chore: simpler const parsing (#10347)

* chore: simpler const parsing

* fix

* fix

---------

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

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

@@ -550,42 +550,17 @@ function special(parser) {
 	}
 
 	if (parser.eat('const')) {
-		// {@const a = b}
-		const start_index = parser.index - 5;
-		parser.require_whitespace();
+		parser.allow_whitespace();
 
-		let end_index = parser.index;
-		/** @type {import('estree').VariableDeclaration | undefined} */
-		let declaration = undefined;
+		const id = read_context(parser);
+		parser.allow_whitespace();
 
-		// Can't use parse_expression_at here, so we try to parse until we find the correct range
-		const dummy_spaces = parser.template.substring(0, start_index).replace(/[^\n]/g, ' ');
-		while (true) {
-			end_index = parser.template.indexOf('}', end_index + 1);
-			if (end_index === -1) break;
-			try {
-				const node = parse(
-					dummy_spaces + parser.template.substring(start_index, end_index),
-					parser.ts
-				).body[0];
-				if (node?.type === 'VariableDeclaration') {
-					declaration = node;
-					break;
-				}
-			} catch (e) {
-				continue;
-			}
-		}
+		parser.eat('=', true);
+		parser.allow_whitespace();
 
-		if (
-			declaration === undefined ||
-			declaration.declarations.length !== 1 ||
-			declaration.declarations[0].init === undefined
-		) {
-			error(start, 'invalid-const');
-		}
+		const init = read_expression(parser);
+		parser.allow_whitespace();
 
-		parser.index = end_index;
 		parser.eat('}', true);
 
 		parser.append(
@@ -593,7 +568,13 @@ function special(parser) {
 				type: 'ConstTag',
 				start,
 				end: parser.index,
-				declaration
+				declaration: {
+					type: 'VariableDeclaration',
+					kind: 'const',
+					declarations: [{ type: 'VariableDeclarator', id, init }],
+					start: start + 1,
+					end: parser.index - 1
+				}
 			})
 		);
 	}