fix(scanner): skip svelte components with #snippet

[Copilot]

Fixes #20511

When a Svelte file exports snippets through its <script module> block, Vite's dependency scanner fails with an esbuild error:

✘ [ERROR] "foo" is not declared in this file

  script:/path/to/Component.svelte?id=0:2:12:
    2 │    export { foo };
      ╵             ~~~

This happens because snippets are defined in the template using {#snippet foo()} syntax, but the scanner only sees the script block where export { foo } references an undeclared identifier.

Solution

This PR adds special handling for Svelte files containing #snippet:

1. Detection: When scanning Svelte files with <script module> blocks, check if the file content contains #snippet
2. Virtual Module Export: For files with snippets, add export * from 'svelte-snippet-dummy:filename' to handle snippet exports that aren't defined in the script block
3. Externalization: Add an esbuild onResolve handler to externalize any modules starting with svelte-snippet-dummy

Example

Before this fix, this Svelte file would cause scanning to fail:

<script module>
  export { mySnippet };
</script>

{#snippet mySnippet()}
  <p>Hello from snippet!</p>
{/snippet}

After the fix, the scanner generates:

export * from 'svelte-snippet-dummy:/path/to/Component.svelte'
import "virtual:Component.svelte?id=0"
export default {}

The virtual module is externalized, preventing esbuild from trying to resolve snippet exports that don't exist in the script block.

Testing

• Added integration test with actual Svelte file containing snippets
• Test verifies that scanning completes without errors for Svelte files with #snippet syntax
• All existing tests continue to pass
• Manual verification confirms the fix resolves the scanning error

This change is backward compatible and only affects Svelte files that contain #snippet syntax.

---

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[sapphi-red]

@copilot update the PR description

[Copilot]

@copilot update the PR description

Updated the PR description to match the actual implementation. The main change was correcting virtual-module-named-svelte-dummy to svelte-snippet-dummy to reflect what was actually implemented in the code.

[Copilot]

Pull Request Overview

This PR fixes a Vite dependency scanner error that occurs when Svelte files export snippets through their <script module> block. The scanner fails because snippets are defined in the template using {#snippet foo()} syntax, but the scanner only sees the script block where export { foo } references an undeclared identifier.

• Added special handling for Svelte files containing #snippet syntax to prevent scanning errors
• Introduced a virtual module export mechanism for snippet exports that aren't defined in the script block
• Added an esbuild onResolve handler to externalize svelte-snippet-dummy modules

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/vite/src/node/optimizer/scan.ts	Adds detection for #snippet syntax and virtual module handling to prevent scanner errors
packages/vite/src/node/tests/scan.spec.ts	Adds integration test for Svelte files with snippets and refactors existing test helpers
packages/vite/src/node/tests/fixtures/scan-svelte/svelte-snippet.svelte	Test fixture demonstrating a Svelte file with snippet exports

[sapphi-red]

/ecosystem-ci run

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vite@20562

commit: 6b13cf1

[vite-ecosystem-ci]

📝 Ran ecosystem CI on aaf92ae: Open

suite	result	latest scheduled
analogjs	✅ success	❌ failure
histoire	✅ success	❌ failure
laravel	❌ failure	❌ failure
one	❌ failure	✅ success
qwik	❌ failure	✅ success

✅ nuxt, astro, ladle, marko, vite-plugin-pwa, quasar, vite-environment-examples, vite-plugin-cloudflare, rakkas, sveltekit, vite-plugin-svelte, unocss, react-router, vitepress, vite-plugin-react, vuepress, vite-setup-catalogue, vike, vitest, vite-plugin-rsc, storybook, waku, vite-plugin-vue

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 6b13cf1: Open

suite	result	latest scheduled
analogjs	✅ success	❌ failure
histoire	✅ success	❌ failure
one	❌ failure	✅ success
laravel	❌ failure	❌ failure
qwik	❌ failure	✅ success

✅ ladle, astro, marko, nuxt, unocss, quasar, rakkas, storybook, vike, sveltekit, react-router, vite-plugin-svelte, vite-plugin-react, vite-plugin-vue, vite-plugin-pwa, vitepress, vite-setup-catalogue, vite-environment-examples, vuepress, vite-plugin-cloudflare, vitest, vite-plugin-rsc, waku