compile time mutation validation

packages/svelte/src/compiler/errors.js

@@ -182,6 +182,8 @@ const runes = {
 		`$props() assignment must not contain nested properties or computed keys`,
 	'invalid-props-location': () =>
 		`$props() can only be used at the top level of components as a variable declaration initializer`,
+	'invalid-props-mutation': () =>
+		'Properties defined by $props() cannot be mutated. Use $props.bindable() instead, or make a copy of the value and reassign it.',
 	/** @param {string} rune */
 	'invalid-state-location': (rune) =>
 		`${rune}(...) can only be used as a variable declaration initializer or a class field`,

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

@@ -436,7 +436,7 @@ export function analyze_component(root, options) {
 			);
 		}
 	} else {
-		instance.scope.declare(b.id('$$props'), 'prop', 'synthetic');
+		instance.scope.declare(b.id('$$props'), 'bindable_prop', 'synthetic');
 		instance.scope.declare(b.id('$$restProps'), 'rest_prop', 'synthetic');
 
 		for (const { ast, scope, scopes } of [module, instance, template]) {
@@ -466,7 +466,10 @@ export function analyze_component(root, options) {
 		}
 
 		for (const [name, binding] of instance.scope.declarations) {
-			if (binding.kind === 'prop' && binding.node.name !== '$$props') {
+			if (
+				(binding.kind === 'prop' || binding.kind === 'bindable_prop') &&
+				binding.node.name !== '$$props'
+			) {
 				const references = binding.references.filter(
 					(r) => r.node !== binding.node && r.path.at(-1)?.type !== 'ExportSpecifier'
 				);
@@ -758,7 +761,7 @@ const legacy_scope_tweaker = {
 					(binding.kind === 'normal' &&
 						(binding.declaration_kind === 'let' || binding.declaration_kind === 'var'))
 				) {
-					binding.kind = 'prop';
+					binding.kind = 'bindable_prop';
 					if (specifier.exported.name !== specifier.local.name) {
 						binding.prop_alias = specifier.exported.name;
 					}
@@ -796,7 +799,7 @@ const legacy_scope_tweaker = {
 			for (const declarator of node.declaration.declarations) {
 				for (const id of extract_identifiers(declarator.id)) {
 					const binding = /** @type {import('#compiler').Binding} */ (state.scope.get(id.name));
-					binding.kind = 'prop';
+					binding.kind = 'bindable_prop';
 				}
 			}
 		}
@@ -871,7 +874,9 @@ const runes_scope_tweaker = {
 							? 'derived'
 							: path.is_rest
 								? 'rest_prop'
-								: 'prop';
+								: rune === '$props.bindable'
+									? 'bindable_prop'
+									: 'prop';
 		}
 
 		if (rune === '$props' || rune === '$props.bindable') {

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

@@ -299,17 +299,19 @@ const validation = {
 			error(node, 'invalid-binding-expression');
 		}
 
+		const binding = context.state.scope.get(left.name);
+
 		if (
 			assignee.type === 'Identifier' &&
 			node.name !== 'this' // bind:this also works for regular variables
 		) {
-			const binding = context.state.scope.get(left.name);
 			// reassignment
 			if (
 				!binding ||
 				(binding.kind !== 'state' &&
 					binding.kind !== 'frozen_state' &&
 					binding.kind !== 'prop' &&
+					binding.kind !== 'bindable_prop' &&
 					binding.kind !== 'each' &&
 					binding.kind !== 'store_sub' &&
 					!binding.mutated)
@@ -328,7 +330,9 @@ const validation = {
 			// TODO handle mutations of non-state/props in runes mode
 		}
 
-		const binding = context.state.scope.get(left.name);
+		if (assignee.type === 'MemberExpression' && binding?.kind === 'prop') {
+			error(node, 'invalid-props-mutation');
+		}
 
 		if (node.name === 'group') {
 			if (!binding) {
@@ -969,7 +973,9 @@ function validate_no_const_assignment(node, argument, scope, is_binding) {
 function validate_assignment(node, argument, state) {
 	validate_no_const_assignment(node, argument, state.scope, false);
 
