A different approach

pyo3-macros-backend/src/method.rs

@@ -481,13 +481,16 @@ impl<'a> FnSpec<'a> {
 
         // The method call is necessary to generate a decent error message.
         let rust_call = quote! {
-            let ret = #rust_name(#self_arg #(#arg_names),*);
+            let mut ret = #rust_name(#self_arg #(#arg_names),*);
 
-            use _pyo3::callback::IntoPyResult;
-            let ret: _pyo3::PyResult<_> = ret.into_py_result();
+            if false {
+                use _pyo3::impl_::ghost::IntoPyResult;
+                ret.into_py_result();
+            }
 
             _pyo3::callback::convert(#py, ret)
         };
+
         let krate = &self.krate;
         Ok(match self.convention {
             CallingConvention::Noargs => {

src/callback.rs

@@ -35,31 +35,6 @@ impl PyCallbackOutput for () {
     const ERR_VALUE: Self = ();
 }
 
-/// A helper trait to result-wrap the return values of functions and methods.
-#[doc(hidden)]
-pub trait IntoPyResult<R> {
-    fn into_py_result(self) -> R;
-}
-
-impl<T> IntoPyResult<PyResult<T>> for T
-where
-    T: IntoPy<PyObject>,
-{
-    fn into_py_result(self) -> PyResult<Self> {
-        Ok(self)
-    }
-}
-
-impl<T, E> IntoPyResult<PyResult<T>> for Result<T, E>
-where
-    T: IntoPy<PyObject>,
-    E: Into<PyErr>,
-{
-    fn into_py_result(self) -> PyResult<T> {
-        self.map_err(Into::into)
-    }
-}
-
 /// Convert the result of callback function into the appropriate return value.
 pub trait IntoPyCallbackOutput<Target> {
     fn convert(self, py: Python<'_>) -> PyResult<Target>;

src/impl_.rs

@@ -9,6 +9,7 @@ pub mod extract_argument;
 pub mod freelist;
 #[doc(hidden)]
 pub mod frompyobject;
+pub mod ghost;
 pub(crate) mod not_send;
 #[doc(hidden)]
 pub mod pyclass;

src/impl_/ghost.rs

@@ -0,0 +1,19 @@
+/// If it does nothing, was it ever really there? 👻
+///
+/// This is code that is just type checked to e.g. create better compile errors,
+/// but that never affects anything at runtime,
+use crate::{IntoPy, PyErr, PyObject};
+
+#[allow(clippy::wrong_self_convention)]
+pub trait IntoPyResult<T> {
+    fn into_py_result(&mut self) {}
+}
+
+impl<T> IntoPyResult<T> for T where T: IntoPy<PyObject> {}
+
+impl<T, E> IntoPyResult<T> for Result<T, E>
+where
+    T: IntoPy<PyObject>,
+    E: Into<PyErr>,
+{
+}

tests/ui/invalid_result_conversion.stderr

@@ -1,12 +1,22 @@
-error[E0599]: the method `into_py_result` exists for enum `Result<(), MyError>`, but its trait bounds were not satisfied
+error[E0599]: no method named `into_py_result` found for enum `Result` in the current scope
   --> tests/ui/invalid_result_conversion.rs:21:1
    |
 21 | #[pyfunction]
-   | ^^^^^^^^^^^^^ method cannot be called on `Result<(), MyError>` due to unsatisfied trait bounds
+   | ^^^^^^^^^^^^^ method not found in `Result<(), MyError>`
    |
-   = note: the following trait bounds were not satisfied:
-           `&Result<(), MyError>: IntoPy<Py<PyAny>>`
-           which is required by `&Result<(), MyError>: IntoPyResult<Result<&Result<(), MyError>, PyErr>>`
-           `&mut Result<(), MyError>: IntoPy<Py<PyAny>>`
-           which is required by `&mut Result<(), MyError>: IntoPyResult<Result<&mut Result<(), MyError>, PyErr>>`
    = note: this error originates in the attribute macro `pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `Result<(), MyError>: IntoPyCallbackOutput<_>` is not satisfied
+   --> tests/ui/invalid_result_conversion.rs:21:1
+    |
+21  | #[pyfunction]
+    | ^^^^^^^^^^^^^ the trait `IntoPyCallbackOutput<_>` is not implemented for `Result<(), MyError>`
+    |
+    = help: the following implementations were found:
+              <Result<T, E> as IntoPyCallbackOutput<U>>
+note: required by a bound in `pyo3::callback::convert`
+   --> src/callback.rs
+    |
+    |     T: IntoPyCallbackOutput<U>,
+    |        ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `pyo3::callback::convert`
+    = note: this error originates in the attribute macro `pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/missing_intopy.stderr

@@ -6,18 +6,14 @@ error[E0599]: the method `into_py_result` exists for struct `Blah`, but its trai
     | |
     | method `into_py_result` not found for this
     | doesn't satisfy `Blah: IntoPy<Py<PyAny>>`
-    | doesn't satisfy `Blah: IntoPyResult<Result<Blah, PyErr>>`
+    | doesn't satisfy `Blah: IntoPyResult<Blah>`
 2   |
 3   | #[pyo3::pyfunction]
     | ^^^^^^^^^^^^^^^^^^^ method cannot be called on `Blah` due to unsatisfied trait bounds
     |
     = note: the following trait bounds were not satisfied:
             `Blah: IntoPy<Py<PyAny>>`
-            which is required by `Blah: IntoPyResult<Result<Blah, PyErr>>`
-            `&Blah: IntoPy<Py<PyAny>>`
-            which is required by `&Blah: IntoPyResult<Result<&Blah, PyErr>>`
-            `&mut Blah: IntoPy<Py<PyAny>>`
-            which is required by `&mut Blah: IntoPyResult<Result<&mut Blah, PyErr>>`
+            which is required by `Blah: IntoPyResult<Blah>`
 note: the following trait must be implemented
    --> src/conversion.rs
     |
@@ -27,3 +23,16 @@ note: the following trait must be implemented
     | | }
     | |_^
     = note: this error originates in the attribute macro `pyo3::pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `Blah: IntoPyCallbackOutput<_>` is not satisfied
+   --> tests/ui/missing_intopy.rs:3:1
+    |
+3   | #[pyo3::pyfunction]
+    | ^^^^^^^^^^^^^^^^^^^ the trait `IntoPyCallbackOutput<_>` is not implemented for `Blah`
+    |
+note: required by a bound in `pyo3::callback::convert`
+   --> src/callback.rs
+    |
+    |     T: IntoPyCallbackOutput<U>,
+    |        ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `pyo3::callback::convert`
+    = note: this error originates in the attribute macro `pyo3::pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)