{"ast_node_type":"variable","tokens":[{"end":77,"kind":"keyword","start":0,"text":"let"}]}
            let label_span = self.mk_sp(line_start, frontmatter_opening_pos);
            let span = self.mk_sp(line_start, frontmatter_opening_end_pos);
            self.dcx().emit_err(errors::FrontmatterInvalidOpeningPrecedingWhitespace {