diff --git a/tests/bindings/lua/test_upb.lua b/tests/bindings/lua/test_upb.lua
index 65439bcc5f..1dc07178f4 100644
--- a/tests/bindings/lua/test_upb.lua
+++ b/tests/bindings/lua/test_upb.lua
@@ -401,53 +401,6 @@ function test_symtab()
   local msgdef3 = symtab:lookup("ContainingMessage2")
   assert_not_nil(msgdef3)
   assert_equal(msgdef3:field("field5"):subdef(), msgdef2)
-
-  -- Freeze the symtab and verify that mutating operations are not allowed.
-  assert_false(symtab:is_frozen())
-  symtab:freeze()
-  assert_true(symtab:is_frozen())
-  assert_error_match("frozen", function() symtab:freeze() end)
-  assert_error_match("frozen", function()
-    symtab:add{
-      upb.MessageDef{full_name = "Foo"}
-    }
-  end)
-end
-
-function test_symtab_add_extension()
-  -- Adding an extension at the same time as the extendee.
-  local symtab = upb.SymbolTable{
-    upb.MessageDef{full_name = "M1"},
-    upb.FieldDef{name = "extension1", is_extension = true, number = 1,
-                 type = upb.TYPE_INT32, containing_type_name = "M1"}
-  }
-
-  local m1 = symtab:lookup("M1")
-  assert_not_nil(m1)
-  assert_equal(1, #m1)
-
-  local f1 = m1:field("extension1")
-  assert_not_nil(f1)
-  assert_true(f1:is_extension())
-  assert_true(f1:is_frozen())
-  assert_equal(1, f1:number())
-
-  -- Adding an extension to an existing extendee.
-  symtab:add{
-    upb.FieldDef{name = "extension2", is_extension = true, number = 2,
-                 type = upb.TYPE_INT32, containing_type_name = "M1"}
-  }
-
-  local m1_2 = symtab:lookup("M1")
-  assert_not_nil(m1_2)
-  assert_true(m1 ~= m1_2)
-  assert_equal(2, #m1_2)
-
-  local f2 = m1_2:field("extension2")
-  assert_not_nil(f2)
-  assert_true(f2:is_extension())
-  assert_true(f2:is_frozen())
-  assert_equal(2, f2:number())
 end
 
 function test_numeric_array()
diff --git a/tests/test_cpp.cc b/tests/test_cpp.cc
index 220e69f81f..2cd9802281 100644
--- a/tests/test_cpp.cc
+++ b/tests/test_cpp.cc
@@ -154,15 +154,12 @@ static void TestSymbolTable(const char *descriptor_file) {
     exit(1);
   }
 
-  upb::reffed_ptr<upb::SymbolTable> s(upb::SymbolTable::New());
+  upb::SymbolTable* s = upb::SymbolTable::New();
 
   for (size_t i = 0; i < files.size(); i++) {
     ASSERT(s->AddFile(files[i].get(), &status));
   }
 
-  ASSERT(!s->IsFrozen());
-  s->Freeze();
-  ASSERT(s->IsFrozen());
   upb::reffed_ptr<const upb::MessageDef> md(s->LookupMessage("C"));
   ASSERT(md.get());
 
@@ -180,6 +177,7 @@ static void TestSymbolTable(const char *descriptor_file) {
 #endif
 
   ASSERT(md.get());
+  upb::SymbolTable::Free(s);
 }
 
 static void TestCasts1() {
diff --git a/tests/test_def.c b/tests/test_def.c
index 93622c166c..d82fddb347 100644
--- a/tests/test_def.c
+++ b/tests/test_def.c
@@ -13,13 +13,13 @@
 const char *descriptor_file;
 
 static void test_empty_symtab() {
-  upb_symtab *s = upb_symtab_new(&s);
+  upb_symtab *s = upb_symtab_new();
   upb_symtab_iter i;
   for (upb_symtab_begin(&i, s, UPB_DEF_ANY); !upb_symtab_done(&i);
        upb_symtab_next(&i)) {
     ASSERT(false);  /* Should not get here. */
   }
-  upb_symtab_unref(s, &s);
+  upb_symtab_free(s);
 }
 
 static void test_noreftracking() {
@@ -39,8 +39,8 @@ static void test_noreftracking() {
   upb_msgdef_unref(md, &md);
 }
 
-static upb_symtab *load_test_proto(void *owner) {
-  upb_symtab *s = upb_symtab_new(owner);
+static upb_symtab *load_test_proto() {
+  upb_symtab *s = upb_symtab_new();
   upb_status status = UPB_STATUS_INIT;
   size_t len;
   char *data = upb_readfile(descriptor_file, &len);
@@ -64,15 +64,12 @@ static upb_symtab *load_test_proto(void *owner) {
 
   upb_gfree(files);
 
-  ASSERT(!upb_symtab_isfrozen(s));
-  upb_symtab_freeze(s);
-  ASSERT(upb_symtab_isfrozen(s));
   return s;
 }
 
 static void test_cycles() {
   bool ok;
-  upb_symtab *s = load_test_proto(&s);
+  upb_symtab *s = load_test_proto();
   const upb_msgdef *m;
   const upb_fielddef *f;
   const upb_def *def;
@@ -84,7 +81,7 @@ static void test_cycles() {
   upb_def_ref(def, &def);
   ASSERT(def);
   ASSERT(upb_def_isfrozen(def));
-  upb_symtab_unref(s, &s);
+  upb_symtab_free(s);
 
   /* Message A has only one subfield: "optional B b = 1". */
   m = upb_downcast_msgdef(def);
@@ -162,18 +159,18 @@ static void test_symbol_resolution() {
   upb_msgdef_unref(m3, &m3);
   upb_msgdef_unref(m2, &m2);
   upb_msgdef_unref(m1, &m1);
-  upb_symtab_unref(symtab, &symtab);
+  upb_symtab_free(symtab);
 }
 
 static void test_fielddef_unref() {
   bool ok;
-  upb_symtab *s = load_test_proto(&s);
+  upb_symtab *s = load_test_proto();
   const upb_msgdef *md = upb_symtab_lookupmsg(s, "A");
   const upb_fielddef *f = upb_msgdef_itof(md, 1);
   upb_fielddef_ref(f, &f);
 
