syntax_data: {"ast_node_type":"other","tokens":[{"end":33,"kind":"keyword","start":0,"text":"Err(tail_ty)"}]}
source_snippet:                 Err(tail_ty) => {
context_before:                 Ok(metadata_ty) => metadata_ty,
context_after:                     let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, DUMMY_SP);