fix(datatypes): disallow empty structs when parsing

ibis/expr/datatypes/parse.py

@@ -195,7 +195,7 @@ def geotype_parser(typ: type[dt.DataType]) -> dt.DataType:
     struct = (
         spaceless_string("struct")
         .then(LANGLE)
-        .then(parsy.seq(spaceless(FIELD).skip(COLON), ty).sep_by(COMMA))
+        .then(parsy.seq(spaceless(FIELD).skip(COLON), ty).sep_by(COMMA, min=1))
         .skip(COMMA.optional())
         .skip(RANGLE)
         .map(dt.Struct.from_tuples)

ibis/expr/datatypes/tests/test_parse.py

@@ -159,14 +159,39 @@ def test_struct_with_string_types():
     )
 
 
-def test_struct_trailing_comma_single():
-    result = dt.dtype("struct<a: int32,>")
-    assert result == dt.Struct.from_tuples([("a", dt.int32)])
+@pytest.mark.parametrize(
+    ("type_string", "expected_dtype"),
+    [
+        ("struct<a: int32,>", {"a": dt.int32}),
+        ("struct<a: int32, b: string  , >", {"a": dt.int32, "b": dt.string}),
+    ],
+    ids=["single_field", "multiple_fields"],
+)
+def test_struct_trailing_comma(type_string, expected_dtype):
+    result = dt.dtype(type_string)
+    assert result == dt.Struct(expected_dtype)
 
 
-def test_struct_trailing_comma_multi():
-    result = dt.dtype("struct<a: int32, b: string  , >")
-    assert result == dt.Struct.from_tuples([("a", dt.int32), ("b", dt.string)])
+@pytest.mark.parametrize(
+    "invalid_type_string",
+    [
+        "struct<,>",
+        "struct<a,>",
+        "struct<b: ,>",
+        "struct<c:in,>",
+        "struct<a:int,b:int64,,>",
+    ],
+    ids=[
+        "missing_key",
+        "missing_colon",
+        "missing_value",
+        "invalid_type",
+        "double_comma",
+    ],
+)
+def test_struct_trailing_comma_invalid(invalid_type_string):
+    with pytest.raises(parsy.ParseError):
+        dt.dtype(invalid_type_string)
 
 
 def test_array_with_string_value_types():