userzhy opened a new pull request, #3066:
URL: https://github.com/apache/fory/pull/3066
## Why?
This PR implements support for `UNION` type ID in Rust tagged enum
serialization as part of the xlang type system enhancement (issue #3027).
Previously, Rust tagged enums used `ENUM` or `NAMED_ENUM` type IDs in xlang
mode, but according to the xlang specification, tagged union types (like Rust
enums, C++ `std::variant`, Go `Union`) should use the `UNION` type ID (38).
This change aligns Rust's tagged enum serialization with C++ variant
serialization (#3025) and ensures cross-language compatibility.
## What does this PR do?
1. **Add UNION and NONE type IDs** (`rust/fory-core/src/types.rs`):
- Added `UNION = 38` - Tagged union type (one of several alternatives)
- Added `NONE = 39` - Empty/unit type (no data)
- Added corresponding constants
2. **Modify enum serializer** (`rust/fory-core/src/serializer/enum_.rs`):
- Modified `write_type_info` to write `UNION` type ID when in xlang mode
- Modified `read_type_info` to expect `UNION` type ID when in xlang mode
3. **Add tests** (`rust/tests/tests/test_enum.rs`):
- `xlang_tagged_enum_uses_union_type_id`: Verifies that `UNION` type ID
(38) is used in xlang mode
- `xlang_complex_tagged_enum_roundtrip`: Verifies roundtrip serialization
works correctly
### Serialization Format Change in xlang Mode
```text
Before:
| ref flag | ENUM/NAMED_ENUM type id | variant index |
After:
| ref flag | UNION type id (38) | variant index |
```
## Related issues
- Closes #3028
## Does this PR introduce any user-facing change?
- [ ] Does this PR introduce any public API change?
- [x] Does this PR introduce any binary protocol compatibility change?
**Note**: This change affects the binary protocol in **xlang mode only**. In
xlang mode, tagged enums now use the `UNION` type ID (38) instead of `ENUM`
(13) or `NAMED_ENUM` (14). This is a breaking change for xlang serialization
but aligns with the xlang specification and other language implementations
(C++, Go).
## Benchmark
This change does not introduce any performance impact as it only changes the
type ID value written during serialization.
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