diff --git a/pyo3-derive-backend/src/module.rs b/pyo3-derive-backend/src/module.rs
index b282c3aa..975b0574 100644
--- a/pyo3-derive-backend/src/module.rs
+++ b/pyo3-derive-backend/src/module.rs
@@ -221,8 +221,7 @@ fn function_c_wrapper(name: &Ident, spec: &method::FnSpec<'_>) -> TokenStream {
 
             #body
 
-            pyo3::callback::cb_convert(
-                pyo3::callback::PyObjectCallbackConverter, _py, _result)
+            pyo3::callback::cb_obj_convert(_py, _result)
         }
     }
 }
diff --git a/pyo3-derive-backend/src/pymethod.rs b/pyo3-derive-backend/src/pymethod.rs
index 1eade4e5..0449c204 100644
--- a/pyo3-derive-backend/src/pymethod.rs
+++ b/pyo3-derive-backend/src/pymethod.rs
@@ -115,8 +115,7 @@ fn impl_wrap_common(
                     pyo3::derive_utils::IntoPyResult::into_py_result(#body)
                 };
 
-                pyo3::callback::cb_convert(
-                    pyo3::callback::PyObjectCallbackConverter, _py, _result)
+                pyo3::callback::cb_obj_convert(_py, _result)
             }
         }
     } else {
@@ -138,8 +137,7 @@ fn impl_wrap_common(
 
                 #body
 
-                pyo3::callback::cb_convert(
-                    pyo3::callback::PyObjectCallbackConverter, _py, _result)
+                pyo3::callback::cb_obj_convert(_py, _result)
             }
         }
     }
@@ -169,8 +167,7 @@ pub fn impl_proto_wrap(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
 
             #body
 
-            pyo3::callback::cb_convert(
-                pyo3::callback::PyObjectCallbackConverter, _py, _result)
+            pyo3::callback::cb_obj_convert(_py, _result)
         }
     }
 }
@@ -237,8 +234,7 @@ pub fn impl_wrap_class(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
 
             #body
 
-            pyo3::callback::cb_convert(
-                pyo3::callback::PyObjectCallbackConverter, _py, _result)
+            pyo3::callback::cb_obj_convert(_py, _result)
         }
     }
 }
@@ -267,8 +263,7 @@ pub fn impl_wrap_static(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
 
             #body
 
-            pyo3::callback::cb_convert(
-                pyo3::callback::PyObjectCallbackConverter, _py, _result)
+            pyo3::callback::cb_obj_convert(_py, _result)
         }
     }
 }
diff --git a/src/callback.rs b/src/callback.rs
index 8061aeea..5abec052 100644
--- a/src/callback.rs
+++ b/src/callback.rs
@@ -10,53 +10,62 @@ use crate::{IntoPy, PyObject, Python};
 use std::os::raw::c_int;
 use std::{isize, ptr};
 
-pub trait CallbackConverter<S> {
-    type R;
+/// Convert the result of callback function into the appropriate return value.
+pub trait CallbackConverter {
+    type Source;
+    type Result;
+    const ERR_VALUE: Self::Result;
 
-    fn convert(s: S, p: Python) -> Self::R;
-    fn error_value() -> Self::R;
-}
-
-pub struct PyObjectCallbackConverter;
-
-impl<S> CallbackConverter<S> for PyObjectCallbackConverter
-where
-    S: IntoPy<PyObject>,
-{
-    type R = *mut ffi::PyObject;
-
-    fn convert(val: S, py: Python) -> *mut ffi::PyObject {
-        val.into_py(py).into_ptr()
-    }
+    fn convert(s: Self::Source, py: Python) -> Self::Result;
 
