Use structs to represent objects instead of uninhabited enums

examples/foundation_custom_class.rs

@@ -6,7 +6,15 @@ use objc::runtime::{Class, Object, Sel};
 use objc::Message;
 use objc_foundation::{INSObject, NSObject};
 
-pub enum MYObject {}
+/// In the future this should be an `extern type`, if that gets stabilized,
+/// see [RFC-1861](https://rust-lang.github.io/rfcs/1861-extern-types.html).
+#[repr(C)]
+pub struct MYObject {
+    /// See the [Nomicon] for details on representing opaque structs.
+    ///
+    /// [Nomicon]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
+    _priv: [u8; 0],
+}
 
 impl MYObject {
     fn number(&self) -> u32 {

objc_exception/src/lib.rs

@@ -24,7 +24,18 @@ extern "C" {
 }
 
 /// An opaque type representing any Objective-C object thrown as an exception.
-pub enum Exception {}
+///
+/// In the future this will be an [`extern type`][rfc-1861], if that gets
+/// stabilized.
+///
+/// [rfc-1861]: https://rust-lang.github.io/rfcs/1861-extern-types.html
+#[repr(C)]
+pub struct Exception {
+    /// See the [Nomicon] for details on representing opaque structs.
+    ///
+    /// [Nomicon]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
+    _priv: [u8; 0],
+}
 
 /// Throws an Objective-C exception.
 /// The argument must be a pointer to an Objective-C object.

objc_foundation/src/macros.rs

@@ -1,8 +1,10 @@
 #[macro_export]
 macro_rules! object_struct {
     ($name:ident) => {
+        // TODO: `extern type`
+        #[repr(C)]
         pub struct $name {
-            _private: (),
+            _private: [u8; 0],
         }
 
         unsafe impl ::objc::Message for $name {}