643,6 +648,10 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   size_t n_ext_range, n_res_range, n_res_name;
   upb_StringView name;
 
+  UPB_DEF_SET_OPTIONS(m->opts, DescriptorProto, MessageOptions, msg_proto);
+  m->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(MessageOptions_features)(m->opts));
+
   // Must happen before _upb_DefBuilder_Add()
   m->file = _upb_DefBuilder_File(ctx);
 
@@ -671,14 +680,12 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   ok = upb_strtable_init(&m->jtof, n_field, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
-  UPB_DEF_SET_OPTIONS(m->opts, DescriptorProto, MessageOptions, msg_proto);
-
   m->oneof_count = n_oneof;
-  m->oneofs = _upb_OneofDefs_New(ctx, n_oneof, oneofs, m);
+  m->oneofs = _upb_OneofDefs_New(ctx, n_oneof, oneofs, m->resolved_features, m);
 
   m->field_count = n_field;
-  m->fields =
-      _upb_FieldDefs_New(ctx, n_field, fields, m->full_name, m, &m->is_sorted);
+  m->fields = _upb_FieldDefs_New(ctx, n_field, fields, m->resolved_features,
+                                 m->full_name, m, &m->is_sorted);
 
   // Message Sets may not contain fields.
   if (UPB_UNLIKELY(UPB_DESC(MessageOptions_message_set_wire_format)(m->opts))) {
@@ -688,7 +695,8 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   }
 
   m->ext_range_count = n_ext_range;
-  m->ext_ranges = _upb_ExtensionRanges_New(ctx, n_ext_range, ext_ranges, m);
+  m->ext_ranges = _upb_ExtensionRanges_New(ctx, n_ext_range, ext_ranges,
+                                           m->resolved_features, m);
 
   m->res_range_count = n_res_range;
   m->res_ranges =
@@ -706,23 +714,29 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   const UPB_DESC(EnumDescriptorProto)* const* enums =
       UPB_DESC(DescriptorProto_enum_type)(msg_proto, &n_enum);
   m->nested_enum_count = n_enum;
-  m->nested_enums = _upb_EnumDefs_New(ctx, n_enum, enums, m);
+  m->nested_enums =
+      _upb_EnumDefs_New(ctx, n_enum, enums, m->resolved_features, m);
 
   const UPB_DESC(FieldDescriptorProto)* const* exts =
       UPB_DESC(DescriptorProto_extension)(msg_proto, &n_ext);
   m->nested_ext_count = n_ext;
-  m->nested_exts = _upb_Extensions_New(ctx, n_ext, exts, m->full_name, m);
+  m->nested_exts = _upb_Extensions_New(ctx, n_ext, exts, m->resolved_features,
+                                       m->full_name, m);
 
   const UPB_DESC(DescriptorProto)* const* msgs =
       UPB_DESC(DescriptorProto_nested_type)(msg_proto, &n_msg);
   m->nested_msg_count = n_msg;
-  m->nested_msgs = _upb_MessageDefs_New(ctx, n_msg, msgs, m);
+  m->nested_msgs =
+      _upb_MessageDefs_New(ctx, n_msg, msgs, m->resolved_features, m);
 }
 
 // Allocate and initialize an array of |n| message defs.
-upb_MessageDef* _upb_MessageDefs_New(
-    upb_DefBuilder* ctx, int n, const UPB_DESC(DescriptorProto) * const* protos,
-    const upb_MessageDef* containing_type) {
+upb_MessageDef* _upb_MessageDefs_New(upb_DefBuilder* ctx, int n,
+                                     const UPB_DESC(DescriptorProto*)
+                                         const* protos,
+                                     const UPB_DESC(FeatureSet*)
+                                         parent_features,
+                                     const upb_MessageDef* containing_type) {
   _upb_DefType_CheckPadding(sizeof(upb_MessageDef));
 
   const char* name = containing_type ? containing_type->full_name
@@ -730,7 +744,8 @@ upb_MessageDef* _upb_MessageDefs_New(
 
   upb_MessageDef* m = _upb_DefBuilder_Alloc(ctx, sizeof(upb_MessageDef) * n);
   for (int i = 0; i < n; i++) {
-    create_msgdef(ctx, name, protos[i], containing_type, &m[i]);
+    create_msgdef(ctx, name, protos[i], parent_features, containing_type,
+                  &m[i]);
   }
   return m;
 }
diff --git a/upb/reflection/message_def.h b/upb/reflection/message_def.h
index a5c85efe23c6..1a92ed4349a2 100644
--- a/upb/reflection/message_def.h
+++ b/upb/reflection/message_def.h
@@ -136,6 +136,8 @@ int upb_MessageDef_RealOneofCount(const upb_MessageDef* m);
 
