fix: case-insensitive filename conflict detection for chunk deduplication by Copilot · Pull Request #8458 · rolldown/rolldown

Copilot · 2026-02-25T06:47:17Z

On case-insensitive filesystems (macOS APFS, Windows NTFS), chunk filenames differing only by case (e.g., Edit.js vs edit.js) resolve to the same inode, causing silent file overwrites and lost modules.

Root Cause

make_unique_name tracked used names with case-sensitive keys, so Edit.js and edit.js were treated as distinct — no suffix was generated, and the second write would overwrite the first on case-insensitive filesystems.

Fix

crates/rolldown_utils/src/make_unique_name.rs — Store lowercase keys in used_name_counts for conflict detection while preserving the original case in the returned filename:

Edit.js         → Edit.js   (registered as "edit.js" in map)
edit.js         → edit2.js  ✅ (detects collision via lowercase key)
EDIT.js         → EDIT3.js  ✅

The map key is now candidate.to_lowercase(), but the returned value retains the original-case candidate.
All existing deduplication behavior (numeric suffix, double-extension, counter propagation) is preserved.

Tests

Added case_insensitive unit test in make_unique_name.rs
Added case_insensitive_chunk_filenames integration fixture: dynamic imports of Edit.js and lowercase/edit.js produce distinct outputs Edit.js and edit2.js, with main.js correctly referencing ./edit2.js

Original prompt

This section details on the original issue you should resolve

<issue_title>[Bug]: Rolldown Case-Insensitive Filename Conflict Reproduction</issue_title>
<issue_description>### Reproduction link or steps

https://github.com/shengnoyi/rolldown-case-conflict-repro

What is expected?

When building with chunkFileNames: '[name].js' on macOS (case-insensitive filesystem), Rolldown should detect filename conflicts where files differ only in case (e.g., Edit.js vs edit.js) and automatically resolve them by adding numeric suffixes, similar to how Rollup handles it:
src/Edit.js         → dist/Edit.js
src/lowercase/edit.js → dist/edit2.js   ✅ (automatically renamed to avoid collision)
src/List.js         → dist/List.js
src/lowercase/list.js → dist/list2.js
src/View.js         → dist/View.js
src/lowercase/view.js → dist/view2.js
Expected output: 13 files, all modules preserved.

What is actually happening?

Rolldown does not detect case-only filename conflicts on case-insensitive filesystems. It attempts to write both Edit.js and edit.js as separate files, but on macOS (APFS case-insensitive), they reference the same inode, causing file overwrites:
src/Edit.js         → dist/Edit.js (written first)
src/lowercase/edit.js → dist/edit.js  ❌ (overwrites Edit.js on macOS)
src/List.js         → dist/List.js
src/lowercase/list.js → dist/list.js  ❌ (overwrites List.js)
src/View.js         → dist/View.js
src/lowercase/view.js → dist/view.js  ❌ (overwrites View.js)
Actual output: Only 10 files, 3 modules lost. The overwritten files contain content from the wrong source module (e.g., Edit.js contains content from src/lowercase/edit.js instead of src/Edit.js).

System Info
System:
    OS: macOS 15.5
    CPU: (10) arm64 Apple M4
    Memory: 240.08 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 18.20.8 - /Users/xxx/.nvm/versions/node/v18.20.8/bin/node
    Yarn: 1.22.22 - /Users/xxx/.nvm/versions/node/v18.20.8/bin/yarn
    npm: 10.8.2 - /Users/xxx/.nvm/versions/node/v18.20.8/bin/npm
    pnpm: 8.15.9 - /Users/xxx/.nvm/versions/node/v18.20.8/bin/pnpm
  Browsers:
    Chrome: 145.0.7632.45
    Safari: 18.5
Any additional comments?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes [Bug]: Rolldown Case-Insensitive Filename Conflict Reproduction #8273

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

netlify · 2026-02-25T06:47:34Z

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Name	Link
🔨 Latest commit	`0157cf4`
🔍 Latest deploy log	https://app.netlify.com/projects/rolldown-rs/deploys/699f07d953ee560008b94d72

Co-authored-by: shulaoda <[email protected]>

Copilot

Pull request overview

This PR fixes a critical bug where chunk filenames differing only by case (e.g., Edit.js vs edit.js) would silently overwrite each other on case-insensitive filesystems (macOS APFS, Windows NTFS), resulting in lost modules.

