fix declarative-modules compile error (#4054)

* fix declarative-modules compile error * add newsfragment

Author: Icxolu

newsfragments/4054.fixed.md

@@ -0,0 +1 @@
+Fixes a compile error when exporting a `#[pyclass]` living in a different Rust module using the declarative-module feature.

pyo3-macros-backend/src/pyclass.rs

@@ -1614,7 +1614,7 @@ impl<'a> PyClassImplsBuilder<'a> {
  quote! {
  impl #cls {
  #[doc(hidden)]
- const _PYO3_DEF: #pyo3_path::impl_::pymodule::AddClassToModule<Self> = #pyo3_path::impl_::pymodule::AddClassToModule::new();
+ pub const _PYO3_DEF: #pyo3_path::impl_::pymodule::AddClassToModule<Self> = #pyo3_path::impl_::pymodule::AddClassToModule::new();
  }
  }
  }

tests/test_declarative_module.rs

@@ -9,6 +9,13 @@ use pyo3::types::PyBool;
 #[path = "../src/tests/common.rs"]
 mod common;
 
+mod some_module {
+ use pyo3::prelude::*;
+
+ #[pyclass]
+ pub struct SomePyClass;
+}
+
 #[pyclass]
 struct ValueClass {
  value: usize,
@@ -50,6 +57,10 @@ mod declarative_module {
  #[pymodule_export]
  use super::{declarative_module2, double, MyError, ValueClass as Value};
 
+ // test for #4036
+ #[pymodule_export]
+ use super::some_module::SomePyClass;
+
  #[pymodule]
  mod inner {
  use super::*;