fix: properly transform destructured $derived.by declarations (#12984)

fixes #12983

Author: Rich-Harris

.changeset/olive-llamas-warn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: properly transform destructured `$derived.by` declarations

packages/svelte/src/compiler/phases/3-transform/client/visitors/VariableDeclaration.js

@@ -173,7 +173,7 @@ export function VariableDeclaration(node, context) {
 					let id;
 					let rhs = value;
 
-					if (init.arguments[0].type === 'Identifier') {
+					if (rune === '$derived' && init.arguments[0].type === 'Identifier') {
 						id = init.arguments[0];
 					} else {
 						id = b.id(context.state.scope.generate('$$d'));

packages/svelte/tests/runtime-runes/samples/destructure-derived-by/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: '<button>clicks: 0</button>',
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, `<button>clicks: 1</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/destructure-derived-by/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let count = $state(0);
+	let fn = () => ({ n: count });
+
+	let { n } = $derived.by(fn);
+</script>
+
+<button onclick={() => (count += 1)}>clicks: {n}</button>