Emit a better error for abi3 inheritance

pyo3-build-config/src/lib.rs

@@ -152,6 +152,10 @@ 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

src/impl_/pyclass.rs

@@ -1102,6 +1102,12 @@ 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 native types cannot be subclassed"
+    )
+)]
 pub trait PyClassBaseType: Sized {
     type LayoutAsBase: PyClassObjectLayout<Self>;
     type BaseNativeType;

tests/diagnostic_namespace/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/diagnostic_namespace/abi3_inheritance.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `PyException: PyClassBaseType` is not satisfied
+ --> tests/diagnostic_namespace/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 native types cannot be subclassed
+  = 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/diagnostic_namespace/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/test_compile_error.rs

@@ -65,3 +65,11 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/invalid_cancel_handle.rs");
     t.pass("tests/ui/pymodule_missing_docs.rs");
 }
+
+#[cfg(all(diagnostic_namespace, not(target_arch = "wasm32")))] // Not possible to invoke compiler from wasm
+#[test]
+fn test_diagnostics() {
+    let t = trybuild::TestCases::new();
+    #[cfg(all(feature = "abi3", not(feature = "experimental-async")))] // output changes with async feature
+    t.compile_fail("tests/diagnostic_namespace/abi3_inheritance.rs");
+}