Merge pull request RustPython#3308 from youknowone/fix-hash

Fix __hash__ not to perform unnecessary downcast

Author: youknowone

vm/src/types/slot.rs

@@ -486,8 +486,8 @@ pub trait Hashable: PyValue {
 
     #[inline]
     #[pymethod]
-    fn __hash__(zelf: PyRef<Self>, vm: &VirtualMachine) -> PyResult<PyHash> {
-        Self::hash(&zelf, vm)
+    fn __hash__(zelf: PyObjectRef, vm: &VirtualMachine) -> PyResult<PyHash> {
+        Self::slot_hash(&zelf, vm)
     }
 
     fn hash(zelf: &PyRef<Self>, vm: &VirtualMachine) -> PyResult<PyHash>;
@@ -499,8 +499,11 @@ impl<T> Hashable for T
 where
     T: Unhashable,
 {
-    fn hash(_zelf: &PyRef<Self>, vm: &VirtualMachine) -> PyResult<PyHash> {
-        Err(vm.new_type_error(format!("unhashable type: '{}'", _zelf.class().name())))
+    fn slot_hash(zelf: &PyObjectRef, vm: &VirtualMachine) -> PyResult<PyHash> {
+        Err(vm.new_type_error(format!("unhashable type: '{}'", zelf.class().name())))
+    }
+    fn hash(_zelf: &PyRef<Self>, _vm: &VirtualMachine) -> PyResult<PyHash> {
+        unreachable!("slot_hash is implemented for unhashable types");
     }
 }