feat: improve treeshaking logic to handle empty parameter list in dynamic import .then() callbacks by Copilot · Pull Request #7781 · rolldown/rolldown

Copilot · 2026-01-07T05:10:48Z

Fix tree shaking for dynamic imports with empty .then() callbacks

Issue

import('./dynamic.js').then(() => {}) should be treated the same as import('./dynamic.js').then(({}) => {}) when tree shaking, but currently the former bails out to complete usage while the latter correctly tree-shakes unused exports.

Root Cause

In crates/rolldown/src/ast_scanner/dynamic_import.rs, the init_dynamic_import_usage_with_member_expr function handles .then() calls. When the callback has no parameters (like () => {}), func.params.items.first() returns None, causing the function to bail out and mark the import as having complete usage. However, when there's an empty destructured object parameter (like ({}) => {}), it correctly identifies this as an empty set of exports being used.

Solution

Modified the init_dynamic_import_usage_with_member_expr function to:

Change func.params.items.first()? to func.params.items.first() (removing the ? operator)
Handle the None case explicitly by returning Some(FxHashSet::default()) which indicates an empty set of used exports
Use idiomatic let-else pattern for cleaner code structure
This matches the behavior of empty destructured object parameters

Changes

Understand the issue and locate root cause in ast_scanner/dynamic_import.rs
Create a test case to reproduce the issue (tree_shaking/dynamic_import_then_empty_param)
Fix the logic to handle empty parameter lists same as empty destructured patterns
Run targeted tests - new test passes with correct behavior
Run full integration test suite - all 682 tests pass
Address code review feedback - simplified test case and used let-else pattern
Update filename hash snapshot after test simplification
Security scan attempted (timed out, but changes are minimal and safe)

Testing

Created focused test case tree_shaking/dynamic_import_then_empty_param that demonstrates the fix
All 682 integration tests pass
All 33 dynamic import tests pass
Test shows .then(() => {}) produces optimized output with complete tree-shaking
Updated snapshot files to reflect test simplification

Original prompt

import('./dynamic.js').then(() => {}) should be treated the same with import('./dynamic.js').then(({}) => {}) when tree shaking, but it doesn't seem to be. Fix that.

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netlify · 2026-01-07T05:11:44Z

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🔨 Latest commit	`7ca847f`
🔍 Latest deploy log	https://app.netlify.com/projects/rolldown-rs/deploys/695dfb53eb79fa000895dd10

Co-authored-by: sapphi-red <[email protected]>

crates/rolldown/src/ast_scanner/dynamic_import.rs

Co-authored-by: sapphi-red <[email protected]>

crates/rolldown/src/ast_scanner/dynamic_import.rs

Signed-off-by: 翠 <[email protected]>

crates/rolldown/tests/rolldown/tree_shaking/dynamic_import_then_empty_param/main.js

Co-authored-by: sapphi-red <[email protected]>

Copilot

Pull request overview

This PR fixes a tree-shaking inconsistency where dynamic imports with empty parameter lists in .then() callbacks were not being tree-shaken properly. The change ensures that import('./dynamic.js').then(() => {}) is treated the same as import('./dynamic.js').then(({}) => {}) for tree-shaking purposes.

Key changes:

Modified the parameter extraction logic to handle empty parameter lists explicitly
Changed from early-return-on-None (? operator) to explicit None handling with let-else pattern
Added test case demonstrating the fix

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`crates/rolldown/src/ast_scanner/dynamic_import.rs`	Core fix: Removes `?` operator from parameter extraction and adds explicit handling for empty parameter lists to return empty usage set instead of bailing out
`crates/rolldown/tests/rolldown/tree_shaking/dynamic_import_then_empty_param/main.js`	Test input demonstrating the issue: dynamic import with empty callback parameters
`crates/rolldown/tests/rolldown/tree_shaking/dynamic_import_then_empty_param/lib.js`	Test library with exports that should be tree-shaken
`crates/rolldown/tests/rolldown/tree_shaking/dynamic_import_then_empty_param/artifacts.snap`	Expected output snapshot showing proper tree-shaking with empty frozen object
`crates/rolldown/tests/rolldown/tree_shaking/dynamic_import_then_empty_param/_config.json`	Empty test configuration file
`crates/rolldown/tests/snapshots/integration_rolldown__filename_with_hash.snap`	Updated hash snapshot including new test case

