From 108a57eaca3307583422c70711a25302594272b5 Mon Sep 17 00:00:00 2001
From: mejrs <59372212+mejrs@users.noreply.github.com>
Date: Tue, 14 May 2024 19:11:25 +0200
Subject: [PATCH] Emit a better error for abi3 inheritance

---
 pyo3-build-config/src/lib.rs                  |  5 ++++
 src/impl_/pyclass.rs                          |  7 ++++++
 .../diagnostic_namespace/abi3_inheritance.rs  | 10 ++++++++
 .../abi3_inheritance.stderr                   | 24 +++++++++++++++++++
 tests/test_compile_error.rs                   | 13 +++++++++-
 5 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 tests/diagnostic_namespace/abi3_inheritance.rs
 create mode 100644 tests/diagnostic_namespace/abi3_inheritance.stderr

diff --git a/pyo3-build-config/src/lib.rs b/pyo3-build-config/src/lib.rs
index 54aff4d10de..0ce808f5b92 100644
--- a/pyo3-build-config/src/lib.rs
+++ b/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
@@ -167,6 +171,7 @@ 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)
diff --git a/src/impl_/pyclass.rs b/src/impl_/pyclass.rs
index 3ec2e329e1a..6dae9567d06 100644
--- a/src/impl_/pyclass.rs
+++ b/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 native types cannot be subclassed"
+    )
+)]
 pub trait PyClassBaseType: Sized {
     type LayoutAsBase: PyClassObjectLayout<Self>;
     type BaseNativeType;
diff --git a/tests/diagnostic_namespace/abi3_inheritance.rs b/tests/diagnostic_namespace/abi3_inheritance.rs
new file mode 100644
index 00000000000..60972e4cf7a
--- /dev/null
+++ b/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() {}
diff --git a/tests/diagnostic_namespace/abi3_inheritance.stderr b/tests/diagnostic_namespace/abi3_inheritance.stderr
new file mode 100644
index 00000000000..b45aa665dcd
--- /dev/null
+++ b/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)
diff --git a/tests/test_compile_error.rs b/tests/test_compile_error.rs
index d31c558f096..4359f03e996 100644
--- a/tests/test_compile_error.rs
+++ b/tests/test_compile_error.rs
@@ -32,7 +32,7 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pyfunctions.rs");
     t.compile_fail("tests/ui/invalid_pymethods.rs");
     // output changes with async feature
-    #[cfg(all(Py_LIMITED_API, feature = "experimental-async"))]
+    #[cfg(all(Py_LIMITED_API, feature = "experimental-async", not(diagnostic_namespace)))]
     t.compile_fail("tests/ui/abi3_nativetype_inheritance.rs");
     #[cfg(not(feature = "gil-refs"))]
     t.compile_fail("tests/ui/invalid_intern_arg.rs");
@@ -65,3 +65,14 @@ 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() {
+    #![allow(unused_variables)]
+
+    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");
+}