I have lost the plot

documentation/docs/98-reference/.generated/server-errors.md

@@ -15,3 +15,11 @@ You (or the framework you're using) used `render` with an async component. Eithe
 ```
 
 Certain methods such as `mount` cannot be invoked while running in a server context. Avoid calling them eagerly, i.e. not during render.
+
+### missing_experimental_flag
+
+```
+Attempted to use `renderAsync` without `experimental.async` enabled
+```
+
+Set `experimental.async: true` in your compiler options to use async server rendering.

packages/svelte/messages/server-errors/async.md

@@ -3,3 +3,9 @@
 > Encountered asynchronous work while rendering synchronously.
 
 You (or the framework you're using) used `render` with an async component. Either use `renderAsync` or wrap the async component in a `svelte:boundary` with a `pending` snippet.
+
+## missing_experimental_flag
+
+> Attempted to use `renderAsync` without `experimental.async` enabled
+
+Set `experimental.async: true` in your compiler options to use async server rendering.

packages/svelte/src/internal/server/errors.js

@@ -14,6 +14,18 @@ export function async_in_sync() {
 	throw error;
 }
 
+/**
+ * Attempted to use `renderAsync` without `experimental.async` enabled
+ * @returns {never}
+ */
+export function missing_experimental_flag() {
+	const error = new Error(`missing_experimental_flag\nAttempted to use \`renderAsync\` without \`experimental.async\` enabled\nhttps://svelte.dev/e/missing_experimental_flag`);
+
+	error.name = 'Svelte error';
+
+	throw error;
+}
+
 /**
  * `%name%(...)` is not available on the server
  * @param {string} name

packages/svelte/src/internal/server/index.js

@@ -20,6 +20,8 @@ import { validate_store } from '../shared/validate.js';
 import { is_boolean_attribute, is_raw_text_element, is_void } from '../../utils.js';
 import { Payload, TreeState } from './payload.js';
 import { abort } from './abort-signal.js';
+import { async_mode_flag } from '../flags/index.js';
+import * as e from './errors.js';
 
 // https://html.spec.whatwg.org/multipage/syntax.html#attributes-2
 // https://infra.spec.whatwg.org/#noncharacter
@@ -116,6 +118,10 @@ export function render(component, options = {}) {
  * @returns {Promise<RenderOutput>}
  */
 export async function render_async(component, options = {}) {
+	if (!async_mode_flag) {
+		e.missing_experimental_flag();
+	}
+
 	var previous_context = ssr_context;
 
 	try {

packages/svelte/src/internal/server/payload.test.ts

@@ -70,7 +70,7 @@ test('creating an async child in a sync context throws', () => {
 			await Promise.resolve();
 			$$payload.push('x');
 		})
-	).toThrow('Encountered an asynchronous component while rendering synchronously');
+	).toThrow('async_in_sync');
 });
 
 test('collect_async allows awaiting payload to get aggregated content', async () => {
@@ -200,7 +200,9 @@ test('subsume refuses to switch modes', () => {
 	b.local.select_value = 'B';
 	b.promises.initial = Promise.resolve();
 
-	expect(() => a.subsume(b)).toThrow('invariant: a payload cannot switch modes');
+	expect(() => a.subsume(b)).toThrow(
+		"invariant: A payload cannot switch modes. If you're seeing this, there's a compiler bug. File an issue!"
+	);
 });
 
 test('TreeState uid generator uses prefix and is shared by copy()', () => {

packages/svelte/tests/runtime-legacy/shared.ts

@@ -167,6 +167,7 @@ export function runtime_suite(runes: boolean) {
 			}
 
 			if (variant === 'async-ssr') {
+				if (!runes) return 'no-test';
 				if (
 					(config.mode && !config.mode.includes('async-server')) ||
 					(!config.test_ssr &&
@@ -334,7 +335,7 @@ async function run_test_variant(
 			// ssr into target
 			const SsrSvelteComponent = (await import(`${cwd}/_output/server/main.svelte.js`)).default;
 			const rendered =
-				variant === 'async-ssr' || variant === 'hydrate'
+				variant === 'async-ssr' || (variant === 'hydrate' && compileOptions.experimental?.async)
 					? await renderAsync(SsrSvelteComponent, {
 							props: config.server_props ?? config.props ?? {},
 							idPrefix: config.id_prefix

packages/svelte/tests/runtime-runes/samples/async-no-pending-throws-sync/_config.js

@@ -3,5 +3,5 @@ import { test } from '../../test';
 export default test({
 	mode: ['server'],
 
-	error: 'Encountered an asynchronous component while rendering synchronously'
+	error: 'async_in_sync'
 });

packages/svelte/tests/server-side-rendering/test.ts

@@ -59,6 +59,8 @@ const { test, run } = suite_with_variants<SSRTest, 'sync' | 'async', CompileOpti
 			...config.compileOptions
 		};
 
+		console.log(compile_options);
+
 		if (!config.load_compiled) {
 			await compile_directory(test_dir, 'server', compile_options);
 		}