fix: emit useful error on invalid binding to derived state (#9497)

closes #9495

Author: Rich-Harris

.changeset/clean-eels-beg.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: emit useful error on invalid binding to derived state

packages/svelte/src/compiler/errors.js

@@ -187,6 +187,7 @@ const runes = {
 				: ''
 		}`,
 	'invalid-derived-assignment': () => `Invalid assignment to derived state`,
+	'invalid-derived-binding': () => `Invalid binding to derived state`,
 	/**
 	 * @param {string} rune
 	 * @param {number[]} args

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

@@ -274,6 +274,10 @@ export const validation = {
 				error(node.expression, 'invalid-binding-value');
 			}
 
+			if (binding.kind === 'derived') {
+				error(node.expression, 'invalid-derived-binding');
+			}
+
 			// TODO handle mutations of non-state/props in runes mode
 		}
 

packages/svelte/tests/compiler-errors/samples/runes-no-derived-binding/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-derived-binding',
+		message: 'Invalid binding to derived state'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-no-derived-binding/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let a = $state(0);
+	let b = $derived({ a });
+</script>
+
+<input type="number" bind:value={b} />