fix: Sourcemap for "/path/to/Component.svelte" points to missing source files

[bfanger]

When using Vite's css.devSourcemap: true and using <style lang="scss"> processed with vitePreprocess will result in broken source maps for the css.

Example vite log:

Sourcemap for "/Volumes/Sites/svelte-project/src/routes/+page.svelte" points to missing source files

This is caused by the the fake filename that is passed to the transform function (which is needed so vite will select the appropriate transformation).

This PR repairs the SourceMap to use the original filename.

[dominikg]

thank you for posting this. Please add a changeset with pnpm changeset and it would also be great if there was at least a reproduction repo or even better a testcase that shows this is working as intended afterwards.

[bfanger]

Interesting, the sourcemap from the unittest i'm writing does use relative paths and a sourceRoot before it is processed by the mapSourcesToRelative():

{
  version: 3,
  sourceRoot: '/Volumes/Sites/vite-plugin-svelte/packages/e2e-tests/css-dev-sourcemap/src',
  sources: [ 'App.svelte.scss' ],
  names: [],
  mappings: 'AAEE;EACC',
  file: 'App.svelte.scss'
}

[bfanger]

I've added an e2e test, so it keeps working as intended #305 😉.
Feel free to contact me when new SourceMap issues arise.

Tip: If you paste the contents of the snapshot from
packages/e2e-tests/css-dev-sourcemap/__tests__/__snapshots__/css-dev-sourcemap.spec.ts.snap
Into the Source Map Visualization tool you'll see it now working as intended:

[dominikg]

Thanks for adding the testcase. I'm not sure using a sourcemap visualizer to be able to understand the snapshot is a good idea as it puts extra work on the maintainers every time the snapshot changes to find out why exactly it suddenly failed.

A unit test calling vitePreprocess().style directly could be better.

The filenames being relative already could be vite having improved in the meantime or because this is css and it's only a simple input. If there was additional config, eg a banner or scss imports leading to more entries in map.sources, the situation could be different.

For this PR I think it is sufficient to make sure that the extra lang suffix is stripped from the file refs in map

[dominikg]

refactored it here #625 . @bfanger please take a look. It should cover your case and then some (eg turning map.file into relative as well)

[bfanger]

Closing in favor of the 625