git: More performance improvements when toggling between diff views (#49400)

- Defer syncing block maps from `set_companion`, eliminating some
redundant recomputations
- Emit one large multibuffer edit from `set_show_deleted_hunks` instead
of many small edits, to avoid bad case for block map

This cuts hangs roughly in half when toggling between views in a large
diff (1000 commits from the chromium repository).

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Release Notes:

- Improved performance with large diffs when toggling between diff
views.

---------

Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>
Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/editor/src/display_map.rs

@@ -473,7 +473,6 @@ impl DisplayMap {
         }
     }
 
-    // TODO(split-diff) figure out how to free the LHS from having to build a block map before this is called
     pub(crate) fn set_companion(
         &mut self,
         companion: Option<(Entity<DisplayMap>, Entity<Companion>)>,
@@ -486,26 +485,31 @@ impl DisplayMap {
             let Some((_, companion)) = self.companion.take() else {
                 return;
             };
-            let (snapshot, edits) = self.sync_through_wrap(cx);
-            let edits = edits.compose([text::Edit {
-                old: WrapRow(0)..snapshot.max_point().row(),
-                new: WrapRow(0)..snapshot.max_point().row(),
+            assert_eq!(self.entity_id, companion.read(cx).rhs_display_map_id);
+            let (snapshot, _edits) = self.sync_through_wrap(cx);
+            let edits = Patch::new(vec![text::Edit {
+                old: WrapRow(0)
+                    ..self.block_map.wrap_snapshot.borrow().max_point().row() + WrapRow(1),
+                new: WrapRow(0)..snapshot.max_point().row() + WrapRow(1),
             }]);
-            self.block_map.write(snapshot, edits, None).remove(
-                companion
+            self.block_map.deferred_edits.set(edits);
+            self.block_map.retain_blocks_raw(|block| {
+                if companion
                     .read(cx)
                     .lhs_custom_block_to_balancing_block
                     .borrow()
                     .values()
-                    .copied()
-                    .collect(),
-            );
+                    .any(|id| *id == block.id)
+                {
+                    return false;
+                }
+                true
+            });
             return;
         };
         assert_eq!(self.entity_id, companion.read(cx).rhs_display_map_id);
 
-        // Note, throwing away the wrap edits because we're going to recompute the maximal range for the block map regardless
-        let old_max_row = self.block_map.wrap_snapshot.borrow().max_point().row();
+        // Note, throwing away the wrap edits because we defer spacer computation to the first render.
         let snapshot = {
             let edits = self.buffer_subscription.consume();
             let snapshot = self.buffer.read(cx).snapshot(cx);
@@ -529,28 +533,16 @@ impl DisplayMap {
             snapshot
         };
 
-        let (companion_wrap_snapshot, companion_wrap_edits) =
+        let (companion_wrap_snapshot, _companion_wrap_edits) =
             companion_display_map.update(cx, |dm, cx| dm.sync_through_wrap(cx));
 
-        let edits = Patch::new(
-            [text::Edit {
-                old: WrapRow(0)..old_max_row,
-                new: WrapRow(0)..snapshot.max_point().row(),
-            }]
-            .into_iter()
-            .collect(),
-        );
+        let edits = Patch::new(vec![text::Edit {
+            old: WrapRow(0)..self.block_map.wrap_snapshot.borrow().max_point().row() + WrapRow(1),
+            new: WrapRow(0)..snapshot.max_point().row() + WrapRow(1),
+        }]);
+        self.block_map.deferred_edits.set(edits);
 
-        let reader = self.block_map.read(
-            snapshot.clone(),
-            edits.clone(),
-            Some(CompanionView::new(
-                self.entity_id,
-                &companion_wrap_snapshot,
-                &companion_wrap_edits,
-                companion.read(cx),
-            )),
-        );
+        let all_blocks: Vec<_> = self.block_map.blocks_raw().map(Clone::clone).collect();
 
         companion_display_map.update(cx, |companion_display_map, cx| {
             for my_buffer in self.folded_buffers() {
@@ -564,7 +556,7 @@ impl DisplayMap {
                     .folded_buffers
                     .insert(*their_buffer);
             }
-            for block in reader.blocks {
+            for block in all_blocks {
                 let Some(their_block) = block_map::balancing_block(
                     &block.properties(),
                     snapshot.buffer(),
@@ -584,16 +576,21 @@ impl DisplayMap {
                         .insert(block.id, their_id);
                 });
             }
-            companion_display_map.block_map.read(
-                companion_wrap_snapshot,
-                companion_wrap_edits,
-                Some(CompanionView::new(
-                    companion_display_map.entity_id,
-                    &snapshot,
-                    &edits,
-                    companion.read(cx),
-                )),
-            );
+            let companion_edits = Patch::new(vec![text::Edit {
+                old: WrapRow(0)
+                    ..companion_display_map
+                        .block_map
+                        .wrap_snapshot
+                        .borrow()
+                        .max_point()
+                        .row()
+                        + WrapRow(1),
+                new: WrapRow(0)..companion_wrap_snapshot.max_point().row() + WrapRow(1),
+            }]);
+            companion_display_map
+                .block_map
+                .deferred_edits
+                .set(companion_edits);
             companion_display_map.companion = Some((this, companion.clone()));
         });
 

