Function coroutine_closure_output_coroutine 

fn coroutine_closure_output_coroutine<'tcx>(
    tcx: TyCtxt<'tcx>,
    obligation: &ProjectionTermObligation<'tcx>,
    goal_kind: ClosureKind,
    env_region: Region<'tcx>,
    def_id: DefId,
    args: CoroutineClosureArgs<TyCtxt<'tcx>>,
) -> Ty<'tcx>

Given a CoroutineClosure(def_id, args), interpret it as a closure, and return its output type for the given goal_kind and env_region.