fix: throw validation error when binding to each argument in runes mode

.changeset/mighty-files-hammer.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: throw validation error when binding to each argument in runes mode

packages/svelte/src/compiler/errors.js

@@ -207,7 +207,8 @@ 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`,
-	'invalid-each-assignment': () => `Cannot reassign each block argument in runes mode`
+	'invalid-each-assignment': () =>
+		`Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. 'array[i] = value' instead of 'entry = value')`
 };
 
 /** @satisfies {Errors} */

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

@@ -1,5 +1,5 @@
 import { error } from '../../errors.js';
-import { extract_identifiers, get_parent, is_text_attribute } from '../../utils/ast.js';
+import { extract_identifiers, get_parent, is_text_attribute, object } from '../../utils/ast.js';
 import { warn } from '../../warnings.js';
 import fuzzymatch from '../1-parse/utils/fuzzymatch.js';
 import { disallowed_parapgraph_contents, interactive_elements } from '../1-parse/utils/html.js';
@@ -341,13 +341,9 @@ const validation = {
 		validate_no_const_assignment(node, node.expression, context.state.scope, true);
 
 		const assignee = node.expression;
-		let left = assignee;
+		const left = object(assignee);
 
-		while (left.type === 'MemberExpression') {
-			left = /** @type {import('estree').MemberExpression} */ (left.object);
-		}
-
-		if (left.type !== 'Identifier') {
+		if (left === null) {
 			error(node, 'invalid-binding-expression');
 		}
 
@@ -373,6 +369,10 @@ const validation = {
 				error(node.expression, 'invalid-derived-binding');
 			}
 
+			if (context.state.analysis.runes && binding.kind === 'each') {
+				error(node, 'invalid-each-assignment');
+			}
+
 			// TODO handle mutations of non-state/props in runes mode
 		}
 

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

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-each-assignment',
+		message:
+			"Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. 'array[i] = value' instead of 'entry = value')"
+	}
+});

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

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

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

@@ -3,6 +3,7 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'invalid-each-assignment',
-		message: 'Cannot reassign each block argument in runes mode'
+		message:
+			"Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. 'array[i] = value' instead of 'entry = value')"
 	}
 });

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

@@ -3,5 +3,5 @@
 </script>
 
 {#each arr as value}
-			<button onclick={() => value += 1}>click</button>
+	<button onclick={() => value += 1}>click</button>
 {/each}