cli: do not attempt to merge op heads if --at-op=@ is specified

The idea is that --at-op specifies a certain operation, so --at-op=@ can be
interpreted as the option to select _the_ known head operation. This helps
eliminate special cases from "op log" which doesn't snapshot nor merge
concurrent ops.

CHANGELOG.md

@@ -30,6 +30,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   preserved. If a checked-out named branch gets deleted locally or remotely, the
   corresponding commits will be abandoned.
 
+* `jj --at-op=@` no longer merges concurrent operations if explicitly specified.
+
 ### Deprecations
 
 * The original configuration syntax for `jj fix` is now deprecated in favor of

cli/src/cli_util.rs

@@ -300,8 +300,7 @@ impl CommandHelper {
         let workspace = self.load_workspace()?;
         let op_head = self.resolve_operation(ui, workspace.repo_loader())?;
         let repo = workspace.repo_loader().load_at(&op_head)?;
-        let loaded_at_head = self.global_args.at_operation == "@";
-        WorkspaceCommandHelper::new(ui, self, workspace, repo, loaded_at_head)
+        WorkspaceCommandHelper::new(ui, self, workspace, repo, self.is_at_head_operation())
     }
 
     pub fn get_working_copy_factory(&self) -> Result<&dyn WorkingCopyFactory, CommandError> {
@@ -328,13 +327,25 @@ impl CommandHelper {
             .map_err(|err| map_workspace_load_error(err, self.global_args.repository.as_deref()))
     }
 
+    /// Returns true if the current operation is considered to be the head.
+    pub fn is_at_head_operation(&self) -> bool {
+        // TODO: should we accept --at-op=<head_id> as the head op?
+        matches!(self.global_args.at_operation.as_deref(), None | Some("@"))
+    }
+
+    /// Resolves the current operation from the command-line argument.
+    ///
+    /// If no `--at-operation` is specified, the head operations will be
+    /// loaded. If there are multiple heads, they'll be merged.
     #[instrument(skip_all)]
     pub fn resolve_operation(
         &self,
         ui: &mut Ui,
         repo_loader: &RepoLoader,
     ) -> Result<Operation, CommandError> {
-        if self.global_args.at_operation == "@" {
+        if let Some(op_str) = &self.global_args.at_operation {
+            Ok(op_walk::resolve_op_for_load(repo_loader, op_str)?)
+        } else {
             op_heads_store::resolve_op_heads(
                 repo_loader.op_heads_store().as_ref(),
                 repo_loader.op_store(),
@@ -365,10 +376,6 @@ impl CommandHelper {
                         .clone())
                 },
             )
-        } else {
-            let operation =
-                op_walk::resolve_op_for_load(repo_loader, &self.global_args.at_operation)?;
-            Ok(operation)
         }
     }
 
@@ -2420,10 +2427,12 @@ pub struct GlobalArgs {
     /// Operation to load the repo at
     ///
     /// Operation to load the repo at. By default, Jujutsu loads the repo at the
-    /// most recent operation. You can use `--at-op=<operation ID>` to see what
-    /// the repo looked like at an earlier operation. For example `jj
-    /// --at-op=<operation ID> st` will show you what `jj st` would have
-    /// shown you when the given operation had just finished.
+    /// most recent operation. If there are concurrent operations, they will be
+    /// merged. You can use `--at-op=<operation ID>` to see what the repo looked
+    /// like at an earlier operation. For example `jj --at-op=<operation ID> st`
+    /// will show you what `jj st` would have shown you when the given operation
+    /// had just finished. `--at-op=@` is pretty much the same as the default
+    /// except that concurrent operations will never be merged.
     ///
     /// Use `jj op log` to find the operation ID you want. Any unambiguous
     /// prefix of the operation ID is enough.
@@ -2435,8 +2444,8 @@ pub struct GlobalArgs {
     /// earlier operation. Doing that is equivalent to having run concurrent
     /// commands starting at the earlier operation. There's rarely a reason to
     /// do that, but it is possible.
-    #[arg(long, visible_alias = "at-op", global = true, default_value = "@")]
-    pub at_operation: String,
+    #[arg(long, visible_alias = "at-op", global = true)]
+    pub at_operation: Option<String>,
     /// Enable debug logging
     #[arg(long, global = true)]
     pub debug: bool,

cli/src/commands/debug/index.rs

@@ -35,7 +35,10 @@ pub fn cmd_debug_index(
     // merge concurrent operations and update the index.
     let workspace = command.load_workspace()?;
     let repo_loader = workspace.repo_loader();
-    let op = op_walk::resolve_op_for_load(repo_loader, &command.global_args().at_operation)?;
+    let op = op_walk::resolve_op_for_load(
+        repo_loader,
+        command.global_args().at_operation.as_deref().unwrap_or("@"),
+    )?;
     let index_store = repo_loader.index_store();
     let index = index_store
         .get_index_at_op(&op, repo_loader.store())

cli/src/commands/debug/reindex.rs

@@ -35,7 +35,10 @@ pub fn cmd_debug_reindex(
     // be rebuilt while loading the repo.
     let workspace = command.load_workspace()?;
     let repo_loader = workspace.repo_loader();
-    let op = op_walk::resolve_op_for_load(repo_loader, &command.global_args().at_operation)?;
+    let op = op_walk::resolve_op_for_load(
+        repo_loader,
+        command.global_args().at_operation.as_deref().unwrap_or("@"),
+    )?;
     let index_store = repo_loader.index_store();
     if let Some(default_index_store) = index_store.as_any().downcast_ref::<DefaultIndexStore>() {
         default_index_store.reinit().map_err(internal_error)?;

