branch: suggest <remote>/<branch> when set-upstream-to slips

"git branch --set-upstream-to origin main" reads the trailing word as
the local branch to operate on and dies with "branch 'main' does not
exist", pointing at the wrong problem.

When that branch is missing and "<remote>/<branch>" names a real
remote-tracking ref, suggest the intended
"git branch --set-upstream-to=<remote>/<branch>" form.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>

Author: HaraldNordgren

builtin/branch.c

@@ -957,6 +957,23 @@ int cmd_branch(int argc,
 		if (!refs_ref_exists(get_main_ref_store(the_repository), branch->refname)) {
 			if (!argc || branch_checked_out(branch->refname))
 				die(_("no commit on branch '%s' yet"), branch->name);
+			if (argc == 1 && !strchr(new_upstream, '/') &&
+			    remote_is_configured(remote_get(new_upstream), 0)) {
+				struct strbuf remote_ref = STRBUF_INIT;
+
+				strbuf_addf(&remote_ref, "refs/remotes/%s/%s",
+					    new_upstream, argv[0]);
+				if (refs_ref_exists(get_main_ref_store(the_repository),
+						    remote_ref.buf)) {
+					int code = die_message(_("--set-upstream-to takes a single <remote>/<branch> argument"));
+					advise_if_enabled(ADVICE_SET_UPSTREAM_FAILURE,
+							  _("Did you mean to use: git branch --set-upstream-to=%s/%s?"),
+							  new_upstream, argv[0]);
+					strbuf_release(&remote_ref);
+					exit(code);
+				}
+				strbuf_release(&remote_ref);
+			}
 			die(_("branch '%s' does not exist"), branch->name);
 		}
 

t/t3200-branch.sh

@@ -1022,6 +1022,53 @@ test_expect_success '--set-upstream-to fails on a missing dst branch' '
 	test_cmp expect err
 '
 
+test_expect_success '--set-upstream-to suggests <remote>/<branch> on slip' '
+	test_when_finished "git remote remove upstream-remote" &&
+	test_config_global protocol.file.allow always &&
+	git init --bare slip-remote.git &&
+	git remote add upstream-remote slip-remote.git &&
+	git push upstream-remote main:slip-feature &&
+	git fetch upstream-remote &&
+	test_must_fail git rev-parse --verify refs/heads/slip-feature &&
+	test_must_fail git branch --set-upstream-to upstream-remote slip-feature 2>err &&
+	test_grep "takes a single <remote>/<branch> argument" err &&
+	test_grep "hint: Did you mean to use: git branch --set-upstream-to=upstream-remote/slip-feature?" err &&
+	test_must_fail git -c advice.setUpstreamFailure=false \
+		branch --set-upstream-to upstream-remote slip-feature 2>err &&
+	test_grep ! "Did you mean" err
+'
+
+test_expect_success '--set-upstream-to does not suggest when no matching remote ref' '
+	test_when_finished "git remote remove upstream-remote" &&
+	git init --bare slip-remote2.git &&
+	git remote add upstream-remote slip-remote2.git &&
+	test_must_fail git branch --set-upstream-to upstream-remote no-such-branch 2>err &&
+	test_grep "branch ${SQ}no-such-branch${SQ} does not exist" err &&
+	test_grep ! "Did you mean" err
+'
+
+test_expect_success '--set-upstream-to to a local branch is not mistaken for a slip' '
+	git branch slip-local-upstream &&
+	git branch slip-local-target &&
+	git branch --set-upstream-to=slip-local-upstream slip-local-target 2>err &&
+	test_grep ! "Did you mean" err &&
+	echo refs/heads/slip-local-upstream >expect &&
+	git config branch.slip-local-target.merge >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '--set-upstream-to slip suggestion keeps a slashed branch name' '
+	test_when_finished "git remote remove upstream-remote" &&
+	test_config_global protocol.file.allow always &&
+	git init --bare slip-remote3.git &&
+	git remote add upstream-remote slip-remote3.git &&
+	git push upstream-remote main:slip/feature &&
+	git fetch upstream-remote &&
+	test_must_fail git rev-parse --verify refs/heads/slip/feature &&
+	test_must_fail git branch --set-upstream-to upstream-remote slip/feature 2>err &&
+	test_grep "hint: Did you mean to use: git branch --set-upstream-to=upstream-remote/slip/feature?" err
+'
+
 test_expect_success '--set-upstream-to fails on a missing src branch' '
 	test_must_fail git branch --set-upstream-to does-not-exist main 2>err &&
 	test_grep "the requested upstream branch '"'"'does-not-exist'"'"' does not exist" err