{"ast_node_type":"other","tokens":[{"end":44,"kind":"keyword","start":0,"text":"//"}]}
    // "{async_fn_env#0}<T1, T2, ...>", etc.
    // Name will be "{closure_env#0}<T1, T2, ...>", "{coroutine_env#0}<T1, T2, ...>", or
    let def_key = tcx.def_key(def_id);