diff --git a/stdlib/src/array.rs b/stdlib/src/array.rs
index ddc6510400..bd5837dce9 100644
--- a/stdlib/src/array.rs
+++ b/stdlib/src/array.rs
@@ -1253,12 +1253,12 @@ mod array {
     }
 
     impl AsMapping for PyArray {
-        fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-            Ok(PyMappingMethods {
+        fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+            PyMappingMethods {
                 length: Some(Self::length),
                 subscript: Some(Self::subscript),
                 ass_subscript: Some(Self::ass_subscript),
-            })
+            }
         }
 
         fn length(zelf: PyObjectRef, vm: &VirtualMachine) -> PyResult<usize> {
diff --git a/vm/src/builtins/bytearray.rs b/vm/src/builtins/bytearray.rs
index 2a65036ab2..9b91061da5 100644
--- a/vm/src/builtins/bytearray.rs
+++ b/vm/src/builtins/bytearray.rs
@@ -756,12 +756,12 @@ impl<'a> BufferResizeGuard<'a> for PyByteArray {
 }
 
 impl AsMapping for PyByteArray {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: Some(Self::ass_subscript),
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/bytes.rs b/vm/src/builtins/bytes.rs
index 53aa561ef3..5131e9abd5 100644
--- a/vm/src/builtins/bytes.rs
+++ b/vm/src/builtins/bytes.rs
@@ -569,12 +569,12 @@ impl BufferInternal for PyRef<PyBytes> {
 }
 
 impl AsMapping for PyBytes {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: None,
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/dict.rs b/vm/src/builtins/dict.rs
index b3d7d78023..17d799880f 100644
--- a/vm/src/builtins/dict.rs
+++ b/vm/src/builtins/dict.rs
@@ -416,12 +416,12 @@ impl PyDict {
 }
 
 impl AsMapping for PyDict {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: Some(Self::ass_subscript),
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/list.rs b/vm/src/builtins/list.rs
index 97af1233de..5ed4f86201 100644
--- a/vm/src/builtins/list.rs
+++ b/vm/src/builtins/list.rs
@@ -418,12 +418,12 @@ impl PyList {
 }
 
 impl AsMapping for PyList {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: Some(Self::ass_subscript),
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/mappingproxy.rs b/vm/src/builtins/mappingproxy.rs
index e4e386f39f..9df9c0b7e8 100644
--- a/vm/src/builtins/mappingproxy.rs
+++ b/vm/src/builtins/mappingproxy.rs
@@ -151,12 +151,12 @@ impl PyMappingProxy {
 }
 
 impl AsMapping for PyMappingProxy {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: None,
             subscript: Some(Self::subscript),
             ass_subscript: None,
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/memory.rs b/vm/src/builtins/memory.rs
index 7ebdb4910d..56b87f8466 100644
--- a/vm/src/builtins/memory.rs
+++ b/vm/src/builtins/memory.rs
@@ -737,12 +737,12 @@ impl BufferInternal for PyRef<PyMemoryView> {
 }
 
 impl AsMapping for PyMemoryView {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: Some(Self::ass_subscript),
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/range.rs b/vm/src/builtins/range.rs
index 751b0b79dd..96e209e048 100644
--- a/vm/src/builtins/range.rs
+++ b/vm/src/builtins/range.rs
@@ -377,12 +377,12 @@ impl PyRange {
 }
 
 impl AsMapping for PyRange {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: None,
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/builtins/tuple.rs b/vm/src/builtins/tuple.rs
index 3f44c75cec..e733143425 100644
--- a/vm/src/builtins/tuple.rs
+++ b/vm/src/builtins/tuple.rs
@@ -306,12 +306,12 @@ impl PyTuple {
 }
 
 impl AsMapping for PyTuple {
-    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        Ok(PyMappingMethods {
+    fn as_mapping(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyMappingMethods {
+        PyMappingMethods {
             length: Some(Self::length),
             subscript: Some(Self::subscript),
             ass_subscript: None,
-        })
+        }
     }
 
     #[inline]
diff --git a/vm/src/protocol/mapping.rs b/vm/src/protocol/mapping.rs
index 174652c456..6b4a1dcfed 100644
--- a/vm/src/protocol/mapping.rs
+++ b/vm/src/protocol/mapping.rs
@@ -36,7 +36,7 @@ impl PyMapping<PyObjectRef> {
         let obj_cls = self.0.class();
         for cls in obj_cls.iter_mro() {
             if let Some(f) = cls.slots.as_mapping.load() {
-                return f(&self.0, vm).unwrap();
+                return f(&self.0, vm);
             }
         }
         PyMappingMethods::default()
diff --git a/vm/src/pyobjectrc.rs b/vm/src/pyobjectrc.rs
index 28b01bd3f2..952f3ecc20 100644
--- a/vm/src/pyobjectrc.rs
+++ b/vm/src/pyobjectrc.rs
@@ -237,6 +237,13 @@ impl PyObjectRef {
         PyRef::from_obj_unchecked(self)
     }
 