-	if (state.analysis.runes && argument.type === 'Identifier') {
+	if (!state.analysis.runes) return;
+
+	if (argument.type === 'Identifier') {
 		const binding = state.scope.get(argument.name);
 		if (binding?.kind === 'derived') {
 			error(node, 'invalid-derived-assignment');
@@ -978,19 +984,24 @@ function validate_assignment(node, argument, state) {
 		if (binding?.kind === 'each') {
 			error(node, 'invalid-each-assignment');
 		}
+	} else if (argument.type === 'MemberExpression') {
+		const id = object(argument);
+		if (id && state.scope.get(id.name)?.kind === 'prop') {
+			error(node, 'invalid-props-mutation');
+		}
 	}
 
-	let object = /** @type {import('estree').Expression | import('estree').Super} */ (argument);
+	let obj = /** @type {import('estree').Expression | import('estree').Super} */ (argument);
 
 	/** @type {import('estree').Expression | import('estree').PrivateIdentifier | null} */
 	let property = null;
 
-	while (object.type === 'MemberExpression') {
+	while (obj.type === 'MemberExpression') {
-		property = object.property;
+		property = obj.property;
-		object = object.object;
+		obj = obj.object;
 	}
 
-	if (object.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
+	if (obj.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
 		if (state.private_derived_state.includes(property.name)) {
 			error(node, 'invalid-derived-assignment');
 		}

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

@@ -251,7 +251,8 @@ export function client_component(source, analysis, options) {
 
 	if (analysis.accessors) {
 		for (const [name, binding] of analysis.instance.scope.declarations) {
-			if (binding.kind !== 'prop' || name.startsWith('$$')) continue;
+			if ((binding.kind !== 'prop' && binding.kind !== 'bindable_prop') || name.startsWith('$$'))
+				continue;
 
 			const key = binding.prop_alias ?? name;
 
@@ -356,7 +357,7 @@ export function client_component(source, analysis, options) {
 		/** @type {string[]} */
 		const named_props = analysis.exports.map(({ name, alias }) => alias ?? name);
 		for (const [name, binding] of analysis.instance.scope.declarations) {
-			if (binding.kind === 'prop') named_props.push(binding.prop_alias ?? name);
+			if (binding.kind === 'bindable_prop') named_props.push(binding.prop_alias ?? name);
 		}
 
 		component_block.body.unshift(
@@ -464,7 +465,8 @@ export function client_component(source, analysis, options) {
 		const props_str = [];
 
 		for (const [name, binding] of analysis.instance.scope.declarations) {
-			if (binding.kind !== 'prop' || name.startsWith('$$')) continue;
+			if ((binding.kind !== 'prop' && binding.kind !== 'bindable_prop') || name.startsWith('$$'))
+				continue;
 
 			const key = binding.prop_alias ?? name;
 			const prop_def = typeof ce === 'boolean' ? {} : ce.props?.[key] || {};

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

@@ -78,7 +78,7 @@ export function serialize_get_binding(node, state) {
 		return typeof binding.expression === 'function' ? binding.expression(node) : binding.expression;
 	}
 
-	if (binding.kind === 'prop') {
+	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
@@ -377,6 +377,7 @@ export function serialize_set_binding(node, context, fallback, options) {
 		binding.kind !== 'state' &&
 		binding.kind !== 'frozen_state' &&
 		binding.kind !== 'prop' &&
+		binding.kind !== 'bindable_prop' &&
 		binding.kind !== 'each' &&
 		binding.kind !== 'legacy_reactive' &&
 		!is_store
@@ -389,7 +390,7 @@ export function serialize_set_binding(node, context, fallback, options) {
 
 	const serialize = () => {
 		if (left === node.left) {
-			if (binding.kind === 'prop') {
+			if (binding.kind === 'prop' || binding.kind === 'bindable_prop') {
 				return b.call(left, value);
 			} else if (is_store) {
 				return b.call('$.store_set', serialize_get_binding(b.id(left_name), state), value);
@@ -467,7 +468,7 @@ export function serialize_set_binding(node, context, fallback, options) {
 					b.call('$.untrack', b.id('$' + left_name))
 				);
 			} else if (!state.analysis.runes) {
-				if (binding.kind === 'prop') {
+				if (binding.kind === 'bindable_prop') {
 					return b.call(
 						left,
 						b.sequence([
@@ -571,7 +572,7 @@ function get_hoistable_params(node, context) {
 				params.push(b.id(binding.expression.object.arguments[0].name));
 			} else if (
 				// If we are referencing a simple $$props value, then we need to reference the object property instead
-				binding.kind === 'prop' &&
+				(binding.kind === 'prop' || binding.kind === 'bindable_prop') &&
 				!binding.reassigned &&
 				binding.initial === null &&
 				!context.state.analysis.accessors

