feat: allow state created in deriveds/effects to be written/read locally without self-invalidation

[Rich-Harris]

This implements the idea in #15300 (comment). It means that state created inside a reaction (i.e. a derived or effect) can be written and read inside that reaction without causing it to re-run.

This means that things like this, which fail on main, because a simple console.log(this.name) causes an 'unsafe read'...

class User {
  name = $state();
  
  constructor(name) {
    this.name = name;
    console.log(this.name);
  }
}

const user = $derived(new User(name));

...work totally fine in this PR.

I don't see any real downside to this, and I don't see it as a breaking change (since the changed behaviour only concerns code that would have previously failed altogether). The only slight trickery concerns proxies — sources that are created lazily as properties are read need to have the same parent as the original $state, otherwise in a situation like this, which should cause an infinite loop...

let object = $state({ count: 0 });

$effect(() => {
  object.count += 1;
});

...no infinite loop would occur, because object.count would 'belong' to the effect. Userland does not have this ability, so anyone implementing their own lazy state mechanism would potentially find that assignments that 'should' cause an infinite loop don't. Personally I'm very okay with this.

Draft because it could probably use a few tests; will try and get to that tomorrow.

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Tests and linting

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[changeset-bot]

🦋 Changeset detected

Latest commit: 82c6f39

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

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svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15553-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15553

[trueadm]

Looks good. I wonder if there might be any unforeseen issues with keeping a round a reference to the parent on state signals. Like if you create a proxied object in an effect with a ton of properties and then pass that to something else outside the effect. That effect will never be GC'd even if it's unmounted because all those state signals will retain that reference forever.

[Rich-Harris]

Good catch — addressed that by reinstating a version of the derived_sources trick, but with the distinction that we only append to it if the effect that owns the source is currently updating. This also fixed a notable performance regression with the previous approach