Fix incorrect mask in BufferChunks::next

crates/language/src/buffer.rs

@@ -5798,7 +5798,9 @@ impl<'a> Iterator for BufferChunks<'a> {
 
             let slice = &chunk[bit_start..bit_end];
 
-            let mask = 1u128.unbounded_shl(bit_end as u32).wrapping_sub(1);
+            let mask = 1u128
+                .unbounded_shl((bit_end - bit_start) as u32)
+                .wrapping_sub(1);
             let tabs = (tabs >> bit_start) & mask;
             let chars = (chars_map >> bit_start) & mask;
             let newlines = (newlines >> bit_start) & mask;

crates/language/src/buffer_tests.rs

@@ -4192,3 +4192,48 @@ fn test_random_chunk_bitmaps(cx: &mut App, mut rng: StdRng) {
         }
     }
 }
+
+#[gpui::test]
+fn test_formatted_chunks(cx: &mut gpui::App) {
+    init_settings(cx, |_| {});
+    let buffer = cx.new(|cx| Buffer::local("use std::cmp::Eq;", cx).with_language(rust_lang(), cx));
+    let snapshot = buffer.read(cx).snapshot();
+
+    let chunks = snapshot.chunks(
+        0..snapshot.len(),
+        LanguageAwareStyling {
+            tree_sitter: true,
+            diagnostics: false,
+        },
+    );
+
+    for chunk in chunks {
+        let chunk_text = chunk.text;
+        let chars_bitmap = chunk.chars;
+
+        // Verify chars bitmap
+        let char_indices = chunk_text
+            .char_indices()
+            .map(|(i, _)| i)
+            .collect::<Vec<_>>();
+
+        assert_eq!(char_indices.len() as u32, chars_bitmap.count_ones());
+
+        for byte_idx in 0..chunk_text.len() {
+            let should_have_bit = char_indices.contains(&byte_idx);
+            let has_bit = chars_bitmap & (1 << byte_idx) != 0;
+
+            if has_bit != should_have_bit {
+                eprintln!("Chunk text bytes: {:?}", chunk_text.as_bytes());
+                eprintln!("Char indices: {:?}", char_indices);
+                eprintln!("Chars bitmap: {:#b}", chars_bitmap);
+            }
+
+            assert_eq!(
+                has_bit, should_have_bit,
+                "Chars bitmap mismatch at byte index {} in chunk {:?}. Expected bit: {}, Got bit: {}",
+                byte_idx, chunk_text, should_have_bit, has_bit
+            );
+        }
+    }
+}