Fix hash's error message when hash method is None (RustPython#4161)

Author: jopemachine

Lib/test/test_dataclasses.py

@@ -2536,8 +2536,6 @@ class C:
         self.assertEqual(hash(C(4)), hash((4,)))
         self.assertEqual(hash(C(42)), hash((42,)))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_hash_no_args(self):
         # Test dataclasses with no hash= argument.  This exists to
         #  make sure that if the @dataclass parameter name is changed

vm/src/protocol/object.rs

@@ -517,10 +517,19 @@ impl PyObject {
     }
 
     pub fn hash(&self, vm: &VirtualMachine) -> PyResult<PyHash> {
+        let hash = self.get_class_attr(identifier!(vm, __hash__)).unwrap();
+        if vm.is_none(&hash) {
+            return Err(vm.new_exception_msg(
+                vm.ctx.exceptions.type_error.to_owned(),
+                format!("unhashable type: '{}'", self.class().name()),
+            ));
+        }
+
         let hash = self
             .class()
             .mro_find_map(|cls| cls.slots.hash.load())
-            .unwrap(); // hash always exist
+            .unwrap();
+
         hash(self, vm)
     }