  We show that the merger and tidal disruption of a C/O white dwarf (WD) by a
neutron star (NS) binary companion provides a natural formation scenario for
the PSR B1257+12 planetary system. Starting with initial conditions for the
debris disk produced of the disrupted WD, we model its long term viscous
evolution, including for the first time the effects of mass and angular
momentum loss during the early radiatively inefficient accretion flow (RIAF)
phase and accounting for the unusual C/O composition on the disk opacity. For
plausible values of the disk viscosity $\alpha \sim 10^{-3}-10^{-2}$ and the
RIAF mass loss efficiency, we find that the disk mass remaining near the planet
formation radius at the time of solid condensation is sufficient to explain the
pulsar planets. Rapid rocky planet formation via gravitational instability of
the solid carbon-dominated disk is facilitated by the suppression of vertical
shear instabilities due to the high solid-to-gas ratio. Additional evidence
supporting a WD-NS merger scenario includes (1) the low observed occurrence
rate of pulsar planets ($\lesssim 1\%$ of NS birth), comparable to the expected
WD-NS merger rate; (2) accretion by the NS during the RIAF phase is sufficient
to spin PSR B1257+12 up to its observed 6 ms period; (3) similar models of `low
angular momentum' disks, such as those produced from supernova fallback, find
insufficient mass reaching the planet formation radius. The unusually high
space velocity of PSR B1257+12 of $\gtrsim 326\,{\rm km\,s}^{-1}$ suggests a
possible connection to the Calcium-rich transients, dim supernovae which occur
in the outskirts of their host galaxies and were proposed to result from
mergers of WD-NS binaries receiving SN kicks. The C/O disk composition implied
by our model likely results in carbon-rich planets with diamond interiors.
