In the context of 4d effective gravity theories with 8 supersymmetries, we propose to unify, strenghten, and refine the several swampland conjectures into a single statement: the structural criterion, modelled on the structure theorem in Hodge theory. In its most abstract form the new swampland criterion applies to all 4d N = 2 effective theories (having a quantum-consistent UV completion) whether supersymmetry is local or rigid: indeed it may be regarded as the more general version of Seiberg-Witten geometry which holds both in the rigid and local cases.As a first application of the new swampland criterion we show that a quantum-consistent N = 2 supergravity with a cubic pre-potential is necessarily a truncation of a higher-Nsugra. More precisely: its moduli space is a Shimura variety of 'magic' type. In all other cases a quantum-consistent special Kahler geometry is either an arithmetic quotient of the complex hyperbolic space SU(1, m)/U(m) or has no local Killing vector.Applied to Calabi-Yau 3-folds this result implies (assuming mirror symmetry) the validity of the Oguiso-Sakurai conjecture in Algebraic Geometry: all Calabi-Yau 3-folds X without rational curves have Picard number rho = 2, 3; in facts they are finite quotients of Abelian varieties. More generally: the Kahler moduli of X do not receive quantum corrections if and only if X has infinite fundamental group. In all other cases the Kahler moduli have instanton corrections in (essentially) all possible degrees.
