When you are using linear histories and rebasing you don't do monolithic feature branches. You land smaller chunks and gate their functionality via some configuration variable. `if (useNewPath) { newPath(); } else { oldPath(); }` and all your new incremental features land in `newPath`. All tests pass on both code paths and nothing breaks. When the feature is fully done then you change the default configuration to move to the `newPath`.
> How are you going to deal with non-trivial feature branches that need to be integrated into master?
That's the point -- this isn't a thing in rebase workflows. That's a feature. You don't have to deal with megapatches for massive features. It's incrementally verified along the way and bisect works flawlessly.