Don't remove double casts in cudf_polars (#20773)

closes pola-rs/polars#25564

This was added back in #16192, but it probably pre-dated some improvements in aggregation expressions.

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #20773

Author: mroeschke

python/cudf_polars/cudf_polars/dsl/translate.py

@@ -1011,11 +1011,8 @@ def _(
     # Push casts into literals so we can handle Cast(Literal(Null))
     if isinstance(inner, expr.Literal):
         return inner.astype(dtype)
-    elif isinstance(inner, expr.Cast):
-        # Translation of Len/Count-agg put in a cast, remove double
-        # casts if we have one.
-        (inner,) = inner.children
-    return expr.Cast(dtype, inner)
+    else:
+        return expr.Cast(dtype, inner)
 
 
 @_translate_expr.register

python/cudf_polars/tests/expressions/test_casting.py

@@ -52,3 +52,9 @@ def test_cast_unsupported(tests):
     assert_ir_translation_raises(
         df.select(pl.col("a").cast(totype)), NotImplementedError
     )
+
+
+def test_allow_double_cast():
+    df = pl.LazyFrame({"c0": [1000]})
+    query = df.select(pl.col("c0").cast(pl.Boolean).cast(pl.Int8))
+    assert_gpu_result_equal(query)