fix: add compiler error for each block mutations in runes mode

.changeset/violet-pigs-jam.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: add compiler error for each block mutations in runes mode

packages/svelte/src/compiler/errors.js

@@ -202,7 +202,8 @@ const runes = {
 		}`,
 	/** @param {string} name */
 	'invalid-runes-mode-import': (name) => `${name} cannot be used in runes mode`,
-	'duplicate-props-rune': () => `Cannot use $props() more than once`
+	'duplicate-props-rune': () => `Cannot use $props() more than once`,
+	'invalid-each-assignment': () => `Cannot reassign each block argument in runes mode`
 };
 
 /** @satisfies {Errors} */

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

@@ -334,6 +334,9 @@ function is_tag_valid_with_parent(tag, parent_tag) {
  * @type {import('zimmerframe').Visitors<import('#compiler').SvelteNode, import('./types.js').AnalysisState>}
  */
 const validation = {
+	AssignmentExpression(node, context) {
+		validate_assignment(node, node.left, context.state);
+	},
 	BindDirective(node, context) {
 		validate_no_const_assignment(node, node.expression, context.state.scope, true);
 
@@ -655,6 +658,9 @@ const validation = {
 			error(child, 'invalid-title-content');
 		}
 	},
+	UpdateExpression(node, context) {
+		validate_assignment(node, node.argument, context.state);
+	},
 	ExpressionTag(node, context) {
 		if (!node.parent) return;
 		if (context.state.parent_element) {
@@ -904,39 +910,35 @@ function validate_no_const_assignment(node, argument, scope, is_binding) {
 function validate_assignment(node, argument, state) {
 	validate_no_const_assignment(node, argument, state.scope, false);
 
-	let left = /** @type {import('estree').Expression | import('estree').Super} */ (argument);
-
-	if (left.type === 'Identifier') {
-		const binding = state.scope.get(left.name);
+	if (state.analysis.runes && argument.type === 'Identifier') {
+		const binding = state.scope.get(argument.name);
 		if (binding?.kind === 'derived') {
 			error(node, 'invalid-derived-assignment');
 		}
+
+		if (binding?.kind === 'each') {
+			error(node, 'invalid-each-assignment');
+		}
 	}
 
+	let object = /** @type {import('estree').Expression | import('estree').Super} */ (argument);
+
 	/** @type {import('estree').Expression | import('estree').PrivateIdentifier | null} */
 	let property = null;
 
-	while (left.type === 'MemberExpression') {
-		property = left.property;
-		left = left.object;
+	while (object.type === 'MemberExpression') {
+		property = object.property;
+		object = object.object;
 	}
 
-	if (left.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
+	if (object.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
 		if (state.private_derived_state.includes(property.name)) {
 			error(node, 'invalid-derived-assignment');
 		}
 	}
 }
 
 export const validation_runes = merge(validation, a11y_validators, {
-	AssignmentExpression(node, { state, path }) {
-		const parent = path.at(-1);
-		if (parent && parent.type === 'ConstTag') return;
-		validate_assignment(node, node.left, state);
-	},
-	UpdateExpression(node, { state }) {
-		validate_assignment(node, node.argument, state);
-	},
 	LabeledStatement(node, { path }) {
 		if (node.label.name !== '$' || path.at(-1)?.type !== 'Program') return;
 		error(node, 'invalid-legacy-reactive-statement');

packages/svelte/tests/compiler-errors/samples/runes-invalid-each-mutation/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-each-assignment',
+		message: 'Cannot reassign each block argument in runes mode'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-invalid-each-mutation/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let arr = $state([1,2,3]);
+</script>
+
+{#each arr as value}
+			<button onclick={() => value += 1}>click</button>
+{/each}