     #[inline]
-    fn error_value() -> *mut ffi::PyObject {
-        ptr::null_mut()
+    fn convert_result(py: Python, value: PyResult<Self::Source>) -> Self::Result {
+        match value {
+            Ok(val) => Self::convert(val, py),
+            Err(e) => {
+                e.restore(py);
+                Self::ERR_VALUE
+            }
+        }
+    }
+}
+
+pub struct PyObjectCallbackConverter<T>(pub std::marker::PhantomData<T>);
+
+impl<T> CallbackConverter for PyObjectCallbackConverter<T>
+where
+    T: IntoPy<PyObject>,
+{
+    type Source = T;
+    type Result = *mut ffi::PyObject;
+    const ERR_VALUE: Self::Result = ptr::null_mut();
+
+    fn convert(s: Self::Source, py: Python) -> Self::Result {
+        s.into_py(py).into_ptr()
     }
 }
 
 pub struct BoolCallbackConverter;
 
-impl CallbackConverter<bool> for BoolCallbackConverter {
-    type R = c_int;
+impl CallbackConverter for BoolCallbackConverter {
+    type Source = bool;
+    type Result = c_int;
+    const ERR_VALUE: Self::Result = -1;
 
     #[inline]
-    fn convert(val: bool, _py: Python) -> c_int {
-        val as c_int
-    }
-
-    #[inline]
-    fn error_value() -> c_int {
-        -1
+    fn convert(s: Self::Source, _py: Python) -> Self::Result {
+        s as c_int
     }
 }
 
 pub struct LenResultConverter;
 
-impl CallbackConverter<usize> for LenResultConverter {
-    type R = isize;
+impl CallbackConverter for LenResultConverter {
+    type Source = usize;
+    type Result = isize;
+    const ERR_VALUE: Self::Result = -1;
 
-    fn convert(val: usize, py: Python) -> isize {
+    fn convert(val: Self::Source, py: Python) -> Self::Result {
         if val <= (isize::MAX as usize) {
             val as isize
         } else {
@@ -64,27 +73,19 @@ impl CallbackConverter<usize> for LenResultConverter {
             -1
         }
     }
-
-    #[inline]
-    fn error_value() -> isize {
-        -1
-    }
 }
 
 pub struct UnitCallbackConverter;
 
-impl CallbackConverter<()> for UnitCallbackConverter {
-    type R = c_int;
+impl CallbackConverter for UnitCallbackConverter {
+    type Source = ();
+    type Result = c_int;
+    const ERR_VALUE: Self::Result = -1;
 
     #[inline]
-    fn convert(_: (), _: Python) -> c_int {
+    fn convert(_s: Self::Source, _py: Python) -> Self::Result {
         0
     }
-
-    #[inline]
-    fn error_value() -> c_int {
-        -1
-    }
 }
 
 pub trait WrappingCastTo<T> {
@@ -112,13 +113,15 @@ wrapping_cast!(i32, Py_hash_t);
 wrapping_cast!(isize, Py_hash_t);
 wrapping_cast!(i64, Py_hash_t);
 
-pub struct HashConverter;
+pub struct HashConverter<T>(pub std::marker::PhantomData<T>);
 
-impl<T> CallbackConverter<T> for HashConverter
+impl<T> CallbackConverter for HashConverter<T>
 where
     T: WrappingCastTo<Py_hash_t>,
 {
-    type R = Py_hash_t;
+    type Source = T;
+    type Result = Py_hash_t;
+    const ERR_VALUE: Self::Result = -1;
 
     #[inline]
     fn convert(val: T, _py: Python) -> Py_hash_t {
@@ -129,23 +132,30 @@ where
             hash
         }
     }
+}
 
-    #[inline]
-    fn error_value() -> Py_hash_t {
-        -1
-    }
+// Short hands methods for macros
+#[inline]
+pub fn cb_convert<C, T>(_c: C, py: Python, value: PyResult<T>) -> C::Result
+where
+    C: CallbackConverter<Source = T>,
+{
+    C::convert_result(py, value)
 }
 
