fix: coarse reactivity, alternative approach

[Rich-Harris]

This is an alternative approach to #16073 that uses the existing ExpressionMetadata approach. One test is failing — will explain all this further on the other PR

Fixes #14351

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[changeset-bot]

🦋 Changeset detected

Latest commit: d2932a5

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@16100

[7nik]

Maybe build_expression should apply to @render's arguments as well.
And changes to the each-index-non-null test now can be reverted.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16100-svelte.vercel.app/

[Rich-Harris]

just realised this doesn't correctly fail in cases like this... working on it

<script>
  let a = 1;
  let b = 2;

  function sum() {
    return a + b;
  }
</script>

<button on:click={() => a++}>a++</button>
<button on:click={() => b++}>b++</button>

<p>{a} + {b} = {sum()}</p>