fix: better error for TS annotations in snippets without lang="ts"

.changeset/gentle-dots-dream.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Improve error message when TypeScript syntax is used in snippet parameters without `lang="ts"`

packages/svelte/src/compiler/phases/1-parse/index.js

@@ -177,10 +177,11 @@ export class Parser {
 
 	/**
 	 * @param {any} err
+	 * @param {string} [suffix]
 	 * @returns {never}
 	 */
-	acorn_error(err) {
+	acorn_error(err, suffix = '') {
-		e.js_parse_error(err.pos, err.message.replace(regex_position_indicator, ''));
+		e.js_parse_error(err.pos, err.message.replace(regex_position_indicator, '') + suffix);
 	}
 
 	/**

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -390,16 +390,28 @@ function open(parser) {
 		const prelude = parser.template.slice(0, params_start).replace(/\S/g, ' ');
 		const params = parser.template.slice(params_start, parser.index);
 
-		let function_expression = matched
+		let function_expression;
-			? /** @type {ArrowFunctionExpression} */ (
+
+		if (matched) {
+			try {
+				function_expression = /** @type {ArrowFunctionExpression} */ (
 					parse_expression_at(
 						prelude + `${params} => {}`,
 						parser.root.comments,
 						parser.ts,
 						params_start
 					)
-				)
+				);
-			: { params: [] };
+			} catch (/** @type {any} */ err) {
+				const suffix =
+					!parser.ts && parser.template[err.pos] === ':'
+						? ` (did you forget to add \`lang="ts"\`?)`
+						: '';
+				parser.acorn_error(err, suffix);
+			}
+		} else {
+			function_expression = { params: [] };
+		}
 
 		parser.allow_whitespace();
 		parser.eat('}', true);

packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'js_parse_error',
+		message: 'Unexpected token (did you forget to add `lang="ts"`?)',
+		position: [18, 18]
+	}
+});

packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/main.svelte

@@ -0,0 +1,3 @@
+{#snippet bar(text: string)}
+	{text}
+{/snippet}