  During the commissioning of {\em JWST}, the Medium-Resolution Spectrometer
(MRS) on the Mid-Infrared Instrument (MIRI) observed the planetary nebula SMP
LMC 058 in the Large Magellanic Cloud. The MRS was designed to provide medium
resolution (R = $\lambda$/$\Delta\lambda$) 3D spectroscopy in the whole MIRI
range. SMP LMC 058 is the only source observed in {\em JWST} commissioning that
is both spatially and spectrally unresolved by the MRS and is a good test of
{\em JWST's} capabilities. The new MRS spectra reveal a wealth of emission
lines not previously detected in this planetary nebula. From these lines, the
spectral resolving power ($\lambda$/$\Delta\lambda$) of the MRS is confirmed to
be in the range R $=$ 4000 to 1500, depending on the MRS spectral sub-band. In
addition, the spectra confirm that the carbon-rich dust emission is from SiC
grains and that there is little to no time evolution of the SiC dust and
emission line strengths over a 17-year epoch. These commissioning data reveal
the great potential of the MIRI MRS for the study of circumstellar and
interstellar material.
