rebase message confusion

[Sune Foldager]

I am trying to fix a bug in hg rebase, which occurs in some cases when 
you try to rebase from a certain point to a descendant of that point; 
you get a cryptic error and the wdir parent is changed.

Before I can do that, however, I had to make sense of what I consider at 
best highly confusing status, debug and error messages from hg rebase. 
For instace, what does this mean?

"cannot rebase an ancestor"

well it makes you think at least. I am trying to rebase x to an ancestor 
of x? or to a descendant of x? the latter is correct. But why not the 
much clearer "source is ancestor of destination"?

Similar with the opposite, "cannot rebase a descendant". A descendant of 
what?? Here I would prefer "source is a descendant of destination", 
analogous to the previous case.

These messages are actually aborts, so the process stops with an error. 
And this happens when the --source option is used. If either --base or 
no source selection option is used, an entirely different code path is 
activated, for some reason, with other, even worse, messages. This time 
they are debug messages, and the command succeeds with "nothing to 
rebase" instead of aborting.

For instance, we have the great message "already working on current".... 
which means... source == destination! why, of course :-)

And what on earth does "already working on branch" mean?? Well, as it 
turns out it means the same as "cannot rebase an ancestor", that is it 
means "source is ancestor of destination".

Now, my bug fix needs to output the opposite message of the above, which 
is why I write this. Because I find it hard to create the opposite of 
"already working on branch" :-p.

So does anyone know why a) we would want these somewhat cryptic 
messages? and b) why we sometimes want to abort, and sometimes to 
ui.debug and otherwise fail almost silently? I hope we can agree on some 
nice messages; suggestions are very welcome :-).

/Sune

(Another issue is the documentation of --source and --base, which IMHO 
fail to make it clear what they each do. But well, that's another issue 
:-).)