Implement unknown field aliasing support

PiperOrigin-RevId: 708077050

upb/message/internal/message.h

@@ -120,7 +120,8 @@ void _upb_Message_DiscardUnknown_shallow(struct upb_Message* msg);
 
 // Adds unknown data (serialized protobuf data) to the given message. The data
 // must represent one or more complete and well formed proto fields.
-// The data is copied into the message instance.
+// If alias is set, will keep a view to the provided data; otherwise a copy is
+// made.
 bool UPB_PRIVATE(_upb_Message_AddUnknown)(struct upb_Message* msg,
                                           const char* data, size_t len,
                                           upb_Arena* arena, bool alias);

upb/message/message.c

@@ -43,11 +43,18 @@ bool UPB_PRIVATE(_upb_Message_AddUnknown)(upb_Message* msg, const char* data,
   if (!UPB_PRIVATE(_upb_Message_ReserveSlot)(msg, arena)) {
     return false;
   }
-  upb_StringView* view = upb_Arena_Malloc(arena, sizeof(upb_StringView) + len);
-  if (!view) return false;
-  char* copy = UPB_PTR_AT(view, sizeof(upb_StringView), char);
-  memcpy(copy, data, len);
-  view->data = copy;
+  upb_StringView* view;
+  if (alias) {
+    view = upb_Arena_Malloc(arena, sizeof(upb_StringView));
+    if (!view) return false;
+    view->data = data;
+  } else {
+    view = upb_Arena_Malloc(arena, sizeof(upb_StringView) + len);
+    if (!view) return false;
+    char* copy = UPB_PTR_AT(view, sizeof(upb_StringView), char);
+    memcpy(copy, data, len);
+    view->data = copy;
+  }
   view->size = len;
   upb_Message_Internal* in = UPB_PRIVATE(_upb_Message_GetInternal)(msg);
   in->aux_data[in->size++] = upb_TaggedAuxPtr_MakeUnknownData(view);

upb/wire/decode.h

@@ -26,8 +26,8 @@ extern "C" {
 #endif
 
 enum {
-  /* If set, strings will alias the input buffer instead of copying into the
-   * arena. */
+  /* If set, strings and unknown fields will alias the input buffer instead of
+   * copying into the arena. */
   kUpb_DecodeOption_AliasString = 1,
 
   /* If set, the parse will return failure if any message is missing any