  We present the optical spectroscopic evolution of SN~2023ixf seen in
sub-night cadence spectra from 1.18 to 14 days after explosion. We identify
high-ionization emission features, signatures of interaction with material
surrounding the progenitor star, that fade over the first 7 days, with rapid
evolution between spectra observed within the same night. We compare the
emission lines present and their relative strength to those of other supernovae
with early interaction, finding a close match to SN~2020pni and SN~2017ahn in
the first spectrum and SN~2014G at later epochs. To physically interpret our
observations we compare them to CMFGEN models with confined, dense
circumstellar material around a red supergiant progenitor from the literature.
We find that very few models reproduce the blended \NC{} emission lines
observed in the first few spectra and their rapid disappearance thereafter,
making this a unique diagnostic. From the best models, we find a mass-loss rate
of $10^{-3}-10^{-2}$ \mlunit{}, which far exceeds the mass-loss rate for any
steady wind, especially for a red supergiant in the initial mass range of the
detected progenitor. These mass-loss rates are, however, similar to rates
inferred for other supernovae with early circumstellar interaction. Using the
phase when the narrow emission features disappear, we calculate an outer dense
radius of circumstellar material $R_\mathrm{CSM,
out}\sim5\times10^{14}~\mathrm{cm}$ and a mean circumstellar material density
of $\rho=5.6\times10^{-14}~\mathrm{g\,cm^{-3}}$. This is consistent with the
lower limit on the outer radius of the circumstellar material we calculate from
the peak \Halpha{} emission flux, $R_\text{CSM,
out}\gtrsim9\times10^{13}~\mathrm{cm}$.