   /* Unref symtab; now fielddef is the only thing keeping the msgdef alive. */
-  upb_symtab_unref(s, &s);
+  upb_symtab_free(s);
   /* Check that md is still alive. */
   ok = strcmp(upb_msgdef_fullname(md), "A") == 0;
   ASSERT(ok);
@@ -210,66 +207,30 @@ static upb_msgdef *upb_msgdef_newnamed(const char *name, void *owner) {
   return m;
 }
 
-static upb_enumdef *upb_enumdef_newnamed(const char *name, void *owner) {
-  upb_enumdef *e = upb_enumdef_new(owner);
-  upb_enumdef_setfullname(e, name, NULL);
-  return e;
-}
-
-static void test_replacement() {
+static void test_replacement_fails() {
+  bool ok;
   upb_symtab *s = upb_symtab_new(&s);
-  upb_enumdef *e2;
-  upb_msgdef *m2;
-  upb_enumdef *e;
   upb_status status = UPB_STATUS_INIT;
-  upb_def *newdefs[3];
-  upb_def *newdefs2[1];
-  const upb_msgdef *m3;
+  upb_def *newdefs[2];
 
   upb_msgdef *m = upb_msgdef_newnamed("MyMessage", &s);
-  upb_msgdef_addfield(m, newfield("field1", 1, UPB_TYPE_ENUM,
-                                  UPB_LABEL_OPTIONAL, ".MyEnum", &s),
-                      &s, NULL);
-  m2 = upb_msgdef_newnamed("MyMessage2", &s);
-  e = upb_enumdef_newnamed("MyEnum", &s);
-  ASSERT_STATUS(upb_enumdef_addval(e, "VAL1", 1, &status), &status);
+  upb_msgdef *m2 = upb_msgdef_newnamed("MyMessage", &s);
 
   newdefs[0] = upb_msgdef_upcast_mutable(m);
   newdefs[1] = upb_msgdef_upcast_mutable(m2);
-  newdefs[2] = upb_enumdef_upcast_mutable(e);
-  ASSERT_STATUS(upb_symtab_add(s, newdefs, 3, &s, &status), &status);
-
-  /* Try adding a new definition of MyEnum, MyMessage should get replaced with
-   * a new version. */
-  e2 = upb_enumdef_newnamed("MyEnum", &s);
-  ASSERT_STATUS(upb_enumdef_addval(e2, "VAL1", 1, &status), &status);
-  newdefs2[0] = upb_enumdef_upcast_mutable(e2);
-  ASSERT_STATUS(upb_symtab_add(s, newdefs2, 1, &s, &status), &status);
-
-  m3 = upb_symtab_lookupmsg(s, "MyMessage");
-  ASSERT(m3);
-  /* Must be different because it points to MyEnum which was replaced. */
-  ASSERT(m3 != m);
+  ok = upb_symtab_add(s, newdefs, 2, &s, &status);
+  ASSERT(ok == false);
+  upb_status_clear(&status);
 
-  m3 = upb_symtab_lookupmsg(s, "MyMessage2");
-  /* Should be the same because it was not replaced, nor were any defs that
-   * are reachable from it. */
-  ASSERT(m3 == m2);
+  /* Adding just one is ok. */
+  ASSERT_STATUS(upb_symtab_add(s, newdefs, 1, &s, &status), &status);
 
-  upb_symtab_unref(s, &s);
-}
+  /* Adding a conflicting one is not ok. */
+  newdefs[0] = upb_msgdef_upcast_mutable(m2);
+  ok = upb_symtab_add(s, newdefs, 1, &s, &status);
+  ASSERT(ok == false);
 
-static void test_cycles_in_replacement() {
-  upb_symtab *s = upb_symtab_new(&s);
-  upb_msgdef *m = upb_msgdef_newnamed("M", &s);
-  upb_status status = UPB_STATUS_INIT;
-
-  upb_msgdef_addfield(m, newfield("m", 1, UPB_TYPE_MESSAGE,
-                                  UPB_LABEL_OPTIONAL, ".M", &s),
-                      &s, NULL);
-  ASSERT_STATUS(upb_symtab_add(s, (upb_def**)&m, 1, &s, &status), &status);
-  ASSERT_STATUS(upb_symtab_add(s, NULL, 0, &s, &status), &status);
-  upb_symtab_unref(s, &s);
+  upb_symtab_free(s);
 }
 
 static void test_freeze_free() {
@@ -368,7 +329,6 @@ static void test_partial_freeze() {
 
 static void test_descriptor_flags() {
   upb_msgdef *m = upb_msgdef_new(&m);
-  upb_msgdef *m2;
   upb_status s = UPB_STATUS_INIT;
 
   ASSERT(upb_msgdef_mapentry(m) == false);
@@ -376,10 +336,7 @@ static void test_descriptor_flags() {
   ASSERT(upb_ok(&s));
   upb_msgdef_setmapentry(m, true);
   ASSERT(upb_msgdef_mapentry(m) == true);
-  m2 = upb_msgdef_dup(m, &m2);
-  ASSERT(upb_msgdef_mapentry(m2) == true);
   upb_msgdef_unref(m, &m);
-  upb_msgdef_unref(m2, &m2);
 }
 
 static void test_mapentry_check() {
@@ -413,7 +370,7 @@ static void test_mapentry_check() {
   upb_symtab_add(symtab, defs, 2, NULL, &s);
   ASSERT(upb_ok(&s));
 
-  upb_symtab_unref(symtab, &symtab);
+  upb_symtab_free(symtab);
   upb_msgdef_unref(subm, &subm);
   upb_msgdef_unref(m, &m);
 }
@@ -470,7 +427,7 @@ static void test_oneofs() {
   lookup_field = upb_oneofdef_ntofz(o, "field1");
   ASSERT(lookup_field != NULL && upb_fielddef_number(lookup_field) == 1);
 
-  upb_symtab_unref(symtab, &symtab);
+  upb_symtab_free(symtab);
   upb_oneofdef_unref(o, &o);
 }
 
@@ -485,8 +442,7 @@ int run_tests(int argc, char *argv[]) {
   test_symbol_resolution();
   test_fielddef();
   test_fielddef_unref();
-  test_replacement();
-  test_cycles_in_replacement();
+  test_replacement_fails();
   test_freeze_free();
   test_partial_freeze();
   test_noreftracking();
diff --git a/upb/bindings/lua/def.c b/upb/bindings/lua/def.c
index 638abb6b15..5133831e8d 100644
--- a/upb/bindings/lua/def.c
+++ b/upb/bindings/lua/def.c
@@ -1090,50 +1090,35 @@ static const struct luaL_Reg lupb_filedef_m[] = {
 
