fix: make immutable option work more correctly

.changeset/giant-ladybugs-thank.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make immutable option work more correctly

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -521,15 +521,15 @@ export function analyze_component(root, source, options) {
 
 	if (root.options) {
 		for (const attribute of root.options.attributes) {
-			if (attribute.name === 'accessors') {
+			if (attribute.name === 'accessors' && analysis.runes) {
 				w.options_deprecated_accessors(attribute);
 			}
 
 			if (attribute.name === 'customElement' && !options.customElement) {
 				w.options_missing_custom_element(attribute);
 			}
 
-			if (attribute.name === 'immutable') {
+			if (attribute.name === 'immutable' && analysis.runes) {
 				w.options_deprecated_immutable(attribute);
 			}
 		}

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -212,7 +212,9 @@ export function client_component(analysis, options) {
 
 	for (const [name, binding] of analysis.instance.scope.declarations) {
 		if (binding.kind === 'legacy_reactive') {
-			legacy_reactive_declarations.push(b.const(name, b.call('$.mutable_state')));
+			legacy_reactive_declarations.push(
+				b.const(name, b.call('$.mutable_state', undefined, analysis.immutable ? b.true : undefined))
+			);
 		}
 		if (binding.kind === 'store_sub') {
 			if (store_setup.length === 0) {
@@ -368,7 +370,9 @@ export function client_component(analysis, options) {
 		...group_binding_declarations,
 		...analysis.top_level_snippets,
 		.../** @type {ESTree.Statement[]} */ (instance.body),
-		analysis.runes || !analysis.needs_context ? b.empty : b.stmt(b.call('$.init')),
+		analysis.runes || !analysis.needs_context
+			? b.empty
+			: b.stmt(b.call('$.init', analysis.immutable ? b.true : undefined)),
 		.../** @type {ESTree.Statement[]} */ (template.body)
 	]);
 

packages/svelte/src/compiler/phases/3-transform/client/visitors/VariableDeclaration.js

@@ -1,7 +1,6 @@
 /** @import { CallExpression, Expression, Identifier, Literal, VariableDeclaration, VariableDeclarator } from 'estree' */
 /** @import { Binding } from '#compiler' */
-/** @import { ComponentContext } from '../types' */
-/** @import { Scope } from '../../../scope' */
+/** @import { ComponentClientTransformState, ComponentContext } from '../types' */
 import { dev } from '../../../../state.js';
 import { extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
@@ -267,7 +266,7 @@ export function VariableDeclaration(node, context) {
 			declarations.push(
 				...create_state_declarators(
 					declarator,
-					context.state.scope,
+					context.state,
 					/** @type {Expression} */ (declarator.init && context.visit(declarator.init))
 				)
 			);
@@ -287,12 +286,17 @@ export function VariableDeclaration(node, context) {
 /**
  * Creates the output for a state declaration in legacy mode.
  * @param {VariableDeclarator} declarator
- * @param {Scope} scope
+ * @param {ComponentClientTransformState} scope
  * @param {Expression} value
  */
-function create_state_declarators(declarator, scope, value) {
+function create_state_declarators(declarator, { scope, analysis }, value) {
 	if (declarator.id.type === 'Identifier') {
-		return [b.declarator(declarator.id, b.call('$.mutable_state', value))];
+		return [
+			b.declarator(
+				declarator.id,
+				b.call('$.mutable_state', value, analysis.immutable ? b.true : undefined)
+			)
+		];
 	}
 
 	const tmp = scope.generate('tmp');
@@ -304,7 +308,9 @@ function create_state_declarators(declarator, scope, value) {
 			const binding = scope.get(/** @type {Identifier} */ (path.node).name);
 			return b.declarator(
 				path.node,
-				binding?.kind === 'state' ? b.call('$.mutable_state', value) : value
+				binding?.kind === 'state'
+					? b.call('$.mutable_state', value, analysis.immutable ? b.true : undefined)
+					: value
 			);
 		})
 	];

packages/svelte/src/internal/client/dom/legacy/lifecycle.js

@@ -1,21 +1,46 @@
 /** @import { ComponentContextLegacy } from '#client' */
 import { run, run_all } from '../../../shared/utils.js';
+import { derived } from '../../reactivity/deriveds.js';
 import { user_pre_effect, user_effect } from '../../reactivity/effects.js';
 import { component_context, deep_read_state, get, untrack } from '../../runtime.js';
 
 /**
  * Legacy-mode only: Call `onMount` callbacks and set up `beforeUpdate`/`afterUpdate` effects
+ * @param {boolean} [immutable]
  */
-export function init() {
+export function init(immutable = false) {
 	const context = /** @type {ComponentContextLegacy} */ (component_context);
 
 	const callbacks = context.l.u;
 	if (!callbacks) return;
 
