Give the FoldMap its own edit type that does not have deleted_lines

We want to use this struct to communicate about edits performed to the fold map itself, and it will be simpler if we don't have to communicate this data.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

zed/src/editor/display_map/fold_map.rs

@@ -1,6 +1,6 @@
 use super::{
     buffer::{AnchorRangeExt, TextSummary},
-    Anchor, Buffer, DisplayPoint, Edit, Point, ToOffset,
+    Anchor, Buffer, DisplayPoint, Point, ToOffset,
 };
 use crate::{
     editor::buffer,
@@ -70,7 +70,6 @@ impl FoldMap {
                 edits.push(Edit {
                     old_bytes: range.clone(),
                     new_bytes: range.clone(),
-                    ..Default::default()
                 });
             }
         }
@@ -116,7 +115,6 @@ impl FoldMap {
                 edits.push(Edit {
                     old_bytes: offset_range.clone(),
                     new_bytes: offset_range,
-                    ..Default::default()
                 });
                 fold_ixs_to_delete.push(*folds_cursor.start());
                 folds_cursor.next(&buffer);
@@ -147,7 +145,10 @@ impl FoldMap {
 
     fn sync(&self, cx: &AppContext) -> MutexGuard<SumTree<Transform>> {
         let buffer = self.buffer.read(cx);
-        let mut edits = buffer.edits_since(self.last_sync.lock().clone()).peekable();
+        let mut edits = buffer
+            .edits_since(self.last_sync.lock().clone())
+            .map(Into::into)
+            .peekable();
         if edits.peek().is_some() {
             self.apply_edits(edits, cx);
         }
@@ -838,6 +839,34 @@ impl<'a> sum_tree::Dimension<'a, TransformSummary> for usize {
     }
 }
 
+struct Edit {
+    old_bytes: Range<usize>,
+    new_bytes: Range<usize>,
+}
+
+impl Edit {
+    pub fn delta(&self) -> isize {
+        self.inserted_bytes() as isize - self.deleted_bytes() as isize
+    }
+
+    pub fn deleted_bytes(&self) -> usize {
+        self.old_bytes.len()
+    }
+
+    pub fn inserted_bytes(&self) -> usize {
+        self.new_bytes.len()
+    }
+}
+
+impl From<buffer::Edit> for Edit {
+    fn from(edit: buffer::Edit) -> Self {
+        Self {
+            old_bytes: edit.old_bytes,
+            new_bytes: edit.new_bytes,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;