Do you have a link to a copy of the video with captions? YouTube autogen doesn't cut it unfortunately. Or perhaps a written-form version (slide deck + transcript)?
The remaining 8% mostly falls into the following categories:
- Code that use OS functionalities that are cumbersome to mock in tests
- Code paths that are triggered relatively rarely and I was simply too lazy to add tests for them
Nothing is impossible to cover, but for whatever reason it was too much work for me when I wrote the code.
However, it's worth mentioning that I only settled on the transcript test pattern fairly recently, and if I were to rewrite or refactor some of the untested code today I would add tests for them, because the cost of adding tests has been lowered considerably. So Elvish's test coverage is still increasing slowly as the cost of testing decreases.
What's in the 8% not covered by testing?