fix: proper typetracer array slicing in BitMaskedArray (#3028)

* fix: proper typetracer array slicing in BitMaskedArray

* test: add test

src/awkward/contents/bitmaskedarray.py

@@ -471,7 +471,9 @@ def mask_as_bool(self, valid_when=None):
                 self._lsb_order,
             )
         )
-        return bytemask.data[: self._length].view(np.bool_)
+        return bytemask.data[
+            : self._backend.index_nplike.shape_item_as_index(self._length)
+        ].view(np.bool_)
 
     def _getitem_nothing(self):
         return self._content._getitem_range(0, 0)

tests/test_3028_mask_bitmaskedarray.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+import numpy as np
+
+import awkward as ak
+from awkward.typetracer import unknown_length
+
+
+def test():
+    arr = (
+        ak.mask([1, 2, 3], [True, False, False], highlevel=False)
+        .to_BitMaskedArray(valid_when=True, lsb_order=True)
+        .to_typetracer(forget_length=True)
+    )
+
+    result = arr.mask_as_bool()
+    assert result.dtype == np.dtype("bool")
+    assert result.ndim == 1
+    assert result.size is unknown_length