fix: don't hang on a tag whose expression ends with a trailing slash (#18350)

close https://github.com/sveltejs/svelte/issues/18349

.changeset/fix-find-matching-bracket-infinite-loop.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent infinite loop when a tag's expression ends with a trailing `/` at the end of the input

packages/svelte/src/compiler/phases/1-parse/utils/bracket.js

@@ -107,7 +107,11 @@ export function find_matching_bracket(template, index, open) {
 				continue;
 			case '/': {
 				const next_char = template[i + 1];
-				if (!next_char) continue;
+				if (!next_char) {
+					// `/` is the last character; advance past it so we don't loop forever
+					i++;
+					continue;
+				}
 				if (next_char === '/') {
 					i = infinity_if_negative(template.indexOf('\n', i + 1)) + '\n'.length;
 					continue;

packages/svelte/tests/compiler-errors/samples/declaration-tag-trailing-slash/_config.js

@@ -0,0 +1,11 @@
+import { test } from '../../test';
+
+// A tag whose expression ends with a bare `/` at the end of the input used to
+// make `find_matching_bracket` loop forever; it should error instead of hanging.
+export default test({
+	error: {
+		code: 'unexpected_eof',
+		message: 'Unexpected end of input',
+		position: [12, 12]
+	}
+});

packages/svelte/tests/compiler-errors/samples/declaration-tag-trailing-slash/main.svelte

@@ -0,0 +1 @@
+{let x = a /