Function borrowck_check_region_constraints 

pub(crate) fn borrowck_check_region_constraints<'diag, 'tcx>(
    root_cx: &mut BorrowCheckRootCtxt<'diag, 'tcx>,
    diags_buffer: &mut BorrowckDiagnosticsBuffer<'diag, 'tcx>,
    _: CollectRegionConstraintsResult<'tcx>,
) -> PropagatedBorrowCheckResults<'tcx>

Using the region constraints computed by borrowck_collect_region_constraints and the additional constraints from BorrowCheckRootCtxt::handle_opaque_type_uses, compute the region graph and actually check for any borrowck errors.