rerun reactive declarations if the reactive declared variable is mutated

[tanhauhau]

Fixes #2444

The demo in #2444 was fixed in v3.2.1 but broken again since v3.16.0

The test case added in #2694 didn't handle a particular case in the demo where there's another declaration that should be invalidated too:

<script>
   let foo;
   $: var_a = some_fn(foo);
   $: var_b = some_fn(foo);
</script>
{#each var_b as b}
   <input bind:value={b} />
{/each}
{var_a.length} {var_b.length}

after #2694 mutating var_b, would $$invalidate var_b & foo.

but because foo wasn't mutated anywhere, var_a is marked as fixed_reactive_declarations instead, and $$invalidate foo will not produce a new var_a.

This fix is to mark foo as mutated if var_b or var_a is mutated.

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

• It's really useful if your PR relates to an outstanding issue, so please reference it in your PR, or create an explanatory one for discussion. In many cases features are absent for a reason.
• This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.
• Ideally, include a test that fails without this PR but passes with it. PRs will only be merged once they pass CI. (Remember to npm run lint!)

Tests

• Run the tests tests with npm test or yarn test)

[Conduitry]

The test failure here looks to be legitimate.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[vercel]

@dummdidumm is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[dummdidumm]

I'm not sure how I feel about this change.

• Con: It could result in a lot of false positive reruns
• Con: The tests currently fail, unsure how reliably we can get this to work with introducing bugs
• Pro: Intuitively it makes sense to me to rerun in this case

In general this looks like one of those general shortcomings / things we haven't quite thought through holistically for bindings. Personally I'd hold off from merging this and wait until Svelte 5 where we should have a deep conversation about the future direction of bindings.

[tanhauhau]

@dummdidumm yes you are right, i think we can hold off or even close this one for now.