Use Cow<&str> for rope grapheme iterator.

src/formatter.rs

@@ -415,7 +415,7 @@ impl<'a> Iterator for BlockVisIter<'a> {
             let mut word_width = 0;
             self.word_buf.truncate(0);
 
-            while let Some(g) = self.grapheme_itr.next().map(|g| Cow::<str>::from(g)) {
+            while let Some(g) = self.grapheme_itr.next() {
                 let width =
                     grapheme_vis_width_at_vis_pos(&g, self.pos.1 + word_width, self.tab_width);
                 self.word_buf.push((g.clone(), width));

src/graphemes.rs

@@ -156,9 +156,9 @@ impl<'a> RopeGraphemes<'a> {
 }
 
 impl<'a> Iterator for RopeGraphemes<'a> {
-    type Item = RopeSlice<'a>;
+    type Item = std::borrow::Cow<'a, str>;
 
-    fn next(&mut self) -> Option<RopeSlice<'a>> {
+    fn next(&mut self) -> Option<std::borrow::Cow<'a, str>> {
         let a = self.cursor.cur_cursor();
         let b;
         loop {
@@ -177,10 +177,22 @@ impl<'a> Iterator for RopeGraphemes<'a> {
                     self.cur_chunk_start += self.cur_chunk.len();
                     self.cur_chunk = self.chunks.next().unwrap_or("");
                 }
+                Err(GraphemeIncomplete::PreContext(idx)) => {
+                    let (chunk, byte_idx) = self.text.chunk(idx.saturating_sub(1));
+                    self.cursor.provide_context(chunk, byte_idx);
+                }
                 _ => unreachable!(),
             }
         }
 
-        Some(self.text.slice(a..b))
+        if a >= self.cur_chunk_start && b <= (self.cur_chunk_start + self.cur_chunk.len()) {
+            let a_internal = a - self.cur_chunk_start;
+            let b_internal = b - self.cur_chunk_start;
+            Some(std::borrow::Cow::Borrowed(
+                &self.cur_chunk[a_internal..b_internal],
+            ))
+        } else {
+            Some(self.text.slice(a..b).into())
+        }
     }
 }