fix: don't crash on an unknown code in a svelte-ignore comment before <script>

[wfatih]

An unknown code in a svelte-ignore comment right before the instance <script> crashes the compiler in runes mode, instead of reporting an unknown_code warning:

<!-- svelte-ignore not_a_code -->
<script>
	let count = $state(0);
</script>

Error: An impossible situation occurred
    at locator (compiler/state.js)
    at w.unknown_code (compiler/warnings.js)
    at extract_svelte_ignore (compiler/utils/extract_svelte_ignore.js)

The parser passes that HTML comment to the analysis as a leading comment of the script's Program. It creates the comment with only type and value, so comment.start + 2 in the analysis is NaN, and the warning can't be located. A known code doesn't need a position, which is why only unknown or legacy codes crash.

This change gives the comment the position of the HTML comment. When reading the leading comment of a Program, the analysis skips <!-- instead of //, so the warning points at the code itself. The comment-before-script parser snapshot changes only by the new start/end of that comment.

If template content follows the <script>, the comment also applies to that content, and the warning is reported for it too. That already happens on main for a comment before <style>, and this PR doesn't change it.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
  • tests/validator/samples/unknown-code-before-script expects the unknown_code warning at line 3, column 19. On main it fails with the crash above.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint
  • pnpm lint is clean. pnpm test: 7894 passed. The 3 failures are the timing-sensitive effect-loop-infinite and error-boundary-20 runtime tests. They fail the same way on main on my machine and pass when run alone.

[pkg-svelte-dev]

Install the latest version of svelte from 0241d58:

pnpm add https://pkg.svelte.dev/svelte/c/0241d58d2d579414ffd69c1c68a580aa5a257b88

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/19037

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0241d58

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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