js_parser: serialize enum types in decorator metadata as Number, String, or Object

[robobun]

Problem

• With experimentalDecorators and emitDecoratorMetadata, a decorated member typed with an enum gets the enum object as design:type. The emitted metadata is typeof Color === "undefined" ? Object : Color. tsc emits Number for a numeric enum, String for a string enum, and Object for a mixed one.
• Libraries such as TypeORM, class-transformer and NestJS call new (Reflect.getMetadata("design:type", ...))() or switch on the constructor. They receive a plain object that is not callable.
• serialize_metadata (src/js_parser/p.rs) only had the identifier path. add emitDecoratorMetadata #5777 mapped a known enum to Number. feat(bundler): implement enum inlining / more constant folding #12144 (enum inlining) dropped that branch.

Fix

• When the type names an enum that the parser knows (declared in the same file, including const enum, merged enums, an enum merged with a namespace, an empty enum, forward declarations, Ns.Deep.Enum, and the member type Enum.Member), classify it from the member values: all numbers give Number, all strings give String, both give Object. A computed member counts as a number, which is the only type tsc allows there.
• The lookup runs when the class is lowered. The parser now records a literal initializer in the member map as it parses the enum, so an enum that is visited after the class (for example inside a later namespace) still classifies. The name is looked up again from the class scope, as the visit pass does for identifiers, so a later declaration in a function body resolves too.
• Metadata::MDot held a heap Vec<Ref> inside arena-owned AST nodes, and the arena reset never dropped it. ASAN reported the leak for every dotted type annotation (Ns.Inner) once the new test used one. The refs now live in the arena as a StoreSlice<Ref>, so Metadata is Copy and the clones and the ptr::read copies go away.
• An enum imported from another file is unchanged. The parser cannot see its members. Enum | number still folds to Object at parse time (a test.todo records it).
• Verified: test/bundler/transpiler/decorator-metadata.test.ts (new enum types block, fails on 1.4.3). Expected values checked against tsc 6.0.2 emit. Also test/bundler/transpiler/, test/bundler/bundler_decorator_metadata.test.ts, test/js/bun/typescript/, and test/integration/typegraphql/, whose same-file enum assertion now expects String.

Background

• emitDecoratorMetadata makes the transpiler attach design:type, design:paramtypes and design:returntype to decorated members. tsc computes them with its type checker. bun computes them from the syntax of the type annotation.
• The parser keeps enum members in ref_to_ts_namespace_member: the enum symbol maps to a member map, and each member is EnumNumber, EnumString, or EnumProperty (a computed value). The enum inliner uses the same map.
• Self-reviewed: 4 concerns raised, 4 addressed (the typegraphql pin, Enum.Member types, the rest-parameter assertion, the PR body). Review on the PR raised 3 more (a late namespace-nested enum, an enum merged with a namespace, an empty nested enum), all addressed.

Notes

Repro from the report (bun main.ts with experimentalDecorators, emitDecoratorMetadata):

enum Color { Red, Green }
(Reflect as any).metadata = (k: string, v: any) => (t: any, p: any) => {
  if (k === "design:type") console.log(p, typeof v, v === Color ? "the enum object" : v?.name);
};
const Prop: any = () => {};
class Dto { @Prop c: Color; @Prop n: number; }

bun 1.4.3: c object the enum object. tsc 6.0.2 + node, and bun with this change: c function Number.

Oracle for the new test block: tsc 6.0.2 with strict: false. tsc 6.0 turns strict on by default, and with strictNullChecks it serializes Enum | null as Object. bun follows the non-strict rule for every T | null union already (string | null is String in the existing test), so the new test expects Number for Numeric | null.

Cases checked against tsc: numeric, string, mixed, const enum, computed members ((1 + 1) * 2, Math.max(1, 2)), B = A aliases, two merged blocks, an enum declared after the class in the same function, Ns.Inner, Ns.Deep.Inner, typeof Ns, Enum.Member, Ns.Deep.Inner.S, parameter and return types.

Known gap kept out of scope: Enum | number and Enum | string give Object, tsc gives the primitive. Union folding happens while the type annotation is parsed, before the enum can be resolved.

The typegraphql integration test pinned design:type for a same-file enum to the enum object under a comment about imported enums. The imported cases stay as they were. The same-file case now expects String.

