Logical assignment operators on $state fields always assign

Describe the bug

<script>
	class Q {
		#p = $state.raw(null);
		get() {
			this.#p ??= fetch('/x');
			return this.#p;
		}
	}
</script>

compiles (5.56.6) to

get() {
	$.set(this.#p, $.get(this.#p) ?? fetch('/x'));
	return $.get(this.#p);
}

Per spec, a ??= b performs no assignment when a is non-nullish. The compiled output assigns on every evaluation, writing the read value back into the source. ||= and &&= compile the same way.

Usually this is invisible because the value is unchanged. It becomes observable in two ways. Reading through a getter that uses ??= inside a $derived throws state_unsafe_mutation even though no mutation should occur (see reproduction). And when the read happens while a batch is pending, the assignment re-commits the pre-batch value into the source, erasing the write the batch had applied. That second case is the mechanism behind sveltejs/kit#16444, where #promise ??= this.#run() on a $state.raw field permanently undid a reset() applied during navigation; the workaround on the kit side is an explicit if (sveltejs/kit#16546).

#17906 made the RHS lazy but left the assignment unconditional. Expected output is something like

if ($.get(this.#p) == null) $.set(this.#p, fetch('/x'));

Reproduction

https://svelte.dev/playground/hello-world?version=5.56.6#H4sIAAAAAAAAA21Ry27CMBD8lZFBIkgo3HkU9dob4thUYMIClpK18W4SIcS_V4YgeujFlmdnZ3bHN8O2JjMz3FSVkzOsiDtxTaywVWevgi46JTETc3QViZl934xeQ-pJgJm8FD5DyKWlShO2t0L_4aVnJVYxM7OQMrqgHwUXWlZWBGvc0qPQQcASQ1GrlEfbZWm48fxZO5Fm4xexUD07yQcBq9USo9ZWDY3mr1okbSKjp_TwPV3pSLaeRXHBEkwd1tnT45I_POZPznSKQYATsO9eDhOIR6AICVQ-9LFbrZa7hB19rBP5T5J5r7N5pIDS1yFFCadQD8touPR8cOo82wrDXEiRCRG-NvCNhkahdj-e9DriQWkQq45P0DPhQNG1dICeo-8Ej-S2DYs90rZu1Cbl_L1xm-Lte7J-3bTvYvr-E76194LN_ef-C4eSIqUkAgAA

The playground shows state_unsafe_mutation on load; the JS output tab shows the unconditional $.set. Per spec the derived is legal: the field is non-nullish, so ??= never assigns.

Logs

System Info

svelte 5.56.6

Severity

annoyance