Function identity_args_with_inferred 

pub fn identity_args_with_inferred<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: RDefId,
) -> GenericArgsRef<'tcx>

Closures and inline consts have special type params that are used as inference variables. After analysis, they can be computed from the rest of the args (modulo some erased regions). This computes the identity args for this def_id with the inference ars properly inferred.