fix: throw compilation error for malformed snippets (#12144)

* fix: throw compilation error for malformed snippets

* fix: support multiline

* tweak

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/polite-ways-serve.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: throw compilation error for malformed snippets

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

@@ -269,28 +269,27 @@ function open(parser) {
 			e.expected_identifier(parser.index);
 		}
 
-		let slice_end = parser.index;
+		const params_start = parser.index;
 
 		parser.eat('(', true);
-
 		let parentheses = 1;
-		let params = '';
 
-		while (!parser.match(')') || parentheses !== 1) {
+		while (parser.index < parser.template.length && (!parser.match(')') || parentheses !== 1)) {
 			if (parser.match('(')) parentheses++;
 			if (parser.match(')')) parentheses--;
-			params += parser.read(/^./);
+			parser.index += 1;
 		}
 
+		parser.eat(')', true);
+
+		const prelude = parser.template.slice(0, params_start).replace(/\S/g, ' ');
+		const params = parser.template.slice(params_start, parser.index);
+
 		let function_expression = /** @type {import('estree').ArrowFunctionExpression} */ (
-			parse_expression_at(
-				parser.template.slice(0, slice_end).replace(/\S/g, ' ') + `(${params}) => {}`,
-				parser.ts,
-				0
-			)
+			parse_expression_at(prelude + `${params} => {}`, parser.ts, params_start)
 		);
 
-		parser.index += 2;
+		parser.eat('}', true);
 
 		/** @type {ReturnType<typeof parser.append<import('#compiler').SnippetBlock>>} */
 		const block = parser.append({

packages/svelte/tests/compiler-errors/samples/malformed-snippet-2/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'expected_token',
+		message: 'Expected token )',
+		position: [31, 31]
+	}
+});

packages/svelte/tests/compiler-errors/samples/malformed-snippet-2/main.svelte

@@ -0,0 +1 @@
+{#snippet children(hi{/snippet}

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

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'expected_token',
+		message: 'Expected token }',
+		position: [20, 20]
+	}
+});

packages/svelte/tests/compiler-errors/samples/malformed-snippet/main.svelte

@@ -0,0 +1 @@
+{#snippet children()hi{/snippet}