worktree add: introduce "try --orphan" hint

Add 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.

Current Behavior:

% git init foo
Initialized empty Git repository in /path/to/foo/
% touch file
% git -C foo commit -q -a -m "test commit"
% git -C foo switch --orphan norefbranch
% git -C foo worktree add newbranch/
Preparing worktree (new branch 'newbranch')
fatal: invalid reference: HEAD
%

New Behavior:

% git init --bare foo
Initialized empty Git repository in /path/to/foo/
% touch file
% git -C foo commit -q -a -m "test commit"
% git -C foo switch --orphan norefbranch
% git -C foo worktree add newbranch/
Preparing worktree (new branch 'newbranch')
hint: If you meant to create a worktree containing a new orphan branch
hint: (branch with no commits) for this repository, you can do so
hint: using the --orphan option:
hint:
hint:   git worktree add --orphan newbranch/
hint:
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
% git -C foo worktree add -b newbranch2 new_wt/
Preparing worktree (new branch 'newbranch')
hint: If you meant to create a worktree containing a new orphan branch
hint: (branch with no commits) for this repository, you can do so
hint: using the --orphan option:
hint:
hint:   git worktree add --orphan -b newbranch2 new_wt/
hint:
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
%

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/advice.txt

@@ -138,4 +138,8 @@ advice.*::
 		checkout.
 	diverging::
 		Advice shown when a fast-forward is not possible.
+	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.
 --

advice.c

@@ -78,6 +78,7 @@ 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[] =

advice.h

@@ -49,6 +49,7 @@ 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);

builtin/worktree.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "abspath.h"
+#include "advice.h"
 #include "checkout.h"
 #include "config.h"
 #include "builtin.h"
@@ -39,6 +40,20 @@
 #define BUILTIN_WORKTREE_UNLOCK_USAGE \
 	N_("git worktree unlock <worktree>")
 
+#define WORKTREE_ADD_ORPHAN_WITH_DASH_B_HINT_TEXT \
+	_("If you meant to create a worktree containing a new orphan branch\n" \
+	"(branch with no commits) for this repository, you can do so\n" \
+	"using the --orphan flag:\n" \
+	"\n" \
+	"	git worktree add --orphan -b %s %s\n")
+
+#define WORKTREE_ADD_ORPHAN_NO_DASH_B_HINT_TEXT \
+	_("If you meant to create a worktree containing a new orphan branch\n" \
+	"(branch with no commits) for this repository, you can do so\n" \
+	"using the --orphan flag:\n" \
+	"\n" \
+	"	git worktree add --orphan %s\n")
+
 static const char * const git_worktree_usage[] = {
 	BUILTIN_WORKTREE_ADD_USAGE,
 	BUILTIN_WORKTREE_LIST_USAGE,
@@ -634,6 +649,7 @@ static int add(int ac, const char **av, const char *prefix)
 	const char *opt_track = NULL;
 	const char *lock_reason = NULL;
 	int keep_locked = 0;
+	int used_new_branch_options;
 	struct option options[] = {
 		OPT__FORCE(&opts.force,
 			   N_("checkout <branch> even if already checked out in other worktree"),
@@ -686,6 +702,7 @@ static int add(int ac, const char **av, const char *prefix)
 
 	path = prefix_filename(prefix, av[0]);
 	branch = ac < 2 ? "HEAD" : av[1];
+	used_new_branch_options = new_branch || new_branch_force;
 
 	if (!strcmp(branch, "-"))
 		branch = "@{-1}";
@@ -728,6 +745,15 @@ static int add(int ac, const char **av, const char *prefix)
 	}
 
 	if (!opts.orphan && !lookup_commit_reference_by_name(branch)) {
+		int attempt_hint = !opts.quiet && (ac < 2);
+		if (attempt_hint && used_new_branch_options) {
+			advise_if_enabled(ADVICE_WORKTREE_ADD_ORPHAN,
+				WORKTREE_ADD_ORPHAN_WITH_DASH_B_HINT_TEXT,
+				new_branch, path);
+		} else if (attempt_hint) {
+			advise_if_enabled(ADVICE_WORKTREE_ADD_ORPHAN,
+				WORKTREE_ADD_ORPHAN_NO_DASH_B_HINT_TEXT, path);
+		}
 		die(_("invalid reference: %s"), branch);
 	}
 

t/t2400-worktree-add.sh

@@ -401,6 +401,43 @@ test_expect_success '"add" worktree with orphan branch, lock, and reason' '
 	test_cmp expect .git/worktrees/orphan-with-lock-reason/locked
 '
 
+# Note: Quoted arguments containing spaces are not supported.
+test_wt_add_orphan_hint () {
+	local context="$1" &&
+	local use_branch=$2 &&
+	shift 2 &&
+	local opts="$*" &&
+	test_expect_success "'worktree add' show orphan hint in bad/orphan HEAD w/ $context" '
+		test_when_finished "rm -rf repo" &&
+		git init repo &&
+		(cd repo && test_commit commit) &&
+		git -C repo switch --orphan noref &&
+		test_must_fail git -C repo worktree add $opts foobar/ 2>actual &&
+		! grep "error: unknown switch" actual &&
+		grep "hint: If you meant to create a worktree containing a new orphan branch" actual &&
+		if [ $use_branch -eq 1 ]
+		then
+			grep -E "^hint:\s+git worktree add --orphan -b \S+ \S+\s*$" actual
+		else
+			grep -E "^hint:\s+git worktree add --orphan \S+\s*$" actual
+		fi
+
+	'
+}
+
+test_wt_add_orphan_hint 'no opts' 0
+test_wt_add_orphan_hint '-b' 1 -b foobar_branch
+test_wt_add_orphan_hint '-B' 1 -B foobar_branch
+
+test_expect_success "'worktree add' doesn't show orphan hint in bad/orphan HEAD w/ --quiet" '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(cd repo && test_commit commit) &&
+	test_must_fail git -C repo worktree add --quiet foobar_branch foobar/ 2>actual &&
+	! grep "error: unknown switch" actual &&
+	! grep "hint: If you meant to create a worktree containing a new orphan branch" actual
+'
+
 test_expect_success 'local clone from linked checkout' '
 	git clone --local here here-clone &&
 	( cd here-clone && git fsck )