git_ui: Scaffold out support for generating commit messages with an LLM (#26161)

This PR adds the rough structure needed to support generating commit
messages using an LLM.

This functionality is not yet surfaced to the user.

This is the current state, if you tweak the source to show the button:


https://github.com/user-attachments/assets/66d1fbc4-09f3-4277-84f4-e9c9ebab274c

Release Notes:

- N/A

Cargo.lock

@@ -5438,6 +5438,7 @@ dependencies = [
  "gpui",
  "itertools 0.14.0",
  "language",
+ "language_model",
  "linkify",
  "linkme",
  "log",

crates/git_ui/Cargo.toml

@@ -30,6 +30,7 @@ git.workspace = true
 gpui.workspace = true
 itertools.workspace = true
 language.workspace = true
+language_model.workspace = true
 linkify.workspace = true
 linkme.workspace = true
 log.workspace = true

crates/git_ui/src/commit_message_prompt.txt

@@ -0,0 +1,15 @@
+You are an expert at writing Git commits. Your job is to write a clear commit message that summarizes the changes.
+
+Only return the commit message in your response. Do not include any additional meta-commentary about the task.
+
+Follow good Git style:
+
+- Separate the subject from the body with a blank line
+- Try to limit the subject line to 50 characters
+- Capitalize the subject line
+- 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*
+
+Here are the changes in this commit:

crates/git_ui/src/git_panel.rs

@@ -12,6 +12,7 @@ use editor::{
     scroll::ScrollbarAutoHide, Editor, EditorElement, EditorMode, EditorSettings, MultiBuffer,
     ShowScrollbar,
 };
+use futures::StreamExt as _;
 use git::repository::{
     Branch, CommitDetails, CommitSummary, PushOptions, Remote, RemoteCommandOutput, ResetMode,
     Upstream, UpstreamTracking, UpstreamTrackingStatus,
@@ -21,6 +22,9 @@ use git::{RestoreTrackedFiles, StageAll, TrashUntrackedFiles, UnstageAll};
 use gpui::*;
 use itertools::Itertools;
 use language::{Buffer, File};
+use language_model::{
+    LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage, Role,
+};
 use menu::{Confirm, SecondaryConfirm, SelectFirst, SelectLast, SelectNext, SelectPrevious};
 use multi_buffer::ExcerptInfo;
 use panel::{
@@ -195,6 +199,7 @@ pub struct GitPanel {
     conflicted_staged_count: usize,
     current_modifiers: Modifiers,
     add_coauthors: bool,
+    generate_commit_message_task: Option<Task<Option<()>>>,
     entries: Vec<GitListEntry>,
     focus_handle: FocusHandle,
     fs: Arc<dyn Fs>,
@@ -318,6 +323,7 @@ impl GitPanel {
                 conflicted_staged_count: 0,
                 current_modifiers: window.modifiers(),
                 add_coauthors: true,
+                generate_commit_message_task: None,
                 entries: Vec::new(),
                 focus_handle: cx.focus_handle(),
                 fs,
@@ -1346,6 +1352,71 @@ impl GitPanel {
         Some(format!("{} {}", action_text, file_name))
     }
 
+    /// 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 {
+            return;
+        };
+        let Some(model) = LanguageModelRegistry::read_global(cx).active_model() else {
+            return;
+        };
+
+        if !provider.is_authenticated(cx) {
+            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,
+        };
+
+        self.generate_commit_message_task = Some(cx.spawn(|this, mut cx| {
+            async move {
+                let stream = model.stream_completion_text(request, &cx);
+                let mut messages = stream.await?;
+
+                while let Some(message) = messages.stream.next().await {
+                    let text = message?;
+
+                    this.update(&mut cx, |this, cx| {
+                        this.commit_message_buffer(cx).update(cx, |buffer, cx| {
+                            let insert_position = buffer.anchor_before(buffer.len());
+                            buffer.edit([(insert_position..insert_position, text)], None, cx);
+                        });
+                    })?;
+                }
+
+                anyhow::Ok(())
+            }
+            .log_err()
+        }));
+    }
+
     fn update_editor_placeholder(&mut self, cx: &mut Context<Self>) {
         let suggested_commit_message = self.suggest_commit_message();
         let placeholder_text = suggested_commit_message
@@ -2013,6 +2084,8 @@ impl GitPanel {
         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);
@@ -2060,8 +2133,18 @@ impl GitPanel {
                             .absolute()
                             .bottom_0()
                             .right_2()
+                            .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(
                                 panel_filled_button(title)