fix(source-maps): avoid loader request in lightningcss source maps

[zalishchuk]

Summary

Fix builtin:lightningcss-loader sourcemap source names so loader-chain requests are not exposed as user-visible sources.

PR #13436 fixed previous-loader sourcemap composition by wrapping Lightning CSS output in SourceMapSource, but the loader used loader_context.context.module.request() as the generated source name. In a loader chain, that request includes entries such as css-loader!...!builtin:lightningcss-loader!...!sass-loader!...!file.scss, which can make Chrome DevTools link injected CSS rules to index.js / loader internals instead of the original CSS or SCSS file.

This change keeps the existing inner_source_map: loader_context.take_source_map() composition behavior, but uses the actual resource filename as the intermediate source name. It also extends the existing Sass previous-source-map regression test to assert that final CSS sourcemap sources do not include loader internals.

Related links

• Fixes [Bug]: injectStyles + lightningcss-loader makes DevTools link CSS rules to index.js rsbuild#7540
• Related PR: fix: lightningcss with previous loader source maps #13436

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull request overview

This PR fixes builtin:lightningcss-loader sourcemap source naming so that loader-chain requests (e.g. ...!builtin:lightningcss-loader!...!sass-loader!...) are not exposed as user-visible sources, improving DevTools source attribution when composing sourcemaps from previous loaders.

Changes:

• Switch Lightning CSS sourcemap “source name” from the full module request (including loader chain) to the actual resource filename/path.
• Extend the existing Sass previous-source-map config case to assert that final CSS sourcemap sources do not contain builtin:lightningcss-loader or sass-loader.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/rspack_loader_lightningcss/src/lib.rs	Uses the resource filename/path (not loader-chain module request) for Lightning CSS sourcemap source naming and composition.
tests/rspack-test/configCases/builtin-lightningcss-loader/sass-previous-source-map/rspack.config.js	Adds regression assertions ensuring final sourcemap sources don’t include loader internals.

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