fix(transformer/legacy-decorator): preserve accessor type annotation for emitDecoratorMetadata

[Dunqing]

Fixes bug 7 of #21922.

The bug

@dec accessor x: T always emitted design:type = Object regardless of T:

class Entity {
  @dec accessor name: string = "";   // tsc: design:type = String
  @dec accessor count: number = 0;   // tsc: design:type = Number
}

Reflect.getMetadata("design:type", entity, "name") returned Object instead of String. Frameworks reading design:type (NestJS Swagger, class-validator, TypeORM column metadata) silently degraded for every typed accessor field.

Cause

lower_accessor_properties (in enter_class) rewrites

@dec accessor x: T = init;

into three members — a private storage field, a synthesized getter, and a synthesized setter — and transfers the decorators to the getter so the existing decorator pipeline picks them up. But it dropped the type annotation: the synth getter was constructed with return_type: NONE.

The metadata pass runs later and takes the getter branch:

if method.kind.is_get() {
    (self.serialize_type_annotation(method.value.return_type.as_ref(), ctx), None)
}

return_type is None because lowering discarded it, so serialize_type_annotation correctly fell through to Object for what it sees as an untyped getter.

The metadata pass's enter_accessor_property dispatch exists, but it never fires because the AccessorProperty no longer exists by the time traversal reaches the class body.

Fix

Stop dropping the type annotation. In lower_accessor_properties:

let decorators = std::mem::replace(&mut accessor.decorators, ctx.ast.vec());
let type_annotation = accessor.type_annotation.take();

Thread type_annotation through create_accessor_method to the getter (setter passes None) and attach it as the synth function's return_type. The existing getter-metadata path then serializes it correctly.

The lowered AST is now self-consistent — the synth getter for accessor x: T reads as a getter that returns T, which is what it semantically is.

Out of scope (architectural limit)

type Uuid = string; @dec accessor x: Uuid still emits Object, same as for non-accessor properties. Resolving type-alias references to their primitive constituent requires a name-based alias-resolution pass (the same one that tsc's checker provides via getTypeReferenceSerializationKind). That's bug 1 of #21922 and a separate change from this fix; flagged here so reviewers don't mistake it for an omission.

Test plan

• New conformance fixture oxc/metadata/accessor covers primitives, untyped (correctly stays Object), string[] → Array, and static accessor
• cargo test -p oxc_transformer — 31 + 5 passed
• cargo run -p oxc_transform_conformance -- --filter legacy-decorators — 73 pass / 23 fail (baseline 72/24, +1 new fixture, 0 regressions)
• cargo fmt -p oxc_transformer

[Dunqing]

• fix(transformer/legacy-decorator): preserve accessor type annotation for emitDecoratorMetadata #21966 👈 (View in Graphite)
• main

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[codspeed-hq]

Merging this PR will not alter performance

✅ 44 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing fix-transformer-accessor-emit-decorator-metadata (192da8b) with main (079cfdd)}

Footnotes

1. 7 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. ↩