Changes:

Modified make_unique_name to use lowercase keys for conflict detection while preserving original case in output filenames
Added unit test case_insensitive to verify the fix handles case variations correctly
Added integration test fixture case_insensitive_chunk_filenames with dynamic imports to validate end-to-end behavior

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_utils/src/make_unique_name.rs	Core fix: uses `to_lowercase()` for map keys to detect case-insensitive conflicts while returning original-case candidate
crates/rolldown/tests/snapshots/integration_rolldown__filename_with_hash.snap	Updated snapshot showing `Edit.js` and `edit2.js` are correctly deduplicated
crates/rolldown/tests/rolldown/function/case_insensitive_chunk_filenames/main.js	Test entry with dynamic imports of modules differing only by case
crates/rolldown/tests/rolldown/function/case_insensitive_chunk_filenames/lowercase/edit.js	Lowercase variant test module
crates/rolldown/tests/rolldown/function/case_insensitive_chunk_filenames/_config.json	Test configuration using `[name].js` pattern to trigger conflict
crates/rolldown/tests/rolldown/function/case_insensitive_chunk_filenames/Edit.js	Capitalized test module

crates/rolldown_utils/src/make_unique_name.rs

github-actions · 2026-02-25T14:08:46Z

Benchmarks Rust

target: main(6ab3c56)
pr: copilot/fix-case-insensitive-filename-conflict(0157cf4)

group                                                        pr                                     target
-----                                                        --                                     ------
bundle/bundle@multi-duplicated-top-level-symbol              1.00     72.0±2.93ms        ? ?/sec    1.07     76.8±2.53ms        ? ?/sec
bundle/bundle@multi-duplicated-top-level-symbol-sourcemap    1.00     77.4±2.78ms        ? ?/sec    1.00     77.7±2.52ms        ? ?/sec
bundle/bundle@rome_ts                                        1.01    102.4±3.16ms        ? ?/sec    1.00    101.4±2.42ms        ? ?/sec
bundle/bundle@rome_ts-sourcemap                              1.02    114.4±2.58ms        ? ?/sec    1.00    112.1±2.61ms        ? ?/sec
bundle/bundle@threejs                                        1.03     37.9±3.12ms        ? ?/sec    1.00     36.8±0.84ms        ? ?/sec
bundle/bundle@threejs-sourcemap                              1.00     40.9±0.87ms        ? ?/sec    1.01     41.4±0.71ms        ? ?/sec
bundle/bundle@threejs10x                                     1.00    370.1±7.12ms        ? ?/sec    1.05    389.1±8.60ms        ? ?/sec
bundle/bundle@threejs10x-sourcemap                           1.00    430.8±7.51ms        ? ?/sec    1.01   433.4±10.15ms        ? ?/sec
scan/scan@rome_ts                                            1.00     76.1±1.71ms        ? ?/sec    1.01     77.0±2.08ms        ? ?/sec
scan/scan@threejs                                            1.02     27.6±1.55ms        ? ?/sec    1.00     27.0±0.50ms        ? ?/sec
scan/scan@threejs10x                                         1.00    277.9±6.52ms        ? ?/sec    1.00    277.6±4.61ms        ? ?/sec

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Copilot · 2026-02-25T15:05:22Z

crates/rolldown_utils/src/make_unique_name.rs

-    match used_name_counts.entry(candidate.clone()) {
+    // Lowercase key for case-insensitive filesystems (macOS APFS, Windows NTFS).
+    // When already lowercase, reuse the `candidate` Arc directly to avoid allocation.
+    let lowercase_candidate = match candidate.as_str().cow_to_ascii_lowercase() {


cow_to_ascii_lowercase() only normalizes ASCII A-Z, so filenames that differ by non-ASCII case (e.g. "Ä.js" vs "ä.js") can still collide on case-insensitive filesystems. This also diverges from other filename-conflict logic in the repo (e.g. to_lowercase() in rolldown_common/src/file_emitter.rs). Consider using full Unicode lowercasing (e.g. to_lowercase() / a cow_to_lowercase() helper) for the map key to make conflict detection truly case-insensitive.

