marshal: round-trip code constants through the runtime bag - #8516

marshal.loads(marshal.dumps(code)) loses or rejects any co_consts entry that the compiler constant enum cannot describe, because a code object's constants are read and written through ConstantBag/BorrowedConstant rather than through the runtime object bag.

>>> import marshal
>>> c = compile("1 + 1", "", "eval").replace(co_consts=([],))
>>> marshal.loads(marshal.dumps(c)).co_consts
# CPython 3.14: ([],)
# before:       thread 'main' panicked at crates/vm/src/builtins/code.rs:254:14:
#               unexpected payload for constant python value

Reader

deserialize_code_value_inner decodes the code fields through the runtime MarshalBag instead of deserialize_code_inner, so co_consts values survive as the objects they were, and the code fields share the single reference index space CPython's r_object() uses. MarshalBag gains make_code_with_constants, code_constant_from_value, bytes_from_value, str_from_value and tuple_elements_from_value; the compiler bags keep the previous behaviour through the defaults and the blanket impl.

PyMarshalBag implements those hooks. Literal wraps any object, so a decoded constant is already its own compiler-side constant and make_code_with_constants keeps the default.

Writer

serialize_code gains a serialize_code_with variant that writes each co_consts entry through a caller-supplied writer; serialize_code passes the BorrowedConstant writer, so compiler callers are unchanged. The VM writer passes its own write_object_depth, which

• reaches the values BorrowedConstant cannot describe, instead of panicking in borrow_obj_constant, and
• gives a constant shared with the enclosing object an entry in the writer's reference table, which the reader now indexes against.

Tests

extra_tests/snippets/stdlib_marshal.py gains a round trip over list/dict/set constants and an identity check for a constant shared with the enclosing tuple. Both pass on CPython 3.14 and on this branch (13/13).

Lib/test/test_marshal.py passes in a release build (75 run, 18 skipped). A debug build segfaults in test_loads_recursion, which nests 2**20 tuples and exhausts the native stack before the 2000 depth limit — unrelated to this change and present on both sides.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Code objects can now preserve runtime constant values during serialization and deserialization.
  • Support added for bytes, strings, tuples, and other literal values in code constants.
  • Shared constants and references are retained across marshal round trips.

• Bug Fixes

  • Improved handling of code objects containing non-constant values, preventing loss or incorrect conversion of runtime data.

• Tests

  • Added coverage for round-trip preservation and shared-reference identity.