 #[inline]
-pub unsafe fn cb_convert<C, T>(_c: C, py: Python, value: PyResult<T>) -> C::R
-where
-    C: CallbackConverter<T>,
-{
-    match value {
-        Ok(val) => C::convert(val, py),
-        Err(e) => {
-            e.restore(py);
-            C::error_value()
-        }
-    }
+pub fn cb_obj_convert<T: IntoPy<PyObject>>(
+    py: Python,
+    value: PyResult<T>,
+) -> <PyObjectCallbackConverter<T> as CallbackConverter>::Result {
+    PyObjectCallbackConverter::<T>::convert_result(py, value)
+}
+
+#[inline]
+pub unsafe fn cb_err<C>(_c: C, py: Python, err: impl Into<crate::PyErr>) -> C::Result
+where
+    C: CallbackConverter,
+{
+    err.into().restore(py);
+    C::ERR_VALUE
 }
diff --git a/src/class/basic.rs b/src/class/basic.rs
index 5d8e60e5..cb920aee 100644
--- a/src/class/basic.rs
+++ b/src/class/basic.rs
@@ -233,8 +233,13 @@ where
 
             let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
             let arg = py.from_borrowed_ptr::<crate::types::PyAny>(arg);
-            let result = call_ref!(slf, __getattr__, arg);
-            crate::callback::cb_convert(PyObjectCallbackConverter, py, result)
+            call_ref_with_converter!(
+                slf,
+                PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData),
+                py,
+                __getattr__,
+                arg
+            )
         }
         Some(wrap::<T>)
     }
@@ -355,8 +360,7 @@ where
         py_unary_func!(
             PyObjectStrProtocol,
             T::__str__,
-            <T as PyObjectStrProtocol>::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -380,8 +384,7 @@ where
         py_unary_func!(
             PyObjectReprProtocol,
             T::__repr__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -444,8 +447,7 @@ where
         py_unary_func!(
             PyObjectHashProtocol,
             T::__hash__,
-            isize,
-            HashConverter,
+            HashConverter::<isize>(std::marker::PhantomData),
             ffi::Py_hash_t
         )
     }
@@ -470,7 +472,6 @@ where
         py_unary_func!(
             PyObjectBoolProtocol,
             T::__bool__,
-            bool,
             BoolCallbackConverter,
             c_int
         )
diff --git a/src/class/descr.rs b/src/class/descr.rs
index 80313dfe..61c2e51c 100644
--- a/src/class/descr.rs
+++ b/src/class/descr.rs
@@ -87,8 +87,7 @@ where
         py_ternary_func!(
             PyDescrGetProtocol,
             T::__get__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -109,13 +108,7 @@ where
     T: for<'p> PyDescrSetProtocol<'p>,
 {
     fn tp_descr_set() -> Option<ffi::descrsetfunc> {
-        py_ternary_func!(
-            PyDescrSetProtocol,
-            T::__set__,
-            (),
-            UnitCallbackConverter,
-            c_int
-        )
+        py_ternary_func!(PyDescrSetProtocol, T::__set__, UnitCallbackConverter, c_int)
     }
 }
 
diff --git a/src/class/iter.rs b/src/class/iter.rs
index d347b65d..c6d69361 100644
--- a/src/class/iter.rs
+++ b/src/class/iter.rs
@@ -80,8 +80,7 @@ where
         py_unary_refmut_func!(
             PyIterIterProtocol,
             T::__iter__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -108,21 +107,22 @@ where
         py_unary_refmut_func!(
             PyIterNextProtocol,
             T::__next__,
-            Option<T::Success>,
-            IterNextConverter
+            IterNextConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
 
-struct IterNextConverter;
+struct IterNextConverter<T>(std::marker::PhantomData<T>);
 
-impl<T> CallbackConverter<Option<T>> for IterNextConverter
+impl<T> CallbackConverter for IterNextConverter<T>
 where
     T: IntoPy<PyObject>,
 {
-    type R = *mut ffi::PyObject;
+    type Source = Option<T>;
+    type Result = *mut ffi::PyObject;
+    const ERR_VALUE: Self::Result = ptr::null_mut();
 