 /* lupb_symtab ****************************************************************/
 
-/* Inherits a ref on the symtab.
- * Checks that narg is a proper lupb_symtab object.  If it is, leaves its
- * metatable on the stack for cache lookups/updates. */
-const upb_symtab *lupb_symtab_check(lua_State *L, int narg) {
-  return lupb_refcounted_check(L, narg, LUPB_SYMTAB);
-}
-
-static upb_symtab *lupb_symtab_checkmutable(lua_State *L, int narg) {
-  const upb_symtab *s = lupb_symtab_check(L, narg);
-  if (upb_symtab_isfrozen(s))
-    luaL_error(L, "not allowed on frozen value");
-  return (upb_symtab*)s;
-}
-
-void lupb_symtab_pushwrapper(lua_State *L, const upb_symtab *s,
-                             const void *ref_donor) {
-  lupb_refcounted_pushwrapper(L, upb_symtab_upcast(s), LUPB_SYMTAB, ref_donor,
-                              sizeof(void *));
-}
+typedef struct {
+  upb_symtab *symtab;
+} lupb_symtab;
 
-void lupb_symtab_pushnewrapper(lua_State *L, const upb_symtab *s,
-                               const void *ref_donor) {
-  lupb_refcounted_pushnewrapper(L, upb_symtab_upcast(s), LUPB_SYMTAB,
-                                ref_donor);
+upb_symtab *lupb_symtab_check(lua_State *L, int narg) {
+  lupb_symtab *lsymtab = luaL_checkudata(L, narg, LUPB_SYMTAB);
+  if (!lsymtab->symtab) {
+    luaL_error(L, "called into dead object");
+  }
+  return lsymtab->symtab;
 }
 
 static int lupb_symtab_new(lua_State *L) {
-  upb_symtab *s = upb_symtab_new(&s);
-  lupb_symtab_pushnewrapper(L, s, &s);
+  lupb_symtab *lsymtab = lua_newuserdata(L, sizeof(*lsymtab));
+  lsymtab->symtab = upb_symtab_new();
+  luaL_getmetatable(L, LUPB_SYMTAB);
+  lua_setmetatable(L, -2);
   return 1;
 }
 
-static int lupb_symtab_freeze(lua_State *L) {
-  upb_symtab_freeze(lupb_symtab_checkmutable(L, 1));
+static int lupb_symtab_gc(lua_State *L) {
+  lupb_symtab *lsymtab = luaL_checkudata(L, 1, LUPB_SYMTAB);
+  upb_symtab_free(lsymtab->symtab);
+  lsymtab->symtab = NULL;
   return 0;
 }
 
-static int lupb_symtab_isfrozen(lua_State *L) {
-  lua_pushboolean(L, upb_symtab_isfrozen(lupb_symtab_check(L, 1)));
-  return 1;
-}
-
 static int lupb_symtab_add(lua_State *L) {
-  upb_symtab *s = lupb_symtab_checkmutable(L, 1);
+  upb_symtab *s = lupb_symtab_check(L, 1);
   int n;
   upb_def **defs;
 
@@ -1161,7 +1146,7 @@ static int lupb_symtab_add(lua_State *L) {
 }
 
 static int lupb_symtab_addfile(lua_State *L) {
-  upb_symtab *s = lupb_symtab_checkmutable(L, 1);
+  upb_symtab *s = lupb_symtab_check(L, 1);
   upb_filedef *f = lupb_filedef_checkmutable(L, 2);
   CHK(upb_symtab_addfile(s, f, &status));
   return 0;
@@ -1201,14 +1186,12 @@ static const struct luaL_Reg lupb_symtab_m[] = {
   {"add", lupb_symtab_add},
   {"add_file", lupb_symtab_addfile},
   {"defs", lupb_symtab_defs},
-  {"freeze", lupb_symtab_freeze},
-  {"is_frozen", lupb_symtab_isfrozen},
   {"lookup", lupb_symtab_lookup},
   {NULL, NULL}
 };
 
 static const struct luaL_Reg lupb_symtab_mm[] = {
-  {"__gc", lupb_refcounted_gc},
+  {"__gc", lupb_symtab_gc},
   {NULL, NULL}
 };
 
diff --git a/upb/bindings/lua/upb.h b/upb/bindings/lua/upb.h
index 84212c5370..88a201cee8 100644
--- a/upb/bindings/lua/upb.h
+++ b/upb/bindings/lua/upb.h
@@ -101,7 +101,7 @@ void *lupb_refcounted_check(lua_State *L, int narg, const char *type);
 const upb_msgdef *lupb_msgdef_check(lua_State *L, int narg);
 const upb_enumdef *lupb_enumdef_check(lua_State *L, int narg);
 const upb_fielddef *lupb_fielddef_check(lua_State *L, int narg);
-const upb_symtab *lupb_symtab_check(lua_State *L, int narg);
+upb_symtab *lupb_symtab_check(lua_State *L, int narg);
 
 void lupb_refcounted_pushnewrapper(lua_State *L, const upb_refcounted *obj,
                                    const char *type, const void *ref_donor);
diff --git a/upb/def.c b/upb/def.c
index 45cd362509..5fe9e142bc 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -122,22 +122,6 @@ bool upb_def_setfullname(upb_def *def, const char *fullname, upb_status *s) {
 
 const upb_filedef *upb_def_file(const upb_def *d) { return d->file; }
 
-upb_def *upb_def_dup(const upb_def *def, const void *o) {
-  switch (def->type) {
-    case UPB_DEF_MSG:
-      return upb_msgdef_upcast_mutable(
-          upb_msgdef_dup(upb_downcast_msgdef(def), o));
-    case UPB_DEF_FIELD:
-      return upb_fielddef_upcast_mutable(
-          upb_fielddef_dup(upb_downcast_fielddef(def), o));
-    case UPB_DEF_ENUM:
-      return upb_enumdef_upcast_mutable(
-          upb_enumdef_dup(upb_downcast_enumdef(def), o));
-    default:
-      UPB_UNREACHABLE();
-  }
-}
-
 static bool upb_def_init(upb_def *def, upb_deftype_t type,
                          const struct upb_refcounted_vtbl *vtbl,
                          const void *owner) {
@@ -394,13 +378,14 @@ bool _upb_def_validate(upb_def *const*defs, size_t n, upb_status *s) {
     } else if (def->type == UPB_DEF_FIELD) {
       upb_status_seterrmsg(s, "standalone fielddefs can not be frozen");
       goto err;
-    } else if (def->type == UPB_DEF_ENUM) {
-      if (!upb_validate_enumdef(upb_dyncast_enumdef(def), s)) {
-        goto err;
-      }
     } else {
       /* Set now to detect transitive closure in the second pass. */
       def->came_from_user = true;
+
+      if (def->type == UPB_DEF_ENUM &&
+          !upb_validate_enumdef(upb_dyncast_enumdef(def), s)) {
+        goto err;
+      }
     }
   }
 
