cli: add --create to branch set to upsert branches

CHANGELOG.md

@@ -62,6 +62,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj rebase` now accepts revsets resolving to multiple revisions with the
    `--revisions`/`-r` option.
 
+* `jj branch set` now accepts `-c`/`--create` option to create the branch if it does not exist.
+
 ### Fixed bugs
 
 * Revsets now support `\`-escapes in string literal.

cli/src/commands/branch.rs

@@ -169,6 +169,10 @@ pub struct BranchSetArgs {
     #[arg(long, short = 'B')]
     pub allow_backwards: bool,
 
+    /// Allow creating the branch if it does not exist.
+    #[arg(long, short = 'c')]
+    pub create: bool,
+
     /// The branches to update.
     #[arg(required = true)]
     pub names: Vec<String>,
@@ -353,13 +357,13 @@ fn cmd_branch_set(
     let branch_names = &args.names;
     for name in branch_names {
         let old_target = repo.view().get_local_branch(name);
-        if old_target.is_absent() {
+        if !args.create && old_target.is_absent() {
             return Err(user_error_with_hint(
                 format!("No such branch: {name}"),
-                "Use `jj branch create` to create it.",
+                "Use `jj branch create` or `jj branch set --create` to create it.",
             ));
         }
-        if !args.allow_backwards && !is_fast_forward(old_target) {
+        if !args.allow_backwards && !old_target.is_absent() && !is_fast_forward(old_target) {
             return Err(user_error_with_hint(
                 format!("Refusing to move branch backwards or sideways: {name}"),
                 "Use --allow-backwards to allow it.",

cli/tests/cli-reference@.md.snap

@@ -355,6 +355,10 @@ Update an existing branch to point to a certain commit
 
   Possible values: `true`, `false`
 
+* `-c`, `--create` — Allow creating the branch if it does not exist
+
+  Possible values: `true`, `false`
+
 
 
 

cli/tests/test_branch_command.rs

@@ -99,7 +99,7 @@ fn test_branch_move() {
     let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "set", "foo"]);
     insta::assert_snapshot!(stderr, @r###"
     Error: No such branch: foo
-    Hint: Use `jj branch create` to create it.
+    Hint: Use `jj branch create` or `jj branch set --create` to create it.
     "###);
 
     let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "create", "foo"]);
@@ -128,6 +128,22 @@ fn test_branch_move() {
     insta::assert_snapshot!(stderr, @"");
 }
 
+#[test]
+fn test_branch_set_create() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "set", "foo"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: No such branch: foo
+    Hint: Use `jj branch create` or `jj branch set --create` to create it.
+    "###);
+
+    let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "set", "foo", "--create"]);
+    insta::assert_snapshot!(stderr, @"");
+}
+
 #[test]
 fn test_branch_move_conflicting() {
     let test_env = TestEnvironment::default();