-    fn convert(val: Option<T>, py: Python) -> *mut ffi::PyObject {
+    fn convert(val: Self::Source, py: Python) -> Self::Result {
         match val {
             Some(val) => val.into_py(py).into_ptr(),
             None => unsafe {
@@ -131,9 +131,4 @@ where
             },
         }
     }
-
-    #[inline]
-    fn error_value() -> *mut ffi::PyObject {
-        ptr::null_mut()
-    }
 }
diff --git a/src/class/macros.rs b/src/class/macros.rs
index 3ba31880..124860d0 100644
--- a/src/class/macros.rs
+++ b/src/class/macros.rs
@@ -3,24 +3,23 @@
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_unary_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {
+    ($trait:ident, $class:ident :: $f:ident, $conv: expr) => {
+        py_unary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject);
+    };
+    // Use call_ref! by default
+    ($trait:ident, $class:ident :: $f:ident, $conv: expr, $ret_type:ty) => {
         py_unary_func!(
             $trait,
             $class::$f,
-            $res_type,
             $conv,
-            *mut $crate::ffi::PyObject
+            $ret_type,
+            call_ref_with_converter
         );
     };
-    // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr, $ret_type:ty) => {
-        py_unary_func!($trait, $class::$f, $res_type, $conv, $ret_type, call_ref);
-    };
-    ($trait:ident,
+    ($trait: ident,
      $class:ident :: $f:ident,
-     $res_type:ty,
-     $conv:expr,
-     $ret_type:ty,
+     $conv: expr,
+     $ret_type: ty,
      $call: ident
     ) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut $crate::ffi::PyObject) -> $ret_type
@@ -30,8 +29,7 @@ macro_rules! py_unary_func {
             let py = $crate::Python::assume_gil_acquired();
             let _pool = $crate::GILPool::new(py);
             let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let res = $call!(slf, $f);
-            $crate::callback::cb_convert($conv, py, res.map(|x| x))
+            $call!(slf, $conv, py, $f)
         }
         Some(wrap::<$class>)
     }};
@@ -40,7 +38,7 @@ macro_rules! py_unary_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_unary_refmut_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut $crate::ffi::PyObject) -> *mut $crate::ffi::PyObject
         where
             T: for<'p> $trait<'p>,
@@ -76,20 +74,14 @@ macro_rules! py_len_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_binary_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {
-        py_binary_func!(
-            $trait,
-            $class::$f,
-            $res_type,
-            $conv,
-            *mut $crate::ffi::PyObject
-        )
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
+        py_binary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject)
     };
     // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr, $return:ty) => {{
-        py_binary_func!($trait, $class::$f, $res_type, $conv, $return, call_ref)
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return:ty) => {{
+        py_binary_func!($trait, $class::$f, $conv, $return, call_ref_with_converter)
     }};
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr, $return:ty, $call:ident) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return:ty, $call:ident) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut ffi::PyObject, arg: *mut ffi::PyObject) -> $return
         where
             T: for<'p> $trait<'p>,
@@ -99,8 +91,7 @@ macro_rules! py_binary_func {
             let _pool = $crate::GILPool::new(py);
             let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
             let arg = py.from_borrowed_ptr::<$crate::types::PyAny>(arg);
-            let result = $call!(slf, $f, arg);
-            $crate::callback::cb_convert($conv, py, result)
+            $call!(slf, $conv, py, $f, arg)
         }
         Some(wrap::<$class>)
     }};
@@ -109,7 +100,7 @@ macro_rules! py_binary_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_binary_num_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
         unsafe extern "C" fn wrap<T>(
             lhs: *mut ffi::PyObject,
             rhs: *mut ffi::PyObject,
@@ -137,7 +128,7 @@ macro_rules! py_binary_num_func {
 }
 