@@ -493,21 +478,6 @@ err2:
   return NULL;
 }
 
-upb_enumdef *upb_enumdef_dup(const upb_enumdef *e, const void *owner) {
-  upb_enum_iter i;
-  upb_enumdef *new_e = upb_enumdef_new(owner);
-  if (!new_e) return NULL;
-  for(upb_enum_begin(&i, e); !upb_enum_done(&i); upb_enum_next(&i)) {
-    bool success = upb_enumdef_addval(
-        new_e, upb_enum_iter_name(&i),upb_enum_iter_number(&i), NULL);
-    if (!success) {
-      upb_enumdef_unref(new_e, owner);
-      return NULL;
-    }
-  }
-  return new_e;
-}
-
 bool upb_enumdef_freeze(upb_enumdef *e, upb_status *status) {
   upb_def *d = upb_enumdef_upcast_mutable(e);
   return upb_def_freeze(&d, 1, status);
@@ -742,42 +712,6 @@ upb_fielddef *upb_fielddef_new(const void *o) {
   return f;
 }
 
-upb_fielddef *upb_fielddef_dup(const upb_fielddef *f, const void *owner) {
-  const char *srcname;
-  upb_fielddef *newf = upb_fielddef_new(owner);
-  if (!newf) return NULL;
-  upb_fielddef_settype(newf, upb_fielddef_type(f));
-  upb_fielddef_setlabel(newf, upb_fielddef_label(f));
-  upb_fielddef_setnumber(newf, upb_fielddef_number(f), NULL);
-  upb_fielddef_setname(newf, upb_fielddef_name(f), NULL);
-  if (f->default_is_string && f->defaultval.bytes) {
-    str_t *s = f->defaultval.bytes;
-    upb_fielddef_setdefaultstr(newf, s->str, s->len, NULL);
-  } else {
-    newf->default_is_string = f->default_is_string;
-    newf->defaultval = f->defaultval;
-  }
-
-  if (f->subdef_is_symbolic) {
-    srcname = f->sub.name;  /* Might be NULL. */
-  } else {
-    srcname = f->sub.def ? upb_def_fullname(f->sub.def) : NULL;
-  }
-  if (srcname) {
-    char *newname = upb_gmalloc(strlen(f->sub.def->fullname) + 2);
-    if (!newname) {
-      upb_fielddef_unref(newf, owner);
-      return NULL;
-    }
-    strcpy(newname, ".");
-    strcat(newname, f->sub.def->fullname);
-    upb_fielddef_setsubdefname(newf, newname, NULL);
-    upb_gfree(newname);
-  }
-
-  return newf;
-}
-
 bool upb_fielddef_typeisset(const upb_fielddef *f) {
   return f->type_is_set_;
 }
@@ -1457,42 +1391,6 @@ err2:
   return NULL;
 }
 
-upb_msgdef *upb_msgdef_dup(const upb_msgdef *m, const void *owner) {
-  bool ok;
-  upb_msg_field_iter i;
-  upb_msg_oneof_iter o;
-
-  upb_msgdef *newm = upb_msgdef_new(owner);
-  if (!newm) return NULL;
-  ok = upb_def_setfullname(upb_msgdef_upcast_mutable(newm),
-                           upb_def_fullname(upb_msgdef_upcast(m)),
-                           NULL);
-  newm->map_entry = m->map_entry;
-  newm->syntax = m->syntax;
-  UPB_ASSERT(ok);
-  for(upb_msg_field_begin(&i, m);
-      !upb_msg_field_done(&i);
-      upb_msg_field_next(&i)) {
-    upb_fielddef *f = upb_fielddef_dup(upb_msg_iter_field(&i), &f);
-    /* Fields in oneofs are dup'd below. */
-    if (upb_fielddef_containingoneof(f)) continue;
-    if (!f || !upb_msgdef_addfield(newm, f, &f, NULL)) {
-      upb_msgdef_unref(newm, owner);
-      return NULL;
-    }
-  }
-  for(upb_msg_oneof_begin(&o, m);
-      !upb_msg_oneof_done(&o);
-      upb_msg_oneof_next(&o)) {
-    upb_oneofdef *f = upb_oneofdef_dup(upb_msg_iter_oneof(&o), &f);
-    if (!f || !upb_msgdef_addoneof(newm, f, &f, NULL)) {
-      upb_msgdef_unref(newm, owner);
-      return NULL;
-    }
-  }
-  return newm;
-}
-
 bool upb_msgdef_freeze(upb_msgdef *m, upb_status *status) {
   upb_def *d = upb_msgdef_upcast_mutable(m);
   return upb_def_freeze(&d, 1, status);
@@ -1793,23 +1691,6 @@ err2:
   return NULL;
 }
 
-upb_oneofdef *upb_oneofdef_dup(const upb_oneofdef *o, const void *owner) {
-  bool ok;
-  upb_oneof_iter i;
-  upb_oneofdef *newo = upb_oneofdef_new(owner);
-  if (!newo) return NULL;
-  ok = upb_oneofdef_setname(newo, upb_oneofdef_name(o), NULL);
-  UPB_ASSERT(ok);
-  for (upb_oneof_begin(&i, o); !upb_oneof_done(&i); upb_oneof_next(&i)) {
-    upb_fielddef *f = upb_fielddef_dup(upb_oneof_iter_field(&i), &f);
-    if (!f || !upb_oneofdef_addfield(newo, f, &f, NULL)) {
-      upb_oneofdef_unref(newo, owner);
-      return NULL;
-    }
-  }
-  return newo;
-}
-
 const char *upb_oneofdef_name(const upb_oneofdef *o) { return o->name; }
 
 bool upb_oneofdef_setname(upb_oneofdef *o, const char *name, upb_status *s) {
@@ -2142,8 +2023,7 @@ bool upb_filedef_adddep(upb_filedef *f, const upb_filedef *dep) {
   }
 }
 
