feat: add partial evaluation

[Rich-Harris]

Noticed while looking into something unrelated that this input...

<script>
  let choice = $state('one');
</script>

<input type="radio" bind:group={choice} value="one" />
<input type="radio" bind:group={choice} value="two" />
<input type="radio" bind:group={choice} value="three" />

<p>{choice}</p>

...yields this output:

input.value = null == (input.__value = 'one') ? '' : 'one';

At minimum, it should be this instead:

input.value = input.__value = 'one' ?? '';

But we can go much further. Something I've been meaning to get round to for a while is to add some simple partial evaluation, which would make it possible to avoid the ?? '' stuff in a lot more cases. @Ocean-OS made some progress on this front in #15374 but I'd like it to be able to accommodate bindings as well (in other words, knowing that message is always a string and count is always a number). I think it makes sense to implement this as an evaluate method on the Scope class.

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🦋 Changeset detected

Latest commit: 696bb2e

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Playground

pnpm add https://pkg.pr.new/svelte@15494

[Rich-Harris]

Marking as ready for review, even though there's room for future improvement:

• marking some bindings as always defined (e.g. each block indices), so they don't need ?? ''
• tracking reassignments and bind: directives (e.g. if count = 0 and count++ then we know that count is always a number
• skipping $.escape calls where unnecessary (e.g. because value is a number)
• baking more stuff into $.template calls
• maybe simplifying gather_possible_values
• hoisting stuff
• (one day) evaluating imports and props

[Ocean-OS]

Is this PR meant to be combined with #15374, or just replace it?

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15494-svelte.vercel.app/

[Rich-Harris]

I think it's probably a replacement given their overlap. Which isn't a commentary on your PR! Convergent evolution; I started working on this before I understood the degree to which we were addressing a similar problem.

The reason I favour this PR over #15374 is that it feels more future-proof to have an Evaluation class with different properties than a single value which could be DYNAMIC. For example it's useful to know that an each index binding is a) defined and b) always a number, because then we can ditch the ?? '' in the client output and the $.escape(...) in the server output. I suspect it will also be useful to know that a ? 'b' : 'c' will always be either "b" or "c" when doing CSS pruning, for example.

Which isn't to say this analysis is particularly sophisticated — it definitely isn't, compared to what a good general purpose optimiser will give you. But it gives us something to build from. (Maybe one day we'll be able to incorporate type information and so on.)