git config --global rebase.autostash # one time configuration

git rebase @{u} # example rebase on upstream branch

The autostash will be applied whenever the rebase is finished. It does not matter whether the rebase finishes successfully, or if it is aborted. Either way, the autostash will be applied. If the rebase was successful, and the base commit therefore changed, then there may be a conflict between the autostash and the new commits. In this case, you will have to resolve the conflicts before committing. This is no different than if you would have manually stashed, and then applied, so there is no downside to doing it automatically.