fix: propagate async @const blockers through closure references (#18309)

Right now, if you have the following:

```svelte
{@const data = await foo}
<p>{(() => data)()}<p>
```

It will blow up during CSR, because it tries to read `data` before it
exists. The solution to this is to consider references inside closures
as blockers for those closures. This _does_ mean we'll overblock in some
circumstances, such as:

```svelte
{@const data = await foo}
<button onclick={() => data}>foo<button>
```

But I wonder if that's actually incorrect? If the user were to click
`onclick` before `data` is ready, it would blow up.

---------

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

.changeset/iife-await-const-blocker.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: propagate async `@const` blockers through closure references so template expressions like `{(() => host)()}` correctly wait for the awaited value

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

@@ -52,7 +52,7 @@ export class Memoizer {
 	 * @param {ExpressionMetadata} metadata
 	 */
 	check_blockers(metadata) {
-		for (const binding of metadata.dependencies) {
+		for (const binding of metadata.references) {
 			if (binding.blocker) {
 				this.#blockers.add(binding.blocker);
 			}

packages/svelte/src/compiler/phases/nodes.js

@@ -102,8 +102,8 @@ export class ExpressionMetadata {
 		if (!this.#blockers) {
 			this.#blockers = new Set();
 
-			for (const d of this.dependencies) {
+			for (const r of this.references) {
-				if (d.blocker) this.#blockers.add(d.blocker);
+				if (r.blocker) this.#blockers.add(r.blocker);
 			}
 		}
 

packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/_config.js

@@ -0,0 +1,14 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Tests that an IIFE referencing an `await` @const inside an {#each} block
+// correctly registers the async dependency so the template waits for the
+// resolved value instead of rendering with `undefined`.
+export default test({
+	mode: ['client', 'hydrate', 'async-server'],
+	ssrHtml: '<p>example.com</p>',
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>example.com</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	async function get_host() {
+		return 'example.com';
+	}
+</script>
+
+{#each { length: 1 } as nothing}
+	{@const host = await get_host()}
+	<p>{(() => host)()}</p>
+{/each}