This is genius, loving it!

If you want, you can also put the git diff shelling behind an experimental Cargo feature toggle, or even a Rustc feature flag.
That way you can turn it on with RUSTFLAGS="--cfg my_custom_flag" (maybe like this, I asked LLM).

That way it might be easier to test in the real world, and we could merge this.

I was hoping that merging behind a flag might be possible and I really like the idea of using RUSTFLAGS! There’s one issue, though, as I had to change the API of gix_blame::file::function::file to include worktree_path as that’s needed by git diff. Is there a way to get worktree_path from any of the other parameters maybe?

Since the gix based diff definitely has that information, I am surprised it's not readily available. But somewhere, a level up, it must definitely be there, so I think this can be cfg_if'ed on that level, with adjustments to the callsite. It's not super pretty to adjust the call site (one with, and one without the worktree path), but it gets the job done and is probably needed to make this testable in the real world.

From what I can tell, the issue is that we pass repo.objects as odb: impl gix_object::Find + gix_object::FindHeader. While repo.objects has .store().path() which we could use to infer the workdir (at that level, we can also get it through repo.workdir()), odb does not provide this piece of information. So I agree that the easiest workaround probably is to feature flag the worktree_path as an additional argument.

Let's try that then. If it doesn't come together, I would also take a look and actually try it myself, so far I was just looking at this from my armchair.