diff --git a/Cargo.lock b/Cargo.lock
index eb986773c..f37607256 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1518,7 +1518,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "syn",
- "syn-ext",
+ "syn-ext 0.3.1",
 ]
 
 [[package]]
@@ -1645,7 +1645,7 @@ dependencies = [
  "rustpython-compiler",
  "rustpython-doc",
  "syn",
- "syn-ext",
+ "syn-ext 0.4.0",
  "textwrap 0.15.0",
 ]
 
@@ -2120,6 +2120,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "syn-ext"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9b86cb2b68c5b3c078cac02588bc23f3c04bb828c5d3aedd17980876ec6a7be6"
+dependencies = [
+ "syn",
+]
+
 [[package]]
 name = "system-configuration"
 version = "0.5.0"
diff --git a/vm/src/function/protocol.rs b/vm/src/function/protocol.rs
index de18517ce..02c179b4a 100644
--- a/vm/src/function/protocol.rs
+++ b/vm/src/function/protocol.rs
@@ -4,6 +4,7 @@ use crate::{
     convert::ToPyObject,
     identifier,
     protocol::{PyIter, PyIterIter, PyMapping, PyMappingMethods},
+    types::AsMapping,
     AsObject, PyObject, PyObjectRef, PyPayload, PyResult, TryFromObject, VirtualMachine,
 };
 use std::{borrow::Borrow, marker::PhantomData};
@@ -110,7 +111,7 @@ impl ArgMapping {
     pub fn from_dict_exact(dict: PyDictRef) -> Self {
         Self {
             obj: dict.into(),
-            mapping_methods: PyDict::MAPPING_METHODS,
+            mapping_methods: PyDict::AS_MAPPING,
         }
     }
 
diff --git a/vm/src/types/slot.rs b/vm/src/types/slot.rs
index b626327e9..d7bbaed1f 100644
--- a/vm/src/types/slot.rs
+++ b/vm/src/types/slot.rs
@@ -896,15 +896,14 @@ pub trait AsBuffer: PyPayload {
 
 #[pyimpl]
 pub trait AsMapping: PyPayload {
+    const AS_MAPPING: PyMappingMethods;
+
     #[inline]
     #[pyslot]
-    fn slot_as_mapping(zelf: &PyObject, vm: &VirtualMachine) -> PyMappingMethods {
-        let zelf = unsafe { zelf.downcast_unchecked_ref::<Self>() };
-        Self::as_mapping(zelf, vm)
+    fn as_mapping(_zelf: &PyObject, _vm: &VirtualMachine) -> PyMappingMethods {
+        Self::AS_MAPPING
     }
 
-    fn as_mapping(zelf: &Py<Self>, vm: &VirtualMachine) -> PyMappingMethods;
-
     fn mapping_downcast<'a>(mapping: &'a PyMapping) -> &'a Py<Self> {
         unsafe { mapping.obj.downcast_unchecked_ref() }
     }