perf: remove unnecessary assignment for default export#8127
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Pull request overview
This PR optimizes the code generation for default exports by eliminating unnecessary intermediate variable assignments when exporting an identifier that doesn't require a snapshot.
Changes:
- Added logic in
ast_scannerto detect whenexport default identifiercan reuse the existing symbol directly - Added logic in
module_finalizersto skip creating export statements when the identifier already points to the default export symbol - Updated test snapshots to reflect the optimized output (removing intermediate variable assignments)
- Added one test to the ignored list for circular default exports (behavioral difference with Rollup)
Reviewed changes
Copilot reviewed 34 out of 34 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
crates/rolldown/src/ast_scanner/mod.rs |
Adds optimization logic to detect when default export identifiers can reuse existing symbols |
crates/rolldown/src/module_finalizers/mod.rs |
Adds logic to skip creating export statements when identifier already is the default export symbol |
packages/rollup-tests/src/ignored-tests.js |
Adds circular-default-exports test to ignored list with explanation |
packages/rollup-tests/src/status.json |
Updates test counts (ignored +1, passed -1) |
packages/rollup-tests/src/status.md |
Updates test counts in markdown format |
| Multiple test snapshot files | Updates snapshots to reflect optimized output without intermediate variables |
crates/rolldown/tests/snapshots/integration_rolldown__filename_with_hash.snap |
Updates hashes due to code changes |
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Benchmarks Rust |
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Pull request overview
Copilot reviewed 38 out of 38 changed files in this pull request and generated no new comments.
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Does this mean this PR will introduce a bug temporarily? |
No, this has been fixed. I added the relevant test cases, but we no longer support the |
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What do you mean by "no longer support"? |
Previously, #5856 supported the Rolldown distinguishes |
Supporting this pattern may be feasible, but it would likely require changes in the link stage. It might be a bit complex. |
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I see 👍 |
crates/rolldown/tests/rolldown/topics/live_bindings/default_export_expr/artifacts.snap
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well done! |
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## [1.0.0-rc.3] - 2026-02-04
⚡ Remove Unnecessary Default Export Assignment
- Eliminates redundant variable assignments when exporting default values
- Reduces output bundle size and improves code clarity
```js
// index.js
var x = 42;
export default x;
// Before
var x = 42;
var entry_default = x;
export { entry_default as default };
// After
var x = 42;
export { x as default };
```
🐛 Avoid Unnecessary Facade Chunks
- Keep user-defined entry modules in their own chunks
- Reduces HTTP round trips and improves startup performance
```js
// main.js (entry module)
import { a } from './lib.js'
console.log(a)
// lib.js
export * from './counter.js'
// counter.js (entry module)
export const a = 100
// Before
dist/
├── main.js → import { a } from './counter-chunk.js'; console.log(a)
├── counter.js → export { a } from './counter-chunk.js' // ❌ facade chunk
└── counter-chunk.js → export const a = 100
// After
dist/
├── main.js → import { a } from './counter.js'; console.log(a)
└── counter.js → export const a = 100
```
### 🚀 Features
- expose `RUNTIME_MODULE_ID` constant for plugin authors (#8199) by @shulaoda
- warn unsupported combination of `preserveValueImports` and `importsNotUsedAsValues` in tsconfig.json (#8169) by @sapphi-red
- sophisticated watch tracking for load dependencies (#8092) by @sapphi-red
- add `inputMap` option to `minify` / `minifySync` functions (#8138) by @sapphi-red
- consolidate same tsconfig errors (#8119) by @sapphi-red
- include tsconfig file path in error messages (#8107) by @Copilot
- lazy-barrel: support incremental build mode (#8114) by @shulaoda