+	let props = () => deep_read_state(context.s);
+
+	if (immutable) {
+		let version = 0;
+		let prev = /** @type {Record<string, any>} */ ({});
+
+		// In legacy immutable mode, before/afterUpdate only fire if the object identity of a prop changes
+		const d = derived(() => {
+			let changed = false;
+			const props = context.s;
+			for (const key in props) {
+				if (props[key] !== prev[key]) {
+					prev[key] = props[key];
+					changed = true;
+				}
+			}
+			if (changed) version++;
+			return version;
+		});
+
+		props = () => get(d);
+	}
+
 	// beforeUpdate
 	if (callbacks.b.length) {
 		user_pre_effect(() => {
-			observe_all(context);
+			observe_all(context, props);
 			run_all(callbacks.b);
 		});
 	}
@@ -35,7 +60,7 @@ export function init() {
 	// afterUpdate
 	if (callbacks.a.length) {
 		user_effect(() => {
-			observe_all(context);
+			observe_all(context, props);
 			run_all(callbacks.a);
 		});
 	}
@@ -45,11 +70,12 @@ export function init() {
  * Invoke the getter of all signals associated with a component
  * so they can be registered to the effect this function is called in.
  * @param {ComponentContextLegacy} context
+ * @param {(() => void)} props
  */
-function observe_all(context) {
+function observe_all(context, props) {
 	if (context.l.s) {
 		for (const signal of context.l.s) get(signal);
 	}
 
-	deep_read_state(context.s);
+	props();
 }

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

@@ -67,12 +67,15 @@ export function state(v) {
 /**
  * @template V
  * @param {V} initial_value
+ * @param {boolean} [immutable]
  * @returns {Source<V>}
  */
 /*#__NO_SIDE_EFFECTS__*/
-export function mutable_source(initial_value) {
+export function mutable_source(initial_value, immutable = false) {
 	const s = source(initial_value);
-	s.equals = safe_equals;
+	if (!immutable) {
+		s.equals = safe_equals;
+	}
 
 	// bind the signal to the component context, in case we need to
 	// track updates to trigger beforeUpdate/afterUpdate callbacks
@@ -86,10 +89,11 @@ export function mutable_source(initial_value) {
 /**
  * @template V
  * @param {V} v
+ * @param {boolean} [immutable]
  * @returns {Source<V>}
  */
-export function mutable_state(v) {
-	return push_derived_source(mutable_source(v));
+export function mutable_state(v, immutable = false) {
+	return push_derived_source(mutable_source(v, immutable));
 }
 
 /**

packages/svelte/tests/runtime-legacy/samples/immutable-before-after-update/ImmutableTodo.svelte

@@ -0,0 +1,24 @@
+<svelte:options immutable />
+
+<script>
+	import { afterUpdate, beforeUpdate } from 'svelte';
+
+	export let todo;
+
+	let btn;
+
+	$: console.log('$:'+ todo.id);
+
+	beforeUpdate(() => {
+		console.log('beforeUpdate:'+ todo.id);
+	})
+
+	afterUpdate(() => {
+		console.log('afterUpdate:'+ todo.id);
+	});
+</script>
+
+<button bind:this={btn} on:click>
+	{todo.done ? 'X' : ''}
+	{todo.id}
+</button>

packages/svelte/tests/runtime-legacy/samples/immutable-before-after-update/_config.js

@@ -0,0 +1,45 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	immutable: true,
+
+	html: '<button>1</button> <button>2</button> <button>3</button>',
+
+	test({ assert, target, logs }) {
+		assert.deepEqual(logs, [
+			'$:1',
+			'beforeUpdate:1',
+			'$:2',
+			'beforeUpdate:2',
+			'$:3',
+			'beforeUpdate:3',
+			'afterUpdate:1',
+			'afterUpdate:2',
+			'afterUpdate:3',
+			'beforeUpdate:1',
+			'beforeUpdate:2',
+			'beforeUpdate:3'
+		]);
+
+		const [button1, button2] = target.querySelectorAll('button');
+
+		logs.length = 0;
+		button1.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>X 1</button> <button>2</button> <button>3</button>'
+		);
+		assert.deepEqual(logs, ['$:1', 'beforeUpdate:1', 'afterUpdate:1']);
+
+		logs.length = 0;
+		button2.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>X 1</button> <button>X 2</button> <button>3</button>'
+		);
+		assert.deepEqual(logs, ['$:2', 'beforeUpdate:2', 'afterUpdate:2']);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/immutable-before-after-update/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	import ImmutableTodo from './ImmutableTodo.svelte';
+
+	let todos = [
+		{ id: 1, done: false },
+		{ id: 2, done: false },
+		{ id: 3, done: false }
+	];
+
+	function toggle(id) {
+		todos = todos.map((todo) => {
+			if (todo.id === id) {
+				return {
+					id,
+					done: !todo.done
+				};
+			}
+
+			return todo;
+		});
+	}
+</script>
+
+{#each todos as todo}
+	<ImmutableTodo {todo} on:click={() => toggle(todo.id)} />
+{/each}

packages/svelte/tests/runtime-legacy/samples/immutable-mutate-object/_config.js

@@ -0,0 +1,20 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	immutable: true,
+
+	html: '<button>0</button> <button>0</button>',
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		button1.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<button>0</button> <button>0</button>');
+
+		button2.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<button>2</button> <button>2</button>');
+	}
+});

packages/svelte/tests/runtime-legacy/samples/immutable-mutate-object/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let name = { value: 0 };
+</script>
+
+<button onclick={() => name.value++}>{name.value}</button>
+<button onclick={() => (name = { value: name.value + 1 })}>{name.value}</button>