crates/editor/src/display_map/block_map.rs

@@ -16,7 +16,7 @@ use multi_buffer::{
 };
 use parking_lot::Mutex;
 use std::{
-    cell::RefCell,
+    cell::{Cell, RefCell},
     cmp::{self, Ordering},
     fmt::Debug,
     ops::{Deref, DerefMut, Not, Range, RangeBounds, RangeInclusive},
@@ -45,6 +45,7 @@ pub struct BlockMap {
     excerpt_header_height: u32,
     pub(super) folded_buffers: HashSet<BufferId>,
     buffers_with_disabled_headers: HashSet<BufferId>,
+    pub(super) deferred_edits: Cell<Patch<WrapRow>>,
 }
 
 pub struct BlockMapReader<'a> {
@@ -620,6 +621,7 @@ impl BlockMap {
             wrap_snapshot: RefCell::new(wrap_snapshot.clone()),
             buffer_header_height,
             excerpt_header_height,
+            deferred_edits: Cell::default(),
         };
         map.sync(
             &wrap_snapshot,
@@ -747,6 +749,11 @@ impl BlockMap {
             .retain(|id, _| !ids_to_remove.contains(id));
     }
 
+    // Warning: doesn't sync the block map, use advisedly
+    pub(crate) fn blocks_raw(&self) -> impl Iterator<Item = &Arc<CustomBlock>> {
+        self.custom_blocks.iter()
+    }
+
     #[ztracing::instrument(skip_all, fields(edits = ?edits))]
     fn sync(
         &self,
@@ -756,6 +763,8 @@ impl BlockMap {
     ) {
         let buffer = wrap_snapshot.buffer_snapshot();
 
+        edits = self.deferred_edits.take().compose(edits);
+
         // Handle changing the last excerpt if it is empty.
         if buffer.trailing_excerpt_update_count()
             != self

crates/editor/src/editor.rs

@@ -20821,11 +20821,7 @@ impl Editor {
 
     fn toggle_single_diff_hunk(&mut self, range: Range<Anchor>, cx: &mut Context<Self>) {
         self.buffer.update(cx, |buffer, cx| {
-            let snapshot = buffer.snapshot(cx);
-            let excerpt_id = range.end.excerpt_id;
-            let point_range = range.to_point(&snapshot);
-            let expand = !buffer.single_hunk_is_expanded(range, cx);
-            buffer.expand_or_collapse_diff_hunks_inner([(point_range, excerpt_id)], expand, cx);
+            buffer.toggle_single_diff_hunk(range, cx);
         })
     }
 

crates/multi_buffer/src/multi_buffer.rs

@@ -2710,7 +2710,25 @@ impl MultiBuffer {
 
     pub fn set_show_deleted_hunks(&mut self, show: bool, cx: &mut Context<Self>) {
         self.snapshot.get_mut().show_deleted_hunks = show;
-        self.expand_or_collapse_diff_hunks(vec![Anchor::min()..Anchor::max()], true, cx);
+
+        self.sync_mut(cx);
+
+        let old_len = self.snapshot.borrow().len();
+
+        let ranges = std::iter::once((Point::zero()..Point::MAX, ExcerptId::max()));
+        let _ = self.expand_or_collapse_diff_hunks_inner(ranges, true, cx);
+
+        let new_len = self.snapshot.borrow().len();
+
+        self.subscriptions.publish(vec![Edit {
+            old: MultiBufferOffset(0)..old_len,
+            new: MultiBufferOffset(0)..new_len,
+        }]);
+
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
     }
 
     pub fn set_use_extended_diff_range(&mut self, use_extended: bool, _cx: &mut Context<Self>) {
@@ -2768,9 +2786,9 @@ impl MultiBuffer {
         ranges: impl IntoIterator<Item = (Range<Point>, ExcerptId)>,
         expand: bool,
         cx: &mut Context<Self>,
-    ) {
+    ) -> Vec<Edit<MultiBufferOffset>> {
         if self.snapshot.borrow().all_diff_hunks_expanded && !expand {
-            return;
+            return Vec::new();
         }
         self.sync_mut(cx);
         let mut snapshot = self.snapshot.get_mut();
@@ -2796,18 +2814,11 @@ impl MultiBuffer {
             }
         }
 
-        let edits = Self::sync_diff_transforms(
+        Self::sync_diff_transforms(
             &mut snapshot,
             excerpt_edits,
             DiffChangeKind::ExpandOrCollapseHunks { expand },
-        );
-        if !edits.is_empty() {
-            self.subscriptions.publish(edits);
-        }
-        cx.emit(Event::DiffHunksToggled);
-        cx.emit(Event::Edited {
-            edited_buffer: None,
-        });
+        )
     }
 
     pub fn expand_or_collapse_diff_hunks(
@@ -2826,7 +2837,14 @@ impl MultiBuffer {
             };
             (range.start..peek_end, end_excerpt_id)
         });
-        self.expand_or_collapse_diff_hunks_inner(ranges, expand, cx);
+        let edits = self.expand_or_collapse_diff_hunks_inner(ranges, expand, cx);
+        if !edits.is_empty() {
+            self.subscriptions.publish(edits);
+        }
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
     }
 
     pub fn resize_excerpt(
@@ -3629,6 +3647,22 @@ impl MultiBuffer {
         );
         did_extend
     }
+
+    pub fn toggle_single_diff_hunk(&mut self, range: Range<Anchor>, cx: &mut Context<Self>) {
+        let snapshot = self.snapshot(cx);
+        let excerpt_id = range.end.excerpt_id;
+        let point_range = range.to_point(&snapshot);
+        let expand = !self.single_hunk_is_expanded(range, cx);
+        let edits =
+            self.expand_or_collapse_diff_hunks_inner([(point_range, excerpt_id)], expand, cx);
+        if !edits.is_empty() {
+            self.subscriptions.publish(edits);
+        }
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
+    }
 }
 
 fn build_excerpt_ranges(