We present detailed observations of ZTF18abukavn (SN2018gep), discovered in
high-cadence data from the Zwicky Transient Facility as a rapidly rising
($1.4\pm0.1$ mag/hr) and luminous ($M_{g,\mathrm{peak}}=-20$ mag) transient. It
is spectroscopically classified as a broad-lined stripped-envelope supernova
(Ic-BL SN). The high peak luminosity ($L_{\mathrm{bol}} \gtrsim 3 \times
10^{44}$ erg $\mathrm{sec}^{-1}$), the short rise time ($t_{\mathrm{rise}}= 3$
days in $g$-band), and the blue colors at peak ($g-r\sim-0.4$) all resemble the
high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast
transients. The early discovery of SN2018gep (within an hour of shock breakout)
enabled an intensive spectroscopic campaign, including the highest-temperature
($T_{\mathrm{eff}}\gtrsim40,000$ K) spectra of a stripped-envelope SN. A
retrospective search revealed luminous ($M_g \sim M_r \approx -14\,$mag)
emission in the days to weeks before explosion, the first definitive detection
of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray
energy release $E_\mathrm{\gamma,iso}<4.9 \times 10^{48}$ erg, a limit on X-ray
emission $L_{\mathrm{X}} < 10^{40}\,$erg sec$^{-1}$, and a limit on radio
emission $\nu L_\nu \lesssim 10^{37}\,$erg sec$^{-1}$. Taken together, we find
that the early ($<10\,$days) data are best explained by shock breakout in a
massive shell of dense circumstellar material ($0.02\,M_\odot$) at large radii
($3 \times 10^{14}\,$cm) that was ejected in eruptive pre-explosion mass-loss
episodes. The late-time ($>10$ days) light curve requires an additional energy
source, which could be the radioactive decay of Ni-56.
