Allow folding buffers inside multi buffers (#22046)

Closes https://github.com/zed-industries/zed/issues/4925


https://github.com/user-attachments/assets/e7b87375-893f-41ae-a2d9-d501499e40d1


Allows to fold any buffer inside multi buffers, either by clicking the
chevron icon on the header, or by using
`editor::Fold`/`editor::UnfoldLines`/`editor::ToggleFold`/`editor::FoldAll`
and `editor::UnfoldAll` actions inside the multi buffer (those were noop
there before).

Every fold has a fake line inside it, so it's possible to navigate into
that via the keyboard and unfold it with the corresponding editor
action.

The state is synchronized with the outline panel state: any fold inside
multi buffer folds the corresponding file entry; any file entry fold
inside the outline panel folds the corresponding buffer inside the multi
buffer, any directory fold inside the outline panel folds the
corresponding buffers inside the multi buffer for each nested file entry
in the panel.


Release Notes:

- Added a possibility to fold buffers inside multi buffers

---------

Co-authored-by: Antonio Scandurra <antonio@zed.dev>
Co-authored-by: Max Brunsfeld <max@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -195,6 +195,7 @@ pub struct ExcerptInfo {
     pub buffer: BufferSnapshot,
     pub buffer_id: BufferId,
     pub range: ExcerptRange<text::Anchor>,
+    pub text_summary: TextSummary,
 }
 
 impl std::fmt::Debug for ExcerptInfo {
@@ -1546,6 +1547,33 @@ impl MultiBuffer {
         excerpts
     }
 
+    pub fn excerpt_ranges_for_buffer(
+        &self,
+        buffer_id: BufferId,
+        cx: &AppContext,
+    ) -> Vec<Range<Point>> {
+        let snapshot = self.read(cx);
+        let buffers = self.buffers.borrow();
+        let mut cursor = snapshot.excerpts.cursor::<(Option<&Locator>, Point)>(&());
+        buffers
+            .get(&buffer_id)
+            .into_iter()
+            .flat_map(|state| &state.excerpts)
+            .filter_map(move |locator| {
+                cursor.seek_forward(&Some(locator), Bias::Left, &());
+                cursor.item().and_then(|excerpt| {
+                    if excerpt.locator == *locator {
+                        let excerpt_start = cursor.start().1;
+                        let excerpt_end = excerpt_start + excerpt.text_summary.lines;
+                        Some(excerpt_start..excerpt_end)
+                    } else {
+                        None
+                    }
+                })
+            })
+            .collect()
+    }
+
     pub fn excerpt_buffer_ids(&self) -> Vec<BufferId> {
         self.snapshot
             .borrow()
@@ -3559,6 +3587,7 @@ impl MultiBufferSnapshot {
                     buffer: excerpt.buffer.clone(),
                     buffer_id: excerpt.buffer_id,
                     range: excerpt.range.clone(),
+                    text_summary: excerpt.text_summary.clone(),
                 });
 
                 if next.is_none() {
@@ -3574,6 +3603,7 @@ impl MultiBufferSnapshot {
                     buffer: prev_excerpt.buffer.clone(),
                     buffer_id: prev_excerpt.buffer_id,
                     range: prev_excerpt.range.clone(),
+                    text_summary: prev_excerpt.text_summary.clone(),
                 });
                 let row = MultiBufferRow(cursor.start().1.row);