By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Only instrument library components, not test-suites/exes/benches Cabal's --ghc-option=-fplugin=... in Setup build's buildFlags applies globally to every component compiled in that invocation. For packages where addManifest wraps a package with both a library and a test-suite (e.g. opt-env-conf-test), this caused the test-suite modules to be instrumented too. Fix: do a two-step build in addManifest's postBuild. Step 1 (main build phase) scopes Setup build to lib:<pname> with the plugin flags; step 2 re-runs Setup build with the same flags but MUTATION_PLUGIN_SKIP=1, so remaining components (test-suite, exe, bench) compile without instrumentation. The plugin honours the env var. Same plugin flags are required in step 2 to avoid Cabal seeing a package-set change and rebuilding the already-instrumented library un-instrumented. Also switch pluginRecompile from impurePlugin to flagRecompile so that the second invocation does not force-recompile the library.