add flag

packages/svelte/package.json

@@ -59,6 +59,9 @@
     "./internal/disclose-version": {
       "default": "./src/internal/disclose-version.js"
     },
+    "./internal/flags/async": {
+      "default": "./src/internal/flags/async.js"
+    },
     "./internal/flags/legacy": {
       "default": "./src/internal/flags/legacy.js"
     },

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

@@ -563,6 +563,10 @@ export function client_component(analysis, options) {
 		);
 	}
 
+	if (options.experimental.async) {
+		body.unshift(b.imports([], 'svelte/internal/flags/async'));
+	}
+
 	if (!analysis.runes) {
 		body.unshift(b.imports([], 'svelte/internal/flags/legacy'));
 	}

packages/svelte/src/index-client.js

@@ -6,7 +6,7 @@ import { is_array } from './internal/shared/utils.js';
 import { user_effect } from './internal/client/index.js';
 import * as e from './internal/client/errors.js';
 import { lifecycle_outside_component } from './internal/shared/errors.js';
-import { legacy_mode_flag } from './internal/flags/index.js';
+import { async_mode_flag, legacy_mode_flag } from './internal/flags/index.js';
 import { component_context } from './internal/client/context.js';
 import { DEV } from 'esm-env';
 

packages/svelte/src/internal/client/render.js

@@ -30,6 +30,7 @@ import * as w from './warnings.js';
 import * as e from './errors.js';
 import { assign_nodes } from './dom/template.js';
 import { is_passive_event } from '../../utils.js';
+import { async_mode_flag } from '../flags/index.js';
 
 /**
  * This is normally true — block effects should run their intro transitions —

packages/svelte/src/internal/flags/async.js

@@ -0,0 +1,3 @@
+import { enable_async_mode_flag } from './index.js';
+
+enable_async_mode_flag();

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

@@ -1,6 +1,11 @@
+export let async_mode_flag = false;
 export let legacy_mode_flag = false;
 export let tracing_mode_flag = false;
 
+export function enable_async_mode_flag() {
+	async_mode_flag = true;
+}
+
 export function enable_legacy_mode_flag() {
 	legacy_mode_flag = true;
 }

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

@@ -173,7 +173,7 @@ async function common_setup(cwd: string, runes: boolean | undefined, config: Run
 		dev: force_hmr ? true : undefined,
 		hmr: force_hmr ? true : undefined,
 		experimental: {
-			async: true
+			async: runes
 		},
 		fragments,
 		...config.compileOptions,