Type Alias RollbackStep 

pub type RollbackStep<S> = Box<dyn FnOnce(&mut S) + Send>;

Boxed rollback step. Invoked only if the matching forward step previously succeeded; receives the same shared state and undoes the forward mutation in place. Failures are logged at error level — rollback failure is a degraded-session signal, not a recoverable caller-facing error.

Aliased Type

pub struct RollbackStep<S>(/* private fields */);