Indent guides (#11503)

Builds on top of existing work from #2249, but here's a showcase:


https://github.com/zed-industries/zed/assets/53836821/4b346965-6654-496c-b379-75425d9b493f

TODO:
- [x] handle line wrapping
- [x] implement handling in multibuffer (crashes currently)
- [x] add configuration option
- [x] new theme properties? What colors to use?
- [x] Possibly support indents with different colors or background
colors
- [x] investigate edge cases (e.g. indent guides and folds continue on
empty lines even if the next indent is different)
- [x] add more tests (also test `find_active_indent_index`)
- [x] docs (will do in a follow up PR)
- [x] benchmark performance impact

Release Notes:

- Added indent guides
([#5373](https://github.com/zed-industries/zed/issues/5373))

---------

Co-authored-by: Nate Butler <1714999+iamnbutler@users.noreply.github.com>
Co-authored-by: Remco <djsmits12@gmail.com>

crates/rope/src/rope.rs

@@ -619,10 +619,12 @@ impl<'a> Chunks<'a> {
     }
 
     pub fn lines(self) -> Lines<'a> {
+        let reversed = self.reversed;
         Lines {
             chunks: self,
             current_line: String::new(),
             done: false,
+            reversed,
         }
     }
 }
@@ -726,6 +728,7 @@ pub struct Lines<'a> {
     chunks: Chunks<'a>,
     current_line: String,
     done: bool,
+    reversed: bool,
 }
 
 impl<'a> Lines<'a> {
@@ -737,13 +740,26 @@ impl<'a> Lines<'a> {
         self.current_line.clear();
 
         while let Some(chunk) = self.chunks.peek() {
-            let mut lines = chunk.split('\n').peekable();
-            while let Some(line) = lines.next() {
-                self.current_line.push_str(line);
-                if lines.peek().is_some() {
-                    self.chunks
-                        .seek(self.chunks.offset() + line.len() + "\n".len());
-                    return Some(&self.current_line);
+            let lines = chunk.split('\n');
+            if self.reversed {
+                let mut lines = lines.rev().peekable();
+                while let Some(line) = lines.next() {
+                    self.current_line.insert_str(0, line);
+                    if lines.peek().is_some() {
+                        self.chunks
+                            .seek(self.chunks.offset() - line.len() - "\n".len());
+                        return Some(&self.current_line);
+                    }
+                }
+            } else {
+                let mut lines = lines.peekable();
+                while let Some(line) = lines.next() {
+                    self.current_line.push_str(line);
+                    if lines.peek().is_some() {
+                        self.chunks
+                            .seek(self.chunks.offset() + line.len() + "\n".len());
+                        return Some(&self.current_line);
+                    }
                 }
             }
 
@@ -1355,6 +1371,21 @@ mod tests {
         assert_eq!(lines.next(), Some("hi"));
         assert_eq!(lines.next(), Some(""));
         assert_eq!(lines.next(), None);
+
+        let rope = Rope::from("abc\ndefg\nhi");
+        let mut lines = rope.reversed_chunks_in_range(0..rope.len()).lines();
+        assert_eq!(lines.next(), Some("hi"));
+        assert_eq!(lines.next(), Some("defg"));
+        assert_eq!(lines.next(), Some("abc"));
+        assert_eq!(lines.next(), None);
+
+        let rope = Rope::from("abc\ndefg\nhi\n");
+        let mut lines = rope.reversed_chunks_in_range(0..rope.len()).lines();
+        assert_eq!(lines.next(), Some(""));
+        assert_eq!(lines.next(), Some("hi"));
+        assert_eq!(lines.next(), Some("defg"));
+        assert_eq!(lines.next(), Some("abc"));
+        assert_eq!(lines.next(), None);
     }
 
     #[gpui::test(iterations = 100)]