fix: put expressions in effects unless known to be static

[Rich-Harris]

This ensures that a template expression/attribute is wrapped in an effect unless we can guarantee that it won't update. It means that we don't lose reactivity in edge cases like this:

<script>
  let count = $state(0);

  let object = {
    toString() {
      return count;
    }
  };
</script>

<button onclick={() => count += 1}>
  {object}
</button>

Draft because scope.evaluate(...) can't currently handle cyclical bindings (which is already a bug, this PR just exposes it), but I don't want to fix that until #15781 is merged since conflicts are likely.

Closes #15289 (as well as many previous issues that were closed as 'not planned', but probably shouldn't have been), closes #15292

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[changeset-bot]

🦋 Changeset detected

Latest commit: 930c532

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

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15792

[Ocean-OS]

What does cyclical bindings mean in this context?

[Rich-Harris]

svelte/packages/svelte/tests/compiler-errors/samples/const-tag-cyclical/main.svelte

Lines 1 to 5 in e40e9eb

	{#if true}
	{@const a = b}
	{@const b = a}
	<h1>hello {a}</h1>
	{/if}

[Ocean-OS]

Wouldn't that just be an error, since b is referenced before being declared?

[Rich-Harris]

Yeah it's an error, but prior to ffc4769 you wouldn't get the error, you would get a stack overflow