Emit a better error for abi3 inheritance (#4185)

* Emit a better error for abi3 inheritance

* Update tests/test_compile_error.rs

---------

Co-authored-by: David Hewitt <mail@davidhewitt.dev>

pyo3-build-config/src/lib.rs

@@ -140,6 +140,10 @@ pub fn print_feature_cfgs() {
     if rustc_minor_version >= 74 {
         println!("cargo:rustc-cfg=invalid_from_utf8_lint");
     }
+
+    if rustc_minor_version >= 78 {
+        println!("cargo:rustc-cfg=diagnostic_namespace");
+    }
 }
 
 /// Registers `pyo3`s config names as reachable cfg expressions
@@ -160,6 +164,7 @@ pub fn print_expected_cfgs() {
     println!("cargo:rustc-check-cfg=cfg(invalid_from_utf8_lint)");
     println!("cargo:rustc-check-cfg=cfg(pyo3_disable_reference_pool)");
     println!("cargo:rustc-check-cfg=cfg(pyo3_leak_on_drop_without_reference_pool)");
+    println!("cargo:rustc-check-cfg=cfg(diagnostic_namespace)");
 
     // allow `Py_3_*` cfgs from the minimum supported version up to the
     // maximum minor version (+1 for development for the next)

src/impl_/pyclass.rs

@@ -1102,6 +1102,13 @@ impl<T> PyClassThreadChecker<T> for ThreadCheckerImpl {
 }
 
 /// Trait denoting that this class is suitable to be used as a base type for PyClass.
+
+#[cfg_attr(
+    all(diagnostic_namespace, feature = "abi3"),
+    diagnostic::on_unimplemented(
+        note = "with the `abi3` feature enabled, PyO3 does not support subclassing native types"
+    )
+)]
 pub trait PyClassBaseType: Sized {
     type LayoutAsBase: PyClassObjectLayout<Self>;
     type BaseNativeType;

tests/test_compile_error.rs

@@ -64,4 +64,7 @@ fn test_compile_errors() {
     #[cfg(any(not(Py_LIMITED_API), Py_3_10))] // to avoid PyFunctionArgument for &str
     t.compile_fail("tests/ui/invalid_cancel_handle.rs");
     t.pass("tests/ui/pymodule_missing_docs.rs");
+    #[cfg(all(Py_LIMITED_API, not(feature = "experimental-async")))]
+    // output changes with async feature
+    t.compile_fail("tests/ui/abi3_inheritance.rs");
 }

tests/ui/abi3_inheritance.rs

@@ -0,0 +1,10 @@
+use pyo3::exceptions::PyException;
+use pyo3::prelude::*;
+
+#[pyclass(extends=PyException)]
+#[derive(Clone)]
+struct MyException {
+    code: u32,
+}
+
+fn main() {}

tests/ui/abi3_inheritance.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `PyException: PyClassBaseType` is not satisfied
+ --> tests/ui/abi3_inheritance.rs:4:19
+  |
+4 | #[pyclass(extends=PyException)]
+  |                   ^^^^^^^^^^^ the trait `PyClass` is not implemented for `PyException`, which is required by `PyException: PyClassBaseType`
+  |
+  = note: with the `abi3` feature enabled, PyO3 does not support subclassing native types
+  = help: the trait `PyClassBaseType` is implemented for `PyAny`
+  = note: required for `PyException` to implement `PyClassBaseType`
+note: required by a bound in `PyClassImpl::BaseType`
+ --> src/impl_/pyclass.rs
+  |
+  |     type BaseType: PyTypeInfo + PyClassBaseType;
+  |                                 ^^^^^^^^^^^^^^^ required by this bound in `PyClassImpl::BaseType`
+
+error[E0277]: the trait bound `PyException: PyClass` is not satisfied
+ --> tests/ui/abi3_inheritance.rs:4:1
+  |
+4 | #[pyclass(extends=PyException)]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `PyClass` is not implemented for `PyException`, which is required by `PyException: PyClassBaseType`
+  |
+  = help: the trait `PyClass` is implemented for `MyException`
+  = note: required for `PyException` to implement `PyClassBaseType`
+  = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/abi3_nativetype_inheritance.stderr

@@ -4,6 +4,7 @@ error[E0277]: the trait bound `PyDict: PyClassBaseType` is not satisfied
 5 | #[pyclass(extends=PyDict)]
   |                   ^^^^^^ the trait `PyClass` is not implemented for `PyDict`, which is required by `PyDict: PyClassBaseType`
   |
+  = note: with the `abi3` feature enabled, PyO3 does not support subclassing native types
   = help: the trait `PyClassBaseType` is implemented for `PyAny`
   = note: required for `PyDict` to implement `PyClassBaseType`
 note: required by a bound in `PyClassImpl::BaseType`