  We forecast the constraints on single-field inflation from the bispectrum of
future high-redshift surveys such as MegaMapper. Considering non-local
primordial non-Gaussianity (NLPNG), we find that current methods will yield
constraints of order $\sigma(f_{\rm NL}^{\rm eq})\approx 23$, $\sigma(f_{\rm
NL}^{\rm orth})\approx 12$ in a joint power-spectrum and bispectrum analysis,
varying both nuisance parameters and cosmology, including a conservative range
of scales. Fixing cosmological parameters and quadratic bias parameter
relations, the limits tighten significantly to $\sigma(f_{\rm NL}^{\rm
eq})\approx 17$, $\sigma(f_{\rm NL}^{\rm orth})\approx 8$. These compare
favorably with the forecasted bounds from CMB-S4: $\sigma(f_{\rm NL}^{\rm
eq})\approx 21$, $\sigma(f_{\rm NL}^{\rm orth})\approx 9$, with a combined
constraint of $\sigma(f_{\rm NL}^{\rm eq})\approx 14$, $\sigma(f_{\rm NL}^{\rm
orth})\approx 7$; this weakens only slightly if one instead combines with data
from the Simons Observatory. We additionally perform a range of Fisher analyses
for the error, forecasting the dependence on nuisance parameter
marginalization, scale cuts, and survey strategy. Lack of knowledge of bias and
counterterm parameters is found to significantly limit the information content;
this could be ameliorated by tight simulation-based priors on the nuisance
parameters. The error-bars decrease significantly as the number of observed
galaxies and survey depth is increased: as expected, deep dense surveys are the
most constraining, though it will be difficult to reach $\sigma(f_{\rm
NL})\approx 1$ with current methods. The NLPNG constraints will tighten further
with improved theoretical models (incorporating higher-loop corrections and
improved understanding of nuisance parameters), as well as the inclusion of
additional higher-order statistics.
