I get a panic as follows when trying to use a plugin with the latest commit on the 8.6 branch.

<no location info>: error:
    ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.1.20180803 for x86_64-unknown-linux):
	mkPluginUsage: file not found
  LiftPlugin /nix/store/q8rg4mca5zqv98arpxd7164pqa72hf1n-ncurses-6.1/lib/libHSlift-plugin-0.1.0.0-LpWZumtLigSEjndVxGCyUH-ghc8.6.1.20180803.so
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable
        pprPanic, called at compiler/deSugar/DsUsage.hs:215:15 in ghc:DsUsage

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

My example is nixified and not very minimal but I can provide it tomorrow if that would help.