-static void upb_symtab_free(upb_refcounted *r) {
-  upb_symtab *s = (upb_symtab*)r;
+void upb_symtab_free(upb_symtab *s) {
   upb_strtable_iter i;
   upb_strtable_begin(&i, &s->symtab);
   for (; !upb_strtable_done(&i); upb_strtable_next(&i)) {
@@ -2154,32 +2034,16 @@ static void upb_symtab_free(upb_refcounted *r) {
   upb_gfree(s);
 }
 
-upb_symtab *upb_symtab_new(const void *owner) {
-  static const struct upb_refcounted_vtbl vtbl = {NULL, &upb_symtab_free};
-
+upb_symtab *upb_symtab_new() {
   upb_symtab *s = upb_gmalloc(sizeof(*s));
   if (!s) {
     return NULL;
   }
 
-  upb_refcounted_init(upb_symtab_upcast_mutable(s), &vtbl, owner);
   upb_strtable_init(&s->symtab, UPB_CTYPE_PTR);
   return s;
 }
 
-void upb_symtab_freeze(upb_symtab *s) {
-  upb_refcounted *r;
-  bool ok;
-
-  UPB_ASSERT(!upb_symtab_isfrozen(s));
-  r = upb_symtab_upcast_mutable(s);
-  /* The symtab does not take ref2's (see refcounted.h) on the defs, because
-   * defs cannot refer back to the table and therefore cannot create cycles.  So
-   * 0 will suffice for maxdepth here. */
-  ok = upb_refcounted_freeze(&r, 1, NULL, 0);
-  UPB_ASSERT(ok);
-}
-
 const upb_def *upb_symtab_lookup(const upb_symtab *s, const char *sym) {
   upb_value v;
   upb_def *ret = upb_strtable_lookup(&s->symtab, sym, &v) ?
@@ -2226,115 +2090,7 @@ const upb_def *upb_symtab_resolve(const upb_symtab *s, const char *base,
   return ret;
 }
 
-/* Starts a depth-first traversal at "def", recursing into any subdefs
- * (ie. submessage types).  Adds duplicates of existing defs to addtab
- * wherever necessary, so that the resulting symtab will be consistent once
- * addtab is added.
- *
- * More specifically, if any def D is found in the DFS that:
- *
- *   1. can reach a def that is being replaced by something in addtab, AND
- *
- *   2. is not itself being replaced already (ie. this name doesn't already
- *      exist in addtab)
- *
- * ...then a duplicate (new copy) of D will be added to addtab.
- *
- * Returns true if this happened for any def reachable from "def."
- *
- * It is slightly tricky to do this correctly in the presence of cycles.  If we
- * detect that our DFS has hit a cycle, we might not yet know if any SCCs on
- * our stack can reach a def in addtab or not.  Once we figure this out, that
- * answer needs to apply to *all* defs in these SCCs, even if we visited them
- * already.  So a straight up one-pass cycle-detecting DFS won't work.
- *
- * To work around this problem, we traverse each SCC (which we already
- * computed, since these defs are frozen) as a single node.  We first compute
- * whether the SCC as a whole can reach any def in addtab, then we dup (or not)
- * the entire SCC.  This requires breaking the encapsulation of upb_refcounted,
- * since that is where we get the data about what SCC we are in. */
-static bool upb_resolve_dfs(const upb_def *def, upb_strtable *addtab,
-                            const void *new_owner, upb_inttable *seen,
-                            upb_status *s) {
-  upb_value v;
-  bool need_dup;
-  const upb_def *base;
-  const void* memoize_key;
-
-  /* Memoize results of this function for efficiency (since we're traversing a
-   * DAG this is not needed to limit the depth of the search).
-   *
-   * We memoize by SCC instead of by individual def. */
-  memoize_key = def->base.group;
-
-  if (upb_inttable_lookupptr(seen, memoize_key, &v))
-    return upb_value_getbool(v);
-
-  /* Visit submessages for all messages in the SCC. */
-  need_dup = false;
-  base = def;
-  do {
-    upb_value v;
-    const upb_msgdef *m;
-
-    UPB_ASSERT(upb_def_isfrozen(def));
-    if (def->type == UPB_DEF_FIELD) continue;
-    if (upb_strtable_lookup(addtab, upb_def_fullname(def), &v)) {
-      need_dup = true;
-    }
-
-    /* For messages, continue the recursion by visiting all subdefs, but only
-     * ones in different SCCs. */
-    m = upb_dyncast_msgdef(def);
-    if (m) {
-      upb_msg_field_iter i;
-      for(upb_msg_field_begin(&i, m);
-          !upb_msg_field_done(&i);
-          upb_msg_field_next(&i)) {
-        upb_fielddef *f = upb_msg_iter_field(&i);
-        const upb_def *subdef;
-
-        if (!upb_fielddef_hassubdef(f)) continue;
-        subdef = upb_fielddef_subdef(f);
-
-        /* Skip subdefs in this SCC. */
-        if (def->base.group == subdef->base.group) continue;
-
-        /* |= to avoid short-circuit; we need its side-effects. */
-        need_dup |= upb_resolve_dfs(subdef, addtab, new_owner, seen, s);
-        if (!upb_ok(s)) return false;
-      }
-    }
-  } while ((def = (upb_def*)def->base.next) != base);
-
-  if (need_dup) {
-    /* Dup all defs in this SCC that don't already have entries in addtab. */
-    def = base;
-    do {
-      const char *name;
-
-      if (def->type == UPB_DEF_FIELD) continue;
-      name = upb_def_fullname(def);
-      if (!upb_strtable_lookup(addtab, name, NULL)) {
-        upb_def *newdef = upb_def_dup(def, new_owner);
-        if (!newdef) goto oom;
-        newdef->came_from_user = false;
-        if (!upb_strtable_insert(addtab, name, upb_value_ptr(newdef)))
-          goto oom;
-      }
-    } while ((def = (upb_def*)def->base.next) != base);
-  }
-
-  upb_inttable_insertptr(seen, memoize_key, upb_value_bool(need_dup));
-  return need_dup;
-
-oom:
-  upb_status_seterrmsg(s, "out of memory");
-  return false;
-}
-
-/* TODO(haberman): we need a lot more testing of error conditions.
- * The came_from_user stuff in particular is not tested. */
+/* TODO(haberman): we need a lot more testing of error conditions. */
 static bool symtab_add(upb_symtab *s, upb_def *const*defs, size_t n,
                        void *ref_donor, upb_refcounted *freeze_also,
                        upb_status *status) {
@@ -2346,13 +2102,11 @@ static bool symtab_add(upb_symtab *s, upb_def *const*defs, size_t n,
   upb_def **add_defs = NULL;
   size_t add_objs_size;
   upb_strtable addtab;
-  upb_inttable seen;
 