---

[human-review] gate passed · iteration 0 · 7 files touched

fails on main (without fix)

ASAN without fix: 2 failed, 1 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/transpiler/decorator-metadata.test.ts test/integration/typegraphql/src/typegraphql.test.ts
bun test v1.4.3 (f42e98025)

test/bundler/transpiler/decorator-metadata.test.ts:
(pass) decorator metadata > type serialization [127.25ms]
(pass) decorator metadata > design: type, paramtypes, returntype [36.93ms]
(pass) decorator metadata > class with only constructor argument decorators [6.88ms]
(pass) decorator metadata > more types [11.29ms]
(pass) decorator metadata > rest parameters and defaults [19.79ms]
559 |   test("enum types", () => {
560 |     function d2(target: any, key: string) {}
561 | 
562 |     class Declared {
563 |       @d2
564 |       later: Later;
            ^
ReferenceError: Cannot access 'Later' before initialization.
      at <anonymous> (/workspace/bun/test/bundler/transpiler/decorator-metadata.test.ts:564:7)
(fail) decorator metadata > enum types [21.33ms]
(todo) decorator metadata > enum in a union with its own primitive

test/integration/typegraphql/src/typegraphql.test.ts:
before run
55 |   // know that
... (truncated)

release without fix: 2 failed, 1 skipped
bun test v1.4.3-canary.1 (f42e98025)

test/bundler/transpiler/decorator-metadata.test.ts:
(pass) decorator metadata > type serialization [1.21ms]
(pass) decorator metadata > design: type, paramtypes, returntype [0.46ms]
(pass) decorator metadata > class with only constructor argument decorators [0.08ms]
(pass) decorator metadata > more types [0.12ms]
(pass) decorator metadata > rest parameters and defaults [0.32ms]
559 |   test("enum types", () => {
560 |     function d2(target: any, key: string) {}
561 | 
562 |     class Declared {
563 |       @d2
564 |       later: Later;
            ^
ReferenceError: Cannot access 'Later' before initialization.
      at <anonymous> (/workspace/bun/test/bundler/transpiler/decorator-metadata.test.ts:564:7)
(fail) decorator metadata > enum types [0.43ms]
(todo) decorator metadata > enum in a union with its own primitive

test/integration/typegraphql/src/typegraphql.test.ts:
before run
55 |   // know that if the enum comes from an import statement. type-graphql doesn't care because the field type
56 |   // is manually specified.
57 |   expect(Reflect.getMetadata("design:type", User.prototype, "enum1")).toBe(Enum1);
58 |   expect(Refl
... (truncated)

passes on PR (with fix)

ASAN with fix: 1 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/transpiler/decorator-metadata.test.ts test/integration/typegraphql/src/typegraphql.test.ts
bun test v1.4.3 (f42e98025)

test/bundler/transpiler/decorator-metadata.test.ts:
(pass) decorator metadata > type serialization [120.19ms]
(pass) decorator metadata > design: type, paramtypes, returntype [35.06ms]
(pass) decorator metadata > class with only constructor argument decorators [5.81ms]
(pass) decorator metadata > more types [11.60ms]
(pass) decorator metadata > rest parameters and defaults [20.53ms]
(pass) decorator metadata > enum types [42.37ms]
(todo) decorator metadata > enum in a union with its own primitive

test/integration/typegraphql/src/typegraphql.test.ts:
before run
(pass) correct reflect.metadata types for getters [6.67ms]

 7 pass
 1 todo
 0 fail
 254 expect() calls
Ran 8 tests across 2 files. [22.47s]
__F:0:S:1

release with fix: 1 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     ab0b511666
  features     baseline

23 deps, 131 codegen, 1172 objects in 725ms

ninja: Entering directory `/workspace/bun/build/release'
[1/1244] gen ErrorCode+*.h
[2/1244] gen bindgenv2
[3/1244] install /workspace/bun
bun install v1.4.3-canary.1 (f42e98025)

Checked 22 installs across 61 packages (no changes) [44.00ms]
[4/1244] gen .bind.ts → GeneratedBindings.cpp
[5/1244] gen bake.{client,server,error}.js
-> bake.client.js, bake.server.js, bake.error.js
[6/1244] fetch libjpeg-turbo
[libjpeg-turbo] up to date
[7/1217] fetch zlib
[zlib] up to date
[8/1217] install /workspace/bun/packages/bun-error
bun install v1.4.3-canary.1 (f42e98025)

