breaking: disallow state mutations in logic block expression

.changeset/eighty-dryers-pretend.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: disallow state mutations in logic block expression

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 is forbidden. If the value should not be reactive, declare it without `$state`
+Updating state inside a derived or logic block expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```

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

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

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -31,7 +31,7 @@ import {
 	pause_effect,
 	resume_effect
 } from '../../reactivity/effects.js';
-import { source, mutable_source, set } from '../../reactivity/sources.js';
+import { source, mutable_source, internal_set } from '../../reactivity/sources.js';
 import { array_from, is_array } from '../../../shared/utils.js';
 import { INERT } from '../../constants.js';
 import { queue_micro_task } from '../task.js';
@@ -455,11 +455,11 @@ function reconcile(array, state, anchor, render_fn, flags, get_key) {
  */
 function update_item(item, value, index, type) {
 	if ((type & EACH_ITEM_REACTIVE) !== 0) {
-		set(item.v, value);
+		internal_set(item.v, value);
 	}
 
 	if ((type & EACH_INDEX_REACTIVE) !== 0) {
-		set(/** @type {Value<number>} */ (item.i), index);
+		internal_set(/** @type {Value<number>} */ (item.i), index);
 	} else {
 		item.i = index;
 	}

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

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

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

@@ -28,7 +28,8 @@ import {
 	BRANCH_EFFECT,
 	INSPECT_EFFECT,
 	UNOWNED,
-	MAYBE_DIRTY
+	MAYBE_DIRTY,
+	BLOCK_EFFECT
 } from '../constants.js';
 import * as e from '../errors.js';
 
@@ -136,14 +137,24 @@ export function set(source, value) {
 	if (
 		active_reaction !== null &&
 		is_runes() &&
-		(active_reaction.f & DERIVED) !== 0 &&
+		(active_reaction.f & (DERIVED | BLOCK_EFFECT)) !== 0 &&
 		// If the source was created locally within the current derived, then
 		// we allow the mutation.
 		(derived_sources === null || !derived_sources.includes(source))
 	) {
 		e.state_unsafe_mutation();
 	}
 
+	return internal_set(source, value);
+}
+
+/**
+ * @template V
+ * @param {Source<V>} source
+ * @param {V} value
+ * @returns {V}
+ */
+export function internal_set(source, value) {
 	if (!source.equals(value)) {
 		source.v = value;
 		source.version = increment_version();

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

@@ -0,0 +1,9 @@
+<script>
+	let items = $state([]);
+</script>
+
+<button onclick={() => items.push(3, 2, 1)}>Add</button>
+{#each items.sort() as item (item)}
+  <p>{item}</p>
+{/each}
+