fix: disallow accessing internal Svelte props (#12207)

closes #12184

.changeset/olive-cobras-wonder.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: disallow accessing internal Svelte props

packages/svelte/messages/compile-errors/script.md

@@ -78,6 +78,10 @@
 
 > Cannot use `$props()` more than once
 
+## props_illegal_name
+
+> Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)
+
 ## props_invalid_identifier
 
 > `$props()` can only be used with an object destructuring pattern

packages/svelte/src/compiler/errors.js

@@ -276,6 +276,15 @@ export function props_duplicate(node) {
 	e(node, "props_duplicate", "Cannot use `$props()` more than once");
 }
 
+/**
+ * Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function props_illegal_name(node) {
+	e(node, "props_illegal_name", "Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)");
+}
+
 /**
  * `$props()` can only be used with an object destructuring pattern
  * @param {null | number | NodeLike} node

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

@@ -341,6 +341,14 @@ function validate_block_not_empty(node, context) {
  * @type {import('zimmerframe').Visitors<import('#compiler').SvelteNode, import('./types.js').AnalysisState>}
  */
 const validation = {
+	MemberExpression(node, context) {
+		if (node.object.type === 'Identifier' && node.property.type === 'Identifier') {
+			const binding = context.state.scope.get(node.object.name);
+			if (binding?.kind === 'rest_prop' && node.property.name.startsWith('$$')) {
+				e.props_illegal_name(node.property);
+			}
+		}
+	},
 	AssignmentExpression(node, context) {
 		validate_assignment(node, node.left, context.state);
 	},
@@ -1255,6 +1263,10 @@ export const validation_runes = merge(validation, a11y_validators, {
 							e.props_invalid_pattern(property);
 						}
 
+						if (property.key.type === 'Identifier' && property.key.name.startsWith('$$')) {
+							e.props_illegal_name(property);
+						}
+
 						const value =
 							property.value.type === 'AssignmentPattern' ? property.value.left : property.value;
 

packages/svelte/tests/compiler-errors/samples/runes-props-illegal-name-1/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'props_illegal_name',
+		message:
+			'Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-props-illegal-name-1/main.svelte

@@ -0,0 +1,3 @@
+<script>
+	let { $$slots: a } = $props();
+</script>

packages/svelte/tests/compiler-errors/samples/runes-props-illegal-name-2/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'props_illegal_name',
+		message:
+			'Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-props-illegal-name-2/main.svelte

@@ -0,0 +1,4 @@
+<script>
+	let props = $props();
+	props.$$slots;
+</script>