tweak error message for readonly rest props

Author: Rich-Harris

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

@@ -211,10 +211,9 @@ export const javascript_visitors_runes = {
 							property.value.type === 'AssignmentPattern' ? property.value.left : property.value;
 						assert.equal(id.type, 'Identifier');
 						const binding = /** @type {import('#compiler').Binding} */ (state.scope.get(id.name));
-						let initial = /** @type {import('estree').Expression | null} */ (binding.initial);
-						if (initial) {
-							initial = /** @type {import('estree').Expression} */ (visit(initial));
-						}
+						const initial =
+							binding.initial &&
+							/** @type {import('estree').Expression} */ (visit(binding.initial));
 
 						if (binding.reassigned || state.analysis.accessors || initial) {
 							declarations.push(b.declarator(id, get_prop_source(binding, state, name, initial)));
@@ -223,6 +222,14 @@ export const javascript_visitors_runes = {
 						// RestElement
 						/** @type {import('estree').Expression[]} */
 						const args = [b.id('$$props'), b.array(seen.map((name) => b.literal(name)))];
+
+						if (state.options.dev) {
+							// include rest name, so we can provide informative error messages
+							args.push(
+								b.literal(/** @type {import('estree').Identifier} */ (property.argument).name)
+							);
+						}
+
 						declarations.push(b.declarator(property.argument, b.call('$.rest_props', ...args)));
 					}
 				}

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

@@ -36,19 +36,18 @@ export function update_pre_prop(fn, d = 1) {
 /**
  * The proxy handler for rest props (i.e. `const { x, ...rest } = $props()`).
  * Is passed the full `$$props` object and excludes the named props.
- * @type {ProxyHandler<{ props: Record<string | symbol, unknown>, exclude: Array<string | symbol> }>}}
+ * @type {ProxyHandler<{ props: Record<string | symbol, unknown>, exclude: Array<string | symbol>, name: string }>}}
  */
 const rest_props_handler = {
 	get(target, key) {
 		if (target.exclude.includes(key)) return;
 		return target.props[key];
 	},
-	set(_, key) {
+	set(target, key) {
 		if (DEV) {
-			throw new Error(
-				`Cannot write to property '${String(key)}' of rest element of $props(). It is always readonly.`
-			);
+			throw new Error(`${target.name}.${String(key)} is readonly`);
 		}
+
 		return false;
 	},
 	getOwnPropertyDescriptor(target, key) {
@@ -73,10 +72,11 @@ const rest_props_handler = {
 /**
  * @param {Record<string, unknown>} props
  * @param {string[]} rest
+ * @param {string} [name]
  * @returns {Record<string, unknown>}
  */
-export function rest_props(props, rest) {
-	return new Proxy({ props, exclude: rest }, rest_props_handler);
+export function rest_props(props, rest, name) {
+	return new Proxy({ props, exclude: rest, name }, rest_props_handler);
 }
 
 /**