Handle empty diff excerpts (#24168)

Release Notes:

- Fix display, revert and undo of deleted hunks when the file is empty.

crates/git/src/diff.rs

@@ -80,6 +80,20 @@ impl BufferDiff {
         let buffer_text = buffer.as_rope().to_string();
         let patch = Self::diff(diff_base, &buffer_text);
 
+        // A common case in Zed is that the empty buffer is represented as just a newline,
+        // but if we just compute a naive diff you get a "preserved" line in the middle,
+        // which is a bit odd.
+        if buffer_text == "\n" && diff_base.ends_with("\n") && diff_base.len() > 1 {
+            tree.push(
+                InternalDiffHunk {
+                    buffer_range: buffer.anchor_before(0)..buffer.anchor_before(0),
+                    diff_base_byte_range: 0..diff_base.len() - 1,
+                },
+                buffer,
+            );
+            return Self { tree };
+        }
+
         if let Some(patch) = patch {
             let mut divergence = 0;
             for hunk_index in 0..patch.num_hunks() {

crates/multi_buffer/src/multi_buffer.rs

@@ -2875,15 +2875,6 @@ impl MultiBuffer {
         // Visit each excerpt that intersects the edit.
         let mut did_expand_hunks = false;
         while let Some(excerpt) = excerpts.item() {
-            if excerpt.text_summary.len == 0 {
-                if excerpts.end(&()) <= edit.new.end {
-                    excerpts.next(&());
-                    continue;
-                } else {
-                    break;
-                }
-            }
-
             // Recompute the expanded hunks in the portion of the excerpt that
             // intersects the edit.
             if let Some(diff_state) = snapshot.diffs.get(&excerpt.buffer_id) {
@@ -3602,8 +3593,10 @@ impl MultiBufferSnapshot {
             value: None,
         });
 
-        if cursor.region().is_some_and(|region| !region.is_main_buffer) {
-            cursor.prev();
+        if let Some(region) = cursor.region().filter(|region| !region.is_main_buffer) {
+            if region.range.start.key > 0 {
+                cursor.prev()
+            }
         }
 
         iter::from_fn(move || loop {
@@ -6044,17 +6037,13 @@ where
     }
 
     fn main_buffer_position(&self) -> Option<D> {
-        if let DiffTransform::BufferContent { .. } = self.diff_transforms.next_item()? {
-            let excerpt = self.excerpts.item()?;
-            let buffer = &excerpt.buffer;
-            let buffer_context_start = excerpt.range.context.start.summary::<D>(buffer);
-            let mut buffer_start = buffer_context_start;
-            let overshoot = self.diff_transforms.end(&()).1 .0 - self.excerpts.start().0;
-            buffer_start.add_assign(&overshoot);
-            Some(buffer_start)
-        } else {
-            None
-        }
+        let excerpt = self.excerpts.item()?;
+        let buffer = &excerpt.buffer;
+        let buffer_context_start = excerpt.range.context.start.summary::<D>(buffer);
+        let mut buffer_start = buffer_context_start;
+        let overshoot = self.diff_transforms.end(&()).1 .0 - self.excerpts.start().0;
+        buffer_start.add_assign(&overshoot);
+        Some(buffer_start)
     }
 
     fn build_region(&self) -> Option<MultiBufferRegion<'a, D>> {

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -989,6 +989,79 @@ fn test_empty_multibuffer(cx: &mut App) {
     );
 }
 
+#[gpui::test]
+fn test_empty_diff_excerpt(cx: &mut TestAppContext) {
+    let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
+    let buffer = cx.new(|cx| Buffer::local("", cx));
+    let base_text = "a\nb\nc";
+
+    let change_set = cx.new(|cx| {
+        let snapshot = buffer.read(cx).snapshot();
+        let mut change_set = BufferChangeSet::new(&buffer, cx);
+        let _ = change_set.set_base_text(base_text.into(), snapshot.text, cx);
+        change_set
+    });
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.set_all_diff_hunks_expanded(cx);
+        multibuffer.add_change_set(change_set.clone(), cx);
+        multibuffer.push_excerpts(
+            buffer.clone(),
+            [ExcerptRange {
+                context: 0..0,
+                primary: None,
+            }],
+            cx,
+        );
+    });
+    cx.run_until_parked();
+
+    let snapshot = multibuffer.update(cx, |multibuffer, cx| multibuffer.snapshot(cx));
+    assert_eq!(snapshot.text(), "a\nb\nc\n");
+
+    let hunk = snapshot
+        .diff_hunks_in_range(Point::new(1, 1)..Point::new(1, 1))
+        .next()
+        .unwrap();
+
+    assert_eq!(hunk.diff_base_byte_range.start, 0);
+
+    let buf2 = cx.new(|cx| Buffer::local("X", cx));
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.push_excerpts(
+            buf2,
+            [ExcerptRange {
+                context: 0..1,
+                primary: None,
+            }],
+            cx,
+        );
+    });
+
+    buffer.update(cx, |buffer, cx| {
+        buffer.edit([(0..0, "a\nb\nc")], None, cx);
+        change_set.update(cx, |change_set, cx| {
+            let _ = change_set.recalculate_diff(buffer.snapshot().text, cx);
+        });
+        assert_eq!(buffer.text(), "a\nb\nc")
+    });
+    cx.run_until_parked();
+
+    let snapshot = multibuffer.update(cx, |multibuffer, cx| multibuffer.snapshot(cx));
+    assert_eq!(snapshot.text(), "a\nb\nc\nX");
+
+    buffer.update(cx, |buffer, cx| {
+        buffer.undo(cx);
+        change_set.update(cx, |change_set, cx| {
+            let _ = change_set.recalculate_diff(buffer.snapshot().text, cx);
+        });
+        assert_eq!(buffer.text(), "")
+    });
+    cx.run_until_parked();
+
+    let snapshot = multibuffer.update(cx, |multibuffer, cx| multibuffer.snapshot(cx));
+    assert_eq!(snapshot.text(), "a\nb\nc\n\nX");
+}
+
 #[gpui::test]
 fn test_singleton_multibuffer_anchors(cx: &mut App) {
     let buffer = cx.new(|cx| Buffer::local("abcd", cx));