breaking: state mutations inside the template are no longer allowed

.changeset/wild-chairs-build.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: state mutations inside the template are no longer allowed

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

@@ -125,5 +125,5 @@ Reading state that was created inside the same derived is forbidden. Consider us
 ### state_unsafe_mutation
 
 ```
-Updating state inside a derived or logic block expression is forbidden. If the value should not be reactive, declare it without `$state`
+Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```

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

@@ -82,4 +82,4 @@
 
 ## state_unsafe_mutation
 
-> Updating state inside a derived or logic block expression is forbidden. If the value should not be reactive, declare it without `$state`
+> Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`

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

@@ -343,12 +343,12 @@ export function state_unsafe_local_read() {
 }
 
 /**
- * Updating state inside a derived or logic block expression is forbidden. If the value should not be reactive, declare it without `$state`
+ * Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
  * @returns {never}
  */
 export function state_unsafe_mutation() {
 	if (DEV) {
-		const error = new Error(`state_unsafe_mutation\nUpdating state inside a derived or logic block expression is forbidden. If the value should not be reactive, declare it without \`$state\``);
+		const error = new Error(`state_unsafe_mutation\nUpdating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without \`$state\``);
 
 		error.name = 'Svelte error';
 		throw error;

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

@@ -325,7 +325,7 @@ export function template_effect(fn) {
 			value: '{expression}'
 		});
 	}
-	return render_effect(fn);
+	return block(fn);
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/side-effect-template/_config.js

@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		assert.throws(() => {
+			button?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+	}
+});

packages/svelte/tests/runtime-runes/samples/side-effect-template/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let items = $state([]);
+</script>
+
+<button onclick={() => items.push(3, 2, 1)}>Add</button>
+{JSON.stringify(items.sort())}

packages/svelte/tests/runtime-runes/samples/store-unsubscribe-not-referenced-after/main.svelte

@@ -1,4 +1,5 @@
 <script>
+	import { untrack } from "svelte";
 	import { writable, derived } from "svelte/store";
 
 	const obj = writable({ a: 1 });
@@ -7,7 +8,9 @@
 
 	function watch (prop) {
 		return derived(obj, (o) => {
-			count++;
+			untrack(() => {
+				count++;
+			});
 			return o[prop];
 		});
 	}