  Recent studies on swampland conjectures (e.g. non-SUSY AdS conjecture or AdS
distance conjecture) predict that the (lightest) neutrino must be Dirac and the
mass must be cosmologically small $m < c \Lambda^{1/4}_{\text{cc}}\sim 10 \
\text{meV}$. The Dirac neutrino naturally accompanies a $U(1)$ symmetry that
can be embedded in the anomaly-free $U(1)_{B-L}$ global symmetry of the
standard model. We point out that the swampland conjectures applied to a circle
compactification with the $U(1)$ symmetry twisting lead to further constraints.
In particular, the $U(1)$ symmetry must be broken down to
$\mathbb{Z}_4$,$\mathbb{Z}_8$ or $\mathbb{Z}_{10}$ in the case of normal
hierarchy, and $\mathbb{Z}_4$ in the case of inverted hierarchy, providing
evidence for the absence of continuous global symmetry in quantum gravity. We
also predict a more constrained upper bound of the neutrino mass for each case.
