fix: error on snippet rest parameters

closes #12464

Author: dummdidumm

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

@@ -250,7 +250,7 @@
 
 ## snippet_invalid_rest_parameter
 
-> snippets do not support rest parameters; use an array instead
+> Snippets do not support rest parameters; use an array instead
 
 ## snippet_shadowing_prop
 

packages/svelte/src/compiler/errors.js

@@ -1108,12 +1108,12 @@ export function snippet_conflict(node) {
 }
 
 /**
- * snippets do not support rest parameters; use an array instead
+ * Snippets do not support rest parameters; use an array instead
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function snippet_invalid_rest_parameter(node) {
-	e(node, "snippet_invalid_rest_parameter", "snippets do not support rest parameters; use an array instead");
+	e(node, "snippet_invalid_rest_parameter", "Snippets do not support rest parameters; use an array instead");
 }
 
 /**

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

@@ -674,6 +674,12 @@ const validation = {
 	SnippetBlock(node, context) {
 		validate_block_not_empty(node.body, context);
 
+		for (const arg of node.parameters) {
+			if (arg.type === 'RestElement') {
+				e.snippet_invalid_rest_parameter(arg);
+			}
+		}
+
 		context.next({ ...context.state, parent_element: null });
 
 		const { path } = context;

packages/svelte/tests/compiler-errors/samples/snippet-rest-args/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'snippet_invalid_rest_parameter',
+		message: 'Snippets do not support rest parameters; use an array instead',
+		position: [19, 26]
+	}
+});

packages/svelte/tests/compiler-errors/samples/snippet-rest-args/main.svelte

@@ -0,0 +1,3 @@
+{#snippet children(...args)}
+	{args}
+{/snippet}