fix: properly lazily evaluate RHS when checking for assignment_value_stale (#17906)

Fixes #17904 by wrapping the RHS in a `() =>`.

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- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/quiet-jars-search.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: properly lazily evaluate RHS when checking for `assignment_value_stale`

packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js

@@ -179,7 +179,7 @@ function build_assignment(operator, left, right, context) {
 	// in cases like `(object.items ??= []).push(value)`, we may need to warn
 	// if the value gets proxified, since the proxy _isn't_ the thing that
 	// will be pushed to. we do this by transforming it to something like
-	// `$.assign_nullish(object, 'items', [])`
+	// `$.assign_nullish(object, 'items', () => [])`
 	let should_transform =
 		dev &&
 		path.at(-1) !== 'ExpressionStatement' &&
@@ -236,7 +236,7 @@ function build_assignment(operator, left, right, context) {
 							? left.property
 							: b.literal(/** @type {Identifier} */ (left.property).name)
 					),
-					right,
+					b.arrow([], right),
 					b.literal(locate_node(left))
 				)
 			)

packages/svelte/src/internal/client/dev/assign.js

@@ -21,12 +21,12 @@ function compare(a, b, property, location) {
 /**
  * @param {any} object
  * @param {string} property
- * @param {any} value
+ * @param {() => any} rhs_getter
  * @param {string} location
  */
-export function assign(object, property, value, location) {
+export function assign(object, property, rhs_getter, location) {
 	return compare(
-		(object[property] = value),
+		(object[property] = rhs_getter()),
 		untrack(() => object[property]),
 		property,
 		location
@@ -36,12 +36,12 @@ export function assign(object, property, value, location) {
 /**
  * @param {any} object
  * @param {string} property
- * @param {any} value
+ * @param {() => any} rhs_getter
  * @param {string} location
  */
-export function assign_and(object, property, value, location) {
+export function assign_and(object, property, rhs_getter, location) {
 	return compare(
-		(object[property] &&= value),
+		(object[property] &&= rhs_getter()),
 		untrack(() => object[property]),
 		property,
 		location
@@ -51,12 +51,12 @@ export function assign_and(object, property, value, location) {
 /**
  * @param {any} object
  * @param {string} property
- * @param {any} value
+ * @param {() => any} rhs_getter
  * @param {string} location
  */
-export function assign_or(object, property, value, location) {
+export function assign_or(object, property, rhs_getter, location) {
 	return compare(
-		(object[property] ||= value),
+		(object[property] ||= rhs_getter()),
 		untrack(() => object[property]),
 		property,
 		location
@@ -66,12 +66,12 @@ export function assign_or(object, property, value, location) {
 /**
  * @param {any} object
  * @param {string} property
- * @param {any} value
+ * @param {() => any} rhs_getter
  * @param {string} location
  */
-export function assign_nullish(object, property, value, location) {
+export function assign_nullish(object, property, rhs_getter, location) {
 	return compare(
-		(object[property] ??= value),
+		(object[property] ??= rhs_getter()),
 		untrack(() => object[property]),
 		property,
 		location

packages/svelte/tests/runtime-runes/samples/assignment-value-stale-lazy-rhs/_config.js

@@ -0,0 +1,19 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test({ assert, target }) {
+		const button = /** @type {HTMLElement} */ (target.querySelector('button'));
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>go</button><p>count1: 0, count2: 0</p>`);
+		button.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>go</button><p>count1: 1, count2: 1</p>`);
+		button.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>go</button><p>count1: 2, count2: 1</p>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/assignment-value-stale-lazy-rhs/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	let count1 = $state(0);
+	let count2 = $state(0);
+	let cache = $state({});
+
+	function go() {
+		count1++;
+		const value = cache.value ??= get_value();
+	}
+
+	function get_value() {
+		count2++;
+		return 42;
+	}
+</script>
+
+<button onclick={go}>go</button>
+<p>count1: {count1}, count2: {count2}</p>