Disallow the string_view optimization if message format is not "binary".

PiperOrigin-RevId: 354168926

Author: brills

struct2tensor/expression_impl/proto.py

@@ -507,6 +507,10 @@ def calculate(
     needed_fields = _get_needed_fields(destinations)
     backing_str_tensor = None
     if options.use_string_view:
+      assert self._message_format == "binary", (
+          "`options.use_string_view` is only compatible with 'binary' message "
+          "format. Please create the root expression with "
+          "message_format='binary'.")
       backing_str_tensor = self._tensor_of_protos
     fields = parse_message_level_ex.parse_message_level_ex(
         self._tensor_of_protos,

struct2tensor/ops/struct2tensor_ops.py

@@ -160,6 +160,8 @@ def parse_message_level(
   # TODO(b/172576749): Once we allow sufficient bake in time for the kernel
   # change, switch to using V3 only.
   if backing_str_tensor:
+    assert message_format == "binary", (
+        "message_format must be 'binary' if a backing_str_tensor is provided")
     values, indices = gen_decode_proto_sparse.decode_proto_sparse_v3(
         tensor_of_protos,
         backing_str_tensor,