fix(rust): Reject non-integral offset and length in AExpr::Slice

crates/polars-plan/src/logical_plan/conversion/type_coercion/mod.rs

@@ -440,6 +440,16 @@ impl OptimizationRule for TypeCoercionRule {
                     options,
                 })
             },
+            AExpr::Slice { offset, length, .. } => {
+                let input_schema = get_schema(lp_arena, lp_node);
+                let (_, offset_dtype) =
+                    unpack!(get_aexpr_and_type(expr_arena, offset, &input_schema));
+                polars_ensure!(offset_dtype.is_integer(), InvalidOperation: "offset must be integral for slice, not {}", offset_dtype);
+                let (_, length_dtype) =
+                    unpack!(get_aexpr_and_type(expr_arena, length, &input_schema));
+                polars_ensure!(length_dtype.is_integer(), InvalidOperation: "length must be integral for slice, not {}", length_dtype);
+                None
+            },
             _ => None,
         };
         Ok(out)

crates/polars/tests/it/lazy/expressions/slice.rs

@@ -15,7 +15,7 @@ fn test_slice_args() -> PolarsResult<()> {
     ]?
     .lazy()
     .group_by_stable([col("groups")])
-    .agg([col("vals").slice(lit(0i64), len() * lit(0.2))])
+    .agg([col("vals").slice(lit(0i64), (len() * lit(0.2)).cast(DataType::Int32))])
     .collect()?;
 
     let out = df.column("vals")?.explode()?;

py-polars/polars/expr/expr.py

@@ -4851,7 +4851,11 @@ def tail(self, n: int | Expr = 10) -> Self:
         │ 7   │
         └─────┘
         """
-        offset = -self._from_pyexpr(parse_into_expression(n))
+        # This cast enables tail with expressions that return unsigned integers,
+        # for which negate otherwise raises InvalidOperationError.
+        offset = -self._from_pyexpr(
+            parse_into_expression(n).cast(Int64, strict=False, wrap_numerical=True)
+        )
         return self.slice(offset, n)
 
     def limit(self, n: int | Expr = 10) -> Self:

py-polars/tests/unit/dataframe/test_df.py

@@ -1976,7 +1976,7 @@ def test_group_by_slice_expression_args() -> None:
 
     out = (
         df.group_by("groups", maintain_order=True)
-        .agg([pl.col("vals").slice(pl.len() * 0.1, (pl.len() // 5))])
+        .agg([pl.col("vals").slice((pl.len() * 0.1).cast(int), (pl.len() // 5))])
         .explode("vals")
     )
 

py-polars/tests/unit/expr/test_exprs.py

@@ -678,7 +678,7 @@ def test_head() -> None:
     assert df.select(pl.col("a").head(10)).to_dict(as_series=False) == {
         "a": [1, 2, 3, 4, 5]
     }
-    assert df.select(pl.col("a").head(pl.len() / 2)).to_dict(as_series=False) == {
+    assert df.select(pl.col("a").head(pl.len() // 2)).to_dict(as_series=False) == {
         "a": [1, 2]
     }
 
@@ -690,7 +690,7 @@ def test_tail() -> None:
     assert df.select(pl.col("a").tail(10)).to_dict(as_series=False) == {
         "a": [1, 2, 3, 4, 5]
     }
-    assert df.select(pl.col("a").tail(pl.len() / 2)).to_dict(as_series=False) == {
+    assert df.select(pl.col("a").tail(pl.len() // 2)).to_dict(as_series=False) == {
         "a": [4, 5]
     }
 
@@ -732,3 +732,16 @@ def test_replace_no_cse() -> None:
         .explain()
     )
     assert "POLARS_CSER" not in plan
+
+
+def test_slice_rejects_non_integral() -> None:
+    df = pl.LazyFrame({"a": [0, 1, 2, 3], "b": [1.5, 2, 3, 4]})
+
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        df.select(pl.col("a").slice(pl.col("b").slice(0, 1), None)).collect()
+
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        df.select(pl.col("a").slice(0, pl.col("b").slice(1, 2))).collect()
+
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        df.select(pl.col("a").slice(pl.lit("1"), None)).collect()