docs: virtual modules internal convention

[patak-dev]

Description

See added docs and test

AFAICS, if plugins in the ecosystem would follow this rollup convention, then we wouldn't have dealt with issues as in #2904 and #5421

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

[patak-dev]

@bluwy are you using virtual modules in vite-plugin-svelte? I'm going to do a PR at one point for vite core to start using \0 for internal virtual modules to see how it works. AFAICS, the only issue is that we'll get slightly worse URLs for them but since \0 is used in Rollup core (for example during sourcemaps processing), I think it is a good idea to use it.

[bluwy]

Yeah vite-plugin-svelte also uses virtual modules for Svelte files, similar to how the Vue plugin processes them. In short, for every Svelte file that has CSS, a /path/to/component.svelte&type=style&lang.css counterpart is created. We have a function for it.

For this case we would need to prepend that path with \0, which is a bit weird and makes things feel separated, but it might be good for consistency anyways.

[patak-dev]

Interesting, I was thinking of the helper virtual modules (like the module preload polyfill). In the case of the modules generated from the SFCs, the file is still there, so maybe this doesn't apply (you want the sourcemap to point to the original .svelte file, no?)

[bluwy]

Hmm yeah putting it that way, perhaps SFCs don't need to follow the convention after all, as long as the virtual module directly derives from the single real module. Maybe it would benefit to have this in the docs too 🤔

[patak-dev]

@bluwy @aleclarson would you review if the added note is clear enough?

Note that modules directly derived from a real file, as in the case of a script module in a Single File Component (like a .vue or .svelte SFC) don't need to follow this convention. SFCs generally generate a set of submodules when processed but the code in these can be mapped back to the filesystem. Using \0 for these submodules would prevent sourcemaps from working correctly.

[bluwy]

Minor typo, but otherwise looks good to me 👍