 // NOTE(kngwyu):
-// Now(2020 2/9) This macro is used only for inplace operation so I used call_refmut here.
+// Now(2020 2/9) This macro is used only for inplace operation so I used call_mut here.
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_binary_self_func {
@@ -155,7 +146,7 @@ macro_rules! py_binary_self_func {
             let _pool = $crate::GILPool::new(py);
             let slf_ = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
             let arg = py.from_borrowed_ptr::<$crate::types::PyAny>(arg);
-            let result = call_refmut!(slf_, $f, arg);
+            let result = call_mut!(slf_, $f, arg);
             match result {
                 Ok(_) => {
                     ffi::Py_INCREF(slf);
@@ -172,16 +163,15 @@ macro_rules! py_binary_self_func {
 #[doc(hidden)]
 macro_rules! py_ssizearg_func {
     // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
         py_ssizearg_func!(
             $trait,
             $class::$f,
-            $res_type,
             $conv,
-            call_ref
+            call_ref_with_converter
         )
     };
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr, $call:ident) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $call:ident) => {{
         unsafe extern "C" fn wrap<T>(
             slf: *mut ffi::PyObject,
             arg: $crate::ffi::Py_ssize_t,
@@ -192,8 +182,7 @@ macro_rules! py_ssizearg_func {
             let py = $crate::Python::assume_gil_acquired();
             let _pool = $crate::GILPool::new(py);
             let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let result = $call!(slf, $f ; arg.into());
-            $crate::callback::cb_convert($conv, py, result)
+            $call!(slf, $conv, py, $f ;arg.into())
         }
         Some(wrap::<$class>)
     }};
@@ -202,16 +191,10 @@ macro_rules! py_ssizearg_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_ternary_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {
-        py_ternary_func!(
-            $trait,
-            $class::$f,
-            $res_type,
-            $conv,
-            *mut $crate::ffi::PyObject
-        );
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
+        py_ternary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject);
     };
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr, $return_type:ty) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return_type:ty) => {{
         unsafe extern "C" fn wrap<T>(
             slf: *mut $crate::ffi::PyObject,
             arg1: *mut $crate::ffi::PyObject,
@@ -228,8 +211,7 @@ macro_rules! py_ternary_func {
             let arg1 = py.from_borrowed_ptr::<$crate::types::PyAny>(arg1);
             let arg2 = py.from_borrowed_ptr::<$crate::types::PyAny>(arg2);
 
-            let result = call_ref!(slf, $f, arg1, arg2);
-            $crate::callback::cb_convert($conv, py, result)
+            call_ref_with_converter!(slf, $conv, py, $f, arg1, arg2)
         }
 
         Some(wrap::<T>)
@@ -239,7 +221,7 @@ macro_rules! py_ternary_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_ternary_num_func {
-    ($trait:ident, $class:ident :: $f:ident, $res_type:ty, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
         unsafe extern "C" fn wrap<T>(
             arg1: *mut $crate::ffi::PyObject,
             arg2: *mut $crate::ffi::PyObject,
@@ -292,7 +274,7 @@ macro_rules! py_ternary_self_func {
             let slf_cell = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
             let arg1 = py.from_borrowed_ptr::<$crate::types::PyAny>(arg1);
             let arg2 = py.from_borrowed_ptr::<$crate::types::PyAny>(arg2);
-            let result = call_refmut!(slf_cell, $f, arg1, arg2);
+            let result = call_mut!(slf_cell, $f, arg1, arg2);
             match result {
                 Ok(_) => slf,
                 Err(e) => e.restore_and_null(py),
@@ -328,7 +310,7 @@ macro_rules! py_func_set {
             } else {
                 let name = py.from_borrowed_ptr::<$crate::types::PyAny>(name);
                 let value = py.from_borrowed_ptr::<$crate::types::PyAny>(value);
-                call_refmut!(slf, $fn_set, name, value)
+                call_mut!(slf, $fn_set, name, value)
             };
             match result {
                 Ok(_) => 0,
@@ -359,7 +341,7 @@ macro_rules! py_func_del {
                 let slf = py.from_borrowed_ptr::<$crate::PyCell<U>>(slf);
                 let name = py.from_borrowed_ptr::<$crate::types::PyAny>(name);
 
