feat: add warning when using $bindable runes without calling it

.changeset/shiny-rats-heal.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: add warning when using `$bindable` rune without calling it

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

@@ -1168,6 +1168,15 @@ export const validation_runes = merge(validation, a11y_validators, {
 			}
 		}
 	},
+	AssignmentPattern(node, { state, path }) {
+		if (
+			node.right.type === 'Identifier' &&
+			node.right.name === '$bindable' &&
+			!state.scope.get('bindable')
+		) {
+			warn(state.analysis.warnings, node, path, 'invalid-bindable-declaration');
+		}
+	},
 	// TODO this is a code smell. need to refactor this stuff
 	ClassBody: validation_runes_js.ClassBody,
 	ClassDeclaration: validation_runes_js.ClassDeclaration,

packages/svelte/src/compiler/warnings.js

@@ -39,7 +39,9 @@ const runes = {
 	'derived-iife': () =>
 		`Use \`$derived.by(() => {...})\` instead of \`$derived((() => {...})());\``,
 	'invalid-props-declaration': () =>
-		`Component properties are declared using $props() in runes mode. Did you forget to call the function?`
+		`Component properties are declared using $props() in runes mode. Did you forget to call the function?`,
+	'invalid-bindable-declaration': () =>
+		`Bindable component properties are declared using $bindable() in runes mode. Did you forget to call the function?`
 };
 
 /** @satisfies {Warnings} */

packages/svelte/tests/validator/samples/runes-bindable-not-called/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/validator/samples/runes-bindable-not-called/input.svelte

@@ -0,0 +1,3 @@
+<script>
+	let { a = $bindable } = $props();
+</script>

packages/svelte/tests/validator/samples/runes-bindable-not-called/warnings.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "invalid-bindable-declaration",
+		"message": "Bindable component properties are declared using $bindable() in runes mode. Did you forget to call the function?",
+		"start": {
+			"column": 7,
+			"line": 2
+		},
+		"end": {
+			"column": 20,
+			"line": 2
+		}
+	}
+]