fix: use the resolved vite root

[ElMassimo]

Description 📖

This pull request fixes a compatibility issue between vite-plugin-svelte and backend integrations like Vite Ruby, by using the root provided by Vite in the configResolved hook.

Background 📜

vite-plugin-svelte needs to infer the root before configResolved in order to pre-scan svelte dependencies, which was last changed in #115.

Vite plugins can override options using the config hook. In particular, Vite Ruby changes the root option based on user configuration, which is typically a subfolder (instead of the project root, process.cwd()).

vite-plugin-svelte does not use the resolved value for root, which can cause Vite and vite-plugin-svelte to use different values of root to resolve paths.

The Bug 🐞

Since the root is different, the check for file existence when creating a CSS virtual module no longer matches the expected result, preventing CSS virtual modules from being served.

Proposed Fix 🔨

Using the root that Vite provides in the configResolved hook fixes this and other similar problems.

Notes ✏️

Since vite-plugin-svelte uses enforce: 'pre', it's not viable for other plugins to workaround this issue without this patch.

Using enforce: 'pre' in other plugins would make the outcome dependent on the order in which the plugins are applied in the user config, which would be error-prone and result in a bad user experience.

[bluwy]

Hey 👋 Thanks for bring this up. This looks like a valid issue, since we have always worked under the assumption that root (and other usual user configs) are never changed by a plugin. Good to know that there's a case like this out there.

This brings another (minor) issue though. We use the root to look for svelte.config.js, and loading the Svelte config has to happen in the config hook since it would return additional Vite config. With this PR, the svelte.config.js would be loaded from the old root instead.

This likely isn't a big issue, but just noting here for future reference.

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Re the change, this looks fine to me. But on our part we might have to re-think how we should load Svelte configs.

[dominikg]

Thank you for this high quality PR. ❤️

Regarding enforce: pre, this has been required for vite-plugin-svelte to be able to resolve svelte correctly. We may be able to either split vite-plugin-svelte into multiple plugins where only the resolve part has 'pre' or check if changes in vite since then made it possible to do it differently.

But in the interim i'm going to merge this, as it doesn't change the behavior if root is unchanged and improves the behavior if it is.

Although i think plugins should not modify root from their config hook. This value is fundamental and should be stable for all plugins regardless or ordering. Users can provide a custom root from their config file or via cli. Custom frameworks or integrations could document how to set that up or provide a wrapper around defineConfig 🤔

Imagine a situation where 2 plugins want to modify root to different locations. Which one "wins"?

[buhrmi]

Just updated to 1.0.0-next.34, works perfectly. Thanks guys