-                call_refmut!(slf, $fn_del, name)
+                call_mut!(slf, $fn_del, name)
             } else {
                 Err(PyErr::new::<exceptions::NotImplementedError, _>(
                     "Subscript assignment not supported",
@@ -393,10 +375,10 @@ macro_rules! py_func_set_del {
             let name = py.from_borrowed_ptr::<$crate::types::PyAny>(name);
 
             let result = if value.is_null() {
-                call_refmut!(slf, $fn_del, name)
+                call_mut!(slf, $fn_del, name)
             } else {
                 let value = py.from_borrowed_ptr::<$crate::types::PyAny>(value);
-                call_refmut!(slf, $fn_set, name, value)
+                call_mut!(slf, $fn_set, name, value)
             };
             match result {
                 Ok(_) => 0,
@@ -407,33 +389,48 @@ macro_rules! py_func_set_del {
     }};
 }
 
-// Utitlities for extract arguments + call method for PyCell<T>
-macro_rules! call_ref {
-    ($slf: expr, $fn: ident $(; $args: expr)*) => {
-        match $slf.try_borrow_unguarded() {
-            Ok(slf) => slf.$fn($($args,)*).into(),
-            Err(e) => Err(e.into()),
-        }
-    };
-    ($slf: expr, $fn: ident, $raw_arg: expr $(,$raw_args: expr)* $(; $args: expr)*) => {
+macro_rules! _call_impl {
+    ($slf: ident, $fn: ident $(; $args: expr)*) => { $slf.$fn($($args,)*).into() };
+    ($slf: ident, $fn: ident, $raw_arg: expr $(,$raw_args: expr)* $(; $args: expr)*) => {
         match $raw_arg.extract() {
-            Ok(arg) => call_ref!($slf, $fn $(,$raw_args)* $(;$args)* ;arg),
+            Ok(arg) => _call_impl!($slf, $fn $(,$raw_args)* $(;$args)* ;arg),
             Err(e) => Err(e.into()),
         }
     };
 }
 
