cli: branch: make "set" do upsert as before

CHANGELOG.md

@@ -21,6 +21,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Dropped support for deprecated `jj branch delete`/`forget` `--glob` option.
 
+* `jj branch set` now creates new branch if it doesn't exist. Use `jj branch
+  move` to ensure that the target branch already exists.
+  [#3584](https://github.com/martinvonz/jj/issues/3584)
+
 ### Deprecations
 
 * Replacing `-l` shorthand for `--limit` with `-n` in `jj log`, `jj op log` and `jj obslog`.

cli/src/commands/branch/set.rs

@@ -21,7 +21,7 @@ use crate::cli_util::{CommandHelper, RevisionArg};
 use crate::command_error::{user_error_with_hint, CommandError};
 use crate::ui::Ui;
 
-/// Update an existing branch to point to a certain commit
+/// Create or update a branch to point to a certain commit
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchSetArgs {
     /// The branch's target revision
@@ -47,13 +47,11 @@ pub fn cmd_branch_set(
         workspace_command.resolve_single_rev(args.revision.as_ref().unwrap_or(&RevisionArg::AT))?;
     let repo = workspace_command.repo().as_ref();
     let branch_names = &args.names;
+    let mut new_branch_names: Vec<&str> = Vec::new();
     for name in branch_names {
         let old_target = repo.view().get_local_branch(name);
         if old_target.is_absent() {
-            return Err(user_error_with_hint(
-                format!("No such branch: {name}"),
-                "Use `jj branch create` to create it.",
-            ));
+            new_branch_names.push(name);
         }
         if !args.allow_backwards && !is_fast_forward(repo, old_target, target_commit.id()) {
             return Err(user_error_with_hint(
@@ -84,5 +82,18 @@ pub fn cmd_branch_set(
             target_commit.id().hex()
         ),
     )?;
+
+    if !new_branch_names.is_empty() {
+        writeln!(
+            ui.status(),
+            "Created branches: {}",
+            new_branch_names.join(", "),
+        )?;
+        // TODO: delete this hint in jj 0.25+
+        writeln!(
+            ui.hint_default(),
+            "Consider using `jj branch move` if your intention was to move existing branches."
+        )?;
+    }
     Ok(())
 }

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

@@ -239,7 +239,7 @@ For information about branches, see https://github.com/martinvonz/jj/blob/main/d
 * `list` — List branches and their targets
 * `move` — Move existing branches to target revision
 * `rename` — Rename `old` branch name to `new` branch name
-* `set` — Update an existing branch to point to a certain commit
+* `set` — Create or update a branch to point to a certain commit
 * `track` — Start tracking given remote branches
 * `untrack` — Stop tracking given remote branches
 
@@ -370,7 +370,7 @@ The new branch name points at the same commit as the old branch name.
 
 ## `jj branch set`
 
-Update an existing branch to point to a certain commit
+Create or update a branch to point to a certain commit
 
 **Usage:** `jj branch set [OPTIONS] <NAMES>...`
 

cli/tests/test_branch_command.rs

@@ -103,18 +103,16 @@ fn test_branch_move() {
     test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
     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` to create it.
-    "###);
     let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "move", "foo"]);
     insta::assert_snapshot!(stderr, @r###"
     Error: No such branch: foo
     "###);
 
-    let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "create", "foo"]);
-    insta::assert_snapshot!(stderr, @"");
+    let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "set", "foo"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Created branches: foo
+    Hint: Consider using `jj branch move` if your intention was to move existing branches.
+    "###);
 
     test_env.jj_cmd_ok(&repo_path, &["new"]);
     let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "create", "foo"]);