[PATCH v4 3/3] worktree add: Add hint to use --orphan when bad ref

[Jacob Abel <jacobabel@nullpo.dev>]

Adds a new advice/hint in `git worktree add` for when the user
tries to create a new worktree from a reference that doesn't exist.

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
---
 Documentation/config/advice.txt |  4 ++++
 advice.c                        |  1 +
 advice.h                        |  1 +
 builtin/worktree.c              |  8 ++++++++
 t/t2400-worktree-add.sh         | 16 ++++++++++++++++
 5 files changed, 30 insertions(+)

diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index a00d0100a8..3e58521613 100644
--- a/Documentation/config/advice.txt
+++ b/Documentation/config/advice.txt
@@ -136,4 +136,8 @@ advice.*::
 		Advice shown when either linkgit:git-add[1] or linkgit:git-rm[1]
 		is asked to update index entries outside the current sparse
 		checkout.
+	worktreeAddOrphan::
+		Advice shown when a user tries to create a worktree from an
+		invalid reference, to instruct how to create a new orphan
+		branch instead.
 --
diff --git a/advice.c b/advice.c
index fd18968943..53e91fdb85 100644
--- a/advice.c
+++ b/advice.c
@@ -75,6 +75,7 @@ static struct {
 	[ADVICE_SUBMODULES_NOT_UPDATED] 		= { "submodulesNotUpdated", 1 },
 	[ADVICE_UPDATE_SPARSE_PATH]			= { "updateSparsePath", 1 },
 	[ADVICE_WAITING_FOR_EDITOR]			= { "waitingForEditor", 1 },
+	[ADVICE_WORKTREE_ADD_ORPHAN]			= { "worktreeAddOrphan", 1 },
 };

 static const char turn_off_instructions[] =
diff --git a/advice.h b/advice.h
index 07e0f76833..919d8c0064 100644
--- a/advice.h
+++ b/advice.h
@@ -50,6 +50,7 @@ struct string_list;
 	ADVICE_UPDATE_SPARSE_PATH,
 	ADVICE_WAITING_FOR_EDITOR,
 	ADVICE_SKIPPED_CHERRY_PICKS,
+	ADVICE_WORKTREE_ADD_ORPHAN,
 };

 int git_default_advice_config(const char *var, const char *value);
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 51b247b2a7..ea306e18de 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -744,6 +744,14 @@ static int add(int ac, const char **av, const char *prefix)
 		 * If `branch` does not reference a valid commit, a new
 		 * worktree (and/or branch) cannot be created based off of it.
 		 */
+		advise_if_enabled(ADVICE_WORKTREE_ADD_ORPHAN,
+			"If you meant to create a worktree containing a new orphan branch\n"
+			"(branch with no commits) for this repository, e.g. '%s',\n"
+			"you can do so using the --orphan option:\n"
+			"\n"
+			"	git worktree add --orphan %s %s\n"
+			"\n",
+			 new_branch, new_branch, path);
 		die(_("invalid reference: %s"), branch);
 	} else if (new_branch) {
 		struct child_process cp = CHILD_PROCESS_INIT;
diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index 6118ace92d..455cddcdd2 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -376,6 +376,22 @@ test_expect_success '"add" worktree with orphan branch, lock, and reason' '
 	test_cmp expect .git/worktrees/orphan-with-lock-reason/locked
 '

+# Helper function to test hints for using --orphan in an empty repo.
+test_wt_add_empty_repo_orphan_hint() {
+	local context="$1" &&
+	local opts="${@:2}" &&
+	test_expect_success "'worktree add' show orphan hint in empty repo w/ $context" '
+		test_when_finished "rm -rf empty_repo" &&
+		GIT_DIR="empty_repo" git init --bare &&
+		test_must_fail git -C empty_repo worktree add $opts 2> actual &&
+		grep "hint: If you meant to create a worktree containing a new orphan branch" actual
+	'
+}
+
+test_wt_add_empty_repo_orphan_hint 'DWIM' foobar/
+test_wt_add_empty_repo_orphan_hint '-b' -b foobar_branch foobar/
+test_wt_add_empty_repo_orphan_hint '-B' -B foobar_branch foobar/
+
 test_expect_success 'local clone from linked checkout' '
 	git clone --local here here-clone &&
 	( cd here-clone && git fsck )
--
2.37.4