From fd2f38c394359af9ee7a8fcb5f2ffb18c6bbb50c Mon Sep 17 00:00:00 2001
From: Essien Ita Essien <34972+essiene@users.noreply.github.com>
Date: Wed, 18 Oct 2023 21:00:10 +0100
Subject: [PATCH] Implement a `rename` subcommand for the `branch` command.

This is really a simple change that does the following in a transaction:
* Set the new branch name to point to the same commit as the old branch name.
* Set the old branch name to point to no commit (hence deleting the old name).

Before it starts, it confirms that the new branch name is not already in use.
---
 CHANGELOG.md               |  2 ++
 cli/src/commands/branch.rs | 51 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6b48d6c3787..04c5bf7af16 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### New features
 
 * Information about new and resolved conflicts is now printed by every command.
+* `jj branch` has gained a new `rename` subcommand that allows changing a branch
+  name atomically. `jj branch help rename` for details.
 
 ### Fixed bugs
 
diff --git a/cli/src/commands/branch.rs b/cli/src/commands/branch.rs
index 35a5a914482..9469ec1a9b1 100644
--- a/cli/src/commands/branch.rs
+++ b/cli/src/commands/branch.rs
@@ -34,6 +34,8 @@ pub enum BranchSubcommand {
     Forget(BranchForgetArgs),
     #[command(visible_alias("l"))]
     List(BranchListArgs),
+    #[command(visible_alias("r"))]
+    Rename(BranchRenameArgs),
     #[command(visible_alias("s"))]
     Set(BranchSetArgs),
     Track(BranchTrackArgs),
@@ -122,6 +124,19 @@ pub struct BranchForgetArgs {
     pub glob: Vec<StringPattern>,
 }
 
+/// Rename `old` branch name to `new` branch name.
+///
+/// The new branch name points at the same commit as the old
+/// branch name.
+#[derive(clap::Args, Clone, Debug)]
+pub struct BranchRenameArgs {
+    /// The old name of the branch.
+    pub old: String,
+
+    /// The new name of the branch.
+    pub new: String,
+}
+
 /// Update an existing branch to point to a certain commit.
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchSetArgs {
@@ -248,6 +263,7 @@ pub fn cmd_branch(
 ) -> Result<(), CommandError> {
     match subcommand {
         BranchSubcommand::Create(sub_args) => cmd_branch_create(ui, command, sub_args),
+        BranchSubcommand::Rename(sub_args) => cmd_branch_rename(ui, command, sub_args),
         BranchSubcommand::Set(sub_args) => cmd_branch_set(ui, command, sub_args),
         BranchSubcommand::Delete(sub_args) => cmd_branch_delete(ui, command, sub_args),
         BranchSubcommand::Forget(sub_args) => cmd_branch_forget(ui, command, sub_args),
@@ -301,6 +317,41 @@ fn cmd_branch_create(
     Ok(())
 }
 
+fn cmd_branch_rename(
+    ui: &mut Ui,
+    command: &CommandHelper,
+    args: &BranchRenameArgs,
+) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let repo = workspace_command.repo().as_ref();
+    let old_branch = &args.old;
+    // call clone because .set_local_branch_target takes ownership
+    // of the target_commit, so we need to create a copy that is no
+    // longer owned by the workspace.
+    let target_commit = repo.view().get_local_branch(old_branch).clone();
+    if target_commit.is_absent() {
+        return Err(user_error_with_hint(format!(
+            "No such branch: {old_branch}"
+        )));
+    }
+
+    let new_branch = &args.new;
+    let mut tx = workspace_command.start_transaction();
+    tx.mut_repo()
+        .set_local_branch_target(new_branch, target_commit);
+    tx.mut_repo()
+        .set_local_branch_target(old_branch, RefTarget::absent());
+    tx.finish(
+        ui,
+        format!(
+            "rename {} to {}",
+            make_branch_term(&[old_branch]),
+            make_branch_term(&[new_branch]),
+        ),
+    )?;
+    Ok(())
+}
+
 fn cmd_branch_set(
     ui: &mut Ui,
     command: &CommandHelper,