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.

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

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.

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