Regression

The build passed. The import failed at runtime.

Rspack v2.1.0 production builds could remove the real default export from a Babel-compiled CommonJS package. The affected import worked in the release candidate, and v2.1.1 restored the behavior about 18 hours later.

The production build completed.The default import did not.

Baselinev2.1.0-rc.0

analyzed change

Releasedv2.1.0

1 release affected

Restoredv2.1.1

The case file

The anatomy of a regression

01 Project
02 Regression & fix
03 Early replay

The project

A fast Rust-based bundler for the web.

Rspack is an open-source Rust bundler with a modernized webpack-compatible API. Its production optimizer shapes the JavaScript that downstream applications execute.

Project repository

The regression & fix

Valid default imports became undefined.

A new side-effect analysis treated a CommonJS export assignment inside an unused variable declaration as removable. Production optimization then discarded the declaration and the export write with it. Rspack v2.1.1 reverted the change.

Babel-compiled CommonJS package → Rspack optimizer → production bundle → default import

The Rspack release candidate preserves a CommonJS default export while v2.1.0 production optimization removes the export write and leaves it undefined. — Removing the unused declaration also removed the default export assignment that consumers needed.

Compared: v2.1.0-rc.0 → v2.1.0
Affected: v2.1.0–v2.1.0 · 1 release
Public report: User · 12 minutes after release
Fixed: v2.1.1 · ~18 hours later

The Early replay

Early found the regression.

1Regression

4Expected changes

Early verdict · REGRESSION

“CJS export-assign now DCE-prunable via declarator”

Early traced the production-only failure to the changed side-effect analysis and the export write it made removable. The independent reproduction passed on v2.1.0-rc.0, failed on v2.1.0, and passed again on v2.1.1.

Primary evidence

Follow the record.

Your case file

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[Let Early trace it]