-macro_rules! call_refmut {
-    ($slf: expr, $fn: ident $(; $args: expr)*) => {
-        match $slf.try_borrow_mut_unguarded() {
-            Ok(slf) => slf.$fn($($args,)*).into(),
-            Err(e) => Err(e.into()),
-        }
-    };
-    ($slf: expr, $fn: ident, $raw_arg: expr $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $raw_arg.extract() {
-            Ok(arg) => call_refmut!($slf, $fn $(,$raw_args)* $(;$args)* ;arg),
+macro_rules! call_ref {
+    ($slf: expr, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
+        match $slf.try_borrow() {
+            Ok(slf) => _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*),
             Err(e) => Err(e.into()),
         }
     };
 }
+
+macro_rules! call_ref_with_converter {
+    ($slf: expr, $conv: expr, $py: ident, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
+        match $slf.try_borrow() {
+            Ok(slf) => $crate::callback::cb_convert($conv, $py, _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*)),
+            Err(e) => $crate::callback::cb_err($conv, $py, e)
+        }
+    };
+}
+
+macro_rules! call_mut {
+    ($slf: expr, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
+        match $slf.try_borrow_mut() {
+            Ok(mut slf) => _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*),
+            Err(e) => Err(e.into()),
+        }
+    };
+}
+
+macro_rules! call_mut_with_converter {
+    ($slf: expr, $conv: expr, $py: ident, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
+        match $slf.try_borrow_mut() {
+            Ok(mut slf) => $crate::callback::cb_convert($conv, $py, _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*)),
+            Err(e) => $crate::callback::cb_err($conv, $py, e)
+        }
+    };
+}
diff --git a/src/class/mapping.rs b/src/class/mapping.rs
index cccc6a22..39373134 100644
--- a/src/class/mapping.rs
+++ b/src/class/mapping.rs
@@ -167,8 +167,7 @@ where
         py_binary_func!(
             PyMappingGetItemProtocol,
             T::__getitem__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
diff --git a/src/class/number.rs b/src/class/number.rs
index f44ef5b8..1be54f54 100644
--- a/src/class/number.rs
+++ b/src/class/number.rs
@@ -764,8 +764,7 @@ where
         py_binary_num_func!(
             PyNumberAddProtocol,
             T::__add__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -791,8 +790,7 @@ where
         py_binary_num_func!(
             PyNumberSubProtocol,
             T::__sub__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -818,8 +816,7 @@ where
         py_binary_num_func!(
             PyNumberMulProtocol,
             T::__mul__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -845,8 +842,7 @@ where
         py_binary_num_func!(
             PyNumberMatmulProtocol,
             T::__matmul__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -872,8 +868,7 @@ where
         py_binary_num_func!(
             PyNumberTruedivProtocol,
             T::__truediv__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -899,8 +894,7 @@ where
         py_binary_num_func!(
             PyNumberFloordivProtocol,
             T::__floordiv__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -926,8 +920,7 @@ where
         py_binary_num_func!(
             PyNumberModProtocol,
             T::__mod__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -953,8 +946,7 @@ where
         py_binary_num_func!(
             PyNumberDivmodProtocol,
             T::__divmod__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -980,8 +972,7 @@ where
         py_ternary_num_func!(
             PyNumberPowProtocol,
             T::__pow__,
-            <T as PyNumberPowProtocol>::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1007,8 +998,7 @@ where
         py_binary_num_func!(
             PyNumberLShiftProtocol,
             T::__lshift__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1034,8 +1024,7 @@ where
         py_binary_num_func!(
             PyNumberRShiftProtocol,
             T::__rshift__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1061,8 +1050,7 @@ where
         py_binary_num_func!(
             PyNumberAndProtocol,
             T::__and__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1088,8 +1076,7 @@ where
         py_binary_num_func!(
             PyNumberXorProtocol,
             T::__xor__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1115,8 +1102,7 @@ where
         py_binary_num_func!(
             PyNumberOrProtocol,
             T::__or__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1620,8 +1606,7 @@ where
         py_unary_func!(
             PyNumberNegProtocol,
             T::__neg__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1647,8 +1632,7 @@ where
         py_unary_func!(
             PyNumberPosProtocol,
             T::__pos__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1674,8 +1658,7 @@ where
         py_unary_func!(
             PyNumberAbsProtocol,
             T::__abs__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1701,8 +1684,7 @@ where
         py_unary_func!(
             PyNumberInvertProtocol,
             T::__invert__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1728,8 +1710,7 @@ where
         py_unary_func!(
             PyNumberIntProtocol,
             T::__int__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1755,8 +1736,7 @@ where
         py_unary_func!(
             PyNumberFloatProtocol,
             T::__float__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -1782,8 +1762,7 @@ where
         py_unary_func!(
             PyNumberIndexProtocol,
             T::__index__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
diff --git a/src/class/pyasync.rs b/src/class/pyasync.rs
index 90a42d83..842219ca 100644
--- a/src/class/pyasync.rs
+++ b/src/class/pyasync.rs
@@ -153,8 +153,7 @@ where
         py_unary_func!(
             PyAsyncAwaitProtocol,
             T::__await__,
-            <T as PyAsyncAwaitProtocol>::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -181,8 +180,7 @@ where
         py_unary_func!(
             PyAsyncAiterProtocol,
             T::__aiter__,
-            <T as PyAsyncAiterProtocol>::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -206,17 +204,20 @@ mod anext {
     use crate::IntoPyPointer;
     use crate::Python;
     use crate::{ffi, IntoPy, PyObject};
+    use std::marker::PhantomData;
     use std::ptr;
 
-    pub struct IterANextResultConverter;
+    struct IterANextResultConverter<T>(PhantomData<T>);
 
-    impl<T> CallbackConverter<Option<T>> for IterANextResultConverter
+    impl<T> CallbackConverter for IterANextResultConverter<T>
     where
         T: IntoPy<PyObject>,
     {
-        type R = *mut ffi::PyObject;
+        type Source = Option<T>;
+        type Result = *mut ffi::PyObject;
+        const ERR_VALUE: Self::Result = ptr::null_mut();
 
