Disallow Drop impls in extern_class!

crates/objc2/src/__macro_helpers/class.rs

@@ -47,6 +47,26 @@ impl<Cls: ?Sized + MainThreadOnly + Send> MainThreadOnlyDoesNotImplSendSync<Impl
 struct ImplsSync;
 impl<Cls: ?Sized + MainThreadOnly + Sync> MainThreadOnlyDoesNotImplSendSync<ImplsSync> for Cls {}
 
+/// Check that class does not implement `Drop`.
+///
+/// This is not needed for soundness, it's just a nice footgun to avoid (since
+/// it wouldn't ever get called).
+///
+/// Check implemented using type inference:
+/// let _ = <MyType as DoesNotImplDrop<_>>::check
+pub trait DoesNotImplDrop<Inferred> {
+    // Required to reference the trait.
+    fn check() {}
+}
+
+// Type inference will find this blanket impl...
+impl<Cls: ?Sized> DoesNotImplDrop<()> for Cls {}
+
+// ... unless this impl also applies, then type inference fails.
+struct ImplsDrop;
+#[allow(drop_bounds)] // We're intentionally using `Drop` as a bound.
+impl<Cls: ?Sized + Drop> DoesNotImplDrop<ImplsDrop> for Cls {}
+
 #[cfg(test)]
 mod tests {
     use super::*;

crates/objc2/src/__macro_helpers/mod.rs

@@ -30,7 +30,7 @@ mod sync_unsafe_cell;
 mod writeback;
 
 pub use self::cache::{CachedClass, CachedSel};
-pub use self::class::{MainThreadOnlyDoesNotImplSendSync, ValidThreadKind};
+pub use self::class::{DoesNotImplDrop, MainThreadOnlyDoesNotImplSendSync, ValidThreadKind};
 pub use self::common_selectors::{alloc_sel, dealloc_sel, init_sel, new_sel};
 pub use self::convert::{ConvertArgument, ConvertArguments, ConvertReturn, TupleExtender};
 pub use self::declare_class::{

crates/objc2/src/macros/extern_class.rs

@@ -300,6 +300,7 @@ macro_rules! __extern_class_inner {
             fn class() -> &'static $crate::runtime::AnyClass {
                 let _ = <Self as $crate::__macro_helpers::ValidThreadKind<<Self as $crate::ClassType>::ThreadKind>>::check;
                 let _ = <Self as $crate::__macro_helpers::MainThreadOnlyDoesNotImplSendSync<_>>::check;
+                let _ = <Self as $crate::__macro_helpers::DoesNotImplDrop<_>>::check;
 
                 $crate::__class_inner!($crate::__fallback_if_not_set! {
                     ($($name)*)

crates/test-ui/ui/extern_class_impls_drop.rs

@@ -0,0 +1,14 @@
+//! Test that implementing Drop for a class created with extern_class! fails.
+use objc2::extern_class;
+use objc2::runtime::NSObject;
+
+extern_class!(
+    #[unsafe(super(NSObject))]
+    struct MyClass;
+);
+
+impl Drop for MyClass {
+    fn drop(&mut self) {}
+}
+
+fn main() {}