Subject: Re: [PATCH RFC] diff --no-index: teach option to exclude files by pattern

From: Junio C Hamano <[email-masked]>
Date: Wed, 14 May 2025 14:10:10 -0700

Jacob Keller <[email-masked]> writes:

> From: Jacob Keller <[email-masked]>
>
> Teach git diff --no-index the ability to exclude files by wildmatch
> pattern when recursing through directories. The '--exclude' option
> builds up a string list containing the patterns. These are checked with
> wildmatch() in the read_directory_contents function. If any pattern
> matches, then the file is not included in the directory contents.

A quite natural question that comes to mind is:

    How would we do this for the normal "git diff" that is not the
    bolted on '--no-index' mode?

but ...

> The --exclude option is only supported by the --no-index mode. Standard
> diff modes support negative pathspecs which is more powerful. I tried to
> see if there was a way to add support for negative pathspecs themselves,
> but haven't yet figured out if this is possible.

... of course you have thought about it already.  I do agree with
you that we should figure out how and teach this mode to also use
pathspec, not necessarily only the negative ones but positive ones.

After all,

    $ git diff --no-index [<option>...] dirA dirB

is like running

    $ diff -r [<option>...] dirA dirB

after preparing these two directories like so:

    $ git archive revA | ( mkdir dirA && tar Cxf dirA - )
    $ git archive revB | ( mkdir dirB && tar Cxf dirB - )

Hence it is natural for users to expect that anything you can do
with

    $ git diff revA revB

should be doable, in

    $ git diff --no-index dirA dirB

and vice versa.  And as you said, when comparing two revisions,
you'd use pathspec for this kind of thing.

    $ git diff revA revB -- Documentation/ t/ ':!po/'

So, I pretty much agree with the need to be able to exclude some
parts of the tree(s) from comparison in "diff --no-index" mode, but
I doubt it is a good idea to tell what to exclude the "--no-index"
mode in a completely different way.

The last time I looked at it, I got an impression that the command
line argument parsing of "git diff --no-index" was messy (which is
sort of inevitable, since unlike the normal "git diff", it can
compare more than just two "collections"---it can take two paths to
regular files, for example, and in such a case pathspec arguments
can play no role), so teaching it pathspec parsing might be a bit of
work, though.

Thanks for starting an interesting topic.