-        fn convert(val: Option<T>, py: Python) -> *mut ffi::PyObject {
+        fn convert(val: Self::Source, py: Python) -> Self::Result {
             match val {
                 Some(val) => val.into_py(py).into_ptr(),
                 None => unsafe {
@@ -225,11 +226,6 @@ mod anext {
                 },
             }
         }
-
-        #[inline]
-        fn error_value() -> *mut ffi::PyObject {
-            ptr::null_mut()
-        }
     }
 
     impl<T> PyAsyncAnextProtocolImpl for T
@@ -241,10 +237,9 @@ mod anext {
             py_unary_func!(
                 PyAsyncAnextProtocol,
                 T::__anext__,
-                Option<T::Success>,
-                IterANextResultConverter,
+                IterANextResultConverter::<T::Success>(std::marker::PhantomData),
                 *mut crate::ffi::PyObject,
-                call_refmut
+                call_mut_with_converter
             )
         }
     }
diff --git a/src/class/sequence.rs b/src/class/sequence.rs
index 643b4d21..79716e84 100644
--- a/src/class/sequence.rs
+++ b/src/class/sequence.rs
@@ -202,8 +202,7 @@ where
         py_ssizearg_func!(
             PySequenceGetItemProtocol,
             T::__getitem__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -283,10 +282,7 @@ mod sq_ass_item_impl {
 
                 match result {
                     Ok(_) => 0,
-                    Err(e) => {
-                        e.restore(py);
-                        -1
-                    }
+                    Err(e) => e.restore_and_minus1(py),
                 }
             }
             Some(wrap::<T>)
@@ -334,10 +330,7 @@ mod sq_ass_item_impl {
 
                 match result {
                     Ok(_) => 0,
-                    Err(e) => {
-                        e.restore(py);
-                        -1
-                    }
+                    Err(e) => e.restore_and_minus1(py),
                 }
             }
             Some(wrap::<T>)
@@ -375,7 +368,7 @@ mod sq_ass_item_impl {
                 let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
 
                 let result = if value.is_null() {
-                    call_refmut!(slf, __delitem__; key.into())
+                    call_mut!(slf, __delitem__; key.into())
                 } else {
                     let value = py.from_borrowed_ptr::<PyAny>(value);
                     match value.extract() {
@@ -388,10 +381,7 @@ mod sq_ass_item_impl {
                 };
                 match result {
                     Ok(_) => 0,
-                    Err(e) => {
-                        e.restore(py);
-                        -1
-                    }
+                    Err(e) => e.restore_and_minus1(py),
                 }
             }
             Some(wrap::<T>)
@@ -420,7 +410,6 @@ where
         py_binary_func!(
             PySequenceContainsProtocol,
             T::__contains__,
-            bool,
             BoolCallbackConverter,
             c_int
         )
@@ -448,8 +437,7 @@ where
         py_binary_func!(
             PySequenceConcatProtocol,
             T::__concat__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -475,8 +463,7 @@ where
         py_ssizearg_func!(
             PySequenceRepeatProtocol,
             T::__repeat__,
-            T::Success,
-            PyObjectCallbackConverter
+            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
         )
     }
 }
@@ -502,10 +489,9 @@ where
         py_binary_func!(
             PySequenceInplaceConcatProtocol,
             T::__inplace_concat__,
-            T,
-            PyObjectCallbackConverter,
+            PyObjectCallbackConverter::<T>(std::marker::PhantomData),
             *mut crate::ffi::PyObject,
-            call_refmut
+            call_mut_with_converter
         )
     }
 }
@@ -531,9 +517,8 @@ where
         py_ssizearg_func!(
             PySequenceInplaceRepeatProtocol,
             T::__inplace_repeat__,
-            T,
-            PyObjectCallbackConverter,
-            call_refmut
+            PyObjectCallbackConverter::<T>(std::marker::PhantomData),
+            call_mut_with_converter
         )
     }
 }