fix: handle reassignment of $$props and $$restProps

.changeset/lucky-teachers-exist.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: handle reassignment of `$$props` and `$$restProps`

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

@@ -447,7 +447,7 @@ export function analyze_component(root, source, options) {
 			);
 		}
 	} else {
-		instance.scope.declare(b.id('$$props'), 'bindable_prop', 'synthetic');
+		instance.scope.declare(b.id('$$props'), 'rest_prop', 'synthetic');
 		instance.scope.declare(b.id('$$restProps'), 'rest_prop', 'synthetic');
 
 		for (const { ast, scope, scopes } of [module, instance, template]) {

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

@@ -418,7 +418,7 @@ export function client_component(source, analysis, options) {
 			b.const(
 				'$$restProps',
 				b.call(
-					'$.rest_props',
+					'$.legacy_rest_props',
 					b.id('$$sanitized_props'),
 					b.array(named_props.map((name) => b.literal(name)))
 				)
@@ -431,8 +431,12 @@ export function client_component(source, analysis, options) {
 		if (analysis.custom_element) {
 			to_remove.push(b.literal('$$host'));
 		}
+
 		component_block.body.unshift(
-			b.const('$$sanitized_props', b.call('$.rest_props', b.id('$$props'), b.array(to_remove)))
+			b.const(
+				'$$sanitized_props',
+				b.call('$.legacy_rest_props', b.id('$$props'), b.array(to_remove))
+			)
 		);
 	}
 

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

@@ -74,6 +74,11 @@ export function serialize_get_binding(node, state) {
 		return node;
 	}
 
+	if (binding.node.name === '$$props') {
+		// Special case for $$props which only exists in the old world
+		return b.id('$$sanitized_props');
+	}
+
 	if (binding.kind === 'store_sub') {
 		return b.call(node);
 	}
@@ -83,12 +88,6 @@ export function serialize_get_binding(node, state) {
 	}
 
 	if (binding.kind === 'prop' || binding.kind === 'bindable_prop') {
-		if (binding.node.name === '$$props') {
-			// Special case for $$props which only exists in the old world
-			// TODO this probably shouldn't have a 'prop' binding kind
-			return node;
-		}
-
 		if (
 			state.analysis.accessors ||
 			(state.analysis.immutable ? binding.reassigned : binding.mutated) ||

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

@@ -88,6 +88,7 @@ export { mutable_source, mutate, source, set } from './reactivity/sources.js';
 export {
 	prop,
 	rest_props,
+	legacy_rest_props,
 	spread_props,
 	update_pre_prop,
 	update_prop

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

@@ -6,9 +6,9 @@ import {
 	PROPS_IS_UPDATED
 } from '../../../constants.js';
 import { get_descriptor, is_function } from '../utils.js';
-import { mutable_source, set } from './sources.js';
+import { mutable_source, set, source } from './sources.js';
 import { derived } from './deriveds.js';
-import { get, is_signals_recorded, untrack } from '../runtime.js';
+import { get, is_signals_recorded, untrack, update } from '../runtime.js';
 import { safe_equals } from './equality.js';
 import { inspect_fn } from '../dev/inspect.js';
 import * as e from '../errors.js';
@@ -79,7 +79,67 @@ const rest_props_handler = {
  * @returns {Record<string, unknown>}
  */
 export function rest_props(props, exclude, name) {
-	return new Proxy(DEV ? { props, exclude, name } : { props, exclude }, rest_props_handler);
+	return new Proxy(
+		DEV ? { props, exclude, name, other: {}, to_proxy: [] } : { props, exclude },
+		rest_props_handler
+	);
+}
+
+/**
+ * The proxy handler for legacy $$restProps and $$props
+ * @type {ProxyHandler<{ props: Record<string | symbol, unknown>, exclude: Array<string | symbol>, special: Record<string | symbol, (v?: unknown) => unknown>, version: import('./types.js').Source<number> }>}}
+ */
+const legacy_rest_props_handler = {
+	get(target, key) {
+		if (target.exclude.includes(key)) return;
+		get(target.version);
+		return key in target.special ? target.special[key]() : target.props[key];
+	},
+	set(target, key, value) {
+		if (!(key in target.special)) {
+			// Handle props that can temporarily get out of sync with the parent
+			/** @type {Record<string, (v?: unknown) => unknown>} */
+			target.special[key] = prop(
+				{
+					get [key]() {
+						return target.props[key];
+					}
+				},
+				/** @type {string} */ (key),
+				PROPS_IS_UPDATED
+			);
+		}
+
+		target.special[key](value);
+		update(target.version); // $$props is coarse-grained: when $$props.x is updated, usages of $$props.y etc are also rerun
+		return true;
+	},
+	getOwnPropertyDescriptor(target, key) {
+		if (target.exclude.includes(key)) return;
+		if (key in target.props) {
+			return {
+				enumerable: true,
+				configurable: true,
+				value: target.props[key]
+			};
+		}
+	},
+	has(target, key) {
+		if (target.exclude.includes(key)) return false;
+		return key in target.props;
+	},
+	ownKeys(target) {
+		return Reflect.ownKeys(target.props).filter((key) => !target.exclude.includes(key));
+	}
+};
+
+/**
+ * @param {Record<string, unknown>} props
+ * @param {string[]} exclude
+ * @returns {Record<string, unknown>}
+ */
+export function legacy_rest_props(props, exclude) {
+	return new Proxy({ props, exclude, special: {}, version: source(0) }, legacy_rest_props_handler);
 }
 
 /**

packages/svelte/tests/runtime-legacy/samples/props-reassign/App.svelte

@@ -0,0 +1,6 @@
+<script>
+	$: $$props.a = $$props.a * 2;
+</script>
+
+<p>{$$props.a} {$$props.b}</p>
+<button on:click={() => $$props.b = 'b'}>update</button>

packages/svelte/tests/runtime-legacy/samples/props-reassign/_config.js

@@ -0,0 +1,35 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<button>increment</button>
+		<p>0 </p>
+		<button>update</button>
+	`,
+
+	async test({ assert, target }) {
+		const [btn1, btn2] = target.querySelectorAll('button');
+
+		await btn1.click();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>increment</button>
+			<p>2 </p>
+			<button>update</button>
+		`
+		);
+
+		await btn2.click();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>increment</button>
+			<p>4 b</p>
+			<button>update</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/props-reassign/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import App from './App.svelte';
+	let a = 0;
+</script>
+
+<button on:click={() => a++}>increment</button>
+<App {a} />

packages/svelte/tests/runtime-legacy/samples/rest-props-reassign/App.svelte

@@ -0,0 +1,6 @@
+<script>
+	$: $$restProps.c = $$restProps.c ?? 'c';
+</script>
+
+<p>{$$restProps.a} {$$restProps.b} {$$restProps.c}</p>
+<button on:click={() => $$restProps.b = 'b'}>update</button>

packages/svelte/tests/runtime-legacy/samples/rest-props-reassign/_config.js

@@ -0,0 +1,35 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<button>increment</button>
+		<p>0 c</p>
+		<button>update</button>
+	`,
+
+	async test({ assert, target }) {
+		const [btn1, btn2] = target.querySelectorAll('button');
+
+		await btn1.click();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>increment</button>
+			<p>1 c</p>
+			<button>update</button>
+		`
+		);
+
+		await btn2.click();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>increment</button>
+			<p>1 b c</p>
+			<button>update</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/rest-props-reassign/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import App from './App.svelte';
+	let a = 0;
+</script>
+
+<button on:click={() => a++}>increment</button>
+<App {a} />