fix: ensure legacy run utility does not cause cycles

.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/legacy-warnings.md

@@ -0,0 +1,5 @@
+### 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`.
+```

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

@@ -0,0 +1,3 @@
+## 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`.

packages/svelte/scripts/process-messages/index.js

@@ -404,3 +404,4 @@ transform('client-errors', 'src/internal/client/errors.js');
 transform('server-errors', 'src/internal/server/errors.js');
 transform('shared-errors', 'src/internal/shared/errors.js');
 transform('shared-warnings', 'src/internal/shared/warnings.js');
+transform('legacy-warnings', 'src/legacy/warnings.js');

packages/svelte/scripts/process-messages/templates/legacy-warnings.js

@@ -0,0 +1,17 @@
+import { DEV } from 'esm-env';
+
+var bold = 'font-weight: bold';
+var normal = 'font-weight: normal';
+
+/**
+ * MESSAGE
+ * @param {string} PARAMETER
+ */
+export function CODE(PARAMETER) {
+	if (DEV) {
+		console.warn(`%c[svelte] ${'CODE'}\n%c${MESSAGE}`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn('CODE');
+	}
+}

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 './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);
+		}
+	});
 }
 
 /**

packages/svelte/src/legacy/warnings.js

@@ -0,0 +1,22 @@
+/* This file is generated by scripts/process-messages/index.js. Do not edit! */
+
+import { DEV } from 'esm-env';
+
+var bold = 'font-weight: bold';
+var normal = 'font-weight: normal';
+
+/**
+ * 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');
+	}
+}