On Tue, Jan 12 2021, Patrick Steinhardt wrote:

A minor doc bug that wasn't spotted before landing. Here we say
"--config-env foo=bar" will work:

But not here, we ask for "--config-env=" (note the "="):

+--config-env=<name>=<envvar>::
+	Like `-c <name>=<value>`, give configuration variable
+	'<name>' a value, where <envvar> is the name of an
+	environment variable from which to retrieve the value. Unlike
+	`-c` there is no shortcut for directly setting the value to an
+	empty string, instead the environment variable itself must be
+	set to the empty string.  It is an error if the `<envvar>` does not exist
+	in the environment. `<envvar>` may not contain an equals sign
+	to avoid ambiguity with `<name>`s which contain one.
++
[...]
+		} else if (skip_prefix(cmd, "--config-env=", &cmd)) {
+			git_config_push_env(cmd);

But as this...

+test_expect_success 'git --config-env=key=envvar support' '
+	cat >expect <<-\EOF &&
+	value
+	value
+	false
+	EOF
+	{
+		ENVVAR=value git --config-env=core.name=ENVVAR config core.name &&
+		ENVVAR=value git --config-env=foo.CamelCase=ENVVAR config foo.camelcase &&
+		ENVVAR= git --config-env=foo.flag=ENVVAR config --bool foo.flag
+	} >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git --config-env fails with invalid parameters' '
+	test_must_fail git --config-env=foo.flag config --bool foo.flag 2>error &&
+	test_i18ngrep "invalid config format: foo.flag" error &&
+	test_must_fail git --config-env=foo.flag= config --bool foo.flag 2>error &&
+	test_i18ngrep "missing environment variable name for configuration ${SQ}foo.flag${SQ}" error &&
+	sane_unset NONEXISTENT &&
+	test_must_fail git --config-env=foo.flag=NONEXISTENT config --bool foo.flag 2>error &&
+	test_i18ngrep "missing environment variable ${SQ}NONEXISTENT${SQ} for configuration ${SQ}foo.flag${SQ}" error
+'
+
+test_expect_success 'git -c and --config-env work together' '
+	cat >expect <<-\EOF &&
+	bar.cmd cmd-value
+	bar.env env-value
+	EOF
+	ENVVAR=env-value git \
+		-c bar.cmd=cmd-value \
+		--config-env=bar.env=ENVVAR \
+		config --get-regexp "^bar.*" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git -c and --config-env override each other' '
+	cat >expect <<-\EOF &&
+	env
+	cmd
+	EOF
+	{
+		ENVVAR=env git -c bar.bar=cmd --config-env=bar.bar=ENVVAR config bar.bar &&
+		ENVVAR=env git --config-env=bar.bar=ENVVAR -c bar.bar=cmd config bar.bar
+	} >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git config --edit works' '
 	git config -f tmp test.value no &&
 	echo test.value=yes >expect &&

...and the tests show we just support the --opt=foo=bar form, not --opt
foo=bar.

Bonus points to anyone sorting out some of the existing inconsistencies
when fixing this, i.e. --exec-path supports either the "=" form, or not,
but various other skip_prefix() in the same function don't, seemingly
(but I have not tested) for no good reason.

It seems to me that having a skip_prefix_opt() or something would be a
good fix for this, i.e. a "maybe trim the last '='" version of
skip_prefix. Then we could just consistently use that.

Or maybe there's some reason we don't want to be as lax as --exec-path
with any other option...