Make mappingproxy's nb_or slot symmetric for dict | mp (#7723)

* Make mappingproxy's nb_or slot symmetric for dict | mp

CPython's mappingproxy_or (Objects/descrobject.c) unwraps a mappingproxy
on either side of '|' to its underlying mapping and delegates back to
PyNumber_Or. This makes 'dict | mp', 'mp | dict', and 'mp | mp' all
produce a plain dict result without dict's own nb_or having to know
about mappingproxy.

RustPython's 'or' slot in 'AsNumber for PyMappingProxy' only handled
the case where the left operand is a mappingproxy. When the slot fired
with a=dict, b=mp (because dict.nb_or returned NotImplemented), it
returned NotImplemented again — so 'dict | mp' raised TypeError. This
propagated to 'UserDict | mappingproxy' (which calls self.data | other).

Rewrite the slot to unwrap a mappingproxy on either side (or both)
before delegating to vm._or on the underlying mappings. The
inplace_or slot is unchanged — '|=' is still rejected.

Unmasks test_userdict.UserDictTest.test_mixed_or.

* Unmask test_types.MappingProxyTests.test_union

The slot fix in this PR also enables this test, which was marked
expectedFailure for the same dict|mappingproxy TypeError. Caught as
UNEXPECTED SUCCESS in CI.

Lib/test/test_types.py

@@ -1362,7 +1362,6 @@ class MappingProxyTests(unittest.TestCase):
         self.assertEqual(view['key1'], 70)
         self.assertEqual(copy['key1'], 27)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; TypeError: unsupported operand type(s) for |: 'dict' and 'mappingproxy'
     def test_union(self):
         mapping = {'a': 0, 'b': 1, 'c': 2}
         view = self.mappingproxy(mapping)

Lib/test/test_userdict.py

@@ -244,7 +244,6 @@ class UserDictTest(mapping_tests.TestHashMappingProtocol):
 
     test_repr_deep = mapping_tests.TestHashMappingProtocol.test_repr_deep
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; TypeError: unsupported operand type(s) for |: 'UserDict' and 'mappingproxy'
     def test_mixed_or(self):
         for t in UserDict, dict, types.MappingProxyType:
             with self.subTest(t.__name__):

crates/vm/src/builtins/mappingproxy.rs

@@ -258,11 +258,19 @@ impl AsNumber for PyMappingProxy {
     fn as_number() -> &'static PyNumberMethods {
         static AS_NUMBER: PyNumberMethods = PyNumberMethods {
             or: Some(|a, b, vm| {
-                if let Some(a) = a.downcast_ref::<PyMappingProxy>() {
-                    a.__or__(b.to_pyobject(vm), vm)
-                } else {
-                    Ok(vm.ctx.not_implemented())
-                }
+                // Mirror CPython's mappingproxy_or: when either side is a
+                // mappingproxy, unwrap to its underlying mapping and delegate
+                // to PyNumber_Or so `dict | mp`, `mp | dict`, and `mp | mp`
+                // all produce a `dict` result.
+                let a_obj = match a.downcast_ref::<PyMappingProxy>() {
+                    Some(mp) => mp.copy(vm)?,
+                    None => a.to_pyobject(vm),
+                };
+                let b_obj = match b.downcast_ref::<PyMappingProxy>() {
+                    Some(mp) => mp.copy(vm)?,
+                    None => b.to_pyobject(vm),
+                };
+                vm._or(a_obj.as_ref(), b_obj.as_ref())
             }),
             inplace_or: Some(|a, b, vm| {
                 if let Some(a) = a.downcast_ref::<PyMappingProxy>() {