It's definitely not 0 because rebase heavy workflows involve the rerere cache which is a minefield of per-repo hidden merge changes. You get the results of "criss-cross merges" as "ghosts" you can't easily debug because there aren't good UI tools for the rerere cache. About the best you can do is declare rerere cache bankruptcy and make sure every repo clears their rerere cache.

I know that worst case isn't all that common or everyone would be scared of rebases, but I've seen it enough that I have a healthy disrespect of rebase heavy workflows and try to avoid them when given the option/in charge of choosing the tools/workflows/processes.

To be honest I've used rebase-heavy workflows for 15 years and never used rerere, so I can't comment on that (been a happy Jujutsu user for a few years — I've always wondered what the constituency for rerere is, and I'm curious if you could tell me!) I definitely agree in general that whenever you have a cache, you have to think about cache invalidation.

rerere is used automatically by git to cache certain merge conflict fixes encountered during a rebase so that you don't have to reapply them more than once rebasing the same branch later. In general, when it works, which is most of the time, it's part of what keeps rebases feeling easy and lightweight despite capturing in the final commit output sometimes a fraction of the data of a real merge commit. The rerere cache is in some respects a hidden collection of the rest of a merge commit.

In git, the merge (and merge commit) is the primitive and rebase a higher level operation on top of them with a complex but not generally well understood cache with only a few CLI commands and just about no UI support anywhere.

Like I said, because the rerere cache is so out-of-sight/out-of-mind that's why problems with it become weird and hard to debug. The situations I've seen that have been truly rebase-heavy workflows with multiple "git flow" long-running branches and even sometimes cherry picking between them. (Generally the same sorts of things that create "criss-cross merge scenarios".) Rebased commits start to bring in regressions from other branches. Rebased commits start to break builds randomly. If what is getting rebased is a long-running branch you probably don't have eyes on every commit, so finding where these hidden merge regressions happen becomes full branch bisects, you can't just focus on merge commits because you don't have them anymore, every commit is a candidate for a bad merge in a rebased branch.

Personally, I'd rather have real merge commits where you can trace both parents and the code not from either parent (conflict fixes), and you don't have to worry about ghosts of bad merges showing up in any random commit. Even the worst "criss-cross merge" commits are obvious in a commit log and I've seen have had enough data to surgically fix, often nearly as soon as they happen. rerere cache problems are things that can go unnoticed for weeks to everyone's confusion and potentially a lot of hidden harm. You can't easily see both parents of the merges involved. You might even have multiple repos with competing rerere caches alternating damage.

But also yes rerere cache problems are so generally infrequent that it might also take weeks of research, when it does happen, just to figure out what the rerere cache is for, that it might be the cause of some of your "merge ghosts" haunting your codebase, and how to clean it.

Obviously by the point where you are rebasing git flow-style long runnning branches and using frequent cherry picks you're in a rebase heavy workflow that is painful for other reasons and maybe that's an even heavier step beyond "rebase heavy" to some, but because the rerere cache is involved to some degree in every rebase once you stop trusting the rerere cache it can be hard to trust any rebase heavy workflow again. Like I said, personally I like the integration history/logs/investigatable diffs that real merge commits provide and prefer tools like `--first-parent` when I need "linear history" views/bisects.

You have to turn rerere on, though, right? I've never done that. I've also never worked with long-running branches — tend to strongly prefer integrating into main and using feature flags if necessary. Jujutsu doesn't have anything like rerere as far as I know.

Hmm, yeah looks like it is default off. Probably some git flow automation tool or other sort of bad corporate/consultant disseminated default config at a past job left the impression that it was default on. It's the solution to a lot of papercuts working with long-running branches as well as the source of new problems as stated above; problems that are visible with merge commits but hidden in rebases.