Add support for the unstable arbitrary_self_types feature

Tracking issue rust-lang/rust#44874.

Especially useful to allow `self: Allocated<Self>` in the future.

crates/objc2/Cargo.toml

@@ -85,6 +85,10 @@ unstable-autoreleasesafe = []
 # Additionally, the message sending improvements is not yet implemented.
 unstable-apple-new = []
 
+# Uses the nightly arbitrary_self_types feature to make initialization more
+# ergonomic.
+unstable-arbitrary-self-types = []
+
 # Deprecated; this is the default on Apple platforms, and not applicable on other platforms.
 apple = []
 

crates/objc2/src/lib.rs

@@ -151,6 +151,10 @@
     feature = "unstable-autoreleasesafe",
     feature(negative_impls, auto_traits)
 )]
+#![cfg_attr(
+    feature = "unstable-arbitrary-self-types",
+    feature(arbitrary_self_types)
+)]
 // Note: `doc_notable_trait` doesn't really make sense for us, it's only shown
 // for functions returning a specific trait.
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg, doc_cfg_hide))]

crates/objc2/src/rc/allocated_partial_init.rs

@@ -56,6 +56,13 @@ pub struct Allocated<T: ?Sized> {
 }
 
 // Explicitly don't implement `Deref`, `Message` nor `RefEncode`.
+//
+// We do want to implement `Receiver` though, to allow the user to type
+// `self: Allocated<Self>`.
+#[cfg(feature = "unstable-arbitrary-self-types")]
+impl<T> core::ops::Receiver for Allocated<T> {
+    type Target = T;
+}
 
 impl<T: ?Sized + Message> Allocated<T> {
     /// # Safety
@@ -349,4 +356,20 @@ mod tests {
         let obj: Allocated<RcTestObject> = unsafe { Allocated::new(ptr::null_mut()) };
         let _ = obj.set_ivars(());
     }
+
+    #[test]
+    #[cfg(feature = "unstable-arbitrary-self-types")]
+    fn arbitrary_self_types() {
+        use crate::rc::Retained;
+        use crate::{extern_methods, AllocAnyThread};
+
+        extern_methods!(
+            unsafe impl RcTestObject {
+                #[method_id(init)]
+                fn init_with_self(self: Allocated<Self>) -> Retained<Self>;
+            }
+        );
+
+        let _ = RcTestObject::alloc().init_with_self();
+    }
 }