Suggested change

let lowercase_candidate = match candidate.as_str().cow_to_ascii_lowercase() {

let lowercase_candidate = match candidate.as_str().cow_to_lowercase() {

Copilot · 2026-02-25T15:05:22Z

crates/rolldown/tests/rolldown/function/case_insensitive_chunk_filenames/_config.json

+    ],
+    "chunkFilenames": "[name].js"
+  },
+  "snapshot": false,


This fixture disables snapshots and output execution, and there is no _test.mjs in the folder, so the regular fixture test run doesn't assert the expected behavior (e.g. that the output contains both Edit.js and edit2.js, and that main references ./edit2.js). Consider adding a small _test.mjs assertion or enabling snapshots for this fixture so the regression is directly validated.

Suggested change

"snapshot": false,

"snapshot": true,

@hyf0

## [1.0.0-rc.6] - 2026-02-26 ### 💥 BREAKING CHANGES - css: remove `css_entry_filenames` , `css_chunk_filenames` and related code (#8402) by @hyf0 - css: drop builtin CSS bundling to explore alternative solutions (#8399) by @hyf0 ### 🚀 Features - rust/data-url: use hash as id for data url modules to prevent long string overhead (#8420) by @hyf0 - validate bundle stays within output dir (#8441) by @sapphi-red - rust: support `PluginOrder::PinPost` (#8417) by @hyf0 - support `ModuleType:Copy` (#8407) by @hyf0 - expose `ESTree` types from `rolldown/utils` (#8400) by @sapphi-red ### 🐛 Bug Fixes - incorrect sourcemap when postBanner/postFooter is used with shebang (#8459) by @Copilot - resolver: disable node_path option to align ESM resolver behavior (#8472) by @sapphi-red - parse `.js` within `"type": "commonjs"` as ESM for now (#8470) by @sapphi-red - case-insensitive filename conflict detection for chunk deduplication (#8458) by @Copilot - prevent inlining CJS exports that are mutated by importers (#8456) by @IWANABETHATGUY - parse `.cjs` / `.cts` / `.js` within `"type": "commonjs"` as CommonJS (#8455) by @sapphi-red - plugin/copy-module: correct hooks' priority (#8423) by @hyf0 - plugin/chunk-import-map: ensure `render_chunk_meta` run after users plugin (#8422) by @hyf0 - rust: correct hooks order of `DataUriPlugin` (#8418) by @hyf0 - `jsx.preserve` should also considering tsconfig json preserve (#8324) by @IWANABETHATGUY - `deferred_scan_data.rs "Should have resolved id: NotFound"` error (#8379) by @sapphi-red - cli: require value for `--dir`/`-d` and `--file`/`-o` (#8378) by @Copilot - dev: avoid mutex deadlock caused by inconsistent lock order (#8370) by @sapphi-red ### 🚜 Refactor - watch: rename TaskStart/TaskEnd to BundleStart/BundleEnd (#8463) by @hyf0 - rust: rename `rolldown_plugin_data_uri` to `rolldown_plugin_data_url` (#8421) by @hyf0 - bindingify-build-hook: extract helper for PluginContextImpl (#8438) by @ShroXd - give source loading a proper name (#8436) by @IWANABETHATGUY - ban holding DashMap refs across awaits (#8362) by @sapphi-red ### 📚 Documentation - add glob pattern usage example to input option (#8469) by @IWANABETHATGUY - remove `https://rolldown.rs` from links in reference docs (#8454) by @sapphi-red - mention execution order issue in `output.codeSplitting` docs (#8452) by @sapphi-red - clarify `output.comments` behavior a bit (#8451) by @sapphi-red - replace npmjs package links with npmx.dev (#8439) by @Boshen - reference: add `Exported from` for values / types exported from subpath exports (#8394) by @sapphi-red - add JSDocs for APIs exposed from subpath exports (#8393) by @sapphi-red - reference: generate reference pages for APIs exposed from subpath exports (#8392) by @sapphi-red - avoid pipe character in codeSplitting example to fix broken rendering (#8391) by @IWANABETHATGUY ### ⚡ Performance - avoid redundant PathBuf allocations in resolve paths (#8435) by @Brooooooklyn - bump to `sugar_path@2` (#8432) by @hyf0 - use flag-based convergence detection in include_statements (#8412) by @Brooooooklyn ### 🧪 Testing - execute `_test.mjs` even if `executeOutput` is false (#8398) by @sapphi-red - add retry to tree-shake/module-side-effects-proxy4 as it is flaky (#8397) by @sapphi-red - avoid `expect.assertions()` as it is not concurrent test friendly (#8383) by @sapphi-red - disable `mockReset` option (#8382) by @sapphi-red - fix flaky failure caused by concurrent resolveId calls (#8381) by @sapphi-red ### ⚙️ Miscellaneous Tasks - deps: update dependency rollup to v4.59.0 [security] (#8471) by @renovate[bot] - ai/design: add design doc about watch mode (#8453) by @hyf0 - deps: update oxc resolver to v11.19.0 (#8461) by @renovate[bot] - ai: introduce progressive spec-driven development pattern (#8446) by @hyf0 - deprecate output.legalComments (#8450) by @sapphi-red - deps: update dependency oxlint-tsgolint to v0.15.0 (#8448) by @renovate[bot] - ai: make CLAUDE.md a symlink of AGENTS.md (#8445) by @hyf0 - deps: update rollup submodule for tests to v4.59.0 (#8433) by @sapphi-red - deps: update test262 submodule for tests (#8434) by @sapphi-red - deps: update oxc to v0.115.0 (#8430) by @renovate[bot] - deps: update oxc apps (#8429) by @renovate[bot] - deps: update npm packages (#8426) by @renovate[bot] - deps: update rust crate owo-colors to v4.3.0 (#8428) by @renovate[bot] - deps: update github-actions (#8424) by @renovate[bot] - deps: update rust crates (#8425) by @renovate[bot] - deps: update oxc resolver to v11.18.0 (#8406) by @renovate[bot] - deps: update dependency oxlint-tsgolint to v0.14.2 (#8405) by @renovate[bot] - ban `expect.assertions` in all fixture tests (#8395) by @sapphi-red - deps: update oxc apps (#8389) by @renovate[bot] - ban `expect.assertions` in fixture tests (#8387) by @sapphi-red - enable lint for `_config.ts` files (#8386) by @sapphi-red - deps: update dependency oxlint-tsgolint to v0.14.1 (#8385) by @renovate[bot] Co-authored-by: shulaoda <[email protected]>

