Function rustc_middle::ty::relate::relate_substs

pub fn relate_substs<R: TypeRelation<'tcx>, 'tcx>(
    relation: &mut R, 
    variances: Option<&[Variance]>, 
    a_subst: SubstsRef<'tcx>, 
    b_subst: SubstsRef<'tcx>
) -> RelateResult<'tcx, SubstsRef<'tcx>>