  We use data from Gaia DR2 to constrain the initial-final mass relation (IFMR)
for field stars with initial masses $0.9 \lesssim m_{\rm in}/M_{\odot} \lesssim
8$. Precise parallaxes have revealed unprecedented substructure in the white
dwarf (WD) cooling sequence on the color-magnitude diagram (CMD). Some of this
substructure stems from the diversity of WD atmospheric compositions, but the
CMD remains bimodal even when only spectroscopically-confirmed DA WDs are
considered. We develop a generative model to predict the CMD for DA WDs as a
function of the initial mass function (IMF), stellar age distribution, and a
flexibly parameterized IFMR. We then fit the CMD of 1100 bright DA WDs within
100 pc, for which atmospheric composition and completeness are well-understood.
The resulting best-fit IFMR flattens at $3.5 \lesssim m_{\rm
in}/M_{\odot}\lesssim 5.5$, producing a secondary peak in the WD mass
distribution at $m_{\rm WD} \sim 0.8 M_{\odot}$. Our IFMR is broadly consistent
with weaker constraints obtained from binaries and star clusters in previous
work but represents the clearest observational evidence obtained to date of
theoretically-predicted non-linearity in the IFMR. A visibly bimodal CMD is
only predicted for mixed-age stellar populations: in single-age clusters, more
massive WDs reach the bottom of the cooling sequence before the first
lower-mass WDs appear. This may explain why bimodal cooling sequences have thus
far evaded detection in cluster CMDs.