cli/src/commands/git/clone.rs

@@ -85,7 +85,7 @@ pub fn cmd_git_clone(
     command: &CommandHelper,
     args: &GitCloneArgs,
 ) -> Result<(), CommandError> {
-    if command.global_args().at_operation != "@" {
+    if command.global_args().at_operation.is_some() {
         return Err(cli_error("--at-op is not respected"));
     }
     let remote_name = "origin";

cli/src/commands/git/init.rs

@@ -76,7 +76,7 @@ pub fn cmd_git_init(
     if command.global_args().ignore_working_copy {
         return Err(cli_error("--ignore-working-copy is not respected"));
     }
-    if command.global_args().at_operation != "@" {
+    if command.global_args().at_operation.is_some() {
         return Err(cli_error("--at-op is not respected"));
     }
     let cwd = command.cwd();

cli/src/commands/init.rs

@@ -55,7 +55,7 @@ pub(crate) fn cmd_init(
     if command.global_args().ignore_working_copy {
         return Err(cli_error("--ignore-working-copy is not respected"));
     }
-    if command.global_args().at_operation != "@" {
+    if command.global_args().at_operation.is_some() {
         return Err(cli_error("--at-op is not respected"));
     }
     let cwd = command.cwd();

cli/src/commands/operation/abandon.rs

@@ -53,11 +53,10 @@ pub fn cmd_op_abandon(
     let op_store = repo_loader.op_store();
     // It doesn't make sense to create concurrent operations that will be merged
     // with the current head.
-    let head_op_str = &command.global_args().at_operation;
-    if head_op_str != "@" {
+    if command.global_args().at_operation.is_some() {
         return Err(cli_error("--at-op is not respected"));
     }
-    let current_head_op = op_walk::resolve_op_for_load(repo_loader, head_op_str)?;
+    let current_head_op = op_walk::resolve_op_for_load(repo_loader, "@")?;
     let resolve_op = |op_str| op_walk::resolve_op_at(op_store, &current_head_op, op_str);
     let (abandon_root_op, abandon_head_op) =
         if let Some((root_op_str, head_op_str)) = args.operation.split_once("..") {

cli/src/commands/operation/log.rs

@@ -54,7 +54,7 @@ pub fn cmd_op_log(
     // operation id to be abandoned.
     let workspace = command.load_workspace()?;
     let repo_loader = workspace.repo_loader();
-    let head_op_str = &command.global_args().at_operation;
+    let head_op_str = command.global_args().at_operation.as_deref().unwrap_or("@");
     let head_ops = if head_op_str == "@" {
         // If multiple head ops can't be resolved without merging, let the
         // current op be empty. Beware that resolve_op_for_load() will eliminate

cli/src/commands/util.rs

@@ -173,7 +173,7 @@ fn cmd_util_gc(
     command: &CommandHelper,
     args: &UtilGcArgs,
 ) -> Result<(), CommandError> {
-    if command.global_args().at_operation != "@" {
+    if !command.is_at_head_operation() {
         return Err(user_error(
             "Cannot garbage collect from a non-head operation",
         ));

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

@@ -166,15 +166,13 @@ To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/d
    This option only affects the check. It does not affect the `immutable_heads()` revset or the `immutable` template keyword.
 * `--at-operation <AT_OPERATION>` — Operation to load the repo at
 
-   Operation to load the repo at. By default, Jujutsu loads the repo at the most recent operation. You can use `--at-op=<operation ID>` to see what the repo looked like at an earlier operation. For example `jj --at-op=<operation ID> st` will show you what `jj st` would have shown you when the given operation had just finished.
+   Operation to load the repo at. By default, Jujutsu loads the repo at the most recent operation. If there are concurrent operations, they will be merged. You can use `--at-op=<operation ID>` to see what the repo looked like at an earlier operation. For example `jj --at-op=<operation ID> st` will show you what `jj st` would have shown you when the given operation had just finished. `--at-op=@` is pretty much the same as the default except that concurrent operations will never be merged.
 
    Use `jj op log` to find the operation ID you want. Any unambiguous prefix of the operation ID is enough.
 
    When loading the repo at an earlier operation, the working copy will be ignored, as if `--ignore-working-copy` had been specified.
 
    It is possible to run mutating commands when loading the repo at an earlier operation. Doing that is equivalent to having run concurrent commands starting at the earlier operation. There's rarely a reason to do that, but it is possible.
-
-  Default value: `@`
 * `--debug` — Enable debug logging
 * `--color <WHEN>` — When to colorize output (always, never, debug, auto)
 * `--quiet` — Silence non-primary command output

cli/tests/test_concurrent_operations.rs

@@ -30,6 +30,13 @@ fn test_concurrent_operation_divergence() {
         &["describe", "-m", "message 2", "--at-op", "@-"],
     );
 
+    // "--at-op=@" disables op heads merging
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["log", "--at-op=@"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: The "@" expression resolved to more than one operation
+    Hint: Try specifying one of the operations by ID: e31015019d90, 48f4a48f3f70
+    "###);
+
     // "op log" doesn't merge the concurrent operations
     let stdout = test_env.jj_cmd_success(&repo_path, &["op", "log"]);
     insta::assert_snapshot!(stdout, @r###"

cli/tests/test_global_opts.rs

@@ -607,7 +607,7 @@ fn test_help() {
       -R, --repository <REPOSITORY>      Path to repository to operate on
           --ignore-working-copy          Don't snapshot the working copy, and don't update it
           --ignore-immutable             Allow rewriting immutable commits
-          --at-operation <AT_OPERATION>  Operation to load the repo at [default: @] [aliases: at-op]
+          --at-operation <AT_OPERATION>  Operation to load the repo at [aliases: at-op]
           --debug                        Enable debug logging
           --color <WHEN>                 When to colorize output (always, never, debug, auto)
           --quiet                        Silence non-primary command output