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crates/rolldown/src/ast_scanner/dynamic_import.rs

github-actions · 2026-01-07T06:13:19Z

Benchmarks Rust

target: main(e5a047d)
pr: copilot/fix-tree-shaking-behavior(7ca847f)

group                                                        pr                                     target
-----                                                        --                                     ------
bundle/bundle@multi-duplicated-top-level-symbol              1.00     63.1±1.37ms        ? ?/sec    1.01     64.0±1.40ms        ? ?/sec
bundle/bundle@multi-duplicated-top-level-symbol-sourcemap    1.00     69.4±1.86ms        ? ?/sec    1.01     70.4±1.90ms        ? ?/sec
bundle/bundle@rome_ts                                        1.00    104.7±2.74ms        ? ?/sec    1.00    104.5±1.75ms        ? ?/sec
bundle/bundle@rome_ts-sourcemap                              1.01    116.7±2.12ms        ? ?/sec    1.00    116.1±1.62ms        ? ?/sec
bundle/bundle@threejs                                        1.01     37.7±2.19ms        ? ?/sec    1.00     37.2±2.17ms        ? ?/sec
bundle/bundle@threejs-sourcemap                              1.00     41.6±0.50ms        ? ?/sec    1.00     41.7±1.02ms        ? ?/sec
bundle/bundle@threejs10x                                     1.00    379.4±2.91ms        ? ?/sec    1.00    381.2±5.52ms        ? ?/sec
bundle/bundle@threejs10x-sourcemap                           1.00    440.0±3.79ms        ? ?/sec    1.01    442.5±3.79ms        ? ?/sec
scan/scan@rome_ts                                            1.00     82.3±1.55ms        ? ?/sec    1.00     82.0±1.48ms        ? ?/sec
scan/scan@threejs                                            1.00     27.4±0.33ms        ? ?/sec    1.01     27.7±1.65ms        ? ?/sec
scan/scan@threejs10x                                         1.00    281.1±3.42ms        ? ?/sec    1.00    281.6±4.31ms        ? ?/sec

sapphi-red · 2026-01-07T06:14:33Z

@copilot update the snapshot that happens with just test

Co-authored-by: sapphi-red <[email protected]>

Copilot · 2026-01-07T06:21:15Z

@copilot update the snapshot that happens with just test

Done in 7ca847f. Updated the filename hash snapshot that changed due to test simplification.

@shulaoda

## [1.0.0-beta.59] - 2026-01-07 ⚡ Inline Dynamic Imports for Statically Imported Modules - When a module is already statically imported, dynamic imports to that same module are now inlined instead of creating a separate chunk ### 🚀 Features - plugin_timings: add 3s threshold and doc link to warning message (#7741) by @shulaoda - improve treeshaking logic to handle empty parameter list in dynamic import .then() callbacks (#7781) by @Copilot - dev/lazy: don't include already executed modules (#7745) by @hyf0 - dev/lazy: support dynamic `import(..)` (#7726) by @hyf0 - inline dynamic imports that imports statically imported modules (#7742) by @IWANABETHATGUY - option: add experimental option to control chunk optimization (#7738) by @IWANABETHATGUY ### 🐛 Bug Fixes - inline dynamic entry to user defined entry with esm wrap kind (#7783) by @IWANABETHATGUY - use canonical namespace reference for property access (#7777) by @IWANABETHATGUY - dynamic entry merged into common chunk with cjs and esm wrap kind (#7771) by @IWANABETHATGUY - tla: should not await non-tla-related modules (#7768) by @hyf0 - dynamic entry captured by common chunk with CJS format (#7757) by @IWANABETHATGUY - module_loader: mark emitted chunks as user-defined entry when already loaded (#7765) by @shulaoda - normalize preserveModulesRoot path (#7737) by @IWANABETHATGUY - linker: resolve race condition in side effects computation for export-star (#7728) by @camc314 ### 🚜 Refactor - plugin_timings: filter out plugins with duration < 1s from timing warnings (#7785) by @shulaoda - module_loader: remove unnecessary collect before extend (#7769) by @shulaoda - rename _id suffixes to _idx for oxc_index types (#7767) by @IWANABETHATGUY - remove duplicate `preserve_entry_signatures` from `AddEntryModuleMsg` (#7762) by @shulaoda - module_loader: pass `user_defined_entries` by reference (#7756) by @shulaoda - dev/lazy: get proxy entry's `ResolvedId` correctly (#7746) by @hyf0 - simplify try_rewrite_import_expression control flow (#7753) by @IWANABETHATGUY - module_loader: remove unnecessary dynamic import handling for runtime module (#7754) by @shulaoda - inline __toDynamicImportESM (#7747) by @IWANABETHATGUY - use From impl for ModuleLoaderOutput conversion (#7732) by @shulaoda - remove duplicate fields from `ModuleLoader` (#7731) by @shulaoda - tweak `resolve_user_defined_entries` (#7727) by @shulaoda ### 📚 Documentation - add rolldown-string reference to native MagicString compatibility section (#7778) by @Copilot - improve comments for export star side effects handling (#7730) by @IWANABETHATGUY ### 🧪 Testing - use assertion instead of console.log for some testcase (#7744) by @IWANABETHATGUY ### ⚙️ Miscellaneous Tasks - tweak some `output.dynamicImportInCjs` related rollup test results (#7776) by @sapphi-red - mark esbuild/dce/dce_of_symbol_ctor_call as passed (#7775) by @sapphi-red - deps: update oxc apps (#7772) by @renovate[bot] - vite-tests: allow running on PRs with `test: vite-tests` label (#7770) by @shulaoda - deps: update oxc apps (#7760) by @renovate[bot] - deps: update rollup submodule for tests to v4.55.1 (#7763) by @sapphi-red - deps: update test262 submodule for tests (#7764) by @sapphi-red - deps: update oxc to v0.107.0 (#7758) by @camc314 - deps: update taiki-e/install-action action to v2.65.13 (#7751) by @renovate[bot] - deps: update rust crates (#7750) by @renovate[bot] - deps: update npm packages (#7749) by @renovate[bot] - deps: update github-actions (#7748) by @renovate[bot] - deps: update dependency oxlint-tsgolint to v0.10.1 (#7729) by @renovate[bot] - deps: update crate-ci/typos action to v1.41.0 (#7725) by @renovate[bot] Co-authored-by: shulaoda <[email protected]>

