diff --git a/BUILD b/BUILD
index b2ac9bbdec..b14bad4a94 100644
--- a/BUILD
+++ b/BUILD
@@ -106,7 +106,6 @@ cc_library(
         "upb/arena.c",
         "upb/decode.c",
         "upb/encode.c",
-        "upb/internal/arena.h",
         "upb/internal/decode.h",
         "upb/internal/table.h",
         "upb/internal/upb.h",
@@ -130,6 +129,7 @@ cc_library(
     copts = UPB_DEFAULT_COPTS,
     visibility = ["//visibility:public"],
     deps = [
+        ":arena_internal",
         ":extension_registry",
         ":fastdecode",
         ":port",
@@ -270,13 +270,13 @@ cc_library(
         "upb/decode.h",
         "upb/decode_fast.c",
         "upb/decode_fast.h",
-        "upb/internal/arena.h",
         "upb/internal/decode.h",
         "upb/msg.h",
         "upb/msg_internal.h",
     ],
     copts = UPB_DEFAULT_COPTS,
     deps = [
+        ":arena_internal",
         ":extension_registry",
         ":port",
         ":table",
@@ -407,7 +407,6 @@ cc_library(
 cc_library(
     name = "json",
     srcs = [
-        "upb/internal/arena.h",
         "upb/internal/upb.h",
         "upb/json_decode.c",
         "upb/json_encode.c",
@@ -708,6 +707,20 @@ sh_test(
 
 # Internal C/C++ libraries #####################################################
 
+cc_library(
+    name = "arena_internal",
+    srcs = [
+        "upb/alloc.h",
+        "upb/arena.h",
+    ],
+    hdrs = ["upb/internal/arena.h"],
+    copts = UPB_DEFAULT_COPTS,
+    visibility = ["//:__subpackages__"],
+    deps = [
+        ":port",
+    ],
+)
+
 cc_library(
     name = "table",
     hdrs = [
@@ -738,6 +751,7 @@ upb_amalgamation(
         "upb.h",
     ],
     libs = [
+        ":arena_internal",
         ":collections",
         ":descriptor_upb_proto",
         ":extension_registry",
@@ -764,6 +778,7 @@ upb_amalgamation(
         "php-upb.h",
     ],
     libs = [
+        ":arena_internal",
         ":collections",
         ":descriptor_upb_proto",
         ":descriptor_upb_proto_reflection",
@@ -793,6 +808,7 @@ upb_amalgamation(
         "ruby-upb.h",
     ],
     libs = [
+        ":arena_internal",
         ":collections",
         ":descriptor_upb_proto",
         ":extension_registry",
diff --git a/upb/alloc.h b/upb/alloc.h
index add70b8799..870a1b1d9e 100644
--- a/upb/alloc.h
+++ b/upb/alloc.h
@@ -28,13 +28,13 @@
 #ifndef UPB_ALLOC_H_
 #define UPB_ALLOC_H_
 
+// Must be last.
 #include "upb/port_def.inc"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-struct upb_alloc;
 typedef struct upb_alloc upb_alloc;
 
 /* A combined `malloc()`/`free()` function.
diff --git a/upb/arena.h b/upb/arena.h
index 81a673bd9a..1fabbf2aef 100644
--- a/upb/arena.h
+++ b/upb/arena.h
@@ -28,7 +28,6 @@
 #ifndef UPB_ARENA_H_
 #define UPB_ARENA_H_
 
-#include <stdbool.h>
 #include <string.h>
 
 #include "upb/alloc.h"
@@ -54,7 +53,6 @@ extern "C" {
 
 typedef void upb_CleanupFunc(void* ud);
 
-struct upb_Arena;
 typedef struct upb_Arena upb_Arena;
 
 typedef struct {
diff --git a/upb/array.c b/upb/array.c
index b4cdc49924..17bec38145 100644
--- a/upb/array.c
+++ b/upb/array.c
@@ -29,8 +29,9 @@
 
 #include <string.h>
 
-#include "upb/internal/table.h"
 #include "upb/msg.h"
+
+// Must be last.
 #include "upb/port_def.inc"
 
 static const char _upb_CTypeo_sizelg2[12] = {
diff --git a/upb/array.h b/upb/array.h
index f7936fa2d9..7117392ed8 100644
--- a/upb/array.h
+++ b/upb/array.h
@@ -28,7 +28,6 @@
 #ifndef UPB_ARRAY_H_
 #define UPB_ARRAY_H_
 
-#include "google/protobuf/descriptor.upb.h"
 #include "upb/message_value.h"
 
 // Must be last.
diff --git a/upb/map.c b/upb/map.c
index d92776f546..6cc785db65 100644
--- a/upb/map.c
+++ b/upb/map.c
@@ -29,8 +29,9 @@
 
 #include <string.h>
 
-#include "upb/internal/table.h"
 #include "upb/msg.h"
+
+// Must be last.
 #include "upb/port_def.inc"
 
 /* Strings/bytes are special-cased in maps. */
diff --git a/upb/map.h b/upb/map.h
index 8ee0ddd05e..ecaaa1d029 100644
--- a/upb/map.h
+++ b/upb/map.h
@@ -28,7 +28,6 @@
 #ifndef UPB_MAP_H_
 #define UPB_MAP_H_
 
-#include "google/protobuf/descriptor.upb.h"
 #include "upb/message_value.h"
 
 // Must be last.