git_ui: Add support for generating commit messages with an LLM (#26227)

This PR finishes up the support for generating commit messages using an
LLM.

We're shelling out to `git diff` to get the diff text, as it seemed more
efficient than attempting to reconstruct the diff ourselves from our
internal Git state.


https://github.com/user-attachments/assets/9bcf30a7-7a08-4f49-a753-72a5d954bddd

Release Notes:

- Git Beta: Added support for generating commit messages using a
language model.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/git_ui/src/commit_message_prompt.txt

@@ -1,4 +1,8 @@
-You are an expert at writing Git commits. Your job is to write a clear commit message that summarizes the changes.
+You are an expert at writing Git commits. Your job is to write a short clear commit message that summarizes the changes.
+
+If you can accurately express the change in just the subject line, don't include anything in the message body. Only use the body when it is providing *useful* information.
+
+Don't repeat information from the subject line in the message body.
 
 Only return the commit message in your response. Do not include any additional meta-commentary about the task.
 
@@ -10,6 +14,6 @@ Follow good Git style:
 - Do not end the subject line with any punctuation
 - Use the imperative mood in the subject line
 - Wrap the body at 72 characters
-- Use the body to explain *what* and *why* vs. *how*
+- Keep the body short and concise (omit it entirely if not useful)
 
 Here are the changes in this commit:

crates/git_ui/src/commit_modal.rs

@@ -234,7 +234,7 @@ impl CommitModal {
     pub fn render_footer(&self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let git_panel = self.git_panel.clone();
 
-        let (branch, can_commit, tooltip, commit_label, co_authors) =
+        let (branch, can_commit, tooltip, commit_label, co_authors, generate_commit_message) =
             self.git_panel.update(cx, |git_panel, cx| {
                 let branch = git_panel
                     .active_repository
@@ -249,7 +249,15 @@ impl CommitModal {
                 let (can_commit, tooltip) = git_panel.configure_commit_button(cx);
                 let title = git_panel.commit_button_title();
                 let co_authors = git_panel.render_co_authors(cx);
-                (branch, can_commit, tooltip, title, co_authors)
+                let generate_commit_message = git_panel.render_generate_commit_message_button(cx);
+                (
+                    branch,
+                    can_commit,
+                    tooltip,
+                    title,
+                    co_authors,
+                    generate_commit_message,
+                )
             });
 
         let branch_picker_button = panel_button(branch)
@@ -316,7 +324,13 @@ impl CommitModal {
             .w_full()
             .h(px(self.properties.footer_height))
             .gap_1()
-            .child(h_flex().gap_1().child(branch_picker).children(co_authors))
+            .child(
+                h_flex()
+                    .gap_1()
+                    .child(branch_picker)
+                    .children(co_authors)
+                    .child(generate_commit_message),
+            )
             .child(div().flex_1())
             .child(
                 h_flex()

crates/git_ui/src/git_panel.rs

@@ -18,8 +18,8 @@ use editor::{
 };
 use futures::StreamExt as _;
 use git::repository::{
-    Branch, CommitDetails, CommitSummary, PushOptions, Remote, RemoteCommandOutput, ResetMode,
-    Upstream, UpstreamTracking, UpstreamTrackingStatus,
+    Branch, CommitDetails, CommitSummary, DiffType, PushOptions, Remote, RemoteCommandOutput,
+    ResetMode, Upstream, UpstreamTracking, UpstreamTrackingStatus,
 };
 use git::{repository::RepoPath, status::FileStatus, Commit, ToggleStaged};
 use git::{RestoreTrackedFiles, StageAll, TrashUntrackedFiles, UnstageAll};
@@ -1393,6 +1393,15 @@ impl GitPanel {
         Some(format!("{} {}", action_text, file_name))
     }
 
+    fn generate_commit_message_action(
+        &mut self,
+        _: &git::GenerateCommitMessage,
+        _window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        self.generate_commit_message(cx);
+    }
+
     /// Generates a commit message using an LLM.
     fn generate_commit_message(&mut self, cx: &mut Context<Self>) {
         let Some(provider) = LanguageModelRegistry::read_global(cx).active_provider() else {
@@ -1406,38 +1415,50 @@ impl GitPanel {
             return;
         }
 
-        const PROMPT: &str = include_str!("commit_message_prompt.txt");
-
-        // TODO: We need to generate a diff from the actual Git state.
-        //
-        // It need not look exactly like the structure below, this is just an example generated by Claude.
-        let diff_text = "diff --git a/src/main.rs b/src/main.rs
-index 1234567..abcdef0 100644
---- a/src/main.rs
-+++ b/src/main.rs
-@@ -10,7 +10,7 @@ fn main() {
-     println!(\"Hello, world!\");
--    let unused_var = 42;
-+    let important_value = 42;
-
-     // Do something with the value
--    // TODO: Implement this later
-+    println!(\"The answer is {}\", important_value);
- }";
-
-        let request = LanguageModelRequest {
-            messages: vec![LanguageModelRequestMessage {
-                role: Role::User,
-                content: vec![format!("{PROMPT}\n{diff_text}").into()],
-                cache: false,
-            }],
-            tools: Vec::new(),
-            stop: Vec::new(),
-            temperature: None,
+        let Some(repo) = self.active_repository.as_ref() else {
+            return;
         };
 
