rebaseUrls runs before Sass variables are resolved

[DavidBiddle]

Describe the bug

I'm importing sass from a package (govuk-frontend) which has a mixin to generate asset URLs.

When using the modern Sass API with Vite, and using an alias in the import path, I find that the assets don't build and I get an error message $govuk-fonts-path + $filename referenced in $govuk-fonts-path + $filename didn't resolve at build time, it will remain unchanged to be resolved at runtime.

When debugging, I can see that the rebaseUrls function is converting the url($govuk-fonts-path + $filename) from the mixin into url("$govuk-fonts-path + $filename"). This is unexpected, because the variables should be replaced with their values before the rebaseUrls function wraps them in quotes.

This doesn't happen if the package is imported using another importer:

• The package is imported using Sass's NodePackageImporter and the pkg: syntax
• The package is imported using its full node_modules URL (i.e. node_modules/govuk-frontend/dist/govuk/all) which I think must use Sass's builtin FileImporter

It alsl doesn't happen when using the legacy Sass API. This suggests to me that there's something different happening in makeModernScssWorker’s internalImporter that causes rebaseUrls to be run before the sass variables have been resolved.

Reproduction

https://github.com/DavidBiddle/vite-issue-test

Steps to reproduce

Run npm install and npx vite build
Check response for didn't resolve at build time, it will remain unchanged to be resolved at runtime message

System Info

System:
    OS: macOS 15.2
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 1.07 GB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.12.1 - ~/.nvm/versions/node/v20.12.1/bin/node
    Yarn: 1.22.19 - ~/.yarn/bin/yarn
    npm: 10.5.0 - ~/.nvm/versions/node/v20.12.1/bin/npm
  Browsers:
    Chrome: 131.0.6778.265
    Safari: 18.2

Used Package Manager

npm

Logs

No response

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