Fixer tail-line deletion lost when an unstaged edit overlaps it (different case from #929/#965)

[LymanBE]

Versions: hk 1.47.0 and 1.48.0 (latest), git 2.x, macOS arm64. Reproduces on both.

Summary

If a file is partially staged, a fixer deletes lines at the end of the staged content, and there's an unstaged edit in the middle of the same file, the stash restore puts the deleted tail lines back in the worktree. The commit itself is fine (the index keeps the fixer's deletion), but the worktree ends up reverted to the full pre-hook content, so the fixer's deletion and the staged edit are both gone from disk.

This is a different case from #929/#931. That fix made diff_hunks emit the tail-deletion hunk, and it does now, but three_way_merge_hunks drops the hunk during overlap resolution (details below). It's also not #965/#966: the worktree edit here is in the middle of the file, not the last line, so it never reaches the git.rs prefix fallback.

Repro

ver=1.48.0
git init t && cd t
git config user.email r@e.c && git config user.name r
cat > hk.pkl <<EOF
amends "package://github.com/jdx/hk/releases/download/v${ver}/hk@${ver}#/Config.pkl"
hooks { ["pre-commit"] { fix = true; stash = "patch-file"; steps {
  ["fixer"] { glob = "f.txt"; fix = "head -2 f.txt > .t && mv .t f.txt"; stage = List("f.txt") }
} } }
EOF
printf 'l1\nl2\nl3\nl4\nl5\n' > f.txt
git add -A && git commit -qm base
printf 'l1 STAGED\nl2\nl3\nl4\nl5\n' > f.txt   # staged edit on line 1
git add f.txt
printf 'l1 STAGED\nl2\nl3 UNSTAGED\nl4\nl5\n' > f.txt   # unstaged edit in the middle
hk run pre-commit
cat f.txt

Expected: the fixer truncates the file to 2 lines, so after the restore the worktree should have the unstaged middle edit merged on top of that, with l4/l5 still deleted. (The index gets this right.)

Actual: the worktree is back to all 5 lines. l3 UNSTAGED is there, but l4/l5 are restored and the staged l1 STAGED edit is reverted too. The log shows stash – Restoring unstaged changes (manual).

Where it happens

In src/merge.rs at v1.48.0, with the index snapshot as base:

• diff_hunks(base, fixer) produces [{start:2, end:5, lines:[]}] (the tail deletion, correctly emitted since fix(stash): preserve fixer tail-line deletions in three-way merge #931)
• diff_hunks(base, worktree) produces [{start:2, end:3, lines:["l3 UNSTAGED\n"]}]

The two hunks overlap at base line 2. three_way_merge_hunks applies the worktree hunk first, advancing idx to 3, and then the skip loop

while fi < fixer_hunks.len() && fixer_hunks[fi].start < idx { fi += 1; }

skips the fixer hunk because its start (2) is less than idx (3). The deletion is never applied, and base[3..] (l4, l5) gets copied to the output. So any fixer deletion hunk that overlaps a worktree hunk starting at or before the same base line is silently discarded.

Workaround attempts

stash = "git" and stash = "patch-file" both go through the same manual merge, so switching modes doesn't help. stash = "none" avoids the corruption but silently commits the unstaged hunks of fixer-touched files, which isn't acceptable for a fix = true pre-commit hook. I didn't find a configuration that runs fixers and preserves this case.

Happy to test a patch; the repro above is self-contained.

[jdx]

#990