Re: [PATCH v3 2/2] diff: add SUBMODULE_DIFF format to display submodule diff

[Junio C Hamano <gitster@pobox.com>]

Jacob Keller <jacob.e.keller@intel.com> writes:

> @@ -2305,6 +2311,15 @@ static void builtin_diff(const char *name_a,
>  	struct strbuf header = STRBUF_INIT;
>  	const char *line_prefix = diff_line_prefix(o);
>  
> +	diff_set_mnemonic_prefix(o, "a/", "b/");
> +	if (DIFF_OPT_TST(o, REVERSE_DIFF)) {
> +		a_prefix = o->b_prefix;
> +		b_prefix = o->a_prefix;
> +	} else {
> +		a_prefix = o->a_prefix;
> +		b_prefix = o->b_prefix;
> +	}
> +

Hmph, is it safe to raise this when SUBMODULE_DIFF is not in effect?
Not objecting, just asking.

>  	if (DIFF_OPT_TST(o, SUBMODULE_LOG) &&
> ...
> +	} else if (DIFF_OPT_TST(o, SUBMODULE_DIFF) &&

This makes clear that SUBMODULE_LOG and SUBMODULE_DIFF should not be
independent bits in the diff-opt flag word.  We used to run
something like "log --oneline --left-right A...B" and your new code
runs "diff A B", but the next month somebody else would want to do
"log -p --left-right A...B" or something else, and they are clearly
mutually exclusive.

> diff --git a/diff.h b/diff.h
> index 6a91a1139686..65df44b1fcba 100644
> --- a/diff.h
> +++ b/diff.h
> @@ -69,7 +69,7 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
>  #define DIFF_OPT_FIND_COPIES_HARDER  (1 <<  6)
>  #define DIFF_OPT_FOLLOW_RENAMES      (1 <<  7)
>  #define DIFF_OPT_RENAME_EMPTY        (1 <<  8)
> -/* (1 <<  9) unused */
> +#define DIFF_OPT_SUBMODULE_DIFF      (1 <<  9)

So I'd really prefer not to see this change; instead, we should move
in the direction where we _REMOVE_ DIFF_OPT_SUBMODULE_LOG from these,
and introduce an enum to hold "how would we show submodule changes"
in the diff_options structure.