fix(datatypes): give a more useful error message when trying to cast structs with different fields (#10214)

Author: NickCrews

ibis/expr/datatypes/cast.py

@@ -116,7 +116,8 @@ def castable(source: dt.DataType, target: dt.DataType, value: Any = None) -> boo
         )
     elif target.is_struct():
         return source.is_struct() and all(
-            castable(source[field], target[field]) for field in target.names
+            field in source and castable(source[field], target[field])
+            for field in target.names
         )
     elif target.is_geospatial():
         return source.is_geospatial() or source.is_array()

ibis/expr/datatypes/tests/test_cast.py

@@ -101,3 +101,21 @@ def test_implicitly_castable_int_to_bool(value):
 )
 def test_implicitly_uncastable_values(source, target, value):
     assert not source.castable(target, value=value)
+
+
+def test_struct_different_fields():
+    x = dt.Struct({"x": dt.int32})
+    x2 = dt.Struct({"x": dt.int64})
+    y = dt.Struct({"y": dt.int32})
+    xy = dt.Struct({"x": dt.int32, "y": dt.int32})
+
+    # Can upcast int32 to int64, but not other way
+    assert x.castable(x2)
+    assert not x2.castable(x)
+    # Can remove a field, but not add one
+    assert xy.castable(x)
+    assert not x.castable(xy)
+
+    # Missing fields entirely from each other
+    assert not x.castable(y)
+    assert not y.castable(x)