switch: mention the --detach option when dying due to lack of a branch

Users who are accustomed to doing `git checkout <tag>` assume that
`git switch <tag>` will do the same thing. Inform them of the --detach
option so they aren't left wondering why `git switch` doesn't work but
`git checkout` does.

Signed-off-by: Alex Henrie <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: alexhenrie

Documentation/config/advice.txt

@@ -85,6 +85,9 @@ advice.*::
 		linkgit:git-switch[1] or linkgit:git-checkout[1]
 		to move to the detach HEAD state, to instruct how to
 		create a local branch after the fact.
+	suggestDetachingHead::
+		Advice shown when linkgit:git-switch[1] refuses to detach HEAD
+		without the explicit `--detach` option.
 	checkoutAmbiguousRemoteBranchName::
 		Advice shown when the argument to
 		linkgit:git-checkout[1] and linkgit:git-switch[1]

advice.c

@@ -42,6 +42,7 @@ static struct {
 	[ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME] 	= { "checkoutAmbiguousRemoteBranchName", 1 },
 	[ADVICE_COMMIT_BEFORE_MERGE]			= { "commitBeforeMerge", 1 },
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead", 1 },
+	[ADVICE_SUGGEST_DETACHING_HEAD]			= { "suggestDetachingHead", 1 },
 	[ADVICE_FETCH_SHOW_FORCED_UPDATES]		= { "fetchShowForcedUpdates", 1 },
 	[ADVICE_GRAFT_FILE_DEPRECATED]			= { "graftFileDeprecated", 1 },
 	[ADVICE_IGNORED_HOOK]				= { "ignoredHook", 1 },

advice.h

@@ -20,6 +20,7 @@ struct string_list;
 	ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME,
 	ADVICE_COMMIT_BEFORE_MERGE,
 	ADVICE_DETACHED_HEAD,
+	ADVICE_SUGGEST_DETACHING_HEAD,
 	ADVICE_FETCH_SHOW_FORCED_UPDATES,
 	ADVICE_GRAFT_FILE_DEPRECATED,
 	ADVICE_IGNORED_HOOK,

builtin/checkout.c

@@ -1397,23 +1397,31 @@ static void die_expecting_a_branch(const struct branch_info *branch_info)
 {
 	struct object_id oid;
 	char *to_free;
+	int code;
 
 	if (dwim_ref(branch_info->name, strlen(branch_info->name), &oid, &to_free, 0) == 1) {
 		const char *ref = to_free;
 
 		if (skip_prefix(ref, "refs/tags/", &ref))
-			die(_("a branch is expected, got tag '%s'"), ref);
-		if (skip_prefix(ref, "refs/remotes/", &ref))
-			die(_("a branch is expected, got remote branch '%s'"), ref);
-		die(_("a branch is expected, got '%s'"), ref);
+			code = die_message(_("a branch is expected, got tag '%s'"), ref);
+		else if (skip_prefix(ref, "refs/remotes/", &ref))
+			code = die_message(_("a branch is expected, got remote branch '%s'"), ref);
+		else
+			code = die_message(_("a branch is expected, got '%s'"), ref);
 	}
-	if (branch_info->commit)
-		die(_("a branch is expected, got commit '%s'"), branch_info->name);
-	/*
-	 * This case should never happen because we already die() on
-	 * non-commit, but just in case.
-	 */
-	die(_("a branch is expected, got '%s'"), branch_info->name);
+	else if (branch_info->commit)
+		code = die_message(_("a branch is expected, got commit '%s'"), branch_info->name);
+	else
+		/*
+		 * This case should never happen because we already die() on
+		 * non-commit, but just in case.
+		 */
+		code = die_message(_("a branch is expected, got '%s'"), branch_info->name);
+
+	if (advice_enabled(ADVICE_SUGGEST_DETACHING_HEAD))
+		advise(_("If you want to detach HEAD at the commit, try again with the --detach option."));
+
+	exit(code);
 }
 
 static void die_if_some_operation_in_progress(void)

t/t2060-switch.sh

@@ -32,6 +32,17 @@ test_expect_success 'switch and detach' '
 	test_must_fail git symbolic-ref HEAD
 '
 
+test_expect_success 'suggestion to detach' '
+	test_must_fail git switch main^{commit} 2>stderr &&
+	grep "try again with the --detach option" stderr
+'
+
+test_expect_success 'suggestion to detach is suppressed with advice.suggestDetachingHead=false' '
+	test_config advice.suggestDetachingHead false &&
+	test_must_fail git switch main^{commit} 2>stderr &&
+	! grep "try again with the --detach option" stderr
+'
+
 test_expect_success 'switch and detach current branch' '
 	test_when_finished git switch main &&
 	git switch main &&