By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Fix `ignore` filter leaking through `$` and ExpandedThingTc Two bugs prevented the plugin's `ignore` config from working in practice: 1. `nix/addManifest.nix` passed the YAML config via `--ghc-options=...` (plural). Cabal treats that as a space-separated multi-flag and the value's embedded `=` and `:` confused the parser, so the plugin was invoked with no opts and `ignore` always stayed at `[]`. Switch to `--ghc-option=...` (singular) so the whole `-fplugin-opt=...` token reaches GHC intact. 2. Once the config is actually loaded, `opOccName` still failed to see through GHC 9.6+ expansions: `mark $ unwords [...]` is a `XExpr (ExpandedThingTc _ (HsApp (HsApp ($) mark) (unwords [...])))`, and `opOccName` returned `Nothing` for it. The ignore filter then fell through to children and mutated the inner list literal. Walk through `ExpandedThingTc`, and treat `($) f x` as having syntactic head `f` rather than `$`. Regression test in `sydtest-mutation-example/src/Example/IgnoreLib.hs`: `mark` is a two-argument no-op with a polymorphic payload (introduces typecheck `WrapExpr` wrappers), called both directly and via `$`; the golden `Example.IgnoreLib.json` asserts no mutations are produced. `mutationManifestCheck.nix` now passes `./mutation.yaml` so the golden actually exercises the `ignore` machinery.