Use target branch tip as base for new independent branches#13626
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Use target branch tip as base for new independent branches#13626
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Pull request overview
This PR adjusts how “independent” branches are based in a workspace by preferring the newest (closest-to-target) merge base among all stacks, instead of always using the workspace lower bound. This better aligns new independent branches with the most recent common history when stacks diverge at different points from the target.
Changes:
- Added
Workspace::newest_base_among_stacks()to compute a newer merge base across stacks vs. the target. - Updated independent-branch creation to prefer
newest_base_among_stacks()and fall back tolower_bound. - Added workspace tests covering expected “newest base” selection and target configuration edge cases.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| crates/but-workspace/src/branch/create_reference.rs | Uses the new workspace API to choose a more recent base for independent branches. |
| crates/but-graph/src/projection/workspace/api.rs | Introduces newest_base_among_stacks() to compute a newer merge base among stacks and the target. |
| crates/but-graph/tests/graph/workspace/newest_base_among_stacks.rs | Adds tests validating newest-base selection and behavior without a configured target. |
| crates/but-graph/tests/graph/workspace/mod.rs | Registers the new test module. |
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Previously, new independent branches were based on the workspace's lower bound (the lowest common ancestor of all stacks). When stacks have different merge bases with the target, this placed the new branch on an unnecessarily old commit. Now we use the stored target commit (target.sha) directly, via a new `Workspace::target_tip_commit_id()` helper, preferring `target_commit` over `target_ref`. Creating an independent branch without a configured target is now an error rather than silently falling back to the lower bound. Also extracts `target_segment_index()` to deduplicate the target resolution logic shared with `merge_base_with_target_branch()`. Tested with four cases in `target_tip_commit_id.rs`: - Two stacks with different bases — returns the target tip - One stack above the target — still returns the target tip - No target configured — returns None - Only extra_target set — returns None (not sufficient)
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Problem
When creating a new independent branch, we used the workspace's lower bound — the lowest common ancestor of all stacks. If stacks have different merge bases with the target, this places the new branch on an unnecessarily old commit:
Solution
Use the stored target commit (
target.sha) as the base for new independent branches. If unavailable, fall back to the target ref tip. Creating an independent branch without any target is now an error.Key changes
but-graph/.../workspace/api.rstarget_tip_commit_id()— returns the storedtarget_commitID, falling back to thetarget_refsegment tiptarget_segment_index()— extracted shared helper to deduplicate target resolution withmerge_base_with_target_branch()but-workspace/.../create_reference.rs— usetarget_tip_commit_id()instead oflower_bound(no fallback)Test plan
Four new tests in
target_tip_commit_id.rs:Noneextra_targetset — returnsNone(not sufficient)