  WFIRST will conduct a coronagraphic program of characterizing the atmospheres
of planets around bright nearby stars. When observed with the WFIRST Wide Field
Camera, these stars will saturate the detector and produce very strong
diffraction spikes. In this paper, we forecast the astrometric precision that
WFIRST can achieve by centering on the diffraction spikes of highly saturated
stars. This measurement principle is strongly facilitated by the WFIRST H4RG
detectors, which confine excess charges within the potential well of saturated
pixels. By adopting a simplified analytical model of the diffraction spike
caused by a single support strut obscuring the telescope aperture, integrated
over the WFIRST pixel size, we predict the performance of this approach with
the Fisher-matrix formalism. We discuss the validity of the model and find that
10 ${\mu}$as astrometric precision is achievable with a single 100 s exposure
of a R = 6 or a J = 5 star. We discuss observational limitations from the
optical distortion correction and pixel-level artifacts, which need to be
calibrated at the level of 10 - 20 ${\mu}$as so as to not dominate the error
budget. To suppress those systematics, we suggest a series of short exposures,
dithered by at least several hundred pixels, to reach an effective per-visit
astrometric precision of better than 10 ${\mu}$as. If this can be achieved, a
dedicated WFIRST GO program will be able to detect Earth-mass exoplanets with
orbital periods of 1 yr around stars within a few pc as well as Neptune-like
planets with shorter periods or around more massive or distant stars. Such a
program will also enable mass measurements of many anticipated direct-imaging
exoplanet targets of the WFIRST coronagraph and a "starshade" occulter.
