Factored upb_map_entry into a shared place.

upb/decode.c

@@ -492,19 +492,7 @@ static bool upb_decode_mapfield(upb_decstate *d, upb_decframe *frame,
                                 const upb_msglayout_field *field, int len) {
   upb_map *map = *(upb_map**)&frame->msg[field->offset];
   const upb_msglayout *entry = frame->layout->submsgs[field->submsg_index];
-
-  /* The compiler ensures that all map entry messages have this layout. */
-  struct map_entry {
-    upb_msg_internal internal;
-    union {
-      upb_strview str;  /* For str/bytes. */
-      upb_value val;    /* For all other types. */
-    } k;
-    union {
-      upb_strview str;  /* For str/bytes. */
-      upb_value val;    /* For all other types. */
-    } v;
-  } ent;
+  upb_map_entry ent;
 
   if (!map) {
     /* Lazily create map. */

upb/def.c

@@ -869,10 +869,9 @@ static size_t upb_msgval_sizeof(upb_fieldtype_t type) {
 
 static uint8_t upb_msg_fielddefsize(const upb_fielddef *f) {
   if (upb_msgdef_mapentry(upb_fielddef_containingtype(f))) {
-    /* Map entries aren't actually stored, they are only used during parsing.
-     * For parsing, it helps a lot if all map entry messages have the same
-     * layout. */
-    return sizeof(upb_strview);
+    upb_map_entry ent;
+    UPB_ASSERT(sizeof(ent.k) == sizeof(ent.v));
+    return sizeof(ent.k);
   } else if (upb_fielddef_isseq(f)) {
     return sizeof(void*);
   } else {

upb/msg.h

@@ -200,6 +200,21 @@ typedef struct {
   upb_strtable table;
 } upb_map;
 
+/* Map entries aren't actually stored, they are only used during parsing.  For
+ * parsing, it helps a lot if all map entry messages have the same layout.
+ * The compiler and def.c must ensure that all map entries have this layout. */
+typedef struct {
+  upb_msg_internal internal;
+  union {
+    upb_strview str;  /* For str/bytes. */
+    upb_value val;    /* For all other types. */
+  } k;
+  union {
+    upb_strview str;  /* For str/bytes. */
+    upb_value val;    /* For all other types. */
+  } v;
+} upb_map_entry;
+
 /* Creates a new map on the given arena with this key/value type. */
 upb_map *_upb_map_new(upb_arena *a, size_t key_size, size_t value_size);