feat: guard 语句 guard x >= 0 else { return -1; }

src/parser/parser.c

@@ -621,6 +621,19 @@ static AstNode* parse_statement(Parser* p, ErrorInfo* error) {
         return parse_match_stmt(p, error);
     }
 
+    if (t->kind == TOK_GUARD) {
+        // guard expr else { ... }  →  if !(expr) { ... }
+        const Token* guard_tok = advance(p);
+        AstNode* cond = parse_expr(p, error);
+        if (!cond) return NULL;
+        if (!expect(p, TOK_ELSE, error, "guard 缺少 'else'")) return NULL;
+        AstNode* body = parse_block(p, error);
+        if (!body) return NULL;
+        // 去糖: if !cond { body }
+        AstNode* not_cond = ast_make_unary(p->arena, OP_NOT, cond, tok_loc(guard_tok));
+        return ast_make_if(p->arena, not_cond, body, NULL, tok_loc(guard_tok));
+    }
+
     if (t->kind == TOK_RETURN) {
         advance(p);
         if (match(p, TOK_SEMICOLON)) {