assistant2: Add support for referencing other threads as context (#22092)

This PR adds the ability to reference other threads as context:

<img width="1159" alt="Screenshot 2024-12-16 at 11 29 54 AM"
src="https://github.com/user-attachments/assets/bb8a24ff-56d3-4406-ab8c-6657e65d8c70"
/>

<img width="1159" alt="Screenshot 2024-12-16 at 11 29 35 AM"
src="https://github.com/user-attachments/assets/7a02ebda-a2f5-40e9-9dd4-1bb029cb1c43"
/>


Release Notes:

- N/A

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -0,0 +1,209 @@
+use std::sync::Arc;
+
+use fuzzy::StringMatchCandidate;
+use gpui::{AppContext, DismissEvent, FocusHandle, FocusableView, Task, View, WeakModel, WeakView};
+use picker::{Picker, PickerDelegate};
+use ui::{prelude::*, ListItem};
+
+use crate::context::ContextKind;
+use crate::context_picker::ContextPicker;
+use crate::message_editor::MessageEditor;
+use crate::thread::ThreadId;
+use crate::thread_store::ThreadStore;
+
+pub struct ThreadContextPicker {
+    picker: View<Picker<ThreadContextPickerDelegate>>,
+}
+
+impl ThreadContextPicker {
+    pub fn new(
+        thread_store: WeakModel<ThreadStore>,
+        context_picker: WeakView<ContextPicker>,
+        message_editor: WeakView<MessageEditor>,
+        cx: &mut ViewContext<Self>,
+    ) -> Self {
+        let delegate =
+            ThreadContextPickerDelegate::new(thread_store, context_picker, message_editor);
+        let picker = cx.new_view(|cx| Picker::uniform_list(delegate, cx));
+
+        ThreadContextPicker { picker }
+    }
+}
+
+impl FocusableView for ThreadContextPicker {
+    fn focus_handle(&self, cx: &AppContext) -> FocusHandle {
+        self.picker.focus_handle(cx)
+    }
+}
+
+impl Render for ThreadContextPicker {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        self.picker.clone()
+    }
+}
+
+#[derive(Debug, Clone)]
+struct ThreadContextEntry {
+    id: ThreadId,
+    summary: SharedString,
+}
+
+pub struct ThreadContextPickerDelegate {
+    thread_store: WeakModel<ThreadStore>,
+    context_picker: WeakView<ContextPicker>,
+    message_editor: WeakView<MessageEditor>,
+    matches: Vec<ThreadContextEntry>,
+    selected_index: usize,
+}
+
+impl ThreadContextPickerDelegate {
+    pub fn new(
+        thread_store: WeakModel<ThreadStore>,
+        context_picker: WeakView<ContextPicker>,
+        message_editor: WeakView<MessageEditor>,
+    ) -> Self {
+        ThreadContextPickerDelegate {
+            thread_store,
+            context_picker,
+            message_editor,
+            matches: Vec::new(),
+            selected_index: 0,
+        }
+    }
+}
+
+impl PickerDelegate for ThreadContextPickerDelegate {
+    type ListItem = ListItem;
+
+    fn match_count(&self) -> usize {
+        self.matches.len()
+    }
+
+    fn selected_index(&self) -> usize {
+        self.selected_index
+    }
+
+    fn set_selected_index(&mut self, ix: usize, _cx: &mut ViewContext<Picker<Self>>) {
+        self.selected_index = ix;
+    }
+
+    fn placeholder_text(&self, _cx: &mut WindowContext) -> Arc<str> {
+        "Search threads…".into()
+    }
+
+    fn update_matches(&mut self, query: String, cx: &mut ViewContext<Picker<Self>>) -> Task<()> {
+        let Ok(threads) = self.thread_store.update(cx, |this, cx| {
+            this.threads(cx)
+                .into_iter()
+                .map(|thread| {
+                    const DEFAULT_SUMMARY: SharedString = SharedString::new_static("New Thread");
+
+                    let id = thread.read(cx).id().clone();
+                    let summary = thread.read(cx).summary().unwrap_or(DEFAULT_SUMMARY);
+                    ThreadContextEntry { id, summary }
+                })
+                .collect::<Vec<_>>()
+        }) else {
+            return Task::ready(());
+        };
+
+        let executor = cx.background_executor().clone();
+        let search_task = cx.background_executor().spawn(async move {
+            if query.is_empty() {
+                threads
+            } else {
+                let candidates = threads
+                    .iter()
+                    .enumerate()
+                    .map(|(id, thread)| StringMatchCandidate::new(id, &thread.summary))
+                    .collect::<Vec<_>>();
+                let matches = fuzzy::match_strings(
+                    &candidates,
+                    &query,
+                    false,
+                    100,
+                    &Default::default(),
+                    executor,
+                )
+                .await;
+
+                matches
+                    .into_iter()
+                    .map(|mat| threads[mat.candidate_id].clone())
+                    .collect()
+            }
+        });
+
+        cx.spawn(|this, mut cx| async move {
+            let matches = search_task.await;
+            this.update(&mut cx, |this, cx| {
+                this.delegate.matches = matches;
+                this.delegate.selected_index = 0;
+                cx.notify();
+            })
+            .ok();
+        })
+    }
+
+    fn confirm(&mut self, _secondary: bool, cx: &mut ViewContext<Picker<Self>>) {
+        let entry = &self.matches[self.selected_index];
+
+        let Some(thread_store) = self.thread_store.upgrade() else {
+            return;
+        };
+
+        let Some(thread) = thread_store.update(cx, |this, cx| this.open_thread(&entry.id, cx))
+        else {
+            return;
+        };
+
+        self.message_editor
+            .update(cx, |message_editor, cx| {
+                let text = thread.update(cx, |thread, _cx| {
+                    let mut text = String::new();
+
+                    for message in thread.messages() {
+                        text.push_str(match message.role {
+                            language_model::Role::User => "User:",
+                            language_model::Role::Assistant => "Assistant:",
+                            language_model::Role::System => "System:",
+                        });
+                        text.push('\n');
+
+                        text.push_str(&message.text);
+                        text.push('\n');
+                    }
+
+                    text
+                });
+
+                message_editor.insert_context(ContextKind::Thread, entry.summary.clone(), text);
+            })
+            .ok();
+    }
+
+    fn dismissed(&mut self, cx: &mut ViewContext<Picker<Self>>) {
+        self.context_picker
+            .update(cx, |this, cx| {
+                this.reset_mode();
+                cx.emit(DismissEvent);
+            })
+            .ok();
+    }
+
+    fn render_match(
+        &self,
+        ix: usize,
+        selected: bool,
+        _cx: &mut ViewContext<Picker<Self>>,
+    ) -> Option<Self::ListItem> {
+        let thread = &self.matches[ix];
+
+        Some(
+            ListItem::new(ix)
+                .inset(true)
+                .toggle_state(selected)
+                .child(thread.summary.clone()),
+        )
+    }
+}