By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Auto-kill mutations that produce non-exhaustive patterns Re-enable Opt_WarnIncompletePatterns (and friends) on the validation HscEnv so deSugarExpr's coverage checker runs. A RemoveCase that drops the only alternative covering a constructor — e.g. the `_` wildcard in `case xs of [] -> ...; [_] -> ...; _ -> ...` — now produces a DsNonExhaustivePatterns message, fails the silent-desugar check in validateAlt, and is dropped before reaching the manifest. The compiler's incomplete-patterns warning would catch these mutations in any normal build, so emitting them is pure noise.