feat(autocast): add autocast for digits argument to ops.Round

Ibis literals default to Int8, which makes this a pain for users to
type. Since substrait only allows Int32 for the `digits` arg, we
autocast to int32 from any int type

ibis_substrait/compiler/translate.py

@@ -1502,3 +1502,17 @@ def _upcast_string_op(op: string_op) -> string_op:
         for newop in op.args
     ]
     return type(op)(*casted_args)
+
+
+@_upcast.register(ops.Round)
+def _upcast_round_digits(op: ops.Round) -> ops.Round:
+    # Substrait wants Int32 for decimal place argument to round
+    if op.digits is None:
+        raise ValueError(
+            "Substrait requires that a rounding operation specify the number of digits to round to"
+        )
+    elif not isinstance(op.digits.dtype, dt.Int32):
+        return ops.Round(
+            op.arg, op.digits.copy(dtype=dt.Int32(nullable=op.digits.dtype.nullable))
+        )
+    return op

ibis_substrait/tests/compiler/test_extensions.py

@@ -358,3 +358,30 @@ def test_extension_arithmetic_multiple_signatures(compiler):
     assert "add:fp32_fp32" in scalar_func_names
     assert "subtract:i64_i64" in scalar_func_names
     assert "subtract:fp32_fp32" in scalar_func_names
+
+
+_TYPE_MAPPING = {
+    "int8": "i8",
+    "int16": "i16",
+    "int32": "i32",
+    "int64": "i64",
+    "float32": "fp32",
+    "float64": "fp64",
+}
+
+
+@pytest.mark.parametrize(
+    "col_dtype", ["float32", "float64", "int8", "int16", "int32", "int64"]
+)
+@pytest.mark.parametrize("digits_dtype", ["int8", "int16", "int32", "int64"])
+def test_extension_round_upcast(compiler, col_dtype, digits_dtype):
+    t = ibis.table([("col", col_dtype)], name="t")
+
+    query = t.mutate(col=t.col.round(ibis.literal(8, type=digits_dtype)))
+    plan = compiler.compile(query)
+
+    scalar_func_names = [
+        extension.extension_function.name for extension in plan.extensions
+    ]
+
+    assert f"round:{_TYPE_MAPPING[col_dtype]}_i32" in scalar_func_names