fix: preserve transform sourcemaps for webpack and rspack

[SyMind]

Fixes #616

Summary

• preserve transform sourcemaps when no input sourcemap is provided to the loader
• combine transformed and upstream sourcemaps for Webpack and Rspack instead of dropping either mapping layer
• add regression coverage for sourcemap passthrough and composition in both adapters

Tests

• pnpm test
• pnpm typecheck
• pnpm lint
• pnpm build

Summary by CodeRabbit

• Bug Fixes
  • Improved source-map handling for Webpack and Rspack transformations.
  • Preserved transformed or incoming source maps when only one is available.
  • Correctly combines source maps when both transformed and incoming maps exist.
• Tests
  • Added coverage for transformed source maps and combined source-map output.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f9763ce4-099e-4ed1-8906-5c6120ab7deb

📥 Commits

Reviewing files that changed from the base of the PR and between 48b18ba and 512e90d.

📒 Files selected for processing (4)

• src/rspack/loaders/transform.ts
• src/webpack/loaders/transform.ts
• test/unit-tests/rspack/loaders/transform.test.ts
• test/unit-tests/webpack/loaders/transform.test.ts

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

Both transform loaders now compose returned and incoming source maps. Tests cover direct forwarding, composition, and native build-context callbacks.

Changes

Transform source map handling

Layer / File(s)	Summary
Compose transform and incoming maps src/rspack/loaders/transform.ts, src/webpack/loaders/transform.ts	Both loaders use @jridgewell/remapping when both maps exist. Each loader forwards the available map when only one exists.
Validate source map results test/unit-tests/rspack/loaders/transform.test.ts, test/unit-tests/webpack/loaders/transform.test.ts	Tests add source-map fixtures and verify direct forwarding, composed mappings, and native build-context callback results.

Estimated code review effort: 2 (Simple) | ~15 minutes

Merge Risk: ⚪ Minimal · up to 512e9

The PR preserves and composes transform sourcemaps for Webpack and Rspack without any supplied merge-blocking concern; no actionable risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving transform source maps for both Webpack and Rspack adapters.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[sxzz]

Please send an issue first.

[SyMind]

Created #616 with a runnable Rspack reproduction: https://github.com/SyMind/unplugin-rspack-sourcemap-repro