Introduce Editor::insert_flaps and Editor::remove_flaps (#12096)

This pull request introduces the ability to add flaps, custom foldable
regions whose first foldable line can be associated with:

- A toggle in the gutter
- A trailer showed at the end of the line, before the inline blame
information


https://github.com/zed-industries/zed/assets/482957/c53a9148-f31a-4743-af64-18afa73c404c

To achieve this, we changed `FoldMap::fold` to accept a piece of text to
display when the range is folded. We use this capability in flaps to
avoid displaying the ellipsis character.

We want to use this new API in the assistant to fold context while still
giving visual cues as to what that context is.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/editor/src/editor_tests.rs

@@ -494,8 +494,8 @@ fn test_clone(cx: &mut TestAppContext) {
         editor.change_selections(None, cx, |s| s.select_ranges(selection_ranges.clone()));
         editor.fold_ranges(
             [
-                Point::new(1, 0)..Point::new(2, 0),
-                Point::new(3, 0)..Point::new(4, 0),
+                (Point::new(1, 0)..Point::new(2, 0), "⋯"),
+                (Point::new(3, 0)..Point::new(4, 0), "⋯"),
             ],
             true,
             cx,
@@ -903,9 +903,9 @@ fn test_move_cursor_multibyte(cx: &mut TestAppContext) {
     _ = view.update(cx, |view, cx| {
         view.fold_ranges(
             vec![
-                Point::new(0, 6)..Point::new(0, 12),
-                Point::new(1, 2)..Point::new(1, 4),
-                Point::new(2, 4)..Point::new(2, 8),
+                (Point::new(0, 6)..Point::new(0, 12), "⋯"),
+                (Point::new(1, 2)..Point::new(1, 4), "⋯"),
+                (Point::new(2, 4)..Point::new(2, 8), "⋯"),
             ],
             true,
             cx,
@@ -3407,9 +3407,9 @@ fn test_move_line_up_down(cx: &mut TestAppContext) {
     _ = view.update(cx, |view, cx| {
         view.fold_ranges(
             vec![
-                Point::new(0, 2)..Point::new(1, 2),
-                Point::new(2, 3)..Point::new(4, 1),
-                Point::new(7, 0)..Point::new(8, 4),
+                (Point::new(0, 2)..Point::new(1, 2), "⋯"),
+                (Point::new(2, 3)..Point::new(4, 1), "⋯"),
+                (Point::new(7, 0)..Point::new(8, 4), "⋯"),
             ],
             true,
             cx,
@@ -3891,9 +3891,9 @@ fn test_split_selection_into_lines(cx: &mut TestAppContext) {
     _ = view.update(cx, |view, cx| {
         view.fold_ranges(
             vec![
-                Point::new(0, 2)..Point::new(1, 2),
-                Point::new(2, 3)..Point::new(4, 1),
-                Point::new(7, 0)..Point::new(8, 4),
+                (Point::new(0, 2)..Point::new(1, 2), "⋯"),
+                (Point::new(2, 3)..Point::new(4, 1), "⋯"),
+                (Point::new(7, 0)..Point::new(8, 4), "⋯"),
             ],
             true,
             cx,
@@ -4548,8 +4548,8 @@ async fn test_select_larger_smaller_syntax_node(cx: &mut gpui::TestAppContext) {
     _ = view.update(cx, |view, cx| {
         view.fold_ranges(
             vec![
-                Point::new(0, 21)..Point::new(0, 24),
-                Point::new(3, 20)..Point::new(3, 22),
+                (Point::new(0, 21)..Point::new(0, 24), "⋯"),
+                (Point::new(3, 20)..Point::new(3, 22), "⋯"),
             ],
             true,
             cx,
@@ -11448,6 +11448,67 @@ async fn test_multiple_expanded_hunks_merge(
     );
 }
 
+#[gpui::test]
+fn test_flap_insertion_and_rendering(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+
+    let editor = cx.add_window(|cx| {
+        let buffer = MultiBuffer::build_simple("aaaaaa\nbbbbbb\ncccccc\nddddddd\n", cx);
+        build_editor(buffer, cx)
+    });
+
+    let render_args = Arc::new(Mutex::new(None));
+    let snapshot = editor
+        .update(cx, |editor, cx| {
+            let snapshot = editor.buffer().read(cx).snapshot(cx);
+            let range =
+                snapshot.anchor_before(Point::new(1, 0))..snapshot.anchor_after(Point::new(2, 6));
+
+            struct RenderArgs {
+                row: MultiBufferRow,
+                folded: bool,
+                callback: Arc<dyn Fn(bool, &mut WindowContext) + Send + Sync>,
+            }
+
+            let flap = Flap::new(
+                range,
+                {
+                    let toggle_callback = render_args.clone();
+                    move |row, folded, callback, _cx| {
+                        *toggle_callback.lock() = Some(RenderArgs {
+                            row,
+                            folded,
+                            callback,
+                        });
+                        div()
+                    }
+                },
+                |_row, _folded, _cx| div(),
+            );
+
+            editor.insert_flaps(Some(flap), cx);
+            let snapshot = editor.snapshot(cx);
+            let _div = snapshot.render_fold_toggle(MultiBufferRow(1), false, cx.view().clone(), cx);
+            snapshot
+        })
+        .unwrap();
+
+    let render_args = render_args.lock().take().unwrap();
+    assert_eq!(render_args.row, MultiBufferRow(1));
+    assert_eq!(render_args.folded, false);
+    assert!(!snapshot.is_line_folded(MultiBufferRow(1)));
+
+    cx.update_window(*editor, |_, cx| (render_args.callback)(true, cx))
+        .unwrap();
+    let snapshot = editor.update(cx, |editor, cx| editor.snapshot(cx)).unwrap();
+    assert!(snapshot.is_line_folded(MultiBufferRow(1)));
+
+    cx.update_window(*editor, |_, cx| (render_args.callback)(false, cx))
+        .unwrap();
+    let snapshot = editor.update(cx, |editor, cx| editor.snapshot(cx)).unwrap();
+    assert!(!snapshot.is_line_folded(MultiBufferRow(1)));
+}
+
 fn empty_range(row: usize, column: usize) -> Range<DisplayPoint> {
     let point = DisplayPoint::new(DisplayRow(row as u32), column as u32);
     point..point