Copilot AI assigned Copilot and shulaoda Feb 25, 2026

Copilot started work on behalf of shulaoda February 25, 2026 06:47 View session

Copilot AI changed the title ~~[WIP] Fix Rolldown case-insensitive filename conflict issue~~ fix: case-insensitive filename conflict detection for chunk deduplication Feb 25, 2026

Copilot finished work on behalf of shulaoda February 25, 2026 06:58

Copilot AI requested a review from shulaoda February 25, 2026 06:58

Copilot AI and others added 3 commits February 25, 2026 21:51

Initial plan

7507865

fix: case-insensitive filename conflict detection in make_unique_name

7f34b81

Co-authored-by: shulaoda <[email protected]>

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shulaoda force-pushed the copilot/fix-case-insensitive-filename-conflict branch from d6ed301 to bd0956e Compare February 25, 2026 13:51

[autofix.ci] apply automated fixes

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shulaoda marked this pull request as ready for review February 25, 2026 13:53

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shulaoda marked this pull request as draft February 25, 2026 14:28

fix

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shulaoda marked this pull request as ready for review February 25, 2026 14:55

Copilot AI review requested due to automatic review settings February 25, 2026 14:55

shulaoda merged commit a23b437 into main Feb 25, 2026
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release: v1.0.0-rc.6 #8473

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fix: case-insensitive filename conflict detection for chunk deduplication#8458

fix: case-insensitive filename conflict detection for chunk deduplication#8458
shulaoda merged 6 commits intomainfrom
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	let lowercase_candidate = match candidate.as_str().cow_to_ascii_lowercase() {
	let lowercase_candidate = match candidate.as_str().cow_to_lowercase() {

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