feat: tree-shake `export default <namespace>` re-exports by IWANABETHATGUY · Pull Request #9574 · rolldown/rolldown

IWANABETHATGUY · 2026-05-26T16:05:14Z

Summary

Extends the existing namespace re-export tree-shaking so that export default ns (where ns is import * as ns from '...') chains through the underlying module, just like the named export { ns as default } shape already does. In the scanner, LocalExport["default"].referenced is rewired to the namespace local, letting bind_imports_and_exports's re-export chain propagate and member-expression resolution descend into only the accessed exports.

Before

// main.js
import api from './middle';
console.log(api.used);
// middle.js
import * as api from './api';
export default api;
// api.js
export const used = 'used';
export const unused = 'unused';

bundled to:

var api_exports = /* @__PURE__ */ __exportAll({ unused: () => unused, used: () => used });
const used = "used";
const unused = "unused";
console.log(api_exports.used);

After

console.log("used");

Test plan

Added integration fixtures under crates/rolldown/tests/rolldown/tree_shaking/:
- default_export_namespace — primary case
- default_export_namespace_with_star — combined with export * from
- default_export_namespace_cjs — CJS importee
- default_export_namespace_member_write — write through the chain
- default_export_namespace_aliased — both export default ns and export { ns as foo }
Full integration suite passes with no regressions in other fixtures
cargo clippy -p rolldown --tests clean

IWANABETHATGUY · 2026-05-26T16:05:30Z

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codspeed-hq · 2026-05-27T06:43:47Z

Merging this PR will not alter performance

✅ 4 untouched benchmarks
⏩ 10 skipped benchmarks¹

_{Comparing 05-26-feat_treeshake_ns_reexport (b4a0ce1) with main (6164676)}

10 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. ↩

shulaoda · 2026-05-27T08:49:09Z

Concern: `sideEffects: false` interaction with `export default <namespace>` rewiring

Context

It rewires LocalExport["default"].referenced so that export default ns (where ns is import * as ns from '...') is treated like a re-export chain, enabling member-expression tree-shaking through the default import.

However, export default <expr> is semantically an own export of the module, not a re-export. This distinction matters when sideEffects: false is involved.

The scenario

// package.json
{
  "sideEffects": false
}

// main.js
import api from './middle';
console.log(api.used);

// middle.js
import * as api from './api';
console.log(123);       // <-- side effect in the module body
export default api;

// api.js
export const used = 'used';
export const unused = 'unused';

Expected behavior

Because export default api is an own export of middle.js (not a re-export like export { api as default }), consuming the default import means middle.js itself is "used". Even with sideEffects: false, the module's own statements should execute, so console.log(123) must appear in the output.

Potential issue

The optimization rewires LocalExport["default"].referenced to point at the namespace local (the api symbol from import * as api from './api'). This effectively makes the default export look like a re-export chain to the linker. Combined with sideEffects: false, the bundler may conclude that middle.js has no own exports being consumed (only pass-through re-exports), and therefore skip the module's own statements, incorrectly dropping console.log(123).

This is the same distinction described in Re-export vs Own export for default:

export { ns as default } is a re-export (the binding stays in sync with the source).
export default ns is an own export (it captures the value into a separate binding).

The optimization collapses the two cases into one, but they have different implications for module inclusion under sideEffects: false.

Suggested test case

Add a fixture that combines this optimization with sideEffects: false and a side-effectful statement in the middle module:

// _config.json
{
  "config": {
    "treeshake": {
      "moduleSideEffects": false
    }
  }
}

// main.js
import api from './middle';
console.log(api.used);

// middle.js
import * as api from './api';
console.log(123);
export default api;

// api.js
export const used = 'used';
export const unused = 'unused';

The expected output should still contain console.log(123), since the module's default export is consumed and export default ns makes it an own export that anchors the module's inclusion.

pkg-pr-new · 2026-05-27T09:57:02Z

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commit: b4a0ce1

github-actions Bot assigned IWANABETHATGUY May 26, 2026

IWANABETHATGUY force-pushed the 05-26-feat_treeshake_ns_reexport branch 2 times, most recently from 9ba18a0 to 77d0d8c Compare May 26, 2026 17:04

feat: treeshake ns reexport

b4a0ce1

IWANABETHATGUY force-pushed the 05-26-feat_treeshake_ns_reexport branch from 77d0d8c to b4a0ce1 Compare May 26, 2026 17:08

IWANABETHATGUY changed the title ~~feat: treeshake ns reexport~~ feat: tree-shake re-exports May 26, 2026

IWANABETHATGUY changed the title ~~feat: tree-shake re-exports~~ feat: tree-shake export default <namespace> re-exports May 26, 2026

IWANABETHATGUY mentioned this pull request May 27, 2026

feat: treeshake namespace aliases via non-reassigned bindings #9578

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5 tasks

IWANABETHATGUY marked this pull request as ready for review May 27, 2026 06:37

IWANABETHATGUY requested review from h-a-n-a, hyfdev and shulaoda May 27, 2026 06:37

shulaoda marked this pull request as draft May 27, 2026 06:58

IWANABETHATGUY added the trigger: preview label May 27, 2026

This was referenced Jun 15, 2026

Support patch releases for older minor versions #9758

Open

Clean up stale remote branches #9888

Closed

logaretm mentioned this pull request Jul 10, 2026

[Bug]: Destructuring a dynamic import namespace bypasses tree-shaking #10212

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Merging this PR will not alter performance

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Concern: sideEffects: false interaction with export default <namespace> rewiring

Context

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Expected behavior

Potential issue

Suggested test case

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Concern: `sideEffects: false` interaction with `export default <namespace>` rewiring