+        let diff = repo.update(cx, |repo, cx| {
+            if self.has_staged_changes() {
+                repo.diff(DiffType::HeadToIndex, cx)
+            } else {
+                repo.diff(DiffType::HeadToWorktree, cx)
+            }
+        });
+
         self.generate_commit_message_task = Some(cx.spawn(|this, mut cx| {
             async move {
+                let _defer = util::defer({
+                    let mut cx = cx.clone();
+                    let this = this.clone();
+                    move || {
+                        this.update(&mut cx, |this, _cx| {
+                            this.generate_commit_message_task.take();
+                        })
+                        .ok();
+                    }
+                });
+
+                let mut diff_text = diff.await??;
+                const ONE_MB: usize = 1_000_000;
+                if diff_text.len() > ONE_MB {
+                    diff_text = diff_text.chars().take(ONE_MB).collect()
+                }
+
+                const PROMPT: &str = include_str!("commit_message_prompt.txt");
+
+                let request = LanguageModelRequest {
+                    messages: vec![LanguageModelRequestMessage {
+                        role: Role::User,
+                        content: vec![format!("{PROMPT}\n{diff_text}").into()],
+                        cache: false,
+                    }],
+                    tools: Vec::new(),
+                    stop: Vec::new(),
+                    temperature: None,
+                };
+
                 let stream = model.stream_completion_text(request, &cx);
                 let mut messages = stream.await?;
 
@@ -2113,6 +2134,32 @@ index 1234567..abcdef0 100644
         self.has_staged_changes()
     }
 
+    pub(crate) fn render_generate_commit_message_button(&self, cx: &Context<Self>) -> AnyElement {
+        if self.generate_commit_message_task.is_some() {
+            return Icon::new(IconName::ArrowCircle)
+                .size(IconSize::XSmall)
+                .color(Color::Info)
+                .with_animation(
+                    "arrow-circle",
+                    Animation::new(Duration::from_secs(2)).repeat(),
+                    |icon, delta| icon.transform(Transformation::rotate(percentage(delta))),
+                )
+                .into_any_element();
+        }
+
+        IconButton::new("generate-commit-message", IconName::ZedAssistant)
+            .shape(ui::IconButtonShape::Square)
+            .tooltip(Tooltip::for_action_title_in(
+                "Generate commit message",
+                &git::GenerateCommitMessage,
+                &self.commit_editor.focus_handle(cx),
+            ))
+            .on_click(cx.listener(move |this, _event, _window, cx| {
+                this.generate_commit_message(cx);
+            }))
+            .into_any_element()
+    }
+
     pub(crate) fn render_co_authors(&self, cx: &Context<Self>) -> Option<AnyElement> {
         let potential_co_authors = self.potential_co_authors(cx);
         if potential_co_authors.is_empty() {
@@ -2201,8 +2248,6 @@ index 1234567..abcdef0 100644
         let panel_editor_style = panel_editor_style(true, window, cx);
 
         let enable_coauthors = self.render_co_authors(cx);
-        // Note: This is hard-coded to `false` as it is not fully implemented.
-        let show_generate_commit_message_button = false;
 
         let title = self.commit_button_title();
         let editor_focus_handle = self.commit_editor.focus_handle(cx);
@@ -2253,16 +2298,8 @@ index 1234567..abcdef0 100644
                             .gap_0p5()
                             .h(footer_size)
                             .flex_none()
-                            .when(show_generate_commit_message_button, |parent| {
-                                parent.child(
-                                    panel_filled_button("Generate Commit Message").on_click(
-                                        cx.listener(move |this, _event, _window, cx| {
-                                            this.generate_commit_message(cx);
-                                        }),
-                                    ),
-                                )
-                            })
                             .children(enable_coauthors)
+                            .child(self.render_generate_commit_message_button(cx))
                             .child(
                                 panel_filled_button(title)
                                     .tooltip(move |window, cx| {
@@ -2927,6 +2964,7 @@ impl Render for GitPanel {
             .on_action(cx.listener(Self::restore_tracked_files))
             .on_action(cx.listener(Self::clean_all))
             .on_action(cx.listener(Self::expand_commit_editor))
+            .on_action(cx.listener(Self::generate_commit_message_action))
             .when(has_write_access && has_co_authors, |git_panel| {
                 git_panel.on_action(cx.listener(Self::toggle_fill_co_authors))
             })