chore: lift "flushSync cannot be called in effects" restriction

Since async-await was introduced into the code base a lot has changed. This lifts the restriction.

Closes #17131 (though I still wonder why Skeleton does that)

.changeset/heavy-lions-tap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: lift "flushSync cannot be called in effects" restriction

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

@@ -136,16 +136,6 @@ Often when encountering this issue, the value in question shouldn't be state (fo
 Cannot use `fork(...)` unless the `experimental.async` compiler option is `true`
 ```
 
-### 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.
-
 ### fork_discarded
 
 ```

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

@@ -104,14 +104,6 @@ Often when encountering this issue, the value in question shouldn't be state (fo
 
 > Cannot use `fork(...)` unless the `experimental.async` compiler option is `true`
 
-## 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.
-
 ## fork_discarded
 
 > Cannot commit a fork that was already discarded

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

@@ -245,22 +245,6 @@ export function experimental_async_fork() {
 	}
 }
 
-/**
- * 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`);
-	}
-}
-
 /**
  * Cannot commit a fork that was already discarded
  * @returns {never}

packages/svelte/src/internal/client/reactivity/batch.js

@@ -528,11 +528,6 @@ export class Batch {
  * @returns {T}
  */
 export function flushSync(fn) {
-	if (async_mode_flag && active_effect !== null) {
-		// We disallow this because it creates super-hard to reason about stack trace and because it's generally a bad idea
-		e.flush_sync_in_effect();
-	}
-
 	var was_flushing_sync = is_flushing_sync;
 	is_flushing_sync = true;
 

packages/svelte/tests/runtime-runes/samples/flush-sync-inside-attachment/_config.js

@@ -1,12 +1,8 @@
-import { async_mode } from '../../../helpers';
 import { test } from '../../test';
 
 export default test({
-	// In legacy mode this succeeds and logs 'hello'
-	// In async mode this throws an error because flushSync is called inside an effect
 	async test({ assert, target, logs }) {
 		assert.htmlEqual(target.innerHTML, `<button>show</button> <div>hello</div>`);
 		assert.deepEqual(logs, ['hello']);
-	},
-	runtime_error: async_mode ? 'flush_sync_in_effect' : undefined
+	}
 });