   if (n == 0 && !freeze_also) {
     return true;
   }
 
-  UPB_ASSERT(!upb_symtab_isfrozen(s));
   if (!upb_strtable_init(&addtab, UPB_CTYPE_PTR)) {
     upb_status_seterrmsg(status, "out of memory");
     return false;
@@ -2390,74 +2144,23 @@ static bool symtab_add(upb_symtab *s, upb_def *const*defs, size_t n,
         upb_status_seterrf(status, "Conflicting defs named '%s'", fullname);
         goto err;
       }
-      /* We need this to back out properly, because if there is a failure we
-       * need to donate the ref back to the caller. */
-      def->came_from_user = true;
-      upb_def_donateref(def, ref_donor, s);
-      if (!upb_strtable_insert(&addtab, fullname, upb_value_ptr(def)))
-        goto oom_err;
-    }
-  }
-
-  /* Add standalone fielddefs (ie. extensions) to the appropriate messages.
-   * If the appropriate message only exists in the existing symtab, duplicate
-   * it so we have a mutable copy we can add the fields to. */
-  for (i = 0; i < n; i++) {
-    upb_def *def = defs[i];
-    upb_fielddef *f = upb_dyncast_fielddef_mutable(def);
-    const char *msgname;
-    upb_value v;
-    upb_msgdef *m;
-
-    if (!f) continue;
-    msgname = upb_fielddef_containingtypename(f);
-    /* We validated this earlier in this function. */
-    UPB_ASSERT(msgname);
-
-    /* If the extendee name is absolutely qualified, move past the initial ".".
-     * TODO(haberman): it is not obvious what it would mean if this was not
-     * absolutely qualified. */
-    if (msgname[0] == '.') {
-      msgname++;
-    }
-
-    if (upb_strtable_lookup(&addtab, msgname, &v)) {
-      /* Extendee is in the set of defs the user asked us to add. */
-      m = upb_value_getptr(v);
-    } else {
-      /* Need to find and dup the extendee from the existing symtab. */
-      const upb_msgdef *frozen_m = upb_symtab_lookupmsg(s, msgname);
-      if (!frozen_m) {
-        upb_status_seterrf(status,
-                           "Tried to extend message %s that does not exist "
-                           "in this SymbolTable.",
-                           msgname);
+      if (upb_strtable_lookup(&s->symtab, fullname, NULL)) {
+        upb_status_seterrf(status, "Symtab already has a def named '%s'",
+                           fullname);
         goto err;
       }
-      m = upb_msgdef_dup(frozen_m, s);
-      if (!m) goto oom_err;
-      if (!upb_strtable_insert(&addtab, msgname, upb_value_ptr(m))) {
-        upb_msgdef_unref(m, s);
+      if (!upb_strtable_insert(&addtab, fullname, upb_value_ptr(def)))
         goto oom_err;
-      }
+      upb_def_donateref(def, ref_donor, s);
     }
 
-    if (!upb_msgdef_addfield(m, f, ref_donor, status)) {
+    if (upb_dyncast_fielddef_mutable(def)) {
+      /* TODO(haberman): allow adding extensions attached to files. */
+      upb_status_seterrf(status, "Can't add extensions to symtab.\n");
       goto err;
     }
   }
 
-  /* Add dups of any existing def that can reach a def with the same name as
-   * anything in our "add" set. */
-  if (!upb_inttable_init(&seen, UPB_CTYPE_BOOL)) goto oom_err;
-  upb_strtable_begin(&iter, &s->symtab);
-  for (; !upb_strtable_done(&iter); upb_strtable_next(&iter)) {
-    upb_def *def = upb_value_getptr(upb_strtable_iter_value(&iter));
-    upb_resolve_dfs(def, &addtab, s, &seen, status);
-    if (!upb_ok(status)) goto err;
-  }
-  upb_inttable_uninit(&seen);
-
   /* Now using the table, resolve symbolic references for subdefs. */
   upb_strtable_begin(&iter, &addtab);
   for (; !upb_strtable_done(&iter); upb_strtable_next(&iter)) {
@@ -2535,15 +2238,9 @@ static bool symtab_add(upb_symtab *s, upb_def *const*defs, size_t n,
   for (i = 0; i < add_n; i++) {
     upb_def *def = (upb_def*)add_objs[i];
     const char *name = upb_def_fullname(def);
-    upb_value v;
     bool success;
-
-    if (upb_strtable_remove(&s->symtab, name, &v)) {
-      const upb_def *def = upb_value_getptr(v);
-      upb_def_unref(def, s);
-    }
     success = upb_strtable_insert(&s->symtab, name, upb_value_ptr(def));
-    UPB_ASSERT(success == true);
+    UPB_ASSERT(success);
   }
   upb_gfree(add_defs);
   return true;
@@ -2551,18 +2248,11 @@ static bool symtab_add(upb_symtab *s, upb_def *const*defs, size_t n,
 oom_err:
   upb_status_seterrmsg(status, "out of memory");
 err: {
-    /* For defs the user passed in, we need to donate the refs back.  For defs
-     * we dup'd, we need to just unref them. */
+    /* We need to donate the refs back. */
     upb_strtable_begin(&iter, &addtab);
     for (; !upb_strtable_done(&iter); upb_strtable_next(&iter)) {
       upb_def *def = upb_value_getptr(upb_strtable_iter_value(&iter));
-      bool came_from_user = def->came_from_user;
-      def->came_from_user = false;
-      if (came_from_user) {
-        upb_def_donateref(def, s, ref_donor);
-      } else {
-        upb_def_unref(def, s);
-      }
+      upb_def_donateref(def, s, ref_donor);
     }
   }
   upb_strtable_uninit(&addtab);
diff --git a/upb/def.h b/upb/def.h
index b2c42a62c3..9194a406d3 100644
--- a/upb/def.h
+++ b/upb/def.h
@@ -44,8 +44,7 @@ UPB_DECLARE_DERIVED_TYPE(upb::OneofDef, upb::RefCounted, upb_oneofdef,
                          upb_refcounted)
 UPB_DECLARE_DERIVED_TYPE(upb::FileDef, upb::RefCounted, upb_filedef,
                          upb_refcounted)
-UPB_DECLARE_DERIVED_TYPE(upb::SymbolTable, upb::RefCounted,
-                         upb_symtab, upb_refcounted)
+UPB_DECLARE_TYPE(upb::SymbolTable, upb_symtab)
 