 const UPB_DESC(MessageOptions) *
     upb_MessageDef_Options(const upb_MessageDef* m);
+const UPB_DESC(FeatureSet) *
+    upb_MessageDef_ResolvedFeatures(const upb_MessageDef* m);
 
 upb_StringView upb_MessageDef_ReservedName(const upb_MessageDef* m, int i);
 int upb_MessageDef_ReservedNameCount(const upb_MessageDef* m);
diff --git a/upb/reflection/method_def.c b/upb/reflection/method_def.c
index 049ef4a81d17..ee4e06efff8a 100644
--- a/upb/reflection/method_def.c
+++ b/upb/reflection/method_def.c
@@ -15,7 +15,8 @@
 #include "upb/port/def.inc"
 
 struct upb_MethodDef {
-  const UPB_DESC(MethodOptions) * opts;
+  const UPB_DESC(MethodOptions*) opts;
+  const UPB_DESC(FeatureSet*) resolved_features;
   upb_ServiceDef* service;
   const char* full_name;
   const upb_MessageDef* input_type;
@@ -68,8 +69,14 @@ bool upb_MethodDef_ServerStreaming(const upb_MethodDef* m) {
 }
 
 static void create_method(upb_DefBuilder* ctx,
-                          const UPB_DESC(MethodDescriptorProto) * method_proto,
+                          const UPB_DESC(MethodDescriptorProto*) method_proto,
+                          const UPB_DESC(FeatureSet*) parent_features,
                           upb_ServiceDef* s, upb_MethodDef* m) {
+  UPB_DEF_SET_OPTIONS(m->opts, MethodDescriptorProto, MethodOptions,
+                      method_proto);
+  m->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(MethodOptions_features)(m->opts));
+
   upb_StringView name = UPB_DESC(MethodDescriptorProto_name)(method_proto);
 
   m->service = s;
@@ -87,18 +94,17 @@ static void create_method(upb_DefBuilder* ctx,
       ctx, m->full_name, m->full_name,
       UPB_DESC(MethodDescriptorProto_output_type)(method_proto),
       UPB_DEFTYPE_MSG);
-
-  UPB_DEF_SET_OPTIONS(m->opts, MethodDescriptorProto, MethodOptions,
-                      method_proto);
 }
 
 // Allocate and initialize an array of |n| method defs belonging to |s|.
-upb_MethodDef* _upb_MethodDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(MethodDescriptorProto) * const* protos, upb_ServiceDef* s) {
+upb_MethodDef* _upb_MethodDefs_New(upb_DefBuilder* ctx, int n,
+                                   const UPB_DESC(MethodDescriptorProto*)
+                                       const* protos,
+                                   const UPB_DESC(FeatureSet*) parent_features,
+                                   upb_ServiceDef* s) {
   upb_MethodDef* m = _upb_DefBuilder_Alloc(ctx, sizeof(upb_MethodDef) * n);
   for (int i = 0; i < n; i++) {
-    create_method(ctx, protos[i], s, &m[i]);
+    create_method(ctx, protos[i], parent_features, s, &m[i]);
     m[i].index = i;
   }
   return m;
diff --git a/upb/reflection/method_def.h b/upb/reflection/method_def.h
index 64b88a66832e..4abfb69e0f60 100644
--- a/upb/reflection/method_def.h
+++ b/upb/reflection/method_def.h
@@ -26,6 +26,8 @@ int upb_MethodDef_Index(const upb_MethodDef* m);
 const upb_MessageDef* upb_MethodDef_InputType(const upb_MethodDef* m);
 const char* upb_MethodDef_Name(const upb_MethodDef* m);
 const UPB_DESC(MethodOptions) * upb_MethodDef_Options(const upb_MethodDef* m);
+const UPB_DESC(FeatureSet) *
+    upb_MethodDef_ResolvedFeatures(const upb_MethodDef* m);
 const upb_MessageDef* upb_MethodDef_OutputType(const upb_MethodDef* m);
 bool upb_MethodDef_ServerStreaming(const upb_MethodDef* m);
 const upb_ServiceDef* upb_MethodDef_Service(const upb_MethodDef* m);
diff --git a/upb/reflection/oneof_def.c b/upb/reflection/oneof_def.c
index 869ca16b96c4..04ce27dc4456 100644
--- a/upb/reflection/oneof_def.c
+++ b/upb/reflection/oneof_def.c
@@ -23,6 +23,7 @@
 
