editor: Show hints for using AI features on empty lines (#20824)

Co-Authored-by: Thorsten <thorsten@zed.dev>
Co-Authored-by: Antonio <antonio@zed.dev>

Screenshot:

![screenshot-2024-11-18-17 11
08@2x](https://github.com/user-attachments/assets/610fd7db-7476-4b9b-9465-a3d55df12340)

TODO:
- [x] docs

Release Notes:

- Added inline hints that guide users on how to invoke the inline
assistant and open the assistant panel. (These hints can be disabled by
setting `{"assistant": {"show_hints": false}}`.)

---------

Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/editor/src/editor.rs

@@ -540,6 +540,15 @@ pub enum IsVimMode {
     No,
 }
 
+pub trait ActiveLineTrailerProvider {
+    fn render_active_line_trailer(
+        &mut self,
+        style: &EditorStyle,
+        focus_handle: &FocusHandle,
+        cx: &mut WindowContext,
+    ) -> Option<AnyElement>;
+}
+
 /// Zed's primary text input `View`, allowing users to edit a [`MultiBuffer`]
 ///
 /// See the [module level documentation](self) for more information.
@@ -667,6 +676,7 @@ pub struct Editor {
     next_scroll_position: NextScrollCursorCenterTopBottom,
     addons: HashMap<TypeId, Box<dyn Addon>>,
     _scroll_cursor_center_top_bottom_task: Task<()>,
+    active_line_trailer_provider: Option<Box<dyn ActiveLineTrailerProvider>>,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Default)]
@@ -2200,6 +2210,7 @@ impl Editor {
             addons: HashMap::default(),
             _scroll_cursor_center_top_bottom_task: Task::ready(()),
             text_style_refinement: None,
+            active_line_trailer_provider: None,
         };
         this.tasks_update_task = Some(this.refresh_runnables(cx));
         this._subscriptions.extend(project_subscriptions);
@@ -2488,6 +2499,16 @@ impl Editor {
         self.refresh_inline_completion(false, false, cx);
     }
 
+    pub fn set_active_line_trailer_provider<T>(
+        &mut self,
+        provider: Option<T>,
+        _cx: &mut ViewContext<Self>,
+    ) where
+        T: ActiveLineTrailerProvider + 'static,
+    {
+        self.active_line_trailer_provider = provider.map(|provider| Box::new(provider) as Box<_>);
+    }
+
     pub fn placeholder_text(&self, _cx: &WindowContext) -> Option<&str> {
         self.placeholder_text.as_deref()
     }
@@ -11844,6 +11865,21 @@ impl Editor {
             && self.has_blame_entries(cx)
     }
 
+    pub fn render_active_line_trailer(
+        &mut self,
+        style: &EditorStyle,
+        cx: &mut WindowContext,
+    ) -> Option<AnyElement> {
+        if !self.newest_selection_head_on_empty_line(cx) || self.has_active_inline_completion(cx) {
+            return None;
+        }
+
+        let focus_handle = self.focus_handle.clone();
+        self.active_line_trailer_provider
+            .as_mut()?
+            .render_active_line_trailer(style, &focus_handle, cx)
+    }
+
     fn has_blame_entries(&self, cx: &mut WindowContext) -> bool {
         self.blame()
             .map_or(false, |blame| blame.read(cx).has_generated_entries())