display map: Fix left shift debug panic (#38656)

Anthony Eid created 5 days ago

Closes https://github.com/zed-industries/zed/issues/38558

The bug occurred because TabStopCursor chunk_position.1 is bounded
between 0 and 128. The fix for this was changing the bound to 0 and 127.

This also allowed me to simplify some of the tab stop cursor code to be
a bit faster (less branches and unbounded shifts).

Release Notes:

- N/A

Change summary

crates/editor/src/display_map/tab_map.rs | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

Detailed changes

crates/editor/src/display_map/tab_map.rs

@@ -1440,7 +1440,7 @@ where
         self.current_chunk.as_ref().and_then(|(chunk, idx)| {
             let mut idx = *idx;
             let mut diff = 0;
-            while idx > 0 && chunk.chars & (1 << idx) == 0 {
+            while idx > 0 && chunk.chars & (1u128.unbounded_shl(idx)) == 0 {
                 idx -= 1;
                 diff += 1;
             }
@@ -1460,7 +1460,7 @@ where
     fn is_char_boundary(&self) -> bool {
         self.current_chunk
             .as_ref()
-            .is_some_and(|(chunk, idx)| (chunk.chars & (1 << *idx.min(&127))) != 0)
+            .is_some_and(|(chunk, idx)| (chunk.chars & 1u128.unbounded_shl(*idx)) != 0)
     }
 
     /// distance: length to move forward while searching for the next tab stop
@@ -1483,18 +1483,20 @@ where
 
                     self.byte_offset += overshoot;
                     self.char_offset += get_char_offset(
-                        chunk_position..(chunk_position + overshoot).saturating_sub(1).min(127),
+                        chunk_position..(chunk_position + overshoot).saturating_sub(1),
                         chunk.chars,
                     );
 
-                    self.current_chunk = Some((chunk, chunk_position + overshoot));
+                    if chunk_position + overshoot < 128 {
+                        self.current_chunk = Some((chunk, chunk_position + overshoot));
+                    }
 
                     return None;
                 }
 
                 self.byte_offset += chunk_distance;
                 self.char_offset += get_char_offset(
-                    chunk_position..(chunk_position + chunk_distance).saturating_sub(1).min(127),
+                    chunk_position..(chunk_position + chunk_distance).saturating_sub(1),
                     chunk.chars,
                 );
                 distance_traversed += chunk_distance;
@@ -1546,8 +1548,6 @@ where
 
 #[inline(always)]
 fn get_char_offset(range: Range<u32>, bit_map: u128) -> u32 {
-    // This edge case can happen when we're at chunk position 128
-
     if range.start == range.end {
         return if (1u128 << range.start) & bit_map == 0 {
             0
@@ -1555,7 +1555,7 @@ fn get_char_offset(range: Range<u32>, bit_map: u128) -> u32 {
             1
         };
     }
-    let end_shift: u128 = 127u128 - range.end.min(127) as u128;
+    let end_shift: u128 = 127u128 - range.end as u128;
     let mut bit_mask = (u128::MAX >> range.start) << range.start;
     bit_mask = (bit_mask << end_shift) >> end_shift;
     let bit_map = bit_map & bit_mask;