Checked 1 install across 2 packages (no changes) [1.00ms]
[9/1217] gen ProcessBindingConstants.lut.h
Generating /workspace/bun/build/release/codegen/ProcessBindingConstants.lut.h from /workspace/bun/src/jsc/bindings/ProcessBindingConstants.cpp
[10/1217] fetch tinycc
[tinycc] up to date
[11/1216] gen JSBuffer.lut.h
Generating /workspace/bun/build/rele
... (truncated)

diff hotspot

src/ast/g.rs                                       |   6 +-
 src/ast/ts.rs                                      |   7 +-
 src/js_parser/lower/lower_decorators.rs            |  11 +-
 src/js_parser/p.rs                                 | 107 ++++++++++++++--
 src/js_parser/parse/parse_skip_typescript.rs       |  26 ++--
 test/bundler/transpiler/decorator-metadata.test.ts | 140 +++++++++++++++++++++
 .../typegraphql/src/typegraphql.test.ts            |   3 +-
 7 files changed, 265 insertions(+), 35 deletions(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                                                  reads  edits  tests
src/ast/g.rs                                              0      0     15
src/ast/ts.rs                                             0      0     15
src/js_parser/lower/lower_decorators.rs                   0      0     15
src/js_parser/p.rs                                        2     10     15
src/js_parser/parse/parse_skip_typescript.rs              1      0     15
test/bundler/transpiler/decorator-metadata.test.ts        1      2     14
test/integration/typegraphql/src/typegraphql.test.ts      0      0      3

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📒 Files selected for processing (5)

• src/ast/g.rs
• src/ast/ts.rs
• src/js_parser/p.rs
• src/js_parser/parse/parse_skip_typescript.rs
• test/bundler/transpiler/decorator-metadata.test.ts

Walkthrough

Summary

The change adds copyable TypeScript metadata with arena-backed dotted references, records enum member metadata during parsing, and resolves enum references during decorator metadata serialization. Tests cover enum and namespace references across declarations and type positions.

Changes

Decorator metadata resolution

Layer / File(s)	Summary
Metadata representation and copying src/ast/g.rs, src/ast/ts.rs, src/js_parser/lower/lower_decorators.rs	Metadata now implements Copy. Dotted references use StoreSlice<Ref>. Metadata copying no longer uses unsafe shallow reads.
Metadata parsing and arena storage src/js_parser/parse/parse_skip_typescript.rs, src/js_parser/parse/parse_typescript.rs	Enum initializers record numeric, string, or property metadata. Union, intersection, and qualified-name handling uses the updated metadata representation.
Enum resolution and validation src/js_parser/p.rs, test/bundler/transpiler/decorator-metadata.test.ts, test/integration/typegraphql/src/typegraphql.test.ts	Decorator serialization resolves enum and namespace references to Number, String, or Object. Tests cover nested, merged, aliased, computed, forward-declared, and type-position references.

Suggested reviewers: jarred-sumner

Merge Risk: 🟠 High · up to b1b63

The metadata representation can cause crashes or memory corruption after AST cloning, while long qualified names can consume excessive parser memory. These issues should be fixed before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: serializing enum types in decorator metadata as Number, String, or Object.
Description check	✅ Passed	The description explains the problem, fix, scope, known gap, implementation details, and verification results. It does not use the exact template headings, but it provides the required change summary …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

---

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[robobun]

^{Updated 5:04 AM PT - Sep 6th, 2026}

❌ @robobun, your commit f628c60 has 2 failures in Build #111026 (All Failures):

• test/regression/issue/10887.test.ts - stalled in the parallel batch on 🐧 25.04 x64
• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 41517

That installs a local version of the PR into your bun-41517 executable, so you can run:

bun-41517 --bun

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Code review found no new issues

No new issues were found in this update; 3 findings from earlier reviews are still open above.

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Actionable comments posted: 2

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Inline comments:
In `@src/ast/g.rs`:
- Line 227: Implement an arena-aware Metadata clone that copies Metadata::MDot
and its StoreSlice<Ref> into the destination bump arena, then use it in
Property::deep_clone, Fn::deep_clone, and Arg::deep_clone instead of copying
ts_metadata directly, matching Data::deep_clone’s arena ownership.

In `@src/js_parser/parse/parse_skip_typescript.rs`:
- Around line 918-924: Update the qualified-name handling around Metadata::MDot
so the full Ref path is accumulated in one ArenaVec across iterations, then
converted to StoreSlice once after traversal; avoid repeatedly allocating,
copying, and replacing StoreSlice values for each component.

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📒 Files selected for processing (8)

• src/ast/g.rs
• src/ast/ts.rs
• src/js_parser/lower/lower_decorators.rs
• src/js_parser/p.rs
• src/js_parser/parse/parse_skip_typescript.rs
• src/js_parser/parse/parse_typescript.rs
• test/bundler/transpiler/decorator-metadata.test.ts
• test/integration/typegraphql/src/typegraphql.test.ts

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