Copilot AI self-assigned this Jan 7, 2026

Initial plan

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Copilot AI assigned sapphi-red Jan 7, 2026

Copilot started work on behalf of sapphi-red January 7, 2026 05:11 View session

Copilot AI and others added 2 commits January 7, 2026 05:20

Fix tree shaking for dynamic imports with empty .then() callbacks

ca5dc0b

Co-authored-by: sapphi-red <[email protected]>

Improve comments based on code review feedback

8e09780

Co-authored-by: sapphi-red <[email protected]>

Copilot AI changed the title ~~[WIP] Fix tree shaking handling for import syntax variations~~ fix(tree-shaking): handle empty parameter list in dynamic import .then() callbacks Jan 7, 2026

Copilot AI requested a review from sapphi-red January 7, 2026 05:29

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crates/rolldown/src/ast_scanner/dynamic_import.rs Outdated Show resolved Hide resolved

Copilot started work on behalf of sapphi-red January 7, 2026 05:42 View session

Use let-else pattern for cleaner code structure

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Co-authored-by: sapphi-red <[email protected]>

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chore: update

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Signed-off-by: 翠 <[email protected]>

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Copilot started work on behalf of sapphi-red January 7, 2026 05:51 View session

Simplify test case to focus on single pattern

c59715d

Co-authored-by: sapphi-red <[email protected]>

Copilot AI requested a review from sapphi-red January 7, 2026 05:56

Copilot finished work on behalf of sapphi-red January 7, 2026 05:56

Merge branch 'main' into copilot/fix-tree-shaking-behavior

0985c8b

sapphi-red changed the title ~~fix(tree-shaking): handle empty parameter list in dynamic import .then() callbacks~~ feat: improve treeshaking logic to handle empty parameter list in dynamic import .then() callbacks Jan 7, 2026

sapphi-red requested review from IWANABETHATGUY and removed request for sapphi-red January 7, 2026 05:58

sapphi-red marked this pull request as ready for review January 7, 2026 05:58

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Copilot started work on behalf of sapphi-red January 7, 2026 06:15 View session

Update filename hash snapshot after test simplification

7ca847f

Co-authored-by: sapphi-red <[email protected]>

Copilot AI requested a review from sapphi-red January 7, 2026 06:21

Copilot finished work on behalf of sapphi-red January 7, 2026 06:21

sapphi-red merged commit bf9cc26 into main Jan 7, 2026
36 checks passed

sapphi-red deleted the copilot/fix-tree-shaking-behavior branch January 7, 2026 06:55

github-actions bot mentioned this pull request Jan 7, 2026

release: v1.0.0-beta.59 #7786

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feat: improve treeshaking logic to handle empty parameter list in dynamic import .then() callbacks#7781

feat: improve treeshaking logic to handle empty parameter list in dynamic import .then() callbacks#7781
sapphi-red merged 8 commits intomainfrom
copilot/fix-tree-shaking-behavior

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Fix tree shaking for dynamic imports with empty .then() callbacks

Issue

Root Cause

Solution

Changes

Testing

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Pull request overview

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Benchmarks Rust

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