fix(bundler): skip barrel optimization for entry points with sideEffects

[robobun]

Summary

• Fix barrel imports optimization incorrectly tree-shaking entry point re-exports when the project's package.json has a sideEffects field
• Entry points have no requested_exports (nothing imports them within the bundle), so the barrel optimization would mark all their imports as unused, producing empty output with dangling references like export{m as B,e as A};
• Add an early return in applyBarrelOptimizationImpl to skip barrel optimization for entry points

Closes #27709

Test plan

• Added regression tests in test/regression/issue/27709.test.ts covering both sideEffects: ["./dist/index.js"] and sideEffects: false
• Tests fail with system bun (v1.3.10) confirming the bug
• Tests pass with debug build confirming the fix
• All 46 existing barrel tests pass (test/bundler/bundler_barrel.test.ts)

🤖 Generated with Claude Code

[robobun]

^{Updated 1:17 AM PT - Mar 3rd, 2026}

❌ Your commit 1213676b has 2 failures in Build #38573 (All Failures):

• annotation error on 🪟 x64-baseline - build-bun
• annotation error on 🪟 x64 - build-bun
• annotation error on 🪟 aarch64 - build-bun
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl - build-cpp
• CMakeLists.txt - cmake error on 🍎 x64 - build-bun
• CMakeLists.txt - cmake error on 🍎 aarch64 - build-bun
• CMakeLists.txt - cmake error on 🐧 x64-baseline - build-bun
• CMakeLists.txt - cmake error on 🐧 x64-musl-baseline - build-bun
• CMakeLists.txt - cmake error on 🐧 x64 - build-bun
• CMakeLists.txt - cmake error on 🪟 x64-baseline - build-bun
• CMakeLists.txt - cmake error on 🐧 aarch64 - build-bun
• CMakeLists.txt - cmake error on 🪟 x64 - build-bun
• CMakeLists.txt - cmake error on 🪟 aarch64 - build-bun
• CMakeLists.txt - cmake error on 🐧 aarch64-musl - build-bun
• CMakeLists.txt - cmake error on 🐧 x64-asan - build-bun

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🧪   To try this PR locally:

bunx bun-pr 27710

That installs a local version of the PR into your bun-27710 executable, so you can run:

bun-27710 --bun

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 6c7e972 and 1213676.

📒 Files selected for processing (2)

• src/bundler/barrel_imports.zig
• test/regression/issue/27709.test.ts

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Walkthrough

Adds a guard in barrel import optimization to skip entry point files, preventing unwanted barrel optimization that affects public API exports. Introduces regression tests validating sideEffects field handling with re-exports during bundling.

Changes

Cohort / File(s)	Summary
Barrel Import Optimization Guard src/bundler/barrel_imports.zig	Adds early return in applyBarrelOptimizationImpl to skip barrel optimization for entry point files, ensuring entry points preserve all exports.
Regression Tests for sideEffects Handling test/regression/issue/27709.test.ts	Adds test suite verifying correct re-export preservation when sideEffects is configured in package.json, covering both array and false configurations.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: skipping barrel optimization for entry points when sideEffects is present.
Description check	✅ Passed	The description covers the problem, root cause, solution, and test verification, though it uses a generated footer that could be considered non-essential.
Linked Issues check	✅ Passed	The code changes directly address issue #27709 by preventing barrel optimization on entry points, which resolves the tree-shaking problem with sideEffects.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the barrel optimization issue for entry points with sideEffects; no out-of-scope modifications detected.

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[claude]

LGTM — small, focused fix with good regression tests.

Extended reasoning...

Overview

This PR adds a 7-line early return in applyBarrelOptimizationImpl (src/bundler/barrel_imports.zig) that skips barrel optimization for entry points. Entry points have no requested_exports since nothing imports them within the bundle, so barrel optimization would incorrectly mark all their imports as unused when sideEffects is configured. The test file (test/regression/issue/27709.test.ts) adds two regression tests covering both sideEffects: ["./dist/index.js"] and sideEffects: false.

Security risks

None — this is a bundler optimization fix with no security implications.

Level of scrutiny

Moderate. The production code change is minimal and follows established patterns (linear scan over entry_points.items is used throughout bundle_v2.zig). The logic is straightforward: entry points should never be barrel-optimized because they define the public API of the bundle.

Other factors

• The entry point check is correctly positioned after the initial guards (optimize_imports, side_effects check, empty records check) but before the barrel verification loop.
• The test is placed in the correct location per CLAUDE.md conventions (test/regression/issue/27709.test.ts).
• Tests use tempDir with using for proper cleanup, and test via Bun.build API which is appropriate for bundler regression tests.
• No CODEOWNERS concerns — this is standard bundler code.
• The PR description notes all 46 existing barrel tests continue to pass.

[Jarred-Sumner]

Can you use std.mem.containsScalar?