feat(split): --reparent for --detach / --discard

When splitting a commit with `--discard` or `--detach`, the `--reparent`
option ensures that the changes from the discarded or detached portion
are squashed to the child commit, just like `amend --reparent`.

Reparenting is not applied to `InsertAfter` or `InsertBefore` modes as
it would simply be a no-op in these cases (the descendants are not
changed in the first place).

Author: bryango

git-branchless/src/commands/split.rs

@@ -80,9 +80,13 @@ pub fn split(
         resolve_merge_conflicts,
         dump_rebase_constraints,
         dump_rebase_plan,
-        reparent: _, // not yet implemented
+        reparent,
     } = *move_options;
 
+    // reparenting is only relevant for non-Insert* modes
+    let should_reparent =
+        reparent && !matches!(split_mode, SplitMode::InsertAfter | SplitMode::InsertBefore);
+
     let target_oid: NonZeroOid = match resolve_commits(
         effects,
         &repo,
@@ -534,10 +538,16 @@ pub fn split(
         for child in dag.commit_set_to_vec(&children)? {
             match (&split_mode, extracted_commit_oid) {
                 (_, None) | (SplitMode::DetachAfter, Some(_)) => {
-                    builder.move_subtree(child, vec![remainder_commit_oid])?
+                    builder.move_subtree(child, vec![remainder_commit_oid])?;
+                    if should_reparent {
+                        builder.reparent_subtree(child, vec![remainder_commit_oid], &repo)?;
+                    }
                 }
                 (_, Some(extracted_commit_oid)) => {
-                    builder.move_subtree(child, vec![extracted_commit_oid])?
+                    builder.move_subtree(child, vec![extracted_commit_oid])?;
+                    if should_reparent {
+                        builder.reparent_subtree(child, vec![extracted_commit_oid], &repo)?;
+                    }
                 }
             }
         }

git-branchless/tests/test_split.rs

@@ -615,6 +615,136 @@ fn test_split_discard() -> eyre::Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_split_reparent() -> eyre::Result<()> {
+    let git = make_git()?;
+    git.init_repo()?;
+    git.detach_head()?;
+
+    git.write_file_txt("test1", "contents1")?;
+    git.write_file_txt("test2", "contents2")?;
+    git.write_file_txt("test3", "contents3")?;
+    git.run(&["add", "."])?;
+    git.run(&["commit", "-m", "first commit"])?;
+
+    git.write_file_txt("test3", "updated contents3")?;
+    git.write_file_txt("test4", "contents4")?;
+    git.write_file_txt("test5", "contents5")?;
+    git.run(&["add", "."])?;
+    git.run(&["commit", "-m", "second commit"])?;
+
+    {
+        let (stdout, _stderr) = git.branchless("smartlog", &[])?;
+        insta::assert_snapshot!(stdout, @r###"
+        O f777ecc (master) create initial.txt
+        |
+        o e48cdc5 first commit
+        |
+        @ 8c3edf7 second commit
+        "###);
+
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD~"])?;
+        insta::assert_snapshot!(&stdout, @"
+            test1.txt | 1 +
+            test2.txt | 1 +
+            test3.txt | 1 +
+            3 files changed, 3 insertions(+)
+        ");
+
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD"])?;
+        insta::assert_snapshot!(&stdout, @"
+            test3.txt | 2 +-
+            test4.txt | 1 +
+            test5.txt | 1 +
+            3 files changed, 3 insertions(+), 1 deletion(-)
+        ");
+    }
+
+    {
+        let (stdout, _stderr) =
+            git.branchless("split", &["HEAD~", "test2.txt", "--discard", "--reparent"])?;
+        insta::assert_snapshot!(&stdout, @r"
+            Attempting rebase in-memory...
+            [1/1] Committed as: 3aec2d3 second commit
+            branchless: processing 1 rewritten commit
+            branchless: running command: <git-executable> checkout 3aec2d3c59e90647cef2f4bbb49a4e09790611c6
+            In-memory rebase succeeded.
+            O f777ecc (master) create initial.txt
+            |
+            o 2932db7 first commit
+            |
+            @ 3aec2d3 second commit
+        ");
+
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD~"])?;
+        insta::assert_snapshot!(&stdout, @"
+            test1.txt | 1 +
+            test3.txt | 1 +
+            2 files changed, 2 insertions(+)
+        ");
+
+        // the discarded test2 is effectively squashed into the second commit
+        // due to --reparent
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD"])?;
+        insta::assert_snapshot!(&stdout, @r"
+            test2.txt | 1 +
+            test3.txt | 2 +-
+            test4.txt | 1 +
+            test5.txt | 1 +
+            4 files changed, 4 insertions(+), 1 deletion(-)
+        ");
+    }
+
+    // similarly for --detach
+    {
+        let (stdout, _stderr) =
+            git.branchless("split", &["HEAD~", "test1.txt", "--detach", "--reparent"])?;
+        insta::assert_snapshot!(&stdout, @r"
+            Attempting rebase in-memory...
+            [1/1] Committed as: 6249d09 second commit
+            branchless: processing 1 rewritten commit
+            branchless: running command: <git-executable> checkout 6249d093020c9d764e62b8bf8e67228a4445b4f0
+            In-memory rebase succeeded.
+            O f777ecc (master) create initial.txt
+            |
+            o 52618ab first commit
+            |\
+            | o 4271e93 temp(split): test1.txt (+1)
+            |
+            @ 6249d09 second commit
+        ");
+    }
+
+    {
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD~"])?;
+        insta::assert_snapshot!(&stdout, @r"
+            test3.txt | 1 +
+            1 file changed, 1 insertion(+)
+        ");
+
+        // the detached test1 is effectively squashed into the second commit
+        let (stdout, _stderr) = git.run(&["show", "--pretty=format:", "--stat", "HEAD"])?;
+        insta::assert_snapshot!(&stdout, @r"
+            test1.txt | 1 +
+            test2.txt | 1 +
+            test3.txt | 2 +-
+            test4.txt | 1 +
+            test5.txt | 1 +
+            5 files changed, 5 insertions(+), 1 deletion(-)
+        ");
+
+        let (split_commit, _stderr) = git.run(&["query", "--raw", "exactly(siblings(HEAD), 1)"])?;
+        let (stdout, _stderr) =
+            git.run(&["show", "--pretty=format:", "--stat", split_commit.trim()])?;
+        insta::assert_snapshot!(&stdout, @r"
+            test1.txt | 1 +
+            1 file changed, 1 insertion(+)
+        ");
+    }
+
+    Ok(())
+}
+
 #[test]
 fn test_split_discard_bug_checked_out_branch() -> eyre::Result<()> {
     let git = make_git()?;