 struct upb_OneofDef {
   const UPB_DESC(OneofOptions) * opts;
+  const UPB_DESC(FeatureSet) * resolved_features;
   const upb_MessageDef* parent;
   const char* full_name;
   int field_count;
@@ -30,9 +31,6 @@ struct upb_OneofDef {
   const upb_FieldDef** fields;
   upb_strtable ntof;  // lookup a field by name
   upb_inttable itof;  // lookup a field by number (index)
-#if UINTPTR_MAX == 0xffffffff
-  uint32_t padding;  // Increase size to a multiple of 8.
-#endif
 };
 
 upb_OneofDef* _upb_OneofDef_At(const upb_OneofDef* o, int i) {
@@ -166,9 +164,15 @@ size_t _upb_OneofDefs_Finalize(upb_DefBuilder* ctx, upb_MessageDef* m) {
 }
 
 static void create_oneofdef(upb_DefBuilder* ctx, upb_MessageDef* m,
-                            const UPB_DESC(OneofDescriptorProto) * oneof_proto,
+                            const UPB_DESC(OneofDescriptorProto*) oneof_proto,
+                            const UPB_DESC(FeatureSet*) parent_features,
                             const upb_OneofDef* _o) {
   upb_OneofDef* o = (upb_OneofDef*)_o;
+
+  UPB_DEF_SET_OPTIONS(o->opts, OneofDescriptorProto, OneofOptions, oneof_proto);
+  o->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(OneofOptions_features)(o->opts));
+
   upb_StringView name = UPB_DESC(OneofDescriptorProto_name)(oneof_proto);
 
   o->parent = m;
@@ -177,8 +181,6 @@ static void create_oneofdef(upb_DefBuilder* ctx, upb_MessageDef* m,
   o->field_count = 0;
   o->synthetic = false;
 
-  UPB_DEF_SET_OPTIONS(o->opts, OneofDescriptorProto, OneofOptions, oneof_proto);
-
   if (upb_MessageDef_FindByNameWithSize(m, name.data, name.size, NULL, NULL)) {
     _upb_DefBuilder_Errf(ctx, "duplicate oneof name (%s)", o->full_name);
   }
@@ -195,14 +197,16 @@ static void create_oneofdef(upb_DefBuilder* ctx, upb_MessageDef* m,
 }
 
 // Allocate and initialize an array of |n| oneof defs.
-upb_OneofDef* _upb_OneofDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(OneofDescriptorProto) * const* protos, upb_MessageDef* m) {
+upb_OneofDef* _upb_OneofDefs_New(upb_DefBuilder* ctx, int n,
+                                 const UPB_DESC(OneofDescriptorProto*)
+                                     const* protos,
+                                 const UPB_DESC(FeatureSet*) parent_features,
+                                 upb_MessageDef* m) {
   _upb_DefType_CheckPadding(sizeof(upb_OneofDef));
 
   upb_OneofDef* o = _upb_DefBuilder_Alloc(ctx, sizeof(upb_OneofDef) * n);
   for (int i = 0; i < n; i++) {
-    create_oneofdef(ctx, m, protos[i], &o[i]);
+    create_oneofdef(ctx, m, protos[i], parent_features, &o[i]);
   }
   return o;
 }
diff --git a/upb/reflection/oneof_def.h b/upb/reflection/oneof_def.h
index 38c7a7002f32..1a62f3dc8b7b 100644
--- a/upb/reflection/oneof_def.h
+++ b/upb/reflection/oneof_def.h
@@ -36,7 +36,9 @@ const upb_FieldDef* upb_OneofDef_LookupNumber(const upb_OneofDef* o,
                                               uint32_t num);
 UPB_API const char* upb_OneofDef_Name(const upb_OneofDef* o);
 int upb_OneofDef_numfields(const upb_OneofDef* o);
-const UPB_DESC(OneofOptions) * upb_OneofDef_Options(const upb_OneofDef* o);
+const UPB_DESC(OneofOptions*) upb_OneofDef_Options(const upb_OneofDef* o);
+const UPB_DESC(FeatureSet*)
+    upb_OneofDef_ResolvedFeatures(const upb_OneofDef* o);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/reflection/service_def.c b/upb/reflection/service_def.c
index 7a0cbc7a09b8..f1d1eee0c0a9 100644
--- a/upb/reflection/service_def.c
+++ b/upb/reflection/service_def.c
@@ -17,11 +17,15 @@
 
 struct upb_ServiceDef {
   const UPB_DESC(ServiceOptions) * opts;
+  const UPB_DESC(FeatureSet) * resolved_features;
   const upb_FileDef* file;
   const char* full_name;
   upb_MethodDef* methods;
   int method_count;
   int index;
+#if UINTPTR_MAX == 0xffffffff
+  uint32_t padding;  // Increase size to a multiple of 8.
+#endif
 };
 
