fix(esm-library): handle CSS modules in preserveModules by JSerFeng · Pull Request #13670 · web-infra-dev/rspack

JSerFeng · 2026-04-09T10:16:03Z

Summary

Fix output.library.preserveModules panic when CSS modules are involved — both native CSS (experiments.css: true) and CssExtractRspackPlugin. The build previously died with "chunk xxx should have at least one file" or "Multiple assets emit different content to the same filename".
Classify modules in preserve_modules by source_types, preserve the .css extension and source path for CSS modules, and set css_filename_template instead of the JS filename_template.
Skip CSS-only modules/chunks in the ESM render phase so it no longer emits __webpack_require__('style.css') calls or import \"__RSPACK_ESM_CHUNK_<id>\" placeholders that point at chunks with no JS file.
Make CssExtractRspackPlugin honor chunk.css_filename_template, so preserveModules' per-chunk CSS filename overrides actually take effect.

Test plan

New case `tests/rspack-test/esmOutputCases/preserve-modules/css` — native CSS with `experiments.css: true` and `exportsOnly: false` (forces a real `.css` asset). Emits `style.css` + `components/button/style.css` alongside `index.mjs`.
New case `tests/rspack-test/esmOutputCases/preserve-modules/css-extract` — same layout via `CssExtractRspackPlugin`; verifies the chunk-filename override path.
All 199 `EsmOutput` tests pass (`pnpm rstest run --include "**/EsmOutput.test.js" --project base`).
Ran `Config.part*.test.js` (1685 tests) and `Normal*.test.js` (1883 tests) — all green.

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crates/rspack_plugin_esm_library/src/render.rs

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Pull request overview

Fixes output.library.preserveModules behavior when CSS-only modules/chunks are present, preventing ESM library rendering from emitting JS imports/requires for chunks that have no JavaScript asset and enabling per-chunk CSS filename overrides to take effect (including with CssExtractRspackPlugin).

Changes:

Classify preserveModules splitting by module source_types, preserving .css extension and applying chunk.css_filename_template for CSS output.
Skip CSS-only modules/chunks during ESM render so JS output no longer references CSS-only chunks as JS imports/requires.
Make CssExtractRspackPlugin respect chunk.css_filename_template and add new ESM output test fixtures for native CSS + extract-css.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/rspack_plugin_extract_css/src/plugin.rs	Honors per-chunk CSS filename overrides (`chunk.css_filename_template`) when emitting extracted CSS assets.
crates/rspack_plugin_esm_library/src/render.rs	Skips emitting JS requires/imports for CSS-only modules and chunks in ESM rendering.
crates/rspack_plugin_esm_library/src/preserve_modules.rs	Splits modules by kind (JS vs CSS), preserves `.css` names, and sets `css_filename_template` for CSS chunks.
crates/rspack_core/src/chunk.rs	Adds `set_css_filename_template` setter to support per-chunk CSS naming updates.
tests/rspack-test/esmOutputCases/preserve-modules/css/test.config.js	New esm-output case config for native CSS + preserveModules.
tests/rspack-test/esmOutputCases/preserve-modules/css/rspack.config.js	Enables `experiments.css` and preserveModules for native CSS fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css/src/index.js	Imports CSS and a nested JS module for the native CSS fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css/src/style.css	Native CSS fixture stylesheet.
tests/rspack-test/esmOutputCases/preserve-modules/css/src/components/button/index.js	Nested JS module used by the native CSS fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css/src/components/button/style.css	Nested native CSS fixture stylesheet.
tests/rspack-test/esmOutputCases/preserve-modules/css/snapshots/esm.snap.txt	Snapshot for native CSS preserveModules fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/test.config.js	New esm-output case config for extract-css + preserveModules.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/rspack.config.js	Configures `CssExtractRspackPlugin` and preserveModules for extract-css fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/src/index.js	Imports CSS and a nested JS module for the extract-css fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/src/style.css	Extract-css fixture stylesheet.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/src/components/button/index.js	Nested JS module used by the extract-css fixture.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/src/components/button/style.css	Nested extract-css fixture stylesheet.
tests/rspack-test/esmOutputCases/preserve-modules/css-extract/snapshots/esm.snap.txt	Snapshot for extract-css preserveModules fixture.

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tests/rspack-test/esmOutputCases/preserve-modules/css/test.config.js

tests/rspack-test/esmOutputCases/preserve-modules/css-extract/test.config.js

codspeed-hq · 2026-04-09T10:24:45Z

Merging this PR will degrade performance by 1.17%

❌ 1 regressed benchmark
✅ 27 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
❌	Simulation	`rust@persistent_cache_restore_after_single_file_change@basic-react-development`	27.6 ms	27.9 ms	-1.17%

_{Comparing fy/friendly-volhard (b3d5c82) with main (c0afc25)}

github-actions · 2026-04-09T10:28:01Z

Rsdoctor Bundle Diff Analysis

Found 6 projects in monorepo, 0 projects with changes.

