squash: support --restore-descendants

Sometimes it is convenient to be able to make changes to a commit in the
past without affecting descendants -- I already have a workflow that
tracks the commits of all children, makes the changes, and then restores
manually, but that performs a lot more rebases than necessary, which
adds up when history is long. Supporting `--restore-descendants` for
`jj squash` helps this workflow greatly.

Author: LoganDark

CHANGELOG.md

@@ -298,6 +298,10 @@ Thanks to the people who made this release happen!
 * `JJ_PAGER` can now override the `ui.pager` config, matching `JJ_EDITOR` for
   callers that need a jj-specific environment override.
 
+* `jj squash` now accepts a `--restore-descendants` flag, matching `jj abandon`,
+  `jj diffedit`, and `jj restore`. When used, descendants of the squashed
+  commits keep their original content rather than being 3-way merged.
+
 ### Fixed bugs
 
 * Improving consistency with `git` handling of `.gitignore`, including `/`

cli/src/commands/squash.rs

@@ -176,6 +176,10 @@ pub(crate) struct SquashArgs {
     /// The source revision will not be abandoned
     #[arg(long, short)]
     keep_emptied: bool,
+
+    /// Preserve the content (not the diff) when rebasing descendants
+    #[arg(long)]
+    restore_descendants: bool,
 }
 
 #[instrument(skip_all)]
@@ -293,7 +297,11 @@ pub(crate) async fn cmd_squash(
                             .collect(),
                     );
                 }
-                let new_commit = rewriter.rebase().await?.write().await?;
+                let new_commit = if args.restore_descendants {
+                    rewriter.reparent().write().await?
+                } else {
+                    rewriter.rebase().await?.write().await?
+                };
                 rewritten.insert(old_commit_id, new_commit);
                 num_rebased += 1;
                 Ok(())
@@ -307,6 +315,12 @@ pub(crate) async fn cmd_squash(
         commit
     };
 
+    let descendants_msg_suffix = if args.restore_descendants {
+        " (while preserving their content)"
+    } else {
+        ""
+    };
+
     let fileset_expression = tx
         .base_workspace_helper()
         .parse_file_patterns(ui, &args.paths)?;
@@ -332,6 +346,7 @@ pub(crate) async fn cmd_squash(
         &source_commits,
         &destination,
         args.keep_emptied,
+        args.restore_descendants,
     )
     .await?
     {
@@ -391,15 +406,22 @@ pub(crate) async fn cmd_squash(
             );
         }
         let commit = commit_builder.write(tx.repo_mut()).await?;
-        let num_rebased = tx.repo_mut().rebase_descendants().await?;
+        let num_rebased = if args.restore_descendants {
+            tx.repo_mut().reparent_descendants().await?
+        } else {
+            tx.repo_mut().rebase_descendants().await?
+        };
         if let Some(mut formatter) = ui.status_formatter() {
             if insert_destination_commit {
                 write!(formatter, "Created new commit ")?;
                 tx.write_commit_summary(formatter.as_mut(), &commit)?;
                 writeln!(formatter)?;
             }
             if num_rebased > 0 {
-                writeln!(formatter, "Rebased {num_rebased} descendant commits.")?;
+                writeln!(
+                    formatter,
+                    "Rebased {num_rebased} descendant commits{descendants_msg_suffix}."
+                )?;
             }
         }
     } else {
@@ -414,7 +436,10 @@ pub(crate) async fn cmd_squash(
                 writeln!(formatter)?;
             }
             if num_rebased > 0 {
-                writeln!(formatter, "Rebased {num_rebased} descendant commits.")?;
+                writeln!(
+                    formatter,
+                    "Rebased {num_rebased} descendant commits{descendants_msg_suffix}."
+                )?;
             }
         }
 

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

@@ -3278,6 +3278,7 @@ An alternative squashing UI is available via the `-o`, `-A`, and `-B` options. U
 * `-i`, `--interactive` — Interactively choose which parts to squash
 * `--tool <NAME>` — Specify diff editor to be used (implies --interactive)
 * `-k`, `--keep-emptied` — The source revision will not be abandoned
+* `--restore-descendants` — Preserve the content (not the diff) when rebasing descendants