 upb_ServiceDef* _upb_ServiceDef_At(const upb_ServiceDef* s, int index) {
@@ -72,37 +76,40 @@ const upb_MethodDef* upb_ServiceDef_FindMethodByName(const upb_ServiceDef* s,
 }
 
 static void create_service(upb_DefBuilder* ctx,
-                           const UPB_DESC(ServiceDescriptorProto) * svc_proto,
+                           const UPB_DESC(ServiceDescriptorProto*) svc_proto,
+                           const UPB_DESC(FeatureSet*) parent_features,
                            upb_ServiceDef* s) {
-  upb_StringView name;
-  size_t n;
+  UPB_DEF_SET_OPTIONS(s->opts, ServiceDescriptorProto, ServiceOptions,
+                      svc_proto);
+  s->resolved_features = _upb_DefBuilder_ResolveFeatures(
+      ctx, parent_features, UPB_DESC(ServiceOptions_features)(s->opts));
 
   // Must happen before _upb_DefBuilder_Add()
   s->file = _upb_DefBuilder_File(ctx);
 
-  name = UPB_DESC(ServiceDescriptorProto_name)(svc_proto);
+  upb_StringView name = UPB_DESC(ServiceDescriptorProto_name)(svc_proto);
   const char* package = _upb_FileDef_RawPackage(s->file);
   s->full_name = _upb_DefBuilder_MakeFullName(ctx, package, name);
   _upb_DefBuilder_Add(ctx, s->full_name,
                       _upb_DefType_Pack(s, UPB_DEFTYPE_SERVICE));
 
+  size_t n;
   const UPB_DESC(MethodDescriptorProto)* const* methods =
       UPB_DESC(ServiceDescriptorProto_method)(svc_proto, &n);
   s->method_count = n;
-  s->methods = _upb_MethodDefs_New(ctx, n, methods, s);
-
-  UPB_DEF_SET_OPTIONS(s->opts, ServiceDescriptorProto, ServiceOptions,
-                      svc_proto);
+  s->methods = _upb_MethodDefs_New(ctx, n, methods, s->resolved_features, s);
 }
 
-upb_ServiceDef* _upb_ServiceDefs_New(
-    upb_DefBuilder* ctx, int n,
-    const UPB_DESC(ServiceDescriptorProto) * const* protos) {
+upb_ServiceDef* _upb_ServiceDefs_New(upb_DefBuilder* ctx, int n,
+                                     const UPB_DESC(ServiceDescriptorProto*)
+                                         const* protos,
+                                     const UPB_DESC(FeatureSet*)
+                                         parent_features) {
   _upb_DefType_CheckPadding(sizeof(upb_ServiceDef));
 
   upb_ServiceDef* s = _upb_DefBuilder_Alloc(ctx, sizeof(upb_ServiceDef) * n);
   for (int i = 0; i < n; i++) {
-    create_service(ctx, protos[i], &s[i]);
+    create_service(ctx, protos[i], parent_features, &s[i]);
     s[i].index = i;
   }
   return s;
diff --git a/upb/reflection/service_def.h b/upb/reflection/service_def.h
index 98cd993b4d72..781932e176fc 100644
--- a/upb/reflection/service_def.h
+++ b/upb/reflection/service_def.h
@@ -30,6 +30,8 @@ int upb_ServiceDef_MethodCount(const upb_ServiceDef* s);
 const char* upb_ServiceDef_Name(const upb_ServiceDef* s);
 const UPB_DESC(ServiceOptions) *
     upb_ServiceDef_Options(const upb_ServiceDef* s);
+const UPB_DESC(FeatureSet) *
+    upb_ServiceDef_ResolvedFeatures(const upb_ServiceDef* s);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/util/required_fields.c b/upb/util/required_fields.c
index c6616ecdd3f7..b93adc0d9958 100644
--- a/upb/util/required_fields.c
+++ b/upb/util/required_fields.c
@@ -196,7 +196,7 @@ static void upb_util_FindUnsetInMessage(upb_FindContext* ctx,
   // Iterate over all fields to see if any required fields are missing.
   for (int i = 0, n = upb_MessageDef_FieldCount(m); i < n; i++) {
     const upb_FieldDef* f = upb_MessageDef_Field(m, i);
-    if (upb_FieldDef_Label(f) != kUpb_Label_Required) continue;
+    if (!upb_FieldDef_IsRequired(f)) continue;
 
     if (!msg || !upb_Message_HasFieldByDef(msg, f)) {
       // A required field is missing.