📊 Quick Summary

Project	Total Size	Change
popular-libs	1.7 MB	0
react-10k	5.7 MB	0
react-1k	826.2 KB	0
rome	984.1 KB	0
react-5k	2.7 MB	0
ui-components	5.0 MB	0

Generated by Rsdoctor GitHub Action

github-actions · 2026-04-09T10:34:47Z

📦 Binary Size-limit

Comparing b3d5c82 to feat: implement compilation.hooks.shouldRecord for NoEmitOnErrorsPlugin (#13630) by pshu

🎉 Size decreased by 4.97KB from 49.39MB to 49.38MB (⬇️0.01%)

JSerFeng · 2026-04-10T07:55:04Z

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chatgpt-codex-connector · 2026-04-10T08:00:38Z

Codex Review: Didn't find any major issues. Hooray!

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cloudflare-workers-and-pages · 2026-04-13T02:50:25Z

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JSerFeng · 2026-04-13T03:16:59Z

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chatgpt-codex-connector · 2026-04-13T03:22:09Z

crates/rspack_plugin_esm_library/src/preserve_modules.rs

+    let base_name: String = if let Some(extension) = file_path.extension() {
+      let suffix = format!(".{}", extension.to_string_lossy());
+      file_path_str
+        .strip_suffix(&suffix)
+        .unwrap_or(&file_path_str)


Preserve unique chunk names across file extensions

Stripping the source extension unconditionally makes foo.js and foo.css both resolve to the same base_name (foo) under preserveModules. That introduces duplicate chunk names/ids for different chunks, and downstream code assumes ids are unique (NamedChunkIdsPlugin uses chunk.name() directly and after_code_generation stores id -> ukey with overwrite). In projects that colocate files like index.js + index.css, JS placeholders can resolve to the CSS-only chunk and later fail in process_assets with "chunk <id> should have at least one file".

Useful? React with 👍 / 👎.

chatgpt-codex-connector · 2026-04-13T03:22:09Z

crates/rspack_plugin_esm_library/src/preserve_modules.rs

+    let file_path_str = file_path
+      .to_slash()
+      .expect("relative path should be valid UTF-8")
+      .into_owned();


Avoid panicking on non-UTF8 module paths

to_slash().expect("relative path should be valid UTF-8") introduces a hard panic when a preserved module path contains non-UTF8 bytes (valid on Unix filesystems). The previous implementation used lossy conversion and continued; with this change, compilation can abort unexpectedly for such repositories instead of returning a normal diagnostic.

Useful? React with 👍 / 👎.

CSS modules (native `experiments.css` or mini-css-extract) under `output.library.preserveModules` caused either "chunk xxx should have at least one file" or "Multiple assets emit different content to the same filename". preserveModules set the JS filename_template on CSS chunks, left chunks nameless so multiple CSS files collapsed onto `.css`, and the ESM render phase emitted `import "__RSPACK_ESM_CHUNK_<id>"` placeholders pointing at CSS-only chunks with no JS file. Classify modules in preserve_modules by source_types and set css_filename_template for CSS modules (preserving the `.css` extension and source path), skip CSS-only modules/chunks in the ESM render paths that emit JS-side requires / cross-chunk imports, and make CssExtractRspackPlugin honor chunk.css_filename_template so preserveModules can override its per-chunk filename.

Copilot AI review requested due to automatic review settings April 9, 2026 10:16

github-actions bot added team The issue/pr is created by the member of Rspack. release: bug fix release: bug related release(mr only) labels Apr 9, 2026

Copilot started reviewing on behalf of JSerFeng April 9, 2026 10:16 View session

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JSerFeng force-pushed the fy/friendly-volhard branch 3 times, most recently from a2e272a to c085726 Compare April 10, 2026 07:53

JSerFeng force-pushed the fy/friendly-volhard branch 2 times, most recently from 49640dc to 5f69f8b Compare April 10, 2026 08:49

chatgpt-codex-connector bot reviewed Apr 13, 2026

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JSerFeng force-pushed the fy/friendly-volhard branch from 2df2b07 to 35fbb90 Compare April 13, 2026 03:40

JSerFeng requested a review from LingyuCoder as a code owner April 13, 2026 03:40

JSerFeng force-pushed the fy/friendly-volhard branch from 35fbb90 to 885ce36 Compare April 13, 2026 11:19

JSerFeng force-pushed the fy/friendly-volhard branch from 885ce36 to b3d5c82 Compare April 14, 2026 03:59

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