Compute scrollbar markers asynchronously (#10080)

Refs #9647
Fixes https://github.com/zed-industries/zed/issues/9792

This pull request moves the computation of scrollbar markers off the
main thread, to prevent them from grinding the editor to a halt when we
have a lot of them (e.g., when there are lots of search results on a
large file). With these changes we also avoid generating multiple quads
for adjacent markers, thus fixing an issue where we stop drawing other
primitives because we've drawn too many quads in the scrollbar.

Release Notes:

- Improved editor performance when displaying lots of search results,
diagnostics, or symbol highlights in the scrollbar
([#9792](https://github.com/zed-industries/zed/issues/9792)).

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Nathan <nathan@zed.dev>

crates/sum_tree/src/tree_map.rs

@@ -5,7 +5,7 @@ use crate::{Bias, Dimension, Edit, Item, KeyedItem, SeekTarget, SumTree, Summary
 #[derive(Clone, PartialEq, Eq)]
 pub struct TreeMap<K, V>(SumTree<MapEntry<K, V>>)
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
     V: Clone + Debug;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -14,18 +14,30 @@ pub struct MapEntry<K, V> {
     value: V,
 }
 
-#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
-pub struct MapKey<K>(K);
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct MapKey<K>(Option<K>);
 
-#[derive(Clone, Debug, Default)]
+impl<K> Default for MapKey<K> {
+    fn default() -> Self {
+        Self(None)
+    }
+}
+
+#[derive(Clone, Debug)]
 pub struct MapKeyRef<'a, K>(Option<&'a K>);
 
+impl<'a, K> Default for MapKeyRef<'a, K> {
+    fn default() -> Self {
+        Self(None)
+    }
+}
+
 #[derive(Clone)]
 pub struct TreeSet<K>(TreeMap<K, ()>)
 where
-    K: Clone + Debug + Default + Ord;
+    K: Clone + Debug + Ord;
 
-impl<K: Clone + Debug + Default + Ord, V: Clone + Debug> TreeMap<K, V> {
+impl<K: Clone + Debug + Ord, V: Clone + Debug> TreeMap<K, V> {
     pub fn from_ordered_entries(entries: impl IntoIterator<Item = (K, V)>) -> Self {
         let tree = SumTree::from_iter(
             entries
@@ -44,7 +56,7 @@ impl<K: Clone + Debug + Default + Ord, V: Clone + Debug> TreeMap<K, V> {
         let mut cursor = self.0.cursor::<MapKeyRef<'_, K>>();
         cursor.seek(&MapKeyRef(Some(key)), Bias::Left, &());
         if let Some(item) = cursor.item() {
-            if *key == item.key().0 {
+            if Some(key) == item.key().0.as_ref() {
                 Some(&item.value)
             } else {
                 None
@@ -162,7 +174,7 @@ impl<K: Clone + Debug + Default + Ord, V: Clone + Debug> TreeMap<K, V> {
 
 impl<K: Debug, V: Debug> Debug for TreeMap<K, V>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
     V: Clone + Debug,
 {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
@@ -173,8 +185,8 @@ where
 #[derive(Debug)]
 struct MapSeekTargetAdaptor<'a, T>(&'a T);
 
-impl<'a, K: Debug + Clone + Default + Ord, T: MapSeekTarget<K>>
-    SeekTarget<'a, MapKey<K>, MapKeyRef<'a, K>> for MapSeekTargetAdaptor<'_, T>
+impl<'a, K: Debug + Clone + Ord, T: MapSeekTarget<K>> SeekTarget<'a, MapKey<K>, MapKeyRef<'a, K>>
+    for MapSeekTargetAdaptor<'_, T>
 {
     fn cmp(&self, cursor_location: &MapKeyRef<K>, _: &()) -> Ordering {
         if let Some(key) = &cursor_location.0 {
@@ -197,7 +209,7 @@ impl<K: Debug + Ord> MapSeekTarget<K> for K {
 
 impl<K, V> Default for TreeMap<K, V>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
     V: Clone + Debug,
 {
     fn default() -> Self {
@@ -207,7 +219,7 @@ where
 
 impl<K, V> Item for MapEntry<K, V>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
     V: Clone,
 {
     type Summary = MapKey<K>;
@@ -219,19 +231,19 @@ where
 
 impl<K, V> KeyedItem for MapEntry<K, V>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
     V: Clone,
 {
     type Key = MapKey<K>;
 
     fn key(&self) -> Self::Key {
-        MapKey(self.key.clone())
+        MapKey(Some(self.key.clone()))
     }
 }
 
 impl<K> Summary for MapKey<K>
 where
-    K: Clone + Debug + Default,
+    K: Clone + Debug,
 {
     type Context = ();
 
@@ -242,16 +254,16 @@ where
 
 impl<'a, K> Dimension<'a, MapKey<K>> for MapKeyRef<'a, K>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
 {
     fn add_summary(&mut self, summary: &'a MapKey<K>, _: &()) {
-        self.0 = Some(&summary.0)
+        self.0 = summary.0.as_ref();
     }
 }
 
 impl<'a, K> SeekTarget<'a, MapKey<K>, MapKeyRef<'a, K>> for MapKeyRef<'_, K>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
 {
     fn cmp(&self, cursor_location: &MapKeyRef<K>, _: &()) -> Ordering {
         Ord::cmp(&self.0, &cursor_location.0)
@@ -260,7 +272,7 @@ where
 
 impl<K> Default for TreeSet<K>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
 {
     fn default() -> Self {
         Self(Default::default())
@@ -269,7 +281,7 @@ where
 
 impl<K> TreeSet<K>
 where
-    K: Clone + Debug + Default + Ord,
+    K: Clone + Debug + Ord,
 {
     pub fn from_ordered_entries(entries: impl IntoIterator<Item = K>) -> Self {
         Self(TreeMap::from_ordered_entries(