fix: do not treat reassigned synthetic binds as state in runes mode

Author: paoloricciuti

.changeset/bright-papayas-sneeze.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: do not treat reassigned synthetic binds as state in runes mode

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -276,6 +276,8 @@ export function analyze_component(root, source, options) {
 	/** @type {Template} */
 	const template = { ast: root.fragment, scope, scopes };
 
+	let synthetic_stores_legacy_check = [];
+
 	// create synthetic bindings for store subscriptions
 	for (const [name, references] of module.scope.references) {
 		if (name[0] !== '$' || RESERVED.includes(name)) continue;
@@ -351,16 +353,21 @@ export function analyze_component(root, source, options) {
 				}
 			}
 
-			// if we are creating a synthetic binding for a let declaration we should also declare
-			// the declaration as state in case it's reassigned
-			if (
-				declaration !== null &&
-				declaration.kind === 'normal' &&
-				declaration.declaration_kind === 'let' &&
-				declaration.reassigned
-			) {
-				declaration.kind = 'state';
-			}
+			// we push to the array because at this moment in time we can't be sure if we are in legacy
+			// mode yet because we are still changing the module scope
+			synthetic_stores_legacy_check.push((/** @type {boolean} */ runes) => {
+				// if we are creating a synthetic binding for a let declaration we should also declare
+				// the declaration as state in case it's reassigned and we are not in runes mode
+				if (
+					declaration !== null &&
+					declaration.kind === 'normal' &&
+					declaration.declaration_kind === 'let' &&
+					declaration.reassigned &&
+					!runes
+				) {
+					declaration.kind = 'state';
+				}
+			});
 
 			const binding = instance.scope.declare(b.id(name), 'store_sub', 'synthetic');
 			binding.references = references;
@@ -373,6 +380,10 @@ export function analyze_component(root, source, options) {
 
 	const runes = options.runes ?? Array.from(module.scope.references.keys()).some(is_rune);
 
+	for (let check of synthetic_stores_legacy_check) {
+		check(runes);
+	}
+
 	if (runes && root.module) {
 		const context = root.module.attributes.find((attribute) => attribute.name === 'context');
 		if (context) {

packages/svelte/tests/runtime-runes/samples/reassigned-store-not-state/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ logs, assert }) {
+		assert.deepEqual(logs, ['world']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/reassigned-store-not-state/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	let style = writable('world');
+	
+	$effect(() => {
+	  console.log($style);
+	});
+
+	function init(){
+		style = writable('svelte');
+	}
+</script>