### 🐛 Bug Fixes
- rust: preserve dependencies added by `this.addWatchFile` (#8198) by @hyf0
- spawn `RuntimeModuleTask` after `build_start` to avoid race condition (#8182) by @shulaoda
- rust/dev: only close after the ongoing task finished (#8147) by @hyf0
- ensure `\0rolldown/runtime.js` will go through transform hook and add test (#8093) by @hyf0
- `[name]` in `assetFileNames` does not include the directory part (#8098) by @IWANABETHATGUY
- handle external module properties in namespace imports (#8124) by @IWANABETHATGUY
- keep user-defined entry modules in their own chunks (#8047) by @IWANABETHATGUY
- avoid `Unknown resolve error` error message (#8111) by @sapphi-red
### 💼 Other
- remove warnings for building rolldown with `not(feature = "experimental")` (#8110) by @coolreader18
### 🚜 Refactor
- move `VERSION` to `constants` directory (#8200) by @shulaoda
- simplify import symbol check using `SymbolFlags` (#8193) by @shulaoda
- extract tsconfig option and transform options merging logic (#8168) by @sapphi-red
- filter empty module_groups before sorting (#8149) by @ShroXd
- lazy-barrel: use single `remove` instead of `contains_key` + `remove` (#8123) by @shulaoda
- lazy-barrel: avoid redundant call and inline `get_barrel_normal_module` (#8122) by @shulaoda
- use logger instead of console.log for warnings (#8117) by @IWANABETHATGUY
- module-loader: remove intermediate ModuleTaskOwnerRef type (#8113) by @shulaoda
- rename ReExportExternalModule to ReExportDynamicExports (#8104) by @IWANABETHATGUY
### 📚 Documentation
- add dynamic OG image generation (#8192) by @sapphi-red
- add dynamic OG image generation (#8191) by @sapphi-red
- add dynamic OG image generation (#8179) by @Copilot
- apis: add links to option descriptions in JSDoc comments (#8167) by @sapphi-red
- apis: clarify parameters of `resolveDynamicImport` hook (#8137) by @sapphi-red
- lazy-barrel: clarify default export behavior (#8128) by @shulaoda
### ⚡ Performance
- remove unnecessary assignment for default export (#8127) by @shulaoda
### 🧪 Testing
- dev: `this.addWatchFile` dependency should be preserved after reload (#8165) by @sapphi-red
- mark flaky `transform_runtime_module` test as ignored (#8178) by @Copilot
- rolldown_sourcemap: add test for coarse segments (#8166) by @sapphi-red
- dev: correctly assert file change (#8164) by @sapphi-red
- rust: `transform_runtime_module` test shouldn't panic inside (#8151) by @hyf0
- rust: fix flakiness of rust tests (#8150) by @hyf0
- mark `output.dynamicImportInCjs` tests as passed (#8125) by @shulaoda
- lazy-barrel: add test cases for default export (#8129) by @shulaoda
- rolldown_plugin_vite_manifest: use relative path for outPath (#8101) by @shulaoda
### ⚙️ Miscellaneous Tasks
- deps: update crate-ci/typos action to v1.43.1 (#8188) by @renovate[bot]
- deps: update rust crate ts-rs to v12 (#8160) by @renovate[bot]
- deps: update crate-ci/typos action to v1.43.0 (#8175) by @renovate[bot]
- deps: update rust crates (#8157) by @renovate[bot]
- deps: update oxc to v0.112.0 (#8171) by @renovate[bot]
- deps: update rollup submodule for tests to v4.57.1 (#8176) by @sapphi-red
- deps: update test262 submodule for tests (#8177) by @sapphi-red
- deps: update dependency oxlint to v1.43.0 (#8173) by @renovate[bot]
- deps: update dependency oxfmt to ^0.28.0 (#8170) by @renovate[bot]
- deps: update dependency rolldown-plugin-dts to v0.21.8 (#8163) by @renovate[bot]
- deps: update dependency vue-router to v5 (#8159) by @renovate[bot]
- deps: update github-actions (#8158) by @renovate[bot]
- deps: update npm packages (#8156) by @renovate[bot]
- deps: update dependency oxlint-tsgolint to v0.11.4 (#8140) by @renovate[bot]
- fix clippy replacement typo (#8136) by @IWANABETHATGUY
- disallow `HashMap::new` and `HashSet::new` by clippy (#8135) by @sapphi-red
- deps: update dependency rolldown-plugin-dts to v0.21.7 (#8126) by @renovate[bot]
- deps: update oxc resolver to v11.17.0 (#8121) by @renovate[bot]
- deps: update dependency oxlint-tsgolint to v0.11.3 (#8109) by @renovate[bot]
### ❤️ New Contributors
* @coolreader18 made their first contribution in [#8110](#8110)
Co-authored-by: shulaoda <[email protected]>

closes #4013, partially revert of #5856, and fixed issue #5847.