# Add support for \`format_on_save\` only changed lines \(\#49964\) · gitcafe/zed

[View on GitCafe](https://git.cafe/gitcafe/zed/commit/b9f3396b68c61e27f2aab38fbe4bcd274968befa)

Repository: [gitcafe/zed](https://git.cafe/gitcafe/zed)

Visibility: public

Requested revision: b9f3396b68c61e27f2aab38fbe4bcd274968befa

Requested commit: b9f3396b68c61e27f2aab38fbe4bcd274968befa

Commit: b9f3396b68c61e27f2aab38fbe4bcd274968befa

Tree: bc4c1d08d5f22210d949be772632042b6b938044

Author: G36maid

Committer: GitHub

## Message

```
Add support for `format_on_save` only changed lines (#49964)

## Summary

Introduces `format_on_save` variants that format only modified lines,
limiting formatting to lines changed since the last commit. This
prevents massive diffs when editing legacy codebases. Aligns with VS
Code's `editor.formatOnSaveMode` naming:

- `"modifications"` — formats only git-diffed lines. Requires source
control; skips formatting if no diff is available.
- `"modifications_if_available"` — formats only git-diffed lines,
falling back to full-file formatting for untracked files, when source
control is unavailable, or when the LSP does not support range
formatting.

Also supports importing equivalent VS Code settings
(`formatOnSaveMode`).

This PR uses the range-based whitespace/newline infrastructure from the
dependency PR above.

## Changes


<details>
<summary><b>New settings, modified ranges computation, VS Code
import</b></summary>

### New settings (`language.rs`, `default.json`, `all-settings.md`)

Two new `FormatOnSave` variants: `Modifications` and
`ModificationsIfAvailable`.

### Modified ranges computation (`items.rs`)

- `compute_format_decision()` — reads `format_on_save` setting across
all buffers, determines whether to use ranged formatting (`Ranges`),
full formatting (`Full`), or skip (`Skip`).
- `compute_modified_ranges()` — extracts modified line ranges from git
diff hunks via `BufferDiffSnapshot`.
- `is_empty_range()` — helper to detect deletion-only hunks that produce
no formatable content.

The `save()` method calls `compute_format_decision()` and dispatches
accordingly.

### Modifications mode in `lsp_store.rs`

- Adds `FormatOnSave::Modifications |
FormatOnSave::ModificationsIfAvailable` to the formatter selection match
arm.
- `ModificationsIfAvailable` falls back to full-file formatting when
ranged formatting produces no results.

### VS Code settings import (`vscode_import.rs`, `settings_store.rs`)

Imports `editor.formatOnSaveMode` mapping:
- `"modifications"` → `FormatOnSave::Modifications`
- `"modificationsIfAvailable"` →
`FormatOnSave::ModificationsIfAvailable`
- `"file"` → `FormatOnSave::On`

</details>

## Tests

- `test_modifications_format_on_save` — basic modifications mode
formatting with dirty buffer
- `test_modifications_format_no_changes` — clean buffer triggers no
formatting
- `test_modifications_format_lsp_no_range_support` — LSP without range
formatting skips entirely for `Modifications`
- `test_modifications_format_lsp_returns_empty_edits` — empty edits
handled gracefully
- `test_modifications_format_multiple_hunks` — two non-adjacent edits
produce two separate range formatting requests

---

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable


Closes #16509
Depends on #53942

Release Notes:

- Added `modifications` and `modifications_if_available` options to
`format_on_save`, which format only git-changed lines instead of the
entire file
- When using modifications mode, `remove_trailing_whitespace_on_save`
and `ensure_final_newline_on_save` are also scoped to changed lines,
preventing unwanted diff jitter in legacy codebases
- Added support for importing VS Code's `editor.formatOnSaveMode`
setting

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>
```

## Parents

- [7dc634124c249c5625da643d94f9fccefad80c56](https://git.cafe/gitcafe/zed/commit/7dc634124c249c5625da643d94f9fccefad80c56?format=markdown)

[Source at this commit](https://git.cafe/gitcafe/zed/tree/b9f3396b68c61e27f2aab38fbe4bcd274968befa?format=markdown)