{"ast_node_type":"variable","tokens":[{"end":81,"kind":"keyword","start":0,"text":"let"}]}
    let env_ty = Ty::new_adt(tcx, pin_adt_ref, tcx.mk_args(&[proxy_ref.into()]));
    let pin_adt_ref = tcx.adt_def(tcx.require_lang_item(LangItem::Pin, span));
    // sig = `fn (Pin<&mut proxy_ty>, &mut Context) -> Poll<()>`