fix: allow `{@html await ...}` and async snippets on the server (#16817)

Fixes #16816
Fixes #16811

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/long-spies-hope.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow `{@html await ...}` and snippets with async content on the server

packages/svelte/src/compiler/phases/3-transform/server/visitors/HtmlTag.js

@@ -9,5 +9,10 @@ import * as b from '#compiler/builders';
  */
 export function HtmlTag(node, context) {
 	const expression = /** @type {Expression} */ (context.visit(node.expression));
-	context.state.template.push(b.call('$.html', expression));
+	const call = b.call('$.html', expression);
+	context.state.template.push(
+		node.metadata.expression.has_await
+			? b.stmt(b.call('$$renderer.push', b.thunk(call, true)))
+			: call
+	);
 }

packages/svelte/src/compiler/phases/3-transform/server/visitors/SnippetBlock.js

@@ -3,6 +3,7 @@
 /** @import { ComponentContext } from '../types.js' */
 import { dev } from '../../../../state.js';
 import * as b from '#compiler/builders';
+import { create_async_block } from './shared/utils.js';
 
 /**
  * @param {AST.SnippetBlock} node
@@ -15,6 +16,10 @@ export function SnippetBlock(node, context) {
 		/** @type {BlockStatement} */ (context.visit(node.body))
 	);
 
+	if (node.body.metadata.has_await) {
+		fn.body = b.block([create_async_block(fn.body)]);
+	}
+
 	// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone
 	fn.___snippet = true;
 

packages/svelte/tests/server-side-rendering/samples/async-html-tag/_expected.html

@@ -0,0 +1 @@
+<div>this should work</div>

packages/svelte/tests/server-side-rendering/samples/async-html-tag/main.svelte

@@ -0,0 +1 @@
+<div>{@html await 'this should work'}</div>

packages/svelte/tests/server-side-rendering/samples/async-snippet/_expected.html

@@ -0,0 +1 @@
+<div>this should work</div>

packages/svelte/tests/server-side-rendering/samples/async-snippet/main.svelte

@@ -0,0 +1,6 @@
+{#snippet foo()}
+    {@const x = await 'this should work'}
+    <div>{x}</div>
+{/snippet}
+
+{@render foo()}