By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Mutation: read interface pragmas while instrumenting The constructor-alias recognition added two commits ago reads the unfolding GHC recorded for an imported binding. -O0 implies -fignore-interface-pragmas, which drops unfoldings as interfaces are read, and addManifest compiles instrumented libraries at -O0 on purpose -- so the recognition silently did nothing in exactly the configuration mutation testing runs in. Every Map.empty in a real codebase still got its unkillable mutant. It looked like it worked because of a second bug. This repo's overlay adds --ghc-options=-O2 to every package in it, and Cabal passes that through after its own optimisation flag, so addManifest's --disable-optimization was overridden and the example -- the only thing the recognition was tested against -- compiled at -O2, where unfoldings are read. addManifest now passes -O0 last, which makes the intent hold whatever the package asked for and puts the example on the same footing as its users. With that alone the example regains the Map.empty -> Tip mutant, reproducing what nix-ci sees. So the driver plugin unsets Opt_IgnoreInterfacePragmas. Reading unfoldings does not run the simplifier, so the compile-time blowup -O0 is there to avoid does not come back with them, and the committed manifests are unchanged: the -O0-with-unfoldings build produces exactly what the -O2 build did.