parseopt: add OPT_NOOP_NOARG

Add OPT_NOOP_NOARG, a helper macro to define deprecated options in a
standard way.  The help text is taken from the no-op option -r of
git revert.

The callback could be made to emit a (conditional?) warning later.  And
we could also add OPT_NOOP (requiring an argument) etc. as needed.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-parse-options.txt

@@ -204,6 +204,11 @@ There are some macros to easily define options:
 	"auto", set `int_var` to 1 if stdout is a tty or a pager,
 	0 otherwise.
 
+`OPT_NOOP_NOARG(short, long)`::
+	Introduce an option that has no effect and takes no arguments.
+	Use it to hide deprecated options that are still to be recognized
+	and ignored silently.
+
 
 The last element of the array must be `OPT_END()`.
 

parse-options-cb.c

@@ -123,3 +123,8 @@ int parse_opt_string_list(const struct option *opt, const char *arg, int unset)
 	string_list_append(v, xstrdup(arg));
 	return 0;
 }
+
+int parse_opt_noop_cb(const struct option *opt, const char *arg, int unset)
+{
+	return 0;
+}

parse-options.h

@@ -153,6 +153,11 @@ struct option {
 	{ OPTION_CALLBACK, (s), (l), (v), "when", (h), PARSE_OPT_OPTARG, \
 		parse_opt_color_flag_cb, (intptr_t)"always" }
 
+#define OPT_NOOP_NOARG(s, l) \
+	{ OPTION_CALLBACK, (s), (l), NULL, NULL, \
+	  "no-op (backward compatibility)", \
+	  PARSE_OPT_HIDDEN | PARSE_OPT_NOARG, parse_opt_noop_cb }
+
 /* Deprecated synonym */
 #define OPT_BOOLEAN OPT_COUNTUP
 
@@ -216,6 +221,7 @@ extern int parse_opt_verbosity_cb(const struct option *, const char *, int);
 extern int parse_opt_with_commit(const struct option *, const char *, int);
 extern int parse_opt_tertiary(const struct option *, const char *, int);
 extern int parse_opt_string_list(const struct option *, const char *, int);
+extern int parse_opt_noop_cb(const struct option *, const char *, int);
 
 #define OPT__VERBOSE(var, h)  OPT_BOOLEAN('v', "verbose", (var), (h))
 #define OPT__QUIET(var, h)    OPT_BOOLEAN('q', "quiet",   (var), (h))

t/t0040-parse-options.sh

@@ -87,7 +87,7 @@ EOF
 test_expect_success 'long options' '
 	test-parse-options --boolean --integer 1729 --boolean --string2=321 \
 		--verbose --verbose --no-dry-run --abbrev=10 --file fi.le\
-		> output 2> output.err &&
+		--obsolete > output 2> output.err &&
 	test ! -s output.err &&
 	test_cmp expect output
 '

test-parse-options.c

@@ -54,6 +54,7 @@ int main(int argc, const char **argv)
 		OPT_STRING(0, "string2", &string, "str", "get another string"),
 		OPT_STRING(0, "st", &string, "st", "get another string (pervert ordering)"),
 		OPT_STRING('o', NULL, &string, "str", "get another string"),
+		OPT_NOOP_NOARG(0, "obsolete"),
 		OPT_SET_PTR(0, "default-string", &string,
 			"set string to default", (unsigned long)"default"),
 		OPT_STRING_LIST(0, "list", &list, "str", "add str to list"),