fix(transformer/legacy-decorator): emit Array for ReadonlyArray<T> in decorator metadata

[kylecannon]

Summary

ReadonlyArray<T> is a TypeScript-only utility type with no runtime value. The legacy decorator metadata emit currently wraps it in the runtime guard typeof (_ref = typeof X !== "undefined" && X) === "function" ? _ref : Object, which always falls through to Object because there is no runtime ReadonlyArray binding. tsc resolves this through the checker and emits Array; OXC's existing handling of the short form readonly T[] already emits Array, so the generic form is the only divergent shape.

The bug

class Entity {
  @D() shortForm!: readonly string[];           // tsc: Array, OXC: Array  (already correct)
  @D() genericForm!: ReadonlyArray<string>;     // tsc: Array, OXC: Object (bug)
  @D() nestedReadonly!: ReadonlyArray<ReadonlyArray<number>>; // same
}

The two surface forms describe the same type but produce different runtime metadata. Downstream consumers (TypeORM @Column array inference, NestJS Swagger schema, AutoMapper field detection) rely on the Array constructor; Object causes silent degradation.

Fix

In serialize_type_reference_node, recognize ReadonlyArray by name when the symbol is unresolved or type-only. Emit global_array(ctx) directly, bypassing the wrapper. A user-shadowed class ReadonlyArray {} would have a value symbol and falls through to the normal class-handling path, so the shadow case is preserved.

if ident.name == "ReadonlyArray"
    && symbol_id.is_none_or(|sid| ctx.scoping().symbol_flags(sid).is_type())
{
    return Self::global_array(ctx);
}

Scope

Only ReadonlyArray is handled in this PR. ReadonlyMap, ReadonlySet, WeakMap, WeakSet, and Readonly<Array<T>> peel are left for a follow-up because tsc's spec emit for those is less unambiguous and warrants case-by-case verification. The same predicate-style approach extends naturally if reviewers want them folded in.

Test plan

• 4 unit tests in decorator_metadata_readonly_array.rs: bare ReadonlyArray, nested ReadonlyArray, regression on readonly T[] short form, and a user-shadowed class ReadonlyArray {} regression
• New conformance fixture oxc/metadata/readonly-array/
• cargo test -p oxc_transformer passes (31 unit + 9 integration)
• cargo run -p oxc_transform_conformance shows 229 OXC fixtures still passing, +1 new fixture (readonly-array); 0 regressions
• cargo fmt -p oxc_transformer and cargo clippy -p oxc_transformer --tests --no-deps clean

AI assistance was used in writing this patch and tests; the contributor has reviewed and tested locally.

[codspeed-hq]

Merging this PR will not alter performance

✅ 52 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing kylecannon:fix/decorator-metadata-readonly-array (485def7) with main (e421ef0)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