Fix niche-optimized enum construction in cc_bindings_from_rs.#1122
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The `enums_test` (`TestQrError`) added in this CL was initially failing because niche-optimized enums were being constructed in C++ using their semantic discriminant (retrieved via `discriminant_for_variant`), which evaluates to `0` for `QrError::DataTooLong` instead of the actual in-memory niche tag value. This meant that `QrError::MakeDataTooLong()` was constructing a value that on the Rust side was interpreted as `Structured(AtLeast2Pieces)` instead of `DataTooLong`. The fix is switching `generate_struct_and_union.rs` to use the compiler's native layout helper `tcx.tag_for_variant(...)`, which correctly returns the exact in-memory niche tag (or direct discriminant) for any given variant index. PiperOrigin-RevId: 925652439
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Fix niche-optimized enum construction in
cc_bindings_from_rs.The
enums_test(TestQrError) added in this CL was initially failingbecause niche-optimized enums were being constructed in C++ using their
semantic discriminant (retrieved via
discriminant_for_variant), whichevaluates to
0forQrError::DataTooLonginstead of the actualin-memory niche tag value. This meant that
QrError::MakeDataTooLong()was constructing a value that on the Rust side was interpreted as
Structured(AtLeast2Pieces)instead ofDataTooLong.The fix is switching
generate_struct_and_union.rsto use thecompiler's native layout helper
tcx.tag_for_variant(...), whichcorrectly returns the exact in-memory niche tag (or direct discriminant)
for any given variant index.