disallow $state(new SvelteSet) and similar

[mustafa0x]

Motivation

SvelteMap, SvelteSet, and SvelteURL don't need to be wrapped in $state. but that's not always obvious

Description

per above

Examples

<!-- ✓ GOOD -->
<script>
const myset = new SvelteSet
</script>

<!-- ✗ BAD -->
<script>
const myset = $state(new SvelteSet)
</script>

Additional comments

No response

[marekdedic]

Hmm, reacting quickly and not thoroughly enough, but this needs a little bit more nuance:

let myOptionalSet = $state(new SvelteSet());

// Somewhere further down
myOptionalSet = null; // This needs the variable to be wrapped in a $state in order for this to be reactive.

See sveltejs/svelte#14799 for more details.

[mikededo]

I agree with @marekdedic. The statement that reactive built-ins do not need to be wrapped with $state is not correct.

[marekdedic]

Yes, at the same time, it actually is a mistake like 95% of the time... So I think it's a great candidate for a lint rule, the rule just needs to be clever and actually figure out when the $state is really not needed.

[mustafa0x]

$state(new Svelte matches 90 files. Most don't seem to reassign. So yes, if the rule was smart, that'd be best.