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

@@ -52,6 +52,7 @@ export const global_visitors = {
 				binding?.kind === 'each' ||
 				binding?.kind === 'legacy_reactive' ||
 				binding?.kind === 'prop' ||
+				binding?.kind === 'bindable_prop' ||
 				is_store
 			) {
 				/** @type {import('estree').Expression[]} */
@@ -64,7 +65,7 @@ export const global_visitors = {
 					fn += '_store';
 					args.push(serialize_get_binding(b.id(name), state), b.call('$' + name));
 				} else {
-					if (binding.kind === 'prop') fn += '_prop';
+					if (binding.kind === 'prop' || binding.kind === 'bindable_prop') fn += '_prop';
 					args.push(b.id(name));
 				}
 

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

@@ -40,7 +40,7 @@ export const javascript_visitors_legacy = {
 				state.scope.get_bindings(declarator)
 			);
 			const has_state = bindings.some((binding) => binding.kind === 'state');
-			const has_props = bindings.some((binding) => binding.kind === 'prop');
+			const has_props = bindings.some((binding) => binding.kind === 'bindable_prop');
 
 			if (!has_state && !has_props) {
 				const init = declarator.init;
@@ -80,7 +80,7 @@ export const javascript_visitors_legacy = {
 						declarations.push(
 							b.declarator(
 								path.node,
-								binding.kind === 'prop'
+								binding.kind === 'bindable_prop'
 									? get_prop_source(binding, state, binding.prop_alias ?? name, value)
 									: value
 							)
@@ -168,7 +168,7 @@ export const javascript_visitors_legacy = {
 
 			// If the binding is a prop, we need to deep read it because it could be fine-grained $state
 			// from a runes-component, where mutations don't trigger an update on the prop as a whole.
-			if (name === '$$props' || name === '$$restProps' || binding.kind === 'prop') {
+			if (name === '$$props' || name === '$$restProps' || binding.kind === 'bindable_prop') {
 				serialized = b.call('$.deep_read_state', serialized);
 			}
 

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

@@ -1367,6 +1367,7 @@ function serialize_event_handler(node, { state, visit }) {
 					binding.kind === 'legacy_reactive' ||
 					binding.kind === 'derived' ||
 					binding.kind === 'prop' ||
+					binding.kind === 'bindable_prop' ||
 					binding.kind === 'store_sub')
 			) {
 				handler = dynamic_handler();

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

@@ -446,6 +446,7 @@ function serialize_set_binding(node, context, fallback) {
 		binding.kind !== 'state' &&
 		binding.kind !== 'frozen_state' &&
 		binding.kind !== 'prop' &&
+		binding.kind !== 'bindable_prop' &&
 		binding.kind !== 'each' &&
 		binding.kind !== 'legacy_reactive' &&
 		!is_store
@@ -1131,7 +1132,7 @@ const javascript_visitors_legacy = {
 				state.scope.get_bindings(declarator)
 			);
 			const has_state = bindings.some((binding) => binding.kind === 'state');
-			const has_props = bindings.some((binding) => binding.kind === 'prop');
+			const has_props = bindings.some((binding) => binding.kind === 'bindable_prop');
 
