Emit a better error message when a patch is truncated

[ExplodingCabbage]

I propose this as a more conservative way to resolve #690. At least this will make clear to the end user what the problem is. Given that the only way we know of so far that one can stumble into generating a truncated patch like this is manually editing it in an editor with trim-on-save behaviour, it should usually be no big deal for an end user to fix the problem after seeing the error message.

Alternatives considered but that I don't really like:

• Making the default behaviour really liberal, like that of Unix patch. Trouble is this behaviour is arbitrary, surprising, and could easily hide more meaningful corruption of patches.
• Adding a strict mode controlled by a boolean option. Doesn't seem worth the extra API complexity to solve a problem that will only affect people who are manually editing patches.

The only further angle for improving the state of the world I have considered was to see if anything can be reasonably changed in editors. In an editor that has trim-on-save enabled by default and also supports syntax-specific settings (and has some such settings configured by default), trim-on-save behaviour should probably be disabled by default for patch files.

Except in my current editor, Zed, that's already the case; default settings contain:

    "Diff": {
      "show_edit_predictions": false,
      "remove_trailing_whitespace_on_save": false,
      "ensure_final_newline_on_save": false,
    },

and I think VSCode has trim-on-save off by default globally.

So I reckon the best I can do is just this. Commentary welcome.

[Copilot]

Pull request overview

This PR improves the diagnostics emitted by parsePatch when a unified diff hunk ends unexpectedly at end-of-file, so users get a clearer “expected vs got” line-count mismatch error instead of a more opaque parse failure.

Changes:

• Drop the trailing "" element produced by uniDiff.split(/\n/) when the input ends with a newline, preventing that sentinel from being misinterpreted as a hunk line.
• Enhance hunk sanity-check errors to include whether the mismatch is for old/new lines and the expected vs actual counts.
• Update/add tests to assert the new, more informative error messages.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/patch/parse.ts	Removes the split sentinel trailing empty string and improves line-count mismatch error messages.
test/patch/parse.js	Updates expected error messages and adds a regression test for premature EOF in the final hunk.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[ExplodingCabbage]

Good point. Makes sense to test both scenarios (both of which should produce exactly the same error message). Done.