Subject: Re: [PATCH] diff --no-index: allow pathspec after --

From: Junio C Hamano <[email-masked]>
Date: Sat, 20 Sep 2014 23:19:19 -0700

Junio C Hamano <[email-masked]> writes:

> Coming back to the command line syntax for the new feature, if I had
> to choose, I would say
>
> 	git diff --no-index [-<options>] [--] <path> <path> <pathspec>
>
> perhaps?  As we never compare anything other than two things,...

Actually, I am not so sure about this anymore.

It is not entirely fair to say that

	git diff --no-index [--options] A B C D

as comparing A and B while using C and D as pathspec and declare a
person who expects otherwise a moron.  To a person whose brain is
not rotten by years of use of Git, "we never compare anything other
than two things with --no-index" is not a given, I am afraid.

It is equally plausible that the same command line may be asking to
compare A with B and C with D, i.e. producing

	diff --no-git a/A b/B
        ...
        diff --no-git a/C b/D
        ...

It also equally plausible (taking a cue from "mv A B C Dir/") that
it is comparing A, B and C with D/A, D/B and D/C, respectively,
producing

	diff --no-git a/A b/D/A
        ...
        diff --no-git a/B b/D/B
        ...
        diff --no-git a/C b/D/C
        ...

The only unambiguous syntax I can think of that avoids such
alternative interpretations is something ugly like

    git diff --no-index [-<options>] --src=<path> --dst=<path> [--] <pathspec>

where "src" and "dst" are in line with the existing src/dst-prefix
options.

Perhaps we could declare that this is the "canonical" way to spell
pathspec-filtered no-index comparison of two directories, but we
allow the syntax suggested in the message I am responding to as a
short-hand, but I am not sure if that would fly well.

This --src/--dst thing could lead to even uglier tangent.  We could
use these options to specify what is compared with what else

    --{src,dst}-path=<directory in the working tree>
    --{src,dst}-index
    --{src,dst}-worktree
    --{src,dst}-tree=<tree object>

which allows us to express funky combinations like

	git diff --src-index --dst-worktree [--] <pathspec>
	git diff --src-tree=HEAD --dst-index [--] <pathspec>
	git diff --src-tree=maint --dst-tree=master [--] <pathspec>

The above three would be ugly ways to spell the traditional "short
hands", e.g.

	git diff [--] <pathspec>
        git diff --cached [--] <pathspec>
        git diff maint master [--] <pathspec>

respectively.  And an obvious fallout of the above is this command
line:

	git diff --src-path=A --dst-path=B [--] <pathspec>

Because this does not involve any index, tree or worktree, it cannot
be a Git operation, so we know that is --no-index mode; the command
line does not need to say --no-index anywhere if we go that route.