branch: advise about ref syntax rules

git-branch(1) will error out if you give it a bad ref name. But the user
might not understand why or what part of the name is illegal.

The user might know that there are some limitations based on the *loose
ref* format (filenames), but there are also further rules for
easier integration with shell-based tools, pathname expansion, and
playing well with reference name expressions.

The man page for git-check-ref-format(1) contains these rules. Let’s
advise about it since that is not a command that you just happen
upon. Also make this advise configurable since you might not want to be
reminded every time you make a little typo.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/config/advice.txt

@@ -94,6 +94,9 @@ advice.*::
 		`pushNonFFCurrent`, `pushNonFFMatching`, `pushAlreadyExists`,
 		`pushFetchFirst`, `pushNeedsForce`, and `pushRefNeedsUpdate`
 		simultaneously.
+	refSyntax::
+		Shown when the user provides an illegal ref name, to
+		tell the user about the ref syntax documentation.
 	resetNoRefresh::
 		Shown when linkgit:git-reset[1] takes more than 2
 		seconds to refresh the index after reset, to tell the user

advice.c

@@ -68,6 +68,7 @@ static struct {
 	[ADVICE_PUSH_UNQUALIFIED_REF_NAME]		= { "pushUnqualifiedRefName" },
 	[ADVICE_PUSH_UPDATE_REJECTED]			= { "pushUpdateRejected" },
 	[ADVICE_PUSH_UPDATE_REJECTED_ALIAS]		= { "pushNonFastForward" }, /* backwards compatibility */
+	[ADVICE_REF_SYNTAX]				= { "refSyntax" },
 	[ADVICE_RESET_NO_REFRESH_WARNING]		= { "resetNoRefresh" },
 	[ADVICE_RESOLVE_CONFLICT]			= { "resolveConflict" },
 	[ADVICE_RM_HINTS]				= { "rmHints" },

advice.h

@@ -36,6 +36,7 @@ enum advice_type {
 	ADVICE_PUSH_UNQUALIFIED_REF_NAME,
 	ADVICE_PUSH_UPDATE_REJECTED,
 	ADVICE_PUSH_UPDATE_REJECTED_ALIAS,
+	ADVICE_REF_SYNTAX,
 	ADVICE_RESET_NO_REFRESH_WARNING,
 	ADVICE_RESOLVE_CONFLICT,
 	ADVICE_RM_HINTS,

branch.c

@@ -370,8 +370,12 @@ int read_branch_desc(struct strbuf *buf, const char *branch_name)
  */
 int validate_branchname(const char *name, struct strbuf *ref)
 {
-	if (strbuf_check_branch_ref(ref, name))
-		die(_("'%s' is not a valid branch name"), name);
+	if (strbuf_check_branch_ref(ref, name)) {
+		int code = die_message(_("'%s' is not a valid branch name"), name);
+		advise_if_enabled(ADVICE_REF_SYNTAX,
+				  _("See `man git check-ref-format`"));
+		exit(code);
+	}
 
 	return ref_exists(ref->buf);
 }

builtin/branch.c

@@ -576,8 +576,12 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 		 */
 		if (ref_exists(oldref.buf))
 			recovery = 1;
-		else
-			die(_("invalid branch name: '%s'"), oldname);
+		else {
+			int code = die_message(_("invalid branch name: '%s'"), oldname);
+			advise_if_enabled(ADVICE_REF_SYNTAX,
+					  _("See `man git check-ref-format`"));
+			exit(code);
+		}
 	}
 
 	for (int i = 0; worktrees[i]; i++) {

t/t3200-branch.sh

@@ -1722,4 +1722,14 @@ test_expect_success '--track overrides branch.autoSetupMerge' '
 	test_cmp_config "" --default "" branch.foo5.merge
 '
 
+test_expect_success 'errors if given a bad branch name' '
+	cat <<-\EOF >expect &&
+	fatal: '\''foo..bar'\'' is not a valid branch name
+	hint: See `man git check-ref-format`
+	hint: Disable this message with "git config advice.refSyntax false"
+	EOF
+	test_must_fail git branch foo..bar >actual 2>&1 &&
+	test_cmp expect actual
+'
+
 test_done