Remove special case for singleton buffers from MultiBufferSnapshot::anchor_at (#36524)

This may be responsible for a panic that we've been seeing with
increased frequency in agent2 threads.

Release Notes:

- N/A

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/editor/src/editor.rs

@@ -4876,11 +4876,7 @@ impl Editor {
         cx: &mut Context<Self>,
     ) -> bool {
         let position = self.selections.newest_anchor().head();
-        let multibuffer = self.buffer.read(cx);
-        let Some(buffer) = position
-            .buffer_id
-            .and_then(|buffer_id| multibuffer.buffer(buffer_id))
-        else {
+        let Some(buffer) = self.buffer.read(cx).buffer_for_anchor(position, cx) else {
             return false;
         };
 
@@ -5844,7 +5840,7 @@ impl Editor {
             multibuffer_anchor.start.to_offset(&snapshot)
                 ..multibuffer_anchor.end.to_offset(&snapshot)
         };
-        if newest_anchor.head().buffer_id != Some(buffer.remote_id()) {
+        if snapshot.buffer_id_for_anchor(newest_anchor.head()) != Some(buffer.remote_id()) {
             return None;
         }
 

crates/multi_buffer/src/multi_buffer.rs

@@ -2196,6 +2196,15 @@ impl MultiBuffer {
             })
     }
 
+    pub fn buffer_for_anchor(&self, anchor: Anchor, cx: &App) -> Option<Entity<Buffer>> {
+        if let Some(buffer_id) = anchor.buffer_id {
+            self.buffer(buffer_id)
+        } else {
+            let (_, buffer, _) = self.excerpt_containing(anchor, cx)?;
+            Some(buffer)
+        }
+    }
+
     // If point is at the end of the buffer, the last excerpt is returned
     pub fn point_to_buffer_offset<T: ToOffset>(
         &self,
@@ -5228,15 +5237,6 @@ impl MultiBufferSnapshot {
             excerpt_offset += ExcerptOffset::new(offset_in_transform);
         };
 
-        if let Some((excerpt_id, buffer_id, buffer)) = self.as_singleton() {
-            return Anchor {
-                buffer_id: Some(buffer_id),
-                excerpt_id: *excerpt_id,
-                text_anchor: buffer.anchor_at(excerpt_offset.value, bias),
-                diff_base_anchor,
-            };
-        }
-
         let mut excerpts = self
             .excerpts
             .cursor::<Dimensions<ExcerptOffset, Option<ExcerptId>>>(&());
@@ -5260,10 +5260,17 @@ impl MultiBufferSnapshot {
                 text_anchor,
                 diff_base_anchor,
             }
-        } else if excerpt_offset.is_zero() && bias == Bias::Left {
-            Anchor::min()
         } else {
-            Anchor::max()
+            let mut anchor = if excerpt_offset.is_zero() && bias == Bias::Left {
+                Anchor::min()
+            } else {
+                Anchor::max()
+            };
+            // TODO this is a hack, remove it
+            if let Some((excerpt_id, _, _)) = self.as_singleton() {
+                anchor.excerpt_id = *excerpt_id;
+            }
+            anchor
         }
     }
 
@@ -6305,6 +6312,14 @@ impl MultiBufferSnapshot {
         })
     }
 
+    pub fn buffer_id_for_anchor(&self, anchor: Anchor) -> Option<BufferId> {
+        if let Some(id) = anchor.buffer_id {
+            return Some(id);
+        }
+        let excerpt = self.excerpt_containing(anchor..anchor)?;
+        Some(excerpt.buffer_id())
+    }
+
     pub fn selections_in_range<'a>(
         &'a self,
         range: &'a Range<Anchor>,
@@ -6983,19 +6998,20 @@ impl Excerpt {
     }
 
     fn contains(&self, anchor: &Anchor) -> bool {
-        Some(self.buffer_id) == anchor.buffer_id
-            && self
-                .range
-                .context
-                .start
-                .cmp(&anchor.text_anchor, &self.buffer)
-                .is_le()
-            && self
-                .range
-                .context
-                .end
-                .cmp(&anchor.text_anchor, &self.buffer)
-                .is_ge()
+        anchor.buffer_id == None
+            || anchor.buffer_id == Some(self.buffer_id)
+                && self
+                    .range
+                    .context
+                    .start
+                    .cmp(&anchor.text_anchor, &self.buffer)
+                    .is_le()
+                && self
+                    .range
+                    .context
+                    .end
+                    .cmp(&anchor.text_anchor, &self.buffer)
+                    .is_ge()
     }
 
     /// The [`Excerpt`]'s start offset in its [`Buffer`]