disallow `flushSync()` inside effects

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

@@ -74,6 +74,16 @@ Effect cannot be created inside a `$derived` value that was not itself created i
 Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
 ```
 
+### flush_sync_in_effect
+
+```
+Cannot use `flushSync` inside an effect
+```
+
+The `flushSync()` function can be used to flush any pending effects synchronously. It cannot be used if effects are currently being flushed — in other words, you can call it after a state change but _not_ inside an effect.
+
+This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
+
 ### hydration_failed
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -48,6 +48,14 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 
 > Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
 
+## flush_sync_in_effect
+
+> Cannot use `flushSync` inside an effect
+
+The `flushSync()` function can be used to flush any pending effects synchronously. It cannot be used if effects are currently being flushed — in other words, you can call it after a state change but _not_ inside an effect.
+
+This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
+
 ## hydration_failed
 
 > Failed to hydrate the application

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

@@ -335,3 +335,18 @@ export function state_unsafe_mutation() {
 		throw new Error(`https://svelte.dev/e/state_unsafe_mutation`);
 	}
 }
+
+/**
+ * Cannot use `flushSync` inside an effect
+ * @returns {never}
+ */
+export function flush_sync_in_effect() {
+	if (DEV) {
+		const error = new Error(`flush_sync_in_effect\nCannot use \`flushSync\` inside an effect\nhttps://svelte.dev/e/flush_sync_in_effect`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/flush_sync_in_effect`);
+	}
+}

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

@@ -706,6 +706,10 @@ export function process_effects(batch, root) {
  * @returns {T}
  */
 export function flushSync(fn) {
+	if (async_mode_flag && active_effect !== null) {
+		e.flush_sync_in_effect();
+	}
+
 	var result;
 
 	const batch = Batch.ensure();

packages/svelte/src/legacy/legacy-client.js

@@ -10,6 +10,7 @@ import * as w from '../internal/client/warnings.js';
 import { DEV } from 'esm-env';
 import { FILENAME } from '../constants.js';
 import { component_context, dev_current_component_function } from '../internal/client/context.js';
+import { async_mode_flag } from '../internal/flags/index.js';
 
 /**
  * Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.
@@ -119,8 +120,9 @@ class Svelte4Component {
 			recover: options.recover
 		});
 
-		// We don't flushSync for custom element wrappers or if the user doesn't want it
+		// We don't flushSync for custom element wrappers or if the user doesn't want it,
-		if (!options?.props?.$$host || options.sync === false) {
+		// or if we're in async mode since `flushSync()` will fail
+		if (!async_mode_flag && (!options?.props?.$$host || options.sync === false)) {
 			flushSync();
 		}
 

packages/svelte/tests/signals/test.ts

@@ -6,7 +6,8 @@ import {
 	effect,
 	effect_root,
 	render_effect,
-	user_effect
+	user_effect,
+	user_pre_effect
 } from '../../src/internal/client/reactivity/effects';
 import { state, set, update, update_pre } from '../../src/internal/client/reactivity/sources';
 import type { Derived, Effect, Source, Value } from '../../src/internal/client/types';
@@ -1079,17 +1080,17 @@ describe('signals', () => {
 	test('nested effects depend on state of upper effects', () => {
 		const logs: number[] = [];
 
-		user_effect(() => {
+		user_pre_effect(() => {
 			const raw = state(0);
 			const proxied = proxy({ current: 0 });
 
 			// We need those separate, else one working and rerunning the effect
 			// could mask the other one not rerunning
-			user_effect(() => {
+			user_pre_effect(() => {
 				logs.push($.get(raw));
 			});
 
-			user_effect(() => {
+			user_pre_effect(() => {
 				logs.push(proxied.current);
 			});
 
@@ -1097,7 +1098,7 @@ describe('signals', () => {
 			// together with the reading effects
 			flushSync();
 
-			user_effect(() => {
+			user_pre_effect(() => {
 				$.untrack(() => {
 					set(raw, $.get(raw) + 1);
 					proxied.current += 1;