Sandbox remains inconsistent after failed Cabal build

Determine whether Cabal should automatically revert a sandbox to its prior state when a package build fails inside it.

The decision must balance performance overhead of tracking sandbox state against the need to avoid leaving an inconsistent environment after a failure, while also preserving user intent when failures are expected, such as when testing deliberately broken packages.

What signals should Cabal treat as a definitive build failure warranting rollback? How could the tool record enough sandbox state to enable a safe revert without significant slowdown? Should automatic rollback be optional, configurable per project or per invocation?