agent: Rejecting agent changes shouldn't discard user edits (#31617)

The fix prevents data loss, but it also results in a somewhat confusing
UX. Specifically, after the user has made changes to an AI-created file,
selecting "Reject" will leave AI changes in place.

This is because there's no trivial way to disentangle user edits from
the edits made by the AI.

A better solution might exist. In the meantime, this change should do.
    
Closes
* #30527 

Release Notes:

- Prevent data loss when reverting changes in an agent-created file

crates/assistant_tool/src/action_log.rs

@@ -415,14 +415,38 @@ impl ActionLog {
                     self.project
                         .update(cx, |project, cx| project.save_buffer(buffer.clone(), cx))
                 } else {
-                    buffer
-                        .read(cx)
-                        .entry_id(cx)
-                        .and_then(|entry_id| {
-                            self.project
-                                .update(cx, |project, cx| project.delete_entry(entry_id, false, cx))
-                        })
-                        .unwrap_or(Task::ready(Ok(())))
+                    // For a file created by AI with no pre-existing content,
+                    // only delete the file if we're certain it contains only AI content
+                    // with no edits from the user.
+
+                    let initial_version = tracked_buffer.version.clone();
+                    let current_version = buffer.read(cx).version();
+
+                    let current_content = buffer.read(cx).text();
+                    let tracked_content = tracked_buffer.snapshot.text();
+
+                    let is_ai_only_content =
+                        initial_version == current_version && current_content == tracked_content;
+
+                    if is_ai_only_content {
+                        buffer
+                            .read(cx)
+                            .entry_id(cx)
+                            .and_then(|entry_id| {
+                                self.project.update(cx, |project, cx| {
+                                    project.delete_entry(entry_id, false, cx)
+                                })
+                            })
+                            .unwrap_or(Task::ready(Ok(())))
+                    } else {
+                        // Not sure how to disentangle edits made by the user
+                        // from edits made by the AI at this point.
+                        // For now, preserve both to avoid data loss.
+                        //
+                        // TODO: Better solution (disable "Reject" after user makes some
+                        // edit or find a way to differentiate between AI and user edits)
+                        Task::ready(Ok(()))
+                    }
                 };
 
                 self.tracked_buffers.remove(&buffer);
@@ -1576,7 +1600,6 @@ mod tests {
                 project.find_project_path("dir/new_file", cx)
             })
             .unwrap();
-
         let buffer = project
             .update(cx, |project, cx| project.open_buffer(file_path, cx))
             .await
@@ -1619,6 +1642,72 @@ mod tests {
         assert_eq!(unreviewed_hunks(&action_log, cx), vec![]);
     }
 
+    #[gpui::test]
+    async fn test_reject_created_file_with_user_edits(cx: &mut TestAppContext) {
+        init_test(cx);
+
+        let fs = FakeFs::new(cx.executor());
+        let project = Project::test(fs.clone(), [path!("/dir").as_ref()], cx).await;
+        let action_log = cx.new(|_| ActionLog::new(project.clone()));
+
+        let file_path = project
+            .read_with(cx, |project, cx| {
+                project.find_project_path("dir/new_file", cx)
+            })
+            .unwrap();
+        let buffer = project
+            .update(cx, |project, cx| project.open_buffer(file_path, cx))
+            .await
+            .unwrap();
+
+        // AI creates file with initial content
+        cx.update(|cx| {
+            action_log.update(cx, |log, cx| log.buffer_created(buffer.clone(), cx));
+            buffer.update(cx, |buffer, cx| buffer.set_text("ai content", cx));
+            action_log.update(cx, |log, cx| log.buffer_edited(buffer.clone(), cx));
+        });
+
+        project
+            .update(cx, |project, cx| project.save_buffer(buffer.clone(), cx))
+            .await
+            .unwrap();
+
+        cx.run_until_parked();
+
+        // User makes additional edits
+        cx.update(|cx| {
+            buffer.update(cx, |buffer, cx| {
+                buffer.edit([(10..10, "\nuser added this line")], None, cx);
+            });
+        });
+
+        project
+            .update(cx, |project, cx| project.save_buffer(buffer.clone(), cx))
+            .await
+            .unwrap();
+
+        assert!(fs.is_file(path!("/dir/new_file").as_ref()).await);
+
+        // Reject all
+        action_log
+            .update(cx, |log, cx| {
+                log.reject_edits_in_ranges(
+                    buffer.clone(),
+                    vec![Point::new(0, 0)..Point::new(100, 0)],
+                    cx,
+                )
+            })
+            .await
+            .unwrap();
+        cx.run_until_parked();
+
+        // File should still contain all the content
+        assert!(fs.is_file(path!("/dir/new_file").as_ref()).await);
+
+        let content = buffer.read_with(cx, |buffer, _| buffer.text());
+        assert_eq!(content, "ai content\nuser added this line");
+    }
+
     #[gpui::test(iterations = 100)]
     async fn test_random_diffs(mut rng: StdRng, cx: &mut TestAppContext) {
         init_test(cx);