fix: allow async destructured deriveds (#16444)

* fix: allow async destructured deriveds

* add test

* tweak

* tweak

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/mighty-balloons-rush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow async destructured deriveds

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

@@ -171,11 +171,14 @@ export function VariableDeclaration(node, context) {
 							context.state.transform[id.name] = { read: get_value };
 
 							const expression = /** @type {Expression} */ (context.visit(b.thunk(value)));
-							const call = b.call('$.derived', expression);
-							return b.declarator(
-								id,
-								dev ? b.call('$.tag', call, b.literal('[$state iterable]')) : call
-							);
+							let call = b.call('$.derived', expression);
+
+							if (dev) {
+								const label = `[$state ${declarator.id.type === 'ArrayPattern' ? 'iterable' : 'object'}]`;
+								call = b.call('$.tag', call, b.literal(label));
+							}
+
+							return b.declarator(id, call);
 						}),
 						...paths.map((path) => {
 							const value = /** @type {Expression} */ (context.visit(path.expression));
@@ -228,19 +231,37 @@ export function VariableDeclaration(node, context) {
 					}
 				} else {
 					const init = /** @type {CallExpression} */ (declarator.init);
+					let expression = /** @type {Expression} */ (
+						context.visit(value, {
+							...context.state,
+							in_derived: rune === '$derived'
+						})
+					);
 
 					let rhs = value;
 
 					if (rune !== '$derived' || init.arguments[0].type !== 'Identifier') {
 						const id = b.id(context.state.scope.generate('$$d'));
+						let call = b.call('$.derived', rune === '$derived' ? b.thunk(expression) : expression);
+
 						rhs = b.call('$.get', id);
 
-						let expression = /** @type {Expression} */ (context.visit(value));
-						if (rune === '$derived') expression = b.thunk(expression);
-						const call = b.call('$.derived', expression);
-						declarations.push(
-							b.declarator(id, dev ? b.call('$.tag', call, b.literal('[$derived iterable]')) : call)
-						);
+						if (is_async) {
+							const location = dev && !is_ignored(init, 'await_waterfall') && locate_node(init);
+							call = b.call(
+								'$.async_derived',
+								b.thunk(expression, true),
+								location ? b.literal(location) : undefined
+							);
+							call = b.call(b.await(b.call('$.save', call)));
+						}
+
+						if (dev) {
+							const label = `[$derived ${declarator.id.type === 'ArrayPattern' ? 'iterable' : 'object'}]`;
+							call = b.call('$.tag', call, b.literal(label));
+						}
+
+						declarations.push(b.declarator(id, call));
 					}
 
 					const { inserts, paths } = extract_paths(declarator.id, rhs);
@@ -250,10 +271,14 @@ export function VariableDeclaration(node, context) {
 						context.state.transform[id.name] = { read: get_value };
 
 						const expression = /** @type {Expression} */ (context.visit(b.thunk(value)));
-						const call = b.call('$.derived', expression);
-						declarations.push(
-							b.declarator(id, dev ? b.call('$.tag', call, b.literal('[$derived iterable]')) : call)
-						);
+						let call = b.call('$.derived', expression);
+
+						if (dev) {
+							const label = `[$derived ${declarator.id.type === 'ArrayPattern' ? 'iterable' : 'object'}]`;
+							call = b.call('$.tag', call, b.literal(label));
+						}
+
+						declarations.push(b.declarator(id, call));
 					}
 
 					for (const path of paths) {

packages/svelte/tests/runtime-runes/samples/async-derived-destructured/Child.svelte

@@ -0,0 +1,13 @@
+<script>
+	let count = $state(1);
+
+	let { squared, cubed } = $derived(await {
+		squared: count ** 2,
+		cubed: count ** 3
+	});
+</script>
+
+<button onclick={() => count++}>increment</button>
+
+<p>{count} ** 2 = {squared}</p>
+<p>{count} ** 3 = {cubed}</p>

packages/svelte/tests/runtime-runes/samples/async-derived-destructured/_config.js

@@ -0,0 +1,31 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>1 ** 2 = 1</p>
+				<p>1 ** 3 = 1</p>
+			`
+		);
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>2 ** 2 = 4</p>
+				<p>2 ** 3 = 8</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-destructured/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+</script>
+
+<svelte:boundary>
+	<Child />
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>

svelte.config.js

@@ -0,0 +1,8 @@
+// we need this so the VS Code extension doesn't yell at us
+export default {
+	compilerOptions: {
+		experimental: {
+			async: true
+		}
+	}
+};