Add notes about object safety of traits

Author: madsmtm

crates/objc2/src/declare/mod.rs

@@ -152,7 +152,9 @@ pub(crate) mod private {
 ///
 /// This is a sealed trait that is implemented for a lot of `extern "C"`
 /// function pointer types.
-pub trait MethodImplementation: private::Sealed {
+//
+// Note: `Sized` is intentionally added to make the trait not object safe.
+pub trait MethodImplementation: private::Sealed + Sized {
     /// The callee type of the method.
     type Callee: RefEncode + ?Sized;
     /// The return type of the method.

crates/objc2/src/mutability.rs

@@ -256,6 +256,8 @@ mod private {
 ///
 /// This is a sealed trait, and should not need to be implemented. Open an
 /// issue if you know a use-case where this restrition should be lifted!
+//
+// Note: `Sized` is intentionally added to make the trait not object safe.
 pub trait Mutability: private::Sealed + Sized {}
 impl Mutability for Root {}
 impl Mutability for Immutable {}

crates/objc2/src/rc/autorelease.rs

@@ -551,6 +551,9 @@ mod tests {
         pool
     }
 
+    #[allow(unused)]
+    fn assert_object_safe(_: &dyn AutoreleaseSafe) {}
+
     #[cfg_attr(
         not(feature = "unstable-autoreleasesafe"),
         ignore = "only stably ZST when `unstable-autoreleasesafe` is enabled"

crates/objc2/src/runtime/nscopying.rs

@@ -1,5 +1,5 @@
 use crate::mutability::{
-    Immutable, ImmutableWithMutableSubclass, InteriorMutable, MainThreadOnly, Mutable,
+    Immutable, ImmutableWithMutableSubclass, InteriorMutable, MainThreadOnly, Mutability, Mutable,
     MutableWithImmutableSuperclass, Root,
 };
 use crate::rc::Id;
@@ -12,7 +12,9 @@ use crate::{msg_send_id, ClassType, ProtocolType};
 /// and because the return type of those differ if the class has a mutable or
 /// an immutable counterpart (as is the case for `NSString` and
 /// `NSMutableString`).
-pub trait Copyhelper<T: ?Sized> {
+//
+// Note: This trait is intentionally not object safe.
+pub trait Copyhelper<T: ?Sized>: Mutability {
     /// The output type of [`NSCopying`] for the given `T`.
     type CopyOutput: ?Sized + ClassType;
     /// The output type of [`NSMutableCopying`] for the given `T`.