feat: make fallback prop values readonly (#9789)

* WIP

* update tests

* only make readonly in runes mode

* remove this for now

* changeset

* ugh

* add reassignment test

* tweak message

---------

Co-authored-by: Rich Harris <[email protected]>

Author: Rich-Harris

.changeset/lazy-months-knock.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: make fallback prop values readonly

packages/svelte/src/internal/client/proxy/readonly.js

@@ -44,7 +44,7 @@ export function readonly(value) {
 
 /** @returns {never} */
 const readonly_error = () => {
-	throw new Error(`Props are read-only, unless used with \`bind:\``);
+	throw new Error(`Props cannot be mutated, unless used with \`bind:\``);
 };
 
 /** @type {ProxyHandler<StateObject<any>>} */

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

@@ -3,6 +3,7 @@ import { subscribe_to_store } from '../../store/utils.js';
 import { EMPTY_FUNC, run_all } from '../common.js';
 import { get_descriptor, get_descriptors, is_array } from './utils.js';
 import { PROPS_CALL_DEFAULT_VALUE, PROPS_IS_IMMUTABLE, PROPS_IS_RUNES } from '../../constants.js';
+import { readonly } from './proxy/readonly.js';
 
 export const SOURCE = 1;
 export const DERIVED = 1 << 1;
@@ -1422,13 +1423,14 @@ export function is_store(val) {
 export function prop_source(props_obj, key, flags, default_value) {
 	const call_default_value = (flags & PROPS_CALL_DEFAULT_VALUE) !== 0;
 	const immutable = (flags & PROPS_IS_IMMUTABLE) !== 0;
+	const runes = (flags & PROPS_IS_RUNES) !== 0;
 
 	const props = is_signal(props_obj) ? get(props_obj) : props_obj;
 	const update_bound_prop = get_descriptor(props, key)?.set;
 	let value = props[key];
 	const should_set_default_value = value === undefined && default_value !== undefined;
 
-	if (update_bound_prop && default_value !== undefined && (flags & PROPS_IS_RUNES) !== 0) {
+	if (update_bound_prop && runes && default_value !== undefined) {
 		// TODO consolidate all these random runtime errors
 		throw new Error('Cannot use fallback values with bind:');
 	}
@@ -1437,6 +1439,10 @@ export function prop_source(props_obj, key, flags, default_value) {
 		value =
 			// @ts-expect-error would need a cumbersome method overload to type this
 			call_default_value ? default_value() : default_value;
+
+		if (DEV && runes) {
+			value = readonly(/** @type {any} */ (value));
+		}
 	}
 
 	const source_signal = immutable ? source(value) : mutable_source(value);

packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/_config.js

@@ -1,36 +1,41 @@
 import { flushSync } from 'svelte';
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	// The component context class instance gets shared between tests, strangely, causing hydration to fail?
 	skip_if_hydrate: 'permanent',
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1, 2]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1, 2]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1, 2, 3]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1, 2, 3]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [
+		assert.deepEqual(log, [
 			0,
 			'class trigger false',
 			'local trigger false',

packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js

@@ -0,0 +1,2 @@
+/** @type {any[]} */
+export const log = [];

packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/main.svelte

@@ -1,17 +1,17 @@
 <script context="module">
-		class SomeLogic {
-			someValue = $state(0);
-			isAboveThree = $derived(this.someValue > 3);
-			trigger() {
-				this.someValue++;
-			}
+	class SomeLogic {
+		someValue = $state(0);
+		isAboveThree = $derived(this.someValue > 3);
+		trigger() {
+			this.someValue++;
 		}
+	}
 
 	const someLogic = new SomeLogic();
 </script>
 
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	function increment() {
 		someLogic.trigger();

packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/_config.js

@@ -1,9 +1,14 @@
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	html: `<button>0</button>`,
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
@@ -12,6 +17,6 @@ export default test({
 		await btn?.click();
 		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
 
-		assert.deepEqual(component.log, [undefined]);
+		assert.deepEqual(log, [undefined]);
 	}
 });

packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js

@@ -0,0 +1,2 @@
+/** @type {any[]} */
+export const log = [];

packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/main.svelte

@@ -1,5 +1,5 @@
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	const logger = (obj) => {
 		log.push(obj.count)

packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/_config.js

@@ -1,9 +1,14 @@
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	html: `<button>0</button>`,
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
@@ -12,6 +17,6 @@ export default test({
 		await btn?.click();
 		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
 
-		assert.deepEqual(component.log, [undefined]);
+		assert.deepEqual(log, [undefined]);
 	}
 });