fix(transpiler): treat emitDecoratorMetadata as implying legacy decorators

[robobun]

Summary

• When emitDecoratorMetadata: true is set in tsconfig.json without experimentalDecorators: true, Bun now correctly uses legacy decorator semantics instead of TC39 standard decorator lowering
• emitDecoratorMetadata only makes sense with TypeScript's legacy decorator system (reflect-metadata), so its presence implies the user expects legacy decorator behavior
• This fixes a regression from ce715b5 where NestJS, TypeORM, Angular, and other legacy-decorator frameworks would crash with descriptor.value undefined

Closes #27526

Test plan

• Added regression test test/regression/issue/27526.test.ts with two cases:
  • Legacy decorators work when emitDecoratorMetadata: true but experimentalDecorators is absent
  • TC39 standard decorators still work when neither option is set
• Verified test fails with system Bun (USE_SYSTEM_BUN=1) and passes with debug build (bun bd test)

🤖 Generated with Claude Code

[robobun]

^{Updated 1:58 AM PT - Feb 28th, 2026}

❌ Your commit 00553cda has 2 failures in Build #38305 (All Failures):

• test/bundler/bundler_compile_autoload.test.ts - 1 failing on 🐧 13 x64-asan
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 13 aarch64

---

🧪   To try this PR locally:

bunx bun-pr 27527

That installs a local version of the PR into your bun-27527 executable, so you can run:

bun-27527 --bun

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Disabled knowledge base sources:

• Linear integration is disabled

You can enable these sources in your CodeRabbit configuration.

📥 Commits

Reviewing files that changed from the base of the PR and between 4a5c20b and 00553cd.

📒 Files selected for processing (3)

• src/bundler/ParseTask.zig
• src/transpiler.zig
• test/regression/issue/27526.test.ts

---

Walkthrough

Adjusts decorator-selection logic so legacy/experimental decorator semantics are used when TypeScript's emitDecoratorMetadata is enabled; adds explanatory comments and adds regression tests covering both emitDecoratorMetadata and no-decorator-flag scenarios.

Changes

Cohort / File(s)	Summary
Core Decorator Logic src/bundler/ParseTask.zig, src/transpiler.zig	Changed standard-decorators condition from (!loader.isTypeScript() or !experimental_decorators) to (!loader.isTypeScript() or !(experimental_decorators or emit_decorator_metadata)). Added comments clarifying that emitDecoratorMetadata implies legacy/experimental decorators and that TC39 standard decorators use a different metadata mechanism.
Regression Tests test/regression/issue/27526.test.ts	Added two tests: one asserting legacy decorator semantics when emitDecoratorMetadata: true, and one asserting TC39 standard decorators when neither emitDecoratorMetadata nor experimentalDecorators is set; both validate observable output and exit status.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: treating emitDecoratorMetadata as implying legacy decorators in the transpiler.
Description check	✅ Passed	The description covers what the PR does and includes a test plan, but lacks explicit documentation of how code changes were verified beyond test execution.
Linked Issues check	✅ Passed	The PR addresses all coding requirements from issue #27526: fixes decorator handling regression, ensures emitDecoratorMetadata implies legacy decorators, preserves TC39 standard decorator behavior, and includes regression tests.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the decorator handling regression: two implementation files modified for consistent decorator logic and one test file added to verify the fix.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[claude]

LGTM — straightforward one-line condition fix applied consistently in both code paths, with a good regression test.

Extended reasoning...

Overview

This PR fixes a regression where emitDecoratorMetadata: true in tsconfig.json (without experimentalDecorators: true) caused Bun to use TC39 standard decorator lowering instead of legacy decorators. The fix adds emit_decorator_metadata to the condition that determines whether to use standard vs. legacy decorators, applied identically in src/bundler/ParseTask.zig and src/transpiler.zig. A regression test validates both legacy and standard decorator paths.

Security risks

None. This is a transpiler semantics fix that only affects how decorators are lowered — no auth, crypto, permissions, or external I/O changes.

Level of scrutiny

This is production-critical transpiler code, but the change is minimal and mechanical: a single boolean condition extended with an or clause, applied identically in two locations. The logic is sound — emitDecoratorMetadata is a TypeScript-only feature tied to reflect-metadata and legacy decorators; TC39 standard decorators have their own metadata mechanism. The change correctly treats emitDecoratorMetadata as implying legacy decorator intent.

Other factors

• The regression test covers both the fixed case (legacy decorators with emitDecoratorMetadata) and the unaffected case (TC39 standard decorators when neither flag is set).
• CodeRabbit found no issues. No bugs were found by the bug hunting system.
• No outstanding reviewer comments or prior reviews to address.
• The PR is well-scoped and self-contained.