 			if (!has_state && !has_props) {
 				declarations.push(/** @type {import('estree').VariableDeclarator} */ (visit(declarator)));
@@ -2258,7 +2259,7 @@ export function server_component(analysis, options) {
 	/** @type {import('estree').Property[]} */
 	const props = [];
 	for (const [name, binding] of analysis.instance.scope.declarations) {
-		if (binding.kind === 'prop' && !name.startsWith('$$')) {
+		if (binding.kind === 'bindable_prop' && !name.startsWith('$$')) {
 			props.push(b.init(binding.prop_alias ?? name, b.id(name)));
 		}
 	}
@@ -2280,7 +2281,7 @@ export function server_component(analysis, options) {
 		/** @type {string[]} */
 		const named_props = analysis.exports.map(({ name, alias }) => alias ?? name);
 		for (const [name, binding] of analysis.instance.scope.declarations) {
-			if (binding.kind === 'prop') named_props.push(binding.prop_alias ?? name);
+			if (binding.kind === 'bindable_prop') named_props.push(binding.prop_alias ?? name);
 		}
 
 		component_block.body.unshift(

packages/svelte/src/compiler/types/index.d.ts

@@ -241,7 +241,8 @@ export interface Binding {
 	node: Identifier;
 	/**
 	 * - `normal`: A variable that is not in any way special
-	 * - `prop`: A normal prop (possibly mutated)
+	 * - `prop`: A normal prop (possibly reassigned)
+	 * - `bindable_prop`: A prop one can `bind:` to (possibly reassigned or mutated)
 	 * - `rest_prop`: A rest prop
 	 * - `state`: A state variable
 	 * - `derived`: A derived variable
@@ -253,6 +254,7 @@ export interface Binding {
 	kind:
 		| 'normal'
 		| 'prop'
+		| 'bindable_prop'
 		| 'rest_prop'
 		| 'state'
 		| 'frozen_state'

packages/svelte/tests/compiler-errors/samples/runes-non-bindable-prop-mutated/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-props-mutation',
+		message:
+			'Properties defined by $props() cannot be mutated. Use $props.bindable() instead, or make a copy of the value and reassign it.'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-non-bindable-prop-mutated/main.svelte

@@ -0,0 +1,4 @@
+<script>
+	let { x } = $props();
+	x.y = 1;
+</script>

packages/svelte/tests/runtime-runes/samples/each-bind-this-member/main.svelte

@@ -1,5 +1,5 @@
 <script>
-	let { items = [{ src: 'https://ds' }] } = $props();
+	let { items = [{ src: 'https://ds' }] } = $props.bindable();
 </script>
 
 {#each items as item, i}

packages/svelte/tests/runtime-runes/samples/non-local-mutation-discouraged/Counter.svelte

@@ -1,6 +1,6 @@
 <script>
 	/** @type {{ object: { count: number }}} */
-	let { object } = $props();
+	let { object } = $props.bindable();
 </script>
 
 <button onclick={() => object.count += 1}>

packages/svelte/tests/runtime-runes/samples/non-local-mutation-inherited-owner-5/sub.svelte

@@ -1,5 +1,6 @@
 <script>
-	let { count, inc } = $props();
+	let { inc } = $props();
+	let { count } = $props.bindable();
 </script>
 
 <button onclick={inc}>{count.a} (ok)</button>

packages/svelte/tests/runtime-runes/samples/non-local-mutation-with-binding/Counter.svelte

@@ -1,6 +1,6 @@
 <script>
 	/** @type {{ object: { count: number }}} */
-	let { object } = $props();
+	let { object } = $props.bindable();
 </script>
 
 <button onclick={() => object.count += 1}>

packages/svelte/tests/runtime-runes/samples/props-bound-fallback/Counter.svelte

@@ -1,5 +1,5 @@
 <script>
-	let { count = 0 } = $props();
+	let { count = 0 } = $props.bindable();
 </script>
 
 <span>{count}</span>

packages/svelte/tests/runtime-runes/samples/props-default-reactivity/Counter.svelte

@@ -1,6 +1,6 @@
 <script>
 	/** @type {{ object?: { count: number }}} */
-	let { object = { count: 0 } } = $props();
+	let { object = { count: 0 } } = $props.bindable();
 </script>
 
 <button onclick={() => object.count += 1}>

packages/svelte/tests/runtime-runes/samples/proxy-prop-bound/Counter.svelte

@@ -1,6 +1,6 @@
 <script>
 	/** @type {{ object: { count: number }}} */
-	let { object } = $props();
+	let { object } = $props.bindable();
 </script>
 
 <button onclick={() => object.count += 1}>