+    /// # Safety
+    /// T must be the exact payload type
+    pub unsafe fn downcast_unchecked_ref<T: PyObjectPayload>(&self) -> &PyRef<T> {
+        debug_assert!(self.payload_is::<T>());
+        &*(self as *const PyObjectRef as *const PyRef<T>)
+    }
+
     pub(crate) fn class_lock(&self) -> &PyRwLock<PyTypeRef> {
         &self.rc.inner.typ
     }
diff --git a/vm/src/types/slot.rs b/vm/src/types/slot.rs
index 39d4e4ad96..4ce18e15d4 100644
--- a/vm/src/types/slot.rs
+++ b/vm/src/types/slot.rs
@@ -128,7 +128,7 @@ impl Default for PyTypeFlags {
 }
 
 pub(crate) type GenericMethod = fn(&PyObjectRef, FuncArgs, &VirtualMachine) -> PyResult;
-pub(crate) type AsMappingFunc = fn(&PyObjectRef, &VirtualMachine) -> PyResult<PyMappingMethods>;
+pub(crate) type AsMappingFunc = fn(&PyObjectRef, &VirtualMachine) -> PyMappingMethods;
 pub(crate) type HashFunc = fn(&PyObjectRef, &VirtualMachine) -> PyResult<PyHash>;
 // CallFunc = GenericMethod
 pub(crate) type GetattroFunc = fn(PyObjectRef, PyStrRef, &VirtualMachine) -> PyResult;
@@ -150,7 +150,7 @@ pub(crate) type DescrSetFunc =
 pub(crate) type NewFunc = fn(PyTypeRef, FuncArgs, &VirtualMachine) -> PyResult;
 pub(crate) type DelFunc = fn(&PyObjectRef, &VirtualMachine) -> PyResult<()>;
 
-fn as_mapping_wrapper(zelf: &PyObjectRef, _vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
+fn as_mapping_wrapper(zelf: &PyObjectRef, _vm: &VirtualMachine) -> PyMappingMethods {
     macro_rules! then_some_closure {
         ($cond:expr, $closure:expr) => {
             if $cond {
@@ -160,7 +160,7 @@ fn as_mapping_wrapper(zelf: &PyObjectRef, _vm: &VirtualMachine) -> PyResult<PyMa
             }
         };
     }
-    Ok(PyMappingMethods {
+    PyMappingMethods {
         length: then_some_closure!(zelf.has_class_attr("__len__"), |zelf, vm| {
             vm.call_special_method(zelf, "__len__", ()).map(|obj| {
                 obj.payload_if_subclass::<PyInt>(vm)
@@ -189,7 +189,7 @@ fn as_mapping_wrapper(zelf: &PyObjectRef, _vm: &VirtualMachine) -> PyResult<PyMa
                     .map(|_| Ok(()))?,
             }
         ),
-    })
+    }
 }
 
 fn hash_wrapper(zelf: &PyObjectRef, vm: &VirtualMachine) -> PyResult<PyHash> {
@@ -763,10 +763,8 @@ pub trait AsBuffer: PyValue {
 pub trait AsMapping: PyValue {
     #[inline]
     #[pyslot]
-    fn slot_as_mapping(zelf: &PyObjectRef, vm: &VirtualMachine) -> PyResult<PyMappingMethods> {
-        let zelf = zelf
-            .downcast_ref()
-            .ok_or_else(|| vm.new_type_error("unexpected payload for as_mapping".to_owned()))?;
+    fn slot_as_mapping(zelf: &PyObjectRef, vm: &VirtualMachine) -> PyMappingMethods {
+        let zelf = unsafe { zelf.downcast_unchecked_ref::<Self>() };
         Self::as_mapping(zelf, vm)
     }
 
@@ -792,7 +790,7 @@ pub trait AsMapping: PyValue {
         })
     }
 
-    fn as_mapping(zelf: &PyRef<Self>, vm: &VirtualMachine) -> PyResult<PyMappingMethods>;
+    fn as_mapping(zelf: &PyRef<Self>, vm: &VirtualMachine) -> PyMappingMethods;
 
     fn length(zelf: PyObjectRef, _vm: &VirtualMachine) -> PyResult<usize>;