fix(rust): index_to_chunked_index's fast path is not correct (#11710)

crates/polars-core/src/chunked_array/ops/any_value.rs

@@ -187,12 +187,12 @@ macro_rules! get_any_value_unchecked {
 
 macro_rules! get_any_value {
     ($self:ident, $index:expr) => {{
-        let (chunk_idx, idx) = $self.index_to_chunked_index($index);
-        let arr = &*$self.chunks[chunk_idx];
-        polars_ensure!(idx < arr.len(), oob = idx, arr.len());
+        if $index >= $self.len() {
+            polars_bail!(oob = $index, $self.len());
+        }
         // SAFETY
         // bounds are checked
-        Ok(unsafe { arr_to_any_value(arr, idx, $self.dtype()) })
+        Ok(unsafe { $self.get_any_value_unchecked($index) })
     }};
 }
 

crates/polars-core/src/chunked_array/ops/downcast.rs

@@ -99,7 +99,13 @@ impl<T: PolarsDataType> ChunkedArray<T> {
     #[inline]
     pub(crate) fn index_to_chunked_index(&self, index: usize) -> (usize, usize) {
         if self.chunks.len() == 1 {
-            return (0, index);
+            // SAFETY: chunks.len() == 1 guarantees this is correct.
+            let len = unsafe { self.chunks.get_unchecked(0).len() };
+            return if index < len {
+                (0, index)
+            } else {
+                (1, index - len)
+            };
         }
         index_to_chunked_index(self.downcast_iter().map(|arr| arr.len()), index)
     }

crates/polars-ops/src/chunked_array/strings/concat.rs

@@ -65,11 +65,6 @@ pub fn hor_str_concat(cas: &[&Utf8Chunked], delimiter: &str) -> PolarsResult<Utf
         ComputeError: "all series in `hor_str_concat` should have equal or unit length"
     );
 
-    let has_empty_ca = cas.iter().any(|ca| ca.is_empty());
-    if has_empty_ca {
-        return Ok(Utf8Chunked::full_null(cas[0].name(), 0));
-    }
-
     // Calculate total capacity needed.
     let tot_strings_bytes: usize = cas
         .iter()