 
 /* The maximum message depth that the type graph can have.  This is a resource
@@ -80,8 +79,6 @@ class upb::Def {
  public:
   typedef upb_deftype_t Type;
 
-  Def* Dup(const void *owner) const;
-
   /* upb::RefCounted methods like Ref()/Unref(). */
   UPB_REFCOUNTED_CPPMETHODS
 
@@ -127,9 +124,6 @@ class upb::Def {
 
 UPB_BEGIN_EXTERN_C
 
-/* Native C API. */
-upb_def *upb_def_dup(const upb_def *def, const void *owner);
-
 /* Include upb_refcounted methods like upb_def_ref()/upb_def_unref(). */
 UPB_REFCOUNTED_CMETHODS(upb_def, upb_def_upcast)
 
@@ -319,13 +313,6 @@ class upb::FieldDef {
   /* Returns NULL if memory allocation failed. */
   static reffed_ptr<FieldDef> New();
 
-  /* Duplicates the given field, returning NULL if memory allocation failed.
-   * When a fielddef is duplicated, the subdef (if any) is made symbolic if it
-   * wasn't already.  If the subdef is set but has no name (which is possible
-   * since msgdefs are not required to have a name) the new fielddef's subdef
-   * will be unset. */
-  FieldDef* Dup(const void* owner) const;
-
   /* upb::RefCounted methods like Ref()/Unref(). */
   UPB_REFCOUNTED_CPPMETHODS
 
@@ -574,7 +561,6 @@ UPB_BEGIN_EXTERN_C
 
 /* Native C API. */
 upb_fielddef *upb_fielddef_new(const void *owner);
-upb_fielddef *upb_fielddef_dup(const upb_fielddef *f, const void *owner);
 
 /* Include upb_refcounted methods like upb_fielddef_ref(). */
 UPB_REFCOUNTED_CMETHODS(upb_fielddef, upb_fielddef_upcast2)
@@ -784,16 +770,6 @@ class upb::MessageDef {
     return FindOneofByName(str.c_str(), str.size());
   }
 
-  /* Returns a new msgdef that is a copy of the given msgdef (and a copy of all
-   * the fields) but with any references to submessages broken and replaced
-   * with just the name of the submessage.  Returns NULL if memory allocation
-   * failed.
-   *
-   * TODO(haberman): which is more useful, keeping fields resolved or
-   * unresolving them?  If there's no obvious answer, Should this functionality
-   * just be moved into symtab.c? */
-  MessageDef* Dup(const void* owner) const;
-
   /* Is this message a map entry? */
   void setmapentry(bool map_entry);
   bool mapentry() const;
@@ -927,7 +903,6 @@ UPB_REFCOUNTED_CMETHODS(upb_msgdef, upb_msgdef_upcast2)
 
 bool upb_msgdef_freeze(upb_msgdef *m, upb_status *status);
 
-upb_msgdef *upb_msgdef_dup(const upb_msgdef *m, const void *owner);
 const char *upb_msgdef_fullname(const upb_msgdef *m);
 const char *upb_msgdef_name(const upb_msgdef *m);
 int upb_msgdef_numoneofs(const upb_msgdef *m);
@@ -1077,10 +1052,6 @@ class upb::EnumDef {
    * first one that was added. */
   const char* FindValueByNumber(int32_t num) const;
 
-  /* Returns a new EnumDef with all the same values.  The new EnumDef will be
-   * owned by the given owner. */
-  EnumDef* Dup(const void* owner) const;
-
   /* Iteration over name/value pairs.  The order is undefined.
    * Adding an enum val invalidates any iterators.
    *
@@ -1108,7 +1079,6 @@ UPB_BEGIN_EXTERN_C
 
 /* Native C API. */
 upb_enumdef *upb_enumdef_new(const void *owner);
-upb_enumdef *upb_enumdef_dup(const upb_enumdef *e, const void *owner);
 
 /* Include upb_refcounted methods like upb_enumdef_ref(). */
 UPB_REFCOUNTED_CMETHODS(upb_enumdef, upb_enumdef_upcast2)
@@ -1218,10 +1188,6 @@ class upb::OneofDef {
   /* Looks up by tag number. */
   const FieldDef* FindFieldByNumber(uint32_t num) const;
 
-  /* Returns a new OneofDef with all the same fields. The OneofDef will be owned
-   * by the given owner. */
-  OneofDef* Dup(const void* owner) const;
-
   /* Iteration over fields.  The order is undefined. */
   class iterator : public std::iterator<std::forward_iterator_tag, FieldDef*> {
    public:
@@ -1267,7 +1233,6 @@ UPB_BEGIN_EXTERN_C
 
 /* Native C API. */
 upb_oneofdef *upb_oneofdef_new(const void *owner);
-upb_oneofdef *upb_oneofdef_dup(const upb_oneofdef *o, const void *owner);
 
 /* Include upb_refcounted methods like upb_oneofdef_ref(). */
 UPB_REFCOUNTED_CMETHODS(upb_oneofdef, upb_oneofdef_upcast)
@@ -1434,10 +1399,8 @@ class upb::SymbolTable {
  public:
   /* Returns a new symbol table with a single ref owned by "owner."
    * Returns NULL if memory allocation failed. */
-  static reffed_ptr<SymbolTable> New();
-
-  /* Include RefCounted base methods. */
-  UPB_REFCOUNTED_CPPMETHODS
+  static SymbolTable* New();
+  static void Free(upb::SymbolTable* table);
 
   /* For all lookup functions, the returned pointer is not owned by the
    * caller; it may be invalidated by any non-const call or unref of the
@@ -1475,21 +1438,12 @@ class upb::SymbolTable {
    * you ask for an iterator of MessageDef the iterated elements are strongly
    * typed as MessageDef*. */
 
