Sigh. As an inconvenient workaround, I tried adding {-# OPTIONS_GHC -dverbose-core2core #-} to GHC.Real, and then doing cabal new-run hadrian -- -c -j4 --flavour=quick --directory=".." in hadrian/ That did recompile GHC.Real - but all the debug output disappeared! I tried adding {-# OPTIONS_GHC -ddebug-output #-} as well, but that didn't work. I'm stuck - any ideas? Simon From: ghc-devs On Behalf Of Simon Peyton Jones via ghc-devs Sent: 30 August 2018 12:12 To: Alp Mestanogullari ; Andrey Mokhov Cc: ghc-devs Subject: Hadrian Alp, Andrey The old build system printed out every command line; and I often copy-paste that info to build single modules. Eg currently, when trying to understand #15570 I see a suspicious GHC.Real.hi. So I want to manually recompile GHC.Real (from base), adding some debug flags. How can I get the right command line to do that from the build log? Where is the "how to use Hadrian" wiki page? I know you've been writing one. Simon