Can you show me what arc which says?

Sure thing, P1845

Does that output make sense in explaining what arc diff is doing, or not so much?

What did you expect arc diff to do?

Oh, I suppose you pretty much answered that:

Ideally, I'd like to set it up so that arc diff will always act against origin/master unless someone explicitly specifies a revision or branch, so let me know if I've configured something incorrectly.

To make arc diff do that, run this command:

arc set-config base 'git:merge-base(origin/master)'

(Then you can use arc which to explain the new behavior.)

This is documented here:

https://secure.phabricator.com/book/phabricator/article/arcanist_commit_ranges/

Your confusion is reasonable and this is something I want to improve, but there's a lot of historical context making it tricky to change since other users rely on the non-obvious, implicit, highly magical default rulesets.

Broadly, if no explicit base rule is configured telling arc what it should actually do, we try to guess a bunch of "reasonable" things, including the tracked upstream branch if one exists. The word "reasonable" here means "someone filed a feature request asking for it early in the lifecycle of the project before we got strict about not accepting this kind of stuff".

Specifically, I think I want arc to have this default behavior instead:

$ arc diff
Specify the beginning of the commit range you want to send for review, like this:

  $ arc diff <commit>

Use `arc which <commit>` to understand what `arc diff <commit>` will do.

To have `arc` select a range automatically, read "Commit Ranges" in the documentation.

In T9216#132880, @epriestley wrote:

Does that output make sense in explaining what arc diff is doing, or not so much?

Yeah, that makes sense, and I see why I was confused... thanks, I forgot about arc which.