-  /* Adds the given mutable defs to the symtab, resolving all symbols
-   * (including enum default values) and finalizing the defs.  Only one def per
-   * name may be in the list, but defs can replace existing defs in the symtab.
+  /* Adds the given mutable defs to the symtab, resolving all symbols (including
+   * enum default values) and finalizing the defs.  Only one def per name may be
+   * in the list, and the defs may not duplicate any name already in the symtab.
    * All defs must have a name -- anonymous defs are not allowed.  Anonymous
    * defs can still be frozen by calling upb_def_freeze() directly.
    *
-   * Any existing defs that can reach defs that are being replaced will
-   * themselves be replaced also, so that the resulting set of defs is fully
-   * consistent.
-   *
-   * This logic implemented in this method is a convenience; ultimately it
-   * calls some combination of upb_fielddef_setsubdef(), upb_def_dup(), and
-   * upb_freeze(), any of which the client could call themself.  However, since
-   * the logic for doing so is nontrivial, we provide it here.
-   *
    * The entire operation either succeeds or fails.  If the operation fails,
    * the symtab is unchanged, false is returned, and status indicates the
    * error.  The caller passes a ref on all defs to the symtab (even if the
@@ -1499,13 +1453,7 @@ class upb::SymbolTable {
    * leave the defs themselves partially resolved.  Does this matter?  If so we
    * could do a prepass that ensures that all symbols are resolvable and bail
    * if not, so we don't mutate anything until we know the operation will
-   * succeed.
-   *
-   * TODO(haberman): since the defs must be mutable, refining a frozen def
-   * requires making mutable copies of the entire tree.  This is wasteful if
-   * only a few messages are changing.  We may want to add a way of adding a
-   * tree of frozen defs to the symtab (perhaps an alternate constructor where
-   * you pass the root of the tree?) */
+   * succeed. */
   bool Add(Def*const* defs, size_t n, void* ref_donor, Status* status);
 
   bool Add(const std::vector<Def*>& defs, void *owner, Status* status) {
@@ -1527,11 +1475,8 @@ UPB_BEGIN_EXTERN_C
 
 /* Native C API. */
 
-/* Include refcounted methods like upb_symtab_ref(). */
-UPB_REFCOUNTED_CMETHODS(upb_symtab, upb_symtab_upcast)
-
-upb_symtab *upb_symtab_new(const void *owner);
-void upb_symtab_freeze(upb_symtab *s);
+upb_symtab *upb_symtab_new();
+void upb_symtab_free(upb_symtab* s);
 const upb_def *upb_symtab_resolve(const upb_symtab *s, const char *base,
                                   const char *sym);
 const upb_def *upb_symtab_lookup(const upb_symtab *s, const char *sym);
@@ -1562,13 +1507,11 @@ UPB_END_EXTERN_C
 #ifdef __cplusplus
 /* C++ inline wrappers. */
 namespace upb {
-inline reffed_ptr<SymbolTable> SymbolTable::New() {
-  upb_symtab *s = upb_symtab_new(&s);
-  return reffed_ptr<SymbolTable>(s, &s);
+inline SymbolTable* SymbolTable::New() {
+  return upb_symtab_new();
 }
-
-inline void SymbolTable::Freeze() {
-  return upb_symtab_freeze(this);
+inline void SymbolTable::Free(SymbolTable* s) {
+  upb_symtab_free(s);
 }
 inline const Def *SymbolTable::Resolve(const char *base,
                                        const char *sym) const {
@@ -1600,9 +1543,6 @@ UPB_INLINE const char* upb_safecstr(const std::string& str) {
 /* Inline C++ wrappers. */
 namespace upb {
 
-inline Def* Def::Dup(const void* owner) const {
-  return upb_def_dup(this, owner);
-}
 inline Def::Type Def::def_type() const { return upb_def_type(this); }
 inline const char* Def::full_name() const { return upb_def_fullname(this); }
 inline const char* Def::name() const { return upb_def_name(this); }
@@ -1652,9 +1592,6 @@ inline reffed_ptr<FieldDef> FieldDef::New() {
   upb_fielddef *f = upb_fielddef_new(&f);
   return reffed_ptr<FieldDef>(f, &f);
 }
-inline FieldDef* FieldDef::Dup(const void* owner) const {
-  return upb_fielddef_dup(this, owner);
-}
 inline const char* FieldDef::full_name() const {
   return upb_fielddef_fullname(this);
 }
@@ -1894,9 +1831,6 @@ inline const OneofDef* MessageDef::FindOneofByName(const char* name,
                                                    size_t len) const {
   return upb_msgdef_ntoo(this, name, len);
 }
-inline MessageDef* MessageDef::Dup(const void *owner) const {
-  return upb_msgdef_dup(this, owner);
-}
 inline void MessageDef::setmapentry(bool map_entry) {
   upb_msgdef_setmapentry(this, map_entry);
 }
@@ -2066,9 +2000,6 @@ inline bool EnumDef::FindValueByName(const char* name, int32_t *num) const {
 inline const char* EnumDef::FindValueByNumber(int32_t num) const {
   return upb_enumdef_iton(this, num);
 }
-inline EnumDef* EnumDef::Dup(const void* owner) const {
-  return upb_enumdef_dup(this, owner);
-}
 
 inline EnumDef::Iterator::Iterator(const EnumDef* e) {
   upb_enum_begin(&iter_, e);
diff --git a/upb/msg.c b/upb/msg.c
index 517b814a1f..5fa9bc866e 100644
--- a/upb/msg.c
+++ b/upb/msg.c
@@ -350,7 +350,6 @@ upb_msgfactory *upb_msgfactory_new(const upb_symtab *symtab) {
   upb_msgfactory *ret = upb_gmalloc(sizeof(*ret));
 
   ret->symtab = symtab;
-  upb_symtab_ref(ret->symtab, &ret->symtab);
   upb_inttable_init(&ret->layouts, UPB_CTYPE_PTR);
   upb_inttable_init(&ret->mergehandlers, UPB_CTYPE_CONSTPTR);
 
@@ -373,7 +372,6 @@ void upb_msgfactory_free(upb_msgfactory *f) {
 
   upb_inttable_uninit(&f->layouts);
   upb_inttable_uninit(&f->mergehandlers);
-  upb_symtab_unref(f->symtab, &f->symtab);
   upb_gfree(f);
 }