fix: thunkify deriveds on the server

[paoloricciuti]

Closes #14954

I took a shot with the idea of thunkify every derived on the server to make them "work" in case of reassignment. As pointed out by @Rich-Harris this have some downside:

1. we re-execute possibly expensive deriveds on access
2. it's still more work than just accessing a variable

But I think regardless of the solution we will go with we will probably need functions in some way so this PR can be a good start.

I got 90% there in 10 minutes and then faced a big problem with destructured deriveds. Since we want every destructured identifier to be a function the only way i thought of is to do something like this.

let stuff = $state({ foo: true, bar: [1, 2, {baz: 'baz'}] });
let { foo, bar: [a, b, { baz }]} = $derived(stuff);

let stuff = {
    foo: true,
    bar: [1, 2, { baz: 'baz' }]
};

let {
        foo: foo_1,
        bar: [a_1, b_1, { baz: baz_1 }]
    } = stuff,
    foo = () => foo_1,
    a = () => a_1,
    b = () => b_1,
    baz = () => baz_1;

it's a bit more generated code but it works fine.

Again, this is a proposal, don't know if this the actual direction we want to go but even if it's not it could be a good base.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14977-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14977

[Rich-Harris]

Brought this up to speed with main but there's a problem: now that deriveds are writable, this transformation results in a syntax error (foo?.() = blah is invalid).

Not really sure how best to resolve that so for now I'm not going to try.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-14977-svelte.vercel.app/

[paoloricciuti]

Brought this up to speed with main but there's a problem: now that deriveds are writable, this transformation results in a syntax error (foo?.() = blah is invalid).

Not really sure how best to resolve that so for now I'm not going to try.

Yeah i planned to get back to this after the writable deriveds PR but wanted to check if that's the road we want to go first. I can work on this if we want.