fix: ensure legacy run utility does not cause cycles (#13643)

* fix: ensure legacy run utility does not cause cycles

* add warning

* add warning

* lint

* feedback

* lint

* lint

.changeset/moody-pugs-tan.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure legacy run utility does not cause cycles

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

@@ -56,6 +56,12 @@ Hydration failed because the initial UI does not match what was rendered on the
 The `render` function passed to `createRawSnippet` should return HTML for a single element
 ```
 
+### legacy_recursive_reactive_block
+
+```
+Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
+```
+
 ### lifecycle_double_unmount
 
 ```

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

@@ -36,6 +36,10 @@ The easiest way to log a value as it changes over time is to use the [`$inspect`
 
 > The `render` function passed to `createRawSnippet` should return HTML for a single element
 
+## legacy_recursive_reactive_block
+
+> Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
+
 ## lifecycle_double_unmount
 
 > Tried to unmount a component that was not mounted

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

@@ -99,6 +99,18 @@ export function invalid_raw_snippet_render() {
 	}
 }
 
+/**
+ * Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
+ */
+export function legacy_recursive_reactive_block() {
+	if (DEV) {
+		console.warn(`%c[svelte] legacy_recursive_reactive_block\n%cDetected a migrated \`$:\` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an \`$effect\`.`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn("legacy_recursive_reactive_block");
+	}
+}
+
 /**
  * Tried to unmount a component that was not mounted
  */

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

@@ -1,10 +1,18 @@
 /** @import { ComponentConstructorOptions, ComponentType, SvelteComponent, Component } from 'svelte' */
+import { DIRTY, MAYBE_DIRTY } from '../internal/client/constants.js';
 import { user_pre_effect } from '../internal/client/reactivity/effects.js';
 import { mutable_source, set } from '../internal/client/reactivity/sources.js';
 import { hydrate, mount, unmount } from '../internal/client/render.js';
-import { component_context, flush_sync, get } from '../internal/client/runtime.js';
+import {
+	active_effect,
+	component_context,
+	flush_sync,
+	get,
+	set_signal_status
+} from '../internal/client/runtime.js';
 import { lifecycle_outside_component } from '../internal/shared/errors.js';
 import { define_property, is_array } from '../internal/shared/utils.js';
+import * as w from '../internal/client/warnings.js';
 
 /**
  * Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.
@@ -169,7 +177,15 @@ class Svelte4Component {
  * @returns {void}
  */
 export function run(fn) {
-	user_pre_effect(fn);
+	user_pre_effect(() => {
+		fn();
+		var effect = /** @type {import('#client').Effect} */ (active_effect);
+		// If the effect is immediately made dirty again, mark it as maybe dirty to emulate legacy behaviour
+		if ((effect.f & DIRTY) !== 0) {
+			w.legacy_recursive_reactive_block();
+			set_signal_status(effect, MAYBE_DIRTY);
+		}
+	});
 }
 
 /**