Remove an `unchecked_error_guaranteed` call. If we abort immediately after complaining about the obsolete `impl Trait for ..` syntax, then we avoid reaching HIR lowering. This means we can use `TyKind::Dummy` instead of `TyKind::Err`. r? `@oli-obk`
Remove an `unchecked_error_guaranteed` call. If we abort immediately after complaining about the obsolete `impl Trait for ..` syntax, then we avoid reaching HIR lowering. This means we can use `TyKind::Dummy` instead of `TyKind::Err`. r? `@oli-obk`