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

A user reaching for "git branch --set-upstream-to origin main" expects
the second token to name the remote branch to track. The option takes a
single <remote>/<branch> argument, so the trailing word is read as the
local branch to operate on and the command dies with "branch 'main'
does not exist", which points at the wrong thing.

When the local branch does not exist, the upstream argument is a bare
remote name, and that remote has a matching remote-tracking ref,
suggest the intended "--set-upstream-to=<remote>/<branch>" form.

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

Author: HaraldNordgren

builtin/branch.c

@@ -957,6 +957,19 @@ 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))
+					die(_("branch '%s' does not exist.\n"
+					      "Did you mean to use: --set-upstream-to=%s/%s?"),
+					    branch->name, new_upstream, argv[0]);
+				strbuf_release(&remote_ref);
+			}
 			die(_("branch '%s' does not exist"), branch->name);
 		}
 

t/t3200-branch.sh

@@ -1022,6 +1022,27 @@ 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 "Did you mean to use: --set-upstream-to=upstream-remote/slip-feature?" 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 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